Monday, October 21, 2013

Past Habitability of Mars

This post is one of two that was inspired by discussion questions I encountered from a week in which there weren't too many questions that caught my eye, but those that did have pretty complicated answers. This question was "Because Mars is just on the edge of the habitable zone of the Sun, is it possible that it did, at one time, fall within the boundaries of the habitable zone and, therefore, support life?"


Aerial image of an ancient Martian riverbed taken with ESA's Mars Express orbiter.
To the first part of the question, yes it did. There is ample evidence that Mars used to have liquid surface water, which is THE criterion for the astronomical habitable zone. Of course, Mars has no flowing water today, largely because its surface temperature is (on average) about -63 Celsius. So what changed? To start, Mars needed a thick atmosphere. A thick atmosphere allows the greenhouse effect to occur and keep the planet warm enough for it to be "habitable". Further, a low-pressure atmosphere makes it harder for liquid water to exist on Mars' surface because it would most likely just end up as water vapor in the Martian atmosphere. This seems weird if you don't remember your high school chemistry (I barely do), so I've included a phase diagram for water below. As you can see, at low pressures, water can go directly from a solid to a gas (in a process called sublimation).

Phase Diagram for water. Note that 1 bar corresponds to Earth's atmospheric pressure.
The Martian atmospheric pressure today is 160 times smaller than that of present-day Earth with a surface temperature too low for liquid water. So Mars must have had a thick atmosphere at some point, but where did it go? The most widely accepted solution is best explained through an analogy with Earth.

One of the single most underrated aspects of Earth that allows us to exist at all is Earth's magnetic field. Our magnetic field is created by the rotation of Earth's liquid metal outer core (famously portrayed as shutting down and being nuked into motion again in The Core). Earth's magnetic field has the important job of shielding Earth from the stream of plasma known as the solar wind. The solar wind specifically consists of a stream of protons and electrons emitted by the upper layers of the Sun's atmosphere (specifically the solar corona). This matters because ionized particles are generally bad news for both Earth's atmosphere and living organisms.
Diagram showing how a planet's magnetic field affects its interactions with the solar wind.
As was covered in class, Mars is small enough that it was able to cool completely. Mars' outer core solidified, so it appears to have lost its magnetic field about 4 billion years ago. Therefore, the atmosphere had no protection from the solar wind and has been eroded away ever since. In this scenario, the particles that make up the solar wind collide with the atoms and molecules in the atmosphere, and give them enough energy to escape Mars' gravity. This process also occurs naturally even with the protection of a magnetic field. In short, if an atom or molecule happens to have enough energy to escape the gravitational pull of the planet (in a thermal distribution, this will happen every once in a while). Not having a magnetic field just speeds up the process.

To summarize the above, it appears that Mars did have an atmosphere in the past. It also briefly had a magnetic field that allowed it to keep that magnetic field for some time. There is also abundant evidence that Mars had liquid water on its surface in the past. As such, we have every reason to believe that Mars used to be habitable.

While this seems straightforward, there's actually a big complicating factor known as the "Faint Young Sun problem". The Faint Young Sun problem was first noted by the famous Carl Sagan and George Mullan in 1972. In short, the Sun used to be less luminous than it is today, which would move the habitable zone closer in to the Sun, which would make young Earth, which we know from geologic evidence to have been quite hot, cooler than it should be. This applies equally well to Mars, and possibly more so because Mars is on the edge of the habitable zone today, which could put it entirely out of the habitable zone in the early solar system. I actually alluded to this idea towards the end of my previous post, Discussion Questions: 9/5.

Well that's problematic. We know that Mars had habitable conditions, but suddenly it's a lot harder for us to re-create them in our models. The typical solution to this problem is that Mars likely had a much thicker atmosphere than we initially thought. It is not unfeasible that Mars once had a thick atmosphere. Whether or not it could have lost that atmosphere over the past 4 billion years is still an open question, though it is an active area of research.

Wednesday, October 16, 2013

Discussion Questions: 9/26

Round three. Fight!

I read an article recently about the supermassive black hole in the center of our galaxy. They discovered it had "erupted". Can you explain what this means?
It seems like you are referring to an article such as this one from Discovery.com.

When someone talks about a supermassive black hole "erupting", they're referring to the black hole accreting matter and emitting vast amounts of radiation as a result. Black holes are very sloppy eaters and always leave a mess. As I vaguely alluded to in my previous post, black holes that are in the process of eating have hot accretion disks formed by the material that was ripped apart by the massive gravity of the black hole. This material is, over time, consumed by the black hole. So a few things to note here. When a black hole is accreting, the material being accreted is emitting a lot of radiation.

An eruption is what happens when an object like a star or a gas cloud falls into a supermassive black hole. The object will be ripped apart and heat up as it accelerates inwards. The accreting material will give off a lot of radiation all across the electromagnetic spectrum. Not all of the emission will be radiation though. As I said before, black holes are very sloppy eaters, and will spew out a lot of gas as well (mostly hydrogen). This will only happen during the accretion process, so when the material is used up, the emission will cease, and the black hole will return to what we call its quiescent phase where it's mostly just sitting there.

Sag A*, the SMBH at the center of the Milky Way is currently quiet and not doing a whole lot. The linked article above mentions a nearby gas cloud that may get eaten within the year, which of course will be pretty exciting, so we'll have to keep our eyes and telescopes open.

If I had a green laser pointer and a red laser pointer, does the frequency of the light cause the green laser to shoot farther and be brighter than the red laser? Could you excite the green laser pointer so the beam starts to burn things on contact?
I chose this question initially to address a few misconceptions, but then I realized that there was actually some fun to this question. So let's get to it.

First off, the frequency of light has nothing to do with how bright it is. Green laser pointers will appear brighter than red laser pointers of the same power because of how our eyes work. Your rods, the receptors in your eye responsible for detecting light in general (without which you would be very bad at seeing in low-light environments) are rather good at detecting green light while they do not detect red light at all. (The cutoff wavelength for rods appears to be around 600 nanometers.) As such, the receptors in your eye combined (rods and cones) will detect more photokns from a green laser than they will from a red laser. Frequency only affects the amount of energy carried by an individual photon.

Can you burn things on contact with a laser if it is "excited" enough? If you make the beam intense enough (increase the number of photons per second), yes. At this point, the color doesn't matter too much (depending on the material). If you hit something with enough highly concentrated energy, it's going to burn so long as it's not purely reflecting all of the light you're shining on it.

As for the beam shooting farther... this is actually interesting, and there are two effects that need to be accounted for. First off, laser beams are not perfect. As the light travels, the beam itself spreads out, so the light is less concentrated at greater distances from the beam source. If we assume totally ideal conditions, then the beam spread is dependent directly on the wavelength of the light. Lower wavelengths diverge less. This means that the red laser would spread out more than the green laser over the same distance, so it won't make as good a beam.

The other effect to contend with on Earth is the atmosphere. Light doesn't just travel through air perfectly because air is made of stuff (good luck breathing if this wasn't the case). Photons travelling through air have a tendency to run into some of these molecules once in a while, and end up going elsewhere. This is what we call scattering, and it's kind of like a microscopic version of a reflection, except the end direction can't be predicted. Anyway, this process also depends on the wavelength of the light you're using. The effect here is reversed from the above though. Scattering has a stronger effect on photons with shorter wavelengths, so it has a stronger effect on the green laser.

Combining these two effects, which beam is the most visible farther away from the source of the two lasers? I haven't the (puts on sunglasses) faintest idea.

