On July 19, 2009, an object struck Jupiter, leaving a scar the size of the Pacific Ocean. This is not the first time that impacts on Jupiter have been observed. During the same week, 15 years earlier, pieces of Comet Shoemaker-Levy repeatedly hit the planet.
The elongated shape of the impact site suggests that the impact was from a relatively shallow angle.
By comparing images of the 1994 comet impact and the 2009 impact, astronomers noted distinct differences. In the 1994 impact sites, ultraviolet images from the Hubble spacecraft showed a distinct halo around the impact sites. This was interpreted as evidence of fine dust arising from the (dusty, icy) comet as it impacted the atmosphere of Jupiter. In contrast, the 2009 images show no halo, suggesting a lack of fine particles. The 2009 impact site disappeared much more rapidly than the 1994 site, and this has also been taken as evidence for a lack of fine particles. The implication is that the impact may have been by a solid, rocky asteroid rather than a dusty icy comet.
This study appeared in the June 1, 2009, issue of The Astrophysical Journal Letters.
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