While this global ocean below the planet’s surface is likely to freeze over time, remnants of it may still be present under a large impact crater on Ceres. The presence of salt may have retained the liquid as a brine, despite cold temperatures.
This new study is based on observations made during Dawn’s orbit of Ceres between 2015 and 2018, including near passes that made it to the dwarf planet just 22 miles above the surface toward the end of the mission.
At the time, Dawn was targeting the 57-mile-wide Occator Crater, a 22-million-year-old feature that appeared to be lit by bright spots. These striking features were discovered to be sodium carbonate, as a compound including oxygen, carbon and sodium.
But it was unclear how those bright spots got into the crater.
Data from the end of Dawn’s mission revealed an extensive and slushy reservoir of brine, like salty liquid, beneath the crater. It is 25 miles deep and extends for hundreds of miles.
When the impact that made the crater hit Ceres, it may have allowed the reservoir to deposit light salts into the crater by breaking the crust of the planet. When the fractures reached the saline reservoirs, the brine was able to reach the surface of the crater floor. When the water evaporated, a light, salty crust remained behind.
And pellets can still rise to the surface today – suggesting that activity on Ceres is not due to melts that may have occurred when the planet was affected.
In fact, Dawn’s data also showed the presence of hydrated chloride salts in the center of the largest bright area near the center of the crater, called Cerealia Facula. This hydrohalite compound is common in marine ice on Earth, but it is the first time that hydrohalite has been found outside our planet.
The salts at least appear to dehydrate rapidly on the surface, at least astronomically. This dehydration occurs over hundreds of years.
But the measurements taken by Dawn showed that water was still present. This suggests that brine could still rise to the surface of the crater and that saline fluid could still exist in Ceres.
“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 said. Raymond, chief Dawn researcher at NASA’s Jet Propulsion Laboratory in California, in a statement.
“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.”
There are also mounds and hills to be seen in the crater that probably formed when streams of water froze in place, suggesting geological activity on Ceres. These conical hills are similar to pingos on Earth, as small mountains made of ice found in the polar regions. Although features like these were also found on Mars, it is the first time they have been detected on a dwarf planet.
An unusual dwarf planet
The pingo-like structures and the water pushing up through fractures in the crater revealed that Ceres actually experienced cryovolcanic activity, like ice volcanoes, beginning about 9 million years ago. The process is likely to be ongoing.
This type of cryovolcanic activity is witnessed by icy moons in the outer solar system, with plume material projecting into space. But it was never expected to occur on dwarf planets as asteroids in the asteroid belt, which are thought to be waterless and inactive.
Ceres changes that theory because it has been proven to be water rich and perfectly active.
A survivor of the early days of the solar system, since it formed 4.5 billion years ago, Ceres was more of an “embryonic planet”; essentially it began to form, but never finished.
Jupiter, the largest planet in our solar system, and the force of its gravity probably matched the growth of Ceres. About 4 billion years ago, Ceres found its home in the asteroid belt along with all the other remnants of the formation of the solar system.
The idea that liquid water can be preserved on dwarf planets and asteroids is a strange one for scientists.
Unlike other icy ocean worlds in our solar system, such as Saturn’s moon Enceladus and Jupiter’s moon Europe, asteroids and dwarf planets do not experience internal warming. Enceladus and Europe benefit from internal warming that occurs when they gravitationally interact with the massive planets they orbit.
The Dawn mission ended in 2018 when the spacecraft ran out of fuel and could no longer communicate with NASA. It was placed in long orbit around Ceres which will prevent impact, protect its organic materials and liquid substrate.
“Dawn has achieved much more than we had hoped when it launched its extraterrestrial exterior expedition,” Marc Rayman, Dawn’s mission director at JPL, said in a statement. “These exciting new discoveries from the end of its long and productive mission are a wonderful tribute to this remarkable interplanetary explorer.”
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