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New NASA research has found that Ceres may have had a lasting source of chemical energy: the right types of molecules needed to fuel some microbial metabolisms. Although there is no evidence that microorganisms ever existed on Ceres, the finding supports theories that this intriguing dwarf planet, which is the largest body in the main asteroid belt between Mars and Jupiter, may have once had conditions suitable to support single-celled lifeforms.

Science data from NASA's Dawn mission, which ended in 2018, previously showed that the bright, reflective regions on Ceres's surface are mostly made of salts left over from liquid that percolated up from underground. Later analysis in 2020 found that the source of this liquid was an enormous reservoir of brine, or salty water, below the surface. In other research, the Dawn mission also revealed evidence that Ceres has organic material in the form of carbon molecules—essential, though not sufficient on its own, to support microbial cells.

The presence of water and carbon molecules are two critical pieces of the habitability puzzle on Ceres. The new findings offer the third: a long-lasting source of chemical energy in Ceres's ancient past that could have made it possible for microorganisms to survive. This result does not mean that Ceres had life, but rather, that there likely was "food" available should life have ever arisen on Ceres.

In the study, in Science Advances on Aug. 20, the authors built thermal and chemical models mimicking the temperature and composition of Ceres's interior over time. They found that 2.5 billion years or so ago, Ceres's subsurface ocean may have had a steady supply of hot water containing dissolved gases traveling up from metamorphosed rocks in the rocky core. The heat came from the decay of radioactive elements within the dwarf planet's rocky interior that occurred when Ceres was young—an internal process thought to be common in our solar system.

"On Earth, when hot water from deep underground mixes with the ocean, the result is often a buffet for microbes—a feast of chemical energy. So it could have big implications if we could determine whether Ceres's ocean had an influx of hydrothermal fluid in the past," said Sam Courville, lead author of the study. Now based at Arizona State University in Tempe, he led the research while working as an intern at NASA's Jet Propulsion Laboratory in Southern California, which also managed the Dawn mission.

The Ceres we know today is unlikely to be habitable. It is cooler, with more ice and less water than in the past. There is currently insufficient heat from radioactive decay within Ceres to keep the water from freezing, and what liquid remains has become a concentrated brine.

The period when Ceres would most likely have been habitable was between a half-billion and 2 billion years after it formed (or about 2.5 billion to 4 billion years ago), when its rocky core reached its peak temperature. That's when warm fluids would have been introduced into Ceres's underground water.

The dwarf planet also doesn't have the benefit of present-day internal heating generated by the push and pull of orbiting a large planet, like Saturn's moon Enceladus and Jupiter's moon Europa do. So Ceres's greatest potential for habitability-fueling energy was in the past.

This result has implications for water-rich objects throughout the outer solar system, too. Many of the other icy moons and dwarf planets that are of similar size to Ceres (about 585 miles, or 940 kilometers in diameter) and don't have significant internal heating from the gravitational pull of planets could have also had a period of habitability in their past.