It has been a year now since the Dawn spacecraft first reached the dwarf planet Ceres in the main asteroid belt between Mars and Jupiter, and during that time has shown Ceres to be a unique and complex little world. At first glance, Ceres just seems to be a heavily battered place, covered in craters like the Moon or Mercury, but a closer look reveals something more interesting: a small rocky world with large fractures, unusual “bright spots” randomly dispersed across the surface and an odd conical “mountain” which sits in isolation with nothing else like it around. Dawn has already acquired an enormous amount of data about Ceres, but now, in its lowest possible orbit, will continue to do for some time to come.
The intriguing features seen on Ceres’ surface suggest that it has been, or perhaps still is, geologically active, not just a solid ball of inert rock and ice.
“Ceres has defied our expectations and surprised us in many ways, thanks to a year’s worth of data from Dawn. We are hard at work on the mysteries the spacecraft has presented to us,” said Carol Raymond, deputy principal investigator for the mission, at NASA’s Jet Propulsion Laboratory in Pasadena, Calif.
Ceres is referred to as a dwarf planet, in between large asteroids and planets, and is the largest object in the main asteroid belt.
“Ceres is so big compared to all the other asteroids that it’s really different,” said Andrew Rivkin, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md. “It’s sort of the penultimate step before a planet.”
It was March 6, 2015, when Dawn first arrived at Ceres and entered orbit to begin a long study of this fascinating place. Dawn recently entered the lowest orbit that it can go—240 miles (385 kilometers) above the surface—and the images sent back so far have been incredible.
One of the most interesting features seen is the so-called “lonely mountain,” Ahuna Mons, a conical hill which is unusual due to its appearance as well as the fact that there is nothing else like it on Ceres. Scientists do not yet know how it formed, but the newest high-resolution images of it taken by Dawn show a steeply sloped surface with a relatively flat summit. There are also numerous “ridges” of bright material running downhill. When first seen from a distance in February 2015, the mound had a bit more of a pyramid-appearing shape to it, but close-up images showed it to actually be conical. So no, not an alien base, but an intriguing feature just the same. There is a large crater adjacent to it, almost touching, which is roughly about the same size as the base of the mound, but whether it is related to the formation of the mountain is unknown. This unusual mound stands out very noticeably from the surrounding cratered, but otherwise relatively flat, terrain. Mountains of any kind weren’t really anticipated on Ceres, since it is so small and far from the Sun, with little, if any, geological activity expected to be seen.
As noted by Chris Russell, Dawn’s principal investigator at the University of California in Los Angeles: “No one expected a mountain on Ceres, especially one like Ahuna Mons. We still do not have a satisfactory model to explain how it formed.”
“As we take the highest-resolution data ever from Ceres, we will continue to examine our hypotheses and uncover even more surprises about this mysterious world,” he added.
Ahuna Mons is about 3 miles (5 kilometers) tall and 12 miles (20 kilometers) across at the base. It is higher than Mount Rainier in Washington and Mount Whitney in California, for comparison.
Some other now well-known oddities on Ceres are the “bright spots” found in various locations across the dwarf planet, with more than 130 counted so far. They stand out as white-ish against the more grey surrounding terrain. The most famous are the brightest and largest spots, in Occator crater, which can be easily seen from some distance away from Ceres. They initially appear as one or two bright spots, but when seen close-up they can be seen to consist of a cluster of multiple spots, at least 10, near the center of the crater. The others appear to be similar, but less pronounced than those in Occator.
Like the mountain, the origin of these spots is still unclear, although they do seem to be composed of salts, not water ice as first theorized. Studies of data from Dawn show that they are consistent with a type of magnesium sulfate called hexahydrite. They might be deposits left over from when briny water or ice came to the surface from below, perhaps excavated by impacts, and subsequently sublimated in the airless space around Ceres; Ceres is thought to have a water-ice mantle surrounding a rocky core. They might even be deposits from cryovolcanoes, where ice was vented to the surface geologically. Haze has been inside Occator crater, which may be related to the spots.
“The global nature of Ceres’ bright spots suggests that this world has a subsurface layer that contains briny water-ice,” said Andreas Nathues at the Max Planck Institute for Solar System Research in Göttingen, Germany. “The whole picture we do not have yet,” he added.
The images sent back by Dawn of the spots in Occator have already been fantastic, but the best ones are expected to be released sometime this month, taken from the lowest orbit. As mentioned in the Dawn twitter feed on Feb. 28:
“Re: Occator bright spots & mountain: New images are being processed and are expected next month. Stay tuned!”
The new mountain images have now been released, so it shouldn’t be too long until we have our best view so far of the bright spots in Occator.
“Dawn began mapping Ceres at its lowest altitude in December, but it wasn’t until very recently that its orbital path allowed it to view Occator’s brightest area. This dwarf planet is very large and it takes a great many orbital revolutions before all of it comes into view of Dawn’s camera and other sensors,” said Marc Rayman, Dawn’s chief engineer and mission director at JPL.
Dawn has also found ammonia on Ceres, suggesting it may have formed farther out in the Solar System before migrating inward.
From a recently published paper:
“Our measurements indicate widespread ammoniated phyllosilicates across the surface, but no detectable water ice. Ammonia, accreted either as organic matter or as ice, may have reacted with phyllosilicates on Ceres during differentiation. This suggests that material from the outer Solar System was incorporated into Ceres, either during its formation at great heliocentric distance or by incorporation of material transported into the main asteroid belt.”
As also noted by Maria Cristina De Sanctis at the National Institute of Astrophysics in Rome: “The presence of ammonia-bearing species suggests that Ceres is composed of material accreted in an environment where ammonia and nitrogen were abundant. Consequently, we think that this material originated in the outer cold Solar System.”
Dawn was the first spacecraft ever to visit a dwarf planet (before New Horizons reached Pluto) and the first to orbit two different Solar System bodies—initially the asteroid Vesta from 2011-2012 and now Ceres in 2015-2016.
Ceres will be in the news again later this month, as there will also be a press briefing on March 22 with further updates, during the 47th Lunar and Planetary Science Conference in The Woodlands, Texas.
“Ceres continues to amaze, yet puzzle us, as we examine our multitude of images, spectra and now energetic particle bursts,” said Russell.
More information about the Dawn mission is available here.
Please check back with AmericaSpace for more updates about the Dawn mission.