Looking at the Ring Nebula, which still has the remnant of its core visible, is that core truly dead and giving off no light so that the nebula can be viewed as an emission spectrum?
Well, someone asked about the Ring Nebula, so I have to put a picture of it in this post because damn, that thing is gorgeous.
Hubble Space Telescope image of the Ring Nebula combined from images taken in multiple filters.
The Ring Nebula is the remnant of a star that was probably more massive than the Sun, but not massive enough to explode as a supernova. Rather, in its old age, the star's core became unstable from fusing hydrogen into helium and helium into carbon in shells around an inactive carbon/oxygen core. This is unstable because helium burning is ignited intermittently, resulting in bursts of energy being released from the core. This causes the star to rapidly pulse, varying in luminosity and size on relatively short timescales (yeah, we astronomers consider 10,000-100,000 years to be short). This phase of a star's life is called the thermally pulsing asymptotic giant branch (TP-AGB), and more information can be found here if you're interested. Ultimately, these pulses become strong enough that the outer layers of the star are lost to the interstellar medium, leaving only the hot, inert former stellar core behind.

This is where we find the Ring Nebula. The little white dot seen in the center of the nebula is the remnant of the stellar core that will one day become a white dwarf (when the outer layers finally drift away entirely). Until then, it is certainly a hot, dense object emitting thermally. So why do we mostly see an emission spectrum from the Ring Nebula (as was asserted in class)? First, the white dwarf's surface temperature is estimated at roughly 125,000 K. That corresponds to peak emission at about 23 nanometers, which is just a factor of two below the UV/X-ray boundary. The VAST majority of the thermal radiation produced by this object is not only not visible light, but is also energetic enough to ionize pretty much anything near it.

Why does that matter? The emission that we see from the gaseous part of the Ring Nebula is a result of the material ionized by the radiation from the white dwarf. Specifically, it comes from free electrons re-joining nuclei to form neutral atoms again. While the radiation doesn't directly come from the recombination of the electron with a nucleus, it comes from the electron rejoining into an excited state (which probably emits a photon that we can't see), then falling down toward the ground level (which emits a photon that we can).

To get to the heart of the question, the emission spectrum of the Ring Nebula far outweighs the visible blackbody emission of the central white dwarf because the VAST majority of the radiation from the white dwarf is high enough energy to ionize the gas surrounding it. As that gas moves away from the white dwarf, it cools significantly, de-ionizes, and the electrons fall down to lower energy levels, emitting visible light photons as they go. So while there is definitely thermal visible light coming from the white dwarf, we mostly just see the emission lines from the nebula gas spectroscopically.

Assuming the Earth is a blackbody since it's solid and opaque with a cold gas (atmosphere) around it, is it possible to see the absorption spectrum of Earth?
Absolutely! You'd probably be surprised at how hard this is to measure though.

First, I should note that the atmospheric spectrum of Earth is actually astrobiologically interesting because it gives us an idea of what we should look for in exoplanet atmospheres if we're looking for some familiar kind of life. Unfortunately, any spectra we would see of distant planets would be from unresolved sources, meaning we wouldn't be able to distinguish individual locations on that planet. So we need a way to look at the Earth as if it were a single point. Because every telescope we have is awfully close to Earth (even the space telescopes), this is not trivial. Fortunately some rather smart people have thought about this and have come up with a very interesting solution: look at light that is reflected from the Moon!

No, I'm not making that up. Yes, it works. The Moon itself has a very featureless spectrum, and is very good at reflecting light. There are two different types of atmospheric spectra you can observe this way: a reflection  spectrum and a transmission spectrum. The reflection spectrum is sunlight that reflects off of Earth's surface, and onto the Moon, then back to telescopes on Earth. The reflection spectrum is best observed during a solar eclipse, but can also be generally observed during a new Moon, you just have to be more careful about the atmosphere. The transmission spectrum comes from light that only passes through Earth's atmosphere in a grazing fashion on its way to the Moon. This spectrum is best observed during a lunar eclipse.

Each of the above is a way that we have measured the atmospheric spectra of exoplanets, mostly Hot Jupiters at this point because they're the easiest to observe in many respects. We would love to make such measurements for more Earth-like planets, but this requires some specialized instruments for which there were, at some point, plans that have since been scrapped (much to the dismay of Jim Kasting).

Of course, you probably want to see what such a spectrum looks like in the infrared. The following graph was adapted from Christiansen & Pearl (1997), and the x-axis is given in the god-awful units of wavenumber, which I hate with a fiery passion. For whatever reason, some people like to talk about spectra with wavenumber, which is just the inverse of the wavelength. For reference, wavenumber of 200 corresponds to 50 microns and wavenumber of 1000 corresponds to 10 microns, so wavelength actually increases to the left on this plot. Yes, I know. It's stupid and unintuitive. Sorry.
Infrared spectrum of the Earth from 6 microns (far right) to 50 microns (far left)

Wednesday, October 2, 2013

Discussion Questions: 9/17

Round two of Suvrath's discussion questions. This time, I didn't even try to answer some of the questions because they involved discussions of ethics that I am definitely not qualified to do. I will stick to the scientific questions that I can actually answer without projecting my opinion into the answer (too much).

Why is an incandescent bulb a good example of a blackbody where a fluorescent light bulb is not?
This all comes down to how the two different bulbs work.

An incandescent bulb works by running a current (moving electrons) through a filament that acts as a resistor. A resistor is just any material that doesn't allow charges to move through it quite so easily. Electrons moving through a resistor will lose energy as they go. This energy heats up the resistor, which will eventually get hot enough to glow. Because the filament is a hot, dense object, it will emit like a blackbody, which means it emits light all across the entire visible spectrum. Of course, this also means it is emitting light at non-visible wavelengths also, which is a huge part of why they are so inefficient.

Fluorescent bulbs work by running a current through a diffuse (generally) mercury gas. And, as you know by now, a hot, diffuse gas gives off an emission spectrum. Why do we use mercury though? (Hint, it's not because someone wants to poison you, though it's a good reason not to inhale too deeply right after you break a fluorescent bulb.) Here's an emission spectrum of mercury.
Mercury is a good choice because it emits light at colors that span the whole visible spectrum. Unfortunately, it also emits an awful lot of UV light as well. But we can fix this! We have the technology The coating on a flourescent bulb (this coating seems to be what causes them to appear that milky whitish color) specifically absorbs UV photons and spits the energy back out as visible light, covering even more of the visible spectrum. So you can get all of your basic colors looking like they're supposed to and get the same amount of light far more efficiently than incandescent bulbs. Sounds good to me.

This is not to say that fluorescent bulbs produce no waste heat. They just produce less of it than equivalent (visual) brightness incandescent bulbs.

Are there any stars whose peak emission is not invisible light. If so, would they still be capable of supporting life?
Absolutely! In fact, we can calculate exactly when the surface temperature of a star makes the peak wavelength of its spectrum no longer corresponding to visible light using Wein's law!
Where T is the surface temperature of the star and λ is the wavelength (color) at which the star emits the most light. The violet edge of the visible spectrum occurs at about 400 nanometers, or 0.4 microns (in the units given above) while the red edge occurs at about 700 nm (0.7 microns). Wein's law tells us that stars with temperatures higher than 7250 K and lower than 4100 K will have peak colors outside of the visible spectrum.

Can these stars have habitable zones? Absolutely! Whether or not a star has a habitable zone doesn't depend on its peak color, but on its total energy output. For main sequence stars, a star's temperature, mass, luminosity, and size are all correlated to one another, so bigger stars tend to be hotter and more luminous. The only difference will be that the cooler stars will have smaller habitable zones that are closer in to the star, while hotter stars have larger habitable zones that are farther away. For more than you probably ever wanted to know on habitable zones, check out my previous post specifically on the topic of habitable zones.

