NASA’s Dawn spacecraft gave scientists extraordinary close-up 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 dipped to less than 22 miles (35 kilometers) above the surface, revealing crisp details of the mysterious bright regions Ceres had become known for.
Scientists had 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 where did that liquid come from?
By analyzing 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 25 miles (40 kilometers) deep and hundreds of miles wide.
Marc Rayman, Mission Director of NASA’s Jet Propulsion Laboratory in La Cañada, said:
Dawn accomplished far more than we hoped when it embarked on its extraordinary extraterrestrial expedition. These exciting new discoveries from the end of its long and productive mission are a wonderful tribute to this remarkable interplanetary explorer.
Solving the Bright Mystery
Scientists had noticed diffuse bright regions on Ceres with telescopes, but their nature was unknown. From its close orbit, Dawn captured images of two distinct, highly reflective areas within Occator Crater.
Scientists knew that micrometeorites frequently pelt the surface of Ceres, roughing it up and leaving debris. Over time, that sort of action should darken these bright areas. So their brightness indicates that they likely are young. Trying to understand the source of the areas, and how the material could be so new, was a main focus of Dawn’s final extended mission, from 2017 to 2018.
The research not only confirmed that the bright regions are young – some less than 2 million years old; it also found that the geologic activity driving these deposits could be ongoing. Scientists made a key discovery: salt compounds (sodium chloride chemically bound with water and ammonium chloride) concentrated in one area of the crater.
In our solar system, icy geologic activity happens mainly on icy moons, where it is driven by their gravitational interactions with their planets. But that’s not the case with the movement of brines to the surface of Ceres, suggesting that other large ice-rich bodies that are not moons could also be active.
Dawn is the only spacecraft ever to orbit two extraterrestrial destinations – Ceres and the giant asteroid Vesta – thanks to its efficient ion propulsion system. When Dawn used the last of a key fuel, hydrazine, for a system that controls its orientation, it was neither able to point to Earth for communications nor to point its solar arrays at the Sun to produce electrical power. Because Ceres was found to have organic materials on its surface and liquid below the surface, planetary protection rules required Dawn to be placed in a long-duration orbit that will prevent it from impacting the dwarf planet for decades.
This article has been edited for clarity and brevity. For further, very interesting details, see dawn.jpl.nasa.gov.