Tuesday, March 03, 2009

Fermi paradox meets the timescale

John over at cosmic variance has been discussing extra-terrestrial life, so I figured I'd put a geologic spin on it. Specifically, look at the Fermi Paradox through the lens of deep time. The Fermi paradox states, "If advanced aliens are common in the galaxy, where are they?" More specifically, why aren't they here. As a geochronologist, I don't wonder where and why, I wonder when. So let's make a few assumptions:

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.


Wayfarer Scientista said...

intesting little excercise here....makes one think.

Anonymous said...

Perhaps that explains where the other half of the Mt Isa metallogenic province went...

Anonymous said...

Very 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

Anonymous said...

Neat. 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.

Also, I would think aliens would find dinosaurs -- or any form of complex multicellular life -- at least mildly interesting.

Doug M.

Dustin said...

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.

DL said...

Dustin'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.

An 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).

thnidu said...

Tre interese! Pointed here by LJ:browngirl.

Earthy said...

And that is why aliens have not visited. Kthx.