Satellite photography helps uncover lost city of Altinum
Satellite imaging is helping unearth ancient ruins. Dan Cossins joins the space archaeology race...
The lagoon city of Altinum was one of the richest in the Roman Empire – a staging post for traders from across the ancient world. Around the middle of the 5th century, however, its residents fled for fear of marauding barbarians, leaving a ghost town of crumbling villas and basilicas. After much of the masonry was used to build a new settlement nearby – known as Venice – the city was buried in fertile floodplains. Historians knew it existed, but it was hidden from view.
Now, more than 1500 years on, Altinum has risen again. Using sophisticated aerial photography, a crack team of earth scientists and archaeologists at the University of Padua have created a picture of how Altinum looked when it was abandoned – a unique time capsule of a city in the final years of the Roman Empire. By revealing the moisture content of the plants growing there today, which varies according to the presence of man-made structures beneath the topsoil, near infrared photographs provide a relief map of a once great city. “It was amazing because we didn’t expect the image to reveal so much,” says Professor Paolo Mozzi, a geomorphologist who devotes his time to wresting the secrets of the past from the soil. “The images were extraordinary – we immediately understood that this was a big step beyond what was known.”
Suspecting that crops growing directly above ruins would reflect near infrared light differently from plants elsewhere, the team commissioned a plane equipped with sensors to photograph the fields during a particularly dry period, when the crops suffer extreme dehydration. The resulting image highlighted areas where the plants could only draw upon a thin layer of soil thanks to the stonework beneath – lighter markings indicated buildings, while dark patches revealed where canals had been. “Before this, it was impossible to imagine the complexity of the city,” explains Mozzi. “But we can now begin to reconstruct exactly what Altinum must have looked like, and all without any real excavation.”
Mozzi and his colleagues are among a growing band of scientists pioneering the use of remote sensing to uncover vanished cities and unearth the mysteries of ancient civilisations. With satellites capable of capturing high-resolution multi-spectral images of the Earth’s surface from space, tech-savvy archaeologists are able to expose ruins invisible to the naked eye. Armed with superhuman powers of vision, this new generation of explorers is shaking up the world of archaeology with awe-inspiring discoveries that will reshape our understanding of the ancient world.
Taking to the skies to get a better view of the past is not a new idea. It began during the first world war, when men engaged in military cartography realised that the land below was dotted with traces of vanished settlements. Today’s pioneers owe this to the likes of Lt Col George Adam Beazeley, who carried out archaeological investigations as he flew reconnaissance missions over Mesopotamia, before being shot down in 1918. Only in the past few years, however, have archaeologists realised the potential of digital sensors that can record data across the electromagnetic spectrum.
In Cambodia, archaeologists from the University of Sydney have used satellite images to prove that Angkor was the largest city in the pre-industrial world, a sprawling metropolis comparable in size to present-day Los Angeles. Ground-penetrating radar images highlighted a vast network of ancient canals stretching well beyond the famous temple, revealing suburbs where up to one million people lived between the 9th and 13th centuries. Further afield, on Easter Island in the Pacific Ocean, scientists from the University of Hawaii have exposed the paths along which the early Polynesian inhabitants dragged maoi, the island’s famous stone statues.
Into the jungle
The prospect of such groundbreaking discoveries led NASA to create a ‘space archaeology’ division under its Research Opportunities in Space and Earth Science programme. Dr Bill Saturno, an archaeologist at the University of Boston, works closely with the space agency to unearth the secrets of the Mayan empire, a once mighty civilisation that thrived in the jungles of Central America for 2000 years before it mysteriously collapsed. Using NASA’s airborne radar and commercial satellite photography, Saturno hit the headlines in 2006 when he uncovered a series of sprawling Mayan sites beneath the thick canopy of the Guatemalan rainforest.
