Surface water ice found within a brilliantly bright crater on the dwarf planet Ceres is being exposed by avalanches reprocessing impact deposits in just the last few years. This new interpretation of survey data from NASA’s Dawn mission provides the youngest evidence of surface activity on the largest body in the asteroid belt.
Oxo Crater is the second-brightest feature on Ceres. Only Occator’s central area is brighter. Oxo lies near the 0 degree meridian that defines the edge of many Ceres maps, making this small feature easy to overlook. NASA’s Dawn spacecraft took this image in its low-altitude mapping orbit, at a distance of 385 km from the surface of Ceres. Image credit: NASA / JPL-Caltech / UCLA / MPS / DLR / IDA / PSI.
As NASA’s Dawn spacecraft arrived at Ceres back in March 2015, a new crater came to the attention of mission controllers back in Houston.
Just under 10 km in diameter and embedded in an ancient, heavily cratered terrain darkened by thermal alteration and billions of years of falling micrometeorites, Oxo was in most way ways nothing spectacular. And yet its brightness made it one of the most striking features on the planet’s surface.
“The human eye would immediately see this difference,” says Andreas Nathues from the Max Planck Institute for Solar System Research, imagining himself crossing the Cerean terrain and coming across Oxo. “It would be like snow against a background which is about as bright as coal.”
Bright spots are a feature of Ceres. Occator Crater, the brightest of all was observed even before Dawn arrived, from the ground-based W. M. Keck Observatory on Mauna Kea earlier in 2015.
The exact origin of these brightest spots is still debated. One idea for Occator, put forward by Nathues back in 2015, was its formation impact triggered a carbonate rich brine to rise from 20-30 km down, reaching the surface through cryovolcanic eruptions.
However, Oxo poses new questions. Earlier this year a new paper reported evidence of water ice at the crater, a first for Ceres.
“Why did we find water ice at Oxo and not at other craters?” asked Nathues.
The finding was particularly surprising as despite surface temperatures below 200K, surface water ice sublimates very rapidly on Ceres – often in just a few years.
With previous studies predicting an age for Oxo of somewhere between 2.1 and 4.3 million years, Nathues concluded there must be something else responsible for delivering fresh ice to the surface.
Oxo Crater is seen in this perspective view. The elevation has been exaggerated by a factor of two. The view was made using enhanced-color images from NASA’s Dawn mission. Image credit: NASA / JPL-Caltech / UCLA / MPS / DLR / IDA / PSI.
To investigate, Nathues and co-authors analyzed data from the Framing Camera and the Visible and Infrared Spectrometer on board Dawn which together collected topographical, mineralogical and elemental abundances from as close as 385 km above the surface. Their results have been published in the Astronomical Journal.
The team began by using the infrared spectra data to map the two major absorption bands of water ice, confirming its exposure within two lobate deposits on Oxo’s pole-facing scarps.
They discovered the availability of water ice was helped by the nature of the impact that formed Oxo, which struck a very low topographic point on the Cerean terrain.
Previous studies had suggested the deeper you drill down into Ceres the more ice you find, and Oxo was shown to sit 4,802 m below the planet’s average elevation, making it one of the lowest-lying surfaces on the northern hemisphere.
So that explains the origin of water ice at Oxo, but how had it survived for millions of years exposed on the surface?
Simple. It hadn’t. A closer inspection of Oxo’s crisp rim morphology, well defined ejecta blanket, and lack of superimposed craters lead Nathues’ team to reevaluate the impact date as just 190,000 years ago — an error they put down to overestimates of the resurfacing ability of impact ejecta. The recent impact limits the time for sublimation, especially on Oxo’s pole-facing slope which would have experienced weaker solar irradiance.
However, a younger impact alone still cannot account for surface ice at Oxo. This is where evidence of even more recent activity comes in. Signs of rim and wall collapse hint at very recent avalanches running down the sides of Oxo.
“We do not exactly know the age but if their exposure is simply by downslope movement of material then we are talking about an age of a few years only,” says Nathues.
This would make these avalanches one of the youngest examples of activity on the Cerean surface.
Similar to rock glaciers and icy landslides on Earth, Nathues believes these falls could produce the very recently exposed ice along the upper pole-facing slope of Oxo’s since failed rim.
“What we see on Oxo is ice that hasn’t yet sublimated.”
These interpretations fit nicely with another phenomenon associated with Oxo.
Shortly after the arrival of Dawn at Ceres, the mission team saw additional brightening associated with Oxo which was observed when the dwarf planet was lined up in front of the Sun.
These were interpreted as the light scattering effects of haze or cloud development that could be accounted for by the very recent water ice sublimation that Nathues’ latest analysis points to.
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A. Nathues et al. 2017. Oxo Crater on (1) Ceres: Geological History and the Role of Water-ice. AJ 154, 84; doi: 10.3847/1538-3881/aa7a04
