by A dwarf planet thought to have some ice between its dirty surface could be much cooler than we ever expected.
Ceres – the largest body in the asteroid belt between Mars and Jupiter – could have a crust that is more than 90 percent water ice. If so, the heavily cratered and scarred object could teach us a lot about ocean worlds, and what they might look like when they completely freeze over.
“We think there’s a lot of water ice near the surface of Ceres, and it’s gradually becoming less icy as you go deeper and deeper,” said planetary geophysicist Mike Sori of Purdue University in the US.
Ceres was first discovered in 1801 and is sometimes called an asteroid because of its location in the solar system; but it is large and spherical enough to be classified as a dwarf planet, just less than half the size of Pluto.
It’s also quite an interesting odd one out. It is the only dwarf planet closer to the Sun than Neptune, and is dotted with bright spots that could be evidence of ice volcanoes on its surface.
So it is likely that there is the least water there, but how much? Previous estimates, based on surface craters, put the amount at no more than 30 percent.
That’s because, if the surface were made of water ice, scientists thought it would gradually deform and become smoother and shallower over time. When NASA’s Dawn spacecraft arrived at Ceres in 2015, it found well-defined craters that were inconsistent with what researchers would have expected to see if Ceres was icy, so they made their estimates accordingly.
‘People used to think that if Ceres was very icy, the craters would quickly deform over time, like glaciers flowing on Earth, or like sticky flowing honey. However, we have shown through our simulations that ice can be much stronger under Earth conditions. Ceres than previously predicted if you mix in just a little bit of solid rock,” says Sori.
Using data from the Dawn mission and computer simulations of an icy world, a team led by planetary scientist Ian Pamerleau of Purdue University tried to investigate whether this assumption was correct.
And they found that it only took a little bit of dirt in the ice to give it enough structural integrity to retain sharp craters.
“Even solids will flow over long timescales, and ice flows more easily than rock. Craters have deep bowls that induce high stresses and then relax to a lower stress state, resulting in a shallower bowl via solid-state flow,” Pamerleau explains.
“Our computer simulations take into account a new way that ice can flow with just a little bit of non-ice impurities mixed in, leaving a very ice-rich crust barely able to flow even over billions of years. Therefore, we could end up with an ice-rich Ceres that is still matches the observed lack of crater relaxation. We tested several crustal structures in these simulations and found that a gradation crust with high ice content near the surface, which increases toward lower ice, was the best mode of constraining the relaxation of Cerean craters. “
As much as 90 percent of the dwarf planet’s crust could be made up of water ice, which the researchers say could provide some insight into the ice-covered ocean worlds. There are quite a few of them in the solar system, including the Jovian moons Europa and possibly Ganymede, the Kronian moons Enceladus and Mimas, and probably the Uranian moons Miranda and Ariel.
These moons have a thick layer of ice, under which an ocean of liquid water is believed to be maintained by the heat generated by the gravitational interaction between the moon and the planet.
Ceres does not orbit a planet, meaning there is no tidal activity to keep its interior warm. Any ocean that ever lapped there would have been completely frozen, according to the researchers.
“Our interpretation of all this is that Ceres was an ‘ocean world’ like Europa, but with a dirty, muddy ocean,” Sori says. “As the muddy ocean froze over time, it formed an ice crust with a little bit of rocky material in it.”
If this is the case, it means that ocean worlds could look very different than we expect. Moreover, NASA has previously sent a spacecraft to Ceres. That could happen again – and the possible frozen status of the dwarf planet’s ocean world makes it a very intriguing research target.
“To me, the exciting part of all this, if we’re right, is that we have a frozen ocean world that’s quite close to Earth. Ceres could provide a valuable point of comparison for the ocean-hosting icy moons of the outer solar system, such as Jupiter’s moon Europa and Saturn’s moon Enceladus,” Sori says.
“We believe Ceres is therefore the most accessible icy world in the universe. That makes it a great target for future spacecraft missions.”
The research was published in Nature Astronomy.
