It sure doesn't pay to underestimate Ceres: NASA's Dawn spacecraft has spotted signs of organic molecules on the frigid dwarf planet.
The findings, published this week in the journal Science, may shed light on the prevalence of pre-life chemistry in the solar system while marking Ceres as one of the worlds that could potentially host microbial life.
"Because Ceres is a dwarf planet that may still preserve internal heat from its formation period and may even contain a subsurface ocean, this opens the possibility that primitive life could have developed on Ceres itself," Michael Küppers of the European Space Agency, who was not involved in the study, wrote in a commentary. "It joins Mars and several satellites of the giant planets in the list of locations in the solar system that may harbor life."
Ceres, one of five dwarf planets in the solar system, is also an asteroid — the largest of them, in fact. Formed around 4.5 billion years ago, it sits in the belt of rocky debris that lies between the orbits of Mars and Jupiter.
Asteroids are the leftover building blocks of planetary formation, largely unchanged by the geologic processes that occur on Earth and other planets. By studying these space fossils, scientists hope to piece together what the early solar system looked like.
Among the asteroids, Ceres is special. As a dwarf planet, it got stuck somewhere along the way to becoming a full-grown world. Frozen in this state, Ceres also offers a snapshot of planetary adolescence.
Scientists have long wondered whether asteroids had not just water but also organic matter that could have been brought to Earth, perhaps providing the right chemical ingredients for life to emerge. Water and organic molecules have been discovered in meteorites that are thought to be chunks of asteroids that fell to Earth. But it's also possible these meteorites were contaminated or transformed by Earth's environment.
Thanks to the Dawn spacecraft, which reached the frigid little world in 2015, scientists have detected super-bright salt deposits in Ceres' craters and identified Ahuna Mons as an ice volcano. But now, using its Visible and Infrared Mapping Spectrometer instrument, the spacecraft has spotted organics lying on the surface.
When light hits any material, that material will absorb certain wavelengths while reflecting the rest. Since the absorbed wavelengths are unique to the material's properties, those missing bands of light serve as a chemical fingerprint that a spectrometer can use to determine the composition of the surface.
The organic matter detected on Ceres lies in a roughly 1,000-square-kilometer area near an approximately 50-kilometer-wide crater named Ernutet. While the scientists aren't sure exactly what the compounds are, the fingerprint is characteristic of material containing carbon-hydrogen bonds, and may include components like methyl and methylene.
"We were not expecting to see something like this on the surface of Ceres," said study coauthor Christopher Russell, a UCLA planetary scientist and Dawn's principal investigator. These simple molecules, he added, are "really pre-biological, but they're in the family of materials that we would expect if Ceres was working its way along the complexity path."
Together with some of the other stuff already known to be on Ceres, this makes for what could theoretically be a life-friendly environment, perhaps even an environment with the right chemical precursors for life.
"The combined presence on Ceres of ammonia-bearing hydrated minerals, water ice, carbonates, salts, and organic material indicates a very complex chemical environment, suggesting favorable environments to prebiotic chemistry," the study authors wrote.
But how did the organics get there?
One possibility is that they were delivered by comets or other asteroids. But the distribution of the organic material doesn't match the pattern that would have been left by an impact. Besides, the authors pointed out, any organic-rich body that slammed into Ceres would probably be superheated by the collision, causing much of that organic matter to break down.
If the organics really did originate on Ceres itself, as the authors suspect, then researchers will have to figure out how this material made it from the interior of the dwarf planet to its surface. For now, that process remains a mystery.
Whatever the explanation, the findings show that Ceres — like Mars and other worlds such as Saturn's moon Enceladus — may also have the right chemical ingredients for life.
Scientists could learn more if they were able to look at the isotopic composition of the water ice, Russell added. This could reveal whether Ceres formed where it lies, or whether it formed farther out and eventually moved in.
But that would require a lander of some sort, he pointed out, not the type of remote sensing instruments on Dawn. That job, he added, would have to wait for a future mission to the dwarf planet.
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