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Lost cities seen from space

— Archaeologists and NASA experts are using satellite images to find jungle-covered ruins that had been hidden almost literally right under their noses. The 21st-century technology, highlighted in the latest installment of PBS' "Nova ScienceNow," led to the discovery of ancient Maya settlements in Guatemala.

But there are still some mysteries left to solve: "We know a lot more about finding the sites than we do about why we're finding the sites," University of New Hampshire archaeologist Bill Saturno told me today.

However it works, the technique has the potential to transform the way archaeology is done, not only in Guatemala, but in Brazil, Bolivia, Cambodia "or anywhere where the forest has traditionally obscured ruins from view," he said.

During the "Nova ScienceNow" segment, Saturno declares that seeing ruins from space "changes the entire way that we approach archaeology in a tropical environment."

The tale had its beginnings in archaeology done the old-fashioned way: by stumbling upon an unexpected find. Back in 2001, Saturno was surveying a potential dig in a remote, uninhabited patch of Guatemalan rain forest, and took refuge from the hot sun in a dark, cavelike chamber. When he turned his flashlight toward the walls, he spotted marvelous murals telling the Maya story of creation.

The paintings at San Bartolo have been called the "Sistine Chapel of the Maya," and the site has also yielded the earliest examples of Mayan writing. It turns out that San Bartolo was a thriving center of Maya culture for centuries - but over the past millennium, many of the signs of habitation have faded into the jungle landscape.

That's where NASA comes into the picture: In 2003, the space agency began providing Saturno's team with satellite imagery, and Saturno bought into the idea that remote sensing from space could reveal ruins that could not be seen at ground level.

The basic concept isn't new, Saturno admitted. "Crop marks have been used in Europe for decades to identify Roman villas, moats, things like that," he told me. "It's a matter of there being slight changes in the soil over those ruins that, in a wheat field, leaves certain marks. Wheat over [the buried ruins of] a wall matures faster than the wheat next to the wall."

In those cases, the differences in the vegetation are clear enough to be seen by the naked eye from the air. "But the tropical forest is a very different environment," Saturno said.

GeoEye / Space Imaging
Maya settlement sites show up as yellowish
splotches in this false-color infrared image
of Guatemalan rain forest, sent from orbit
by the Ikonos satellite.

The color differences are invisible to the naked eye. You have to identify far more subtle changes in the reflectivity of infrared wavelengths, as seen in multispectral satellite imagery. That's just what NASA archaeologist Tom Sever and scientist Dan Irwin were able to help Saturno do, using data from the Ikonos satellite. The trees that grew up over buried ruins showed up as yellowish splotches amid the surrounding shades of red and blue.

Saturno recalled how amazed he was the first time he used the imagery as a guide and walked right up onto a Maya temple: "We'd been in the forest every day for five years and walked by it every day, but we had no idea it was there until we saw it from space," he said.

"As far as being able to use remote sensing in this way in a tropical environment, that's completely unexpected."

Saturno said researchers are still trying to figure out exactly what accounts for the different colors in the infrared. The vegetation may be a slightly different color because of differences in the soil's water retention, as was the case with the European ruins. But that's not the only possibility.

"It could also be a micro-environment created by the decay of those ruins," Saturno said. "The chemicals of that may be actually taken up into the leaves themselves - from the decaying lime plaster, the calcium carbonate there."

Right now, it takes a human's trained eyes to identify the signature of buried Maya sites in the satellite imagery - but a computer could conceivably take on the job if the data set can be tweaked just right, Saturno said.

"All of a sudden, for the first time, we're able to look at the big picture, and understand the extent to which the Maya expanded these cities," Saturno says during the PBS program. "If you think about how many sites are out there ... and we can see how many there are from these satellite images ... how many San Bartolos are out there, waiting to be discovered?"

"Nova ScienceNow," hosted by the Hayden Planetarium's Neil deGrasse Tyson, delves into other subjects as well:

  • Research into factors that may slow the aging process, including animal studies involving sirtuin genes, caloric restriction and resveratrol, a substance found in red wine.
  • The grass-roots effort to build the elements of a space elevator - with a particular focus on last October's Space Elevator Games.
  • The work of Princeton molecular biologist Bonnie Bassler, who has discovered ways in which bacteria use chemical pathways to "talk" to each other - even across species lines.

The show is due for its first airing on Tuesday - but if you miss it, don't fret. Beginning Wednesday, all the segments will be available over the Web.