Meteorite found on Mars yields clues about planet's past

The rock, dubbed "Block Island," is larger than any other known meteorite on Mars. Scientists calculate it is too massive to have hit the ground without disintegrating unless Mars had a much thicker atmosphere than it has now when the rock fell. An atmosphere slows the descent of meteorites. Additional studies also may provide clues about how weathering has affected the rock since it fell.

Two weeks ago, Opportunity had driven approximately 180 meters (600 feet) past the rock in a Mars region called Meridiani Planum. An image the rover had taken a few days earlier and stored was then transmitted back to Earth. The image showed the rock is approximately 60 centimeters (2 feet) in length, half that in height, and has a bluish tint that distinguishes it from other rocks in the area. The rover team decided to have Opportunity backtrack for a closer look, eventually touching Block Island with its robotic arm.

"There's no question that it is an iron-nickel meteorite," said Ralf Gellert of the University of Guelph in Ontario, Canada. Gellert is the lead scientist for the rover's alpha particle X-ray spectrometer, an instrument on the arm used for identifying key elements in an object. "We already investigated several spots that showed elemental variations on the surface. This might tell us if and how the metal was altered since it landed on Mars."

The microscopic imager on the arm revealed a distinctive triangular pattern in Block Island's surface texture, matching a pattern common in iron-nickel meteorites found on Earth.

"Normally this pattern is exposed when the meteorite is cut, polished and etched with acid," said Tim McCoy, a rover team member from the Smithsonian Institution in Washington. "Sometimes it shows up on the surface of meteorites that have been eroded by windblown sand in deserts, and that appears to be what we see with Block Island."

This view of a rock called "Block Island," the largest meteorite yet found on Mars, comes from the panoramic camera (Pancam) on Opportunity. Analysis of Block Island's composition using the rover's alpha particle X-ray spectrometer confirmed that it is rich in iron and nickel. The rock is about 60 centimeters (2 feet) across. This is a false-color, red-green-blue composite view generated from images taken through the Pancam's 750-nanometer, 530-nanometer and 430-nanometer filters. The exaggeraged color is used for enhancing the visibility of differences among the types of rock and soil materials. High resolution image (2.4 MB). Credit: NASA/JPL-Caltech/Cornell University




Opportunity used its microscopic imager to get this view of the surface of a rock called "Block Island" during the 1,963rd Martian day, or sol, of the rover's mission on Mars (Aug. 1, 2009). The triangular pattern of small ridges seen at the upper right in this image and elsewhere on the rock is characteristic of iron-nickel meteorites found on Earth, especially after they have been cut, polished and etched. Block Island has been identified as an iron-nickel meteorite based on this surface texture and analysis of its composition by Opportunity's alpha particle X-ray spectrometer. At about 60 centimeters (2 feet) across, it is the largest meteorite yet found on Mars.

This image shows a patch 32 millimeters by 32 millimeters (1.3 inches by 1.3 inches) on the surface of Block Island while the target was fully illuminated by the sun. This target on the rock is informally named "New Shoreham." The vertical white streaks, especially near the top and bottom of the image, are artifacts caused by saturation of the camera's CCD (charge-coupled device, or image recorder) where sunlight glinted off metallic facets.

The triangular pattern in the texture of iron-nickel meteorites, called the Widmanstatten pattern, formed more than 4.5 billion years ago as the metal cooled. One iron-nickel mineral, kamacite, formed thin layers along the surface of crystals of another, taenite, which contains less nickel. The two minerals differ in their resistance to either etching by acid or erosion by wind-blown sand, so those processes can make the pattern visible.

Credit: NASA/JPL-Caltech/Cornell University/USGS




This iron-nickel meteorite found near Fort Stockton, Texas, in 1952 shows a surface texture similar to some portions of the surface of an iron-nickel meteorite that NASA's Mars Exploration Rover Opportunity found on Mars in July 2009. The Fort Stockton meteorite is about 15 centimeters (6 inches) across and is in the collection of Texas Christian University. High resolution image (1.5 MB). Credit: Smithsonian Institution

Opportunity found a smaller iron-nickel meteorite, called "Heat Shield Rock," in late 2004. At about a half ton or more, Block Island is roughly 10 times as massive as Heat Shield Rock and several times too big to have landed intact without more braking than today's Martian atmosphere could provide.

"Consideration of existing model results indicates a meteorite this size requires a thicker atmosphere," said rover team member Matt Golombek of NASA's Jet Propulsion Laboratory in Pasadena, Calif. "Either Mars has hidden reserves of carbon-dioxide ice that can supply large amounts of carbon-dioxide gas into the atmosphere during warm periods of more recent climate cycles, or Block Island fell billions of years ago."

Spectrometer observations have already identified variations in the composition of Block Island at different points on the rock's surface. The differences could result from interaction of the rock with the Martian environment, where the metal becomes more rusted from weathering with longer exposures to water vapor or liquid.

"We have lots of iron-nickel meteorites on Earth. We're using this meteorite as a way to study Mars," said Albert Yen, a rover team member at JPL. "Before we drive away from Block Island, we intend to examine more targets on this rock where the images show variations in color and texture. We're looking to see how extensively the rock surface has been altered, which helps us understand the history of the Martian climate since it fell."

When the investigation of Block Island concludes, the team plans to resume driving Opportunity on a route from Victoria Crater, which the rover explored for two years, toward the much larger Endeavour Crater. Opportunity has covered about one-fifth of the 19-kilometer (12-mile) route plotted for safe travel to Endeavour since the rover left Victoria nearly a year ago.

Source: Jet Propulsion Laboratory
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