NASA probe reveals strange hallows on Mercury

Mercury, the barren planet closest to the sun, may seem like a dead world, but new images taken by a NASA probe nearing the end of its life reveal the planet may still be undergoing geological activity, the Discovery News reports.

The new images — taken by NASA's MESSENGER spacecraft — show that Mercury has strange features known as "hollows" (irregularly shaped, flat-floored depressions) that are only a few tens of meters deep and no more than a kilometer in diameter, scientists with the mission said. That finding suggests that Mercury may have lost some of its lighter elements, such as sodium and potassium —a phenomenon that may still be happening today, the scientists said.

"MESSENGER revealed many surprising things about Mercury," said MESSENGER participating scientist David Blewett, of the Johns Hopkins Applied Physics Laboratory. Blewett was part of the team that investigated the unusual features on Mercury as part of the low-altitude campaign, which makes up the last six weeks of MESSENGER's orbit around the planet. These and other findings from the campaign were presented at the 46th Lunar and Planetary Science Conference in Texas on March 16.

"Hollows are one of the surprises in terms of the geological processes that shape the surface at small scales," Blewett told Space.com via email.


When NASA's Mariner 10 flew by Mercury three times in the mid-1970s, it spotted what Blewett described as "certain odd, bright patches" within the impact craters. The patches showed up again during MESSENGER's flybys of the planet. It wasn't until 2011, whenMESSENGER entered orbit and began to capture higher-resolution images, that the bright areas were revealed as shallow, irregularly shaped depressions on the surface.

Hollows are relatively small landforms — shallow features on the surface that stretch at most 0.6 miles (1 kilometer). This small size implies a relatively young age, as cratering would have eroded them away over time. The hollows do not contain many, if any, impacts within them — another characteristic that suggests that they are fairly young. Finally, the hollows' sharp edges also likely indicate these features were made relatively recently.

"The hollows are remarkable because they have a crisp, fresh appearance," Blewett said. "They are probably younger than a few tens of millions of years, but some hollows are probably forming today. It is amazing to find ongoing geological activity modifying the surface of Mercury."

While Earth's moon lacks similar features, the southern polar cap of Mars and some of the icy satellites in the outer solar system have shapes that resemble the hollows. Water ices on the Red Planet and distant moons mix with nonice material such as dirt and rock. When the sun heats these regions, the ice sublimates — meaning it changes directly from a solid to a gas. Voids grow within the features, and the remaining dirt/rock mixture collapses, creating pits.