Suppose a human observer was standing on Mars during the early Solar System, looking up into the night sky, in the approximate direction of the young Earth, as the Theia planet hypothesized in the Giant Impact Formation Theory of our Moon struck the Earth.

Would it have been bright enough to see with the naked eye?

If so, what kind of details would the observe see with naked eye?

  • $\begingroup$ Random musing I had today; I know that things such as the Shoemaker-Levy impact on Jupiter were not visible with the naked eye, but that planet is much further and impact much dimmer in comparison to the Theia impact. I wonder if there was a spectacular flash if an observer would even notice something was "really wrong" on Earth at the time. $\endgroup$ Oct 31, 2023 at 18:07
  • $\begingroup$ well, I think the only way to figure this out, is to throw a planet at earth. Jokes aside, you would certainly be able to see the dot of the earth, and the dot of theia, get real close, and become one dot. $\endgroup$
    – Topcode
    Oct 31, 2023 at 18:45

1 Answer 1


How bright would a 6000 km radius blob of magma be? A quick luminosity calculation assuming the surface to be 1000 C gives an absolute magnitude of 21.63 and an apparent magnitude of -9.056 at 225 million km away. Currently the max Earth magnitude from mars is -2.5. The magma Earth is about as bright as Phobos is from the Martian surface. So at least the collision would have produced a very noticeable brightening. Also, 1000 C might be an underestimate and the initial splash no doubt had a larger surface area, so the initial hours would have been pretty noticeable.

Given that a human observer on Mars can see Earth and the moon as separate points today with binoculars, presumably the imagined observer would be able to see the Theia approach, the major brightening, and then likely some bright blobs and clouds of ejecta. Not sure how easy it is to see any details without optical aid.

  • $\begingroup$ Exactly what I was looking for! Thanks Anders! :) $\endgroup$ Nov 1, 2023 at 13:01
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    $\begingroup$ I'd imagine the debris ring that became the moon would be significantly visible for decades or centuries even after it cooled, just because it makes for a larger reflecting surface. $\endgroup$ Nov 1, 2023 at 19:00
  • $\begingroup$ Would mars surface temperature and atmosphere at that time allow this observation? $\endgroup$
    – user16500
    Nov 2, 2023 at 5:07
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    $\begingroup$ @jkztd - That would be the pre-Noachian. Not much is known about that. But my suspicion is that it might have been a mess of volcanism. $\endgroup$ Nov 2, 2023 at 15:18

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