Asteroids Ahoy! Jupiter Scar Likely from Rocky Body

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.

Cassini Rocks Rhea Rendezvous

NASA's Cassini spacecraft has successfully completed its closest flyby of Saturn's moon Rhea, returning raw images of the icy moon's surface.





Pictures of the Rhea surface taken around the time of closest approach at 4:53 a.m. UTC on Jan. 11, 2011, which was 10:53 p.m. PST, Jan. 10, show shadowy craters at a low sun angle. A portrait of bright, icy Rhea also captures Saturn's rings and three other moons clearly visible in the background.

Images obtained by Cassini's imaging science subsystem show an old, inert surface saturated with craters, just like the oldest parts of Earth's moon. But there appear to be some straight faults that were formed early in Rhea's history, which never developed the full-blown activity seen on another of Saturn's moons, Enceladus.

The flyby of Rhea also presented scientists with their best available chance to study how often tiny meteoroids bombard the moon's surface. Scientists are now sifting through data collected on the close flyby by the cosmic dust analyzer and the radio and plasma wave science instrument. They will use the data to deduce how often objects outside the Saturn system contaminate Saturn's rings, and to improve estimates of how old the rings are.

Scientists using Cassini's fields and particles instruments are also looking through their data to see if they learned more about Rhea's very thin oxygen-and-carbon-dioxide atmosphere and the interaction between Rhea and the particles within Saturn's magnetosphere, the magnetic bubble around the planet.

At closest approach, Cassini passed within about 69 kilometers (43 miles) of the surface.

Spirit's Quiet Anniversary

Spirit's Quiet Anniversary

Space diehards took a moment yesterday to observe an exploration milestone. It was seven years ago, January 3, 2004, that a rolling robot named Spirit landed on Mars.

Together with its twin, Opportunity, which arrived 22 days later, these Mars Exploration Rovers easily exceeded their basic 90-day mission plans and have since surpassed even the most optimistic expectations.

But 2010 was not an easy year for Spirit. Having become entrenched — literally and unfortunately — in a patch of soft sand in May 2009, the "elder" rover struggled to free itself despite having lost the use of two of its six wheels. It was beginning to make some hard-won progress when, a year ago, Martian winter set in. The craft fell silent in March and hasn't been heard from since.

Engineers at the Jet Propulsion Laboratory know that Spirit put itself into electronic hibernation to conserve energy until enough sunlight could fall on its solar-cell panels to recharge its batteries. But the depths of winter have come and gone at its current location, and the rover should have awakened by now.


Orbital view of Spirit and Home Plate
On June 13, 2009, the HiRISE camera aboard NASA's Mars Reconnaissance Orbiter captured this view of the Spirit rover next to the broad plateau named Home Plate, which is about 300 feet (90 m) across. The rover became stuck there in May 2009. Click on the image for a larger view.
NASA / JPL / Univ. of Arizona

The team now believes the rover got so cold (down to at least –148°F (–100°C) that the internal clock lost power and the spacecraft has lost track of time. This is the first time winter's cold has forced either rover to hibernate, so engineers are in untried territory. But according to the MER playbook, Spirit should now be waiting for a command from Earth to wake up and report in. It's just that the rover is only listening for 20 minutes each day — and it's impossible to know when that window is occurring.

Although Spirit's handlers remain optimistic that it will revive, the mission's periodic status reports have been discouragingly headlined "Spirit Remains Silent at Troy" since August.

A dust storm swept over the area in October, and the winds might have either partially cleaned the power panels of accumulated grit (a plus) or added more to what's already there (a definite minus). The sunlight's strength should peak in about two months, and the wake-up calls from Earth will continue until then and beyond.


Opportunity next to Santa Maria crater
The HiRISE camera aboard NASA's Mars Reconnaissance Orbiter snapped this view of the rover Opportunity perched on the rim of Santa maria, a relatively fresh impact crater about 300 feet (90 m) across. Click here for a larger image.
NASA / JPL / Univ. of Arizona

Meanwhile, Opportunity is faring somewhat better. It's more than midway through a long-distance drive to Endeavour, a largish crater about 14 miles (22 km) across. Recently Opportunity has been rolling along backward, an attempt to reduce wear on its wheels' aging gear trains.

Mission scientists have interrupted Opportunity's trek a couple of times to take in the local scenery. A few months ago it chanced upon a meteorite, which scientists dubbed Oileán Ruaidh (the Gaelic name of an island in northwestern Ireland). Right now the rover is checking out Santa Maria, a relatively fresh crater about the size of a football field.


Endeavour's distant rim
On October 31, 2010, the rover Opportunity viewed the upraised rim of its long-term objective, the large crater Endeavour.
NASA / JPL / Cornell Univ.

Opportunity is still about 4 miles (6½ km) from Endeavour, but the spacecraft's cameras can already see its uplifted rim. Once the scientific study of Santa Maria ends, the rover will make a beeline for what promises to be the mission's grand prize: a geologic candy store of outcrops along the rim that could reveal much about the role of water in the planet's past.

Saturn's New Bright Storm

The two largest planets in our solar system are now putting on an exciting show for observers — and they're perfectly placed for those of you hoping to try out that new telescope or eyepiece holiday gift.

On Jupiter, the revival of the South Equatorial Belt has spread to almost completely encircle the planet. Jupiter is a snap to find blazing high in the south at nightfall.

Meanwhile, a new white storm has burst forth on Saturn. Perhaps the ringed planet was feeling neglected due the attention heaped on Jupiter recently.

First spotted by amateurs in Japan around mid-December, Saturn's storm now spans roughly 100° of longitude in the North Tropical Zone (roughly 34° north). The bright "head," or leading edge, is located at System II longitude 157° (293° in System III).

Saturn is now well up in the southeastern sky before dawn, far to the upper right of bright Venus. Be on the lookout for rapid changes to the feature's shape and extent. It will next be positioned in the middle of the planet's disk at 00:21, 11:01, and 21:40 December 29th Universal Time; and at 8:19 (favoring eastern North America) and 18:59 on the 30th UT. Here are transit-time predictions through January 13th (in Easterm Standard Time, which is UT minus 5 hours).

This disturbance is not the same one noted by amateurs earlier this year, which appeared at a dynamic southern latitude band nicknamed "Storm Alley."

Veteran planet-watcher Thomas Dobbins notes the last time such a large storm appeared on Saturn was 1994, but that one had much lower contrast with its surroundings than the current event. This disturbance is easily the brightest feature on the globe — it even rivals the brightness of the planet's ring system.

NASA's Cassini spacecraft has a "ringside" seat for the roiling clouds, as seen in the shot at right, which was taken on December 24th and transmitted to Earth on the 27th.