OVERVIEW

In early 2004, Mars received two new visitors. Both are explorers, geologists, chemists, meteorologists, and photographers. They wander the Martian surface trying to discover whether water was once present and what role it may have played in the formation of local geology.

From our experiences here on Earth, we believe that water is essential to the formation of life. Today, there does not appear to be enough surface water on Mars to sustain life as we know it. The Mars Exploration Rovers, or MERs, are trying to answer questions about the past suitability of Mars for the evolution of life.

The MER is the most ambitious mission yet sent to the red planet. The two MERs, named Spirit and Opportunity, landed in different places on the planet. Designed to operate for a minimum of 90 Martian days, they have lasted much longer. In one day, each MER is able to cover more distance than the 1997 Pathfinder mission covered in its entire 85-day life.

THE ROVERS

The MERs are very different from the Vikings or Pathfinder before it. The two Viking landers (1976) were fixed in place. They set down on the surface of Mars and performed their experiments right where they landed. The 1997 Pathfinder mission consisted of a lander (Sagan Station) and a small, 23-pound mobile rover (Sojourner) that had to stay within radio communication of its base station.

By contrast, the two MERs landed on Mars in January 2004, weigh 380 pounds each. Unlike Sojourner, they are not dependent on a home base. They can wander as far as scientists on Earth want them to—up to about 100 meters per day. Powered by a bank of solar cells on its upper surface, each MER has advanced communication capabilities that will allow it to keep in touch with orbiting probes and directly with Earth. In addition, each rover carries an array of seven tools it will use to explore the surface of Mars.

THE TOOLS

Cameras
Each rover is equipped with a Pancam (short for Panoramic Camera), and each Pancam (contains two digital cameras. Set 30 cm (11.8") apart, the cameras function like left and right "eyes," so we’re getting back breathtaking, 360-degree stereo color panoramas from each MER.

Each camera in the Pancam has a CCD (charged-coupled device) sensor with a 1024 x 1024 pixel array. That's a total of one megapixel—low resolution compared to today's consumer camera specs! These CCDs only detect the presence or absence of light; in other words, they’re black-and-white cameras. To create those beautiful full-color images we’ve come to expect from our space missions, each camera has a wheel with eight positions for colored filters. By taking multiple pictures through different filters, scientists back on Earth will be able to construct color images.

There are a few specialized filters, too. The Pancam’s "left eye" specializes mostly in visible colors, while its "right eye" specializes mostly in infrared wavelengths invisible to the naked eye. The left eye has a clear filter (without color), but both eyes have dark solar filters (for short-wavelength violet light on the left, and long-wavelength, near-infrared light on the right) for solar observations. These solar observations can be used to help determine the rover's position and orientation, as well as to image the Sun.

To get a bird’s-eye view of the terrain, the Pancams are mounted on Pancam Mast Assemblies, or PMAs—vertical poles that raise the MERs’ eyes 1.3 meters (4'3") above the Martian surface. (OK, it's a low-flying bird. . . .) The cameras are motorized and can pivot up and down 90 degrees, and around 360 degrees; the field of view of the camera is 16.8 degrees wide and high. As they work, each Pancam returns its data to the computers inside the body of the rover. There, the data undergo preliminary processing and compression before being sent to Earth.
The Pancams aren’t the only eyes on the rovers. There are actually nine cameras on each. Most are pairs of black-and-white cameras making up stereo pairs. Besides the two in the Pancam, there are two pairs (four total) under the solar panels, one pair in the front and one in the rear of the rover. Used for navigation, these very-wide-angle Hazard Cameras, or Hazcams, have 120-degree fields of view. They map out surface features within 3 meters (10') of the rovers. Although the rovers are designed to drive over small obstacles, the scientists want to avoid larger rocks. The Hazcams help them do this.

There’s one more pair of cameras up on the PMA that is used for navigation. The pair is called (you guessed it) the Navcam. With its elevated 45-degree field of view, the Navcam is designed to look at the area ahead of the rover and help plan future routes for the MER. The last and ninth camera is a scientific instrument we'll discuss later, the Microscopic Imager.

Calibration Target and Sundial

Have you ever noticed that pictures taken outdoors in sunlight can often look different from pictures taken indoors under incandescent or fluorescent light? Cameras only report what they see. They cannot compensate for lighting conditions the way your eyes and brain can. On the surface of Mars, the same thing happens. The light in the morning is different from the light at noon. Sometimes there’s lots of dust in the air, and that can change the lighting, too.

Because lighting conditions vary widely on the surface of Mars, it's important to be able to calibrate the cameras we send there. To do this, a calibration target marked with different values of gray and four colored patches is placed on the rover. When the Pancam looks at these colors, scientists back on Earth—who know the exact values of the colored patches and targets—can compare the light reflected and received from them. Then they can use these data to adjust the images received back on Earth so the pictures we see truly represent the conditions on the Martian surface.

To further calibrate the images, a black target ball on a metal post protruding from the device acts as a sundial, casting shadows that can also be analyzed. The artwork on the instrument was created by students; the word for Mars appears on the dial in seventeen different languages.