There is one critical difference between hotter and cooler stars that would affect the potential for life to develop. Hotter stars live shorter lives. Why does this matter? Life as we know it on Earth took 4.6 billion years from the formation of the solar system to develop. Could intelligent life develop faster? Possibly, but it may also take longer than it did for us. With a sample size of 1, there's no way to tell. But it's probably a safe bet that life needs a decent amount of time to come into existence in the first place, thereby favoring smaller, cooler stars.

What do the spectra of non-blackbody radiators look like?
Conveniently, I was able to cover some of these in class when I subbed for Suvrath. Of course, I only really covered the basics that we know of from (you guessed it) Kirchoff's Laws of Spectroscopy. There is another type of spectrum that isn't covered in typical undergrad astronomy: the power law. A "power law" spectrum is a spectrum whose shape is given by the mathematical relationship
The symbol Iν stands for the specific intensity, or the amount of light emitted at a given frequency, while the symbol ν is generally used to represent frequency, for some reason. α here is just used as a stand-in for some positive number Ultimately, a power-law spectrum will look like Figure (number this appropriately).
General shape of a power law shown on a linear plot. Image courtesy Wikipedia.
Now, the math-savvy readers may note that the equation above should asymptote (approach infinity) as ν approaches zero. This is generally fixed by limiting the range of the power law at low values of whatever variable you're considering. In typical jargon, this is to say that the power law "turns over". Power laws occur an awful lot in nature, but that's a subject for another time.

So what kind of weird objects produce a continuous spectrum that isn't a blackbody? Anything with jets. A "jet", in astronomical terms, is a beam of radiation and energetic particles travelling in the same direction. Generally, objects that emit jets are emitting two jets in opposite directions and are actively accreting matter from a disk. The most interesting of these are black holes. A stellar-mass black hole that is stealing matter from a nearby star with which it shares an orbit (low-mass x-ray binary) or a supermassive black hole that is eating anything unfortunate enough to come too close will have a disk of rather hot material orbiting it. As shown in the artist's depiction below, the two jets will be emitted perpendicular to the hot accretion disk.

While the precise origin of jets is still a bit uncertain, there is reason to believe that they, to some degree, are related to the magnetic fields created from the rotating disk of material around the black hole. The evidence for this is in the spectrum of the radiation. Synchotron radiation is emitted from charged particles rotating in a magnetic field. We expect that particles in the jet will, themselves, have a power-law distribution of energies, which gives the emitted radiation a power-law spectrum as well, where the emitted power (energy emitted every second) at a given frequency goes as
P just stands for the emitted power and p is a variable whose value depends exactly on the source of the synchotron radiation. In general, synchotron emission comes out at radio wavelengths.

Edit: We also get power law spectra from what is known as inverse Compton scattering. In inverse Compton scattering, the hot accretion disk emits photons thermally that bump into very energetic electrons that are flying around in the accretion disk's corona. The corona in this case is just the region above and below the accretion disk which contains very high energy particles flying around. The photons will crash into a relativistic electron and actually gain energy from the collision, which makes them into x-ray photons. However, this has nothing to do with the jets. It's just another source of continuous power-law emission.

Man, I had to dig up my 502 notes for that question. If any of my friends see fit to correct me on what I've said about jets, I'll post updates.

What is it about "green" stars that causes them to appear white to the human eye? Also, why aren't there purple stars?
A star whose peak color (see Wein's Law above) occurs at green wavelengths (~550 nanometers) isn't just emitting green light. In fact, it isn't even emitting mostly green light. It just happens to emit more green light than it does light at any other individual color, but not by much. To illustrate this, I've created a little plot in Excel that shows the blackbody spectrum of an object whose emission peaks at 550 nm (about 5270 K) that spans the whole visible spectrum of light.
That's not a huge difference. While there is definitely a maximum at 550 nm, emission at other colors isn't too far behind.

Now we get into how the human eye works a bit. We have two different types of receptors: rods and cones. Your rods just detect light while your cones are responsible for detecting color. The cones in human eyes, in particular, detect light at red, green, and blue wavelengths, and blend the combination of these three into any color we can perceive. This is why old cathode tube TVs and computer monitors have pixels comprised of red, green, and blue bars, or why Photoshop or MS Paint create colors as a mixture of red, green, and blue. If you have roughly equal amounts of red, green, and blue, the object will look white. This is what happens for "green" stars.

This is also why we can't see purple stars. A given star's spectrum could very well peak at purple wavelengths, but the detectors in our eyes will only see them as blue.

Tuesday, September 24, 2013

9/18 Colloquium: Thirty Meter Telescope

Today's Astronomy and Astrophysics colloquium was from Dr. Warren Skidmore of the Thirty Meter Telescope (TMT) Observatory Corporation. Long story short, Dr. Skidmore was mostly trying to sell us on the idea that the Thirty Meter Telescope project is actually going to happen. Much of astronomy thinks of the TMT as a far-off pipe dream, something too big or too expensive or too likely to be plagued by persistent mechanical failures.  Obviously the issue couldn't be discussed in too much detail in a one-hour talk, but the TMT Corp says the design is doable and that the TMT could be online and gathering scientific data in as little as 10 years.

So... why do we want to build a telescope that big in the first place?

To start, I should talk about exactly what I'm talking about when I refer to a telescope's size. The size of a telescope is usually given as the full diameter of the telescope's aperture, the opening through which it gathers light. In most basic telescopes, this will be the same as the diameter of its mirror, but not always There are designs where the mirror is bigger than the aperture, like the design used on the Kepler spacecraft that has its own special benefits (turns out that telescope design is complicated as hell!). So the Thirty Meter Telescope is the project to built a telescope whose mirror is thirty meters across. That's one third of a football field, or more than the distance between home plate and first base. That's also three times the diameter of the largest optical telescope in the world, the Canaries Great Telescope.

"Size matters not. Look at me. Judge me by my size do you?"
From this quote alone, we can tell that Yoda was not an astronomer. For telescopes, size matters a lot. For a telescope, actually for any light-collecting instrument like binoculars or a camera, aperture size gains you two things. First, bigger mirrors can gather more light simply by having a larger surface area, so you can see fainter objects more easily. Second, a larger telescope allows a higher angular resolution. A telescope's angular resolution is just a fancy way of saying how well it can tell that two individual, but close-together, objects are actually two distinct objects or how well it can distinguish the small features of an object.

While these sound great all-around, bigger isn't always better. For one, as telescopes get bigger, they get very expensive to build (cost estimates of the TMT are roughly 1 billion dollars). Second, you don't always need to gather as much light as physically possible. If you're studying nearby stars that are relatively bright, for instance, you can get away with using a smaller telescope so you don't overload your detectors.

Big telescopes getting better images isn't automatically the case though. Large telescopes still have to deal with the #1 source of bad images in all Earth-based telescopes: the atmosphere! In fact, for big telescopes the problem becomes even worse because of the the telescope is looking through a larger column of atmosphere. This wouldn't be a problem if the atmosphere was perfectly still. So naturally, just to spite us, it isn't. We have constant air currents caused by temperature differences between the air and the ground or between different parts of the atmosphere. Figure 1 below compares an image of a galaxy in the Hickson Compact Group 87 taken with the Hubble Space Telescope (2.4 meters) to the same galaxy as imaged by Gemini South, an 8-meter telescope in the mountains of Chile.
Image courtesy AURA: http://www.aura-astronomy.org/news/archive/hst_vs_ao_2.pdf
So why bother with large ground based telescopes at all if they're just going to be trumped by those in space? Because we can, in fact, beat the atmosphere.

