PerspectivePlanetary Science

Dwarf planet Ceres and the ingredients of life

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Science  17 Feb 2017:
Vol. 355, Issue 6326, pp. 692-693
DOI: 10.1126/science.aal4765

Enhanced color composite of Oxo crater, the only illuminated location on Ceres where water ice was found. Elevation is exaggerated by a factor of 2.

PHOTO: NASA/JPL-CALTECH/UCLA/MPS/DLR/IDA

A fundamental question in the evolution of the early Earth is the origin of the oceans and of some of the organic molecules that were required for the formation of life. Earth formed in the protoplanetary disk, a mixture of gas and dust. At the location of Earth, temperatures were too high for water vapor and some more volatile organic components to condense. This led to the idea that those materials may have been delivered to Earth by asteroids and/or comets from the outer solar system. Recent spacecraft studies of Comet 67P/Churyumov-Gerasimenko with Rosetta (1, 2), and of Ceres on page 719 of this issue by De Sanctis et al. (3) and by Prettyman et al. (4) with the Dawn space probe, provide evidence that complex organic molecules and even amino acids are ubiquitous on small bodies in the solar system and that water ice is abundant in the asteroid belt.

The solar system formed from a cloud of gas and dust that collapsed into a disk, which is now the ecliptic plane. A boundary called the snowline divides the disk into two parts: Below (inside) the snowline, the material was heated up by the protosun to temperatures that did not allow gas to condense and mix with the dust grains. In this region, the rocky terrestrial planets and the (inner) asteroid belt formed. Above (outside) the snowline, the water vapor condensed and, mixed with the dust component, formed the icy satellites of the giant planets and the comets. Whereas the classical view of asteroids as dry rocky bodies implies their formation inside the snowline, evidence from meteorites collected on Earth suggests that asteroidal material from the outer belt contains organic material and some water. Furthermore, models suggest that the dwarf planet Ceres, the largest object in the asteroid belt with a diameter of 940 km and comprising about a third of the total mass of the belt, may consist of 20 to 30% water ice. Therefore, either the snowline went through today's asteroid belt, or asteroids in the outer belt formed further out and moved inward into their current location, disturbed in their orbits by migration of the giant planets (5). In any case, Earth is thought to have formed dry, and the water in Earth's oceans—less than 0.1% of the mass of the planet—was then either collected locally from grains containing hydrated molecules (6) or has been brought in by asteroids (7) and/or comets (8).

So far, the evidence was theoretical. In recent years, different lines of direct evidence for water in the asteroid belt were found. (i) Active asteroids were detected, objects in the asteroid belt that showed a dust coma or dust tail (9). In some cases, activity is recurring during perihelion passage, suggesting that it is caused by sublimation of a volatile, most likely water ice. (ii) Near-infrared spectra of the asteroids Themis and Cybele show signatures of both water ice and organic material (1012). (iii) A cloud of water vapor around Ceres was detected, with sublimation of near-surface ice being the most plausible explanation (13).

Those discoveries set the stage for the arrival of the Dawn spacecraft at Ceres. Dawn has been orbiting Ceres since spring 2015, mapping its surface with cameras and a visible and near-infrared imaging spectrometer. Although the detection of salts and hydrated minerals provided ample evidence of a wet Ceres, those observations revealed surface ice coverage only in shadowed, cold craters (14) and in a geologically very young crater at lower latitudes (15) (see the photo), which is not enough to explain the observed water activity (13). This is where the third instrument on Dawn, a gamma-ray and neutron detector investigating Ceres' subsurface, comes in: Analysis of the mapping data provided convincing evidence for globally distributed ice in the subsurface, ∼1 m (or less) below the surface in warm equatorial regions and very close to or at the surface in the colder polar regions (4). The calculated ice coverage, estimated to be ∼10% in the near-surface layer, cannot explain the observed water vapor from Ceres because temperatures in the ice layer will be too low for substantial sublimation of ice. However, even small impacts will locally expose ice to the surface and may create sporadic activity. At the same time, this process poses the question of the stability of the ice layer: Those impacts remove ice from the upper surface layer, and the resulting water vapor will eventually escape from Ceres. It is not obvious how the ice can remain close to the surface over geological time scales. The problem is similar to the question of how cometary activity is maintained, with little ice found on the surface of cometary nuclei. In spite of the different size and activity of the bodies (cometary nuclei are only a few kilometers in size, and the sublimation rate per area is orders of magnitude higher than for Ceres), the physical process behind the solution of both problems may be the same.

De Sanctis et al. provide the first observations of organic material on Ceres, confirming the presence of such material in the asteroid belt. Furthermore, because Ceres is a dwarf planet that may still preserve internal heat from its formation period and may even contain a subsurface ocean (16), this opens the possibility that primitive life could have developed on Ceres itself. It joins Mars and several satellites of the giant planets in the list of locations in the solar system that may harbor life. Future missions to asteroids, comets, and the outer solar system may provide answers to the fundamental questions concerning the origin of water and organic material on Earth and the possible formation of prebiotic material and primitive life in the outer solar system.

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