What Science and Discoveries Has the Dawn Spacecraft Found on Dwarf Planet Ceres?
The Dawn space probe had previously visited Vesta, a large asteroid in the belt, before it began its new mission to Ceres. After years of space travel, Dawn began to make its final approach to Ceres in January 2015. On the 13th of that month Dawn officially took the best image of the dwarf planet ever, surpassing Hubble's benchmark established in 2003/2004. It also hints at an interesting surface feature: a pair of bright spots! What could they be? Three main theories were presented at the time as to what was reflecting the light. One is that it was subsurface ice that has been exposed by an impact (which makes sense since we have seen water vapor emissions from Ceres). Another was that a cryovolcano went off, releasing ice onto the surface instead of lava. A final though less likely theory was that magnesium silicates, found on other asteroids, may be present and reflecting light. Or maybe something else was emitting the lights.... Poor Michael Bland, a Dawn team member at USGS, felt that Ceres would be...bland. But we are happy for this not being the case (JPL "Dawn Delivers," WIRED UK, Betz "Dawn" 46).
March 6th was the big day as Dawn finally entered orbit around Ceres, becoming the first probe to orbit a dwarf planet (though New Horizons, launched before Dawn, will be second later this year). It was captured by Cere's gravity when it was about 38,000 miles away. Surface maps seem to indicate that the dwarf planet was once an active object, changing its surface frequently by bringing material from within to the surface. This was determined by scientists when they noticed that fewer large craters were present than expected for an object that is so old. Temperature maps also seem to indicate that the bright regions and their surroundings match in composition, possibly indicating they were - or currently are - the source of newer material (NASA/JPL "Spacecraft," JPL "Dawn's Ceres").
The nature of the bright spots was somewhat focused after early May. Images taken by Dawn on May 3 and 4 from an altitude of 8,400 miles have shown that the bright spots are more fractured than previously thought. Also, some reflective material is causing us to see the light and not something that is being emitted by the surface of the dwarf planet. The mystery vapor that scientists thought came from cryovolcanoes also was traced back to the bright spots. Vishno Reddy (from the Planetary Science Institute in Tuscon) even wondered if solar wind interactions could cause the vapor to be released from the bright spots. Sorry folks, no aliens here, but the mystery of the material that is causing the bright spots is not known (JPL "Ceres," Betz "Dawn" 46).
But it seems Ceres wants to keep the alien rumors alive. In late June of 2015, NASA released images of what appears to be a 3-mile high "pyramid" on the surface of Ceres. Later named Ahuna Mons, it was revealed to be more of a mound with a round top and steep sides. What makes it even stranger is how the mound seems to arise from a smooth plain of the dwarf planet. It is likely a remnant of an impact from the opposite side of the object, with the impact waves colliding after traveling around the surface. It couldn't have been from a direct impact because no crater rim is visible. We also know its not a traditional volcano (for no other one has such a shape as odd as Ahuna) but that it could be a water source based one when one looks at similar features on other Kuiper Belt objects. Finally, the bugger is 21,000 feet tall! (Grenoble, Betz "Dawn" 47, JPL "Dawn's First," Coral 31).
But scientists developed an interesting theory. What if Ahuna Mons is not a volcano but a cryovolcano, and that others once existed on Ceres? Where did they go? Michael Sori (Lunar and Planetary Laboratory) and colleagues postulate that the process of viscous relaxation may be at play. This is when solids flow like liquids, but over a large span of time. Ceres is certainly old, so any cryovolcanoes on its surface could have slowly flowed back into the dwarf planet and may have even collapsed into craters. Ahuna Mons is the only mountain left because of its young age, clocking in at 200 million years old. If the surface of Ceres does indeed contain as much water as speculated, then as Ceres orbits and hits perihelion, Ahuna Mons should shrink in size by 10-50 meters every few million years (Klesman "The Case," Wenz "Ceres," Coral 31-2).
Of course it was inevitable that the dwarf planet needed to get a map done in order to have a frame of reference for identifying features. Detailed surface readings show a height differential from the lowest to the highest points of 9 miles and overall the dwarf planet has echoes of Dione and Tethys, which are other icy bodies in the solar system. The crater containing the mysterious bright spots is now called Occator (the Roman deity of harrowing, keeping with the theme of agriculture) and is 60 miles wide with a depth of 2 miles. Here is but a sampling of the new craters with the inspiration for the name in parenthesis:
- Haulani, 20 miles wide (Hawaiian plant goddess)
- Dantu, 75 miles wide and 3 miles deep (Ghanaian God who has connections to corn)
- Ezino, about 75 miles wide (Sumerian goddess of grain)
- Kerwan (Hopi spirit of sprouting maize)
- Yalode (African Dahomey who was prayed to during harvest rites)
- Uvrara, 100 miles wide and 3 miles deep ("Indian and Iranian deity of plants and fields"
Many of the craters are deep but a few are shallow also, possibly having implications for the ice material that is thought to be on the surface. If truly present, then we would expect to see the crater walls deform as constant solar bombardment would melt the ices. The fact that we don't see that and also that many craters have inner craters implying an old age hints at an iceless surface. Based on the average depth of craters on the surface of Ceres, something 100x the viscosity of water ice must be present, like clathrates (salty mix) or porous rock, otherwise we would expect to see more large craters than there are currently. And after mapping the polar regions, Dawn data showed that many craters (totally less than 1% the surface area of the northern hemisphere) in that area exist in permanent shadow, raising the possibility of water ice being stored there as opposed to the aforementioned craters that receive direct sunlight. In January of 2017, a study confirmed that at least one of these craters, PSR2, does have sheets of frozen water in it. Located near the north pole of the dwarf planet, infrared data pointed to its existence. Its possible that the water ice brought to the surface of the bottoms of craters could exist until bombarded by radiation, sublimating away and leaving no traces behind (NASA/JPL "Ceres Gets," Betz "NASA," Betz "Dawn" 48, Timmer, American Geophysical Union, MacDonald, Wenz "Ceres Has," Coral 30).