Well, more accurately said, we can sometimes correct out the flaws in our images caused by turbulence in the atmosphere if we do it very carefully. The general name for the systems that perform these operations is "adaptive optics". There are a number of different ways such a system can work. At its essence though, AO systems use a series of small re-positionable or flexible mirrors to undo all of the light-bending that comes from our atmosphere. In practice, real AO systems are VERY complicated (as I'm learning right now in Larry Ramsey's seminar), so I'll spare you the gory details. If you want to read about AO in some detail, I highly recommend Claire Max's ASTR 289 class page from UC Santa Cruz.

What can adaptive optics do for us? The following figure shows two images of the IW Tau system taken with the 200-inch Hale Telescope on Mount Palomar with and without their adaptive optics system.
IW Tau imaged with and without adaptive optics. Image courtesy http://www.astro.caltech.edu/palomar/AO/
Correcting for the atmosphere allows you to approach what we call the "diffraction limit", the telescope's natural resolution based on its size. I say "approach" because you never quite get to the diffraction limit itself (yet?), but you can get pretty close with the right systems. Suddenly space-based telescopes look pretty obsolete huh?

Not so fast. As good as adaptive optics can get these days, there's one part of the atmosphere that we can't correct for no matter how hard we try: absorption. Our atmosphere is made up of a number of molecules that like to do annoying things such as absorbing radiation that we astronomers would really like to see. Figure 3 below shows how atmospheric transmission varies with wavelength from the part of the spectrum we see to the far infrared. You'll probably want to click on the image to see the full version. Anything beyond visible tends not to be that interesting as the transmissivity just drops off quickly as you get to higher and higher energies, with basically no transmission past mid-ultraviolet.
While we do okay in the visible, anything in infrared pretty much sucks. There are clearly some parts of IR where we can observe from the ground and do a decent job of it. But for major infrared observations, particularly mid- or far-infrared, we really want to go to space. The catch, of course: putting telescopes in space is way more expensive.

Let's bring this back to the topic originally at hand. We've established that for large telescopes, many of the problems that result from Earth's atmosphere can be fixed with adaptive optics, which makes large optical ground-based telescopes scientifically useful. The amount of science we could get from such a massive telescope is pretty exhaustive, and I recommend checking out some of the documents available at http://tmt.org/documents. I'm also not going to get too involved in the engineering aspect of such a telescope, because I'm an astronomer. I hope I've done a decent job outlining some of the benefits and challenges faced by large telescopes. It's also good to finally finish this post, because I've clearly been working on it for nearly a week, and I have at least 2 other drafts and one idea waiting to be finished, including student questions Round 2. Thanks for reading!

Wednesday, September 11, 2013

Discussion Questions: 9/5

As part of doing the stuff that pays my tuition for this semester, I am TAing for Suvrath Mahadevan's AST-140: Life in the Universe class. Suvrath takes attendance on a semi-random basis by having his students submit "discussion questions" relating to the material covered in class that day. My job is to grade the question based on how insightful it is, and how relevant it is to the material covered that day. After the test round of questions, I decided that some of them were so interesting that they deserved answers, so I scribbled answers on the papers. Some questions deserved much longer answers, so I thought to make answering such questions part of my blog. All questions will be anonymous, and the students may opt out simply by indicating as such when submitting the question. I may answer some questions in combination with other questions when convenient. Without further ado, I present the first round of questions.

Why is it that elements such as hydrogen, helium, carbon, and oxygen are common within the universe when elements like lithium and beryllium are not?
I actually got this question from a few different students, and it's a good one. The three elements that occur between helium and carbon on the periodic table, lithium, beryllium, and boron, exist in noticeably smaller quantities throughout the universe than the elements surrounding them (as you can see in Figure 1 below). The answer to this question is based on how these elements are created.
Figure 1: The full solar abundance pattern of naturally-occurring elements.
As I have mentioned briefly in my previous post on cosmology, a number of elements were created in the Big Bang. The most common element in the universe, hydrogen, was solely created in the Big Bang; it has only been destroyed in stellar fusion since then. Helium was also created in fairly large amounts; 1 helium nucleus was created for every 12 hydrogen nuclei in the early universe. Very small amounts of lithium, beryllium, and boron also came out of the Big Bang, but far less of them than hydrogen or helium. Helium's abundance has increased a bit for the same reason that hydrogen has decreased, but the changes are mostly negligible (1 to 2 percent). Today, Li, Be, and B are actually created when high-energy cosmic rays slam into other atoms (like carbon) and actually knock protons out of the nucleus which turns it into boron. This process has the kind of awesome name of "cosmic ray spallation".

The reason carbon, nitrogen, and oxygen are so abundant relative to the previous three elements is because they are all created in stars. I'll discuss the major processes for this a bit later on, because they're relevant to another question, but these elements are created in the cores of stars that have evolved off the main sequence. This process occurs in every star as it grows old and dies. We expect that, over the lifetime of the universe, enough stars have gone through their life cycles that the universe has been able to accumulate a relatively large amount of these common, relatively easy to make, elements.

Ultimately, the processes that create carbon, nitrogen, and oxygen are far more common in the universe than those that can create lithium, beryllium, and boron, so we get more C, N, and O in the universe.

How rare is lithium compared to gold or silver? It seems that using it for batteries is wasteful if there isn't that much of it.
We can actually answer this by looking back at Figure 1. Lithium (Li) and gold (Au) are both marked on the figure I grabbed off the Internet, while silver (Ag) occurs at atomic number 47 (on the x-axis). You should be able to tell fairly quickly that lithium is actually more abundant than both gold and silver. Of course, that's just elements in the universe as a whole. If we look at the amount of each Earth's crust, you'll see that... there's still more lithium than gold or silver. I was actually interested to learn that even uranium is more abundant in Earth's crust than either gold or silver.

Also, if we weren't using lithium for batteries and such, what would it used be for? OK, maybe there would be a market for shiny rock-looking things that can float in oil (it can float in water, but it also tends to chemically react with water). The value of an elements isn't just based on how rare it is, but also how useful it happens to be. So if it was useless, who cares that it's rare?

Can massive stars fuse helium into other elements like they can fuse hydrogen into helium?
Is there a limit on how many elements can form in a star? If so, how are the other elements formed?
These two questions are just literally begging to be answered together. Before I dive into this question head-on, I should clarity that it doesn't take a massive star to fuse helium into heavier elements. Our own sun will eventually get to a point where it can fuse helium into carbon, but that won't happen for another 5 billion years or so.

As main sequence stars age, they slowly "burn" (when I say "burn" with respect to stars, I mean nuclear fusion, not chemical burning) through all of the hydrogen in their cores. Now, keep in mind that stars are stable bodies because the energy released from nuclear fusion works to balance out the gravitational force that would otherwise cause the star to collapse on itself. When you run out of hydrogen fuel for fusion, things become unstable, and the core shrinks rapidly. Squeezing the now mostly helium core causes its internal temperatures and pressures to increase. Under these conditions, you can squeeze three helium nuclei together to create carbon.

I need to pause my narrative here to explain a few things. Helium fusion can only occur under such extreme temperatures because helium has a more positively charged nucleus than hydrogen does (two protons rather than one). This means that squeezing two helium nuclei together takes a lot more energy because the repulsive force is much stronger. Helium fusion is also discouraged because the nucleus that would be formed from two helium nuclei coming together (beryllium-8) is hilariously unstable. A beryllium-8 nucleus will break apart into two helium nuclei within about billionth of a billionth of a second. Therefore, you need to actually bring three helium nuclei together within a very, very short period of time to get a stable nucleus out (carbon).

So post-main sequence stars can stay stable by "burning" helium into carbon. Now we get carbon building up in the star's core (with some carbon combining with helium to create oxygen). At this point, we get two very different paths in stellar evolution. To make a long story short, stars with masses similar to that of the Sun will stop here, become unstable, and blow off their outer layers forming a planetary nebula with the former core of the star eventually becoming a white dwarf.

