Comet Shows Evidence of Space Weathering

aerogel track
Image of aerogel sample with track #170. Credit: Bridges/University of Leicester

The traditional picture of comets as cold, icy, unchanging bodies throughout their history is being reappraised in the light of analyzes of dust grains from Comet Wild 2. A team led by the University of Leicester has detected the presence of iron in a dust grain, evidence of space weathering that could explain the rusty reddish color of Wild 2’s outer surface. The results were presented by John Bridges at the National Astronomy Meeting in Manchester on Tuesday 27th March.

The Wild 2 grains were collected by the NASA Stardust mission and returned to Earth in 2006. The fast-moving dust grains were collected in arrays of aerogel, a silicon-based foam that is 99 percent empty space, which slowed the particles from velocities of 6 kilometers a second to a halt over just a few millimeters. Since then, an international team of scientists has been analyzing the samples and the carrot-shaped tracks that they left in the aerogel. Microscopic samples dissected from the grains have been analyzed at facilities around the U.K., and in particular this work was performed at the Diamond Light Source synchrotron in Oxfordshire and Leicester University. Through a range of analytical techniques, scientists in the U.K. have been able to fully analyze the mineralogy and isotopes of the samples.

Aerogel panel used to capture comet particles. Credit: JPL/Tsou

“The total mass of Comet Wild 2 grains returned is less than a milligram, so these samples are incredibly precious and a considerable analytical challenge,” said Bridges, of the University of Leicester.

The analysis from the Microfocus Spectroscopy beamline at the Diamond synchrotron shows that the surface of Comet Wild 2 has been bombarded by particles in the solar wind and micrometeorites throughout its 4.5 billion year history. This space weathering has deposited nanometer-size grains of iron metal and reddened the surface of the comet.

“This is the first mineralogical evidence for space weathering that has been identified in the Wild 2 samples that was hinted at by other spectroscopic observations of the comet,” said Bridges. “It adds another piece of the puzzle to our understanding of the life history of comets.”

Studying the composition of comets is important for astrobiologists because some theories suggest that these objects of space could have impacted the early Earth and delivered material essential for the origins of life.