Vesta, more than an asteroid. Indeed, almost a planet

Even though it doesn’t quite qualify as a ‘proper’ planet, the second most massive asteroid in the Solar System, Vesta – which has a diameter of approximately 530 kilometres – exhibits numerous planetary characteristics. This is just one of the many significant results of NASA’s Dawn mission, published in the journal Science on 11 May 2012. The Dawn spacecraft has been orbiting Vesta since 16 July 2011.

Three impact craters of different sizes, arranged in the shape of a snowman, make up one of the most striking features on Vesta. In this view the three 'snowballs' are upside down, so that the shadows make the features easily recognisable. North is to the lower right in the image, which has a resolution of 70 metres per pixel. The image is composed of many individual photographs taken during the high-altitude mapping orbit, at about 680 kilometres above Vesta's surface. The largest of the three craters, Marcia, has a diameter of 60 kilometres. The central crater, which is about 50 kilometres in diameter, is named Calpurnia, and the lower crater, named Minucia, has a diameter of about 22 kilometres. Marcia and Calpurnia are possibly the result of an impact by doublet asteroids, whereas Minucia was formed by a later impact. To derive the colour information, images acquired by the German camera system on the Dawn spacecraft in two near-infrared channels (917 nanometres and 749 nanometres) and an ultraviolet channel (438 nanometres) were combined to create what is referred to as a pseudo-true colour image. The true colours of the surface of Vesta appear somewhat different, but the subtle changes in material properties across the craters and impact ejecta can be detected. In both Marcia and Calpurnia, landslides can be seen; also, dark material has been exposed below the rim of Marcia. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

“Vesta is more like the Moon than other asteroids,” explains Ralf Jaumann, from the DLR Institute of Planetary Research in Berlin-Adlershof. “Its internal structure, range of geological surface features, heterogeneous composition and, above all, the alteration of the surface due to material mass movement suggest a dynamic, long lasting, planet-like evolution.” Jaumann heads the Dawn science team at DLR. The planetary geologist is one of the principal authors of a series of articles being published in Science by the US/German/Italian research group.

"One completely unexpected discovery has been a number of troughs several hundred kilometres long that sometimes run parallel to the equator and sometimes at an angle to it, as if carved with a gigantic plough," says Jaumann. "Then, beneath the already suspected huge impact basin near the south pole, we also discovered traces of an equally large but even older collision." The researchers working with Jaumann are certain that these two mega impacts are the cause of the troughs on Vesta's equator. In the midst of the larger basin, there is a central massif over 20 kilometres high.

"It is probable that Vesta was once even bigger than it is today," explains Chris Russell, Principal Investigator of the Dawn mission, from the University of California in Los Angeles. “Enormous fragments were blasted off this protoplanet by collisions. Nevertheless, Vesta was big enough to ‘differentiate’ – that is, to form a metallic core surrounded by a rocky mantle.” This had been suspected previously and has now been confirmed by mineralogical analyses of the surface of Vesta. Observations with the specialised German camera and measurements taken with the US/Italian spectrometers on board Dawn indicate the same composition as that of rare meteorites found on Earth.

‘HED’ meteorites – named after the initials of three classes of rocky meteorite, Howardites, Eucrites and Diogenites – also originate from a differentiated asteroid mother body and have had a ‘hot’ past, meaning that they were at least partly molten when they formed. Two large bodies impacted with Vesta in its south hemisphere, blasting off many thousands of cubic kilometres of rock fragments. Referred to as Vestoids, these now follow Vesta’s orbital path. “Two impact basins that almost completely overlap were left behind and are now named after priestesses of the Roman goddess Vesta – Rheasilvia and Veneneia,” explains Russell. “Fragments of Vesta and the Vestoids were flung into space as HED meteorites, and pieces landed on the Earth.”

The fact that Vesta had been heavily impacted twice became apparent in the topographical maps generated by the DLR researchers. “Vesta has had to endure a lot in its history,” says Jaumann. “Veneneia, the older basin, is almost as large as Rheasilvia and has a diameter of 400 kilometres. The greatly disturbed topography and extremely steep mountain and crater walls show that the asteroid consists of solid rock beneath its topmost layer of dust.”

The gigantic impacts shocked Vesta to the core. Several dozen enormous troughs running along the equator are proof of these asteroid-quakes. “We could establish a significant geometric connection to the centres of the impact basins Rheasilvia and Veneneia. The simultaneous formation of basins several hundred kilometres wide and global structures shows that all of Vesta was shocked to breaking point by the cosmic collisions,” says Ralf Jaumann about the analysis of the results.

But Vesta still holds a few mysteries; no structures clearly indicating volcanism have been identified on the surface, even though it is expected in theory. “The reason for this may be that the surface is covered by a thick layer of debris and dust – the regolith – that has been gradually generated by meteorite impacts, covering up traces of earlier volcanism,” Jaumann suggests. Some areas with conspicuously dark material might in fact indicate volcanism, but this might also be a carbon-rich substance carried there by comets or asteroids.

Source: 

• Vesta – a planet-like asteroid, DLR, May 10, 2012