Thursday, September 9, 2010

Paper: Detection of a "Superbolide" on Jupiter

A paper was published today online in the Astrophysical Journal Letters on the June 3 fireball on Jupiter.  The impact produced a bright flash that was seen all the way from Earth by two amateur astronomers: Christopher Go in Cebu, Philippines and Anthony Wesley in Murrumbateman, Australia.  We discussed the impact at the time as not one but two detections of this impact were confirmed.  This new paper is titled, "First Earth-based Detection of a Superbolide on Jupiter," by Ricardo Hueso, several co-authors include astronomers who observed the site using Hubble, Keck, and other large telescopes, and the two amateur astronomers who detected the impact.  The paper discusses the circumstances of the observations of this impact, measurements of the energy released and consequently the size of the impactor, and observations by Hubble and other telescopes of the site in the days following the June 3, 2010 impact.

Prior to June 3, 2010, only a few extraterrestrial impacts or meteors had been directly observed.  These included small flashes on the nightside of the Moon, a meteor streak across the Martian night sky by the rover Spirit, the faint flash that Voyager 1 saw in Jupiter's atmosphere, and the Shoemaker-Levy 9 impacts in 1994.  Since June 3, two flashes have been seen in Jupiter's atmosphere, the impact on June 3 that is the subject of this paper and another on August 20 that was observed by several astronomers in Japan.  These impacts produced a brief, bright 2-second flash in Jupiter's atmosphere.  Subsequent observations failed to find the kind of visible scars that had resulted from the larger SL-9 impacts in 1994 and an asteroid impact in 2009.  The discoveries this year by Wesley, Go, and the astronomers in Japan were aided by their use of webcam technology to record their observations of Jupiter.  They sum multiple frames from the videos they record to produce spectacular color images of Jupiter and other planetary targets by reducing the signal-to-noise ratio of their data.  These videos also allow for the detection of transient events like meteor fireballs that might otherwise go unnoticed or unconfirmed with additional observations.

In this new paper, Hueso et al. used the two videos taken by Wesley and Go to measure lightcurves of the June 3 fireball.  By measuring how bright the bolide was compared to the brightness of the area before and after the impact, and by calibrating the photometric response of the filters and camera systems used, the authors were able to estimate the amount of energy released by the meteor.  They estimated that the bolide released 1.0–4.0× 1015 Joules, or the equivalent of 0.25–1.0 megatons.  This is about 5-50 times less energy than the June 30, 1908 Tunguska airburst, which flattened 2,150 square kilometers (830 sq mi) of forest in Siberia. Bolides with the energy of the June 3 event occur ever 6–15 years on Earth.  Assuming an impact velocity of 60 kilometers (37 miles) per second and a density of 2000 kg per meter, Hueso estimated that the impactor had a mass of 500–2000 tons and was 8–13 meters (26–43 feet) across.  This fits nicely within a gap in our knowledge of Jovian impactors, as the July 2009 asteroid had a mass that was 105 times larger while the meteor that caused the flash seen by Voyager 1 was 105 times smaller.  According to the NASA press release, the August 20 impactor was on the same scale, though that event occurred a month after this paper was submitted.

Analysis of the bolide's optical flash reveals a number of characteristics that are similar to meteors here on Earth.  The lightcurve of the event, which was visible for 1.5 seconds, is asymmetric as the event slowly brightened for one second, produced a bright central flash, then quickly faded.  Analysis of both the blue filter data taken by Christopher Go and red filter data taken by Anthony Wesley also showed that the flash had three distinct peaks, again similar to bolides on Earth.

Anyway, the big result from this paper was the note that observations of Jovian bolides could help place constraints on the impactor (asteroids and comets) flux in the Jupiter system.  Using similar systems as Go and Wesley, Jovian events five times less luminous than the June 3 impact should be detectable as well as events that involving slightly larger impactors on Saturn.  Based on the impacts seen this year, it would appear that models predicting 30-100 collisions of this magnitude on Jupiter, like the dynamical model by Levison et al. 2000, maybe more accurate than those extrapolating from crater counts on the Galilean satellites.  However, as always, more data is need.  More than two data points will be needed to pin the impactor flux down.

For more details, definitely check out the original paper by Hueso et al. over on the European Southern Observatory website from their press release.

R. Hueso, A. Wesley, C. Go, S. Perez-Hoyos, M. H. Wong, L. N. Fletcher, A. Sanchez-Lavega, M. B. E. Boslough, I. de Pater, G. S. Orton, A. A. Simon-Miller, S. G. Djorgovski, M. L. Edwards, H. B. Hammel, J. T. Clarke, K. S. Noll, and P. A. Yanamandra-Fisher (2010). First Earth-based Detection of a Superbolide on Jupiter The Astrophysical Journal Letters, 721 (2) : 10.1088/2041-8205/721/2/L129

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