[meteorite-list] Asteroids Ahoy! Jupiter Scar Likely from Rocky Body

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 26 Jan 2011 17:21:21 -0800 (PST)
Message-ID: <201101270121.p0R1LL4u027957_at_zagami.jpl.nasa.gov>

http://www.jpl.nasa.gov/news/news.cfm?release=2011-028

Asteroids Ahoy! Jupiter Scar Likely from Rocky Body
Jet Propulsion Laboratory
January 26, 2011

A hurtling asteroid about the size of the Titanic caused the scar that
appeared in Jupiter's atmosphere on July 19, 2009, according to two
papers published recently in the journal Icarus.

Data from three infrared telescopes enabled scientists to observe the
warm atmospheric temperatures and unique chemical conditions associated
with the impact debris. By piecing together signatures of the gases and
dark debris produced by the impact shockwaves, an international team of
scientists was able to deduce that the object was more likely a rocky
asteroid than an icy comet. Among the teams were those led by Glenn
Orton, an astronomer at NASA's Jet Propulsion Laboratory, Pasadena,
Calif., and Leigh Fletcher, researcher at Oxford University, U.K., who
started the work while he was a postdoctoral fellow at JPL.

"Both the fact that the impact itself happened at all and the
implication that it may well have been an asteroid rather than a comet
shows us that the outer solar system is a complex, violent and dynamic
place, and that many surprises may be out there waiting for us," said
Orton. "There is still a lot to sort out in the outer solar system."

The new conclusion is also consistent with evidence from results from
NASA's Hubble Space Telescope indicating the impact debris in 2009 was
heavier or denser than debris from comet Shoemaker-Levy 9, the last
known object to hurl itself into Jupiter's atmosphere in 1994.

Before this collision, scientists had thought that the only objects that
hit Jupiter were icy comets whose unstable orbits took them close enough
to Jupiter to be sucked in by the giant planet's gravitational
attraction. Those comets are known as Jupiter-family comets. Scientists
thought Jupiter had already cleared most other objects, such as
asteroids, from its sphere of influence. Besides Shoemaker-Levy,
scientists know of only two other impacts in the summer of 2010, which
lit up Jupiter's atmosphere.

The July 19, 2009 object likely hit Jupiter between 9 a.m. and 11 a.m.
UTC. Amateur astronomer Anthony Wesley from Australia was the first to
notice the scar on Jupiter, which appeared as a dark spot in visible
wavelengths. The scar appeared at mid-southern latitudes. Wesley tipped
off Orton and colleagues, who immediately used existing observing time
at NASA's Infrared Telescope Facility in Mauna Kea, Hawaii, the
following night and proposed observing time on a host of other
ground-based observatories, including the Gemini North Observatory in
Hawaii, the Gemini South Telescope in Chile, and the European Southern
Observatory's Very Large Telescope in Chile. Data were acquired at
regular intervals during the week following the 2009 collision.

The data showed that the impact had warmed Jupiter's lower stratosphere
by as much as 3 to 4 Kelvin at about 42 kilometers above its cloudtops.
Although 3 to 4 Kelvin does not sound like a lot, it is a significant
deposition of energy because it is spread over such an enormous area.

Plunging through Jupiter's atmosphere, the object created a channel of
super-heated atmospheric gases and debris. An explosion deep below the
clouds ??? probably releasing at least around 200 trillion trillion ergs
of energy, or more than 5 gigatons of TNT -- then launched debris
material back along the channel, above the cloud tops, to splash back
down into the atmosphere, creating the aerosol particulates and warm
temperatures observed in the infrared. The blowback dredged up ammonia
gas and other gases from a lower part of the atmosphere known as the
troposphere into a higher part of the atmosphere known as the stratosphere.

"Comparisons between the 2009 images and the Shoemaker-Levy 9 results
are beginning to show intriguing differences between the kinds of
objects that hit Jupiter," Fletcher said. "The dark debris, the heated
atmosphere and upwelling of ammonia were similar for this impact and
Shoemaker-Levy, but the debris plume in this case didn't reach such high
altitudes, didn't heat the high stratosphere, and contained signatures
for hydrocarbons, silicates and silicas that weren't seen before. The
presence of hydrocarbons, and the absence of carbon monoxide, provide
strong evidence for a water-depleted impactor in 2009."

The detection of silica in this mixture of Jovian atmospheric gases,
processed bits from the impactor and byproducts of high-energy chemical
reactions was significant because abundant silica could only be produced
in the impact itself, by a strong rocky body capable of penetrating very
deeply into the Jovian atmosphere before exploding, but not by a much
weaker comet nucleus. Assuming that the impactor had a rock-like density
of around 2.5 grams per cubic centimeter (160 pounds per cubic foot),
scientists calculated a likely diameter of 200 to 500 meters (700 to
1,600 feet).

Scientists computed the set of possible orbits that would bring an
object into Jupiter in the right range of times and at the right
locations. Then they searched the catalog of known asteroids and comets
to find the kinds of objects in these orbits. An object named 2005 TS100
??? which is probably an asteroid but could be an extinct comet ??? was one
of the closest matches. Although this object was not the actual
impactor, it has a very chaotic orbit and made several very close
approaches to Jupiter in computer models, demonstrating that an asteroid
could have hurtled into Jupiter.

"We weren't expecting to find that an asteroid was the likely culprit in
this impact, but we've now learned Jupiter is getting hit by a diversity
of objects," said Paul Chodas, a scientist at NASA's Near-Earth Object
Program Office at JPL. " Asteroid impacts on Jupiter were thought to be
quite rare compared to impacts from the so-called 'Jupiter-family
comets,' but now it seems there may be a significant population of
asteroids in this category."

Scientists are still working to figure out what that frequency at
Jupiter is, but asteroids of this size hit Earth about once every
100,000 years. The next steps in this investigation will be to use
detailed simulations of the impact to refine the size and properties of
the impactor, and to continue to use imaging at infrared, as well as
visible wavelengths, to search for debris from future impacts of this
size or smaller.

JPL is managed for NASA by the California Institute of Technology in
Pasadena.

Jia-Rui C. Cook 818-354-0850
Jet Propulsion Laboratory, Pasadena, Calif.
jccook at jpl.nasa.gov

2011-028
Received on Wed 26 Jan 2011 08:21:21 PM PST