When discovered in 1801 through the telescope, Ceres was thought to be a new planet, like Uranus, discovered just 20 years earlier. Within a few years, however, more such objects were found, and it became clear that many orbited the Sun between Mars and Jupiter in what became understood as the solar system’s main asteroid belt. Ceres happens to be its largest member.
Asteroids are too small to be shaped by their own gravity—hence their irregular forms. They are thought to be the remnants of planet formation. In the early 1990s, the first detailed images of asteroids were returned by the Galileo spacecraft on its way to Jupiter and revealed gray, potato-shaped bodies whose craters indicated ancient surfaces. Up to that point, telescopic observations of asteroids had only provided estimates of their size and shape from their brightness variations as they spun and their surface composition by spectroscopy. For Ceres, those observations pointed to a rather round body with a hydrated surface mineralogy.
But those observations, which would prove quite accurate, did not uncover how much a part liquid water has played, and continues to play, in shaping Ceres’s surface. Nor did they hint that Ceres could teach us how life’s ingredients may come together.
Getting out there
What initially motivated the exploration of Ceres was the prospect of seeing up close what a large planetary building block looked like at the dawn of the solar system. In 2001, what became the Dawn mission team took advantage of a happy convergence of two trends. First, ion engines were becoming a reliable means of energy-efficient space propulsion (see the article by Igor Levchenko, Dan Goebel, and Katia Bazaka, Physics Today, September 2022, page 38). A spacecraft could orbit one planet and then move on to orbit another, without breaking the propellant bank.
Second, Ceres and Vesta—another large asteroid with a dry mineralogy akin to some meteorites found on Earth—were converging along their orbits, a once-in-17-year occurrence enabling fast travel from one to the other. NASA seized the chance to investigate two very different protoplanets in one go, selecting, out of two dozen competitors, the Dawn proposal for an ion-engine-enabled mission.
The spacecraft launched in 2007 and studied Vesta in 2011–12. It arrived at Ceres in 2015, carrying cameras and spectrometers able to discern minerals, molecules, and chemical elements at its surface. Equipped with a radio antenna, it was able to probe the distribution of mass inside Ceres by precisely tracking its own orbit.
More than meets the eye
Dawn images immediately revealed two surprises. First, Ceres is heavily cratered, as shown in figure 1. Its shape (volume) and orbit (mass) had constrained its bulk density to be consistent with a half-rock, half-ice object, suggesting Ceres to be the closest ice-rich world to the Sun. The relatively warmer temperatures expected at Ceres’s surface from the balance of solar illumination and blackbody reradiation, about 170 K, should have enabled surface topography such as crater rims to be smoothed out over geologically relatively short times. Persistent craters suggested instead a strong, rock-rich crust inconsistent with an ice-rich body.
Ceres shown in false-color renderings that highlight differences in surface materials, with blue material richer in carbonate minerals and brown material richer in clays. (Courtesy of NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.)
Ceres shown in false-color renderings that highlight differences in surface materials, with blue material richer in carbonate minerals and brown material richer in clays. (Courtesy of NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.)
Second, against that seemingly dull backdrop is a blindingly bright spot in the center of a low-latitude crater named Occator (figure 2). In planetary exploration, bright material can suggest recent activity, because over time surfaces get coated with dust, and fresh ices sublimate to space. Indeed, Occator’s central and secondary bright spots appear to cover the underlying surface and are essentially crater-free. Their absence indicates an age of millions of years at most—the blink of an eye in geological time.
The bright spot, 15 km in diameter at the center of Occator Crater, thought to be salty material left behind from eruption of subsurface liquid. (Courtesy of NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/PSI.)
The bright spot, 15 km in diameter at the center of Occator Crater, thought to be salty material left behind from eruption of subsurface liquid. (Courtesy of NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/PSI.)
Dawn’s instruments revealed at least 10% of the matter blanketing Ceres’s surface is made of iron and carbon and the rest is ammonium clays and magnesium carbonates. On Earth, those clays and carbonates form when volcanic rock is altered by liquid water with dissolved carbon dioxide; the nitrogen in the ammonium is sourced from organic matter. Ceres therefore appears to have harbored liquid water with soluble carbon and nitrogen—two elements on which life as we know it is based.
The bright spots are much richer in carbonates and sodium chloride. If those recent deposits are left behind by erupted liquid water known as cryolava, their liquid source reservoirs are likely cold and briny, because water with sodium chloride fully freezes at 251 K.
Water ice generally sublimes to space, but it seems abundant in Ceres’s crust. Indeed, surface images show morphologies evoking flows of ice, perhaps solidified cryolava, including an apparently volcanic edifice—Ahuna Mons—and several potentially older, smoothed-out relatives. Reconciling their presence with the crustal strength apparent from the persistence of craters is a challenge; researchers have invoked the mechanical strength of salts and hypothesized the presence of gas hydrates (see the article by Wendy Mao, Carolyn Koh, and Dendy Sloan, Physics Today, October 2007, page 42).
The spacecraft’s radio tracking investigation of Ceres’s gravity revealed the dwarf planet’s deep interior to be differentiated into a denser rocky core and an outer layer. Detailed analysis of the topography showed that outer layer to be weak at several dozen kilometers depth, consistent with the presence of liquid-filled porous rock.
A window into habitability
Emerging from those findings is an overall picture of a world that, not unlike ocean moons such as Europa and Enceladus, harbors subsurface liquid water in contact with rock that can be erupted through the overlying icy crust onto the surface. (See the Quick Study by Michael Manga and Max Rudolph, Physics Today, January 2023, page 62.) On Ceres, any liquid is likely a brine that upon eruption leaves behind bright salt deposits. Unlike the ocean moons, Ceres does not experience heating by the dissipation of tides, since it is so distant from the body it orbits (the Sun). And its relatively small size implies that the heat from the decay of naturally occurring radioisotopes in its rock has largely dissipated. Thus, its inferred liquid water reservoir is not an extensive global ocean; it’s more likely a set of local pockets of fluid-filled porous rock.
On Earth, clay-forming reactions between liquid water and rock produce chemical energy that microbial communities exploit to thrive in the absence of sunlight. More generally, in any environment on Earth where liquid water, energy, and a source of carbon, nitrogen, and other elements such as sulfur and phosphorus exist, so does life. Life is more abundant where fluxes of chemical energy and nutrients are higher, and less abundant if stressed by temperature extremes (hot or cold), salinity, pH (acidic or alkaline), or radiation. The possible presence of life motivates the exploration of such habitable environments found to be or have been present on Mars’s surface and subsurface, and in the oceans surrounding the rocky cores of some icy moons.
The habitability of environments inside Ceres today is likely precluded by the expected high salinity and low temperature of any liquid, as well as the little chemical energy remaining now that water–rock reactions appear to have largely reached equilibrium. Researchers can only speculate about the presence of any significant biosphere in a warmer past. Ceres is the only ice-rich world, however, with abundant rock mineralogy expressed at the surface. Farther out in the solar system, that mineralogy is masked by ubiquitous ice. Ceres is thus the only place where investigations can tease out, even from orbit, the details of interactions between liquid water, rock, and organic compounds. On Earth those interactions eventually led to the emergence of life.
Additional resources
Marc Neveu is a planetary scientist and astrobiologist at NASA’s Goddard Space Flight Center and the University of Maryland in College Park.