Published: 08 October 2020
What is the nature of the asteroid belt? Is it a failed planet or a remnant belt-like structure from the times of the formation of the solar system? What are the properties of its major bodies and how did they evolve? Do the bodies retain a record of the chemical gradients of the protoplanetary disk? Those were questions that the NASA Dawn mission was set to answer when it was launched in September 2007 to the two largest bodies in the main asteroid belt between Mars and Jupiter, protoplanet Vesta and dwarf planet Ceres. The mission, equipped with a framing camera, a visible and infrared spectrometer and a g-ray and neutron detector, achieved a number of firsts during its lifetime. It was the first mission to orbit an object in the main asteroid belt, the first to orbit two extraterrestrial destinations, and the first to orbit a dwarf planet, a class of planets the IAU had introduced in 2006, with Pluto being the most prominent member. The spacecraft arrived at Vesta, the smaller of the two in July 2011 and orbited the asteroid for 14 months before it left for Ceres where it arrived in March 2015. The spacecraft exhausted all the available hydrazine fuel two years ago in October 2018 and went silent, but is still in orbit around the dwarf planet. The spacecraft used ion propulsion to reach its destinations in the asteroid belt and for all orbit transfers at the bodies.
Dawn confirmed that Vesta is the source of a particular class of meteorites, the HED (howardite-eucrite-diogenite) meteorites that comprise about 6% of all meteorites found to date. The meteorites most likely escaped from Vesta when large impacts created two major basins of several hundred kilometers diameter near the asteroid’s south pole, which also resulted in a trough system circling near the equator. By mapping Vesta’s gravity field, the mission further showed that the asteroid was consistent with a differentiated rocky body with a dense iron-rich core, and the images showed that it had a complex geological history. Generalizing the findings at Vesta, it is thought that early-forming planetesimals could have been differentiated before they were accreted onto proto-planets. Deposits of hydrated minerals discovered on Vesta’s surface provided evidence that water-and carbon-rich planetesimals delivered volatiles to Vesta, and likely were a major source of volatile delivery to the growing terrestrial planets.
Orbiting at a larger distance to the sun, and about twice as large, Ceres was found to be quite different from Vesta with an ice-rich surface and evidence for a subsurface ocean, at least in the past. The distribution of bright deposits on its surface suggest that Ceres is active or was most recently so. The bright material is identified as deposits of mostly sodium carbonate that came from liquid percolating up from subsurface brines. Even organics were identified on its surface. Judging from the cratering record, Ceres’ surface is varied in age. Ammonia found on Ceres suggests that the dwarf planet may have originally formed at a larger orbital distance and was later transferred further in by a large-scale orbital instability as has been suggested by some models of solar system formation.
Carol Raymond is a principal scientist at the Jet Propulsion Laboratory of the California Institute of Technology. She was the Deputy Principal investigator for the Dawn mission and became the Principal Investigator during the extended mission. She is a member of the Europa Clipper Magnetometer team and a Co-I on the NASA Psyche mission. Carol Raymond started her career studying terrestrial paleomagnetism and became focused on planetary science when the magnetized crust of Mars was discovered. She is a highly cited and renowned planetary physicist with a wide-range of interests.
Seminar was recorded on October 8, 2020
