Shaken on impact

A single recent impact may have modified the craters on the asteroid Eros into the pattern we see today. This finding has implications for how we view the structure of asteroids — and for addressing any hazards they present.

Asteroids seem to get stranger with every passing year. Thomas and Robinson's finding (page 366 of this issue)1 — that impact-induced vibrations of an asteroid may be the dominant mechanism reshaping its surface — shakes things up still further. In the case of the well-studied asteroid Eros, the authors link this resurfacing mechanism to the recent impact of a meteoroid that left a particularly large crater. They thereby make the first detailed mechanical connection between surface observations and an asteroid's global geology. The authors conclude that Eros, a rocky asteroid 33 by 13 by 13 kilometres in size, has a relatively homogeneous interior that transmits seismic shocks efficiently and is mantled by a hundred metres or more of regolith. (Regolith is the loose soil-like material familiar from pictures of the surface of the Moon.) This might not come as a surprise, given Eros's appearance2, but for the first time, the authors provide convincing evidence that makes this conclusion more than just reasonable conjecture.

Thomas and Robinson's discovery marks another stage in the journey asteroids have taken from insignificance, through notoriety, into the mainstream of scientific interest. The turning point came in the 1980s, when an asteroid was found to be responsible for the greatest calamity to befall Earth's biosphere since the Permian era — an impact 65 million years ago in present-day Mexico that is postulated, among other things, to have wiped out the dinosaurs. That got people's attention. But the geological subtleties of asteroids remained largely unappreciated for a further ten years. This situation began to change with the first detailed ground-based radar observations3, and the Galileo mission's fly-by of the asteroids Ida and Gaspra4. Now, a new generation of scientists is appreciating asteroids as geological entities2,5,6.

If Thomas and Robinson's hypothesis of seismic shaking1 is correct, then the cratering history of any asteroid is complex. Impacts of small meteoroids make the surface heavily cratered, giving it an ‘old’ look, whereas impacts of larger meteoroids — by causing the surface to vibrate — erase smaller craters, making the asteroid appear ‘young’. This asteroidal Botox calls into question the habit of dating asteroid surfaces through their cratering record: although the passage of time is indeed recorded here, so too is internal structure. A young asteroid of the type that resembles a rubble pile, for instance, is more capable of damping vibrations, and might retain more craters — and so appear older — than an ancient, ‘competent’ asteroid that has a more monolithic interior and thus transmits seismic energy more effectively. But Thomas and Robinson's work also opens up a new way of looking at asteroids generally. It shows how we might gauge interior structure from surface observations: craters and other landforms, and their degradation, could be used as proxies for seismic data.

The idea of seismic processes resurfacing asteroids is not itself new. The formation of the large crater Stickney on Phobos (Fig. 1), a martian moon about the size of Eros and perhaps a captured asteroid, was modelled7 12 years ago using a computational tool called a hydrocode to simulate the effect of the high-velocity impact. The simulation showed that seismic resurfacing could erase craters smaller than about 100 metres in diameter, and significantly degrade larger craters. The same method was later used to show8 that the jolting of the asteroid Gaspra by large impacts could lead to the unusual distribution of its crater sizes. In an argument analogous to that used by Thomas and Robinson for Eros, the asteroid Ida was suggested9 to have a relatively monolithic deep interior, given evidence that stress energy was transmitted from a large impact structure at one end to form a series of impact-induced fracture grooves at the other. And recently, the most detailed seismological model for asteroids so far was developed by James Richardson and colleagues10 at the University of Arizona to explain the lack of small craters on Eros.

Figure 1: Captured asteroid.


The pitted surface of the martian moon Phobos, about the size of Eros — the subject of Thomas and Robinson's study1 — and thought to be an asteroid caught by Mars's gravitational pull. The giant crater Stickney, 10 kilometres in diameter, is clearly visible at the top of the picture. Modelling of the impact that formed this crater7 led to the idea that seismic processes can resurface asteroids — a process now examined in detail1,11 that may help us to determine the mechanical structure of asteroids.

So the stage was already set for Thomas and Robinson. They seek1 to make specific correlations and so explain why certain areas of Eros are almost devoid of small craters, whereas other, often adjacent, areas are heavily cratered. They argue that these discontinuities in crater density (Fig. 2 of ref. 1, page 367) cannot be interpreted as patterns caused by debris thrown out during the formation of other, larger craters; nor are they easily explained by any continuum degradation process. They apply a model for seismic shaking in which the vibration of Eros's surface drops off as a simple function of distance from a single specific impact site. This admittedly naive approach provides a surprisingly good fit to the observed density of small craters, assuming that Eros is able to transmit stresses efficiently throughout its interior, and is covered in loose material about 100 metres in depth. Such assumptions are in agreement with earlier geological interpretations for Eros2,6 and also for Ida9, an asteroid quite similar in outward appearance.

These first steps in the ‘passive seismology’ of asteroids are particularly encouraging as we move rapidly towards a new era of space exploration. This era has in no small part been compelled by the discovery of many small bodies, asteroids and comets, collectively known as near-Earth objects or NEOs, that pass alarmingly close to us. Over the years this list is likely to be narrowed to a few dozen seriously hazardous asteroids. Still, it will certainly be useful to say more about these objects — which could potentially become as notorious as Vesuvius or Popocatépetl — than their equivalent destructive power in millions of megatonnes of explosive. A little knowledge could go a long way to ensure that disaster movies remain in the realm of science fiction.

There is a lot more to be done before surface observations can be used to provide definitive knowledge about asteroid interiors: direct data on interior structure, derived from radar or seismological observations, are almost certainly required to validate these ideas. Even now, as we sift through the debris from the encounter of the space-probe Deep Impact with comet Tempel 1 (ref. 11), one cannot help but think that this excellent experiment would be perfectly complemented by a couple of seismological geophones anchored to the surface of the comet. Those who recall the heyday of lunar spaceflight remember that we began learning about the Moon by crashing things into it, too.


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Asphaug, E. Shaken on impact. Nature 436, 335–336 (2005) doi:10.1038/436335a

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