Wednesday, October 30, 2013

Shockwaves from the Rheasilvia Impact

Vesta is one of the largest asteroids in the Solar System, measuring 573 km by 557 km by 446 km in size. It is a member of the main asteroid belt and it circles the Sun between the orbits of Mars and Jupiter. From July 2011 to September 2012, NASA’s Dawn spacecraft was in orbit around Vesta and the spacecraft conducted numerous observations of this large asteroid. Centred on the south pole of Vesta is a large impact feature known as the Rheasilvia basin. The basin has a depth of ~20 km and a diameter of ~500 km, nearly as large as Vesta itself.

 Figure 1: This topographic map from NASA’s Dawn spacecraft shows the two large impact basins in the southern hemisphere of Vesta. The map is colour-coded by elevation, with red showing the higher areas and blue showing the lower areas. Rheasilvia, the largest impact basin on Vesta, is ~500 km in diameter. It is estimated to have formed no more than ~1 billion years ago by counting the number of smaller craters that have formed on top of it. The other basin, Veneneia, is ~400 km in diameter and it lies partially beneath Rheasilvia. Veneneia is estimated to be at least ~2 billion years old. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/PSI.

Figure 2: This image obtained by the framing camera on NASA’s Dawn spacecraft shows the south pole of the giant asteroid Vesta. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.

Observations by NASA’s Dawn spacecraft suggests that the impact which excavated the Rheasilvia basin may have been sufficiently large to create disrupted terrains at the impact antipode, which is the area on Vesta opposite to the point of impact. Compared to the age of the Solar System, the Rheasilvia basin is relatively young, estimated to be no more than ~1 billion years old. Modelling work performed by Bowling et al. (2013) show that the degree of antipodal deformation is very sensitive to the mantle porosity and core strength of Vesta.

In the “control” model with zero mantle porosity and a strong rock-like core, the shockwaves from the Rheasilvia impact passes through the mantle and core of Vesta with little attenuation. The shockwaves eventually converge around the antipode with sufficient energy to uplift enough material to create a ~6 km tall antipodal peak. More realistically, the presence of mantle porosity and/or a weaker core would result in a smaller degree of antipodal deformation. In fact, the models show that unless the mantle porosity is relatively low and the core is relatively strong, no antipodal deformation would occur.

Figure 3: Modelled antipodal topography 1500 seconds after the Rheasilvia impact. All simulations in this series were run with a strong, rock-like core. (T. J. Bowling et al., 2013)

Topographic maps of Vesta’s north pole, acquired by NASA’s Dawn spacecraft, show an area near the impact antipode that is ~5 to 10 km higher than the surrounding plains. However, the antipodal point itself lies within a ~63 km diameter crater named Pomponia. Pomponia is believed to have formed more recently than the Rheasilvia basin and its formation would have obliterated much of the predicted antipodal topographic uplift from the Rheasilvia impact. Additionally, a ~90 km diameter crater named Albana lies right next to Pomponia.

Figure 4: Topography at the north pole of Vesta. The white dot represents the approximate location of the impact antipode corresponding to the Rheasilvia basin. The region marked ‘2’ indicates the area in which a crater size frequency distribution was produced. (T. J. Bowling et al., 2013)

If the ~5 to 10 km elevated area near the impact antipode is a product of the Rheasilvia impact, it would suggest that Vesta has a low mantle porosity and a core of considerable strength. Unfortunately, the presence of the craters Pomponia and Albana make it difficult to determine what portion of the elevated area is a product of topographic uplift from the Rheasilvia impact and what portion is due to more recent impacts.

Nevertheless, a study of the crater size frequency distribution in an area near the impact antipode shows a deficiency of smaller craters with diameters between 3 km to 9 km. In comparison, craters with diameters larger than ~10 km are as common around the impact antipode as elsewhere on Vesta. The deficiency of smaller craters is evidence that some degree of antipodal deformation from the Rheasilvia impact did occur. This is because the powerful converging shockwaves from the Rheasilvia impact around the antipodal point would have erased small craters more effectively than larger ones.

The very presence of antipodal deformations from the Rheasilvia impact indicates that Vesta must have relatively low mantle porosity and a relatively strong core. This study by Bowling et al. (2013) show that the features observed at the antipode of the Rheasilvia impact can serve as a crude method of constraining the internal structural properties of Vesta. Finally, this method may also be used to constrain the internal properties of some other objects in the Solar System that have craters large enough to have perhaps produced deformation features at their antipodes. An example of one such object is Saturn’s icy moon Mimas with its relatively large crater named Herschel.

Figure 5: An image of Saturn’s moon Mimas taken by the Cassini spacecraft on 13 February 2010. The large crater on the left is Herschel. In the background is the enormous bulk of the planet Saturn. With a diameter of 396 km, Mimas is thought to be about the smallest an object can be and still crunch itself into a near-spherical shape. Credit: NASA/JPL/Space Science Institute.

T. J. Bowling et al., “Antipodal terrains created by the Rheasilvia basin forming impact on asteroid 4 Vesta”, Journal of Geophysical Research: Planets, Volume 118, Issue 9, pages 1821-1834, September 2013