Panspermia and Transfer of Life Between Planets

by Dr. Tahir Yaqoob on May 24, 2012

The idea that life is compellingly prevalent throughout the Galaxy (panspermia), and the spreading of life throughout the Galaxy by means of transfer of life between planets by means of traveling microbes is not new. However a preprint of a paper that appeared in April 2012 by Hara et al., entitled Transfer of Life-Bearing Meteorites from Earth to Other Planets makes fascinating reading. One thing that makes this paper stand out is that because English is not the first language of the authors, the excitement and sheer wonder at the implications of the results oozes out of every paragraph. This is rare to see because in the normal process of following scientific writing protocol, such enthusiasm is killed even though the author may be overjoyed about the results. I highly recommend reading the original paper even if you don’t understand the math because the thrill behind the narrative is raw, especially around one of the chilling conclusions that it could be raining alien life on Earth right now, life that originated elsewhere on another planet a long time ago. But that’s getting ahead too quickly, let’s start at the beginning.

The basic premise is straightforward. Certain phenomena such as solar storms could eject microbes from the outer atmosphere of Earth, which could potentially embark on an interplanetary or interstellar journey. More potent than that is the possibility that an asteroid impact could disperse a huge amount of fragmentary matter (tiny rocks) into space. Obviously, micro life-forms could easily hitch a ride by such a mechanism. The authors take the specific example of the impact suffered by Earth 65 million years ago, responsible for the Chicxulub crater in Mexico. From here the authors proceed to construct a chain of estimates, lumping unknowns into probabilities, creating a kind of Drake’s equation for the spread of life following a catastrophic impact. Although this is very crude, it is interesting to follow the chain of reasoning and in fact the unknowns are less uncertain the those in Drake’s equation. The critical factor right at the beginning is the total number of fragments created by the impact, of which some fraction get to launch into space and potentially arrive at another planet. The authors estimate this number to be huge, 0.1 billion billion (1017) rocks of about 1 centimeter in size. This seems arbitrary but on the other hand not unreasonable. They estimate 0.3 for the fraction of the initial impactor mass that goes into these rocks that get hurled back into space.

Next, the authors estimate the numbers of these rocks that might land on other members of our solar system and actually give a table for six objects including the Moon, Mars, and Europa. These estimates involve dynamics under gravity. Obviously, not all of the rocks are going to be able to escape and the details depend on the uncertain ejection velocities. Both low and high velocity ejection scenarios are considered and for the former, Mars gets a whopping 40 billion fragments from the ejection. The Moon and Europa get about half a billion. Another caveat is of course that the life-forms will not survive in space if they are not protected from the radiation and cosmic-ray environment, but this is possible if they are shielded by ice.

After concluding that the transfer of life within our solar system is very easy, the authors move on to transfers involving interstellar travel. Transfers between exoplanets (including the Earth, which is of course an exoplanet to potential residents of other star systems). Ultimately the goal is to estimate probabilities of seeding the entire Galaxy with life once it begins somewhere.

To illustrate, the authors focus on the star system Gliese 581 (or Gl 581) which has earned fame and fortune for harboring at least three close-in, potentially low-mass planets, at least one of which may favor habitability (the super-Earth Gl 581d). (Note: there are severe ambiguities in multiplanet systems, to the extent that even the number of planets may be uncertain.) The system is about 20 light years away from Earth. (By the way, did you know that if you type “20 light years in miles” into the Google search box, it gives you the answer?) At 10,000 miles per hour it would take about 1.3 million years to get there from Earth.

The authors do a grand probability calculation of life being captured by Gl 581 in the context of two scenarios: direct collision between the life-carrying rock and the planet, and swing-by with gravitational capture by the alien solar system. The former scenario gives an estimated number of rocks that is much less than 1 (by at least a factor of 1000). The second scenario gives a much larger number, even after accounting for the (estimated) probabilities of the newly arrived rocks actually falling onto a planet and landing there. This is mainly because now we talking about the gravitational “reach” of something the size of a planetary orbit, dominated by the mass of the star, as opposed to something the size of a planet, dominated only by the gravity of the planet. The estimate comes out to be about 10,000 rocks potentially landing on an exoplanet in the Gl 581 system, rocks that originated from an Earth-asteroid collision line the one responsible for the Chicxulub crater. Of these, about 100 rocks could land and become new seeds of life in a new home. The authors point out that the number would be even larger if the typical rock size was less than 1 centimeter. They also point out that a possible mechanism to survive the interstellar journey could be burial of the tiny rocks in material from comets or other icy bodies such as those in our outer solar system.

The timescales involved are around a million years for the interstellar journey,and several million years at each end, first to escape our solar system, and finally to be captured by the new star. The authors then take the estimates to the next logical step. Calculations are performed under three different models and sets of assumptions. Most of them do not favor life spreading throughout the Galaxy on a timescale that is less than the age of the Galaxy. But one of them does. That scenario is the one in which after each life transfer event, life on the new planet proliferates and eventually becomes the source of a new ejection event. This does not seem to me to be too unreasonable. After describing this most favorable calculation, the authors say, “The problem may be that many types of life which evolved differently from the same origin are falling to Earth nowadays.” That one sentence is enough to send a chill down your spine once you realize what it says, which is not at first obvious because of the english. Clearly, the authors are saying it could be raining life on Earth right now, life that has traveled to us from other planets. But why do they say it is a problem if, as is indicated by the rest of the paper, they are very happy about the possibility? I realized that what they mean is that the implication may be contradicted by observations which don’t support the scenario. In other words, why haven’t we detected this raining life? What do the authors mean by “evolved differently from the same origin?” I think they mean that all forms of life in the Universe may have a common origin, but after that different evolutionary paths can lead to very different life-forms. Well, in answer to the possibility of raining life being a problem, I have a question for you. By what criteria or metric would we determine whether any life-form on Earth originated from a seed that was ejected from an exoplanet? In other words, how would we know? Next time you see an ant, a bug, a fly, etc., look at it really, really closely. Aren’t they just the weirdest creatures you have ever seen? They look nothing like you or your dog do they?

The authors also did a worst-case variation of the proliferation scenario in which the multiplication factor of life-forms after they are seeded on each planet is very small. The last sentence in the paper before the conclusions describes the result of this as, “Then the probability is almost one that our solar system is visited by organisms originated in extra solar system” (sic). Another chiller of a sentence. Dramatic sentences are a hallmark of this paper and I particularly like the last sentence in the abstract which consists of just two words. “Organisms disperse.”

Until next time. Don’t forget to examine your local life-forms carefully!

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