Ancient Impacts Mysteriously Erased From Asteroid Vesta
Asteroid Vesta likely had enough impacts to erase a flurry of space rock collisions about four billion years ago.
A little less than four billion years ago, as the theory goes, the inner solar system was pummeled by space rocks. It caused craters on the moon and likely left a few marks on the skin of our own planet, which erased over time due to erosion. Called the Late Heavy Bombardment (LHB), a leading theory for why it happened was the outer planets (such as Jupiter and Saturn) adjusting their orbits.
Vesta - an airless, large asteroid that orbits in the belt between Mars and Jupiter - doesn't have much evidence of the LHB. We know this from 2011-12 pictures from the Dawn spacecraft, which visited it up close. But why? A new simulation suggests that despite its airless surface, there have been enough rocks pummeling the surface in the last four billion years to erase most of the evidence.
"We computed an increased number of impacts during the LHB that leads to an erosion of the crust of few meters. Since the crust of Vesta is about 20 to 30 km (12.5 to 18.6 miles) deep, this is ... compatible with the survival of the crust," wrote lead researcher Simona Pirani, a Ph.D. student at Lund Observatory in Sweden.
"We investigated also if we could find any traces of these LHB craters on the surface of the asteroid and we found out that craters produced after the LHB, that is in last 4 Gyr (billion years), efficiently erased the crater population of the LHB event."
This could have implications for how the LHB affected the solar system, Pirani pointed out. Our best models from it come from looking at our nearby moon, but the LHB may have acted differently inside the main asteroid belt, where Vesta resides.
Vesta is one of only a handful of small solar system bodies to be looked at up close. Ceres, where Dawn is orbiting right now, is a dwarf planet with a completely different surface. The ice suspected to lie there, for example, likely causes evidence of craters to erase quite quickly, Pirani said. As for how the erasing took place on Vesta, there are at least a couple of mechanisms that could have caused it, Pirani wrote.
"A possible resurfacing process that could have contributed to erase LHB craters is the wide spread ejecta blanketing, for example. The bigger a target body is, the more ejecta are expected to be retained on its surface after an impact that could affect older craters in the neighborhood," Pirani said.
"Another process to take into account is the seismic shaking following an impact. Vesta showed two big impact basins on the south pole, so the seismic shacking could have been an important mechanism of resurfacing."
Vesta is a unique asteroid in the main belt because it is the only one where we know we have samples on Earth that came from meteorites. Called HEDs (or Howardite-Eucrite and Diogenite), this gives another window by which researchers can look at Vesta's past.
Simulations of the early solar system's formation indicate there could be a lack of mass in the inner solar system, particularly seen in the case of Mars (a small planet). Pirani said a natural next step in research will be to look at how the mass disappeared, which possibly could have happened when planetary embryos interacted.
A paper based on the research, which has been accepted in the journal Icarus, is now available in preprint version on Arxiv.
Vesta's disk seen by the Dawn spacecraft. There isn't much evidence of the Late Heavy Bombardment on its surface.
NASA's Dawn spacecraft orbited the massive asteroid Vesta in 2011 and 2012, giving us an unprecedented look at the protoplanet's landscape, craters and mineral composition. The probe, which is now on its way to dwarf planet Ceres, not only revealed the evolution of Vesta, it also provided vital clues as to the evolution of our solar system. Now,
, an unusually colorful Vesta landscape is on display. Using data from the mission, scientists at Max Planck Institute for Solar System Research in Katlenburg-Lindau, Germany have produced a rather psychedelic view of this otherwise bland landscape. Dawn's camera system is equipped with seven filters, each filter sensitive to a specific wavelength of light. Normally, Vesta would look gray to the naked eye, but when analyzing the ratios of light through Vesta's filters, the landscape pops with color. Shown here, the flow of material inside and outside a crater called Aelia is demonstrated. As different minerals reflect and absorb different wavelengths of light, this composite image is alive with color, each shade representing different kinds of minerals littering Vesta's landscape.
This is Antonia, a crater located inside the huge Rheasilvia basin in the southern hemisphere of Vesta. From this image, planetary scientists have been able
"the light blue material is fine-grain material excavated from the lower crust. The southern edge of the crater was buried by coarser material shortly after the crater formed. The dark blue of the southern crater rim is due to shadowing of the blocky material."
The impact crater Sextilia can be seen in the lower right of this image. The mottled dark patches are likely impact ejecta from a massive impact and the redish regions are thought to be rock that melted during the impact. The diversity of the mineralogy is obvious here. "No artist could paint something like that. Only nature can do this," said Martin Hoffman, a member of the framing camera team at Max Planck Institute.
Earlier images of Vesta have shown an unusual "pitted terrain" on the floors of the craters named Marcia (left) and Cornelia (right). Once again, the varied colors demonstrate the different minerals and processes that cover Vesta's surface.
of Vesta shows the abundance of hydrogen on Vesta's surface. Note that the hydrogen signal is enhanced near the asteroid's equator. The hydrogen is likely from hydroxyl or water bound to minerals in Vesta's surface.
Another, earlier view of Antonia crater, demonstrating the mineral diversity of the region.
This is the distinctive Oppia crater on Vesta, an impact that occurred on a slope. This produced an asymmetric ejecta distribution around the crater -- the red/orange ejecta material is more abundant around the downward slope than around the upward portion.