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!

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