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.

References:
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

Follow-up on Friday's Impact on Jupiter

As I reported yesterday, Japanese astronomer Masayuki Tachikawa recorded on video the impact of a small asteroid or comet on Jupiter's northern hemisphere on August 20 (early August 21 in Japan).  The optical flash of the meteor streaking across the Jovian sky was also seen by two other Japanese astronomers, Kazuo Aoki and Masayuki Ichimaru.  Both astronomers imaged Jupiter, again using webcams connected to their telescopes, during the impact event and recorded the optical flash of the fireball.  Kazuo Aoki's recording allows a more precise estimate of the timing of flash to within a second of 18:21:56 UTC on August 20.  Having more than observation of the event provides a confirmation of the observation and eliminates other potential sources of the flash, such as nearby artificial satellite.

Isshi Tabe has a webpage up where he is collecting the observations from various astronomers across the western Pacific of this impact event.  More information on the methodology of these observations as well as links to images and videos can be found on his website.

Kelly Beatty reports over at the Sky and Telescope website that Imke de Pater and Heidi Hammel were also observing Jupiter using the 10-meter Keck II telescope in Hawaii. They did image the impact region during their two-day run, but in their initial look at the data, they didn't see anything new such as an impact scar. The impact occurred when the Sun was already up in Hawaii, so they most likely didn't observe the actual impact.

As I pointed out yesterday, this is the second fireball to be seen in Jupiter atmosphere this year, after only two previous impact event seen from Earth, in 1994 and 2009 (though the latter was only seen after the impact).  This doesn't mean that impact events are somehow occurring more frequently.  The discoveries this year are helped by the method many amateur astronomers use to record their observations and built up high-quality images.  Because they tend to use smaller telescopes than most professionals, they are limited by the amount of light their telescopes can collect.  This lowers the signal-to-noise of their observations.  Increasing the exposure times is fine for faint targets, but for bright targets like Jupiter, that will just over-exposing the data, limiting their usefulness.  Instead, they use USB cameras (webcams) to record video of their target of interest at 20-60 frames per second.  Software is then used to stack the numerous frames they record to build up images with a much higher signal-to-noise ratio than each separate frame.  This is akin to how we take images of Titan's surface, where we acquire 3 images of Titan's surface at 938 nanometers and then sum them on the ground.  These webcam videos, taken by numerous amateur astronomers from around the world also have the side benefit of allowing them to detect transient events like meteor fireballs in Jupiter's atmosphere, which would have been difficult to detect otherwise.  With so many amateur astronomers taking long videos of Jupiter, the likelihood that an impact event is detected is improved.

On that note, I want to repost something the head of the Jupiter section of the Association of Lunar and Planetary Observers, John Rogers, sent around today on the potential of this data set:

It would really be worth determining the frequency of these events. Some ideas:
1) Regular observers: Please can you tell me: On a typical night, how many minutes of video do you record and look through?
2) From now on, could regular observers record the start and end times of video they view each night, and could anyone volunteer to collect this information? Perhaps regional associations (ALPO, ALPO-Japan, etc.) might be able to collect this info? I hope this would not be a burden for observers -- Obviously it is wonderful when you process and send the best of your images as soon as possible, and I certainly would not ask you to delay until you have completed the paperwork! A list of video times sent once a month would be fine. But it would be worthwhile, so the amateur observers' network could make an important measurement of the frequency of these events
3) Maybe someone could devise software for scanning the webcam videos and identifying these fireballs automatically??

With these measures in place, perhaps more impact events will be recorded and detected going forward. This information will scientists to estimate the impactor flux in the Jupiter system. With some modeling, this can further improve our age estimates for the surfaces of Io and Europa.

