[5] Fireball on Jupiter, 2012 Sep.10.

Jupiter in 2012/13: Report no.5 (2012 Sep.20):

Fireball on Jupiter, 2012 Sep.10

John Rogers (BAA)

A bright fireball has been observed in the atmosphere of Jupiter, on 2012 Sep.10 at 11:35 UT. It was similar to fireballs observed on 2010 June 3 and 2010 Aug.20, and like them, left no detectable trace on the planet’s surface.

It was discovered by a visual observer, Dan Petersen in Racine, Wisconsin, observing at sunrise, who quickly notified two Jupiter contacts as follows:

“Subject: I just observed a large explosion on Jupiter this morning!

This morning at 11:35:30 UT I witnessed an explosion on Jupiter, it was located

at Long1 = 335 and Lat = +12 degrees. This location at the time was just inside

Jupiter's eastern limb and just north and inside of  the southern edge of the

NEB. The flash was bright white in color, maybe about 6th magnitude in

brightness and lasted for just 2 seconds. I observed this event through my

Meade 12" LX200 telescope at a power of 400X, seeing was good.”

[After recalibrating his clock, the time was revised to 11:35:04 UT.]

Observers worldwide were notified at once.  Within hours another amateur, George Hall in Dallas, Texas, reported the flash was clearly visible on a video that he was recording at the time. His video has been posted at:

 http://www.flickr.com/photos/19299984@N08/7976507568/

The video was taken through a red filter, showing the flash at approx.11:34:52 UT, in the EZ.  Although the flash appears as multiple spots, this is partly a speckle pattern from atmospheric turbulence, and partly a diffraction pattern from the telescope; the actual flash would have been a point.  It appears to be brighter than mag.6, which may represent the visual impression against the glare of Jupiter.  Estimates of the longitude have ranged from L1 = 335 (Petersen) to 345 (as marked in Fig.2) but the adopted value, from the processed image in Fig.1, is L1 = 340, lat. 0 deg.

Dan Petersen’s report is a perfect model of how visual observers even now can make and report valuable discoveries.  His estimates of position and time were pretty close for an unexpected flash only 2 seconds long, and his report enabled George Hall to find it in a video that might otherwise have gone unexamined.  The immediacy of his report also enabled it to be used as independent data, so there was no doubt about the reality of the fireball on Jupiter.  Scientists might not have been convinced by a single video alone,  nor by a visual report after an announcement had already been made. 

Eleven hours after the impact, observers were already looking for a visible ‘scar’. However, none was ever seen.  Figs.1 & 2 show the impact site before the fireball, and on the first four rotations after it.  The impact site was in, or on the edge of, the orange Equatorial Band, and no changes can be seen, except the pre-existing small blue-grey streaks drifting along it.  Images were also taken in the methane absorption band – in which impact debris clouds look very bright – and also showed nothing. These were taken on Sep.11 by Gary Walker (09:15 UT; see Fig.3) and Anthony Wesley (18:20 UT).

So this impact was well characterised and left no ‘scar’. This was also the case with the two fireballs in 2010 (see our reports, references below*).  Scientists concluded at the time that these were impacts by miniature asteroids or comets, with diameters in the 10-metre range, which incinerated in the atmosphere above the clouds – unlike the larger impactors in 1994 and 2009, which produced much larger, deeper explosions, converting a substantial mass of atmospheric methane into dark soot.  The frequency of these events is in line with previous (very rough) estimates: smaller impacts are of course more frequent than larger ones. They are currently being observed about once a year, but because it is so easy to overlook them, they could be occurring much more frequently (perhaps once a month?). 

*Our reports on the fireballs in 2010:

On our web site:

http://www.britastro.org/jupiter/2010report02.htm

http://www.britastro.org/jupiter/2010report07.htm

In the Journal of the BAA:

Rogers JH (2010 Oct.) JBAA 120 (5), cover & pp. 267-268: News: ‘Jupiter in

2009-2010: an interim report’, & p.263: ‘New fireball impacts seen on Jupiter’.

_________________________________

John H. Rogers, Ph.D. Jupiter Section Director,
British Astronomical Association
jhr11@cam.ac.uk
http://www.britastro.org/jupiter/