Icy Secrets From the Dawn of Time
As NASA's Dawn spacecraft spirals closer toward the surface of dwarf planet Ceres, a UA planetary researcher is hoping for new insights into how our solar system came to be.
Most of the time, Shane Byrne can be found poring over images taken by the University of Arizona-led HiRISE Mars camera, looking for clues about what climatic changes on Mars can tell us about the effects of climate change here on Earth.
But for the moment, he has his eyes on yet another world: Ceres, a dwarf planet and the largest known object in the main asteroid belt, a vast field of rocks and debris tumbling through the space stretching between Mars and Jupiter.
Byrne, associate professor in the UA's Lunar and Planetary Laboratory, was selected by NASA to participate as a guest investigator on the Dawn mission, specifically during the spacecraft's encounter with Ceres. After visiting giant protoplanet Vesta in 2012, Dawn made its way to Ceres, where it swung into a high orbit in April this year. Since then, the probe has gradually lowered its orbit, most recently embarking on a descent into "High-Altitude Mapping Orbit" just 910 miles above Ceres' surface.
Ceres and Vesta are the two most massive residents of the asteroid belt and thought to be fossils left over from the early days of the solar system — hence the name of the mission. About 4.6 billion years ago, chunks of rocks, metal and ice coalesced under the influence of gravity in a disc of hot gas and dust swirling around the young sun. While some grew into planets and dwarf planets, others remained as bits and pieces until today. Because the asteroid belt has been stirred up by Jupiter's gravity, these bodies keep hitting each other very fast, tending to break down into smaller and smaller pieces.
"Ceres and Vesta are both interesting objects to explore, because they are similar to the building blocks that we believe formed the inner, rocky planets in our solar system — Mercury, Venus, Earth and Mars," Byrne said. "At that time, there were many objects the size of Vesta and Ceres. If you think of the planets made up of Lego blocks, Ceres and Vesta would be individual Lego pieces. By studying those objects, we can learn a lot about how our own planets formed from those pieces."
At 590 miles across, Ceres is the largest known object in the main asteroid belt and about a quarter the diameter of Earth’s moon. Already, Dawn has revealed stunning images of the small world and thrown many challenges at planetary scientists, such as the mysterious bright spots, whose nature has yet to be elucidated.
The research proposal by Byrne, an expert in ice deposits on alien worlds, is one of nine winning applications selected from a pool of 48. Analyzing data from Dawn's imaging instruments and spectrometer, he will study how ice deposits form and change on the surface of Ceres.
"I'm very interested in the possibility that perhaps there is water frost on the surface," Byrne said, "and with its small seasonal cycle, Ceres could have water molecules moving from pole to pole."
Because the dwarf planet's orbital plane is tilted very slightly — three degrees, compared with Earth's 23 degrees that cause pronounced seasons over the course of the year — Ceres may experience similar, if much weaker, changes during its trip of four and one-half years around the sun. Such seasonal changes in conditions might be enough to shift the distribution of water and potentially other ices on Ceres' surface. As icy regions warm up, they are subject to sublimation, a process by which water molecules go directly from the frozen state into the gaseous state, bypassing the liquid intermediate stage that is familiar on Earth but impossible in the frigid vacuum on Ceres.
"In the absence of an atmosphere, water molecules move around the planet in ballistic hops," Byrne explained. "A water molecule can leave the pole in the summertime and hop around Ceres until it comes into a cold area, and then stay there."
Byrne said that Ceres is unique because it has seasonal cycles like Earth and Mars but also permanently shadowed areas that never receive sunlight. Such places, usually on the bottom of steep-walled craters, have been found on the moon and Mercury.
"Water molecules will hop around the surface until they land in those areas, where they get trapped permanently," Byrne said. "This happens on the moon and on Mercury as well. If water is delivered, say, by a comet impact, then all the water molecules hop around the surface like crazy until they find a resting place at the poles."
Unlike on the moon and Mercury, where gravity is comparatively strong and prevents water molecules from escaping into space, gravity on Ceres is quite low, leading scientists to believe that the giant, spherical asteroid may be leaking water molecules into space.
"To figure out how much water makes it to the polar regions, we have to know how much escapes into space," Byrne said. "Because we don't have instruments on the spacecraft that can observe that directly, I will look for patches of frost and permanently shaded areas, so I can calculate the dwarf planet's water dynamics."
Ceres' mysteries reach far below its surface, however.
"At some point, Ceres may have had liquid water in its interior, and hopefully we'll be able to determine if that is still the case, for example, through gravity data as we get closer," Byrne said.
While Vesta is predominantly made of rock, Ceres is a transitional object, thought to contain large amounts of ice — making it an interesting steppingstone from the building blocks of smaller to giant planets, according to Byrne.
"Ceres shares ingredients with the inner, rocky planets as well as with the icy planets in the outer solar system," he said. "It has some metals and some rock, the stuff that went into forming the rocky planets, along with ice, which initially bulked up the cores of Saturn and Jupiter."
Because Dawn is powered by an energy-efficient yet weak ion propulsion drive, the mission "runs in slow motion compared with other missions," as Byrne put it.
"It means you can do lots of great things, like visiting two asteroids in one mission, but it requires more patience," he said.
"We haven't seen things up close yet. As the spacecraft continues to transfer to lower and lower orbits, we'll get better and better imagery. The best images will start coming in the fall and close to the end of the year, when we'll descend to the low-altitude phase."
During that phase, Dawn will cruise less than 250 miles above Ceres, roughly the same altitude as the International Space Station. Circling Ceres in an orbit that takes it over both poles, it will image close to 100 percent of the surface, as the dwarf planet spins around its axis once every nine hours.
"The mission's strategy is simple in that the instruments take data all the time," Byrne said. "It's a continuous stream of observations, and we try to cover the body with as many observations as we can."
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