The bright spots of Ceres. They were hard to miss when Dawn first saw them as it approached the dwarf planet. What are they? It’s a question that has left scientists scratching their heads ever since they first saw them. Scientists came up with two leading theories. Ice or salt.
Today, the Dawn team is leaning heavy in the salt camp. “We believe this is a huge salt deposit,” Dawn’s principal investigator Chris Russell said on Monday at the European Planetary Science Congress. “We know it’s not ice, and we’re pretty sure it’s salt, but we don’t know exactly what salt at the present time.”
These salt deposits are really bright. When Dawn first imaged them, the reflectivity of the bright spots was saturating the camera. But, the Dawn team was able to figure out the albedo (reflectivity) of these bright spots. And it rules out ice.
That doesn’t mean Ceres doesn’t have ice. Quite the opposite actually. Ceres “seems to be a wet and icy world,” according to Russell. Its surface is dry, but the interior is believed to contain a lot of water molecules and ice.
Ok, it’s salt. What does that mean?
According to Russell, the salt deposits are an indication of an active surface. “Some comet or asteroid did not come in carrying salt to this object,” says Russell. “This is derived from the interior somehow.”
Russell also points out another curious feature in the Occator crater. There’s what appears to be a small hill at the center of it. Russell and his team aren’t sure what’s going on there, but they hope to learn more soon.
The salt deposits may not be limited to the Occator crater. Ceres’ other prominent feature, a mountain, has what looks like salt deposits streaking down its slope.
This mountain points to more activity. How it formed is another mystery. Russell talks about how New Horizons recently discovered numerous mountains on Pluto that look more or less like the one on Ceres. “Maybe it’s an ice mountain,” Russell said. “Somehow ice is frozen and lifting itself up.” Right now, the Dawn team doesn’t have any answer as to what forces are behind the mountain’s formation.
There are still many questions about Ceres that scientists want to answer. Are its craters impact craters? How did the mountain form? And what role does subsurface ice play? Those are just some of the questions Russell and his team hope to answer during Dawn’s mission.
Here are some timestamps and the subjects Russell touches on in his speech.
Getting to Ceres (5:40) Ceres interior structure (8:54) Water on Ceres (10:39) How Dawn explores Ceres (12:10) Ceres’ bright spots (14:43) Ceres’ mountain (17:15) Crater comparison (20:21) Ground ice discussion (22:23) Summary (23:53)