Even more massive stars can reach higher still temperatures and pressures, which allows them to fuse even heavier elements. Stars with masses higher than about 8 times the mass of the Sun can fuse elements up to iron in their cores, with shells of previously fused elements surrounding the core like the layers of an onion (ogres are like onions and post-main sequence stellar cores). An illustration of this (with the core shown dramatically expanded with respect to the remainder of the star) is shown in Figure 2.
Figure 2: Illustration of the layers that form around the core of an evolved high-mass star. Not to scale at all.
There is actually a limit on what elements can be created in stars. Stellar fusion can only create elements as heavy as iron. Why iron? Iron happens to be the most stable nucleus in the universe. It takes more energy to stick anything to an iron nucleus than you would get out of the resulting fusion reaction. Therefore, high-mass stars build up an iron core and basically stop there. How the other elements are formed will be discussed in the next questions.

If a star is constantly contracting, then when does it expand before it goes supernova?
How is uranium formed?
I am so happy that someone asked these questions, in part because they make an excellent follow-up to the previous two questions, in part because I f***ing love supernovae.

To start, stars are not constantly contracting. As I described above, once stars begin fusing hydrogen into helium, they are actually very stable until the stars start fusing much heavier elements. But once enough iron builds up in the center of a star, you have a problem. Because you can't fuse iron to make heavier elements, you lose the star's main source of energy. NOW the star's core begins to collapse because the star's gravity finally wins. The core is too massive to be supported by normal gas pressure, and everything gets squeezed so close together that the free electrons flying around are forced to combine with protons, yielding neutrons. The core of this massive star will eventually become a neutron star (or, under the right circumstances, a black hole).

Once the core has collapsed, there's literally nothing holding up the rest of the star. The inner layers collapse first, because they're the first parts of the star to realize that something has gone horribly wrong. The inner layers fall onto the proto-neutron star (PNS). However, the proto-neutron star is as dense as physically possible without becoming a black hole (this takes much more mass than a typical proto-neutron star has). The material that collapses onto the PNS will essentially bounce back outwards away from the core. This all happens very fast, and the rebounding material will quickly run into other layers that are still falling towards the PNS. This collision will create an outward-moving shockwave that ultimately shoves out the remaining layers of the star in what we call a supernova explosion.

At some point during this process (exactly when is still a matter of much study and debate), conditions are just right for the r-process to occur. The r-process, or rapid neutron capture process, occurs when nuclei are literally bombarded with free neutrons. The nuclei will capture these neutrons and become very unstable. Instead of just breaking apart though, the captured neutrons will change into protons and thus change the nucleus into a heavier element. This process works to create elements as heavy as uranium. Creating elements beyond this point doesn't work because the nuclei will break apart under neutron bombardment. And this is how pretty much all elements heavier than iron are formed, uranium included!

How long will it take before the Sun grows so large that Earth will no longer be habitable? Please explain.
A little less than 1 billion years.

As the Sun ages, it is actually getting steadily brighter. Over time, as the Sun fuses its core hydrogen into helium, the helium steadily builds up in the core. Because each single helium nucleus takes the place of what were once four separate particles, this decreases the gas pressure in the core very slightly. To adjust for this, the core contracts, heats up, and increases its fusion rate. Increasing the fusion rate also increases the energy output of the Sun. The predicted increase in brightness is shown graphically in Figure 3. Under the current circumstances, in a little less than 1 billion years, the inner edge of the Sun's habitable zone will have moved out beyond Earth's orbit, so Earth would become too hot for liquid water to exist on its surface.
Figure 3: The solid black line shows how the Sun's energy output has changed over time as a fraction of its current brightness. The shaded gray region shows the temperature range of the Earth if Earth's atmosphere remained unchanged. Figure is adapted from Kasting & Catling 2003.
Long story short, we've got a billion-year timer to get off this rock.

Why do the eccentricities of planets vary so greatly?
Planets' orbits are eccentric because planets don't form or exist in isolation. Planets don't just feel the gravitational pull of the stars they orbit; they're also getting tugged every which way by the other objects in their planetary systems. In many cases, planets appear to have migrated since their formation. This means pretty much what you think it would; planets may not necessarily have formed where we find them today. The different types of migrations, classified by what type of interaction causes them can be found here. When planets move into different orbits, the eccentricity of the orbit will almost certainly change as well. Interactions with other objects can both increase and decrease a planet's eccentricity. It just depends on the specific interaction that takes place.

To finally answer your question, the reason that all of the planets in our solar system have different eccentricities is because each planet experiences different sets of interactions that every other planet.

Tuesday, September 3, 2013

Some Planetary Science in Mass Effect

In my last post, I talked about a not-so-accurate description of the star Sheol in Mass Effect 2. But when reading more about the verbal description of the planet, I saw a few other things that interested me. Rather than try to stuff all of that into one post, I decided that I'd write this one up separately so I can focus on the details more than I would have been able to otherwise.

We'll start with the same image I used yesterday to show what Mass Effect 2 tells us about the planet Gei Hinnom in the Sheol system.
This time, instead of focusing on the numbers (though they will still be relevant), we're going to look at the first paragraph of the planet's description. The Codex entry describes Gei Hinnom as "nearly atmosphere-less" and "tidally locked".

Being tidally locked means that the same side of the planet is always facing the star. You can actually see this in the numbers characterizing the planet. First, the planet's orbital period is the same length as its year, so it rotates (on its axis) once for every time it revolves (around the central star). Second, look at the planet's surface temperatures. I say temperatures (plural) because there are three listed: temperatures of the day side (108 Celsius), the night side (-120 Celsius), and the so-called "habitable zone" (35 Celsius). Note that this is not the "habitable zone" as I have discussed it previously; this is just the region on the planet between the day and night sides of the planet where it is neither too hot or too cold. Because Gei Hinnom doesn't rotate with respect to Sheol, the energy from the star doesn't get distributed over the entire surface of the planet. Therefore once side stays very hot, one side stays very cold, and there's a strip in the middle that is actually a pretty decent temperature for settlements (mining settlements it seems).

Being "nearly atmosphere-less" also contributes to the large difference in the temperature of each side. On a planet like Venus, which rotates VERY slowly but has a VERY thick atmosphere (about 90 times as much atmosphere as Earth), the energy from the Sun is mostly distributed over the surface of the planet. As such, the surface of Venus is more or less a uniform temperature all over. Because our fictional planet has no atmosphere, its two sides remain at very different temperatures.

Let's return to Gei Hinnom being tidally locked for a bit. Tidal locking typically occurs when the orbiting body is particularly close to the object it is orbiting. The reason for this can be found on Wikipedia just from looking at the formula for the approximate "tidal locking timescale", which is how long it takes for an object to become tidally locked (I have re-created the formula here using LaTeX). I recommend looking at the Wikipedia entry to see exactly what each symbol in the equation means, but I'm going to focus in on the most important variables just to illustrate a point.
Two variables should really stand out here: a (the distance between the two objects) and R (the size of the smaller object), because they're both raised to huge powers (6 and 5, respectively). This tells us that the time for a planet to become tidally locked is MUCH shorter if that planet is close to its star. Also, larger planets become tidally locked faster than smaller ones (though the range of planetary sizes isn't nearly as large as the range of planetary orbital distances).

