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Physical Chemistry

Carbonates hint at liquid water on Ceres

Spectroscopic observations suggest the compounds formed from liquid water in the recent geologic past

by Sam Lemonick
March 14, 2018 | A version of this story appeared in Volume 96, Issue 12

The Occator crater with bright spots indicating water ice on the dwarf planet Ceres
Carbonates found in the Occator crater (top left) and elsewhere on Ceres suggest the dwarf planet had liquid water in recent past.

Astronomers have detected small amounts of hydrated sodium carbonates on Ceres, suggesting that liquid water has flowed near the dwarf planet’s surface in the recent geologic past (Sci. Adv.2018. DOI: 10.1126/sciadv.1701645).

Ceres is about one-quarter the diameter of the Moon and resides in the asteroid belt beyond Mars. It’s the largest dwarf planet inside Neptune’s orbit.

Filippo Giacomo Carrozzo of the Institute for Space Astrophysics & Planetology and colleagues used visible and infrared spectroscopic data collected by NASA’s Dawn orbiter to map carbonates on Ceres’s surface. They found that magnesium-calcium carbonates cover most of the dwarf planet, but kilometer-sized areas associated with craters can also contain sodium carbonates like Na2CO3. Carrozzo’s group is the first to identify both hydrated and dehydrated sodium carbonates on Ceres.

The researchers propose two ways hydrated sodium carbonates could form on Ceres. Some astronomers have hypothesized that the dwarf planet may have a subsurface ocean. The brine from such an ocean could seep upwards through cracks in the crust. Alternately, meteor impacts could melt frozen brine below the surface. In either case, ions in the brine could precipitate to form carbonates as the water freezes or evaporates. Carrozzo says his group cannot rule out either mechanism; it is possible both are occurring.

Hydrated carbonates lose their water in a vacuum over the course of millions of years, suggesting these processes happened in the recent geologic past. “Because hydrated sodium carbonates are unstable on the surface, we believe that hydration is occurring today,” Carrozzo says.

“This an important piece work to understand the composition and processes on Ceres,” says Michael Küppers, a planetary scientist at the European Space Agency.



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