Dawn has been in orbit of Ceres since March, 2015 and its three-year examination of the dwarf planet has vastly expanded our understanding of its nature and composition. In February 2017, NASA announced it had detected organic molecules on the surface of Ceres. Now, new evidence suggests there are more of these molecules than previously expected.
The molecules detected on Ceres are tholins — a variety of organic compounds found across the solar system, though not on the modern Earth. They are created by the interaction of ultraviolet radiation or cosmic rays with surface elements like carbon dioxide, ethane, methane, and nitrogen. Tholins are thought to be responsible for the orange and reddish hues across multiple bodies in the solar system, including Europa, Titan, and Sputnik Planitia on Pluto.
Finding a larger-than-expected concentration of tholins on Ceres helps us understand the environment of the early solar system. Ceres is thought to be a protoplanet that either formed in the asteroid belt in its present location or that may have formed in the Kuiper Belt before transitioning inwards towards the sun. Either way, it can be thought of as a sort-of time capsule. There are only three protoplanets remaining in the inner solar system — Ceres, Vesta, and Pallas, and of the three, Ceres is the only one large enough to have rounded itself under its own gravity.
There was a period of time when the asteroid belt was thought of as a shattered planet or the remnants of the same after a cataclysmic impact destroyed a larger parent body. Research throughout the 20th century revealed that this was unlikely to be true. Instead, the asteroid belt represents a remnant population of the same materials that formed the rest of the solar system. In some cases, asteroids have preserved information about the state of the solar system in its earliest days, thereby giving us a window into what our solar system looked like as it formed.
One of the things that makes Ceres strange is that the organic molecules in question weren’t found equally distributed across the planet. They’re strongly concentrated in the Emutet crater. There are two models for how the rich concentration of molecules might have arrived at its current location: Either they were deposited by a low-energy impact (Ceres’ gravity is quite low relative to Earth) or they were produced by processes on or within Ceres itself. The latter seems more likely, given that the deposits are quite concentrated to start with. The impact of an organic-rich comet or asteroid can’t be ruled out, but the idea that these compounds were produced within the protoplanet is far more interesting, given that it would shed light on the internal processes within the dwarf planet and its overall formation.
Based on the material we’ve found to-date, it’s possible that Ceres was formed by the same processes that produced the C chondrite meteor family, but this depends in part on whether certain other compounds are also distributed across the surface or mixed with the organics we’ve detected — and some of those compounds are impossible for the spacecraft’s sensors to measure effectively. The puzzle of how the organics came to be concentrated so heavily in one specific area of the planet may not be something we can definitively answer just now — but it’s an important and interesting examination of the conditions in the early solar system.