During its decade-long journey, Dawn has observed planet-like worlds Vesta and Ceres, collecting detailed data about the two fascinating bodies in the main asteroid belt. Initial data from ground-based telescopes and NASA’s Hubble Space Telescope provided some information about what we might find at the two bodies, though they had never been explored up-close until Dawn. The high-resolution data Dawn has collected offer a clearer picture of the geologies of Vesta and Ceres, providing great insight into their histories. Here we briefly explore how Dawn has shaped and changed our understanding of these distant worlds.
Before Dawn visited Vesta, the first stop on its interplanetary journey, scientists were able to make inferences about what it might be like based on meteorites believed to be from Vesta and data from telescopes. Basalt in such meteorites suggested that it had a magmatic history. Such a hot history also implied that its interior would be differentiated into compositionally distinct layers based on density. Overall, Dawn was expected to find a dry, rocky world with a hot history in line with other terrestrial inner solar system bodies.
These predictions were mostly correct, though Dawn’s data revealed several unforeseen features. Images from Hubble captured its irregularly shaped southern hemisphere, due to the colossal impact that formed Rheasilvia basin. Dawn’s camera found that its southern hemisphere had been resurfaced by this collision, leaving it with far fewer craters than its northern hemisphere. Dawn also found troughs encircling Vesta’s equator that are concentric with its two giant basins, Rheasilvia and Veneneia. The energy of the impacts that formed these basins sent waves through Vesta’s interior that created troughs. In order to form, these troughs require internal layers that react differently to stress, supporting Vesta’s hypothesized differentiation. Dawn’s gravity data confirm this hypothesis. Dawn also confirmed another hypothesis -- that howardite-eucrite-diogenite meteorites (HED), a class of meteorites that makes up a whopping 6 percent of those found on Earth -- originated at Vesta. These meteorites were originally present in different layers of Vesta’s interior before being excavated and ejected into space by impacts like Rheasilvia’s. Impacts are likely also responsible for bringing foreign material to Vesta’s surface. Though Hubble had observed variation in Vesta’s albedo (surface reflectivity), Dawn found unexpectedly distinct dark patches. Their non-random distribution seems to indicate that they are the result of low-energy impacts by carbon-bearing asteroids that delivered the darker material to Vesta and other impacts that melted Vesta’s surface, darkening the material there. Brighter material on Vesta’s surface was likely exposed and scattered by impacts. Asteroids also likely delivered the unexpected hydrated material that Dawn found.
Beyond craters and troughs, Dawn’s lower orbits also offered views of a variety of surface features, including dramatic slopes and landslides, which could not be seen from the distant telescopes before Dawn. Overall, Dawn’s data support many of the previous hypotheses about Vesta, while illuminating Vesta’s striking features and revealing unexpected processes.
Whereas Dawn’s observations at Vesta generally supported existing hypotheses, providing greater detail to fill in the gaps, less was known about Ceres. In fact, most of what we know about the dwarf planet was contributed by Dawn.
Initial calculations suggested that Ceres might be differentiated into layers, though before Dawn the composition of these layers was unknown. With a low average density, Ceres was expected to have large amounts of water ice beneath its surface. Distant observations also showed Ceres to have a relatively smooth surface.
Dawn complicated this picture. Gravity data confirmed that Ceres is, in fact, differentiated into a rocky core and a water ice crust covered by a dusty outer layer. Dawn also found evidence of clathrate hydrates, gas trapped within a crystal structure of water molecules, which contribute to the remarkable strength and low density of Ceres’ crust. While most of Ceres is relatively smooth due to its semi-fluid subsurface ice layer, Dawn did find one large mountain that hadn’t been seen before. Around 2.5 miles (4 kilometers) high, this mountain, called Ahuna Mons, is almost as high as Mount Whitney in California. Its well-defined domed shape, similar to volcanoes on Earth, suggests that it likely formed due to cryovolcanic activity (that is, when muddy ice was expelled from Ceres’ interior and froze on its surface). Though cryovolcanism was hypothesized on Ceres and may be present on other icy worlds, Dawn’s observations make Ahuna Mons the closest known cryovolcano in the solar system. Ahuna Mons is also evidence of recent geological activity (within the past 100 million years or less), making Ceres one of a few bodies in the solar system that show signs of recent activity.
Hubble’s images showed evidence of several brighter areas on Ceres’ surface. When Dawn took a closer look, finding hundreds more than the initial images had captured, scientists hypothesized about what these bright spots might be. As Dawn moved into lower orbits, it became clear that these spots were actually salt deposits, likely left behind when briny water escaped from below Ceres’ surface and sublimated away. Other observations, taken by the European Space Agency’s Herschel Space Observatory, showed a small amount of water vapor around parts of Ceres, leading some scientists to hypothesize that it may have a weak atmosphere or even ongoing cryovolcanic activity. Dawn discovered that this gas might be produced by energetic particles from the Sun colliding with water ice on Ceres, which is then released as vapor, resulting in a temporary weak atmosphere.
Dawn’s spectroscopic data also confirmed the presence of ammonia on Ceres’ surface, which had been suggested from telescopic observations in the 1990s. Conditions in the main asteroid belt should have been too warm for ammonia, which requires much colder conditions to form, raising questions about its origins. Ceres may have formed farther into the colder outer solar system before migrating to its current position, or ammonia could have been brought to Ceres by bodies from the outer solar system, complicating previous ideas about Ceres’ history.
Dawn also confirmed the presence of carbonates on Ceres, which had been detected a decade earlier in telescopic data. Carbonates, which generally form in warm liquid, are often found in meteorites believed to come from asteroids formed with a large amount of water. Dawn’s detection of a variety of carbonates on Ceres further supports the existence of a large ocean in Ceres’ early history. Ceres may even be warm enough to have a small amount of remaining liquid water underground. This potential for liquid water, in combination with the organics that Dawn found in Ernutet Crater, makes Ceres interesting to those who study the chemistry that leads to the formation of life.
While Dawn has answered many questions about Vesta and Ceres, its findings have also raised many questions that Dawn alone cannot answer. Though Dawn will end, its accomplishments and findings will continue to inform future study of these two worlds.