“We used a high-resolution, multi-spectral satellite called IKONOS,” explains Saturno. “It records data in four bands of light – blue, green, red and near infrared.” In the near IR images, forest shows up red and swampy areas appear blue, but an uncharted region around San Bartolo showed some areas of unexpected yellow colouration. Saturno spotted a correlation between these yellow areas and the location of known archaeological sites. This effect is caused by the decaying remains of the limestone and lime-plaster the Mayans used to build. The lack of moisture and nutrition in the ruins keeps some plants away, while those that survive are slightly discoloured due to the altered chemical content of the soil. At near infrared wavelengths, this discolouration becomes visible, as in Saturno’s pictures.
So by using sensors in space to highlight this ‘chemical signature’ of ancient building materials, Saturno can pinpoint the presence of Mayan architecture beneath the forest floor. “Over the centuries these changes in the chemical make-up of the soil became dramatic,” he explains. “It’s impossible to see when you’re on the ground or even flying above, but this pattern seen from space turned out to be a virtual roadmap to ancient Mayan sites.”
Saturno then left the lab and ventured deep into uncharted rainforest, braving the jungle to see if remotely generated predictions linked up with on-the-ground reality. He wasn’t disappointed. “It never ceases to amaze,” says Saturno, who splits his time between Boston, NASA’s Marshall Space Flight Centre in Alabama and the depths of the Central American jungle. “Every time you do it, you think this has to be the time it’s not going to work. You say, ‘We should be approaching a ruin in about 10 metres’, but you don’t see anything. Then you get up closer and there it is.”
Saturno found all kinds of structures, from ornate palaces to small cobbled platforms where thatched huts once stood. “It doesn’t surprise us when we use Google Earth to zoom in on our house,” he reflects. “It’s more remarkable when we can see the outline of a house built 1400 years ago by a Neolithic farmer with stone tools, farming corn on the outskirts of Mayan cities.”
He might be the most aptly named, but Saturno isn’t the only ‘space archaeologist’ racking up impressive discoveries. Dr Sarah Parcak, an archaeologist at the University of Alabama at Birmingham, has mastered the analysis of satellite images to reveal over 100 previously unknown sites in Egypt’s Nile Delta, from monasteries, temples and tombs to entire towns.
These ancient buildings were built on sandy mounds to protect them from the rising waters of the Nile. The river hasn’t flooded for years thanks to the Aswan Dam, but modern towns, many of them constructed with the same mud bricks used by the ancient Egyptians, have smothered the traces of the past. The challenge for Parcak was to utilise remote sensing methods to differentiate between the ancient and modern. “With urbanisation and looting, many of the ancient sites in the regions I work on are obscured,” she explains. “But with satellite imagery I’m able to identify shapes and patterns that indicate the presence of sites, even if they’re buried beneath modern towns.”
To do this, Parcak downloads coarse resolution multi-spectral images from NASA’s Landsat satellite. She loads these into an image manipulation programme that allows her to isolate areas of differing moisture content. Archaeological sites absorb moisture differently from natural features, so it’s possible to highlight their ‘spectral signatures’. She then processes the new images so that the ‘tells’ – areas with higher than usual organic matter and phosphorous content – show up in a particular colour. At that point she’s ready to start digging. “I can’t believe that so few other Egyptologists are doing this kind of work,” she says. “There are still so many sites to be found.”
Only the beginning
Despite its achievements, the field of satellite remote sensing archaeology is still in its infancy. At the moment, even the most cutting-edge practitioners are using borrowed technology. With increasing awareness, however, Parcak expects archaeologists to sharpen their vision of the past. “The resolutions, both spatial and spectral, will keep improving,” she says. “Archaeologists are really waking up to this. They’re seeing how these technologies can help them to find new sites and better understand those sites they’re investigating.”
Sunhat and sandals archaeologists needn’t hang up their trowels yet: remote sensing will never completely replace the sweat and dirt of hands-on excavation. Nevertheless, for a new generation of scientists, ‘space archaeology’ offers the stirring possibilities of finding entire lost cities and securing a place among the pantheon of great explorers.
“This is just the beginning,” says Bill Saturno. “There are hundreds of thousands of ancient ruins out there just waiting to be discovered.”