My latest book on ocean worlds is barely off the press and already a new study has surfaced shedding light on one of the ocean world candidates: Ceres, the closest dwarf planet to Earth and the largest object in the asteroid belt. Ceres is one of the most intriguing planetary objects in our Solar System, with a subsurface that remains poorly understood but is known to hold a substantial amount of water, some of which we believe exists in liquid form.
Since the Dawn spacecraft visited Ceres between 2015 and 2018, we’ve been treated to stunning images of the dwarf planet's surface, showcasing brine volcanoes: massive outpourings of brines (salty water) that occurred there in the past. The biggest brine volcano on Ceres is Ahuna Mons. This volcano stands approximately 4 kilometres (2.5 miles) high and about 20 kilometres (12 miles) wide and is thought to be a few hundred million years old. Its origin is believed to stem from hot subsurface brine (of salts whose nature is still unknown) surging to the surface via fissures in the crust, gradually accumulating over time to construct the massive volcanic dome. Ahura Mons and other brine volcanoes, such as the Occator Crater brine volcano seen in the middle of the image above, suggest the presence of subsurface liquid in the dwarf planet’s past as well as salts and other minerals. Thus, Ceres was thought to have had until recently, and maybe still today, small bodies of liquid water trapped under the icy crust.
Studying Ceres has proven challenging. The difficulty faced by planetary scientists lies in the fact that its internal structure, composition, and formation remain poorly understood due to two conflicting observations: while the morphology of its craters and how Ceres' gravity influenced the Dawn spacecraft’s trajectory suggest an ice-rich interior, the lack of crater relaxation (the process where impact crater shapes change over time due to surface material flow or deformation) points to a low-ice content.
The new study addresses this discrepancy and, in doing so, offers a clearer understanding of what might lie beneath the surface. The explanation seems to come down to dirt. Indeed, the scientists have added into their models of the dwarf planet's interior the effects of impurities trapped between ice grains (this has been observed in terrestrial lake settings and Antarctic ice), and in doing so, show that Ceres can maintain the types of craters it has while still having an ice-rich crust. The simulations show that the ice content diminishes from approximately 90% pure ice near the surface to 0% at a depth of 117 km within the interior, indicating that the ice becomes increasingly impure the deeper we go. This gradual differentiation of water and impurities can only take place in a liquid environment; in other words, a relic ocean. Most likely, the ancient ocean froze out to form a crust with a high ice content, while the rest of the ocean started to freeze out over millions of years, still leaving pockets of liquid ice at present.
This study not only reinforces the idea of Ceres as a relic ocean world with liquid water across its entire surface, but it also suggests that Ceres is the nearest "ocean world" to us. Therefore, we should prioritize sending future spacecraft missions to this giant asteroid to gain a deeper understanding of its interior and reevaluate existing crustal models. Sending spacecraft to other ocean worlds orbiting Jupiter or Saturn is costly and requires long mission times. Ceres can be reached and studied with a relatively smaller budget.
Another reason to revisit Ceres is its astrobiological potential. Given that we can now consider the possibility that Ceres has maintained a global ocean for millions, if not billions, of years, complete with impurities and likely undiscovered compounds, there is a chance that life could have originated within this massive planetary body. Moreover, the concept of panspermia suggests that fragments of planetary bodies ejected by asteroid impacts tend to drift toward the Sun. This raises the possibility that life (in its microbial form) could have started on Ceres and spread to the terrestrial planets such as Mars or Earth. Perhaps we are all, in a sense, descendants of Ceres. Would that make us Cereans? Maybe one day we'll know.
As usual, onwards and upwards. Bernard
(Image credit: NASA and JPL)
Relic oceans? Nanoo nanoo, fellow Cerean.