Rheasilvia

• 1. Introduction

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  Rheasilvia is a wound in the surface of the asteroid Vesta so vast that it covers most of an entire hemisphere. Measuring 505 kilometers across, it spans roughly 90 percent of Vesta's own diameter. That comparison alone stops you cold: imagine a single scar nearly as wide as the world it sits on. At the center of this basin rises a mountain almost 200 kilometers in diameter at its base, ranking among the tallest peaks known anywhere in the Solar System. How did a collision of this scale happen, and what did it send flying into the rest of the solar system? Those are the questions that drew scientists to Vesta in the first place.

• 2. A Basin Unlike Any Other

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  At 75 degrees south latitude, Rheasilvia occupies most of Vesta's southern hemisphere. The crater floor lies about 13 kilometers below the surrounding surface, a depression deep enough to swallow the landscape. Along part of its perimeter runs an escarpment that rises sharply above the terrain outside it, marking the ancient boundary of the impact.

  Beneath the undulating floor of the basin, Rheasilvia partially buries an earlier crater called Veneneia, which stretches 395 kilometers across. Veneneia was already enormous before the younger, larger impact piled on top of it, obscuring its shape. The two overlapping scars tell a story of a southern hemisphere battered not once but at least twice by catastrophic collisions.

  The central mound that rises from the basin floor is one of the defining features of Rheasilvia. It is almost 200 kilometers in diameter at its base, and scientists think a planetary-scale impact could have driven rock upward to form it, in the same way that smaller craters on the Moon produce central peaks.

• 3. Cracks Across the Equator

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  The force that carved Rheasilvia did not stop at the crater rim. Vesta carries a series of troughs in its equatorial region, arranged concentrically around the impact site. Scientists interpret these as large-scale fractures that propagated through the asteroid's interior when the collision struck.

  The largest of these troughs is Divalia Fossae, which runs approximately 465 kilometers long and 22 kilometers wide. Divalia Fossae is, in other words, a crack in the crust of Vesta that stretches nearly as far as the crater itself is wide. The fact that such fractures reached all the way to the equator, thousands of kilometers from the impact point, gives some sense of the energy involved.

• 4. What the Crater Tells the Solar System

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  Spectroscopic analysis of Hubble Space Telescope images revealed that Rheasilvia punched through several distinct layers of Vesta's crust. The spectral signatures of olivine suggest the impact may have reached into the mantle itself, exposing material that had been locked deep inside the asteroid since the early solar system.

  Scientists estimate the impact excavated about one percent of Vesta's total volume. That ejected material had to go somewhere. The leading hypothesis is that the Vesta family of asteroids and the so-called V-type asteroids are the debris scattered by this single event. Known V-type asteroids account for about six percent of the calculated ejected volume. The remainder is thought to have been either too small to detect, or removed from the asteroid belt by the 3:1 Kirkwood gap, by the Yarkovsky effect, or, for the smallest fragments, by radiation pressure.

  The HED meteorites, a class found on Earth, are also thought to originate from this collision. The fact that ten-kilometer fragments from the impact have survived bombardment to the present day gives scientists a rough upper bound on the crater's age: it is estimated to be at most about one billion years old.

• 5. Discovery and Name

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  Rheasilvia first appeared in Hubble Space Telescope images in 1997, years before any spacecraft reached Vesta. Those early images were good enough to reveal that something enormous occupied the southern hemisphere, but the crater had no official name yet.

  The Dawn spacecraft arrived at Vesta in 2011 and confirmed what the Hubble data had suggested. The International Astronomical Union approved the name Rheasilvia on the 30th of September 2011. The name honors Rhea Silvia, a figure from Roman mythology: a vestal virgin who became the mother of Romulus and Remus, the legendary founders of Rome. The choice connected the crater to the naming tradition already established for Vesta, which itself takes its name from the Roman goddess of the hearth whose virgin priestesses, the Vestals, held a central place in Roman religion. Rhea Silvia was counted among those priestesses in myth, making the name a precise fit for a feature on a world already tied to that tradition.