Mercury flyby photo reveals first-ever glimpse of 'hidden hemisphere'

1/14/2008 4:44:00 PM - Peter McMahon

UPDATED 1/15/2008 6:34:00 PM -- Since our first close-up glimpses of Mercury in the 1970s, we've been left to wonder: "What does the rest of Mercury look like?" When it imaged the first planet from the Sun in 1974, NASA's Mariner 10 space probe was only able to capture 45 per cent of Mercury's surface as it flew by. Now, the US space agency's latest craft - MESSENGER - has photographed the planet closer and in more detail than ever before, revealing what looks to be an extreme version of Earth's Moon, with craters inside craters, inside craters.

GALLERY: Mercury Then And Now...

Image caption: The first of MESSENGER's ultra-high-res images of previously-unseen areas of the surface of Mercury. Taken January 14, on the upper right is the faint outline of the giant Caloris impact basin, including its western portions - never before seen by spacecraft. Caloris is one of the largest, youngest basins in the solar system. (enlarge image...)

Shrouded in mystery
Mercury rotates slowly so one side of the planet has always been turned away from a passing probe. As a result, until now, we've known only what poor-quality Earth-based telescopes showed us of other areas of Mercury, as it constantly grazes or passes in front of the blinding light of the Sun.

As of 2:00 pm ET on Jan 14, Messenger passed within 200 km of Mercury's surface - far closer than the 700 km above the surface that Mariner 10 passed 33 years ago. NASA's latest ambassador into the solar system will have far more advanced instruments trained on Mercury this time around, than its distant cousin did back in the Apollo era.

Unlike Mariner 10, MESSENGER will stop and stay a while, eventually settling into orbit in 2011. Before that, it will make two more flybys: later in 2008 and 2009. During this series of flybys and orbits, MESSENGER will try to solve more than a few mysteries about the closest planet to the Sun...

Fire and ice
While in the area, MESSENGER will image and scan Mercury to find out:

if - as has been suspected - there is ice tucked away at the bottom of Mercury's polar craters, which never see the scorching 450 C heat of Mercury's day side

whether 'Vulcanoids', a theoretical swarm of asteroids hidden in the glare of the Sun, exist further inward towards our local star

whether Mercury has an atmosphere or not - Hydrogen and helium floating above the planet may just be captured by the Sun, but there's also the possibility that gasses evaporated from near Mercury's surface

what makes Mercury the densest planet - chunk-by-chunk - in the solar system

if Mercury is shrinking - Are hundred-kilometre-high ridges on its surface a sign that the planet is buckling under a slow implosion?