Ceres is a key object in understanding the evolution of small bodies and is the only dwarf planet to have been orbited by a spacecraft, NASA’s Dawn mission. Dawn data paint an inconclusive picture of Ceres’ internal structure, composition and evolutionary pathway. New research shows that a crust with nearly 90% ice near the surface, which gradually decreases to 0% at 117 km depth, simultaneously matches the Dawn observations. This crustal structure results from a frozen ocean that became more impurity rich as it solidified top-down. Therefore, the Dawn data are consistent with an icy Ceres that evolved through freezing of an ancient, impure ocean.
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.
“Ceres is the largest object in the asteroid belt, and a dwarf planet. I think sometimes people think of small, lumpy things as asteroids (and most of them are!), but Ceres really looks more like a planet,” said Purdue University researcher Mike Sori.
“It is a big sphere, diameter 950 km or so, and has surface features like craters, volcanoes, and landslides.”
“We think that there’s lots of water-ice near Ceres surface, and that it gets gradually less icy as you go deeper and deeper.”
“People used to think that if Ceres was very icy, the craters would deform quickly over time, like glaciers flowing on Earth, or like gooey flowing honey.”
“However, we’ve shown through our simulations that ice can be much stronger in conditions on Ceres than previously predicted if you mix in just a little bit of solid rock.”
This discovery is contradictory to the previous belief that Ceres was relatively dry.
The common assumption was that Ceres was less than 30% ice, but Sori’s team now believes the surface is more like 90% ice.
“Our interpretation of all this is that Ceres used to be an ocean world like Europa (one of Jupiter’s moons), but with a dirty, muddy ocean,’” Dr. Sori said.
“As that muddy ocean froze over time, it created an icy crust with a little bit of rocky material trapped in it.”
The authors used computer simulations to model how relaxation occurs for craters on Ceres over billions of years.
“Even solids will flow over long timescales, and ice flows more readily than rock,” said Purdue University Ph.D. student Ian Pamerleau.
“Craters have deep bowls which produce high stresses that then relax to a lower stress state, resulting in a shallower bowl via solid state flow.”
“So the conclusion after NASA’s Dawn mission was that due to the lack of relaxed, shallow craters, the crust could not be that icy.”
“Our computer simulations account for a new way that ice can flow with only a little bit of non-ice impurities mixed in, which would allow for a very ice-rich crust to barely flow even over billions of years.”
“Therefore, we could get an ice-rich Ceres that still matches the observed lack of crater relaxation.”
“We tested different crustal structures in these simulations and found that a gradational crust with a high ice content near the surface that grades down to lower ice with depth was the best way to limit relaxation of Cerean craters.”
“To me the exciting part of all this, if we’re right, is that we have a frozen ocean world pretty close to Earth,” Dr. Sori said.
“Ceres may be a valuable point of comparison for the ocean-hosting icy moons of the outer Solar System, like Jupiter’s moon Europa and Saturn’s moon Enceladus.”
“Ceres, we think, is therefore the most accessible icy world in the Universe. That makes it a great target for future spacecraft missions.”
“Some of the bright features we see at Ceres’ surface are the remnants of Ceres’ muddy ocean, now mostly or entirely frozen, erupted onto the surface.”
“So we have a place to collect samples from the ocean of an ancient ocean world that is not too difficult to send a spacecraft to.”
The findings were published in the journal Nature Astronomy.
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I.F. Pamerleau et al. An ancient and impure frozen ocean on Ceres implied by its ice-rich crust. Nat Astron, published online September 18, 2024; doi: 10.1038/s41550-024-02350-4
This article was adapted from an original release by Purdue University.