On Isshi Tabe's website, he also points out that Christopher Go found a reference to a Voyager 1 observation of a fireball.  The paper is by A. Cook and T. Duxbury and is titled, "A Fireball in Jupiter's Atmosphere".  It was found while the spacecraft was eclipsed by the Sun, shortly after the Io encounter on March 5, 1979. The Voyager narrow-angle-camera was using this opportunity to image Jupiter's night-side with limited contamination from sunlight to search for lightning and meteor fireballs.  They were able to find one fireball, in Jupiter's high-northern latitudes, with an absolute magnitude of -12.5 and a path length of 75 km (image c1639630 from 03/05/1979 17:45:24 UTC).  Cook and Duxbury mass of the impactor was 11 kg. Assuming an impact velocity of 64 km/sec., the flash occurred over a period of 1.17 seconds, which is on the same order as the meteors seen this year from Earth.  From their observations, they suggest that the number density for objects larger than 3 kg is a factor of 6 less than the estimate obtained from terrestrial meteors.  Additional modeling and observations from ground-based telescopes should help to pin down this estimate.

Link: Isshi Tabe - Fireball on Jupiter in 2010 20th August UT [yokohama.cool.ne.jp]
Link: Paper - A Fireball in Jupiter’s Atmosphere [dx.doi.org]

Meteor fireball spotted in Jupiter's Atmosphere - Again

Remember a time when Jupiter only had 16 satellites?  Or when the only extra-solar planets know where a trio around a pulsar and a recently discovered handful?  Or when the only known impacts on Jupiter seen by astronomers (or its after effects) were the Shoemaker-Levy 9 impacts in 1994?  I do.  Sometimes in science, once a discovery is made or observation recorded, it unleashes a torrent.  First, one new moon is discovered out beyond Callisto (Themisto) in 1999, then another, then another at Saturn, and the next thing you know, both Jupiter and Saturn have 60+ known moons.  Same for extra-solar planets, we've found so many, they are merely just part of a statistic for the Kepler team.  Last July, amateur astronomers found an impact site on Jupiter, formed after an asteroid struck Jupiter's southern hemisphere. Then this year, on June 3, astronomers Anthony Wesley and Christopher Go spotted a meteor impact the planet's upper atmosphere, burning up before it could leave its own mark on the solar system's largest planet.  Well, it has happened again.

On Friday, August 20 at 18:22 UTC (Saturday morning in Japan), Japanese astronomer Masayuki Tachikawa detected an optical flash, likely from a meteor striking Jupiter's upper atmosphere using a Philips Toucam Pro2 USB camera attached to his telescope.  The impact is the first to be seen over Jupiter's northern hemisphere, occurring on the northern edge of the North Equatorial Belt (south is up in the image above) just to the east of the Great Red Spot's longitude.  A detailed report on the discovery can be found over at the ALPO-Japan website (scroll down to the bottom for an English description of the observation).  A video of the optical flash can be found on that site as well, though I found it best to save the file first before playing it.  A French description (the first link I saw with this news, posted by Eric Soucy, tip o' the hat to him for this news) can be found at Ciel et Espace.

The ALPO-Japan website also has images from after the impact, taken yesterday.  So far there doesn't appear to be any evidence for an impact scar, like the June 3 fireball.

Anyways, soon, observations of meteors in Jupiter's atmosphere will be just statistics used to determine the current impact flux in the Jupiter system...

UPDATE: Nick Previsich found a great English language link describing how this discovery was made from the Sky and Telescope's website.
Another Update: Two more excellent blog posts from Daniel Fischer and Emily Lakdawalla (who has a Youtube version of the optical flash video up) I would remiss to point out.

UPDATE 08/23 2:38 am: Thanks to Kelly Beatty and Emily Lakdawalla for pointing out some independent confirmation that this optical flash is in fact an impact on Jupiter.

Link: Optical flash on the surface of the Jupiter by M.Tachikaw [alpo-j.asahikawa-med.ac.jp]
Link: Another Flash on Jupiter? [www.skyandtelescope.com]
Link: Offenbar schon wieder ein Impaktblitz auf Jupiter von Amateur gefilmt [skyweek.wordpress.com]
Link: Yet another Jupiter impact!? August 20, seen from Japan [www.planetary.org]

No Debris Field from the June 3 Impact

A new set of images of Jupiter, taken a few days after the June 3 impact of a small asteroid or comet, was released on the website for the Hubble Space Telescope today.  The research group, which includes Mike Wong, Heidi Hammel, and Amy Simon-Miller, focused on finding a small debris field that might have resulted from the impact, similar to the ones seen from last year's asteroid impact and the Shoemaker-Levy 9 impacts in July 1994.

