A team of researchers led by a NASA Lunar Science Institute (NLSI) member based at Southwest Research Institute has discovered evidence that the giant impact crater Rheasilvia on Asteroid Vesta was created in a collision that occurred only about 1 billion years ago, much more recently than previously thought. This result is based on the analysis of high-resolution images obtained with the Dawn spacecraft, which entered orbit around Vesta in July 2011.
Vesta, the second-most massive body in the main asteroid belt, is believed to have formed within the first few million years after the earliest solar system solids (~4.6 billion years ago). According to models, its early evolution occurred in an environment where collisions with other asteroids were much more frequent than they are today. It was thought that one such early collision on Vesta created a swarm of fragments, which we now call an asteroid family. Although Vesta and its family are located between Mars and Jupiter, smaller pieces of these asteroids can be found in meteorite collections on Earth, including most eucrite, howardite and diogenite meteorites.
This video takes viewers on a virtual tour of Vesta’s south polar basin,
the “snowman” set of craters and a crater called Oppia. Credits:
NASA/JPL
The Dawn survey revealed high-resolution details of these craters, allowing scientists to estimate their ages on the basis of the number of younger craters that have been superposed on the crater’s floor since their formation.
Rheasilvia, the youngest of these impact structures, is about 505 kilometers (314 miles) across. The number of smaller craters found within Rheasilvia can be used like a clock to estimate its formation age. The best estimates suggest it is only about 1 billion years old. For reference, this is nearly 3 billion years after the barrage of comets and asteroids that produced the so-called Late Heavy Bombardment of the Moon (and Solar System). Before this time, the asteroid belt is believed to have been substantially larger than it is today.
The volume of material excavated by the impact that formed Rheasilvia is larger than the estimated volume of known asteroidal members of Vesta’s dynamical family, suggesting that most of the observed family was formed in this single event.
Vesta now has been revealed as a special fossil of the early solar system with a more varied, diverse surface than originally thought. Scientists have confirmed a variety of ways in which Vesta more closely resembles a small planet or Earth’s moon than another asteroid.
Scientists now see Vesta as a layered, planetary building block with an iron core – the only one known to survive the earliest days of the solar system. The asteroid’s geologic complexity can be attributed to a process that separated the asteroid into a crust, mantle and iron core with a radius of approximately 68 miles (110 kilometers) about 4.56 billion years ago. The terrestrial planets and Earth’s moon formed in a similar way.
Dawn observed a pattern of minerals exposed by deep gashes created by space rock impacts, which may support the idea the asteroid once had a subsurface magma ocean. A magma ocean occurs when a body undergoes almost complete melting, leading to layered building blocks that can form planets. Other bodies with magma oceans ended up becoming parts of Earth and other planets.
Data also confirm a distinct group of meteorites found on Earth did, as theorized, originate from Vesta. The signatures of pyroxene, an iron- and magnesium-rich mineral, in those meteorites match those of rocks on Vesta’s surface. These objects account for about 6 percent of all meteorites seen falling on Earth.
This makes the asteroid one of the largest single sources for Earth’s meteorites. The finding also marks the first time a spacecraft has been able to visit the source of samples after they were identified on Earth.
Scientists now know Vesta’s topography is quite steep and varied. Some craters on Vesta formed on very steep slopes and have nearly vertical sides, with landslides occurring more frequently than expected.
Another unexpected finding was that the asteroid’s central peak in the Rheasilvia basin in the southern hemisphere is much higher and wider, relative to its crater size, than the central peaks of craters on bodies like our moon. Vesta also bears similarities to other low-gravity worlds like Saturn’s small icy moons, and its surface has light and dark markings that don’t match the predictable patterns on Earth’s moon.
Dawn has revealed details of ongoing collisions that battered Vesta throughout its history. Dawn scientists now can date the two giant impacts that pounded Vesta’s southern hemisphere and created the basin Veneneia approximately 2 billion years ago and the Rheasilvia basin about 1 billion years ago. Rheasilvia is the largest impact basin on Vesta.
“The large impact basins on the moon are all quite old,” said David O’Brien, a Dawn participating scientist from the Planetary Science Institute in Tucson, Ariz. “The fact that the largest impact on Vesta is so young was surprising.”
Launched in 2007, Dawn began exploring Vesta in mid-2011. The spacecraft will depart Vesta on August 26 for its next study target, the dwarf planet Ceres, in 2015.
Sources:
- Jia-Rui Cook, Dwayne Brown, NASA Dawn Mission Reveals Secrets of Large Asteroid, NASA JPL, May 10, 2012
- The Violent Collisional History of Asteroid 4 Vesta, Science, May 11, 2012, Vol. 336 no. 6082 pp. 690-694