In the image above, we see that Gei Hinnom orbits about 0.83 AU away from Sheol. That makes Gei Hinnom a bit farther away from Sheol than Venus is from the Sun. That's definitely too far for Gei Hinnom to be tidally locked under normal circumstances. Mercury and Venus aren't tidally locked to the Sun (well, Venus is kind of a special case, but it isn't tidally locked. See Venus: Orbit and Rotation.) so why should this theoretical planet that is farther away from its star be tidally locked? It may easily come down to something as simple as the formation of the planetary system itself. Maybe Gei Hinnom formed with a naturally slow rotation (represented by the symbol ω in the equation above), or maybe something slowed it down (a large impact early in the planet's history?). Of course, I can only speculate on the evolutionary history of a totally fictional planetary system, but hey, speculation is fun!

Thursday, August 29, 2013

Mass Effect and Kepler's Third Law

For those of you who may be unfamiliar with the games, the Mass Effect series consists of 3 science-fiction action-RPGs (role-playing games) set in the not-quite-so-distant future (~1250 years into the future), at which point humanity has long since become a spacefaring race and quickly learned that we were not alone in the universe, much less the galaxy. Needless to say, because this is a science fiction game, there is a fair amount of really hilariously bad "science" in the game invoked to make things like FTL travel and the like work (just read the Mass Effect Wiki entry on Element Zero for an example). But that's not what I'm here to talk about. Rather, I'm going to talk about some of the science that they do right... sort of.

As part of Mass Effect 2 in particular, you fly your spaceship around to explore planets in other star systems (this happens in the other two games as well, though in a different sense). While I could probably do a whole post on the featured planets of the Mass Effect universe, I'll refrain for now, mostly because that would take a lot of work on my part. Some of the planets (a definite minority) are story-critical inhabited planets that you have to visit in order to progress the plot. There are still other planets on which you can find side-quests (for you non-gamers, a side-quest is optional and not really important to the story overall). The vast majority of planets, kind of unsurprisingly, are totally uninhabitable but can be mined for various resources (easily the most mind-numbingly dull part of the game, hence why I just edit my save files to give me more resources than I'll ever need) that are used for upgrading weapons/armor/your ship and so forth.

Most interestingly for me, all planets come with some sort of description. This can be as dull as "[planet name] is a standard Jovian gas giant with [some feature]..." or can feature an interesting description of the planet's history, interactions with other bodies in the system, and, once in a while, a description of the central star of the system. The best of these (as uncommon as they may be) even list the spectral type of the star, or give some indicator of what it may be. Beneath the verbal descriptions, you often get some numerical descriptions of the planet that can include its orbital period, semi-major axis (orbital distance from the star), surface gravity (measure of the gravitational force on the planet's surface; requires planet to be solid), surface temperature, the radius of the planet, day length, and so on.

What makes this really cool is that, through the full form of Kepler's Third Law, which you can derive from Newton's Law of Universal Gravitation (getting Kepler's laws right was one of the early successes of Newtonian gravity), we can actually calculate the mass of the star itself! Like I continuously told my students this past summer, the best way to measure an object's mass is to put something in orbit around it.

This requires the approximation that the mass of the star is much greater than the mass of the planet if the mass of the planet is unknown (it generally is not given in the Mass Effect entries) which is usually a pretty good assumption. As an example, Jupiter is about 1000 times less massive than the Sun. For an astronomer, a massless Jupiter is a pretty good approximation (and boy do we love our approximations!), so for this calculation, we'll pretend that planets are actually massless points.

So, we start with the Newtonian form of Kepler's Third Law.
where G is Newton's Gravitational Constant, T is the orbital period of the planet, R is the distance between the planet and the star it's orbiting, and M is the mass of the star. You can derive this yourself if you really want to, or you can take my word for it. G, π, and 4 are all constants, and T and R are given for the system in question, so the only unknown is the star's mass. When we cross-multiply and solve for M, we get
So we can measure the masses of the stars in the Mass Effect universe! As you could probably guess, I was very excited to figure this out because I am a huge geek.

Naturally, as soon as I realized this, I had to put it to the test. So let's take the example of the planet Gei Hinnom in the Sheol system. (Yeah, I have no clue how they came up with the names of most of these, though apparently "Sheol" translates as "grave" in Hebrew.) This is a direct screenshot from Mass Effect 2, except it has been modified so the full text description is shown. The description begins in the left panel and ends in the right panel (with a little overlap between the two).

So this planet is 0.83 Astronomical Units away from the star Sheol and orbits once every 0.8 years. We are also told that Sheol is a red dwarf star, which likely means it is an M-dwarf, though it could also be a late K-star. (Again, see the Wikipedia entry for stellar classification.) If we use the above equation (and Wolfram Alpha for our calculations), we find that Sheol has a mass of 0.89 times that of the Sun.

Wait, what?

A star with a mass of 0.89 M☉  is not a red dwarf. A reasonable upper limit on the mass of a red dwarf is probably around 50-60% the mass of the Sun. I think the writer for this blurb, whoever they may be, just didn't do their research properly. There are other systems in Mass Effect 2 that actually get their numbers right, but I don't remember what they were called or where to find them. Oh well.

One of the great things about the Mass Effect series is Bioware's attention to little details about the characters, the individual worlds, the continuity (turns out Conrad Verner has a Ph.D., as you learn if "Shepard bothered to interact with Conrad twice before and Conrad didn't die horrifically both times"). But naturally, they can't get everything right all the time, and this seems to be one of the cases where they didn't. Of course, this is something that almost no one would ever notice, and certainly no one reasonable. So in terms of doing stuff wrong, it may as well be something you can get away with, unlike the ending to Mass Effect 3.

Tuesday, June 25, 2013

Ask a Scientist: Are there any other planets that have an atmosphere?

Recently the Eberly College of Science here at Penn State has put together an "Ask a Scientist" program, where anyone can ask a scientific question and it will get relayed to a volunteer with expertise in that area of science. Because I (obviously) signed up and noted that I'm a planet geek, I got forwarded the question in the title. Because I've been lacking for posts recently and this seemed like a good kickoff point for my return, my response was as follows.

Question: Is there any other planets that have an atmosphere. (sic)

In short, yes!

Within our solar system, all of the inner planets except Mercury have atmospheres. But both Venus and Mars have atmospheres that are very different from Earth's and very different from each other's as well. Venus has an atmosphere that is about 90 times heavier than Earth's and mostly made of carbon dioxide. Because of this, Venus has a surface temperature of around 850° Fahrenheit, which is way hotter than we ever get on Earth. Mars is the exact opposite, with a much thinner atmosphere than Earth. Mars probably once had an atmosphere similar to Earth's, but that was about 3.5 billion years ago.

The gas giant planets like Jupiter, Saturn, Uranus, and Neptune all have atmospheres as well, but they're different from Earth because they have no solid surface. All four of them still have weather though. Jupiter in particular has what we call the Great Red Spot. The Great Red Spot is actually like a giant hurricane, big enough that you could fit three Earths inside it. Neptune also used to have a large visible storm in its atmosphere, but it seems to have gone away since our last probes flew by.

Something else you may find cool is that we even know of a moon with an atmosphere in our own solar system! Saturn's moon Titan has an atmosphere  just slightly thicker than Earth's, except it's mostly made out of methane. What's really interesting is that Titan also has lakes of methane on its surface and probably even rains methane! On Earth, methane is usually just a gas, but Titan is cold enough that it can easily be either a liquid or a gas, just like water here on Earth.

To take your question even further, we've even seen planets around other stars that have atmospheres! Because the easiest ones to see are the biggest exoplanets, the atmospheres we've seen on exoplanets tend to resemble that of Jupiter so far. Of course, what we're really interested in is finding planets whose atmospheres look a lot more like Earth, and that's probably only a few years into the future.


