Bill DunfordJun 25, 2013

Worlds in Collision

When he saw the first atomic bomb explosion turn darkness into false daylight, Robert Oppenheimer famously thought of a phrase from the Hindu scripture The Bhagavad Gita: "I am become Death / The shatterer of Worlds."  It turns out that in the Solar System, sometimes worlds really are shattered, literally blown to bits.

The rings of Saturn are thought to be the glittering remains of a moon that was either blasted by a collision or torn apart when it drifted too close to the giant planet. The most prevalent theory of our moon's formation holds that a world the size of Mars once collided with the early Earth, shearing off material that later coalesced into the moon we see in tonight's sky.

How do we know for sure that such cataclysms are even possible? Because in the past few years we have explored several worlds up close that bear the scars of huge impacts, collisions so powerful they very nearly broke these worlds apart. Here are some views of two places that came just shy of being shattered when objects slammed into them.

The first is Mimas, a tiny ice moon of Saturn. Mimas is famous for looking like a twin of the Death Star battle station. ("That's no moon...oh wait, yes it is.") But instead of detroying planets, it's Mimas itself that was almost blown away. Its signature feature is Herschel Crater, about 140 kilometers (88 miles) wide. Its walls tower 5 kilometers (3 miles) high, and the floor reaches as far as 10 kilometers (6 miles) deep. The asteroid or passing comet that struck Mimas not only carved out Herschel, which is a third as wide as the entire moon, it also sent shock waves through its icy interior, opening fractures on the opposite hemisphere.

If the impactor were just a little bigger--or maybe moving a little faster--no more Mimas. Saturn might have had an extra ring instead.

Cassini views of Mimas
Cassini views of Mimas Saturn's moon Mimas, as seen by the robotic spacecraft Cassini. The crater Herschel makes a striking impression in the top image. At middle, Mimas drifts in front of Saturn's rings, and Herschel is seen from the side as the flat area on the left side of Mimas. Below, a detailed look at the layers exposed in Herschel's cliffsides.Image: NASA / JPL-Caltech / SSI

Another nearly broken world is Vesta, second largest of all the asteroids, more protoplanet than space rock. Vesta received its first ever explorer from Earth when the Dawn spacecraft arrived in 2011, mapping its surface in detail for the first time. The largest landmark on Vesta is the Rheasilvia impact basin, near the south pole, named after one of the mythical Vestal Virgins. Rheasilvia boasts some frightening stats: cliffs 15 to 20 kilometers high, and at 500 kilometers (310 miles) wide, it's nearly the same width as Vesta itself! The awesome energy unleashed by that collision left huge ripple marks in the rock around Vesta's equator.

The impact carved out so much of the asteroid's material that some of it escaped Vesta's gravity and eventually found its way to Earth. In fact, about one in 16 meteorites found on the ground here are actually little pieces of Vesta, hand-held souvenirs of almost unimaginable violence in the asteroid belt.

Rheasilvia The asteroid Vesta as seen by the Dawn spacecraft. What could have caused the ripples around its equator seen in the top image? Possibly the massive impact that carved out Rheasilvia Basin at the south pole, shown in the middle image. Finally, an elevation map showing the feature's extreme topography.Image: NASA/JPL/UCLA/MPS/DLR/IDA/PSI

Looking at the night sky, you might be tempted to think of the Solar System as a dark and silent place. But obviously that's not the whole story. It's way more complicated and interesting than that.

The Bhagavad Gita is right when it says: "The worlds are filled with wonder."

The Time is Now.

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