Optical flashes caused by impact events on the surface of Jupiter have been within the detection capabilities of amateur astronomers for some time, with six such flashes reported between 2010 and 2020. These events have the potential to provide us with information about the abundance of small objects in the Outer Solar System, which are close to impossible to detect by direct observations, as well as the consequences of such impacts on planetary atmospheres, information which can be used to improve models of such impacts on Earth.
Models suggest that the majority of impacts on Jupiter are caused by Jupiter Family Comets (bodies with orbital periods of less than 20 years and relatively low orbital inclinations), while the majority of detections are of bodies with sizes calculated to be in the tens of metres range. However, these detections have all been essentially made by luck, when a professional or amateur telescope happened to be pointing in the right direction, rather than as a result of organised monitoring, which would provide statistically useful data on the frequency of such events.
In a paper published on the arXiv database at Cornell University and submitted to the Astrophysical Journal Letters, Ko Arimatsu of the Astronomical Observatory at Kyoto University, Kohji Tsumuru of the Department of Natural Science at Tokyo City University, Fumihiko Usui of the Department of Space Astronomy and Astrophysics at the Japan Aerospace Exploration Agency, and Jun-Ichi Watanabe of the Astronomy Data Center at the National Astronomical Observatory of Japan, report the first detection of an impact event on Jupiter by the Planetary ObservatioN Camera for Optical Transient Surveys observation system, which was set up to continuously monitor the surface of Jupiter for impact flashes, and commenced operating on 9 September 2021.
The event was detected at 1.24 pm GMT on 15 October 2021 (i.e. 36 days after the system began continuous observations of Jupiter), in both the visual and methane-absorbance bands of the spectrum, in the north tropical zone of Jupiter. The flash was also seen by two amateur astronomers in Japan, and one in Singapore; these observations provide a corroboration of the event which confirms it occurred on Jupiter, not in the Earth's atmosphere. No subsequent after-effects of the impact could be observed from Earth. An observation by the Juno Spacecraft made 28 hours after the impact showed some darkening at the site of the event, although it is impossible to determine if this was related.
The impact had a maximum apparent magnitude of 4.7 in the visual part of the spectrum (comparable to the Jovian moon Io at it's brightest), and lasted for about 5.5 seconds. This is brighter and longer lasting than any Jovian impact recorded since the Shoemaker Levy 9 impact observed by the Hubble Space Telescope in 1994.
The impact was significantly brighter at visible wavelengths than it was in the methane-absorbance part of the spectrum, which Arimatsu et al. attribute to reflection of light at these wavelengths by the Jovian clouds. Arimatsu et al. further calculate that the impact released an energy of about 740 000 000 megajoules, roughly equivalent to the detonation of 1.8 megatons of TNT. This is roughly equivalent to the amount of energy thought to have been released by the Tunguska explosion in 1908, and an order of magnitude greater than any Jovian impact previously recorded, with the exception of the Shoemaker Levy 9 impact.
Based upon this, Arimatsu et al. calculate that the impactor would have had a mass of about 4 100 000 kg, which would equate to a diameter of roughly 15-30 m, depending on the density and composition of the object.
By calculating the amount of time known to have been dedicated to the direct observation of Jupiter by telescopes capable of detecting large impacts since 2010, and the number of impacts observed in this time, Arimatsu et al. calculate that such impacts are 2-3 orders of magnitude more common on Jupiter than on Earth. This is an order of magnitude greater than previous estimates based upon observations of impact craters on the Jovian moons.
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