New images from NASA’s Dawn mission are allowing scientists to peel back the layers of giant asteroid Vesta, revealing a world of varied surface compositions, sharp temperature changes and hints of layering beneath its surface. The results were presented today at the European Geosciences Union meeting in Vienna, Austria. 

These composite images from the framing camera aboard NASA’s Dawn spacecraft show three views of a terrain with ridges and grooves near Aquilia crater in the southern hemisphere of the giant asteroid Vesta. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Dawn is currently skimming the asteroid’s surface at an altitude of 210 kilometres, giving scientists their closest look yet at a planet embryo that formed early in the Solar System’s 4.6 billion year history, but which never grew into a fully-fledged planet.

“We’re very excited to see Vesta close up,” said Harald Hiesinger, a Dawn participating scientist at Münster University in Germany, during today’s dedicated press conference where the results were announced. “What we see now is just the tip of the iceberg. We’ll get to understand Vesta in much greater detail once we work through all the data.”

Colour-coded images of the surface taken by Dawn reveal stark contrasts in composition. Sites around impact craters show the largest range of colours, demonstrating the chaotic mixing of many different materials on and just below the surface during the impact event.

The colours correspond to iron content, with bright green indicating a high iron content and material that may once have been molten. Material along the edge of crater rims appears blue and represents a different, fresher material. It is also seen on the inside of crater walls where landslides have revealed material once hidden underneath. Streaks of fresh material have also been blasted out of craters in impact events to create ray patterns on the surface.

Elsewhere the red colours indicate that space weathering processes, such as the impact of micrometeorites and solar wind particles, have contaminated the surface materials and over time, slowly mixed it up.

“It’s like Vesta’s “skin” is constantly getting a facelift,” said Maria Cristina De Sanctis, lead of the visible and infrared mapping spectrometer team based at Italy’s National Institute for Astrophysics in Rome.

Dawn has also been taking Vesta’s temperature, finding extremes from -100 degrees Celsius in the shade to a relatively warm -23 degrees Celsius in exposed sunny spots. The variation means that the surface responds quickly to illumination.

Meanwhile, looking at how Vesta’s gravity tugs on the spacecraft’s orbit is revealing secrets of the asteroid’s internal structure. Dense material is seen at the site of the large Rheasilvia impact basin that dominates the asteroid’s south pole, suggesting that the impact blasted away lighter material on top. Around the basin rim the gravity readings are saying that there is low density material made from pulverized rock created in the impact.

As more gravity data is collected and processed, scientists will be able to infer if Vesta is made up of distinct layers like a planet, with a dense core at its heart. Finding out what lies beneath the surface will help scientists learn about how asteroids and planets first formed in the Solar System.

Dawn will soon begin to spiral away from Vesta, snapping images of the asteroid’s as yet unseen north pole as it comes into view. Scientists are keen to see if the force of the giant impact basin forming event at the south pole has caused any strange surface markings on the other side of the asteroid.

After leaving Vesta in late August Dawn will head to its second target, Ceres, for a 1 February 2015 rendezvous.