Scientists add the mysterious bright spots on Ceres, the largest object in the main asteroid belt, to a giant reservoir of salt water beneath the crust of the dwarf planet.
Oceans at the bottom are the kind of thing we expect to see in the outer solar system, specifically on the icy moons orbiting Jupiter, Saturn, Uranus and Neptune. But according to seven (yes, seven) new papers published to a special Nature collection, oceans below the ground can also appear on objects without a host planet, as shown by Ceres, a dwarf planet in the main asteroid belt between Mars and Jupiter.
Ceres was, as new research shows, recently active and may still be, with a large reservoir of groundwater and an exposure of a form of cryovolcanism (in which groundwater reaches the surface) not previously seen in a celestial object. Oceans on the surface of icy moons, such as Jupiter’s Europa and Saturn’s Enceladus, are kept warm by tidal interactions practiced by their host planets, but the same cannot be said for planetless objects in the asteroid belt. At Ceres, this phenomenon is more a matter of chemistry, as the groundwater remains in a slushy state due to its high salinity.
The new study, described in articles published in Natural Astronomy, Natural Geoscience, and Natural Communication, included scientists from NASA, the Lunar and Planetary Institute (LPI), the University of Münster in Germany, and the National Institute for Science Education and Research (NISER) in India, among many other institutions.
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At 590 miles wide (950 kilometers), Ceres is the largest object in the asteroid belt. NASA’s Dawn spacecraft visited Ceres from 2015 to 2018, collecting critically important data during the last five months of the mission, when the orbiter swept within 35 km of the surface.
High resolution images returned to Earth reveal Occator Crater in unusual detail. This crater, formed by a gigantic impact, is the most distinctive feature of the dwarf planet, and measures 57 km wide (92 km), which is even nice according to Earth standards. Occator Crater was revealed as a complex structure, with a central depression covered in a dome-like structure, several cracks and furrows, and light mineral deposits and smaller domes scattered around.
That water may have been responsible for Ceres’ clear surface features before the Dawn mission was suspected, but data collected by the orbiter suggest this is quite the case.
A number of small impact craters on Ceres indicate a relatively young surface. Occator Crater formed about 22 million years ago, with some of the latest surface features on Ceres forming just 2 million years ago.
A common feature of impact craters is a peak that forms in the center. Such a function formed within Occator, but it collapsed in, creating a depression within the depression. Then, about 7.5 million years ago, water – or more of a brine – came to the surface, leaking through this collapsed peak. This salt water evaporated, leaving reflective deposits in the form of sodium carbonate – a mixture of sodium, carbon and oxygen. The bright white splotch in the center of Okkator, Cerealia Facula, is the remnant of this process.
Similar deposits appear elsewhere in the crater, including a prominent feature called Vinalia Faculae. At these locations, the brine went to the surface via cracks and furrows.
About 2 million years ago, Cerealia Facula became active again, spewing out more salt, forming a central dome of clear material. These processes continued from about 1 million years ago, and they may still occur today, although the cryovolcanic processes have weakened greatly over time.
Evaporation and sublimation (when liquid directly transforms into a gas) forced the water to the surface, in a form of cryovolcanism, not otherwise seen in the solar system, according to the researchers. Scientists have good reason to believe that this process may exist elsewhere on other seemingly inert bodies.
“The evidence for very recent geological activity on Ceres contradicts the general belief that bodies of solar systems are not geologically active,” said Guneshwar Thangjam, co-author of Natural Astronomy paper and a researcher from NISER, in a press release.
Importantly, the ocean beneath the ground was probably formed as a result of the impact event that Occator Crater created, but the sustained slushiness is due to dissolved salt in the groundwater.
“For the large castle at Cerealia Facula, most of the salt was supplied from a slushy area just below the surface that was melted by the heat of the impact that formed the crater about 20 million years ago,” Carol Carol , first author of Natural Astronomy to study and chief investigator of Dawn, said in a NASA press release. “The impulse flow subsided after a few million years; However, the impact also caused large fractures that could reach the deep, long life reservoir, allowing brine to continue percolating to the surface. “
Ceres has hundreds and possibly thousands of smaller burial sites, most of which are less than 10 feet thick. Domes and pits appear on the surface, and also form from groundwater motion.
By studying Ceres’ gravity, scientists were able to deduce its internal structure. The salt reservoir lies about 25 miles (40 km) below the surface and is hundreds of miles wide. Given that Ceres itself is just 590 miles wide, it’s fair to call Ceres an ocean world.
Apparently overnight, Ceres has become a hot target for astrobiologists. With its complex chemistry, liquid water, and sustained dynamics for surface and subsoil, it may have been habitable at some point in recent history. A mission to send a probe to the surface suddenly seems like a very good idea.
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