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Mercury: Unusual insides and active history

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The $446 million Messenger (MErcury Surface, Space ENvironment, GEochemistry and Ranging) spacecraft launched in 2004. It then took a circuitous route to the solar system's innermost planet, becoming the first probe ever to orbit Mercury in March 2011. [Latest Mercury Photos from Messenger]

Since then, Messenger has been zipping around the baking-hot planet — which orbits the sun from just 36 million miles (58 million kilometers) away, compared to 93 million miles (150 million km) for Earth — once every 12 hours.

The probe is mapping Mercury's surface and gathering data on the planet's composition, magnetic environment and tenuous atmosphere, among other features. To date, Messenger has taken nearly 100,000 images and made more than 4 million measurements of the planet's surface, researchers said.

Messenger's original science campaign was designed to last one Earth year, but NASA announced in November that it had granted the spacecraft a one-year mission extension. Messenger officially began its extended mission earlier this week.

Surprising findings

The two new studies, which both appear in the March 23 issue of the journal Science, detail findings that should help scientists better understand Mercury's murky past.

In one study, researchers used observations made by Messenger's laser altimeter to map the topography of Mercury's northern hemisphere. They found that the range of elevations was smaller than that found on either Mars or the moon.

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