To date, no such debris field has been spotted by amateur, ground-based telescopes, but it had been hoped that with Hubble's superior resolution, it might be able to spot a small, dark spot from the impact.  Instead, as you can see above, even with Hubble's larger eyes, no impact scar is visible.  This suggests that the impactor was too small to penetrate very deep into Jupiter's atmosphere, but instead burned up in the upper atmosphere, akin to meteor fireballs on Earth.  Since these events are so brief, it is possible that similar Jovian meteors maybe fairly common, but because of their brief duration (< 3 seconds), they just hadn't been noticed before.  Leigh Fletcher on his Twitter feed has noted that the Keck, Gemini, and IRTF observatories at Hawaii's Mauna Kea and VLT in Chile have also obtained views of the impact site, so maybe they will be able to spot a residual thermal hotspot from the site.  He seems to suggest that no such scar has been seen though.

While it failed to detect an impact scar from the Jovian meteor, the Hubble data was able to provide images of the changes that have occurred in Jupiter's atmosphere over the last six months.  The South Equatorial Belt (SEB), normally the reddish-brown southern complement to the similar North Equatorial Belt (NEB) which bracket the bright Equatorial Zone (EZ), brightened during the end of 2009.  From ground-based scopes, this makes Jupiter appear as if it only has one dark belt, as opposed to the normal two.  This also makes the Great Red Spot more visible since it is surrounded by mostly bright clouds.  Hubble detected a high-altitude layer of ammonia clouds over the SEB, obscuring the darker clouds below.  The Hubble image also points to the beginning of the end for these high clouds, as a series of dark spots along the southern margin of the SEB were also seen.  Similar spots were seen near the end of earlier SEB brightenings.

Link: Hubble - Mysterious Flash on Jupiter Left No Debris Cloud [www.hubblesite.org]

Latest on Yesterday's Jupiter Impact

Yesterday at 20:31:29 UTC, astronomers Anthony Wesley and Christopher Go observed a bright flash within Jupiter's faded south equatorial belt.  This bright flash is thought to be the fireball of an impacting meteor in Jupiter's upper atmosphere.  Since the impact, both astronomers have posted videos and processed images online showing this flash.  Wesley's images and video have been posted to his website.  The link to his video is at the bottom of the page.  Note that his video is an AVI container and you may have issues viewing it in Windows Media Player.  It played just fine in VLC.  Additional news from Wesley going forward maybe posted on the Ice in Space forum first.  Christopher Go has posted processed images and video on his website.

The question now is whether this impact left a dark scar like the Shoemaker-Levy 9 impacts in 1994 and the July 2009 asteroid impact.  Gary Spiers posted times when the impact site cross the central meridian as viewed from Earth on his blog.  The first opportunity at 5:04 UTC was imaged in western and southern Europe by several observers.  While the seeing wasn't as good as what those further south like Wesley and Go obtained the previous Jovian day, they don't seem to show any sign of an impact scar.  A summary by John Rogers of the British Astronomical Association has been posted on ALPO-Japan with several "after" images from the UK, France, and Italy.  The next opportunity at 15:00 UTC was observed by several observatories on Mauna Kea in Hawaii (Gemini, Keck, and IRTF), according to Leigh Fletcher (who was involved in the followup observations at Mauna Kea after last year's impact) on Twitter.  No results have been posted yet, so hopefully their keener eyes will be able to spot a small impact scar.

