"Shooting Stars" On Mars: Messages From A Meteorite

Mars Rover's Meteorite Discovery Triggers Questions
Iron Meteorite on Mars, the first meteorite of any type ever identified on another planet. The pitted, basketball-size object is mostly made of iron and nickel. Readings from spectrometers on the Opportunity rover determined that composition. Opportunity used its panoramic camera to take the images used in this approximately true-color composite on the rover's 339th martian day, or sol on January 6, 2005. Image
(Image: © NASA/JPL/Cornell)

NASA'sOpportunity Mars rover has completed its inspection of "Heat Shield Rock" -- aniron meteorite the robot came across at Meridiani Planum, acratered flatland that the machine has called home since landing on the redplanet over a year ago.

The pitted, basketball-size meteorite is mostly made of ironand nickel, as detected by the rover's set of onboard spectrometers -- devicesthat map the presence of different elements on the surface of Mars.

Meteors, often called "shooting stars" have been seen blazingthrough the Martian sky by both Opportunityand its sister ship, Spirit, now rolling through the Columbia Hills at GusevCrater.

But finding the meteorite was a surprise to scientistsrunning the dual Mars Exploration Rover effort. The researchers now wonder justhow prolific meteorites might be, perhaps sitting there strewn across theexpanse of Meridiani Planum.

Sitting there a very long time

"We're donewith the meteorite," said Steve Squyres from Cornell University,and the leader of the science team for the Mars Exploration Rover (MER) mission.

Squyres toldSPACE.com that further study ofnearby entry shield hardware that slammed onto Mars during Opportunity'slanding is on tap, followed by setting sail to the southtoward a circular feature called "Vostok". 

Discoveringthe meteorite has raised some speculation it might have been dislodged from adifferent locale -- "unmarsed" so to speak -- andtossed to its present position by the high-speed impact of Opportunity'sentry shield, but Squyres said there was "nomorphologic evidence for disturbance of soil around the meteorite."

"It hasbeen sitting there a very long time," Squyres said.

Difficult questions to answer

There are anumber of things that make the meteorite find a very scientifically interestingand important find, said planetary geologist, Bradley Jolliff of Washington Universityin St. Louis, Missouri and a MER science team member.


"From the well-preserved shape and form of cuspate marks and 'thumb-print'-likecavities, it is possible to say something about the velocity and atmosphericeffects that this meteorite experienced when it fell," Jolliff said.

How fastthe meteorite struck Mars, and just how thick was the martianatmosphere during its fall are difficult questions to answer. "But carefulstudy will likely provide some constraints. The external shapes andoverall morphology suggest that this was not a piece broken off from a largerobject, but that this is the entire meteorite," Jolliff told SPACE.com.

Recent event or ancient?

Roverscientists are also studying the microscopic images of the meteorite taken by Opportunity. They're on the lookout for any features thatmight record the level of shock experienced when the object hit the surface tohelp bracket possible velocity ranges, Jolliff said.

"We canlook for any tell-tale signs of an impact to determine if perhaps this was arecent event or an ancient one," Jolliff explained. "It may be that thismeteorite has been buried and re-exhumed by inflation and deflation of theMeridiani surface regolith over time."

Importantquestions: If the meteorite is very old, is the external surface weathered oroxidized? Also, what has been the interplay between the meteorite and possibleweathering and abrasion by wind-blown sand particles? 

Ground-truth learning tool

Scientistsare taking a hard look at the nooks and crannies of the object using Opportunity imagery and other data. By inspecting differentparts of the meteorite's surface and its "protected" hollows, the tale of whatreactions have occurred between the metal and the thin martianatmosphere may be gleaned. 

"In thiscase, study of the meteorite is providing clues about the surface andatmosphere of Mars," Jolliff added. Close-up exam of the object by the rover'scamera and Mini-Thermal Emission Spectrometer (Mini-TES) has meant giving thema sort of a ground-truth learning tool, he said.

"Now we canrecognize these objects, which might be common on the Meridiani surface,without having to drive up close to them. Their spectral signature andcontrast are quite unique," Jolliff explained.


Fascinating place to explore


Opportunity's discovery of the meteorite is avery important scientific find for another reason.

"Considerthe Apollo samples from the Moon. With the exception of a few very tinybits of meteorite and meteoritic metal, there have been no 'large' meteoritesfound," Jolliff recalled. "Of course, there is no atmosphere to slow themdown. Yet we suspect that they must be plentiful, even if small, and oneday, when we can sample and analyze [lunar] surface materials routinely, wewill likely find lots of them."

They mayrepresent a different "sampling" of meteorite populations in space and time, Jolliffcontinued, "and thus give us new insights to the origins of specific types ofmeteorites." 

Onequestion in Jolliff's mind is whether the distribution of meteorite types foundtoday on Earth is representative of the meteorite distribution of past epochsas well, and of different locations in the solar system. 

"This findon Mars reinforces that Mars' surface also holds great potential in thisregard, especially a flat surface such as Meridiani Planum. It has beenlikened to some of the ablating ice surfaces in Antarcticawhere many -- and in some places nearly all -- of the rocks at the surface aremeteorites," Jolliff pointed out. "What a spectacularly fascinating place toexplore!"

Unknowncycles and intensities

There's yetanother incentive to get a handle on the "rainfall" of meteors plowing into andthrough the thin martian atmosphere.

Just how vulnerable are spacecraft orbiting Mars, as well ascrews staying for extended stays on the planet?

"Thereis pure scientific interest in knowing the frequency, intensity, and radiantsof martian meteor showers. Being in a different orbitthan Earth and closer to the asteroid belt, Mars has unknown cycles andintensities of meteoroid hazards. Knowledge of these hazards can help us managerisk in future missions, particularly extended and crewed missions."

That's the view of the Oregon L5 Society in a research paperadvocating a Mars meteor survey, presented back in July 2000 to a Concepts andApproaches for Mars Exploration workshop, sponsored by the Lunar and Planetary Institute in Houston, Texas.

Great meteoritical museum

Theresearch group has advocated outfitting instruments on one ormore Mars landers to identify and characterize the meteoroid flux at Mars.

Alongwith a camera, the survey gear would include a radio (or microwave) device topick up a radio frequency signal created by meteoroid entry; a microphone toperhaps detect any sonic boom or other sound generated by a meteoroid entry; aswell as a seismometer to record a seismic signal stemming from a meteor impact.

Such equipment could help assess the nature of martian meteor showers, how big are they and where do theycome from. Also, the gear might determine whether or not meteor storms can bepredicted on Mars. Moreover, could surface operations of human expeditions beexposed to periodic "rains of rock"?

According to Bob McGown of Portland, Oregon, leader of the researchteam on the Mars meteor survey idea, the red planet may well be a storehouse offallen shooting stars.

"There isthe idea that the red dusty surface of Mars is from the rusting ofiron-nickel meteorites on the surface, McGown told SPACE.com. "Also...a rock from Venus or from Mercury has never beenfound. The deserts of Mars could be like a great meteoritical museum or spacelaboratory...a collecting ground of rocks from other worlds."

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