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, June 4, 2011

Nematodes, Mars, and Moons

There's been a lot of interesting stuff going on recently, many items worthy of more detailed posts that will come in the future. The discovery of multi-cellular life in the Earth's subsurface is a huge one. Several species of nematode, including a new one adapted to extreme environments, have been identified in rock fracture water at kilometer depths in a South African gold mine. These are the same environments that are already known to host some remarkable microbial life, including the lonesome D. Audaxviator that relies on natural radioactivity to generate the chemical energy it requires. The tiny nematodes feast on such single-celled organisms and have been sealed off from other subsurface and surface environments for at least 10,000 years. Suddenly the prospects for "complex" life in planetary subsurfaces seem a whole lot better; especially on Mars.

Which brings us to an exceedingly interesting new result from the study of the formation rate of solar system planets. Mars in particular may have formed fast and early, in a mere 2 to 4 million years or so, as told by the hafnium-182 isotopic clock in martian meteorite samples. This places it into a very different category of object than the Earth. In effect it was and still is a large planetary embryo, even though we've all been calling it a small planet. Embryos have not undergone major collision with other similar bodies. By contrast, the Moon-forming impact that likely occurred on our homeworld (although modern reanalysis of lunar rock water content hints at possible complications) was part of the agglomeration process 4.5 billion years ago that puts Earth into a different section of the planetary zoo.

How fast different planets assemble is something we don't always remember to think about, but there are important implications. For example, if Mars did form this fast it would have coincided with a timespan in the young solar system where elements such as highly radioactive aluminum 26 would still be abundant (having been left behind from earlier supernova events that helped generate the very nebula out of which we formed). A young Mars could have been extra hot with internal radioactive decay, opening up the possibility of extensive interior melting and setting up a very different geophysical pathway compared to the Earth. This in turn could influence the nature of subsurface environments today, and their potential occupants.

Finally, on another related topic. An interesting paper appeared recently that employed a large ensemble of gravitational simulations to explore just how rare Earth-Moon type systems might be for planets in habitable zones around Sun-like stars. The Moon is a big satellite, and as such contributes significantly to the dynamical evolution of our planet - from its day-length to the gravitational tides that sweep across us every twelve hours. It also plays a role in the spin-axis stability of our planet (where our poles point) and therefore the long-term climate variation. It's a tricky problem to study for sure, but the results are intriguing. The kinds of giant impacts that can create Earth-Moon type systems may occur at rates of around 1 in 12 systems (with a range of possibly as many as 1 in 4, or as few as 1 in 45). This isn't hugely frequent, but in a galaxy with potentially millions of Earth-analogs that certainly adds up.

All three of these discoveries or studies are noteworthy because they help crack the door open a little further into our fast developing picture of not only our particular context as a habitable planet, but the opportunities for life elsewhere. Worms, isotopes, and collisions, it's a great mix.


wise said...

Given Al-26's HL of on the order of 10^5 years, it seems that it is generated contineuosly near the surface of objects in the solar system by cosmic rays, and is not a remnant of supernovas form nearby and early in the history of the solar system.

But i may be wrong.


Caleb Scharf said...

Right, but I think in this case (4.5-4.6 Gyr ago) there was a *lot* more Al-26 around, produced in prodigious quantities by a slightly earlier supernova. This is indicated by the amount of the decay product (magnesium-26) kicking around today and also provides an energy source for melting planetary interiors that is indicated by what we see in rocks. So this was a *lot* of Al-26 and it was incorporated directly into planetary embryos - so distinct from the much smaller amounts produced by cosmic rays today.

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iconoclastcymru said...

You miss the crucial astrobiology importance of Mars meteorites, that they could have carried viable seeds of life to Earth and in general establish as near as makes no odds that interplanetary panspermia is a reality

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