英語(yǔ)時(shí)差:火星上的海洋
Don: Hey, Ya?l, Have you heard? We've discovered two new oceans? Ya?l: Where? D: On Mars. Well, they aren't there any longer, but they were there a few billion years ago. Y: I thought scientists weren't sure if Mars had oceans or not. D: They weren't. The Mars Opportunity Rover found sulfates and other salts in the soil that were left behind when standing water evaporated. It also discovered cross-bedding in the soil which was created by water ripples, and tiny mineral deposits called blueberries that only appear if water is present. The northern lowlands of Mars looked like there might have been an ocean there, but there wasn't definite proof. Y: So, what has changed their minds? D: Gamma Ray evidence. Y: That sounds exciting. D: It is. The Mars Odyssey spacecraft had a gamma-ray spectrometer onboard that could detect elements buried as much as a third of a meter, or thirteen inches, below the soil. Scientists used the spectrometer to measure the amount of potassium, thorium, and iron above and below what they thought to be the shoreline of ancient oceans. They expected to see low concentrations above the shoreline and larger concentrations below. That would indicate that the elements had been leached out of the soil by the water. Y: So, what did they find? D: They found the shorelines of a younger ocean about ten times the size of the Mediterranean Sea, and an older ocean twice that size. Y: It's too bad those oceans still aren't there. D: Yeah. It sure would be a great spot for a cruise.
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