Link: Anthony Wesley's 2010 Jupiter Impact Page [jupiter.samba.org]
Link: Christopher Go's Jupiter Imaging site [jupiter.cstoneind.com]
Link: New impact on Jupiter before & after by John H.Rogers [alpo-j.asahikawa-med.ac.jp]

Meteor fireball spotted in Jupiter's Atmosphere

Less than a year after he first spotted an impact scar on Jupiter's south polar region, astronomer Anthony Wesley spotted a bright flash at 20:30 UTC today (about three hours ago from the time of this post) that lasted about 2 seconds in Jupiter's faded south equatorial belt.  The brightness of the flash indicates that it was mostly likely caused by a small asteroid or comet striking Jupiter's upper atmosphere and burning up.  Wesley indicates that don't seem to be any markings left by the impact, like those seen after the SL9 impacts in 1994 and last July's impact, though it occurred late in the day at the impact site, and we may not see anything until the area rotates into view three hours from now.

The original thread where Wesley posted this image is located at the Ice in Space web forum.  He plans to post a video of the impact in the next few hours as well.  I want to give a tip o' the hat to Emily Lakdawalla for alerting us to this observation on her blog.  Coincidentally, she points out that the Hubble telescope team has released several images of the site of last year's impact, as the scar became sheared by the planets's winds.

Another place to check for up-to-date news on this is at the Unmannedspaceflight.com forum, where several regulars are already discussing the impact.

UPDATE 5:18 PM MST: A video by Christopher Go in the Philippines is now online.  Didn't seem to last very long.

Link: Jupiter impact, June 3 2010 [www.iceinspace.com.au]

Highlights from the Mercury Global Mosaic

Last night, I posted my completed version of the Departure #4 global mosaic from MESSENGER's second encounter with Mercury in October 2008.  If you haven't had a chance to check it out, I definitely encourage you to do so: Original Post | Full-resolution Global Mosaic.

This morning, I thought I would discuss some of the interesting Mercury geology you can see in this mosaic.  The first area I wanted to discuss is the large impact basin, Raphael, shown at left.  The floor of this 350-kilometer (217-mile) wide impact basin was nearly complete filled by lava flows shortly after its formation.  The lavas pretty much obliterated any trace of the initial impact structures within the basin, like multiple rings, for example.  However, in addition to the lava flows, a debris flow is seen in the southern portion of the basin.  This 185-kilometer (115-mile) long debris flow was likely the result of the collapse of a 200-kilometer (124-mile) long section of the basin's southern rim shortly after Raphael's formation.  It is astonishing just how far this material reached after the 2-kilometer (1.24-mile) tall rim collapsed.  It reached the mid point of the basin!  Now, why did this crater fascinate me?  Well, it got me thinking about which feature came first: the lava flows or the landslide deposit?  The scenario that sounds best to me is that the landslide came first and the lava flows later filled the rest of the basin, lapping up onto the margins of the landslide deposit.  How can I say this?  First, the landslide deposit seems to have an older cratering age than the rest of Raphael's floor, meaning it has more impact craters (though not that much more) superimposed on its surface.  Secondly, the embayment relationship between the lava flows and the landslide deposit suggests that the lava flows filled the floor of the basin and were thick enough to cover up much of the lobate margin of the deposit and lapping up onto its edge before stopping.

Anyways, I thought that was neat.

The other feature I wanted to point out is the impact crater Matabei, shown at right in the center of this crop from the mosaic.  What makes Matabei interesting is the intriguing albedo markings both inside the crater and outside.  Inside, portions of the crater are quite bright.  Outside, several dark "rays" can be seen radiating out from the crater, mostly to its south, suggesting that the crater excavated dark material from below Mercury's surface.  A paper published in Science by the MESSENGER science team (Denevi et al. 2009) back in May suggests that such patterns are the result of layering of regolith (basically impact pulverized rock), old magmatic sills and dikes, and lava flows.  Beneath the surface of Matabei there must have been both dark, "Low-Reflectance Material" (LRM) and bright material that was then excavated by the impact and deposited on the surface.  LRM, according to the Denevi paper, is not uncommon within impact ejecta, but the distinct rays see at Matabei certainly are amusing to look at.

Again, I hope you all liked the mosaic.  The positive response to this mosaic has encouraged me to complete the higher resolution, northern hemisphere mosaic from the second flyby.

