[BRIEF NOTE] Life on Ceres?

[rfmcdonald]

In the past, I've been quite enthusiastic about the asteroid Ceres and rather pleased about the redefinition of Ceres as a dwarf planet. (I've a soft spot for space objects which I feel have been unfairly neglected simply because of nomenclature.) The continuing success of the Dawn space probe, as it travels to first the asteroid Vesta then the dwarf planet Ceres over the next eight years using its ion drive, pleases me. The recent article in Discover pointing out that, since Ceres' surface is known to include clays and abundant water ice and Ceres formed like a planet and may have enjoyed internal heating, there might be a subsurface ocean like Europa's and even life, really interests me. Bruce Moomaw wrote about it last July.

Even if Ceres no longer has a liquid-water layer, though, it seems likely that it had one for a very long time -- perhaps most of its 4.5-billion year life. And that, in turn, raises the question of just how far prebiotic organic evolution could have gone in such an ocean.

Such hydrothermal reactions of liquid water with rock are exactly the sort of thing that synthesizes large amounts of methane and other simple organic compounds in the volcanic vents on Earth's deep ocean floors, and they may have been what generates both the large amount of methane in Titan's atmosphere and the tiny traces of it that we think we've found in Mars' air.

Mix enough nitrogen-containing compounds with that brew (as would happen if Ceres' ice contained even a small amount of ammonia), and no one is sure quite how far these processes could have gone in creating very complex organic molecules.

It's exactly this same set of hydrothermal, organic-producing chemical reactions that have stirred the recent great interest of astrobiologists in the newly discovered water-vapor jets still erupting from the south pole of Saturn's little moon Enceladus, which may be hooked up to a large still-existing pocket of subsurface water below its ice.

Even if Ceres' water is now frozen solid, could it have stayed liquid long enough -- billions of years -- for primitive microbes to start to evolve in its buried ocean? In that case, their frozen remains would still be preserved in its current-day ice layer, and could have been transported up to its surface by those very slow convective currents of slightly plastic warm ice (as may also be the case for Europa).

We have already found some fairly sophisticated organic compounds -- including amino acids -- that must have been produced by the same type of processes, in the small number of carbonaceous meteorites that have been recovered on Earth, but none of them comes anywhere near the complexity of the compounds that must have allowed development of the first living cell. But Ceres' ocean probably lasted much longer than any such subsurface liquid-water pockets on the other protoplanets, which were all either a good deal smaller or shattered early on by the Belt's collisions.

It's just these thoughts that have led Dawn's planners, in the last couple of years, to drop their plans to have it orbit at any low altitude above Ceres' surface. The craft is unsterilized, and -- if it crashes into the surface and embeds pieces of itself in Ceres' near-surface ice, and if Ceres does still have an ammonia-sustained subsurface liquid ocean -- any still-living germs riding on it just might get transported by the solid-state convection of its upper ice crust down into that ocean and quickly and disastrously biocontaminate it, ruining any search for native Cerean germs.

This is exactly why NASA plans to sterilize the first Europa orbiter mission -- and Dawn, if it crashed onto Ceres from orbit, would hit at a much slower speed than the Europa orbiter and would not have been exposed first to a bath of intense sterilizing radiation from Jupiter.

We are virtually sure now that Europa does still have a subsurface ocean; we don't know whether Ceres still does, and indeed the odds seem to be against it. But why take the chance? So Dawn will remain in that final "quarantine orbit" 700 km above Ceres' surface, keeping it orbiting for at least the 50 years prescribed by NASA's biological protection rules for non-sterilized spacecraft orbiting a world that might possibly possess biological interest.

And the more one thinks about Ceres, the more potentially biologically interesting it looks. Even if its ocean has been long-frozen, it probably remained liquid for billions of years first.