Permanently shadowed regions capable of accumulating surface ice were identified in the northern hemisphere of Ceres using images taken by NASA’s Dawn mission combined with sophisticated computer modeling of illumination. Image credit: NASA/JPL-Caltech
The journal Science just published six significant papers about the dwarf planet Ceres. We pored over them to see what has planetary scientists so excited.
Ceres is the only dwarf planet in the asteroid belt, located between Mars and Jupiter. On June 30, the Dawn spacecraft over Ceres completed its prime mission, and NASA has since extended its mission through at least April 2017, at which time the vessel will possibly run out of hydrazine fuel. (Yesterday it moved to a higher orbit, where it will have less of Ceres's gravity dragging on it, and can thus conserve fuel.) Dawn's continuing mission will involve studying Ceres as the world reaches perihelion—that is, as it reaches its closest point to the sun its elliptical orbit.
Since Dawn's arrival at Ceres last year—after first spending a year orbiting Vesta, a minor planet in the asteroid belt—the dwarf planet has proven captivating beyond anyone's expectation. Its mysterious white spots garnered unprecedented public interest. NASA even launched a poll for people to guess what they might be. (Those who suspected alien beacon were, sadly, incorrect; scientists believe the correct answer is salt.)
The spots are but a tiny part of the scientific bonanza delivered by Dawn, however, and 16 months after its arrival at Ceres, scientists have finally been able to get a grip on the libraries of data being returned from the spacecraft. Here are some of their key findings.
1. IT HAS ICE VOLCANOES.
Ahuna Mons is an isolated mountain on the surface of Ceres that is, according to one paper published today, "distinct in its size, shape, and morphology." The mountain's formation, scientists suspect, is as such: cryomagma (ice lava!) erupted to the surface of Ceres, causing the development and spreading of an cryovolcanic (ice volcano!) dome. This occurred much in the same manner as a volcano forms on Earth. As material erupted, it followed the grooves and fractures already present on the surface of Ceres. The ice volcano's formation and composition suggest that the surface of Ceres is warmer than that of icy moons, and within Ceres, there is or was some long-term heat source. Moreover, other "topographic rises" on Ceres might well "share a common formation process and imply that volcanic activity occurred over an extended period." Differing structures and shapes of said rises might be attributed to changes in flow and ascent over time.
Dawn's Framing Camera has mapped Ceres, and the revealed geology is a scientific bonanza, giving scientists clues necessary to piece together the history and geologic activities of the mysterious world. Debra Buczkowski is one of those scientists. She is the lead author of one of the papers published today. She tells mental_floss that what surprised her most about Ceres was the discovery of "floor-fractured craters," or craters with shallow floors cut by fractures of various shape.
"These are features found on the Moon and Mars," she said, "where they are thought to form due to magma upwelling beneath impact craters, pushing their floors upward and causing them to fracture." Such features were not predicted on Ceres. As to the implications, Buczkowski says, "Finding these features on Ceres means that there was at some point in time magmatic processes occurring on Ceres." She notes that evidence of magmatism (along with the identification of Ahuna Mons as a cryovolcanic feature) indicates that Ceres was at some point in time geologically active, "although we have yet to find evidence that it is still an active body."
Another of the Science papers found a relatively even composition (but unequal abundance) of clay-like phyllosilicate minerals—which need water to form—on Ceres' surface. This suggests that "widespread and extensive aqueous alteration processes have affected the dwarf planet at some point in its history."
Planetary scientists have long believed that the mantle of Ceres is rich in water ice (and perhaps water). According to another paper published today, data from the visible and infrared mapping spectrometer (VIR) identified "without ambiguity" H2O absorption bands in a young crater on Ceres called Oxo: "These bands are most likely due to surface materials," the scientists say. In other words, in at least this 10-kilometer crater, there is water ice on the surface of Ceres—and not billion-year-old ice that probably tastes awful, but the young, fresh stuff. At Oxo's latitude, water ice could at best last a few hundred years before disappearing, and would be undetectable in tens of years because of dust in the ice, which would quickly become "the dominant material within the optical thickness (a few micrometers at most)."
So where did this ice come from? The authors of the paper suggest four possible origins: exposure of internal ice due to a surface impact (i.e. giant rock smashing into Ceres); re-condensed water vapor that originated from within Ceres, much in the way the nuclei of comets release water vapor; a water-rich rock crashed into Ceres; or water molecules formed due to "implantation of protons" by solar winds. (This happens on our Moon, too.)
4. … BUT IT MAY BE ROCKY ICE (OR ICY ROCK).
Craters tell an astonishing amount about celestial objects, from their age and composition to their history and internal processes. Ceres is heavily cratered and yet lacks craters larger than 300 kilometers, which is a bit unexpected. Moreover, the suspected composition of Ceres suggests that many of those craters should have "relaxed" over time, which by and large has not happened. This leads scientists to believe that the crust of Ceres possesses less ice than was expected or might be thicker than once thought. Rather than possessing a solid ice shell just below the surface, Ceres might have more of a rocky-ice (or icy-rock) shell.
5. IT MAY HAVE AN EXOSPHERE.
An exosphere is the very outer limits of an atmosphere. (For reference, the Earth's exosphere begins around 300 kilometers beyond the altitude of the International Space Station.) It's where particles are gravitationally bound to an object but are very heavily influenced by the Sun. Before Dawn arrived at Ceres, instruments on the Hubble Space Telescope were unsuccessful in detecting an exosphere at the dwarf planet. Data returned by Dawn now tell a slightly different story. The Dawn spacecraft carries an instrument called the Gamma Ray and Neutron Detector (GRaND). On multiple orbits, GRaND detected bursts of energetic electrons. At those moments, scientists suspect that the weak atmosphere at Ceres was "ionized by the energetic particles in the solar wind, producing a bow shock as the solar wind was deflected," as the paper authors write. When the solar event stopped, ionization of the atmosphere ceased, and the exosphere vanished.