An international group of planetary scientists has produced the first global geologic and tectonic map of Vesta, and constructed the geologic time scale for this asteroid.
Global geologic map of Vesta, showing the spatial distribution of major types of materials. Image credit: D.A. Williams et al.
Vesta is the second largest asteroid of the asteroid main belt and the fourth such object to be discovered.
Discovered by Heinrich Wilhelm Olbers on 29 March 1807, it is the only main-belt asteroid visible to the unaided eye.
Vesta rotates once in 5.34 hours and orbits the Sun in 3.63 years. It has an ellipsoidal shape with radial dimensions of 286 × 279 × 223 km.
It appears to have a surface of basaltic rock – frozen lava – which oozed out of the asteroid’s presumably hot interior shortly after its formation 4.5 billion years ago.
The geologic mapping of Vesta relied on images taken by the framing camera aboard NASA’s Dawn spacecraft.
“Geological mapping was crucial for resolving Vesta’s geologic history, as well as providing geologic context to understand compositional information from Dawn’s Visible and Infrared spectrometer and Gamma Ray and Neutron Detector,” said Dr Carol Raymond, Dawn’s deputy principal investigator.
“The geologic mapping campaign at Vesta took about 2.5 years to complete. The resulting maps enabled us to construct a geologic time scale of Vesta for comparison to other planets and moons,” said the mappers – Dr David Williams of Arizona State University and his colleagues from Germany and the United States.
They found that Vesta’s geologic time scale has been shaped by a sequence of large impact events. The biggest of these were the impacts that blasted the large Veneneia and Rheasilvia craters early in the asteroid’s history, and the Marcia crater late in its history.
An age in years is quite difficult to determine because the samples scientists have from Vesta – the so-called howardite-eucrite-diogenite meteorites – do not show a clear formation age that can be linked to specific features on the asteroid.
“So figuring out an actual date in years is a step-by-step-by-step process. We work with rock samples from the Moon, mostly from Apollo missions decades ago. These give actual dates for large lunar impacts,” said Dr Williams, who is the lead author of the paper published in the journal Icarus.
“The tricky part lies in creating a model that links the lunar impact time scale to the rest of the Solar System.”
Proposed geologic time scale for Vesta: the age dates at left are cratering model ages derived from the asteroid flux-derived chronology function; the age dates at right are cratering model ages derived from the lunar-derived chronology system. Image credit: D.A. Williams et al / Sci-News.com.
In the case of Vesta, Dr Williams and his co-authors have developed two different models to estimate surface ages.
One is based on the lunar impact rate, the other on the frequency of asteroid impacts.
Thus they can use two approaches with crater statistics to date Vesta’s surface, but these yield two different age ranges.
Applying the models to Vesta, they concluded that the oldest surviving crust on Vesta predates the Veneneia impact, which has an age of 2.1 billion years (asteroid system) or 3.7 billion years (lunar system).
The Rheasilvia impact likely has an age of around 1 billion years (asteroids) or 3.5 billion years (lunar).
“Vesta’s last big event, the Marcia impact, has an age that’s still uncertain. But our current best estimates suggest an age between roughly 120 and 390 million years,” Dr Williams said.
“The difference comes from which cratering model is used.”
_____
D.A. Williams et al. 2014. The chronostratigraphy of protoplanet Vesta. Icarus, vol. 244, pp. 158-165; doi: 10.1016/j.icarus.2014.06.027
