the face of Mercury no one has ever seen before

Click image to enlarge.

Mercury is one of the planets which human beings on Earth can see in the night sky with the unaided eye. (The others are Venus, Mars, and the gas giants Jupiter and Saturn.) The ancient Greeks called them planets because they "wander" among the fixed stars and constellations from night to night.

So, when Sun, Earth and Mercury are in a favorable configuration, we've always seen Mercury and always known it was there. The Greeks and Romans named it for the swift messenger god Mercury because its wanderings were the swiftest and most active of all the planets.

Before the telescope, we saw Mercury at best as a bright star-like dot. Because it's so small and so near the Sun, four centuries of telescopes -- even telescopes above the Earth's atmosphere and cameras aboard space probes -- haven't revealed much more about it.

Until the day before yesterday, we'd imaged less than half of its surface.

This is humankind's first look at the unseen side of Mercury, photographed from 27,000 kilometers, about 5.5 Mercury diameters away.

The camera which sent us the image is aboard NASA's MESSENGER probe -- an acronym for MErcury Surface, Space ENvironment, GEochemistry and Ranging. This is a close flyby that is a phase of a long, complicated journey through the inner Solar System. In 2011 MESSENGER will return to Mercury and insert itself into orbit to continue surveying and investigating the planet.

Mercury is about twice as large as our Moon, and at first glance its surface seems closely to resemble the Moon's surface. Both objects lack gas atmospheres, so they have no shield or protection from the full brunt of collisions with smaller massive objects flying through the Solar System. The visual evidence of every surface impact remains largely undisturbed for the rest of time. The most prominent such crater and basin in this view of Mercury is Caloris.

Details as small as 10 km / 6 miles can be seen. If there's a town down there the size of my town, we'll soon see it. If they live on the surface (not a good or safe idea), the Mercurians can hide from us no longer.

No living thing, on or beyond Earth, was harmed or diminished to bring us this remarkable new knowledge. It really deserves to go on a lot of walls and t-shirts. Some of us -- with incredible skill and cleverness -- are reaching out to learn about our Solar System, our Milky Way galaxy, and our universe in a spirit of peaceful curiosity and scientific inquiry.

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Image: NASA
Johns Hopkins University Applied Physics Laboratory
Carnegie Institution of Washington (DC)

When Mariner 10 flew past Mercury three times in 1974 and 1975, the same hemisphere was in sunlight during each encounter. As a consequence, Mariner 10 was able to image less than half the planet.

Planetary scientists have wondered for more than 30 years about what spacecraft images might reveal about the hemisphere of Mercury that Mariner 10 never viewed.

On January 14, 2008, the MESSENGER spacecraft observed about half of the hemisphere missed by Mariner 10. This image was snapped by the Wide Angle Camera, part of the Mercury Dual Imaging System (MDIS) instrument, about 80 minutes after MESSENGER's closest approach to Mercury (2:04 pm EST), when the spacecraft was at a distance of about 27,000 kilometers (about 17,000 miles). The image shows features as small as 10 kilometers (6 miles) in size. This image was taken through a filter sensitive to light near the red end of the visible spectrum (750 nanometers), one of a sequence of images taken through each of MDIS’s 11 filters.

Like the previously mapped portion of Mercury, this hemisphere appears heavily cratered. It also reveals some unique and distinctive features. On the upper right is the giant Caloris basin, including its western portions never before seen by spacecraft. Formed by the impact of a large asteroid or comet, Caloris is one of the largest, and perhaps one of the youngest, basins in the Solar System. The new image shows the complete basin interior and reveals that it is brighter than the surrounding regions and may therefore have a different composition. Darker smooth plains completely surround Caloris, and many unusual dark-rimmed craters are observed inside the basin.

Several other multi-ringed basins are seen in this image for the first time. Prominent fault scarps (large ridges) lace the newly viewed region.

Other images obtained during the flyby will reveal surface features in color and in much more detail. Collectively, these images and measurements made by other MESSENGER instruments will soon provide a detailed global view of the surface of Mercury, yielding key information for understanding the formation and geologic history of the innermost planet.