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For most of us, meteors are by far the ultimate planet killers. All it takes is one direct hit to suddenly wipe all life from the face of a world. But, rather than taking life away from a planet, what if a meteor impact is the reason that the world is habitable in the first place?

In the story I'm writing, humanity discovers a habitable exo-world known as Elysium. It's an Earth-sized rocky moon orbiting elliptically around a Saturn like gas giant (known as Aphrodite) that is just over 4 times the mass of Jupiter.

Elysium is well outside of the habitable zone of it's binary parent stars. But the moon's surface is kept nice and warm thanks to a giant impact crater that dominates it's face (known as the Cinder Fields).

Although the crater should have cooled and sealed up thousands of years ago, the tidal force of Aphrodite on the moon's surface has kept it open and hot, warming up the planet's atmosphere and surface through the Cinder Fields.

Could a world like this actually work in real life?

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  • $\begingroup$ Quibble: I would need to check, but off hand I think a planet with just over 4x the mass of Jupiter would collapse into a star under its own gravity. $\endgroup$ – Joel Harmon Jun 16 '16 at 2:06
  • $\begingroup$ @JoelHarmon Fortunately, that's not the case. The lower limit of a brown dwarf (not even a star) is about 13 $M_J$, while the least massive stars are about 80 $M_J$. $\endgroup$ – HDE 226868 Jun 16 '16 at 2:12
  • $\begingroup$ @HDE226868 Success! I learned a thing today. Thanks. $\endgroup$ – Joel Harmon Jun 16 '16 at 2:15
  • $\begingroup$ Nevertheless, 4xJ would not be Saturn-like. As gas giants get more mass, they don't tend to grow much in diameter, they just get denser. Jupiter has many times the density of Saturn, and a 4xJ planet would have even higher density but not necessarily be much larger physically. Mind you, it does depend on what the author means by "Saturn-like"? $\endgroup$ – Simba Jun 17 '16 at 15:13
  • $\begingroup$ @Simba By Saturn-like, I mean a planet with a smoother, more uniform looking atmosphere and a spectacular, large ring system. In a way, a lot like the one in my profile picture. $\endgroup$ – Mattias Jun 18 '16 at 16:35
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Yes! And even without the tidal heating.

O'Brien et al. (2005) modeled impact craters on Titan using finite-difference methods for various impact scenarios, including a wide range of sizes and temperature profiles. Here are some of their findings:

A 15 km diameter crater in water ice with a depth/diameter (d/D) ratio of 0.1 and a volume fraction 0.05 of liquid can sustain the liquid for about 1,000 years. For the same crater in ammonia dihydrate, the liquid can persist for about 2,000 years. A 150 km crater in water ice with d/D of 0.05 and a volume fraction 0.1 of liquid can sustain the liquid for about 50,000 yr, and for the same crater in ammonia dihydrate, liquid can persist for about 100,000 yr.

Even 1,000 years seems pretty good. Yes, this isn't going to be nearly enough time for life to evolve, and neither will 100,000 years. But it's really good to sustain intelligent life that's already evolved - in this case, humans.

I should note, though, that any body large enough to cause a crater taking up that much of the planet is liable to have terrible consequences, destroying much on the surface - if not the entire planet. This means that it isn't feasible for the crater to cover a substantial portion of the planet.

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