Space Archaeology and the Search for Ubar: How Satellites Found Lost Places

For centuries archaeologists looking for a lost site walked deserts and forests with little more than rumours, old texts and hand-drawn maps. In the late 20th century some began using remote sensing, images and data gathered from aircraft and satellites, and uncovered a wealth of new finds. Few stories show the change better than the search for Ubar, a trading town in the deserts of southern Arabia.

"Can I please speak with someone who can help me find a lost city?"

That, more or less, is what documentary filmmaker Nicholas Clapp asked a telephone operator at NASA's Jet Propulsion Laboratory in 1983. He was put through to geologist and remote-sensing specialist Ron Blom. Clapp had spent months in old manuscripts. Ubar, known in legend as Ad or Iram, grew rich on the frankincense trade: the fragrant resin of a southern Arabian tree, burned in religious ceremonies and used in medicine. The town rose roughly 2,000 years ago and fell between 300 and 500 CE; in the legend, its wicked people were destroyed by a storm. The Greek geographer Ptolemy mentioned an "Omanum Emporium", and the British explorer Bertram Thomas marked a rough location on a map in the 1930s.

To see through the sand, Clapp needed radar. Blom had been a principal scientist on the Shuttle Imaging Radar-A, which flew on Columbia in 1981 and revealed buried features under Egypt's Selima sand sheet; its signals reached as deep as 3 meters under dry sand. "Nick clearly knew his stuff," Blom said. Charles Elachi, principal investigator for the next radar, SIR-B, agreed to image the Omani desert as a "target of opportunity".

Shuttle Imaging Radar-A saw dry stream channels beneath the sand of southern Egypt in 1981. Image from NASA/JPL.

SIR-B flew on Challenger on October 5, 1984, the first space flight explicitly meant to search for an archaeological site. A faulty antenna cut its results to 40 percent of its goals, and Elachi told reporters the lost city would "have to be lost for another year or so." But in spring 1985 an envelope at JPL held an image labelled "JPL Data Take 96.1", showing part of the Rub' al-Khali, the "Empty Quarter". Beneath the modern sand it hinted at an older landscape that could have supported people.

Radar reflects and bends at and below the ground, letting the shuttle radar "see" features buried under sand. NASA Earth Observatory illustration by Joshua Stevens, adapted from JPL/USGS material.

Spy satellites and 4,096 shades of gray

Other tools filled in the picture. From 1960 to 1972 the Corona spy satellites dropped film capsules that aircraft caught in mid-air. Declassified in 1996, their images let archaeologists "go somewhen", in Blom's words, seeing landscapes since bulldozed, bombed or rebuilt; researchers have used them to find dwellings and tombs in Syria and ancient routes in Iraq and Syria.

At NASA's Stennis Space Center, Tom Sever, who had once spent three months walking just one and a half of 41 ancient lines in Peru, taught himself to map archaeology from above. With airborne scanners he found roads 30 feet wide and "straight as an arrow" at Chaco Canyon, New Mexico; teams on the ground confirmed them, "ten for ten". Then came Landsat, from NASA and the USGS, imaging the same place every 16 days at resolutions that improved from 79 to 30 to 15 meters. Early satellites recorded 64 shades of gray; modern sensors record 4,096 in a single band.

Digging at Shisur

The Ubar team's first reconnaissance was in the summer of 1990: Clapp, Blom, the archaeologist Juris Zarins and the explorer Sir Ranulph Fiennes among them, stranded one night on a desert mesa when their helicopter went to refuel. In 1993 they drove across the dunes with grainy images and an early GPS receiver that found a signal only for half an hour each morning: "a compass and dead reckoning," Blom said. They reached Shisur, a hill beside a Bedouin camp, once an oasis on the road to Ubar, and dug for a month.

The area around Ubar in false colour… NASA Earth Observatory image.

… and in natural colour, with the dunes, rocky desert floor, dry stream bed and old tracks marked. NASA Earth Observatory image.

A wall appeared, then the curves of towers. The fortress was larger and older than expected. "There was never any 100 percent proof," Clapp said. Blom is more cautious: probably not the sprawling city of legend but a trading post, and "without question, the most significant archaeological site in that area." He suggested trade there ended as frankincense lost importance with the Roman Empire's conversion to Christianity, as groundwater fell and oases dried, and as sea transport became reliable. Clapp believed the fortress, weakened as its well drew down, finally collapsed into a sinkhole after a small earthquake.

Ubar, deep in the sands of Oman. NASA Earth Observatory map by Joshua Stevens.

Lidar, and a new worry

The most powerful tool came later: lidar, laser pulses from aircraft that map the ground in three dimensions, even under forest. At Caracol in Belize, Arlen and Diane Chase used it to find more than 1,500 reservoirs, evidence of a city far larger than most Maya scholars had believed, which changed ideas about Maya trade and society. Lidar is costly, though, while Landsat images are free.

Remote sensing can help looters too. At Caracol the Chases found hundreds of looters' trenches visible in the lidar; at Chaco, mining and souvenir hunters have disturbed sites. Many archaeologists now urge keeping such data private until sites are studied on the ground. As Sever put it, the point is also "to know where things were before they were destroyed."

Chaco Culture National Historical Park from space. Freely available images may make hidden sites too easy to find. NASA Earth Observatory image by Joshua Stevens, using Landsat data from the USGS.

Chetro Ketl, a Puebloan great house at Chaco built between 945 and 1070. Photo by the National Park Service.

Sources

• NASA Earth Observatory, "Peering through the Sands of Time: Searching for the Origins of Space Archaeology"; rewritten in hubnx's own words. Images from NASA, JPL, the USGS and the National Park Service.