Combine two WorldChanging obsessions -- online map systems and the planet Mars -- and you have the potential for something that could keep us happily clicking and playing for hours. Google has now unleashed Google Mars, a Google Maps site using satellite imagery of the Red Planet. It's not as powerful as Google Earth, but it's by far the most easily-accessible way to get to know the fourth planet from the sun. (Google suggests that a plug-in to bring Mars data to the Google Earth engine may soon be on its way.)

The site includes three different presentations of the Martian surface:

• Elevation - A shaded relief map, generated with data from the Mars Orbiter Laser Altimeter (MOLA) on NASA's Mars Global Surveyor spacecraft. This map is color-coded by altitude, so you can use the color key at the lower left to estimate elevations.
• Visible - A mosaic of images taken by the Mars Orbiter Camera (MOC) on NASA's Mars Global Surveyor spacecraft. MOC is like the digital camera you have at home. Basically, this is what your eyes would see if you were in orbit around Mars.
• Infrared - A mosaic of infrared images taken by the Thermal Emission Imaging System (THEMIS) on NASA's Mars Odyssey spacecraft. Warmer areas appear brighter, and colder areas are darker. Clouds and dust in the atmosphere are transparent in the infrared, making this the sharpest global map of Mars that's ever been made.

You can choose to view flags pointing out the planet's physical features, as well as the locations of the various landers (both successful and otherwise).

When you first hit Google Mars, you're presented with a colorful elevation map. Blue represents land below the Martian average elevation, rising to green, yellow, and orange, with red representing the higher elevations, and white the peaks of mountains. It's no coincidence that this color range strongly suggests what a lightly terraformed Mars might aspire to look like; the elevation map, despite having the least-realistic colors of the three, does the best job at making Mars look like a world, not just a rocky planet.

The announcement of Google Mars was well-timed; the Mars Reconnaissance Orbiter (MRO) successfully reached Martian orbit on Friday, and when it finishes its months-long aerobraking maneuver (using the Martian atmosphere to slow its speed), it will give us by far the most detailed images of Mars yet. Among MRO's tools are a camera with a resolution of one foot per pixel and a broadband transceiver, giving the Mars science fleet a high-speed orbiting router. Let's hope that Google gets the MRO pictures added to the Google Mars dataset as quickly as they can.