The Dawn spacecraft just recently entered its lowest and final orbit around the dwarf planet Ceres, providing the closest look ever at the puzzling world. Dawn will, of course, be taking thousands more high-resolution photographs, but what else will it be doing during the remainder of its mission? Various aspects of the mission will study Ceres in unprecedented detail.
Dawn is now orbiting Ceres at an average altitude of only 240 miles (385 kilometers), which is closer than the International Space Station is to Earth. The images sent back so far have been incredible, but from this close vantage point, they will be even better. More than 16,000 images were sent back by Dawn in 2015, including 2,000 since it entered its lowest orbit. There will be many more to come in 2016.
The images have shown Ceres, the largest body in the asteroid belt, to be a complex world, with craters, valleys, mountains, and, of course, those odd white spots that everyone has been talking about.
“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.”
Ceres has been geologically active, at least in the past, and may still be today, the evidence suggests. There may even be hydrothermal activity; the white spots are now thought to be salt deposits left behind after a layer of briny water-ice has been exposed by impacts and sublimated into space (as there is no atmosphere to speak of). The salts identified are consistent with a brine containing magnesium sulphate hexahydrite. Analysis of data also shows that Ceres has a water-ice mantle surrounding a rocky core, and that there may still be at least pockets of liquid water beneath the surface, raising the prospect of potential habitability for microorganisms, as seemingly unlikely as that may sound for a world so far from the Sun.
“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,” said Chris Russell, principal investigator of the Dawn mission, based at the University of California, Los Angeles.
So now that Dawn is in its lowest orbit, called the low altitude mapping orbit (LAMO), what kind of investigations will it be conducting? Dawn will examine two kinds of radiation—gamma ray spectra and neutron spectra—as well as measure Ceres’ gravity field. The instruments used look below the surface of Ceres, rather than the surface as with the camera, visible mapping spectrometer and infrared mapping spectrometer.
By studying the radiation using the gamma ray and neutron detector (GRaND), scientists can measure various chemical elements within Ceres. With 21 sensors, GRaND can measure the energy of each gamma ray photon and each neutron. Some gamma rays can be produced by radioactive elements within Ceres itself, but most are byproducts of cosmic rays originating from elsewhere in space. The radiation is also very faint compared to the bright surface seen by the camera, infrared spectrometer, and visible spectrometer. The “images” taken of the radiation require long exposure times of weeks.
Dawn will also measure the gravity field of Ceres, which will provide more clues to the internal structure of the dwarf planet. Measurements had already been done in the higher orbits, which were already highly accurate, but the new measurements will be even better. The gravitational effects on the spacecraft are strongest when it is close to areas of Ceres with greater density. By measuring the variations, scientists can calculate the distribution of mass within Ceres. Scientists can also measure tiny variations in the velocity of the spacecraft, as accurate as only one foot (0.3 meters) per hour, by using NASA’s Deep Space Network (DSN) to track the radio signal coming from Dawn. By combining all these results, scientists can “see” into the inside of Ceres and better understand what it is composed of. Similar techniques were used when Dawn previously visited the protoplanet Vesta, which showed it to be composed primarily of a dense iron and nickel core surrounded by a less dense mantle and crust.
By learning more about the interior of Ceres, as with other bodies in the Solar System, scientists can determine how it formed and how geologically active it has been, or still is. As with other smaller rocky worlds such as Pluto, Ceres has had its surprises, showing evidence for more geological activity than had previously been thought.
Dawn has also found evidence for clay minerals, or phyllosilicates, on Ceres, using the visible and infrared mapping spectrometer instrument (VIR). They are very common on Earth, and there are likely several different kinds on Ceres as well. One of the surprises found on Ceres was the presence of ammonia in the phyllosilicates. That presents a puzzle, since it is too warm where Ceres is now for ammonia to easily form. It is now theorized that Ceres actually formed farther out in the Solar System, and then gradually migrated inward, as some other bodies have been known to do. Another possibility is that Ceres did form more or less where it orbits now, but other ammonia-rich debris from farther out drifted in and fell on Ceres.
“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,” said Maria Cristina De Sanctis, lead author of the study, at the National Institute of Astrophysics in Rome.
From the newly 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.”
While taking detailed measurements of the subsurface, Dawn will also, of course, continue to observe the surface itself in incredible detail. The bright spots have been a subject of much discussion, with various theories as to how they formed. While the brightest and best-known spots are in Occator crater, there are over 130 such spots which have been found on Ceres altogether. They appear quite reflective, leading to the idea that they were ice or salt. The latest analysis from Dawn shows them to most likely be salt deposits, such as magnesium sulphate (similar to Epsom salts). The leading theory now is that they are left over from asteroid impacts on Ceres, which briefly excavate a layer of briny water-ice just below the surface. The images of the spots have been increasingly high-resolution as Dawn entered lower orbits, and now, coming soon from this new lowest orbit, they will be even better.
Dawn has also observed haze within Occator crater, most likely water vapor, which seems to 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,” Nathues said. He notes, however, that “The whole picture we do not have yet.”
The “Lonely Mountain” is another oddity, a conical-shaped mountain about 4 miles (6 kilometers) tall which sits all by itself with no other similar features near it. It is also tangent to a similar-sized crater. How did it get there?
As Dawn science team member Paul Schenk, a geologist at the Lunar and Planetary Institute in Houston, Texas, described it: “This mountain is among the tallest features we’ve seen on Ceres to date. It’s unusual that it’s not associated with a crater. Why is it sitting in the middle of nowhere? We don’t know yet, but we may find out with closer observations.”
With Dawn now firmly locked into its lowest and final orbit, scientists will be able to study Ceres, both inside and out, as never before. Jan. 1 was also the 215th anniversary of Giuseppe Piazzi’s first glimpse of this world as nothing more than a dot of light from his observatory in Sicily. Now, in 2016, we can visit and see this place in a way that he could never have imagined.
More information about the Dawn mission is on the official website.
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Just a thought, but if for some reason Mars proves to be a bad choice for human exploration and/or colonization — if we think there are living organisms that ought to be studied only by robots, for example — then we might want to contemplate human expeditions to life-free asteroids instead. To Ceres, for instance, or Juno. So the Dawn mission may be extraordinarily valuable.