Even a dwarf planet can harbor big surprises.
Ever since it was discovered floating between Mars and Jupiter in 1801, Ceres has perplexed astronomers and defied easy categorization. Telescopic studies over the years showed it was roughly the size of Texas, with up to a third of its weight in water—too small to be a true planet, too wet and icy to be an asteroid and too big and rocky to be a comet. Astronomers chalked up Ceres’s oddities to its being a relic from an early, formative epoch of our solar system, when planets coalesced from many Ceres-like objects caroming around the sun. For lack of any better ideas, they also chose to include Ceres in the “dwarf planet” category they created for Pluto in 2006.
Now, new results from NASA’s Dawn spacecraft, which has been orbiting Ceres since March, hint that the body may have much more in common with its diminutive dwarf-planet cousin Pluto than once thought. Dawn co-investigator Maria Cristina De Sanctis of the National Institute of Astrophysics in Rome, Italy and colleagues report their detection of abundant ammonia-rich minerals on Ceres’s surface, suggesting that it was born closer to the vicinity of Pluto even though it now orbits in the Asteroid Belt. Separately, a team led by Andreas Nathues of the Max Planck Institute for Solar System Research in Germany, has found what appears to be cloud-like hazes of water vapor emanating from one of the mysterious bright spots that dot Ceres’s asphalt-dark surface.
In keeping with all the rest of Ceres’s oddball uncertainties, the findings hold major albeit nebulous implications for our understanding of the dwarf planet and its relationship to the other large objects in our solar system. Both studies are published in Nature. (Scientific American is part of Nature Publishing Group)
An import from the outer solar system?
Astronomers have been telescopically studying Ceres for decades, using spectrometers to measure how certain wavelengths of light are reflected or absorbed by substances on the surface. In all that time, they have failed to pin down exactly what that world’s surface is made of, in large part because the key wavelengths of light they need for definitive detections are in the infrared, a region of the spectrum mostly blocked by Earth’s atmosphere.
Armed with data from Dawn’s Visible and Infrared Mapping Spectrometer, De Sanctis and colleagues examined a broad range of spectra for the entire Cereian surface, including the crucial infrared wavelengths. The best match for the spectra proved to be something De Sanctis and her team hadn’t even been looking for—clay-like minerals called ammoniated phyllosilicates.
An abundance of minerals containing ammonia on a relatively rocky world like Ceres is surprising, De Sanctis says, because the volatile compound of nitrogen and hydrogen cannot persist long in the relatively warm and sunny regions of the inner solar system where Ceres now resides. Consequently, ammonia is more commonly associated with comets. Explaining an ammonia-rich Ceres may require either pushing the dwarf planet’s birthplace much farther out from the sun or importing showers of ammonia-rich pebbles from the outer solar system to help form Ceres where it now resides. Fortunately, one mechanism can do either of these things. “Ceres or ammonia-rich material from the outer solar system could have been implanted in the Asteroid Belt as a result of orbital perturbations exerted by the giant planets” early in the solar system’s history, De Sanctis says.
Based on a great deal of circumstantial evidence, many theorists now believe that in our solar system’s infancy its giant planets shifted over hundreds of millions of kilometers, propelled by gravitational interactions between the planets and orbiting disks of debris. Those wanderings are thought to have seeded our own world with another delicate volatile, some of the water that fills Earth’s oceans, and could in theory have flung Ceres or, more probably, ammonia-rich building material sunward from the outer darkness. Ceres’s ammonia-rich surface, De Sanctis says, could be yet another piece of evidence confirming our solar system’s wild, disturbed youth.
“If borne out it would be potentially a very interesting result that would constrain Ceres’s history,” says Andy Rivkin, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory in Baltimore. “But I’m not fully convinced about the interpretation of ammoniated phyllosilicates.” Claims of ammonia on Ceres date back to ground-based observations from the 1990s, and may yet evaporate under closer, more rigorous scrutiny of all the available spectra, Rivkin says. Instead, he has suggested the dwarf planet might be covered not with comet-like minerals but with magnesium- and iron-rich carbonates thought to be more typical of a mixture of water and rock like Ceres. And even if the ammonia is real, Rivkin notes that it could have come from closer to home, perhaps from the nitrogen-rich amino acids prevalent in certain types of meteorites.
