Researchers on the Dawn mission released a series of six groundbreaking papers on the dwarf planet Ceres today in the journal Science. The new results reveal that Ceres has craters, cracks, cryovolcanos and other markers of geological processes.
This false-color image shows the dwarf planet Ceres. Scientists use false color to examine differences in surface materials. The color blue on Ceres is generally associated with bright material, found in more than 130 locations, and seems to be consistent with salts. Image credit: NASA / JPL-Caltech / UCLA / MPS / DLR / IDA.
In the first of the six papers, Ottaviano Ruesch of NASA’s Goddard Space Flight Center and co-authors argue that they have identified a cryovolcanic formation on Ceres, named Ahuna Mons.
“Ahuna is the one true ‘mountain’ on Ceres. After studying it closely, we interpret it as a dome raised by cryovolcanism,” said co-author Dr. David Williams, of Arizona State University.
Ahuna Mons is a volcano that rises 2.5 miles (4 km) high and spreads 11 miles (17.8 km) wide at its base. This would be impressive for a volcano on Earth. But Ahuna Mons stands on Ceres and isn’t built from lava the way terrestrial volcanoes are – it’s built from ice.
“Ahuna is truly unique, being the only mountain of its kind on Ceres. It shows nothing to indicate a tectonic formation, so that led us to consider cryovolcanism as a method for its origin,” Dr. Williams said.
The team applied models to determine the age of Ahuna Mons, finding it to have formed after the craters surrounding it, which suggests that it came into existence relatively recently.
There is no evidence for compressional tectonism, nor for erosional features. It appears that extrusion is a main driver behind the formation of Ahuna Mons.
Although the exact material driving the cryovolcano cannot be determined without further data, the team proposes that chlorine salts, which have been previously detected in a different region of Ceres, could have been present with water ice below Ceres’ surface and driven the chemical activity that formed Ahuna Mons.
“This is the only known example of a cryovolcano that potentially formed from a salty mud mix, and which formed in the geologically recent past,” Dr. Ruesch said.
Volcanic dome Ahuna Mons rises above a foreground impact crater, as seen by NASA’s Dawn spacecraft with no vertical exaggeration. Eruptions of salty, muddy water built the mountain by repeated eruptions, flows, and freezing. Streaks from falls of rocks and debris run down its flanks, while overhead views show fracturing across its summit. Image credit: Dawn Science Team / NASA / JPL-Caltech / GSFC.
In a second study, Jean-Philippe Combe of Bear Fight Institute and co-authors describe the detection of water ice – exposed on the surface of Ceres.
The dwarf planet was known to contain water ice, but water ice is also expected to be unstable on its surface, so scientists were unsure whether it could be detected there.
The team used Dawn’s Visible and InfraRed (VIR) mapping spectrometer to analyze a highly reflective zone in the young crater Oxo.
“The data reveal water-containing materials in an area covering less than one km squared, which most likely is water ice, although hydrated minerals are also a possibility,” Dr. Combe and his colleagues said.
Given the conditions on Ceres, water ice should be removed from the surface within tens of years; consequently, only a relatively recent exposure or formation of water would explain Dawn’s findings.
The team proposes several explanations for how this water appeared on Ceres’ surface, saying the most plausible is exposure of near-surface, H2O-rich materials from an impact or local landslide.
In a third paper, Christopher Russell of the University of California, Los Angeles, and co-authors discuss the unexpected detection of energetically charged solar particles at Ceres.
The Dawn Gamma Ray and Neutron Detector detected rapid bursts of solar wind interacting with the dwarf planet.
The team proposes two possible explanations for the wind: most likely, the dwarf planet has a weak atmosphere that was ionized by the energetic particles in the solar wind, producing a bow shock as the solar wind was deflected.
“Another, less likely, possibility is that a salty interior for the planet drives an electric current there, creating a magnetic field that would deflect the solar wind,” the scientists said.
In a fourth study, Harald Hiesinger of the Westfälische Wilhelms-Universität in Germany and co-authors analyze the craters on Ceres, using imaging data from Dawn’s Framing Camera.
Ceres was previously thought to have a layer of ice just below the surface. If the dwarf planet contains such an ice-dominated layer, it was predicted that even small craters would ‘relax’ within 10 to 100 million years, but this is not consistent with Dawn observations of the depths and shapes of the craters.
Those analyses indicate that Ceres’ outer shell is neither pure ice nor pure rock, but a mixture between the two.
By counting the number and size of craters on the surface, the team was also able to date various regions on the surface of Ceres.
In a fifth study, Debra Buczkowski of Johns Hopkins University Applied Physics Laboratory and co-authors highlight the diverse types of formations observed on Ceres, including craters, domes, lobate flows and linear structures.
While some of these features are caused by impacts, others hint at geological processes such as subsurface faulting.
Some features seem to be formed by cryomagmatism or cryovolcanism processes, driven by molten ice from beneath the surface.
“Patterns in the structures identified on the surface of Ceres are suggestive of salt upwelling,” the scientists said.
Finally, in a sixth study, Eleonora Ammannito of the University of California, Los Angeles, and co-authors analyze the distribution Ceres’ clay-like phyllosilicate minerals, which contain magnesium and ammonium.
The team used Dawn’s visible and infrared mapping spectrometer to determine that that the composition of these phyllosilicates across Ceres is fairly uniform, but that their abundance varies.
Because these minerals require the presence of water to form, the scientists propose that widespread and extensive aqueous alteration processes have affected the dwarf planet at some point in its history.
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O. Ruesch et al. 2016. Cryovolcanism on Ceres. Science 353 (6303); doi: 10.1126/science.aaf4286
Jean-Philippe Combe et al. 2016. Detection of local H2O exposed at the surface of Ceres. Science 353 (6303); doi: 10.1126/science.aaf3010
C. T. Russell et al. 2016. Dawn arrives at Ceres: Exploration of a small, volatile-rich world. Science 353 (6303): 1008-1010; doi: 10.1126/science.aaf4219
H. Hiesinger et al. 2016. Cratering on Ceres: Implications for its crust and evolution. Science 353 (6303); doi: 10.1126/science.aaf4759
D.L. Buczkowski et al. 2016. The geomorphology of Ceres. Science 353 (6303); doi: 10.1126/science.aaf4332
E. Ammannito et al. 2016. Distribution of phyllosilicates on the surface of Ceres. Science 353 (6303); doi: 10.1126/science.aaf4279
