Dwarf planet Ceres is an ocean world: study

The dwarf planet Ceres -- long believed to be a barren space rock -- is an ocean world with reservoirs of sea water beneath its surface, the results of a major exploration mission showed Monday. Ceres is the largest object in the asteroid belt between Mars and Jupiter and has its own gravity, enabling the NASA Dawn spacecraft to capture high-resolution images of its surface. Now a team of scientists from the United States and Europe have analysed images relayed from the orbiter, captured around 35 kilometres (22 miles) from the asteroid. They focused on the 20-million-year-old Occator crater and determined that there is an "extensive reservoir" of brine beneath its surface. Several studies published Monday in the journals Nature Astronomy, Nature Geoscience and Nature Communications also shed further light on the dwarf planet, which was discovered by Italian polymath Giuseppe Piazzi in 1801. Using infrared imaging, one team discovered the presence of the compound hydrohalite -- a material common in sea ice but which until now had never been observed off of Earth. Maria Cristina De Sanctis, from Rome's Istituto Nazionale di Astrofisica said hydrohalite was a clear sign Ceres' used to have sea water. "We can now say that Ceres is a sort of ocean world, as are some of Saturn's and Jupiter's moons," she told AFP. The team said the salt deposits looked like they had built up within the last two million years -- the blink of an eye in space time. This suggests that the brine may still be ascending from the planet's interior, something De Sanctis said could have profound implications in future studies. "The material found on Ceres is extremely important in terms of astrobiology," she said.

"We know that these minerals are all essential for the emergence of life."

Writing in an accompanying comment article, Julie Castillo-Rogez, from the California Institute of Technology's Jet Propulsion Laboratory, said the discovery of hydrohalite was a "smoking gun" for ongoing water activity.

"That material is unstable on Ceres' surface, and hence must have been emplaced very recently," she said.

In a separate paper, US-based researchers analysed images of the Occator crater and found that its mounds and hills may have formed when water ejected by the impact of a meteor froze on the surface.

The authors said their findings showed that such water freezing processes "extend beyond Earth and Mars, and have been active on Ceres in the geologically recent past".

NASA Dawn Reveals Recent Changes in Ceres' Surface

Observations of Ceres have detected recent variations in its surface, revealing that the only dwarf planet in the inner solar system is a dynamic body that continues to evolve and change. NASA's Dawn mission has found recently exposed deposits that give us new information on the materials in the crust and how they are changing, according to two papers published March 14 in Science Advances that document the new findings. Observations obtained by the visible and infrared mapping spectrometer (VIR) on the Dawn spacecraft previously found water ice in a dozen sites on Ceres. The new study revealed the abundance of ice on the northern wall of Juling Crater, a crater 12 miles (20 kilometers) in diameter. The new observations, conducted from April through October 2016, show an increase in the amount of ice on the crater wall. "This is the first direct detection of change on the surface of Ceres," said Andrea Raponi of the Institute of Astrophysics and Planetary Science in Rome. Raponi led the new study, which found changes in the amount of ice exposed on the dwarf planet. "The combination of Ceres moving closer to the sun in its orbit, along with seasonal change, triggers the release of water vapor from the subsurface, which then condenses on the cold crater wall. This causes an increase in the amount of exposed ice. The warming might also cause landslides on the crater walls that expose fresh ice patches."

By combining chemical, geological and geophysical observations, the Dawn mission is producing a comprehensive view of Ceres. Previous data had shown Ceres has a crust about 25 miles (40 kilometers) thick and rich in water, salts and, possibly, organics.

In a second study, VIR observations also reveal new information about the variability of Ceres' crust, and suggest recent surface changes, in the form of newly exposed material.

Dawn previously found carbonates, common on the planet's surface, that formed within an ocean. Sodium carbonates, for example, dominate the bright regions in Occator Crater, and material of similar composition has been found at Oxo Crater and Ahuna Mons.

This study, led by Giacomo Carrozzo of the Institute of Astrophysics and Planetary Science, identified 12 sites rich in sodium carbonates and examined in detail several areas of a few square miles that show where water is present as part of the carbonate structure. The study marks the first time hydrated carbonate has been found on the surface of Ceres, or any other planetary body besides Earth, giving us new information about the dwarf planet's chemical evolution.

Water ice is not stable on the surface of Ceres over long time periods unless it is hidden in shadows, as in the case of Juling. Similarly, hydrated carbonate would dehydrate, although over a longer timescale of a few million years.

"This implies that the sites rich in hydrated carbonates have been exposed due to recent activity on the surface," Carrozzo said.

The great diversity of material, ice and carbonates, exposed via impacts, landslides and cryovolcanism suggests Ceres' crust is not uniform in composition. These heterogeneities were either produced during the freezing of Ceres' original ocean - which formed the crust - or later on as a consequence of large impacts or cryovolcanic intrusions.

"Changes in the abundance of water ice on a short timescale, as well as the presence of hydrated sodium carbonates, are further evidence that Ceres is a geologically and chemically active body," said Cristina De Sanctis, VIR team leader at the Institute of Astrophysics and Planetary Science.