EDIT 10/05/2009 01:05 PM: Added a bit more explanation for Matabei.

New Hubble Images of Jupiter

On Wednesday, NASA released a number of images acquired by the Hubble Space Telescope as part of its re-commissioning period following the Shuttle Repair and Service mission back in May. One of the images released is a full-disk color image of Jupiter taken four days after the impact of a ~0.3 kilometer (330-yard) wide comet or asteroid in the giant planet's south polar region. The temporary impact scar can be seen as dark splotch at bottom right. A portion of this image was released back in July as part of the global campaign to monitor the impact scar. In addition to the impact scar, this image also shows some beautiful atmospheric waves in Jupiter's northern hemisphere.

Another great image released by the Hubble team shows the Butterfly Nebula as captured by the new Wide Field Camera 3. The image shows the glowing gas and dust cast off by the dying star at the center of the image (unseen because of the large amount of dust between the star and us). Very striking!

Link: Collision Leaves Giant Jupiter Bruised [www.hubblesite.org]

Jupiter Impact Confirmed by IRTF

The impact of a small asteroid or comet into Jupiter's atmosphere, first observed by astronomer Anthony Wesley (let's be honest, anyone who takes the kinds of pictures he does of Jupiter is not an amateur), has been confirmed in observations taken by NASA's Infrared Telescope Facility (IRTF) atop Mauna Kea on the island of Hawaii. The image shown at left reveals the impact to be glowing quite brightly in the near-IR, in a methane absorption band at 1.65 microns

Based on additional images taken by ground-based telescopes, the impactor came in from below Jupiter, striking the South polar region sometime between 07:00 and 14:11 UTC on Jupiter's nightside. Several dark spots in addition to the main impact site are visible with a faint, fan-like plume deposit to the west and north of the impact site. Similar plume deposits were seen at Shoemaker-Levy 9 impacts 15 years ago this week in 1994.

EDIT 07/20/2009 06:23 PM: New Scientist has an article with an image taken by Keck II. The IR data from Keck seems to suggest the possibility of multiple impactors.

EDIT 07/20/2009 11:33 PM: Looks like the image from Keck II in the New Scientist article is a bit of a double exposure, making it look like multiple impact sites.

In case you missed it, an hour ago I posted some of my thoughts on this, the 40th Anniversary of the Apollo 11 landing. Don't forget to post a comment there about when you think the first humans will land on Io (never is a possible answer, but not one I necessarily agree with).

Link: New NASA Images Indicate Object Hits Jupiter [jpl.nasa.gov]

Impact observed on Jupiter

15 years ago, the world watched as broken-apart comet Shoemaker-Levy 9 impacted Jupiter's atmosphere in a series of event in July 1994. Well, a similar event seems to have occurred in the last day in Jupiter's south polar region. A new dark spot, similar in size and color to the Shoemaker-Levy 9 impacts was seen by ground-based observers on July 19.

Hopefully more observations will be acquired over the next few days to help confirm this discovery, but it looks quite plausible to me :)

The image at left was captured by Anthony Wesley on 19th July 2009 at 1554UTC from Murrumbateman Australia. The south pole is up, north pole down. The impact site is near the central meridian about an eighth of the way down.

EDIT 07/19/2009 9:23 PM: Wesley's website has been slashdotted so he has mirrored the page to another server. So if you are having trouble accessing the link above, check out http://jupiter.samba.org/

Road Trip up to Meteor Crater

Today, my friends Roni, Audrey, and I drove up to Meteor Crater from Tucson. This was actually my first visit to this tiny impact crater, but certainly worth taking photos of. Tomorrow, we are going to head over to Albuquerque and later Los Alamos. We ended up holing up in Holbrook, Arizona for the night. Holbrook is a pretty small town, but the Italian Mesa Restaurant had some great food. I will post most of my pictures when I get back to Tucson Monday, but at right is a great shot of Meteor Crater that I took today. I also took quite a few images that can be used to make a huge panorama of the crater that I will put together when I get back home.

