The Mars Rover on work
When NASA sent the twin Viking landers to Mars in the 1970s, they had the three basic components of any interplanetary robot
They could produce the power they needed to execute their missions.
They could gather information with their sensors.
They could send the sensor information back to Earth.
The one thing the Viking landers could not do is move, although they did have robotic arms that could reach out and scoop up soil.
NASA first solved the movement problem with the Pathfinder mission in 1997. A tiny rover (just 25 pounds/11 kg) could leave the lander and travel up to 5 meters (15 feet) away from it to look at rocks.
The MER robots are the largest rovers to ever successfully land on another planet. On this mission, NASA has designed the MER robots to act as robotic geologists. The instruments and equipment packed into the rovers are primarily designed to look at rocks.
Here's what each rover can do
The rovers can generate power with their solar panels and store it in their batteries.
The rovers can take color, stereoscopic images of the landscape with a pair of high-resolution cameras mounted on the mast.
They can also take thermal readings with a separate thermal-emission spectrometer that uses the mast as a periscope.
Scientists can choose a point on the landscape and the rover can drive over to it. The rovers are autonomous -- they drive themselves, because the time lag for radio signals to travel between Earth and Mars is too great for the rovers to be radio controlled. Three pairs of black-and-white cameras on the front, back and mast of the rover let the robot see its surroundings and navigate around obstacles. The rovers have six wheels, with a motor in each wheel, to move around.
The rovers can use a drill, mounted on a small arm, to bore into a rock. This drill is officially known as the Rock Abrasion Tool (RAT).
The rovers have a magnifying camera, mounted on the same arm as the drill, that scientists can use to carefully look at the fine structure of a rock.
The rovers have a mass spectrometer that is able to determine the composition of iron-bearing minerals in rocks. This spectrometer is mounted on the arm, as well.
Also on the arm is an alpha-particle X-ray spectrometer that can detect alpha particles and X-rays given off by soil and rocks. These properties also help to determine the composition of the rocks.
There are magnets mounted at three different points on the rover. Iron-bearing sand particles will stick to the magnets so that scientists can look at them with the cameras or analyze them with the spectrometers.
The rovers can send all of this data back to Earth using one of three different radio antennas.