Two nights ago an Australian amateur astronomer, Anthony Wesley, observed a dark spot in the South Polar region of Jupiter. He quickly realized that this feature was out-of-the-ordinary and notified the astronomical community. His announcement allowed professional astronomers with access to some of the largest and best telescopes in the world to observe the new spot. So far the consensus points to a recent impact by a comet or asteroid as the cause of the spot.
Amateur astronomers around the world have been able to observe and image the spot. In most images it is not obvious what the spot is. Luckily Anthony Wesley has an excellent set-up for planetary imaging and was able to recognize the spot as something unusual.
Bob Lunsford (San Diego, CA) was able to image the Jupiter and its new spot with his 9.25″ SCT telescope. The image nicely shows the impact spot. It is located near the upper right limb of Jupiter. Though hard to see, the large but pale circle near the upper left limb of Jupiter is its famous Great Red Spot. These days the Great Red Spot is still great but not as red as it once was.
Jupiter and its new dark impact spot as imaged by Bob Lunsford with his 9.25" SCT on 2009 July 21 at 1106 UT. Image credit: Bob Lunsford.
Professionals on the Keck 10-meter telescope and NASA’s Infrared Telescope Facility (IRTF) were able to observe the spot. According to Keck observations published on Central Bureau Electronic Telegram 1882, the spot covers 200 million square kilometers which is nearly half of the Earth’s surface area. The impact site consists of 2 feature located 2 degrees apart and an ejecta field 10 degrees long. The image below is from a NASA press release on observations taken with the IRTF in Hawaii.
This image shows a large impact on Jupiter's south polar region captured on July 20, 2009, by NASA's Infrared Telescope Facility in Mauna Kea, Hawaii. Image credit: NASA/JPL/Infrared Telescope Facility.
How do we know that this spot was caused by an impact?
Back in 1993, astronomers Carolyn and Gene Shoemaker and David Levy discovered one of the most unusual comets ever seen. Rather than the usual “puffball” with a tail, Comet Shoemaker-Levy 9 appeared to be a fuzzy bar. Observations with larger telescopes revealed a string of individual comets all traveling in roughly the same orbit. Further observations, like the Spacewatch image below, helped uncover the origin of this weird object.
Comet P/Shoemaker-Levy 9 as it appeared on 1993 March 30, a few days after discovery. This image was obtained with the Spacewatch telescope. Image credit: Jim Scotti, Spacewatch, University of Arizona.
Comet Shoemaker-Levy 9 had been captured onto a temporary orbit around Jupiter. Some studies found that it may have been orbiting Jupiter for ~20 years before it was discovered. Its orbit about Jupiter was very elongated and took many months to complete one circuit. In July of 1992 the comet passed so close to Jupiter that Jupiter’s gravity ripped it into pieces. When the comet was first seen by the Shoemaker-Levy team, it was still in dozens of pieces and still in orbit around Jupiter.
Gravitational perturbations caused the comet’s orbit to change to the extent that the comet would not only come close to Jupiter at its next swing-by but would actually hit the planet. Between the days of July 16 and 22, 1994, the many pieces of Shoemaker-Levy 9 impacted the planet. Each impact left a brownish scar on the upper atmosphere of Jupiter. The scars are the result of the fireball caused by the impacting comets pulling material up from deep within Jupiter’s atmosphere. The Hubble Space Telescope picture below shows some of the marks left by SL9 in 1994.
Brown spots mark the places where fragments of Comet Shoemaker-Levy 9 tore through Jupiter's atmosphere in July 1994. Credit: Hubble Space Telescope Comet Team and NASA.
Because we have seen impacts on Jupiter before, it is much easier to recognize impacts today. It would be interesting to see what astronomers would think of today’s dark spot if we hadn’t witnessed the impact of SL9. Would we even take much notice?
So what was it that hit Jupiter (and how did we miss it)?
Based on the SL9 impact scars, the recent impactor was probably on the order of a kilometer or so across. We don’t know if it was a comet or asteroid though I suspect a thorough analysis of the large telescope data may reveal the answer. At the distance of Jupiter, most of the objects on stable orbits are asteroids (or old comets that are no longer active) while the objects that are on unstable orbits are comets. In order to impact Jupiter, an object must be on an unstable orbit so it is likely that it was a comet.
At the distance of Jupiter (5.2 AU or 5.2 times further from the Sun than Earth is) few comets are active. In the case of SL9, the comet was active due to its very close approach to Jupiter. Assuming a small, dark, inactive 1-km comet nucleus, it would have been no brighter than 25th magnitude. Even today’s advanced asteroid survey telescopes have trouble going much fainter than 22nd magnitude which is 16 times brighter than 25th magnitude. It is very likely that the impactor was simply too faint to be detected by the surveys. Many of the largest telescopes around the world, as well as the Hubble Space Telescope, can see faint enough to have discovered the impactor. But… these scopes have such small fields of view that it is impractical to use them to survey the sky for new objects. So we may never know exactly what hit Jupiter.
[In case your wondering, a 1-km wide asteroid orbiting near Earth is much brighter and easier to find. In fact, it is estimated that we have already found close to 90% of the 1-km asteroids and comets that threaten Earth. With more capable asteroid surveys coming online in the next 5 years or so we may find nearly all of the large Earth threatening asteroids over the next 2 decades.]
There is little chance that the recent impactor is related to SL9 even though they impacted almost exactly 15 years apart. The exactly 15 year interval is a fluke since Earth years mean nothing to Jupiter. It takes Jupiter just under 12 years to orbit the Sun so Jupiter is located in a different part of its orbit than it was in 1994. The 15 year interval does tell us that Jupiter impacts may happen often. Right after SL9 a study found that a Jupiter impact should happen once every 20 years or so.
