Can some of the ageing effects on asteroid surfaces be caused by an interplanetary rain of carbon-rich Solar System debris? Observations from the Dawn space mission suggest that the answer is yes. See Letters p.79 & p.83
One might expect the asteroids of our Solar System to show their age in similar ways. After all, asteroids are simply rocks that orbit the Sun through interplanetary space, and they are all subject to the same ageing processes: solar wind, micrometeorite bombardment and the occasional major impact. But asteroids do not all age in the same way, claim McCord et al.1 and Pieters et al.2 in this issue. Their analyses of observations of asteroid Vesta, obtained by the Dawn space mission, indicate that the asteroid's surface is not coloured with age in the same way as other bodies that, like Vesta, lack an atmosphere. Rather, Vesta shows its age by incorporating carbon-rich material from impactors (Fig. 1).
Perhaps it is not too surprising that carbonaceous material of external origin (exogenic material) is found on Vesta, one of the largest bodies in the main asteroid belt that lies between the orbits of Mars and Jupiter. Extraterrestrial spherules and micrometeorites found in Earth's stratosphere, which is just above the lowest portion of the atmosphere, have long been known to be compositionally related to carbonaceous chondrite meteorites3. (Chondrite meteorites contain spherules of igneous material thought to have originated in the primitive solar nebula, from which the Sun and planets formed.) However, it is surprising that the material is abundant enough to change the remotely sensed optical properties of the Vestan surface.
McCord et al. present compelling evidence based on Vesta's colour and brightness, as well as modelling of the putative population of impactors, to support their hypothesis that Vesta's surface is contaminated with carbonaceous material that is rich in volatile elements. Pieters et al. describe an analysis of the spectral differences between fresh and mature Vestan surface patches, and suggest that maturation is linked with contamination — the older the Vestan surface patch, the greater the abundance of carbonaceous exogenic material incorporated. Thus, it seems that, on Vesta, dark, carbon-rich impactor material falls to the surface and darkens the location of the impact. Due to subsequent impacts, the dark material then spreads out over time and mixes with uncontaminated surface areas.
But where does this material come from, and does it coat other asteroids in the main belt? The material almost certainly comes from dark asteroids in the main belt, because asteroids can be ground down to micrometeorite particles by mutual collisions over billions of years. The idea that similar exogenic material coats other large asteroids has yet to be tested. However, perplexing spectral signatures of hydrated minerals, consistent with volatile-rich carbonaceous materials, have been found on objects that would otherwise be interpreted as metallic4,5 or inconsistent with the presence of stable volatile compounds6. In fact, almost a decade ago, astronomers using ground-based telescopes detected evidence of hydrated minerals on Vesta itself7. At the time, those measurements were considered suspect, but they now seem to be vindicated by the Dawn results.
Are the carbonaceous materials found on Vesta the result of low-velocity impactors? The fate of a rocky impactor depends in part on its velocity relative to the speed of sound in the rock. Carbonaceous materials have relatively high porosities8. Therefore, sound waves travel through them slowly and impact-associated shock-wave pressures may be quite low, favouring the survival of large fractions of such impactors. Without high shock-wave pressures, much less material might be vaporized and lost to space9. McCord et al. present estimates of impactor fluxes and of the amount of impactor material that Vesta has accumulated (see Supplementary Information to the paper1). On the basis of these estimates, they conclude that sufficient material has been deposited on the Vestan surface to cover it with a blanket up to about 1–2 metres deep.
The delivery of exogenic material is not generally what comes to mind when considering space weathering. The term is used to refer to processes that change the optical properties of the remotely sensed surface of an airless body. Studies of lunar soils and rocks brought back by the Apollo-mission astronauts have provided important information about space weathering on the Moon. Furthermore, direct evidence of space weathering on asteroids was supplied by the NEAR mission to the near-Earth asteroid 433 Eros10 and by the Hayabusa mission to asteroid 25143 Itokawa11.
Before the Dawn-mission findings, the consensus was that some lunar-like space weathering occurs on asteroids12,13,14, and that its strength depends on the composition of the target material that is, the material from which the asteroid is made. In the leading model of asteroidal space weathering, condensates bearing submicroscopic iron are deposited on grain surfaces after the target material has been vaporized by solar-wind sputtering and micrometeorite bombardment. Space weathering is known to cause surface darkening and spectral changes, and so these processes and their effects must be considered when interpreting the spectral properties of airless bodies. According to Pieters and colleagues, two other processes should now be considered when trying to explain the Dawn observations of Vesta: the mobility of regolith (powdery rubble that covers a planetary body) and fine-scale mixing of surface material. Therefore, the results prompt two questions. Why does Vesta not exhibit lunar-like space weathering? And was our model wrong, or do the weathering processes compete with each other?
The goal of the Dawn mission is to characterize the conditions and processes that were active during the Solar System's earliest epoch by investigating Vesta and Ceres, two of the largest asteroids that are still intact. Dawn has completed its tour of Vesta and will arrive at Ceres in February 2015 to send back data on that asteroid's low-brightness surface. It will be interesting to see whether these observations allow us to distinguish Ceres' bulk material from the 'rain' of carbonaceous material that may be contaminating its surface.
McCord, T. B. et al. Nature 491, 83–86 (2012).
Pieters, C. M. et al. Nature 491, 79–82 (2012).
Brownlee, D. E., Bates, B. & Schramm, L. Meteorit. Planet. Sci. 32, 157–175 (1997).
Ockert-Bell, M. E. et al. Icarus 210, 674–692 (2010).
Rivkin, A. S., Howell, E. S., Lebofsky, L. A., Clark, B. E. & Britt, D. T. Icarus 145, 351–368 (2000).
Rivkin, A. S., Davies, J. K., Clark, B. E., Trilling, D. E. & Brown, R. H. Lunar Planet. Sci. Conf. XXXII, 1723 (2001).
Hasegawa, S. et al. Geophys. Res. Lett. 30, 2123 (2003).
Britt, D. T. & Consolmagno, G. J. Icarus 146, 213–219 (2000).
Rivkin, A. & Bottke, W. Lunar Planet. Sci. Conf. XXVII, 1077–1078 (1996).
Clark, B. E., Hapke, B., Pieters, C. & Britt, D. in Asteroids III (eds Bottke, W. F. Jr, Cellino, A., Paolicchi, P. & Binzel, R. P.) 585–599 (Univ. Arizona Press, 2002).
Noguchi, T. et al. Science 333, 1121–1125 (2011).
Hapke, B. J. Geophys. Res. 106, 10039–10073 (2001).
Pieters, C. M. et al. Meteorit. Planet. Sci. 35, 1101–1107 (2000).
Sasaki, S., Nakamura, K., Hamabe, Y., Kurahashi, E. & Hiroi, T. Nature 410, 555–557 (2001).