The Mars Exploration Rover mission seeks to determine the history of climate and water at sites on Mars where conditions may once have been favorable to life. Each rover is equipped with a suite of science instruments that will be used to read the geologic record at each site, to investigate what role water played there, and to determine how suitable the conditions would have been for life.

Science objectives
Based on priorities of the overall Mars Exploration Program, the following science objectives were developed for the 2003 rovers:

• Search for and characterize a diversity of rocks and soils that hold clues to past water activity (water-bearing minerals and minerals deposited by precipitation, evaporation, sedimentary cementation, or hydrothermal activity).
  
  
• Investigate landing sites, selected on the basis of orbital remote sensing, that have a high probability of containing physical and/or chemical evidence of the action of liquid water.
  
  
• Determine the spatial distribution and composition of minerals, rocks and soils surrounding the landing sites.
  
  
• Determine the nature of local surface geologic processes from surface morphology and chemistry.
  
  
• Calibrate and validate orbital remote-sensing data and assess the amount and scale of heterogeneity at each landing site.
  
  
• For iron-containing minerals, identify and quantify relative amounts of specific mineral types that contain water or hydroxyls, or are indicators of formation by an aqueous process, such as iron-bearing carbonates.
  
  
• Characterize the mineral assemblages and textures of different types of rocks and soils and put them in geologic context.
  
  
• Extract clues from the geologic investigation, related to the environmental conditions when liquid water was present and assess whether those environments were conducive for life.

Science Instruments
The package of science instruments on the rovers is collectively known as the Athena science payload. Led by Dr. Steven Squyres, professor of astronomy at Cornell University, Ithaca, N.Y., the Athena package was originally proposed to fly under different Mars lander and rover mission concepts before being finalized as the science payload for the Mars Exploration Rovers.

The package consists of two instruments designed to survey the landing site, as well as three other instruments on an arm designed for closeup study of rocks. Also on the arm is a tool that can scrape away the outer layers of rocks. Those instruments are supplemented by magnets and calibration targets that will enable other studies.

The two instruments that will survey the general site are:

Panoramic Camera will view the surface using two high-resolution color stereo cameras to complement the rover's navigation cameras. Delivering panoramas of the martian surface with unprecedented detail, the instrument's narrow-angle optics provide angular resolution more than three times higher than that of the Mars Pathfinder cameras. The camera's images will help scientists decide what rocks and soils to analyze in detail, and will provide information on surface features, the distribution and shape of nearby rocks, and the presence of features carved by ancient waterways.

The Mini-Thermal Emission Spectrometer is an instrument that sees infrared radiation emitted by objects. It will determine from afar the mineral composition of martian surface features and allow scientists to select specific rocks and soils to investigate in detail. Observing in the infrared allows scientists to see through dust that coats many rocks, allowing the instrument to recognize carbonates, silicates, organic molecules and minerals formed in water. Infrared data will also help scientists assess the capacity of rocks and soils to hold heat over the wide temperature range of a martian day. Besides studying rocks, the instrument will be pointed upward to make the first-ever high-resolution temperature profiles through the martian atmosphere's boundary layer. The data from the instrument will be complement that obtained by the thermal emission spectrometer on the Mars Global Surveyor orbiter.

The instruments on the rover arm are:

The Microscopic Imager is a combination of a microscope and a camera. It will produce extreme closeup views (at a scale of hundreds of microns) of rocks and soils examined by other instruments on the rover arm, providing context for the interpretation of data about minerals and elements. The imager will help characterize sedimentary rocks that formed in water, and thus will help scientists understand past watery environments on Mars. This instrument will also yield information on the small-scale features of rocks formed by volcanic and impact activity as well as tiny veins of minerals like the carbonates that may contain microfossils in the famous Mars meteorite, ALH84001. The shape and size of particles in the martian soil can also be determined by the instrument, which provides valuable clues about how the soil formed.

Because many of the most important minerals on Mars contain iron, the Mossbauer Spectrometer is designed to determine with high accuracy the composition and abundance of iron-bearing minerals that are difficult to detect by other means. Identification of iron-bearing minerals will yield information about early martian environmental conditions. The spectrometer is also capable of examining the magnetic properties of surface materials and identifying minerals formed in hot, watery environments that could preserve fossil evidence of martian life. The instrument uses two pieces of radioactive cobalt-57, each about the size of a pencil eraser, as radiation sources. The instrument is provided by Germany.

The Alpha Particle X-Ray Spectrometer will accurately determine the elements that make up rocks and soils. This information will be used to complement and constrain the analysis of minerals provided by the other science instruments. Through the use of alpha particles and X-rays, the instrument will determine a sample's abundances of all major rock-forming elements except hydrogen. Analyzing the elemental make-up of martian surface materials will provide scientists with information about crustal formation, weathering processes and water activity on Mars. The instrument uses small amounts of curium-244 for generating radiation. It is provided by Germany.

The arm-mounted instruments will be aided by a Rock Abrasion Tool that will act as the rover's equivalent of a geologist's rock hammer. Positioned against a rock by the rover's instrument arm, the tool uses a grinding wheel to remove dust and weathered rock, exposing fresh rock underneath. The tool will expose an area 4.5 centimeters (2 inches) in diameter, and grind down to a depth of as much as 5 millimeters (0.2 inch).

In addition, the rovers are equipped with the following that work in conjunction with science instruments:

Each rover has three sets of Magnet Arrays that will collect airborne dust for analysis by the science instruments. Mars is a dusty place, and some of that dust is highly magnetic. Magnetic minerals carried in dust grains may be freezedried remnants of the planet´s watery past. A periodic examination of these particles and their patterns of accumulation on magnets of varying strength can reveal clues about their mineralogy and the planet´s geologic history. One set of magnets will be carried by the rock abrasion tool. As it grinds into martian rocks, scientists will have the opportunity to study the properties of dust from these outer rock surfaces. A second set of two magnets is mounted on the front of the rover for the purpose of gathering airborne dust. These magnets will be reachable for analysis by the Mössbauer and alpha particle X-ray spectrometers. A third magnet is mounted on the top of the rover deck in view of the panoramic camera. This magnet is strong enough to deflect the paths of wind-carried, magnetic dust. The magnet arrays are provided by Denmark.

Calibration Targets are reference points that will help scientists fine-tune observations not only from imagers but also other science instruments. The Mössbauer spectrometer, for example, uses as a calibration target a thin slab of rock rich in magnetite. The alpha particle X-ray spectrometer uses a calibration target on the interior surfaces of doors designed to protect its sensor head from martian dust. The miniature thermal emission spectrometer has both an internal target located in the mast assembly as well as an external target on the rover's deck.

The panoramic camera's calibration target is, by far, the most unique the rover carries. It is in the shape of a Sundial and is mounted on the rover deck. The camera will take pictures of the sundial many times during the mission so that scientists can make adjustments to the images they receive from Mars. They will use the colored blocks in the corners of the sundial to calibrate the color in images of the Martian landscape. Pictures of the shadows that are cast by the sundial's center post will allow scientists to properly adjust the brightness of each camera image. Children provided artwork for the sides of the base of the sundial.