Scientists have found that Ceres, the largest object in the asteroid belt between Mars and Jupiter, is an “ocean world” with a big reservoir of salty water under its frigid surface. The findings raise interest in this dwarf planet as a possible outpost for life.

What are the features of Ceres?

Ceres is the only dwarf planet located in the inner solar system. It was the first member of the asteroid belt to be discovered when Giuseppe Piazzi spotted it in 1801. 

Called an asteroid for many years, Ceres is so much bigger and so different from its rocky neighbors that scientists classified it as a dwarf planet in 2006. Ceres has a diameter of about 950 km. Even though Ceres comprises 25 percent of the asteroid belt’s total mass, tiny Pluto is still 14 times more massive.

Ceres is named for the Roman goddess of corn and harvests. 

On March 6, 2015, NASA’s Dawn became the first spacecraft to enter orbit around a dwarf planet.

What are the new findings?

Long before Dawn reached the orbit of Ceres in 2015, scientists had noticed diffuse bright regions with telescopes, but their nature was unknown.

The spacecraft, which flew as close as 35 km from the surface in 2018, provides a new understanding of Ceres, including evidence indicating it remains geologically active with cryovolcanism — volcanoes oozing icy material.

Scientists figured out that the bright areas were deposits made mostly of sodium carbonate — a compound of sodium, carbon, and oxygen. They likely came from liquid that percolated up to the surface and evaporated, leaving behind a highly reflective salt crust. But what they hadn’t yet determined was where that liquid came from.

By analysing data collected near the end of the mission, Dawn scientists have concluded that the liquid came from a deep reservoir of brine, or salt-enriched water. By studying Ceres’ gravity, scientists learned more about the dwarf planet’s internal structure and were able to determine that the brine reservoir is about 40 km deep and hundreds of miles wide.

Ceres doesn’t benefit from internal heating generated by gravitational interactions with a large planet, as is the case for some of the icy moons of the outer solar system. But the new research confirms that Ceres is a water-rich world like these other icy bodies.

This elevates Ceres to ‘ocean world’ status.

The study was published in the journals Nature Astronomy, Nature Geoscience and Nature Communications.

Other solar system bodies beyond Earth where subsurface oceans are known or appear to exist include Jupiter’s moon Europa, Saturn’s moon Enceladus, Neptune’s moon Triton and the dwarf planet Pluto. 

What are dwarf planets?

Planets and dwarf planets are two distinct classes of objects.

Dwarf planets are a lot like regular planets. They both have enough mass and gravity to be nearly round — unlike odd-shaped asteroids. They both travel through space in a path around the Sun.

The big difference is that a dwarf planet could be in for a bumpy ride as it travels — its path is full of other objects like asteroids. A regular planet has a clear path around the Sun. 

There may be dozens of dwarf planets in our solar system. Pluto is the most famous dwarf planet. Eris, Haumea, Makemake and Ceres are other identified dwarf planets.