While, yes, I did give a bit of a simplified answer, I didn't want to write a whole essay on this, and it was targeted at a 4th grader. I should note that there is plenty of other cool stuff out there, even in our own solar system. As I learned while doing some research during my Climate Dynamics course last semester, both Venus and Titan have Hadley cells in their atmospheres, very much like Earth, but also different in a number of respects. I may do a post about Hadley cells in general later on. Just wanted to spark your interest!

Wednesday, April 24, 2013

Happy Birthday, Hubble Space Telescope!

23 years ago today, NASA launched the space shuttle Discovery which carried the Hubble Space Telescope into low-Earth orbit. Since then, despite the infamous initial problems with Hubble's primary mirror, Hubble has been a workhorse of the astronomical community and is probably the most well-known telescope in human history. It's hard to encounter someone who hasn't seen one of the classic images from Hubble at some point in their lives, whether they know it or not. Even with its many scientific successes, I think that Hubble's greatest success has been its unmatched ability to bring the universe to the public through the beautiful, iconic images it has taken over the years. Without any doubt in my mind, Hubble lives up to being one of NASA's Great Observatories and may well be the greatest observatory we've ever had (for now).

In 2001 NASA conducted an online poll to determine what object Hubble should image to celebrate its eleventh year of operation, with the overwhelming winner being the Horsehead Nebula, in the Orion Molecular Cloud Complex. The result was the following image, taken in visible light (with some very near infrared).
Image Credit: Space Telescope Science Institute. http://www.spacetelescope.org/images/heic0105a/
The Horsehead Nebula is a cloud of cool, mostly molecular, gas and dust in which star formation is currently occurring. The gas and dust of the Horsehead Nebula are dark because, unlike stars or the brilliant planetary nebulae that form when stars like the Sun die (see: Ring Nebula), are cold. The average temperature for the cool molecular gas and dust found in star formation regions is between 10 and 50 Kelvin or so. That doesn't mean that all of the gas is this temperature. A large portion of the gas in the Horsehead Nebula is ionized because of young, massive nearby stars whose intense radiation are essentially blowing the nebula apart. One of these stars can just be seen through the clouds of the nebula toward the upper left side of the nebula.

The almost uniformly bright background that can be seen in the image against which the nebula is silhouetted is actually a bright background emission nebula known as IC-434. The Wikipedia page on this object isn't particularly enlightening, except to point out that it was first observed by William Herschel. Unlike the Horsehead Nebula, IC-434 is an emission nebula, which means that it is giving off light because it is being heated by some source. This source could be a particularly hot star or a white dwarf (as in a planetary nebula).

On Friday, April 19, the Space Telescope Science Institute released another anniversary image of the Horsehead Nebula that was taken in purely near-infrared light. Because it was taken at completely different wavelengths of light than the image taken in 2001, you'll see that the image will look very different.
Image Credit: Space Telescope Science Institute. http://www.spacetelescope.org/images/heic1307a/
But why does it look so different? Astronomers use infrared light to see through clouds of gas and dust because its longer wavelength means it doesn't get scattered as easily as visible light. So where you previously saw a dark cloud of gas and dust obscuring everything inside and behind it, now you see way more stars than you did before purely because their light can make it through the gas. (The wider field of vision may also aid the number of stars you can see compared to the previous image).

You've probably also noticed that the background cluster IC-434 no longer provides a luminous background  for the Horsehead Nebula. Instead, you can see through the optically bright gas to see the stars (and even some galaxies!) that were otherwise not visible in or behind IC-434 for the same reason as is stated above.

All of this talk of pretty astronomical pictures is not to diminish Hubble's importance as a scientific instrument, of course! Hubble has been involved in some of the most important scientific work of the past decade. Hubble is responsible for the first precision measurement of the Hubble Constant, the deepest visible light image of the universe (shown below), obtaining spectra of exoplanet atmospheres, detecting exoplanets through both direct and indirect techniques, and some of the best images of our neighbors in the solar system (like the picture of Mars also shown below). Hubble discovered the existence of dark energy, the ubiquity of central supermassive black holes in galaxies, protoplanetary disks, optical counterparts of gamma-ray bursts,  and the many moons of Pluto. And that's probably just a very small taste of what Hubble has accomplished during its time.

The Hubble Ultra Deep Field. To really get an idea of how awesome this picture is, you absolutely must view it in its full resolution, completely zoomed in. Image Credit: Space Telescope Science Institute
Hubble image of Mars. Image Credit: Space Telescope Science Institute
The Hubble Space Telescope is due to be replaced in the not-too-distant future by the James Webb Space Telescope, which is still on schedule to launch in 2018. JWST will have increased capabilities in the infrared part of the spectrum, along with a much larger collecting area than Hubble. Of course, this is very exciting for astronomers everywhere, because the advances we expect to make with JWST are on par with those the astronomical community was able to achieve with Hubble.

The future of the Hubble Space Telescope itself is a bit more sad. Hubble was placed in low Earth orbit so it could easily be serviced and upgraded with shuttle missions (which has been a major part of its longevity). However, because NASA's shuttle program is now defunct, Hubble will not be retrieved at the end of its life, and will most likely be de-orbited to safely burn up in Earth's atmosphere. After all it's accomplished for humanity, I think Hubble deserves better. I, for one, would love to see a one-off mission designed to retrieve Hubble and bring it safely back to Earth. Hubble is part of our history that deserves its place in a museum for future generations. It was the first truly and universally great space telescope, which paved the way for greater still missions like JWST.

But let's save the sadness for another time. For now, as long as it's still chugging away, it deserves to be celebrated for what it has accomplished over the years, and what it will continue to accomplish well into the foreseeable future.

Happy birthday, Hubble Space Telescope!
Image Credit: NASA (except for the birthday hat; that was all me.)

Thursday, April 18, 2013

Kepler Press Conference: 4/18/2013

Today at 2:00 PM, NASA is holding a press conference to announce the latest results from the Kepler Mission. I will be watching the press conference live and updating this post as we go. When the press conference is completed, I will provide a summary of the information.

2:00. Panel members are Paul Hertz, NASA's astrophysics director, Roger Hunter, Kepler project manmager, William Borucki, principal science investegator for Kepler, Thomas Barclay, Kepler scientist, and Lisa Kaltenegger, research group leader at the Max Planck Institute for Astronomy.

2:05. And we're live. Opening remarks from Dr. Simon Worden "This is really cool."

2:11. Roger Hunter: Introduction and motivation of the mission. Outlining basic mission goals and mission findings so far. Announces two planetary systems. One with two habitable zone planets that are larger than Earth. One with one planet solidly in the habitable zone and one on the edge of the habitable zone. Kepler 62e and Kepler 62f are 1.6 and 1.4 times the size of the Earth respectively. Images from the press conference shown below.

(Yeah, clearly I lifted these images directly from the video presentation, as you can tell from the "Press Esc to exit full screen mode" dialog that I didn't wait to clear when screencapping.)

Image courtesy NASA.
Proper version of this image. Credit NASA and Kepler Mission.
William Borucki: "We have not measured the masses of these planets."

Other system announced: Kepler 69. Planet Kepler 69c is on inner edge of habitable zone as shown below, and is 1.7 times the radius of the Earth.
Image credit: NASA and Kepler Mission.
Kepler 62 is smaller than our Sun, and Kepler 69 is roughly the size of our Sun, but a tad smaller.

Image Courtesy NASA
From Lisa Kaltenegger: Kepler 62e needs clouds to be properly habitable. Kepler 62f requires a thicker carbon dioxide atmosphere because it is farther out.

Of course, determining the mass of the planet is one of the critical points remaining. However, these planets are definitely too small to be detected with the radial velocity method, which is how we typically determine the mass of an extrasolar planet. This looks more like a situation in which transit timing variations will be helpful.

