Dwarf planet Ceres might be hiding something from us: reservoirs of brine deep in its rocky interior.
In a recent paper in The Planetary Science Journal, a team of planetary scientists suggest that shortly after Ceres formed, internal heat caused rocks in its mantle to release water, salts, and other compounds. Those changes left holes, or pores, in the surrounding rock, which filled with the newly-released fluid. If they’re right, the briny mixture may have reshaped Ceres’ surface and could raise interesting questions about habitability on the inner Solar System’s only dwarf planet.
What’s New — The study authors modeled what happened to the rocks and ices that make up Ceres as it heated and cooled over the course of 4.5 billion years since its formation. They found that certain rocks in the mantle, like serpentine and carbonate minerals, experienced physical and chemical changes when heated — the same process that creates metamorphic rocks like slate (which forms from shale) here on Earth. As part of those chemical changes, the rocks released compounds like water and an assortment of salts that had once been part of their basic structures.
According to the model, those chemicals may have formed pockets of briny liquid (a brine is water mixed with a salt, and that means familiar table salt but also various combinations of elements like potassium, sulfur, calcium, and oxygen) in Ceres’ mantle. That liquid, which geologists call metamorphic fluids, probably contains large amounts of sodium, sulfur, and carbon-based compounds like carbonate, bicarbonate, and carbon dioxide, as well as a lot of methane. And it’s alkaline, meaning it has a high pH, between 8 and 10.
“The overall composition of the brines appears to be quite similar to the composition of soda lakes in the East African Rift Valley in Kenya,” Jet Propulsion Laboratory geophysicist Mohit Daswani tells Inverse, adding that the only major difference is the fluids’ origin. “In the East African Rift Valley, evaporation concentrates the salts, and microbial communities produce the methane biologically. On the other hand, in Ceres, the fluid composition is a result of heating and devolatilizing the rocks of the mantle over time.”
Most of Ceres’ deeply buried liquid should eventually have worked its way upward through kilometers of rock, where it may have left salty, frozen deposits on the surface at a site called Ahuna Mons.
"It is likely too cold for liquid brines to be stable directly beneath these surface features, but the brines that led to the surface deposits may be present below, in the mantle," Daswani tells Inverse. Other data from the Dawn mission suggests that those brines probably come from about 40 kilometers below the surface, in the upper layers of Ceres’ mantle.
And some of those metamorphic fluids may even have broken through the rocky bottom of a sub-surface ocean that formed even earlier in Ceres’ history, causing chemical reactions that could have produced some key building blocks of life.
Here’s the Background — When Dawn flew past Ceres in 2015, it measured the dwarf planet’s gravity field and mapped its surface topography. That data told planetary scientists that the strange little world wasn’t as dense as some earlier models, based on much more limited information, had predicted. Since then, a couple of possible explanations have been floating around. One is that Ceres’ mantle might be made of very porous rock, and the pores might be full of briny liquids.
Daswani and his colleagues used their computer simulations to test those ideas. They found that Ceres’ mantle probably doesn’t contain a very high proportion of organic compounds, but that the planet’s long evolutionary history could have essentially baked the brines out of the rocks that originally settled into its mantle.
Based on their model, Ceres’ starting materials sorted themselves into a dense, rocky mantle and an ice-rich crust early in the Solar System’s history. At the same time, water from melting ices collected beneath the surface of the crust in a subterranean ocean.
By about 3 billion years ago, radioactive decay deep inside the planet had heated the mantle to its peak temperature. That’s when Daswani and his colleagues’ model predicts that the original rocky mantle gradually transformed into metamorphic rocks and subterranean pools of hot, briny fluids. The model suggests that it was a drawn-out, gradual process, punctuated by sudden bursts of rocky metamorphosis when the mantle’s temperature reached the breaking point for certain minerals. A mineral called antigorite, for instance, becomes olivine, talc, and water at about 725 Kelvin.
By now, the process of producing new metamorphic fluids on Ceres is geologically ancient history, since the dwarf planet has been slowly cooling for the last 2 million years. Any fluids that were going to well up to the surface, through the cracks and fissures that spiderweb through the planet’s crust, will have long since finished their journey. But some small pockets, or even large reservoirs, of liquid brine may remain deep beneath the surface.