Hope you are all having a great week!

Significant Day in Astronomy

Yesterday was a significant day in astronomy. Yesterday evening, Tucson time, an asteroid 5 meters across streaked across the sky of Chad, Egypt, and Sudan. This kind of event happens routinely, an object 5 meters across encounters our planet about once every few months. What made this rock, 2008 TC3, significant was that for the first time, ever, an asteroid was discovered on a collision course with Earth. We observed it as it approached our planet, and according to some early reports on Spaceweather.com, observed it streak across our sky. No longer is the term "meteoroid" just the name we give to what a meteorite WAS before it hit our planet, we have actually studied a meteoroid in space, before impact.

On the Bad Astronomy blog, a commenter asked, "What is the distinction between a meteoroid and an asteroid?" The answer is that there is none. An asteroid is just a small, rocky body that orbits the sun. A meteoroid is just the term for a meteorite's state before it impacts earth, there is no size distinction. In other words, for the first time, we have observed a meteoroid.

Very cool stuff. Wished it streaked through our sky here in Tucson, but I made do with watching it on simulated on Celestia. Hopefully, some folks in southern Egypt caught it on tape :)

Transfer of mass from Io to Europa and beyond due to cometary impacts

Yesterday I posted that the new issue of the journal Icarus is now online. Two papers in the issue are directly related to Io. The first is titled, "Transfer of mass from Io to Europa and beyond due to cometary impacts," by José Luis Alvarellos, Kevin J. Zahnle, Anthony R. Dobrovolskis and Patrick Hamill. In this paper, the authors describe their modeling of ejecta from impacts onto the surface of Io, determining where that ejecta eventually end up in the Jovian system, and calculating how much mass is transferred from Io to the other major satellites in the system.

They modeled the ejection process two different ways. First, they assumed the surface of Io is a regolith and an impact will fling pre-existing blocks or rubble out from the crater. Second, they used a model by Jay Melosh et al. (1986), where the impact ejects chunks known as spalls out from a more competent or hard rock target. Based on what is known of Io's surface, where volcanic resurfacing occurs at a faster rate than impact gardening, the authors state that the latter model is likely more applicable.

The authors simulated impacts at the apex (center of the leading hemisphere), the antapex (the center of the trailing hemisphere), the sub-jovian point, the anti-jovian point, and the south pole. From the two models described above, the authors were able to calculate the velocities of particles generated by the impact as well as how much ejecta is generated by the impact of a generic 1.5 km wide Jupiter-family comet. Finally, 600 test particles with velocities fast enough to escape Io's Hill Sphere were injected into a model of the Jovian system to see where these particles would end up after an integration of 10,000 years.

The authors determined that the vast majority of the ejecta would wind up reimpacting Io, 93% in the rubble model, 86% in the spall model. This is expected since the particles start out in Io-crossing orbits. Their simulations found that between 5% ("Rubble" model) and 9% ("Spall" model) of the test particles end up impacting Europa, typically in a few decades to a couple of centuries. In the spall model, 4.6% of the test particles impacted Ganymede and less than one percent impacted Callisto. Only a handful in that same model ended up either impact Jupiter or one of the small inner satellites, entered heliocentric space, or survived the 10,000 year simulation. Typically, more ejecta reimpacted Io in the rubble model (where initial velocities are less so they start out in less eccentric orbits) than in the spall model, where their higher initial velocities put them in orbits that crossed that of the other Galileans.

Based on the percentage of material transferred from Io to one of the other satellites, estimates of the amount of ejecta per impact (typically 3x the mass of the impactor), and the impact rate on Io, the authors estimated the amount of mass transferred from Io to one of the other Galileans per million years. They estimate that approximately 3.1×10¹⁴ g and 1.6×10¹⁴ g are transferred to Europa and Ganymede via impact ejecta every million years, making Io a significant source of minerals and nutrients to the surfaces of those moons. That's in addition to the mass transferred via sputtering.

Link: Transfer of mass from Io to Europa and beyond due to cometary impacts [dx.doi.org]