Mapping the Solar System: Mercury and Titan
It wasn’t so long ago that our world maps had parts that were either left empty or left to conjecture. “Here be dragons.” We haven’t had to worry about unmapped, unknown parts of the Earth — terra incognita — for a while, but this isn’t true for the rest of the solar system. Other worlds are still in the process of being mapped. We have to wait until 2015 for dwarf planets Ceres and Pluto to receive their first visits from space probes, but even places we’ve already sent probes to have not yet been completely mapped.
Take Titan, for example — Saturn’s largest moon. Discovered in 1655, it was first visited by Voyager 1 in 1980, but its dense, smog-like atmosphere prevented a clear view of the surface. That had to wait for the Cassini-Huygens mission, which arrived at Saturn in 2004. The Huygens lander sent back images, but since then the Cassini probe has been making passes of the moon, using filtered imagery and radar to map the planet, which is now revealed as an ice world with liquid hydrocarbon lakes. NASA has maps of Titan (as of December 2006) with and without labels, as well as more recent radar images of the northern and southern polar regions.
Mercury’s situation is even more unusual: it was visited by the Mariner 10 probe in 1974 and 1975, but its passes by the innermost planet only allowed half of its surface to be imaged. The MESSENGER probe, which makes its first pass of Mercury tomorrow, will rectify that: after three flybys, it will eventually orbit the planet in 2011. The Planetary Society explains the mission and includes an equatorial map of the planet — well, half of it.