[LINK] Two notes on possible environments for extraterrestrial life

[rfmcdonald]

I learned, via io9's George Dvorsky, that a new theory explaining the origin of Martian clays (covered by the Los Angeles Times' Amina Khan) might make the independent development of life on Mars in the planet's hypothesized warm youth that much less likely.

A paper published online Sunday by the journal Nature Geoscience argues that [Martian] clays might have been formed in hot Martian magma rich in water. If so, that water would have been far too hot to support microbial life.

The argument stands in contrast to two more common theories, said study coauthor Bethany Ehlmann, a planetary geologist at Caltech. One of them is that liquid water flowing across the Martian surface would have interacted with surrounding minerals, forming the clays. In another scenario, underground water warmed by the planet's internal heat could have provided a comfortable living before it got bound up in the mineral structure of clays.

On Earth, clays are remarkably good at trapping organic material. So if organic compounds existed on Mars, clays would be a good place to find them.

If either of the prevailing theories about water is true, the Martian environment could have been hospitable for life, Ehlmann said. Superheated water and magma? Not so much.

"The clays would form as the lava cools from 1,500 degrees Celsius," she said. "That would not be a good habitat."

[. . .]

It's possible that all three models could be right, depending on where you're looking, said Ralph Milliken, a planetary scientist at Brown University who was not involved in the study.

"It's certainly a different take on trying to explain the origin of some clay minerals on Mars," he said. "It does have some merit, and alternative hypotheses need to be considered fully."

But he said the story laid out in the new paper doesn't explain why the Martian surface appears to have tracks cut by flowing liquid. Nor does it account for blueberry-shaped mineral deposits of hematite that scientists believe may have formed when water ran past them.

Elsewhere, Universe Today's Nancy Atkinson noted new calculations for planetary habitability that take into account the habitability of subsurface environments. It should be noted that these environments--solid rock, more or less--are suitable for bacterial life, as on Earth, not for more complex organisms.

We know that a large fraction of the Earth’s biomass is dwelling down below, and recently microbiologists discovered bacterial life, 1.4 kilometers below the sea floor in the North Atlantic, deeper in the Earth’s crust than ever before. This and other drilling projects have brought up evidence of hearty microbes thriving in deep rock sediments. Some derive energy from chemical reactions in rocks and others feed on organic seepage from life on the surface. But most life requires at least some form of water.

“Life ‘as we know it’ requires liquid water,” said Sean McMahon, a PhD student from the University of Aberdeen’s (Scotland) School of Geosciences. “Traditionally, planets have been considered ‘habitable’ if they are in the ‘Goldilocks zone’. They need to be not too close to their sun but also not too far away for liquid water to persist, rather than boiling or freezing, on the surface. However, we now know that many micro-organisms—perhaps half of all living things on Earth—reside deep in the rocky crust of the planet, not on the surface.”

While suns warm planet surfaces, there’ also heat from the planets’ interiors. Crust temperature increases with depth so planets that are too cold for liquid water on the surface may be sufficiently warm underground to support life.

“We have developed a new model to show how ‘Goldilocks zones’ can be calculated for underground water and hence life,” McMahon said. “Our model shows that habitable planets could be much more widespread than previously thought.”

In the past, the Goldilocks zone has really been determined by a circumstellar habitable zone (CHZ), which is a range of distances from a star, and depending on the star’s characteristics, the zone varies. The consensus has been that planets that form from Earth-like materials within a star’s CHZ are able to maintain liquid water on their surfaces.

But McMahon and his professor, John Parnell, also from Aberdeen University who is leading the study now are introducing a new term: subsurface-habitability zone (SSHZ). This denote the range of distances from a star within which planets are habitable at any depth below their surfaces up to a certain maximum, for example, they mentioned a “SSHZ for 2 km depth”, within which planets can support liquid water 2 km or less underground.