RAT

When you picture a geologist out in the field doing research, the one tool that always comes to mind is the trusty hammer. Geologists need to study rocks that have not been exposed to the changing forces of weather. By comparing the internal, untouched rock with its weathered surface, a geologist can often determine the processes the rock has undergone. Appropriately, geologists wanted a tool onboard the MER that could expose fresh, unspoiled rock below the weathered surfaces of samples. A hammer was not accurate or practical for a machine to use, so they invented a better tool: the RAT. No, this is not a rodent. RAT stands for Rock Abrasion Tool.

Mounted on a marvelously flexible robotic arm called the Instrument Deployment Device, or IDD, the RAT grinds away the surface of a rock with two rotating diamond-tipped grinders. The RAT can create holes 45 mm (about 2") in diameter and 5 mm (1/5") deep. It takes about two hours if the rock is tough volcanic basalt, less if the rock is softer. Once fresh rock is exposed, other instruments mounted on the robotic arm can examine and contrast fresh versus weathered material.

Microscopic Imager

You'd never think of sending a geologist out without a hammer. But the other tool the geologist needs is a magnifying glass for examining the detailed structure of the freshly cleaved stone. The MER is no different. It, too, has a magnifying glass. It’s called the Microscopic Imager, or MI, and each rover has one.

Like the RAT, the MI is mounted on the robotic arm, which places the MI in contact with the rock surface to take pictures. And like the Pancam, the MI has a one-megapixel sensor. When placed against a rock surface, this sensor can detect details slightly larger than the diameter of a human hair (30 microns). The MI does not have its own light source to illuminate its samples. Instead, it uses available skylight and sunlight to take pictures. It takes stereoscopic images by moving the camera for a second shot of the surface from a different angle.

Alpha-Particle X-Ray Spectrometer

Now that our robotic geologist has chipped away at the rock and examined it microscopically, it's time to do a little chemical analysis. Unfortunately, it's difficult to break out the flasks, beakers, and Bunsen burners on Mars, so the rover has to be clever about its chemistry.
One of the instruments available is the Alpha-Particle X-Ray Spectrometer, or APXS. The APXS can determine what elements are in the rock and in what proportion. The instrument is mounted on the robotic arm and must be placed in contact with the rock surface for its analysis.

The APXS contains six small radioactive sources that pelt a rock with alpha particles (helium nuclei) and X rays. By looking at the energy of the alphas and X rays that bounce back from the surface, the APXS can determine the elemental composition of the rock. It takes quite a while to do this—up to ten hours—so these observations are made during the Martian night, when the rover is not moving. An additional advantage of nocturnal observation is that the temperature is much lower, which helps increase the accuracy of the APXS’s observations.

Mössbauer Spectrometer

The red planet is red for a reason. Scientists believe that Mars is an iron-rich world, and the familiar reddish color comes from iron rust. Because the surface of Mars may contain many iron compounds, it makes sense to send along an instrument to analyze and characterize the iron in the rocks. This is the job of the Mössbauer Spectrometer.

Like the APXS (see above), the Mössbauer Spectrometer is mounted on the robotic arm. When it comes in contact with a rock’s surface, it illuminates the rock with a radioactive source. The APXS does this too, but the Mössbauer Spectrometer gives off gamma rays instead of the alpha particles generated by the APXS. The Mössbauer Spectrometer then uses the energies of the returning gamma rays to determine the composition and abundance of iron-bearing minerals in the surface rocks.

It takes the Mössbauer Spectrometer about twelve hours to do one observation. Iron's magnetic properties depend highly on temperature. Because the observation is so temperature dependent, comparative observations are made day and night while trying to hold the temperature during a given observation at plus or minus 10 degrees Celsius.

Mini-Thermal Emission Spectrometer

A third spectrometer, the Mini-Thermal Emission Spectrometer, or Mini-TES, looks at the infrared light given off by the surface materials of Mars and is able to tell scientists the composition and abundance of minerals. This instrument is located inside the body of the rover. It uses the Pancam Mast Assembly (PMA) as a periscope. Light entering the elevated position of the PMA is reflected down the inside of the mast into the Mini-TES telescope and spectrometer. The Mini-TES looks at right angles to the Pancam and has its own scanning mirror to reflect light down the mast.

The Mini-TES also looks up through the Martian atmosphere to take temperature profiles of the atmospheric layers. By looking at the heat given off by the rocks at night, Mini-TES learns about heat retention in the surface materials of the planet. The Mini-TES tells geologists about carbonates, silicates, organic molecules, and minerals formed in water by past processes.

Magnet Arrays

Not technically a tool, another simple experiment on the rover was designed to look for iron. Both rovers carry three sets of strong magnets to collect magnetic particles of dust. One pair of magnets is mounted on the RAT and can collect dust generated from the grinding operations on specific rocks. Another set of magnets is mounted on the front of the rover to collect airborne dust. These are placed at an angle so that nonmagnetic dust will fall away. Both the APXS and the Mössbauer Spectrometer will be able to observe the material caught by these magnets.

The last magnet is very strong. It's mounted on top of the rover so the Pancam can observe the deflection of wind-blown magnetic dust over the surface of the magnet. We also see what kind of dust gets stuck there.

Altogether, the MERs have an amazing array of tools to explore the Martian surface. In the many days they wander the Martian wilderness, we increase our knowledge of the red planet many times over what we know now. It's not quite like having human explorers, geologists, chemists, meteorologists, and photographers up there, but it’s close.