It has been a tremendous privilege to be involved in the Dawn mission. I was invited to join the team in 2010, a member of the cadre of Participating Scientists NASA provided to help synthesize the fascinating data being returned from Vesta during the 14 months the spacecraft orbited the protoplanet in 2011 and 2012.
Vesta is an intermediate-sized solar system body, between larger planetary objects like the Moon, and small asteroids like those that have been visited by other spacecraft (Gaspra, Ida, Matilda, Lutetia, Eros, etc.). It orbits the Sun in the main asteroid belt, between Mars and Jupiter. For me, the most exciting results from the Vesta phase of the mission are: 1. the general appearance of Vesta, and 2. findings related to one of my specific areas of research interest, Dawn’s investigation of the nature of surface modification on Vesta—or how the giant asteroid’s surface has changed over time.
This is a wonderful global mosaic of Vesta, a composite of many images Dawn’s framing camera captured as the spacecraft orbited the asteroid. Before Dawn, all we had were low-resolution views from the Hubble Space Telescope that revealed the rough shape of the asteroid and some brightness variations. We really didn’t know what the surface would look like up close. Would there be large deposits of volcanic plains, like the maria (“seas”) on the Moon, for example? It turns out that Vesta’s surface is really ancient, and impact cratering long ago eroded any volcanic features that might have existed. To a degree, Vesta is a battered rock, yet it is large enough to have experienced some processes (tectonic faulting, core formation) that occur on larger bodies like the Moon and Mercury. It is so exciting to see a new world for the first time.
I am interested in the ways in which exposure to the harsh space environment changes the materials on the surface of airless planetary bodies like the Moon, Mercury, and asteroids. What data do we have? From the Moon, we have samples returned by the Apollo astronauts and Luna robots, and extensive data collected with Earth-based telescopes and orbiting spacecraft. Thus we have a pretty good understanding of how “space weathering” on the Moon operates to alter the colors of the surface and causes bright rays extending from young impact craters to fade with time. Though Vesta is many hundreds of millions of kilometers away, we have actual samples of the asteroid here on Earth in the form of a common class of meteorite, and some limited telescopic data. Until the Dawn mission arrived at Vesta, missing were high-resolution images and color data that allow the geology of the surface to be examined in detail.
Let’s take a close look at new findings from Vesta. This image of the impact crater Vibidia illustrates several points concerning the processes that work to modify Vesta’s surface. First, a bright ejecta blanket and rays surround the crater. Older craters that have been worn down by smaller impacts lack the
bright rays. This tells us that some form of space weathering is taking place – to slowly erase the rays. However, detailed measurements of color by the Dawn framing camera (FC) and visible-infrared imaging spectrometer (VIR) reveal that Vesta’s surface does not become progressively “redder” as a result of space weathering. In this respect, Vesta’s response to its space environment is very different than the Moon’s: the Moon’s surface strongly reddens with increasing exposure age. We think that the reason for the difference has to do with the reduced flux of the solar wind in the asteroid belt compared to that at the Moon (closer to the Sun). Also, the average speed with which micrometeoroids strike Vesta is perhaps four times slower than those that hit the Moon. Bombardment of the lunar surface by the solar wind and micrometeoroids produces much melting and vaporization of the surface rocks and soils, leading to chemical changes and thus the redder color. Thus Vesta’s location in the solar system dictates the way in which its surface materials evolve.
The second key point about Vesta illustrated in the Vibidia image has to do with the dark material on Vibidia’s wall and rim. There is good evidence from FC, VIR, and the Dawn gamma ray and neutron detector (GRaND), that this is material like that found in carbonaceous chondrite meteorites. The dark rim and floor material is probably residue from the carbonaceous impactor that hit Vesta to form Vibidia crater. It appears that such carbonaceous material has been mixed globally into Vesta’s surface by countless impacts during Vesta’s history, and also is present in larger amounts in more recent impacts like the one that created Vibidia. These conclusions were presented in several papers by the Dawn science team.*
There are still a number of questions concerning the details of space weathering on Vesta, but Dawn has shown that a distinctive style of space weathering takes place on Vesta. The findings from Vesta are important as scientists seek to understand space weathering as a general phenomenon that takes place throughout the Solar System.
*C. M. Pieters and others (2012), Distinctive space weathering on Vesta from regolith mixing processes, Nature 491, 79-82; T. B. McCord and others (2012), Dark material on Vesta from the infall of carbonaceous volatile-rich material, Nature 491, 83-86; T. H. Prettyman and others (2012), Elemental mapping by Dawn reveals exogenic H in Vesta’s regolith, Science 338, 242-246; M. C. De Sanctis and others (2012), Detection of widespread hydrated materials on Vesta by the VIR imaging spectrometer on board the Dawn mission (2012), Astrophys. Journal Letters 758, L36.