A thicker crust on Vesta questions how it was baked

Billing its mission as a journey to the beginning of the Solar System, the NASA Dawn probe has revealed more information about the asteroid Vesta that has scientists both eager and cautious about what they have learned.

The second largest in the belt of bodies between the orbits of Mars and Jupiter, Vesta is almost as old as the Solar System itself. Of late, its internal structure has spurred more interest because it is similar to that of Earth’s: with a crust, mantle and core.

One team, headed by Harold Clenet, a scientist at the Earth and Planetary Science Laboratory, Ecole Polytechnique Federale de Lausanne, has concluded that the asteroid’s newly discovered features defy conventional beliefs of how they may have formed.

Because Vesta’s formation and internal composition are thought to be similar to Earth’s, Dr. Clenet asserts that based on his team’s findings, we rethink some aspects of how the Solar System was formed, too.

On the other hand, the science team behind Dawn, led by principal investigator Christopher Russell, a professor of space physics at the University of California, Los Angeles, contends the Clenet et al team’s reliance on “simplistic” models to come to broad conclusions without sorting out other possibilities first.

Missing olivine

Dr. Clenet and his colleagues from the Universities of Bern, Brittany and Arizona used observations made by Dawn between July 2011 and September 2012 of two large craters near Vesta’s south pole. These craters were formed by meteorite impacts so powerful that the material they dislodged comprises 5% of the meteorites that fall on Earth. And what information we had of Vesta pre-Dawn came from their fragments.

More pertinently, the impacts also dug out enough material to provide scientists with a glimpse of what they thought was Vesta’s mantle.

But they were in for a surprise. They found that a common silicate mineral of the mantle, called olivine, was missing in observations of the southern hemisphere craters. “Olivine is a very common mineral on Earth and represents about 60% of Earth’s upper mantle,” Dr. Clenet said.

In the absence of this signature material in the craters, Dr. Clenet was led to believe Vesta’s mantle has not been exposed at all, and what they were observing might still be the crust. That would mean the crust is some 20 km thick in the northern regions of Vesta, and about 80 km thick in parts of the southern.

“This does not fit with the chondritic models of planet formation,” he added, chondrites being the most primitive material that formed at the beginning of the Solar System, “and thus question the nature of the initial material that formed Vesta.”.

Because of the shared principles of their origins, Dr. Clenet’s findings, published online in Nature on July 16, also cast doubt on what Earth’s early years may have been like, he thinks.

However, Prof. Russell advised more caution because observations of Vesta by Dawn have proved the chondritic model more simplistic than correct.

Not the last word

Planetary bodies that have a hot, inner core also have distinct layers of material: the crust and the mantle. The mantle is formed by cooling magma, and olivine is the first mineral to crystallize when magma cools. In this picture, Prof. Russell said, “One of the predictions is that the differentiation would make a deep magma ocean in which olivine was the major constituent of the mantle, but a pure olivine mantle clearly does not exist.”

“Why? Is there a different chemistry?”

He contends the Clenet et al team’s reliance on the “simplistic” absence of olivine to come to such broad conclusions without sorting out other possibilities. Dr. Clenet argues that olivine may have been superficially removed from Vesta’s upper mantle by “huge impacts”, but the fact that the dislodged mineral can’t be found anywhere else in the asteroid belt lends credence to the hypothesis that the crust is thicker.

Prof. Russell, on the other hand, thinks maybe the body didn’t heat up enough to make an olivine-rich mantle in the first place, or the composition of hot radioactive substances was different, or the overlying material didn’t percolate the way we think it might have.

Similarly, Maria Cristina De Sanctis, Dawn co-investigator at the National Institute of Astrophysics, Rome, is also wary. She said that although Vesta is similar to Earth, “it is much smaller and it is difficult to have such a small object similar to our planet, and mass is an important factor in the evolution of planets.”

While the research community sorts out the possibilities, “the present paper in my opinion contains little new real insight into the problem,” Prof. Russell concluded. “Its publication has puzzled a number of us on the Dawn team.”

The probe is currently on its way to Ceres, the largest asteroid in the belt between Mars and Jupiter, and will get there in March 2015.

Interested in Vesta’s colorful history? Read this. Featured image credit: Once Dawn arrives at Ceres, it will spiral in (from blue to red) toward the asteroid’s surface and map it. Photo: NASA/JPL