Interpretations of recent infrared observations might be changing our view of the Moon. About 4.5 billion years ago, our Earth was utterly shattered-the victim of a giant impact with an object the size of Mars. The collision that was powerful enough to vaporize rock and throw a massive plume of Earth's mantle into space was not all bad, though. The impactor soon merged with the Earth giving it a fast spin, while chunks of Earth's mantle settled into a disk around our planet. Within a year or so, a large moon was formed out of this debris. The left-over rocks continued to circle around the sun over the next million years, occasionally colliding and creating a flow of dust, until it was all cleaned up by gravity and solar radiation.
Many scientists are interested in knowing how common such impacts are in other young solar systems because the heavy tidal mixing driven by the moon’s gravity may have played an important role in making conditions favorable for the origins of life on Earth. Recently Nadya Gorlova of the University of Florida and her colleagues at the Steward observatory in Tucson, Arizona and the European Southern Observatory in Santiago, Chile reported in The Astrophysical Journal that they may not be very common at all. Using the cryogenically-cooled infrared orbiting Spitzer Space Telescope, Gorlova and her colleagues surveyed the 30-million-year old star cluster NGC 2547. They selected this cluster because of its age. The planetary building process usually ends by approximately 50 million years, making the odds of a giant impact unlikely to occur outside this window.
The other advantage of NGC 2547 is that it is old enough for the material left out from the original cloud of which solar systems formed to dissipate (this takes about 3-10 million years). By focusing on radiation at a wavelength of about 8 microns, they could detect the heat they would expect from dust at a distance of about one astronomical unit (1 AU) from a solar-type star. The NGC 2547 cluster was previously surveyed spectroscopically, so they could cross-check to make sure that the emission they detected was not due to gas (which would be evident by spectral emission lines). Out of about 400 stars in the NGC 2547 cluster, they found only one that showed evidence of dust from a massive impact. From this they conclude that collisions like the one that gave rise to our moon don't happen in every system. This means that moons like ours may be rare. (The Astrophysical Journal, 20 November 2007)