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.

1 comment:

  1. I'm quite intrigued by this sort of post. I wish I had something worthwhile to offer to the conversation other than "ooooooooh, exciting!" but alas. Thanks for posting.