Evrim Yazgin
Cosmos science journalist
Astronomers have overturned long-held beliefs about the composition of dwarf planet Ceres.
Ceres was the first asteroid to be discovered, when in 1801 it was spotted by Italian astronomer Giuseppe Piazzi. It is the only dwarf planet in the inner solar system and is the largest object in the main asteroid belt between the orbits of Mars and Jupiter.
Scientists previously argued that the visible craters on Ceres’s surface meant that the dwarf planet is not very icy – less than 30%.
The new research published in Nature Astronomy shows that not only does Ceres have lots of ice – roughly 90% – but that it might have once been a muddy, ocean world in the ancient past.
“We think that there’s lots of water-ice near Ceres’s surface, and that it gets gradually less icy as you go deeper and deeper,” says co-author Mike Sori from Purdue University in the US.
The dwarf planet has a radius of about 476km, making it about a quarter the width of our Moon.
The researchers used computer simulations to show how the craters on Ceres might have deformed over billions of years due to the presence of water.
“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,” Sori says. “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.
“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,” Sori says.
“Even solids will flow over long timescales, and ice flows more readily than rock,” adds co-author Ian Pamerlau, a PhD student at Purdue. “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.”
The team tested different crust compositions and found a high ice content near the surface was best at recreating the “relaxed” craters visible on the surface of Ceres.
“To me the exciting part of all this, if we’re right, is that we have a frozen ocean world pretty close to Earth,” Sori adds. “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.”
He is hopeful that the discovery will prompt future space missions to go to Ceres.
“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’s 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.”
