When Worlds Collide

Jupiter’s getting slammed, Venus is getting slammed, now two innocent planets in the HD 172555 system apparently slammed into each other. From a JPL press release:

Astronomers say that two rocky bodies, one as least as big as our moon and the other at least as big as Mercury, slammed into each other within the last few thousand years or so — not long ago by cosmic standards. The impact destroyed the smaller body, vaporizing huge amounts of rock and flinging massive plumes of hot lava into space.

Spitzer’s infrared detectors were able to pick up the signatures of the vaporized rock, along with pieces of refrozen lava, called tektites.

“This collision had to be huge and incredibly high-speed for rock to have been vaporized and melted,” said Carey M. Lisse of the Johns Hopkins University Applied Physics Laboratory, Laurel, Md., lead author of a new paper describing the findings in the Aug. 20 issue of the Astrophysical Journal. “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.”

Lisse and his colleagues say the cosmic crash is similar to the one that formed our moon more than 4 billion years ago, when a body the size of Mars rammed into Earth.

…

The astronomers used an instrument on Spitzer, called a spectrograph, to break apart the star’s light and look for fingerprints of chemicals, in what is called a spectrum. What they found was very strange. “I had never seen anything like this before,” said Lisse. “The spectrum was very unusual.”

After careful analysis, the researchers identified lots of amorphous silica, or essentially melted glass. Silica can be found on Earth in obsidian rocks and tektites. Obsidian is black, shiny volcanic glass. Tektites are hardened chunks of lava that are thought to form when meteorites hit Earth.

Large quantities of orbiting silicon monoxide gas were also detected, created when much of the rock was vaporized. In addition, the astronomers found rocky rubble that was probably flung out from the planetary wreck.

The mass of the dust and gas observed suggests the combined mass of the two charging bodies was more than twice that of our moon.

Their speed must have been tremendous as well — the two bodies would have to have been traveling at a velocity relative to each other of at least 10 kilometers per second (about 22,400 miles per hour) before the collision.

An animation of the collision is available here on YouTube (easier than the 25 meg quicktime linked on the JPL site). And here’s another video, with an astronomer describing what happened and what they saw. Note however that the astronomer here is dead wrong about at least one fact. He says that the animation has been slowed down to allow you to see the shockwave propogate across the larger planet. But it’s just the opposite. The smaller planet was said to be about the size of the moon, about 2000 miles in diameter, about 3200 km IIRC. And the impact velocity was said to be about 10 km per second. Consequently, rather than the quick smack shown in the video, it would take at least 320 seconds for the “moon” to cross it’s own diameter while plowing into the planet. Rather than an impact taking less than a second, it would take five and a third minutes.

Events on a planetary scale are a lot slower than Hollywood would suggest.