Results from Dawn’s final mission phase upgrade Ceres to the realm of ocean worlds
News from Aug 20, 2020
When speaking of extra-terrestrial ocean worlds, the first images that come into one’s mind are those of the geologically active moons Enceladus and Europa with their icy crusts and underlying layers of liquid water. Recent discoveries on dwarf planet Ceres made by the Dawn Science Team, however, require reconsidering and broadening our idea of what an ocean world is.
The new findings were made possible by high resolution remote sensing of the geologically young 92 km Occator crater during Dawn’s second and final extended mission (XM2). Previous studies based on lower resolution imaging and compositional data from earlier mission phases had already provided information about the chemical composition of the unique bright deposits Cerealia Facula and Vinalia Faculae and their geomorphologic context. With the newly acquired data that almost entirely covers the faculae at resolutions between 3 and 6 m/px at visible wavelengths, up to ten times higher than data from the nominal mission, scientists were able to confirm previous assumptions about the faculae’s cryovolcanic origin and recent activity.
As a relict ocean world, Ceres provides insights into ocean-crust interaction as ocean worlds near the end of their lives
It was in 2004 when scientists first identified a bright region in Hubble Space Telescope images on the otherwise very dark Ceres which has an average low albedo of 0.09, typical of that of carbonaceous chondrites or C-type asteroids. The Dawn mission revealed that these bright deposits are primarily made of sodium carbonate (Na2CO3), a residue of salt-enriched water that ascended to the surface along fractures and evaporated. However, until the XM2 observations, the source of that liquid remained to be determined.
As implied by global scale topographic and gravitational analysis of Ceres, the dwarf planet shares commonalities with other ocean worlds, in particular an interior layered in a rock-rich mantle and water rich crust. However, Dawn data revealed that the mantle is weaker and of low density than typically found at icy moons. This has been interpreted as evidence for the presence of brines in a rock matrix. Also, while icy moons benefit from continuous tidal heating, produced from gravitational interactions with their planets, Ceres heat budget is determined by long-lived radioisotopes. Additionally, in contrast to large icy moons which experience resurfacing even at present, Ceres surface is heavily cratered. And yet also displays freshly exposed, pristine salt deposits within Occator crater.
With the new high-resolution Dawn gravity data, scientists were able to identify a negative anomaly several hundreds of kilometers wide located below Occator’s southeastern crater rim. Combined with thermal modelling this observation is interpreted as a deep brine reservoir beneath Occator. While these brines would usually not travel through Ceres’ 40 km thick crust, they could percolate up to the surface along factures.
Evidence for recent hydrothermal activity form spectral and geomorphological analyses
Further evidence for recent cryovolcanic activity comes from XM2 data acquired by the visual and infrared mapping spectrometer (VIR). The new data covering the Cerealia Facula dome (Cerealia Tholus) at ground resolutions of about 10 m/px (a factor of 10 higher than data acquired during Dawns nominal mission) resulted in fine compositional mappings. Based on these data, VIR scientists detected hydrated sodium chloride or hydrohalite (NaCl • 2H2O) within the radial system of fractures crosscutting the peak of the dome. Modelling of the dehydration rates of hydrohalite shows that its lifetime is less than about 100 years. Therefore, the deposits found within the fractures are a recent manifestation of cryovolcanic activity within Occator crater.
While the primary goal of the XM2 was to study the evolution of Occator with a focus on the faculae and their origin, a number of other features observed within Occator crater, revealed hydrothermal and periglacial-style activity. Thousands of isolated small bright spots, endogenic pits and troughs were identified on the high resolution XM2 data, especially within the impact melt deposits. High resolution stereo photogrammetrically derived digital topographic models (DTMs) allowed for morphometric measurements of endogenic conical hills and mounds (sometimes exhibiting central depressions or cracks), probably of pingo-like origin. Pingos are ice-cored mounds that form in permafrost environments on Earth by uplifting water under pressure. Their detection is thus additional evidence of subsurface volatiles.
Detailed geomorphologic mapping based on the XM2 data of the interior of Occator confirmed and refined results from studies conducted during the nominal mission. Furthermore, the high resolution of the XM2 Framing Camera data allowed the investigation of small craters on the young and small geomorphologic units. The spatial densities of these craters are commonly used to determine absolute model ages of planetary surfaces. While crater statistics could be translated into model formation ages of the faculae ranging between 2 and 8 million years, results still carry large uncertainties associated with obtaining reliable small crater counts especially in terrains with high brightness contrasts, as well as intrinsic model-related uncertainties. Nevertheless, spatial variations of impact craters accumulated on different units indicate post impact hydrothermal and tectonic activity within Occator crater.
The findings appear in a special collection of seven research articles on the Dawn XM2 observations and three supporting papers published in Nature Astronomy, Nature Geoscience, and Nature Communications.
JPL, a division of Caltech in Pasadena, California, manages Dawn’s mission for NASA’s Science Mission Directorate in Washington. Dawn is a project of the directorate’s Discovery Program, managed by NASA’s Marshall Space Flight Center in Huntsville, Alabama. JPL is responsible for overall Dawn mission science. Northrop Grumman in Dulles, Virginia designed and built the spacecraft. The German Aerospace Center, Max Planck Institute for Solar System Research, Italian Space Agency and Italian National Astrophysical Institute are international partners on the mission team.
The figure shows a FC XM2-based panchromatic image mosaic (superimposed by FC LAMO-based RGB data) of Cerealia Tholus and its radial fracture pattern exposing Hydrohalit.