Wednesday, August 31, 2016

Life Origination on Super-Earths

Super-Earths, meaning planets with somewhat more mass than Earth but not enough to be a gas giant, have been found frequently in the exo-planet searches that Earth’s astronomers are doing. Planets as small as Earth are hard to see, but this may change soon. There seems to be a great interest in finding life on exo-planets, or even on planets or satellites in our solar system, so asking the question about life on super-earths might be interesting.

When you ask, why do we care about life on other planets, various answers come back. One is that this will help us understand the origin of life. Unfortunately, if this was really a motivation, we would be spending lots of money in Earth laboratories trying to test hypotheses about the origin of life, but we are not. The astronomy budget is huge compared to the laboratory origination of life budget. So the real reason must be different.

Another reason is that we want to meet other intelligent life-forms. That doesn’t make too much sense either, as we are not looking for other civilizations so much as for life itself on some planet. Why do we care if there is non-intelligent life on some satellite or exo-planet? It seems to be similar to the reason people like to travel to other places on Earth and see what’s there. An instinct for learning about the universe, or a desire to find new places to visit that might be more interesting that a sphere of rock could be part of the answer. It is hard to imagine building a starship to go visit some amoebas a hundred light years out, however. My guess is that it is solely the entertainment value.

Be that as it may, let’s discuss finding life on a super-earth. As the gravity goes up, a few important things happen. One, we go to a different point on the equation of state curve for water. The melting point changes little, but the boiling point at 10 times the atmospheric pressure of earth is almost another hundred degrees C higher. This means the liquid water zone (LWZ) for a super-earth is wider than for an earth.

Using the hypothetical origin of life theory espoused in this blog, the organic oceans concept, there would have to be organic oceans for a period in the history of the planet to form the first cells. There are some organic compounds which are fragile in higher pressures, but most are not. Similarly, higher temperatures, made possible by a wider LWZ, decompose some, but not too many. Thus, the organic ocean zone, where an organic ocean could exist for long periods without evaporating or decomposing, might also be wider for a super-earth.

The energy generated by gravitational collapse stays with the planet, and keeps the rocky core hot. On Earth, this is not sufficient to heat up the surface and boil off the oceans, but there is a hypothesis that on larger planets, such as the cores of gas giants, it does. A super-earth is somewhere on the continuum of collapse-generated heating between the earth and the gas giants, so it may have enough to keep the two oceans from forming. The structure of Jupiter is still largely unknown, because its composition is only guessed to be mostly hydrogen, and the equation of state of hydrogen at extremely high pressures is not certain as well. The current probe circling Jupiter, Juno, was sent in part to shed some light on Jupiter’s core. Estimates of the top of the liquid layer on Jupiter are around ten thousand degrees C, which indicate that even a super-earth might have a lot of thermal flux coming our of its center.

This implies that super-earths with much more mass than earth may be too hot for an ocean of either kind. This in turn spells no life.

Perhaps the biggest hurdle for smaller super-earths is the formation of massive amounts of organics. They do not inhabit the gas cloud that forms a solar system in sufficient abundance to make oceans on a super-earth. Lightning does form them, and if the super-earth was like a gas giant planet, with great amounts of lightning, there might be enough if they simply accumulated for long periods. Observations to date do not bode well for this. Currently, Jupiter has about as much lightning as Earth, so the amount of organic production would be insufficient on a super-earth, assuming it also has about as much lightning as earth does, and that amount was the same billions of years ago.

Impacts of planetesimals also create the heat and shock waves that make organics out of the atmosphere’s CO2 and H2O, but they wouldn’t have made enough for earth, and likely not enough for a super-earth. If the distant solar system had a much larger number of planetesimals, this could change, or if for some reason the super-earth was located closer to the belt or belts where planetesimals form, there could be more. Perhaps a factor of ten seems reasonable, and then there is the larger size of the super-earth to add some tens of percent more. Still fairly sparse.

The organic ocean hypothesis for the origin of life on earth involves a monumental impact from a large planetesimal, planet-sized, coming from a Lagrangian point on the same orbit. This tears up the crust, rips off much of the atmosphere, and begins a long period of volcanism while the newly mixed core material goes through a resettling and re-separation of materials, reformation of the crust and emergence of any crustal material driven deep into the core by the impact. Thus, if something of this magnitude happened to the proto-super-earth, organics would be present in abundance. Thus, if we want to find a super-earth with life, we need an impact similar to one that an earth-sized exo-planet would need.

This requirement eliminates another obstacle: too much atmosphere for photosynthesis. If the shock of impact depletes the atmosphere down to something where visible photons can eventually penetrate down to the liquid ocean, evolution can produce something like chlorophyll, which means the atmosphere can become oxygenated. This doesn’t mean that intelligent life follows, as a super-earth might maintain enough liquid water to completely cover the surface. There is a chance that intelligent life might evolve underwater, but it would be limited and civilization such as we define it would not emerge. Thus, no star travel from a super-earth.

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