And their presence — today or in Ceres’ distant past — could have interesting implications for the possibility of life — or at least little patches of habitable environments — on a very unlikely little world.
Why It Matters — If Ceres’ mantle still contains open spaces full of salty liquid, those places “could even be habitable niches,” Daswani says.
If these brines found their way upward, through cracks in the rocky, icy crust, into the subsurface ocean, they could have helped refill that ocean. In the first 3 billion years’ of Ceres’ history, when radioactive decay deep inside the planet was heating things up, metamorphosis in the mantle may have released quintillions of kilograms of water.
At least some of that water could have found its way into Ceres’ subsurface ocean through cracks and fissures in the planet’s crust. The metamorphic fluids would have been much hotter than the relatively cool ocean, making the places where they flowed up from the mantle something like deep-sea hydrothermal vents here on Earth — except with a different mix of chemicals. A better understanding of how those chemicals interacted with the contents of the original ocean could tell us something interesting about the prospect of habitability on Ceres.
And today, pockets of metamorphic fluids in Ceres’ mantle “could even be habitable niches,” Daswani says.
Deep, underground reservoirs of alien life aside, if Ceres’ mantle is mostly porous rock filled in with briny fluids, that could explain a lot about why Ceres looks the way it does today.
Daswani and his colleagues say that enough brine in the planet’s middle layer could have caused the rock above it to relax, slowly rebounding from the dents left by meteor impacts. That could have effectively erased the large craters that should have marked the dwarf planet’s surface around 4 billion years ago, during a period of the Solar System’s history called the Late Heavy Bombardment, which may help explain why Ceres’ surface has fewer large, old craters than every model of the early Solar System suggests it should.
What’s Next — One of the biggest questions for future research will be how much brine is sloshing around beneath Ceres' icy crust — and where it all is.
We could be talking about small pockets of fluid scattered throughout the mantle, which may or may not be an interconnected system like some magma chambers here on Earth. Or future missions could one day discover a global subsurface ocean tens of kilometers deep hidden in Ceres' mantle.
"A future mission would be able to test for the present of brines on a global scale via orbital magnetometer observations or in situ electromagnetic sounding," JPL planetary scientist Julie Castillo-Rogez, a coauthor of the study, tells Inverse. "This is how deep oceans have been found at icy moons like Europa and Ganymede."
Meanwhile, Daswani and his colleagues are working with other researchers at the University of Texas to create more computer models, which will simulate how fluids could move around, or stay in place, in the rocky interior of a planet like Ceres.
They've also worked on simulating how Ceres' original subsurface ocean would have formed and changed over time. The next step will be to simulate what kinds of chemical reactions happen when the sodium carbonate-rich waters of that ocean meet hot metamorphic fluids welling up from deep below.
Abstract — Recent work has sought to constrain the composition and makeup of the dwarf planet Ceres's mantle, which has a relatively low density, between 2400 and 2800 kg m−3, as inferred by observations by the Dawn mission. Explanations for this low density have ranged from a high fraction of porosity-filled brines to a high fraction of organic matter. We present a series of numerical thermodynamic models that yield the mineralogy and fluid composition in the mantle as a function of Ceres's thermal evolution. We find that the resulting phase assemblage could have changed drastically since the formation of Ceres, as volatile-bearing minerals such as serpentine and carbonates would partially destabilize and release their volatiles as temperatures in the mantle reach their maximum about 3 Gyr after Ceres's formation. These volatiles consist mainly of aqueous fluids containing Na+ and HS− throughout the metamorphic evolution of Ceres and, in addition, high concentrations of CO2 at high temperatures relatively recently. The predicted present-day phase assemblage in the mantle, consisting of partially devolatilized minerals and 13–30 vol% fluid-filled porosity, is consistent with the mantle densities inferred from Dawn. The metamorphic fluids generated in Ceres's mantle may have replenished an ocean at the base of the crust and may even be the source of the Na2CO3 and NaHCO3 mineral deposits observed at Ceres's surface.
