We now have the first optical photos ever of water ice on the surface of the planet Mercury.

NASA's MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) recently took the photos, which confirm previous observations from 20 years ago by Earth-based telescopes, when signs of water ice was first spotted near Mercury's two poles.

What makes this so interesting is that Mercury is the closest planet to the sun, meaning it's hot, reaching temperatures close to 800 degrees Fahrenheit. This means that ice only exists in parts of the planet that stay in shadow. This also makes water ice difficult to image.

MESSENGER, however, was able to observe scattered light from a crater called Prokofiev near Mercury's north pole in 2012. This is the largest crater in that region on the planet. Scientists analyzed those observations and combined that data with thermal models.

Now, however, MESSENGER has images of this ice, taking advantage of the small amounts of sunlight that bounce off Prokofiev's sides. In the images, this water ice is shown as reflections of that sunlight, indicating where the ice is present.

Perhaps most interesting about these images, though, is that they show water ice that's a lot younger than scientists expected. MESSENGER's images show sharp edges around the crater, which denote dark deposits of organic compounds.

"This result was a little surprising, because sharp boundaries indicate that the volatile deposits at Mercury's poles are geologically young, relative to the time scale for lateral mixing by impacts," says lead researcher Nancy Chabot.

Scientists have long wondered about the age of Mercury's ice, but assumed it was older. Now, however, it seems they have a more definitive answer. And that answer could lead to an understanding about how water came to other planets in the solar system, including Earth.

The Earth's moon also has water ice inside craters near its poles, but the moon's ice is radically different. The relative young age of Mercury's ice may explain why.

"If you can understand why one body looks one way and another looks different, you gain insight into the process that's behind it, which in turn is tied to the age and distribution of water ice in the solar system," says Chabot. "This will be a very interesting line of inquiry going forward."

NASA's MESSENGER spacecraft recently celebrated 10 years in space. Its ongoing mission is to study Mercury and it is only the second of two spacecrafts ever sent to the planet's orbit.

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