Ceres, the largest object in the asteroid belt, is composed of rock and ice. NASA’s Dawn spacecraft, which orbited Ceres between 2015 and 2018 gave scientists lots of new insights into its shape and internal structure, surface morphology, and composition. A special collection in Journal of Geophysical Research: Planets focuses on evidence of ice in Ceres’ subsurface and its dynamical behavior. I asked Hanna Sizemore, guest editor of the special collection, some questions about the mission and what has been discovered.
Ceres was one of two bodies in the asteroid belt visited by NASA’s Dawn mission. Why was Ceres a focal point of this exploration mission?
Ceres and Vesta are generally viewed as two extreme cases of the possible evolution of large asteroids. Before the Dawn mission, both objects were thought to have formed in the same neighborhood but at different times. Because Vesta formed early, it accreted Aluminium-26 (26Al), which produced abundant internal heat and rapid loss of water and other volatiles. Ceres formed later, experienced less heating, and was able to retain much more of its water [McCord and Sotin, 2005].
An important goal of the Dawn mission was linking large asteroid interiors to early conditions in the protoplanetary disk, with Ceres representing one extreme scenario. There has been a long debate about how much of Earth’s water was delivered from the main belt, and whether it came in the form of hydrated minerals or ice. The amount of water in the main belt is only beginning to be inventoried.
There are big challenges in linking surface composition information derived from telescope data to information about asteroid interiors [Rivkin et al., 2019]. These challenges make having a spacecraft in orbit around an ice-rich main belt object extremely valuable. As both the largest and the most ice-rich object in the main belt, Ceres was a prime target for a mission.
The special collection has a focus on water ice. How has ice been an important factor in shaping the evolution of the surface of Ceres?
At the most basic level, Ceres owes its internal structure and its spherical shape to ice. It was able to partially differentiate (to form a rocky interior surrounded by an icy outer shell) because ice melts at lower temperatures than rock. It was able to retain enough mass to be spherical because it was cool enough to hang on to most of its H2O and other volatiles, unlike Vesta.
Of course, ice also plays a key role in modern landscape development on Ceres. There are eight broad classes of surface features that have been specifically linked to subsurface ice and are observed over most of the dwarf planet’s surface [Sizemore et al., 2019b].
Large domical mountains, such as Ahuna Mons [Reusch et al., 2019], were some of the first to be recognized in the earliest images to be returned by Dawn. Some of these mountains may be cryovolcanoes, or they may be formed by massive ice diapirs [Sori et al., 2018; Sizemore et al., 2019b].
Lobate landslides and ejecta were also recognized as ice-controlled features early in the Dawn mission [Schmidt et al, 2017].
Some show similarities to rock glaciers on Earth and Mars, some have characteristics in common with landslides on Iapetus.
Ice in Ceres’ subsurface contributes to the formation of fluidized, lobate, and layered ejecta around craters.
In some cases, Cerean ejecta is similar to layered ejecta formed in icy terrains on Mars and Ganymede [Hughson et al., 2019].
Intermingling of ice-rich ejecta and landslides creates unique Cerean terrains [Duarte et al., 2019]. Some smooth ejecta on Ceres is also pitted, probably due to water vapor and other gasses escaping soon after the impact [Sizemore et al., 2017].
Large-scale fractures in crater interiors have been linked to upwelling of ice and brines in the subsurface [Buczkowski et al., 2019]. Ice also causes craters themselves to relax on Ceres, sometimes leading to fracturing around crater rims that retain distinct topography [Otto et al., 2019] and, more rarely, flattening the topography of large craters [Bland et al., 2018].
Isn’t there also evidence that Ceres is active – including losing material to space? Why is this happening and why is it important?
There are two main lines of evidence that Ceres outgasses water to space, at least periodically.
The first line of evidence comes from astronomical observations of Ceres from Earth. In the early 1990s, the International Ultraviolet Explorer (IUE) detected OH near Ceres’ limb [A’Hearn & Feldman, 1992]. In 2014, immediately prior to Dawn’s arrival at Ceres, water vapor was detected by the Herschel Space Observatory [Küppers et al., 2014]. Both of these detections spurred speculation about cometary style outgassing and/or possible cryovolcanism on Ceres.
The second line of evidence for episodic outgassing comes from Ceres’ geomorphology and minerology, as observed by Dawn. In Occator Crater, we see bright carbonate deposits (faculae) that likely formed via extrusion or even fountaining of brines on to the surface in the last several million years [Scully et al., 2019]. Problematically, Dawn did not observe any direct evidence for outgassing during its mission at Ceres or active brine extrusion. Any outgassing from faculae formation would have stopped millions of years ago, so it cannot explain the IUE and Herschel detections.
Explaining the Herschel data in particular has been challenging. Ice in Ceres’ subsurface is slowly receding to greater depths, which produces a steady background rate of water vapor loss much lower than the Herschel rate [Landis et al., 2017]. Any time ice is exposed on the surface, it will quickly sublimate. So stochastic events, landslides and small impacts that expose ice, can cause short bursts of outgassing. A cluster of these events might have produced the water vapor detected by Herschel [Landis et al., 2018].
These passive, near-surface processes most likely drove the outgassing events observed in 1992 and 2014, although we can’t be certain.
Near-surface processes fall under the umbrella of cometary style outgassing. Understanding these processes is important, because they likely operate on numerous small, volatile rich bodies throughout the solar system, and dominate water vapor production at Ceres today.
Regular outgassing from stochastic, passive processes does not preclude more dramatic outbursts of water from deeper in the interior in the recent geologic past.
There is ongoing work to understand the formation of the bright spots in Occator, and the development of the large cryovolcano Ahuna Mons.
With NASA’s Dawn mission having come to an end what are the next steps in the exploration of Ceres now that we know how important water ice has been for the body?
In the near term, there is work to do on Earth to understand the Dawn data better. In particular, laboratory experiments to determine the material properties of the unique mixtures of ice, salts, and clays that occur on Ceres. Knowing more about the properties of these mixtures will allow us to build better models of Ceres’ structure and evolution [Sizemore et al. 2019a].
Longer term, of course, we hope there will be new spacecraft missions. These might include orbiters with instrumentation designed specifically to interrogate the ice and brine content of Ceres’ interior at depths we couldn’t probe effectively with Dawn’s instrument payload.
For example, an orbiter carrying a sounding radar could help us quantify the ice content of lobate landslides and ejecta, helping us understand stratigraphy at the 100 meter to 1 kilometer scale. An orbital magnetometer could constrain whether or not there is a deep brine layer. And of course, a future lander might be able to directly sample the structure and chemistry of the ice – much like the Phoenix lander did on Mars.
—Steven A. Hauck, II ([email protected]; 0000-0001-8245-146X), Department of Earth, Environmental, and Planetary Sciences, Case Western Reserve University, Ohio; and Hanna Sizemore ( 0000-0002-6641-2388), Planetary Science Institute, Arizona