A constant development on Ceres seems to be landslides, with different ones arising from different sources. Type I are "round, large, and occur at higher altitudes" in locations where water ice is suspected to live. Type II (the most frequently spotted type) are at the middle latitudes and are "thinner and longer" than Type I. But Type III may be the most interesting, for they form as water ice melts from impactors. Most have been seen at the lower altitudes near large craters. Based on the patterns of the landslides seen, a 10-50% water ice by volume could be possible for Ceres (Kiefert).
Later on, as more data was analyzed, scientists noticed that many gravity readings were not quite right. Some locations did have the expected patterning of craters but other locations had too much of a pull or too little. The study, led by Anton Ermakov (JPL), also hinted at a crust density closer to ice than to rock yet the crust is known for its hardness. Another study led by Roger Fu (Harvard University) looked at the makeup of the crust for some clues and found that ice, salts, rocks and clathrate hydrates are present. The latter most molecule is interesting, because it traps gases inside a lattice of water molecules and could only form from...liquid water. Perhaps surface water froze into the crust, goofing up the density readings we have found (Klesman "Finding").
Time passed along as Dawn continued to gather data. Eventually, enough infrared readings were collected of the surface to finally gather detailed spectroscopic information. Earth's atmosphere blocks this portion, so any space-based view is crucial. And the data collected by the Visible and Infrared Mapping Spectrometer on Dawn offered quite a few surprises.
Maria De Sanctis (from the National Institute of Astrophysics in Rome) and her team found that the surface was abundant in ammoniated phyllosilicates, a clay-like material, giving it much in common with Kuiper Belt objects. Why is that? Because at the distance Ceres is to the Sun, the nitrogen and hydrogen present in those bonds should have broken up long ago. Objects like comets, which travel from the far reaches of our solar system, have plenty of them. Either Ceres was born elsewhere or the material was deposited. Perhaps the Nice Model can explain this away (Billings, BEC).
The same team also took a look at those bright spots and came up with an answer to their nature, but not the one most people wanted to hear according to a December 10, 2015 issue of Nature. Turns out, those salts were concentrations of hydrated magnesium sulfate known as hexahydrite and sodium carbonate that once mixed with water ice not only causes it to be reflective but also to be a different color from the crater surrounding it. In fact, the sunlight causes some sublimation and therefore releases a haze! The cryovolcano theory died right there but in its place we have a new idea about what Ceres is: a mix between a comet and an asteroid. But how the carbonate got there is a mystery, for that is not something that is common for either objects but instead for icy moons. Yet it came from within the dwarf planet. Again, the Nice Model provideth a potential solution (Scharping, Timmer, Klotz, Wenz "New", Betz "Dawn Explains," BEC, Stacey).
To add to the mystery, gravity readings taken from differentials in Dawn data transmission as it orbited Ceres offered scientists clues about the internal layout of Ceres. Turns out, scientists were justified in relabeling the asteroid as a dwarf planet, for it exhibits hydro-static equilibrium, meaning that the object indeed has roundness and the internal layers reflect that. They also hint at a low density which points to water ice as a major contributor to the interior of the dwarf planet, for even mountains are pushing down on Ceres to the point where the mantle is disfigured. How could such a complex object form? Does it possibly being a KBO resolve anything? Stay tuned (Rice).
Party On at Ceres
July 1, 2016 was a big day for the future of Dawn. NASA scientists released their plans for the space probe, with the possible end for Dawn, as it finished its principle mission to Ceres the day before. Some were even talking about sending Dawn to asteroid 145 Adeona for a 2019 flyby. But it was decided that Ceres has so much more to offer and has many outstanding mysteries, and who can argue with that? So Dawn got its extension for a longer study of the dwarf planet, much in thanks to conservation efforts of saving fuel. The mission did last a long time, but finally ended on November 1, 2018 after Dawn ran out of fuel, thus ending one of the most fascinating missions in recent years (Boyle, Foust, Berger).
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© 2015 Leonard Kelley