Suppose that Earth has been visited by aliens 50 times since our solar system's accretion disk started to cool 4,567 million years ago. What would the aliens have seen? In order to simulate this, I generated 50 random alien arrival times in between then and now, sorted them, and put them in geologic context. They are listed below, in stratigraphic order.
| Time (Ma) | Time (name) | My comment | Alien's comments |
| 125 | Cretaceous | Dinosaurs! | |
| 270 | Permian | Gondwanan glaciers and funky reptiles | |
| 352 | Carboniferous | Swamps and really big insects | |
| 668 | Cryogenian | Pre-Marinoan- no sponges yet | |
| 675 | Cryogenian | ||
| 701 | Cryogenian | ||
| 748 | Neoproterozoic | ||
| 750 | Neoproterozoic | ||
| 808 | Neoproterozoic | ||
| 925 | Neoproterozoic | ||
| 1021 | Mesoproterozoic | Grenville | Big Mountains |
| 1049 | Mesoproterozoic | Grenville | Big Mountains |
| 1300 | Mesoproterozoic | ||
| 1355 | Mesoproterozoic | ||
| 1533 | Mesoproterozoic | Mt Isa is forming | If these aliens came for resources, they didn't want base metals |
| 1684 | Paleoproterozoic | ||
| 1857 | Paleoproterozoic | ||
| 1888 | Paleoproterozoic | ||
| 2159 | Paleoproterozoic | Trans-Amazonian orogeny | |
| 2247 | Paleoproterozoic | Various poorly constrained glaciations in this general timeframe | |
| 2272 | Paleoproterozoic | Various poorly constrained glaciations in this general timeframe | |
| 2355 | Paleoproterozoic | ||
| 2358 | Paleoproterozoic | ||
| 2400 | Paleoproterozoic | ||
| 2459 | Paleoproterozoic | Oxygen just starting to leak into the atmosphere, Manganese and BIFs | |
| 2610 | Neoarchean | ||
| 2612 | Neoarchean | ||
| 2631 | Neoarchean | ||
| 2661 | Neoarchean | ||
| 2682 | Neoarchean | ||
| 2745 | Neoarchean | ||
| 2948 | Mesoarchean | ||
| 2956 | Mesoarchean | These two mesoarchean visitors missed each other by only 95,000 years. | |
| 2956 | Mesoarchean | These two mesoarchean visitors missed each other by only 95,000 years. | |
| 2972 | Mesoarchean | ||
| 2990 | Mesoarchean | ||
| 3152 | Mesoarchean | ||
| 3281 | Paleoarchean | ||
| 3609 | Eoarchean | ||
| 3614 | Eoarchean | ||
| 3641 | Eoarchean | ||
| 3647 | Eoarchean | ||
| 3669 | Eoarchean | ||
| 3828 | Eoarchean | ||
| 3837 | Eoarchean | ||
| 3875 | Eoarchean | LHB | Dynamical instability of this system precludes the development of complex life |
| 3947 | Eoarchean | LHB | Dynamical instability of this system precludes the development of complex life |
| 4011 | Hadean | ||
| 4266 | Hadean | ||
| 4425 | Hadean | Moon forming impact | "That's not a moon, that's a battlestation" |
As you can see, for aliens looking for 'Earthlike' planets, the actual Earth was easy to overlook for msot of its history. In this simulation, there was only macroscopic life for 3 of 50 visits. From another POV, three visits were eaither during the Late Heavy Bombardment, or during the moon forming impact- both of which would appear (to the casual alien visitor) to make long-term viability of life on Earth pretty unlikely.
So as we start to find 'earth-like' planets in our sky surveys, it is important to remember that Earth has only been Earthlike for a relatively short period of time.
intesting little excercise here....makes one think.
ReplyDeletePerhaps that explains where the other half of the Mt Isa metallogenic province went...
ReplyDeleteVery cool! But I think the date for sponges is wrong, they just recently pushed it back to 751 million years. http://www.thenakedscientists.com/HTML/content/news/news
ReplyDeleteNeat. Though the distribution of visits is such that the era of modern life is mostly skipped -- you'd expect 5 or 6 visits since the Cambrian explosion, not 3.
ReplyDeleteAlso, I would think aliens would find dinosaurs -- or any form of complex multicellular life -- at least mildly interesting.
Doug M.
Question: how did you account for the fact that the aliens take some amount of time to come into being? Surely they need a few millenia of their for their homeworld to form and to evolve.
ReplyDeleteDustin's question is very convenient for that timescale. The answer lies in the fact that there are plenty of Sun-like stars older than our Sun. So, it is plausible that some interstellar civilizations could exist in our Hadean period.
ReplyDeleteAn important point is to understand that the habitability of one planet is not a binary question (yes or not) but a question of Myr of habitability. For example, Earth may have a total (past and future) 6-7 Gyr habitability-span. Of course a restricted definition of habitability (e.g. habitability of complex life forms) reduces that span (to perhaps a mere 1-2 Gyr).
Fermi paradox is probably "solved" by a dilution effect. Galaxies are not only very big for a reduced number of space-faring civilizations but also very older. Civilization density must not only be expressed by number of civilizations per galaxy (or cubic megaparsec) but also by number of civilization per galaxy·Myr (or cubic megaparsec·Myr).
Tre interese! Pointed here by LJ:browngirl.
ReplyDeleteAnd that is why aliens have not visited. Kthx.
ReplyDelete