Bright areas on Ceres come from salt water below

NASA’s Dawn spacecraft gave scientists extraordinary views of the dwarf planet Ceres, which lies in the main asteroid belt between Mars and Jupiter. By the time the mission ended in October 2018, the orbiter had sunk less than 22 miles (35 kilometers) above the surface, revealing crisp details of the mysterious bright regions for which Ceres was known.

Scientists had discovered that the bright areas of deposits were mostly made of sodium carbonate – a compound of sodium, carbon and oxygen. They probably came from liquid that percolated and evaporated to the surface, leaving a highly reflective salt body. But what they had not yet determined was where this liquid came from.

By analyzing data collected near the end of the mission, Dawn scientists concluded that the liquid came from a deep reservoir of brine, like salt-rich water. By studying the gravitational signs of Ceres, scientists knew more about the internal structure of the dwarf planet and were able to determine that the brine reservoir is about 25 miles (40 kilometers) deep and hundreds of miles wide.

Ceres does not benefit from internal heating generated by gravity interactions with a large planet, as is the case with some of the icy moons of the outer solar system. But the new study, which focuses on Ceres’ 57-kilometer-wide (92-kilometer-wide) Occator Crater – home to the most extensive light areas – confirms that Ceres is a water-rich world like these other icy bodies.

The findings, which also revealed the extent of geological activity in Occator Crater, appear in a special collection of papers published by Natural Astronomy, Nature Geoscience, en Nature communication on August 10th.

“Dawn has achieved much more than we had hoped when it began its extraordinary alien expedition,” said Mission Director Marc Rayman of NASA’s Jet Propulsion Laboratory in Southern California. “These exciting new discoveries from the end of its long and productive mission are a wonderful tribute to this remarkable interplanetary explorer.”

Solve the clear mystery

Long before Dawn arrived at Ceres in 2015, scientists had noticed diffuse bright regions with telescopes, but their nature was unknown. From its narrow orbit, Dawn took images of two distinct, highly reflective areas within the Occator Crater, which later became known as Cerealia Facula and Vinalia Faculae. (“Facades” means light areas.)

Scientists knew that micrometeorites often coat the surface of Ceres fur, roughen it and leave debris. Over time, such action would have to darken these bright areas. That their brightness indicates that they are probably young. Trying to understand the source of the areas, and how the material could be so new, was a major focus of Dawn’s definitively expanded mission, from 2017 to 2018.

The study not only confirmed that the bright regions are young – some less than 2 million years old; it also found that the geological activity that drove these deposits could continue. This conclusion was dependent on scientists who made an important discovery: salt compounds (sodium chloride chemically associated with water and ammonium chloride) concentrated in Cerealia Facula.

On the surface of Ceres dehydrate salt with water rapidly, within hundreds of years. But Dawn’s measurements show they still have water, so the liquids must have reached the short surface. This is evidence of both the presence of fluid under the region of Occator Crater and continuous transfer of material from the deep interior to the surface.

The scientists found two main pathways through which liquid can reach the surface. “For the large deposit 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,” Dawn said. Principal Investigator Carol Raymond. “The impulse flow subsided after a few million years; however, the impact also created large fractures that could penetrate the deep, long-lived reservoir, allowing brine to continue percolating to the surface.”

Active geology: recent and unusual

In our solar system, icy geological activity occurs mainly on icy moons, where it is driven by its gravitational interactions with its planets. But that is not the case with the movement of brine to the surface of Ceres, suggesting that other large, icy bodies that are not moons could also be active.

Some evidence for recent fluids in Occator Crater comes from light deposits, but other evidence comes from an assortment of interesting conical hills reminiscent of Earth’s pingos – small icebergs in polar regions formed by frozen pressurized groundwater. Such features have been detected on Mars, but the discovery of them on Ceres marks the first time they have been observed on a dwarf planet.

On a larger scale, scientists were able to map the density of Ceres’ crustal structure as a function of depth – a first for an ice-rich planetary body. Using gravity short measurements, they found that the crust density of Ceres increases significantly with depth, beyond the simple effect of pressure. Researchers found that at the same time, Ceres’ reservoir freezes, salt and mud accumulate in the lower part of the crust.

Dawn is the only spacecraft that ever orbited two alien destinations – Ceres and the giant asteroid Vesta – thanks to its efficient ion propulsion system. When Dawn used the last of an important fuel, hydrazine, for a system that controls its orientation, it could not yet use Earth’s means for communication, nor to point its solar systems at the sun to produce electrical power. Because Ceres had organic matter on its surface and liquid beneath the surface, planetary protection rules required that Dawn be placed in a long-term orbit, which would prevent it from affecting the dwarf planet for decades.