Ceres’ Spots in the Spotlight

Water and organics are reported. Is there energy, too?

Ceres false color PIA20180.jpg
Occator Crater on Ceres, in false color, with the spots shining brightly. Blue is generally associated with bright material, found in more than 130 locations, and seems to be consistent with salts such as sulfates. The crater is 60 miles across.

The mystery of the bright spots, more than 130 in total, on the surface of the dwarf planet Ceres might have been solved—sort of. Two research teams have published new results based on data from the Dawn spacecraft currently orbiting Ceres, the largest object in the asteroid belt between Mars and Jupiter.

The first study, led by Andreas Nathues from the Max Planck Institute for Solar System Research in Germany, suggests that the bright spots, especially the brightest ones in a large crater called Occator, consist of magnesium sulfate containing bound water. Interestingly, this crater, which is thought to be only about 78 million years old, also shows abundant terraces and landslides on the crater rims—features you would not expect on a geologically inactive planet. 

Perhaps even more surprising is a second study, led by De Sanctis from the Institute of Astrophysics and Planetary Science in Rome, who found that the surface of Ceres holds ammonia-rich clays. This would imply that Ceres originated in the far outer Solar System (the Kuiper Belt?), and that ammonia-containing organics were incorporated into the clay at that time.

Ceres contains enough organic compounds and water to stir astrobiological interest. It is thought to contain 30 percent water by mass, and water vapor has already been detected. It apparently forms as a haze in crater floors during daytime and vanishes at dusk, which would indicate that there is at least some unbound water close to the surface. And the wide distribution of bright spots on Ceres suggests the presence of a global subsurface layer of salt-rich water ice

It’s cold on the surface of Ceres, with temperatures below -36 degrees C (-26 Fahrenheit), so we wouldn’t expect to find liquid water near the surface. There is also no tidal heating on Ceres, such as there is at Europa. However, Ceres, with its diameter of 950 kilometers (590 miles), is fairly large and is thought to have differentiated into a core, mantle, and crust. If there is some internal heating, due to the presence of radioactive elements, would it be possible to have pools or even seas of liquid water within Ceres?

The high salt content would certainly help to lower the melting point. Do the landslides and terraces and some of the smooth floors, which are similar to crater floors on Mars, suggest that there’s some soft material underneath, likely water ice that’s been partially or totally liquefied by an asteroid impact? Could these landforms really form without any liquid water present, at least temporarily?

On Ceres we know that there are enough organic compounds and water for life. Now the big question is whether there is enough energy to make it a high-priority astrobiological target.

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