Transit timing variations are the small changes that occur in the exact orbital period of a planet based on other planets in the system pulling on one another through their own gravity. While these changes tend to be very small, Kepler is good enough to measure these, and it has apparently become the leading way in which the masses of Kepler planets have been confirmed (according to Dr. Eric Ford).

3:00 update: William Borucki addresses transit timing variations: the planets are not close enough to have significant gravitational influence on one another. Guess TTVs won't work after all. Damn.

Wednesday, April 17, 2013

Neutrinos (Part 2)

If you have not read the first part of this post, I highly recommend you read it first. I'll even provide you a convenient link, because I'm nice like that. Neutrinos (part 1)

We shall pick up the neutrino story from the perspective of understanding the physics of the Sun. In 1957, Margaret and Geoffrey Burbidge, William Fowler, and Fred Hoyle published the groundbreaking paper "Synthesis of the Elements in Stars." Neil DeGrasse Tyson routinely cites this paper as one of the most important works in all of astrophysics. "Synthesis of the Elements in Stars" (often abbreviated B2FH, after the authors' last initials) proposed the existence of the nuclear fusion reactions necessary to create all of the elements we see in the universe apart from those initially created in the Big Bang (which I discuss briefly in my History of the Universe post).

To make a rather long story short, the Sun exists, and continues to exist, because it is fusing hydrogen into helium in its very hot, very dense core. The net reaction (ignoring the intermediate steps) is graphically shown below. This process releases energy and keeps the Sun hot, which is incidentally what prevents the Sun from collapsing on itself from its own weight. There are a lot of nuclear reactions that contribute to the production of helium in the Sun (occupies multiple pages of An Introduction to Modern Astrophysics), the most important point is that a number of these reactions result in beta-plus decays where protons are forced to change into neutrons to keep newly formed nuclei stable. This is the opposite of the reaction that was shown in Part 1, so rather than getting electrons and anti-electron neutrinos, we get positrons and electron neutrinos. (Recall an electron neutrino is just the type of neutrino that is specifically associated with electrons.)
Image slightly modified to include neutrinos. Image credit: Addison Wesley Publishing
If you crunch the numbers, you find that the Sun should be producing something like 1034 neutrinos every second, and that's just the ones that are a high enough energy for us to detect. It's pretty natural that astrophysicists would want to confirm the existence of these neutrinos, because that would tell us whether or not we were right about the physics going on deep inside the Sun. Remember, neutrinos don't like interacting with anything, so they can fly right out of the center of the Sun no problem. Unfortunately, the same generally holds true for our detectors.

Fortunately, we know what reactions need to take place for us to detect a neutrino. Therefore, we can do experiments to measure the probability that a neutrino interacts with your detector before you actually put it to use.

The first dedicated solar neutrino detector was the Raymond Davis and John Bacall's Homestake detector, which came online in 1970. Like most neutrino detectors, the Homestake detector was placed in an old mine in North Dakota. This shielded the detector from cosmic rays entering Earth's atmosphere that can produce neutrinos. The detector itself was quite literally a giant tank of dry cleaning fluid, a molecule made up of 2 carbon and 4 chlorine atoms. The chlorine atoms are the part that matters. When an energetic neutrino interacts with chlorine, the chlorine atom can be changed into a radioactive isotope of argon. Every few weeks, Davis would extract the argon and count how much was created, which would tell him about how many neutrinos had interacted with his detector.

While Davis was in charge of the experimental measurements, Bacall calculated the number of argon atoms that the detector was expected to see. When the data were compared to the calculations, it turns out that Davis' measurements came up with roughly one third of the neutrinos that Bacall predicted.

At first, everyone thought that Bacall's calculations were just wrong, or that he hadn't accounted for something properly. Eventually, more neutrino detectors were built: Kamiokande in 1983 which became Super-Kamiokande in 1996, GALLEX in 1991, SNO in 1999. While I won't discuss the finer points of these detectors, each of them works in a slightly different way, but saw the same result; the Sun was producing only one third of the electron neutrinos it was expected to. The distinction of electron neutrino is important because that is both the type of neutrino the Sun creates and the type that our detectors are primarily sensitive to. In textbooks, this is routinely referred to as the "solar neutrino problem".

Not all neutrino experiments use solar neutrinos though. As I mentioned above, cosmic rays entering Earth's atmosphere can produce neutrinos that can either be annoying background noise or a useful signal! For obvious reasons, we call these "atmospheric neutrinos" and we've been observing these since 1965. Neutrinos are also created in nuclear reactors from beta decays that occur in the reactors. Because these "reactor neutrinos" tend to be pretty high energy, they can be used for neutrino physics experiments. We can also use particle accelerators to produce "neutrino beams". Basically, this involves creating a bunch of particles travelling in the same direction that will eventually break apart and create neutrinos (generally muon neutrinos, actually) that are all travelling in the same direction as one another. Then you stick a detector in their way (or better yet, aim the beam at a detector that already exists) and you have your very own neutrino experiment!

"Well that's cool and all, but what does this have to do with the solar neutrino problem?" you may be asking. This goes back to work done (and promptly ignored) by Italian physicist Bruno Pontecorvo in 1957. Pontecorvo suggested that neutrinos could spontaneously change flavors as they traveled through space. But for this to be the case, neutrinos had to have mass, which, according to the Standard Model of particle physics, they did not. Because the Standard Model had been so bloody successful, there was no reason to believe the theory was wrong.

Until 1998. In 1998, Super-Kamiokande began to see hints of neutrinos changing their flavor in its atmospheric and solar neutrino data. When SNO came online a year later, it also saw evidence that neutrinos were changing flavors. This marks the first, and so far only, time that the Standard Model was shown to be wrong, which required an extension of the theory to allow for neutrinos with mass whose flavors could change more or less spontaneously. Now we even have long baseline (hundreds of miles) neutrino beam experiments such as MINOS and T2K that are designed to detect these flavor changes (and they do). This phenomenon is now referred to as "neutrino oscillations".

This may seem like a strange term for "changing flavors", but it actually makes sense, in an odd sort of way. In particle physics jargon, neutrinos can change their flavor because neutrino mass eigenstates don't line up with neutrino flavor eigenstates. This means that when a neutrino of a certain flavor, let's say an electron neutrino, is created, it is a pure electron neutrino in flavor, but its mass is some combination of electron, muon, and tau neutrino masses. As the neutrino moves through space, as shown in the diagram below for a simple two-flavor case, the mass combination changes a bit (for the life of me, I couldn't tell you why). The change in the mass combination will change the flavor of the neutrino, so rather than being a pure electron neutrino, it has some probability of being a muon or tau neutrino as well.

The oscillation comes in because (in a simple two-flavor picture), as the neutrino travels through space, it will flip back and forth between the two flavor states. Not shown in the diagram, the rate of oscillation depends largely on the energy of the neutrino.
Figure adapted from Scientific American, March 19, 2013 "Neutrino Experiments Light the Way to New Physics"
Oscillations can also occur as neutrinos travel through not-empty space. In fact, they happen much quicker. This is called the Mikheyev-Smirnov-Wolfenstein (yeah, I just wanted to write out those names) or MSW effect. It turns out that the MSW effect is mostly responsible for the oscillation of solar neutrinos rather than their trip through empty space to Earth. This explains the solar neutrino problem because we expect that the neutrinos will become roughly evenly divided amongst all three flavors during their trip from the center of the Sun to our detectors on Earth.

Phew. That was probably the hardest three paragraphs I've ever tried to write. These last paragraphs alone were why this post took so damn long to write. On the bright side, you now all know why neutrinos are so weird and part of why I love them so much. I hope this post was understandable to most of you, and there will be a Part 3 in the works. But before that, I expect you'll be getting some exoplanet news some time tomorrow. Stay tuned!