Suppose that both Earth and Mars was booming with life in a similar way. Except that, on Earth, there was no hominid family, and no humans evolved. Instead, humans evolved on Mars.

Assuming we had similar levels of technology, civilization, and science at similar times (e.g. agriculture ~10K years ago, telescopes ~400 years ago, physical turing machines ~80 years ago, etc), and that life on Earth was almost exactly the same, except without hominids, how soon would we have discovered life on Earth?

Also, more broadly, how quickly would we have figured out that something strange is going on with Earth that makes it different from the other planets? (Without necessarily knowing that it's life.)

  • $\begingroup$ ...hit single "Is the Life on Earth?" ~50 years ago... $\endgroup$
    – TzeraFNX
    Oct 23, 2019 at 14:22
  • $\begingroup$ They would suspect that there's life on Earth as soon as they realize that Earth is just another planet. However, they are unlikely to be able to prove it before reaching 1800s technological level. $\endgroup$
    – Alexander
    Oct 23, 2019 at 18:27
  • $\begingroup$ Well, 1898 is when the Martians attacked, and Earth had been under observation for some time before that.... $\endgroup$
    – user535733
    Oct 24, 2019 at 16:41

2 Answers 2


It's slightly harder to image Earth from Mars than the reverse because Earth is closer to the sun, thus, by simple geometry, at closest approach and best viewing conditions you'd be looking at the dark side of the planet. Of course, Earth is bigger, so there's some benefit there. Still, probably the 17th century equivalents. In 1600s, astronomers like Huygens and Cassini were able to distinguish some surface features and the polar ice caps on Mars, so that by 1666 Cassini was able to determine the length of the Martian day (he was only off by three minutes). Certainly using the same equipment on Mars looking at Earth, they'd be able to easily see the polar ice caps (much larger on Earth than Mars), oceans, land, and clouds. That means atmosphere and liquid water. And if you saw land and had a clear day on Earth, with a large enough telescope when Earth and Mars were at an optimum viewing angle, you'd see that massive swath of green in the equatorial regions that could only be plants. By no later than the mid-18th century equivalent, it would be obvious Earth had life.

  • $\begingroup$ If Earth was in a thin crescent phase WRT Mars could we see green on that crescent with the naked eye? $\endgroup$ Oct 23, 2019 at 15:16
  • 2
    $\begingroup$ Green does automatically not mean life. It could be copper oxides. $\endgroup$
    – Bald Bear
    Oct 23, 2019 at 15:19
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    $\begingroup$ @BaldBear, copper oxides don't undergo seasonal color variations, particularly not of the green -> brown -> white variety. $\endgroup$
    – Mark
    Oct 23, 2019 at 23:02

150 years ago, Earth telescopes gave such blurry images of Mars that people thought they saw green canals on Mars: https://en.wikipedia.org/wiki/Martian_canal. The illusion had its doubted and was finally disproven with better telescopes 100 years ago.

So basically at Early 20th century technology, Martians will be able to see seasonal changes in color of continents. I am not sure if they will interpret it as life or just weather. The big clue will be the fall's change from green to yellow to black/brown. Green could be copper oxides, and I am sure there are minerals with yellow color, so question is if some right mix of minerals and acid rain could cause the pattern of color changes.

We sent first robotic probes to marsh in 1960's: https://en.wikipedia.org/wiki/Exploration_of_Mars#1960s I think the imaging and chemical instruments could prove presence of life.

  • $\begingroup$ 300 years ago, Jacques Maraldi made observations of seasonal changes in Mars's colors. $\endgroup$
    – Mark
    Oct 23, 2019 at 23:00
  • $\begingroup$ No, there isn't any reasonable natural explanation for changing the colours by "acid rain". More to the point, someone would need to explain why the "acid rain" somehow only effected temperate regions while warm regions that were wet all year round stayed green. $\endgroup$ Oct 24, 2019 at 3:51

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