Dawn at Ceres

March 2015

The Dawn spacecraft, having completed its lengthy survey of the asteroid 4 Vesta, and having survived the interplanetary cruise from Vesta to Ceres, is now safely in orbit around the largest asteroid in the Belt, 1 Ceres.  One of the first results from Dawn’s survey of Ceres is the discovery of small, intensely bright spots on its surface.

Vesta and Ceres, though nearly at the same distance from the Sun, are not twins; in fact, they are very different creatures.  Vesta is unique among the large (>100 km diameter) asteroids in the Belt: it is a thoroughly reworked body, having undergone extensive melting and differentiation into layers with different composition and density, with a surface dominated by rocks closely similar to terrestrial basalts.  Ceres, in contrast, is a modestly altered body that is genetically related to the very dark, volatile-rich C-type asteroids that dominate the outer half of the Belt.

Back in 1977 Larry Lebofsky studied the infrared reflection spectrum of Ceres and found an absorption feature near a wavelength of 3 micrometers (µm).  This is a region in which water, in all its many chemical forms, is a strong absorber.  Articles in the press tend to assume that any mention of “water” means liquid water, which equates to a well-watered Eden for life.  But the 3-micron feature is simply due to excitation of the stretching mode of the O-H chemical bond: water vapor, liquid water, solid water-ice polymorphs, clay minerals, micas, and hydrated salts such as gypsum all have broad absorption bands in this same spectral region.  Liquid water, if present today, could not occur stably close to the surface (too cold; hard vacuum), but might persist at modest depths if some solute is present to lower the freezing point and depress the vapor pressure.  A plausible candidate for that role is ammonium chloride, NH₄Cl, which I regard as a far more plausible solute than the often-quoted ammonia.  Many years ago (in Low-Temperature Condensation from the Solar Nebula, Icarus 16, 241 (1972) I pointed out that chemical synthesis of ammonia is strongly favored by high pressures, suppressing ammonia synthesis in the Solar Nebula.  My colleagues Ron Prinn and Bruce Fegley showed that the higher pressure in dense protoplanetary nebulae favored ammonia formation there: ammonia should be an important constituent of ices in planetary satellite systems, but not in asteroids, because they are formed in the low-pressure regime of the Solar Nebula. 

Indeed, the freshly fallen Orgueil CI chondrite, which contains all the minerals mentioned above, as well as veins of soluble salts deposited from solution in water, was reported to give off a strong odor of “smelling salts”, ammonium chloride. 

And what about the bright white spots on Ceres?  Liquid water released from the interior of Ceres would boil well below the surface, producing a jet of rapidly cooling vapor.  Water vapor vented from a warm interior into the frigid vacuum of Ceres’ surface would expand irreversibly to produce a jet of snow, which would fall to the ground near the vent.  This would happen whether the source of the water vapor is a shallow layer containing liquid water or deep, hot rocks containing –OH minerals.  Linking water venting on Ceres to the local origin of life is sufficiently far-fetched to deserve skepticism.