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Spitzer sees
planetary pile-up


Posted: August 11, 2009

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NASA's Spitzer Space Telescope has found evidence of a high-speed collision between a Mercury-sized planet and a Moon-sized body within the last few thousand years.

The smash annihilated the smaller body, vaporizing rock and flinging burning lava into space. Using its infrared detectors, Spitzer was able to recognise the signatures of the vaporized rock and seek out pieces of the lava that had subsequently refrozen into what are known as tektites.

This artist's concept shows molten debris being flung from two rocky bodies as they collide head on. Image: NASA/JPL-Caltech.

"This collision had to be huge and incredibly high-speed for rock to have been vaporized and melted," says Carey Lisse of the Johns Hopkins University Applied Physics Laboratory and lead author of a paper that features in the 20 August issue of the Astrophysical Journal describing the findings. "This is a really rare and short-lived event, critical in the formation of Earth-like planets and moons. We're lucky to have witnessed one not long after it happened."

Scientists are likening the planetary pile-up to the collision between a Mars-sized object with the nascent Earth 4.6 billion years ago that spawned our Moon. "The collision that formed our Moon would have been tremendous, enough to melt the surface of Earth," says co-author Geoff Bryden of NASA's Jet Propulsion Laboratory. "Debris from the collision most likely settled into a disc around Earth that eventually coalesced to make the Moon. This is about the same scale of impact we're seeing with Spitzer – we don't know if a moon will form or not, but we know a large rocky body's surface was red hot, warped and melted."

Our Solar System's history is defined by similar tales of destruction, and no planet is exempt. From giant collisions that are thought to have stripped Mercury of its outer crust and Mars of its northern hemisphere – leaving a striking dichotomy between the northern lowlands and southern highlands – more slow moving strikes are thought to have tipped Uranus on its side and sent Venus spinning backward relative to the rest of the Solar System family. Although such drama is expected in the early years of solar system formation, and is a crucial aspect of planet formation, impact events still occur today, as demonstrated by the recent impact into Jupiter.

Click image for animation of what this cosmic collision may have looked like. A huge flash of light would have been emitted as the two bodies struck each other, and by the end, most of the smaller body is consumed by the larger one. Credit: NASA/JPL-Caltech.

The new collision was observed around a 12 million year old star known as HD 172555 and located in the southern constellation Pavo. The curious spectrum of the star attained by Spitzer caught the observers' attention. "I had never seen anything like this before," says Lisse. "The spectrum was very unusual."

The spectrum was filled with the fingerprints of melted glass – amorphous silica – which on Earth is found in volcanic rocks and in tektites that form when meteorites strike the Earth. Spitzer also picked up the signature of silicon monoxide gas, created when the rocky component of the planet was vaporized, along with the rocky debris itself that would have been flung into space as shock waves from the collision rippled through the planet.

The rocky mass that Spitzer detected implies that the combined mass of the two colliding bodies was more than twice that of our Moon, and careered into each other at a velocity of at least 10 kilometres per second. Although Spitzer has observed the aftermath of cosmic collisions before, none have exhibited such violence as this. "Almost all large impacts are like stately, slow-moving Titanic-versus-the-iceberg collisions, whereas this one must have been a huge fiery blast, over in the blink of an eye and full of fury," adds Lisse.

The data collected at the HD 172555 crime scene was made in 2004 before Spitzer began its 'warm' mission .

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