It looks like there’s competition for which dwarf planet is the most interesting in the Solar System: Ceres or Pluto? Pluto, with its heart and floating mountains, might seem ahead, but this week Ceres revealed something unique: salt volcanoes.

A study, published in Nature, indicates that the bright spots at the center of the Occator Crater on Ceres are made mostly of sodium carbonate, a mineral that on Earth is formed in underwater hydrothermal vents. This is the largest amount of sodium carbonate discovered elsewhere in the Solar System.

"The minerals we have found at the Occator central bright area require alteration by water," said Maria Cristina De Sanctis, lead author of the study, in a statement. "Carbonates support the idea that Ceres had interior hydrothermal activity, which pushed these materials to the surface within Occator."

Astronomers think that the bright area is an outflow of a halovolcano, a salt-spewing volcano powered by the underground forces of the planet. This could be associated with an ancient underground ocean, now gone, or a local water reservoir. The Occator Crater is geologically very young, only 80 million years old, so the halovolcano could have been powered by the heat of the impact.

There could still be residual heat in the planet today, but it is unlikely that it has enough water to form enough sodium carbonate. According to a second study, published in Nature Geoscience, the surface and immediate underground of Ceres are drier than we thought.

The study modeled how quickly craters should be erased depending on the subsurface composition of the dwarf planet. They found that the largest craters are not deformed much over billions of years, and the models indicate that the ground of Ceres is at least 1,000 times more viscous than water ice.

"It’s amazing how much we have been able to learn about Ceres' interior from Dawn's observations of chemical and geophysical properties. We expect more such discoveries as we mine this treasure trove of data," said Carol Raymond, deputy principal investigator for the Dawn mission, which is exploring Ceres.

Ceres can’t be more than 40 percent ice by volume, with the rest likely a mixture of rocks and low-density minerals. The dwarf planet also might not be uniform, which could explain the presence of the bright spots in certain locations.

"We will need to research whether Ceres' many other bright areas also contain these carbonates," said De Sanctis, who is based at the National Institute of Astrophysics in Rome.

Dawn will continue to orbit Ceres for at least another year, with suggestions of the probe mission being extended, but no formal announcement has been made yet.