An image of Vesta taken by the Dawn spacecraft two days ago from a distance of 15,000 kilometres, when (as you probably know) it went into orbit around the asteroid. Dawn will slowly spiral inward, and will take many more images at closer range, but this is a great early look at the ravaged worldlet, the second largest body in the asteroid belt (bigger version here). We're looking down at the south pole, which about a billion years ago was hit by a large body. Some debris spalled off by the impact resurfaced Vesta; the rest, about 1% of Vesta's original mass, went into orbit around the sun. HED meteorites are part of this debris, so we already have samples of Vesta's crust. The big whack left behind a big crater. It's about 500 kilometres across, almost as wide as Vesta's mean diameter. The lump in the centre is an uplifted central peak; there are also huge cliffs, and ridges forming chevron-like features. Over at the Planetary Society blog, Emily Lakdawalla has posted a nice analysis, comparing the chevron features inside Vesta's south pole crater with those of Uranus's moon Miranda. Miranda's chevrons were probably formed by diapirs or plumes of upwelling warm ice; if the chevrons sit at the top of the plumes, those ridges may be the edges of uptilted blocks. Since we know that Vesta was once geologically active and almost certainly has an iron core that was once molten (all the HED meteorites are igneous material), it's tempting to speculate that big whack may have triggered some kind of residual geological activity. Could there be ancient volcanoes, on the opposite side?
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