coondoggie writes "While its sister rover Spirit has garnered most of the attention lately, NASA's other Mars traveler, Opportunity, is chewing up Martian dirt and unearthing the mineral and chemical makeup of the red planet. NASA scientists said this week the rover uncovered 'one of the coolest things Opportunity has found in a very long time:' a dark, basketball-sized rock known as 'Marquette Island.' According to NASA, the Marquette Island rock is a coarse-grained rock that indicates it cooled slowly from molten rock, allowing crystals time to grow. Such composition suggests it originated deep in the crust, not at the surface where it would cool quicker and have finer-grained texture, NASA stated."

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