Bright spots in the search for life
There is less controversy over what De Sanctis and her colleagues did not see in their study of the dwarf world's entire surface: water. Even though density measurements suggest that Ceres is roughly one third water by weight, water ice should rapidly sublimate away into space on the dwarf planet’s airless, sun-soaked surface, so its absence at first would seem to be no surprise. But Ceres apparently does have water percolating at or near its surface, albeit in small, isolated patches too small to show up in the broadband global spectra. Last year the Herschel Space Observatory detected wisps of water vapor around the dwarf planet, and since its arrival at Ceres, Dawn has imaged oodles of highly reflective bright spots on the Cereian surface that may be sites of exposed water ice.
Impacts exposing subsurface ice deposits and generating hydrothermal activity are one possible explanation for the bright spots and Herschel’s water vapor; “cryovolcanoes” that erupt volatiles such as water rather than rock are another. Both scenarios offer hopes that beneath Ceres’s barren surface there may be regions warm, wet and accessible enough to investigate for signs of past or even present extraterrestrial life.
In a second, complementary study using data from Dawn’s Framing Camera, Nathues and colleagues sought answers to the mystery by examining images and spectra of more than a hundred of the bright spots. They found that most of them occur in impact craters, and range in brightness from that of concrete to ocean ice. The spots seem to be mixtures of salt-rich water ice. The brighter spots, they argue, must be fresher ice from more recent impacts. In the same study, Nathues and his co-workers looked for plumes of water vapor produced by erupting cryovolcanoes, but found none.
“Ongoing cryovolcanism at present seems to be unlikely, because of missing plumes of water and a missing mechanism for creating such powerful activity,” Nathues says. That is, Ceres seems too inert to readily produce vapor-belching volcanoes.
But the dwarf planet is not entirely inactive. Of all the spots investigated, the strangest by far was found in a 90-kilometer-wide, four-kilometer-deep crater called Occator. A 10-kilometer-wide pit in the crater’s center holds a spot several times brighter than all the others on Ceres. While looking for plumes over Occator in the Dawn data, Nathues and his coworkers instead glimpsed something very surprising, a sort of daily weather cycle: A thin cloud or haze filled the crater in the mornings and afternoons, then dissipated at sunset. The haze, they argue, must be sun-lofted dust and water vapor, suggesting that somehow fresh, exposed ice lurks in Occator’s depths, despite the sunlight that has been baking the crater for millions of years.
According to another Dawn co-investigator, Mark Sykes, the director of the Planetary Science Institute in Tucson, these sorts of unexpected, counterintuitive findings suggest there’s much more to learn about the bright spots. Cryovolcanism, perhaps generated by impacts, could still play a role in their formation. “Nothing is a slam dunk; what exactly is going on is still up in the air,” Sykes says. Dawn is now entering a low-altitude mapping orbit around Ceres, and will soon quadruple the resolution of its best images of the surface. “When the data’s spatial resolution increases by a factor of four, the one thing I have complete confidence in is surprises…. God knows there will be many more questions to scratch our heads about!” he adds.
“The mystery of the bright spots continues,” says Dawn’s deputy principal investigator Carol Raymond of NASA’s Jet Propulsion Laboratory in California. “What is becoming clear is that the surface of Ceres shows evidence of activity, but the exact processes that are occurring are still not worked out.”
Dawn’s primary mission is slated to end next summer but researchers are already planning observations of Ceres that could come afterward. Guided by Dawn’s detailed surface mapping, Rivkin says, NASA’s James Webb Space Telescope could target Occator and other bright spots for further detailed studies after it launches in 2018. But what he and other researchers really want is to visit Ceres again. “Ultimately, of course, I’d hope to see a lander or even a rover put down in Occator,” Rivkin says. “That probably couldn’t happen before the mid-late 2020s… Hopefully, Ceres will continue to share its secrets until then.”