Pebbles in comets

Comet Hartley 2’s orbit extends from Earth’s orbit to just beyond Jupiter’s. In November 2010, when Hartley 2 neared perihelion, NASA’s EPOXI spacecraft flew within 700 km of the 2-km-long comet, shown here. Among the mission’s findings was a surprise: The gas and dust spewing from Hartley 2 contained gravitationally bound pebbles. Erosion could conceivably account for the pebbles, but Katherine Kretke and Harold Levison of the Southwest Research Institute in Boulder, Colorado, have proposed a different explanation. Although comets are thought to form through the stepwise agglomeration of ever-bigger pieces, a competing hypothesis has emerged: The collapse of a cloud of dust and larger particles through gravitational instability. Kretke and Levison asked themselves whether the pebbles that EPOXI detected could be the uncoalesced leftovers of a putative collapse. To answer that question, they created a family of models of the Sun’s protoplanetary disk. They then identified the zones in the disks where orbiting particles are susceptible to the so-called streaming instability that triggers collapse. Because EPOXI’s camera couldn’t resolve the pebbles, Kretke and Levison assumed a range of sizes dependent on the likely range of albedo. For reasonable albedo assumptions, it turned out that most of the observed pebbles matched predictions for the size of pebbles expected within the Sun’s protoplanetary disk. The researchers’ analysis also suggested that Hartley 2 formed close to the Sun, a finding consistent with the comet’s chemical composition. (K. A. Kretke, H. F. Levison, Icarus 262, 9, 2015, doi:10.1016/j.icarus.2015.08.017 . See the related story on page 16 .)