Sound Science Bite: October 25. The Gentle (?) Birth of Comets
The early solar system was a chaotic place with numerous collisions between bodies of all sizes, including those as large as planets. Comets, which formed far from the Sun in the cold were supposed to have taken part in the chaos. However, the recently concluded Rosetta mission, as reported in Science 30 Sep 2016 and other places, appears to tell a different story. The comet 67P ("P" for "periodic"), a comet that was somehow redirected into the inner solar system from its location in the Kuiper Belt beyond the orbit of Neptune, possibly by a close encounter with another object, and "captured" into a new orbit by the effect of Jupiter's gravity, was the last subject of study for this spacecraft.
The comet sports a dumbell shape, which appears to be the result of a gentle collision between two separate comets, not a bone-jarring one. Further, this comet, rather than being a "dirty snowball", as comets have been described, is rather more like an icy dirtball. The "dirt" eroded off the comet by solar radiation and detected by the spacecraft is extremely fragile and porous. A picture is emerging of a comet that gradually formed by the gentle accumulation of pebble-sized material. If so this has profound implications for theories of solar system formation. However, so-called spherically shaped "goosebumps" three meter across found in pits formed by jets of material spewing out of the comet due to heating by the Sun are thought by other researchers to be the actual building blocks of the comet. Maybe the question will be answered as the Rosetta data is examined in the months ahead.