“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,” said Sam Courville, lead author of a new study at NASA. The same recipe, scientists now think, may have once bubbled deep below the icy crust of Ceres, the biggest object in the asteroid belt.
Information from NASA’s Dawn spacecraft, which orbited Ceres between 2015 and when it ran out of fuel in 2018, had already uncovered brilliant, shiny patches on the surface of the dwarf planet. They were found to be salt deposits, the leftovers of salty liquid that had risen up from a massive reservoir deep beneath the surface. Later examination validated that this reservoir held not only water, but dense brines salted liquids that can exist in liquid form below lower temperatures and carbon-containing organic molecules, a necessary, but not adequate, ingredient for life.
The new study, reported Aug. 20 in Science Advances, brings the third essential piece to the puzzle of habitability: a steady supply of chemical energy. Thermal and chemical models of Ceres’ interior indicate that from around 2.5 to 4 billion years ago, the planet’s rocky core was sufficiently hot to power hydrothermal circulation. Heat was produced by radioactive decay of elements in the core, which made the hot, gas-rich fluids ascend through metamorphosed rock into the overlying brine-rich ocean. This activity, typical in the early histories of rocky bodies, might have continued for hundreds of millions of years.
Similar environments on Earth exist at hydrothermal vents at the bottom of the oceans, where mineral-rich fluids burst from the crust into cold ocean water. Such vents support close-packed microbial communities that grow in the absence of light, deriving energy from chemical reactions between vent fluids and seawater. Examination of the Lost City Hydrothermal Field and other sites indicates that in metal-deficient, alkaline environments, microbes can evolve to tap energy from molecules like hydrogen, methane, and formate. The mineral assemblages at the vents brucite, aragonite, calcite capture the heat and chemical gradients that organize microbial populations, providing a terrestrial counterpart for the conditions under which Ceres’ interior could have existed.
Detection of organics at Ceres by the Dawn mission, and specifically near the Ernutet Crater, supports the theory of ancient habitability. The organics at Ceres are native to the planet and were likely created or changed in a warm, wet environment. Carbonates, clays, and salts on Earth usually form in hydrothermal systems, and in them, they are able to trap and preserve organic molecules for billions of years.
Ceres’ primordial sea, charged with dissolved minerals and gases from the core, potentially hosted chemosynthetic microbes like Earth’s vent microbes. Such ecosystems employ enzymes to capture energy from chemical disequilibria like oxidizing hydrogen or reducing sulfur compounds and these processes do not involve sunlight. Long-lived chemical gradients are essential; without them, metabolism is not possible.
Now, Ceres is a cold planet. Radioactive decay has cooled, and whatever liquid remains would be in isolated pockets of dense brine. Ceres does not enjoy tidal heating from its large parent planet, like icy moons Europa or Enceladus, to maintain its innards at warmth. NASA models suggest its optimal habitability window shut down long ago, billions of years ago, when the core cooled and hydrothermal processes stopped.
Yet the implications extend beyond Ceres. Many mid-sized icy bodies roughly 940 kilometers in diameter, like Ceres may have undergone similar thermal histories. Without tidal heating, their oceans would eventually freeze, but during their early evolution, radioactive decay could have powered hydrothermal systems long enough for life to gain a foothold. This raises the possibility that the solar system’s inventory of once-habitable worlds is larger than previously thought.
For astrobiologists, Ceres provides a natural laboratory in which to investigate the interactions of geology, chemistry, and biology within confined ocean worlds. Its archived surface salts and organics are time capsules from a time when liquid water, heat, and chemical energy all came together beneath its crust. As Courville explained, how to decide whether hydrothermal fluids ever supplied Ceres’ ocean “could have big implications” not only for the understanding of this small world, but in terms of identifying similar signatures elsewhere in the solar system.
