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Making Sense of the Mayan Collapse

Who Are NASA's Earth Explorers?

The elementary school student questioning if El Niño occurs anywhere besides the Pacific Ocean. The researcher investigating connections between Arctic ozone depletion and global climate change. The citizen scientist interested in how changing land cover and use affects animal migration patterns. And the businessperson projecting future needs for harvest, delivery and storage of crops. All of these people are Earth Explorers -- they are all connected by their curiosity about Earth system processes. This series will introduce you to NASA Earth Explorers, young and old, with a variety of backgrounds and interests.

Two men in backpacks kneeling down to get a closer look at a large, moss-covered rock
Eighteen centuries ago, the vast region stretching from southern Mexico to Central America was home to the Maya, an advanced civilization with a mastery of astronomy, architecture and an elaborate hieroglyphic language. The powerful empire collapsed in the ninth century, and now the area is covered by rainforest so thick you can't see more than 10 to 15 feet around you.

Image to left: Deep in the Guatemalan jungle, NASA archaeologist Tom Sever (right) and Rob Griffin, a graduate student at Penn State University, study a crumbled "stele," a stone pyramid used by the Maya to record information or display ornately carved art. Credit: NASA

Scientists studying the civilization can spend days hacking through the dense jungle with machetes, and still walk right past ancient cities. Even in aerial photographs, the forest canopy hides the ruins.

Related Resources
+ Remote Sensing Archaeology Research at NASA

+ Earth Observatory: Mayan Mysteries

+ Exploring the Environment: Tropical Poison

+ Previous Earth Explorers Articles
To find hidden Mayan relics, archaeologist Tom Sever at NASA's Marshall Space Flight Center in Huntsville, Ala., and NASA scientist Dan Irwin at the National Space Science and Technology Center, also in Huntsville, use satellite sensors. The sensors detect energy that is invisible to the human eye, such as heat, microwave and infrared. This technique, called remote sensing, relies on the fact that various soils, vegetation and rocks emit energy at different wavelengths on the electromagnetic spectrum. For instance, remote sensing can detect loose earth that covers ruins, because the dirt has a slightly different temperature and chemical composition than other soil.

Sever, a pioneer of using satellite data in archaeology, said remote sensing allows us to see evidence of past human occupation that we could not see with our own eyes. Interested in both land and sky, Sever has found a career that combines archaeology and remote sensing, as well as architecture and astronomy, to be a natural fit for him. With the help of satellites, he has found invisible ancient roadways, canals and dams.

A satellite picture of a Mayan site showing hot and cold spots with different colors
Image to left: This satellite image shows a Guatemalan "bajo," or a broad, lowland area that is often partially submerged during the rainy season. The yellowish areas, which denote discolorations of the dense forest canopy, pinpoint ancient Mayan building sites. Credit: IKONOS

The latest breakthrough using the combination of remote sensing and traditional archaeology resulted from Sever and Irwin's teaming up in 2001 with William Saturno, an archaeologist at the University of New Hampshire. Saturno is most famous for finding the oldest known Mayan mural.

When Saturno took a look at Sever and Irwin's high-resolution infrared satellite images, taken from the commercial satellite IKONOS, he was surprised to find that trees around known Mayan sites showed up discolored in the images. It didn't take long for Saturno and his team of graduate students to take advantage of this discovery. Trekking deep into the tropical forest, they used the images as a map to find a series of previously unknown sites much faster and more easily than they could have before.

Two men standing in a cave-like opening
"You can predict where you're going to see things. Then you navigate using [the global positioning system] to get there," 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."

Image to right: NASA scientist Daniel Irwin (left) and archaeologist William Saturno of the University of New Hampshire in Durham explore a trench below an ancient Mayan pyramid. Credit: NASA

Why are trees around prehistoric Mayan ruins discolored in satellite images? One explanation is that Mayan agriculture changed the nutritional content and moisture levels of the land. Also, the use of limestone and lime plasters to build Mayan cities changed the soil's chemistry, thus altering the local vegetation.

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

According to most scientific theories, ecological impact, resource overuse and the subsequent environmental degradation played a big role in known Mayan migrations, as well as the society's eventual breakdown.

Large stone buildings on top of a grassy hill
"The Maya had one of the largest population densities in prehistory... similar to that of the most densely populated areas of China and Java today," Sever said.

Image to left: Ruins of the ancient Mayan city named Tikal. Credit: NASA

Archaeological evidence shows that by A.D. 800 the Maya had cut down most of their trees -- they used the wood to build large cities and monuments. Signs of drought and a dramatic drop in population followed this destruction of the forest. Sever, Irwin and Saturno want to find out whether Maya deforestation could have exacerbated natural drought conditions and led to the civilization's collapse.

A modern-day reenactment of the scene is already happening in the Petén region of Guatemala near the border with Mexico, according to Sever.

"Within sight of the Maya ruins... population is growing again, and rain forest is being cut to make farmland," he said. Moreover, recent research has shown that cumulus clouds are forming higher and later in the day, resulting in less rainfall in the region, and that this could be caused by a decreasing number of trees.

The researchers believe that understanding the possible Mayan impact on local climate could help people living in Central America today avoid mistakes of the past. Working together, and using the latest applications of remote sensing technology, they hope to learn more and make new discoveries about the Maya.

"I went into the forest because I wanted to find these ruins, and I did," said Saturno, who has been interested in learning about old cultures and becoming an archaeologist ever since he was a little kid. "Now I have something that helps me do that even better."

Prachi Patel-Predd, Institute for Global Environmental Strategies