Dawn’s Virtual Flight over Vesta. This movie uses data from NASA’s Dawn spacecraft to simulate the view from the spacecraft flying over the surface of the giant asteroid Vesta.

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 (4) 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.

In addition to creating the crater, the impact is believed to have launched a large number of fragments into space, some of which later escaped the main belt and possibly hit the Earth.

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.

Several large craters on Vesta were first inferred by Hubble Space Telescope imaging. However, a photographic survey by the Dawn spacecraft revealed a collision-dominated history, as well as a strong north-south dichotomy in the asteroid’s cratering record. Vesta’s heavily cratered northern terrains retain much of their earliest history, but the southern hemisphere was reset by two major collisions in more recent times.

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.

“An age of about 1 billion years for Rheasilvia is unexpectedly young. This result has important implications for our understanding of the evolution of the Vesta, its asteroid family and the inner main asteroid belt in general. We have just started exploring Vesta’s secrets, and I’m sure other intriguing results will come along shortly,” said NLSI team member Dr. Simone Marchi, lead author of “The Violent Collisional History of Asteroid (4) Vesta,” published in the May 11 issue of the journal Science.

NASA’s Dawn spacecraft has provided researchers with the first orbital analysis of the giant asteroid Vesta, yielding new insights into its creation and kinship with terrestrial planets and Earth’s moon.

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. Results appear in today’s edition of the journal Science.

“Dawn’s visit to Vesta has confirmed our broad theories of this giant asteroid’s history, while helping to fill in details it would have been impossible to know from afar,” said Carol Raymond, deputy principal investigator at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “Dawn’s residence at Vesta of nearly a year has made the asteroid’s planet-like qualities obvious and shown us our connection to that bright orb in our night sky.”

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.


Touring Vesta’s Craters. This video takes viewers on a virtual tour of Vesta’s south polar basin, the “snowman” set of craters and a crater called Oppia.

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.

“We know a lot about the moon and we’re only coming up to speed now on Vesta,” said Vishnu Reddy, a framing camera team member at the Max Planck Institute for Solar System Research in Germany and the University of North Dakota in Grand Forks. “Comparing the two gives us two storylines for how these fraternal twins evolved in the early solar system.”

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.

Dawn’s mission to Vesta and Ceres is managed by JPL for NASA’s Science Mission Directorate in Washington. Dawn is a project of the directorate’s Discovery Program managed by NASA’s Marshall Space Flight Center in Huntsville, Ala. UCLA is responsible for overall Dawn mission science. Orbital Sciences Corp. in Dulles, Va., designed and built the spacecraft. The German Aerospace Center, the Max Planck Institute for Solar System Research, the Italian Space Agency and the Italian National Astrophysical Institute are international partners on the mission team. The California Institute of Technology in Pasadena manages JPL for NASA.

For more information about the Dawn mission, visit: http://www.nasa.gov/dawn and http://dawn.jpl.nasa.gov .

Posted by: Soderman/NLSI Staff
Source: NASA/NLSI team; http://www.nasa.gov/mission_pages/dawn/news/dawn20120510.html

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