ASU Research E-Magazine: Invisible Mars

Phil Christensen is looking for Martians.

Don’t worry. He knows he isn’t going to find any little green men with antennae and ray guns. The Martians he seeks are most likely microbes, like bacteria, that made their home on Mars millions of years ago.

It’s not easy to find microscopic creatures on a planet no human has ever visited. Even so, Christensen and other scientists have managed to collect a lot of information about Mars. They have learned much about what Mars it is made of, and about its geologic history. This information provides strong evidence that Mars could have supported life a long time ago.

Christensen is a planetary geologist at Arizona State University. He designs instruments that fly to Mars aboard spacecraft built by human engineers and scientists. The instruments collect and transmit information about the planet back to Earth for study.

One of Christensen’s devices is called TES. The device is orbiting Mars right now as part of the Mars Global Surveyor spacecraft.

TES is short for Thermal Emission Spectrometer. TES measures the amount of thermal—or infrared—energy given off by Martian rocks and atmosphere. These measurements can tell us what the rocks and atmosphere are made of on Mars.

“Everything emits infrared light,” explains Christensen. “Infrared energy is very similar to visible light. It’s just a longer wavelength. The electromagnetic spectrum goes from ultraviolet, which burns your skin, to visible light that your eye sees, out to infrared and on out to radio waves.”

Think of a rainbow. The rainbow is the part of the electromagnetic spectrum that our eyes can see. We call it visible light.

The colors of the rainbow appear in a specific order based on their wavelengths. The colors appear as red, orange, yellow, green, blue, indigo, and violet. Red has the longest wavelength of all the colors. Violet has the shortest. If our eyes could see infrared energy, it would appear just before red on the rainbow because it has a longer wavelength. Ultraviolet—the light that gives us a sunburn—is on the opposite end after violet.

Like visible light in its many colors, infrared light includes a range of different wavelengths. Scientists can measure the amount of energy an object gives off at various wavelengths. To study those measurements, they make a graph of how much energy is produced at each wavelength. This graph is called a spectrum.

“Every known mineral, or ice, or plants, or anything else, has a unique spectrum of the energy it emits. They’re all different,” Christensen says.

Because every mineral on Mars produces a different spectrum, scientists can use them like signatures or fingerprints to identify what kind of rocks are really there.

“We’ve collected hundreds of minerals in our laboratory over the past 15 years. We’ve measured their spectra. And we’ve created a library of spectra of every known mineral,” Christensen says.

“When we go to Mars, we measure the spectra of individual points on the surface of the planet. Then we compare those spectra to all the minerals in our library to identify which mineral was there.”

In 1998, ASU’s TES team made one of the most important discoveries in the search for life on Mars. They found deposits of the mineral hematite.

Hematite is an iron-containing mineral that can take on several forms. You can often find it in jewelry. It looks like a metallic, dark-gray stone.

The TES team discovered a type of hematite that only forms in the presence of water. The formation takes a very long time. So what is the big deal? Finding this kind of hematite offers evidence that there were once oceans or lakes on Mars.

“One of the things we’re very interested in with Mars is looking for evidence of ancient water,” Christensen says. “The conventional wisdom says that in order for life to have existed there had to have been water. I can’t go back two or three billion years in time to see if Mars had lakes or oceans, but I can look at really old rocks,” he adds.

“There are places on Mars where we see what look like big river channels or gullies, but those could form quite quickly in a flash flood,” he continues. “If you are a little bug trying to get started or survive, that’s not long enough.”

Finding hematite provided scientists with a very big clue.

“Wherever we find hematite there once had to have been water, and that water had to have been liquid for a long period of time,” the ASU scientist explains.

By compiling TES images, Christensen and his team have produced a mineral map of Mars, showing exactly where on the planet hematite is concentrated.

“Amazingly enough, it occurs in just a couple of places,” says Christensen. These areas will be the focus of closer investigation on future Mars missions.

The Mars 2001 Odyssey Orbiter was launched from Cape Canaveral in Florida on April 7, 2001. It is scheduled to reach Mars in October 2001. The spacecraft carries a device called THEMIS, also designed by Christensen and his team. THEMIS works a lot like TES, with a couple of differences.

TES measures 143 different wavelengths of infrared energy. Using so many wavelengths, TES can identify minerals with amazing accuracy, but only on a large scale.

“In order to get enough energy to do this, we have to collect that spectrum over a three-kilometer area. The instrument is just not sensitive enough to look at really tiny areas and get enough energy from the surface,” explains Christensen.

What this means is that TES can view and make images of areas about half the size of Tempe, Arizona.

“What does that really mean?” asks Christensen. “If you measure the infrared spectrum of half of Tempe you’ll get a huge mishmash, a big mix of stuff. There might be something really interesting on the surface of Mars, but it might be only the size of a football field. That small area gets lost in all the spectra of the rest of the stuff.”

That’s where THEMIS comes in. The new device only measures 10 wavelengths in the infrared spectrum.

“I can’t tell you exactly what mineral is there, but I can still get a darn good idea,” says Christensen. “But now the spatial resolution is 100 meters. So I can see things that are the size of a football field.”

After THEMIS zooms in on areas of interest, scientists will have a good idea of where to send the two rovers on the Mars 2003 Surveyor mission.

“Imagine you are a Martian studying the Earth. If you had one rover to send to Earth to study and take pictures, where would you send it?” Christensen asks.

“We’re trying to do a global reconnaissance of Mars from orbit, trying to find the most interesting places. But you can only do so much in orbit.”

The scientists have completed the global recon of Mars. The next step is to send the rovers to some of these interesting places identified from orbit.

“It is time to get a good, close-up look at these places,” says Christensen.

The rovers will land on the surface of Mars and study rocks and other features up close. Both of the rovers will carry miniature versions of TES, called, of course, Mini-TES.

Mini-TES will make images of all the rocks around the rover. The rovers can drive over to those that look most interesting and give them a close look. The rovers will have equipment to take photos, measure the elements in a rock, and take microscopic images. They also will be equipped with drills so that scientists can study the inside of the rocks.

Mini-TES is very important to the missions because it saves a lot of travel time for the rovers.

“Have you ever driven one of those little remote-controlled cars?” asks Christensen. “Now imagine doing this where you told it to do something and you didn’t find out what it did for 40 minutes. It’s very hard to drive something when there’s a 40-minute delay. We don’t want to waste our time just randomly going around.”

To date, none of the Mars spacecraft have returned to Earth. In the future, scientists intend to collect rocks from Mars and actually bring them back to our planet.

“Here on Earth, scientists can use extremely sophisticated instruments to measure the age and origin of these rocks. We can’t do that remotely on the surface of Mars,” says Christensen.

Even better, scientists are busy figuring out how to send people to Mars sometime in the next 25 years. The first human explorers on Mars will study its rocks—and maybe its inhabitants—right in their own environment. An elementary school student somewhere on Earth today will probably be the first person to walk on Mars. It could be you.—Diane Boudreau