Wednesday 9 April 2014

The nature and origin of the July 2009 Jovian Impactor.

In 2009 the remains of the comet Shoemaker-Levy 9 were observed impacting the Jovian atmosphere, the first time a body had been directly observed colliding with a planet other than the  Earth, and the first time a comet had ever been seen impacting a planet. At the time this was thought to be an extremely rare event, possibly happening as infrequently as once every 500 years. However in July 2009 a scar similar to that caused by the Shoemaker-Levy 9 impact was observed in the southern hemisphere of Jupiter, suggesting that a similar impact had occurred once again.

In a paper published in the Astrophysics Journal Letters on 12 May 2010 and on the arXiv database at Cornell University Library on 13 May 2010, a team of scientists led by Augustin Sánchez-Lavega of the Universidad del País Vasco in Bilbao discuss the July 2009 Jovian impact, and try to determine the nature and origin of the impactor.

Composite image of Jupiter, with the impact scar as seen at the times indicated at beside the boxes; note this is one scar seen at different times in different positions, not a series of scars as seen with the Shoemaker-Levy impact. Sánchez-Lavega et al. (2010).

The impact scar was first detected at 1.02 am GMT on 20 July 2009, as a dark spot as it rotated into view from the west. The most recent previous image of the same spot was taken at 7.40 am GMT on 19 July 2009, constraining the time of the event that caused it to a 17 hour 22 minute window. Images taken by NASA’s Infrared Telescope Facility at 10.13 am GMT on 20 July 2009 showed a bright spot in the methane and hydrogen absorption bands, reaching high above the surrounding clouds; this was the same pattern observed after the Shoemaker-Levy 9 impacts, suggesting this was another impact of a similar nature.

The July 2009 impact scar seen (a) in visible wavelengths, and (b) in infrared. Sánchez-Lavega et al. (2010)

The scar extended 4800 km east-west and 200 km north-south, though it was tilted at 12˚ to true latitude. This is more elongate than the Shoemaker-Levy 9 scars, which Sánchez-Lavega et al. interpret as a sign that the impacting body had a shallower incidence angle relative to the horizon. A thin debris crescent extended 4800 km northwest of the western edge of the scar; a similar crescent structure was seen after the Shoemaker-Levy 9 impacts, and was interpreted as being the result of Coriolis force on the falling material plus a sliding in the atmosphere that conserves the tangential velocity. 

Working from the size of the impact scar, Sánchez-Lavega et al. calculate the original body to have been between 500 m and 1 km in diameter, and that it struck Jupiter at a speed of between 54.52 and 55.1 kms¯¹. Attempts to model the previous path of the impactor prior to the collision by working backwards from the impact site suggest there was a 47% chance that the body was on its original, Sun orbiting path when it hit Jupiter, and a 53% chance that it had previously been captured by the planet into  a Jovicentric orbit, probably more recently than 1989, as was the case with Shoemaker-Levy 9. Furthermore the previous orbit of the body was equally likely to have been a Main Asteroid Belt body (either a Hilda Group Asteroid or a Quasi-Hilda Comet), or a Jupiter Family Comet. Shoemaker-Levy 9 is thought to have originally been a Quasi-Hilda Comet.

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