A meteorite the size of a huge watermelon on Mars is revealing new clues to the planet's environment.

The Martian meteorite weighs at least half a ton, making it much too large to have plunged through Mars' current thin atmosphere and hit the ground without being obliterated upon impact, rover scientists said. Either the atmosphere was thicker than expected some time in the relatively recent past, scientists figure, or the rock fell to the surface billions of years ago when the atmosphere was thicker.

NASA's Opportunity rover discovered the metallic meteorite - which scientists now call Block Island - in late July, then drove up to take a closer look.

"Consideration of existing model results indicates a meteorite this size requires a thicker atmosphere," said Matt Golombek, a rover team member at NASA's Jet Propulsion Laboratory in Pasadena, Calif., in a statement. "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."

A planet's atmosphere can slow a meteorite's fall due to the friction the rock encounters as it plows through the atmosphere after flying in the relative vacuum of space.

Block Island is about 2 feet (60 cm) long, about half as tall and has a bluish tint that sets it apart from its rust-colored surroundings, mission managers said. It is 10 times as massive as Heat Shield Rock, another Martian meteorite found by Opportunity in late 2004.

The meteorite would have to be several times smaller than its current size to have survived landing on Mars today, rover scientists said.

Martian meteorite up close

Space rocks come in several varieties, ranging from stone to iron and other heavy metals. They're typically fragments of asteroids chipped off in collisions or through other natural weathering processes. Some land on Earth, and others hit the moon, Mars and other bodies.

Since finding Block Island, Opportunity has used its robotic arm to reach out and touch it, examining the object with a set of instruments at the tip, including an alpha particle X-ray spectrometer that can identify elements of its composition.

"There's no question that it is an iron-nickel meteorite," said the spectrometer's lead scientist Ralf Gellert of the University of Guelph in Ontario, Canada. "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."

Close-up images of the meteorite from Opportunity's arm-mounted microscopic imager have revealed triangular patterns on Block Island's surface that match ones seen in iron-nickel meteorites found on Earth, mission scientists said.

"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."

Weather on Mars

By studying variations in the composition of Block Island's exterior and comparing the meteorite with similar ones found on Earth, researchers hope to determine how the rock may have withstood the rigors of time and exposure to any liquid or water vapor.

"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," said JPL rover team member Albert Yen. "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."

Rover scientists plan to spend a good deal of time studying Block Island before resuming Opportunity's trek toward its next stop Endeavour Crater. The giant crater 13.7 miles (22 km) across and is about 12 miles away (19 km) from Victoria Crater, Opportunity's last pit stop. The rover has managed to complete about one-fifth the trip after a year of driving on Mars.

While Opportunity studies Block Island, engineers on Earth continue to come up with an escape plan for its robotic twin Spirit, which has been stuck in deep Martian sand since early May. The two rovers have been exploring different parts of Mars since they landed in early 2004.

• SPACE.com Video Show - Rover Tracks on Mars
• Video - Opportunity's Mars Marathon
• Video - Meteor's Fall Caught on Camera