AN ASTEROID that is the source of an annual meteor shower may owe its weird crumbliness to intense cooking by the sun.
Most meteor showers are thought to come from comets, whose icy surfaces vaporise easily during close encounters with the sun. Dust that is liberated in the process burns up in Earth's atmosphere, creating "shooting stars".
However, the debris stream responsible for the annual Geminid shower in December follows the orbit of a 5-kilometre-wide object called 3200 Phaethon, which appears to be an asteroid.
So how does the rocky body cast off so much material without any ice to vaporise? A clue came in June 2009, when NASA's STEREO-A spacecraft watched the asteroid double in brightness at its closest point to the sun, which lies just 14 per cent of Earth's distance from the star.
Now David Jewitt and Jing Li, both at the University of California, Los Angeles, say the asteroid's sunward face should have reached a searing 480 to 780 °C at that point. That is hot enough to make its rocks expand and crack, generating dust that reflected sunlight and caused the asteroid to brighten (The Astronomical Journal, in press).
A second effect might also contribute to Phaethon's shedding. Although the asteroid appears to be devoid of ice, some water molecules may be chemically bound within some of its rocks. If a mineral called serpentinite is present, for example, it would break down at 630 °C and release its chemically bound water as vapour.
The process can be violent, according to a 2009 experiment by Jay Melosh of Purdue University in West Lafayette, Indiana, and colleagues. "The fragments of serpentinite actually popped off the surface at high speed, presumably driven by steam exploding from the dehydrating minerals," Melosh says.
The breakdown of minerals like serpentinite could cause the asteroid to shed debris, he says.
"Witnessing mass loss from Phaethon is a great advance," adds Yan Fernández of the University of Central Florida in Orlando, who has studied Phaethon but was not involved in the new study. "It'll be great if someone can find more of these outburst events."
Source: New Scientist [September 24, 2010]
