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Who Are NASA's Earth Explorers? The elementary school student wondering how El Niño will affect tomorrow's weather. The scientist studying connections between ozone and climate change. And the farmer using satellite pictures to keep track of crops. All of these people are Earth Explorers -- they are all curious about the Earth system. This monthly series will introduce you to NASA Earth Explorers, young and old, with many backgrounds and interests. Nominate an Earth Explorer! Tell us about the Earth Explorers you know. We're looking for students, teachers, scientists and others who are working with NASA Earth science data and imagery to better understand our home planet. Send your nominations to Dan Stillman: dan_stillman@strategies.org.

Related Resources + Remote Sensing Archaeology Research at NASA + Earth Observatory: Mayan Mysteries + Previous Earth Explorers Articles

The year was A.D. 800. Their society thrived for more than 2,000 years. Their population was at an all-time high. They were experts in astronomy and architecture. And they had mastered a complex language of written symbols.

Less than 200 years later, the Maya were no more. An empire that had stretched across southern Mexico and Central America was all but gone.

Scientists want to know what led to the Maya's collapse. Finding clues 1,200 years after the fact is not easy. The area where Mayan cities once dotted the land is now filled with rain forest. The jungle is so dense that it's possible to walk right past ancient cities without knowing it. Even in aerial photographs, the treetops hide the ruins.

Image to left: Tom Sever (right) and a graduate student study a crumbled "stele" (stee lee) in the Guatemalan jungle. Steles were stone pyramids used by the Maya to record information or display carved art. Credit: NASA

To find hidden Mayan relics, scientists have turned to satellites. A technique called remote sensing allows satellite sensors to see what lies below the trees. The sensors detect heat and other invisible forms of energy coming from the land beneath. How can the sensors tell the difference between different kinds of soils and rocks? They emit energy at different wavelengths. For example, the energy emitted by loose dirt covering old Mayan buildings is different than that given off by other soil.

Tom Sever was one of the first people to use satellite data in archaeology. He's an archaeologist at NASA's Marshall Space Flight Center. He has always been interested in both the land and sky. So naturally he would study the ground by looking at it from high above.

Sever said remote sensing allows us to see evidence of past human occupation that we could not see with our own eyes. He and another NASA scientist, Dan Irwin, study satellite images of where the Maya used to live. The images have helped them find ancient Mayan roads, canals and dams.

Image to right: This satellite image shows a "bajo" (bah ho) in Guatemala. Bajos are broad, lowland areas that are often partially submerged during the rainy season. The yellowish areas pinpoint ancient Mayan building sites. Credit: IKONOS

The benefits of satellite data have also caught the eye of William Saturno. He, too, is an archaeologist. He works at the University of New Hampshire. Saturno's fascination with old cultures dates back to when he was a kid. He is most famous for finding the oldest known Mayan mural. He found it in Guatemala.

Saturno teamed up with Sever and Irwin in 2001. Saturno was surprised by what he saw in their satellite images. He noticed that trees around known Mayan sites looked different from other trees. They showed up discolored. It didn't take long for Saturno to take advantage of this discovery.

Image to left: Daniel Irwin (left) and William Saturno explore a trench below an ancient Mayan pyramid. Credit: NASA

He and his team of graduate students used the images as a map. They trekked deep into the Guatemalan forest. They found a series of Mayan sites -- right where the images said they would be. And they did so much faster and more easily than ever before.

"You can predict where you're going to see things," Saturno said. "Even within an archaeological site, you can tell where structures are, based on the tree images. So you can tell the site's layout."

Why do trees near Mayan sites show up discolored in satellite images? For one thing, Mayan agriculture changed the nutrients and moisture levels of the land. Also, the Maya's use of limestone to build cities altered the chemicals in the soil. These changes affect the energy emitted by trees, which changes the way they look from space.

"Being able to spot trees 2,000 years after the civilization [collapsed] is a sign of the impact [the Maya] had on the forest," Saturno said.

Sever, Irwin and Saturno are curious about what happened to the Maya. They wonder if the lack of trees could have made a naturally occurring drought even worse. They wonder how big a role deforestation played in the society's demise.

Image to right: These are the ruins of the ancient Mayan city named Tikal. Credit: NASA

The lands once occupied by the Maya are now a sea of green forest. But that wasn't always the case. Historians believe that by A.D. 800 the Maya had cut down most of their trees, which may have taken years to regrow. They used the wood to build huge cities and monuments. Evidence suggests that drought and a sharp drop in population followed soon after.

These are important questions. The answers could help people today avoid mistakes of the past.

History is already repeating itself in Guatemala. "Within sight of the Maya ruins ... population is growing again, and rain forest is being cut to make farmland," Sever said. The shrinking number of trees can be seen in satellite images. This decrease in trees may be causing clouds to form higher and later in the day. This change may be resulting in less rainfall in the region.

The scientists plan to keep using remote sensing in their research. They hope it will help them learn more about the Maya and their downfall. They also hope to show present and future generations the need to take good care of the land.

Dan Stillman, Institute for Global Environmental Strategies