Discussion and news about the modern effort to understand the nature of life on Earth, finding planets around other stars, and the search for life elsewhere in the universe

Saturday, December 18, 2010

The ten most important questions for astrobiology: Number 6

This next question for astrobiology has a fairly long history, going back at least 1500 years or so, with occasional flurries of interest over the past two hundred years. Interestingly, for every controversial idea put forward to address it there are also many sober and reasonable ideas. Question number 6 is:

Can life spread through space itself?

Like the other questions this one requires some further qualification. The typical context is the idea that life originates, or is incubated, on or in planetary bodies. Subsequent events lead to the transport of viable organisms, or at the very least viable biochemical 'seeds', across the gulf of space - either within a single planetary system or between distinct stellar systems. The term most generally used is 'panspermia', coined by the ancient Greeks and meaning 'all seed', more modern studies often refer to 'lithopanspermia', meaning the transport of life-containing material from planetary lithospheres.

It is a very interesting idea. Remarkably, the more we learn about the resilience of life here on Earth, from super-tough microbes to multi-cellular organisms like tartigrades, the more plausible it seems that organisms could in principle survive long-term exposure to interplanetary, and just possibly interstellar, space. This is especially true if they are tucked away inside big chunks of rock or ice, shielded from disruptive electromagnetic and particle radiation. The rub, and there always is one, is that the physical processes that might loft life away from a cozy planetary surface or enable their descent onto a new planetary body are pretty awfully violent - from asteroid impact induced spallation of lithospheric material (big collison=ejected planetary rocks), to hyper-velocity re-entry through a planetary atmosphere. This is such a make-or-break problem that tests have been undertaken, both in the lab and with space-born experiments to scope out how extreme pressures and temperatures weed out organisms. So far nothing suggests that the violent circumstances initiating or ending panspermia are complete deal-breakers.

Then there are issues of actual transportation within a planetary system, how long does it take and how often can a piece of planet A arrive on planet B? Detailed gravitational simulacra of our own solar system provide some insight. Earth-Mars/Mars-Earth exchange (as we know from meteoritic evidence) occurs, and can be on reasonably 'quick' timescales of a few years to a few tens of thousands of years. Even channels that carry material from, for example Earth to Europa, exist - but the efficiency is very low. Out of roughly 100 million bits of ejecta from suitable asteroid impacts on the Earth perhaps only 100 might eventually intersect Europa. So the orbital architecture of a planetary system plays a dual role - determining the rate of ejecta-producing impacts on planetary bodies, and determining the transport efficiencies of these lumps between worlds.

Then there is interstellar panspermia. Solar winds may be able to both accelerate and decelerate tiny crumbs of dust or ices carrying microbial organisms, or at least DNA fragments. A speck is propelled from one star and then if it intersects the stellar wind of another may be slowed down to drift inwards to the planetary zone. Microscopic material can sink down into a planetary atmosphere, as we see here on Earth with high-altitude interplanetary dust particles. The biggest challenge is for anything of any remote biological function to survive the interstellar environment for the huge lengths of time required.

Which gets to the last, and most speculative and abused piece of this. If life, in varied and splendid forms, is common in the universe then wouldn't at least some of it have evolved to exploit the greater terrain offered by migrating through space? Not by building machinery, but either by natural selection or just sheer biological grit. Given the apparent propensity for lithospheric material to get exchanged between planets in our solar system and the potential diversity of planetary systems out there, then it seems plausible that somewhere is a place that could be analogous to one of our terrestrial archipelagos. This Indonesia of planets could offer the kind of back and forth that would, over time, select for organisms best suited to space travel - a little evolutionary honing before they set off into the galaxy.

So the answer to Number 6 remains 'possibly', and by exploring our own solar system there is both a chance of finding our relatives and finding the misplaced chunks of planets that might give us some more clues.


Mark said...

Interesting stuff. One question I have is how to asteroids from a planet get off the surface, for example the Mars/Earth exchanges - are these relics from when the plantoids were colliding a few billion years ago, or is this an ongoing process? I thought hits on the Earth are pretty uncommon, and can't imagine they very often throw bits back out into space.

Caleb Scharf said...

Yes, it takes a pretty big asteroid collision with a planet to eject material to escape velocities, but those kinds of collision seem to come along every few tens of millions of years on Earth, so potentially a few hundred over 4 billion years of life on the planet. The rate of impact was highest around 4 Gyr ago (the late heavy bombardment). Mars may have a higher impact rate overall, plus it's thinner present day atmosphere makes it slightly easier for stuff coming and going.

JohnL said...

Could we have sent life to Mars via say the Viking landers ? I know there was some writing at the time about these units being assembled in sterile environments but one would have to think there are limitations to sterility on a space capsule. These units also contained organic material for life testing experiments.

Caleb Scharf said...

So called 'forward contamination' is a real issue. The Vikings were heavily sterilized before launch, though of course since then we've learned a lot more about microbial life that doesn't show up readily in lab cultures. The Galileo probe was deliberately burnt up in Jupiter's atmosphere after it's mission in order to avoid any possible contamination of the inner Jovian moons. I think it's correct to assume though that even with the best protocols it is likely that at least a few bacteria/archaea even if in spore form will have made the journey to Mars. Though of course if interplanetary exchange of organics/organisms occurs in nature then it may be a moot point.

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