It's been nearly four years since the Messenger spacecraft arrived at Mercury, and during that time it's exceeded its mission goals, sending more than 250,000 images back to Earth. Faced with an unexpected surplus of real estate, the International Astronomical Union wants your help naming the planet's craters.

Thus far, the IAU has given official designations to 372 of the craters discovered, but several hundred thousand others remain nameless. A competition, launched by the IAU and the Messenger Education and Public Outreach Team, is inviting "all Earth citizens" to name five of them, and runs through January 15th.

Each of the five craters possesses characteristics that make them stand out from the thousands of others. For instance, in the case of Crater B:

When a meteoroid crashed into Mercury to form this crater, the explosion brought previously buried material to the surface and forced it outward to form the bright rays you see in this picture [below]. Interestingly, the rock exposed on the central peak of the crater is different from the rock of the surrounding plains. Messenger's high-resolution images have revealed unique features on Mercury that we now call "hollows." There are hollows on the steep slopes of this crater's central peak. Finding out more about this crater will play an important role in understanding how hollows form.

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The rules of the contest stipulate that the impact craters can only be named in honor of people who have made outstanding or fundamental contributions to the Arts and Humanities, such as visual artists, writers, poets, dancers, architects and musicians. The person must have been recognized as a significant figure for more than 50 years and must have been dead for at least three years. The contest runners are "particularly interested in submissions that honor people from nations and cultural groups that are under-represented amongst the currently named craters."

All the rules and submission forms can be found at the website of the Name a Crater on Mercury Competition.