Click on the image for movie
This artist's concept shows a celestial body about the size of our moon
slamming at great speed into a body the size of Mercury. NASA's Spitzer
Space Telescope found evidence that a high-speed collision of this sort
occurred a few thousand years ago around a young star, called HD 172555,
still in the early stages of planet formation. The star is about 100
light-years from Earth.
Spitzer detected the signatures of vaporized and melted rock, in addition
to rubble, all flung out from the giant impact.
The animation begins by showing the intact moon-sized body charging toward
the larger orb. Evidence from Spitzer shows that these two bodies must
have been traveling at a velocity relative to each other of at least 10
kilometers per second (about 22,400 miles per hour). Time has been speed up
in this artist's depiction.
As the bodies slam into each other, a huge flash of light is emitted.
Rocky surfaces are vaporized and melted, and hot matter is sprayed
everywhere. Spitzer detected the vaporized rock in the form of silicon
monoxide gas, and the melted rock as a glassy substance called obsidian.
On Earth, silica can be found around volcanoes in black glassy rocks
called obsidian, and around meteor craters in small rocks called tektites.
Shock waves from the collision can be seen traveling through the planet,
throwing rocky rubble into space. Spitzer also detected the signatures of
this rubble.
The final result is a skinned planet, stripped of its outer layers. The
core of the smaller body and most of its surface have been absorbed by the
larger one. This merging of rocky bodies is how planets like Earth are
thought to form.
Astronomers say a similar type of event stripped Mercury of its crust
early on in the formation of our solar system, flinging the removed
material away from Mercury, out into space and into the sun. Our moon was
also formed by this type of high-speed impact: a body the size of Mars is
thought to have slammed into a young Earth about 30 to 100 million years
after the sun formed (the sun is now 4.5 billion years old). According to
this theory, the resulting molten rock, vapor and shattered debris mixed
with debris from Earth to form a ring around our planet. Over time, this
debris coalesced to make the moon.