Radar reveals ghostly footprints at White Sands

 arstechnica.com  12/04/2019 16:57:04   Kiona N. Smith
Radar reveals ghostly footprints at White Sands

Ground-penetrating radar could help archaeologists spot otherwise invisible ancient footprints, suggests a recent experiment at White Sands National Monument, New Mexico.

Tracks left behind in layers of hardened mud and sand at the site record where humans crossed paths with giant sloths and mammoths during the last Ice Age. But some of the tracks appear only when conditions are just rightusually after a rainwhich makes them difficult to study. Archaeologist Thomas Urban of Cornell University and his colleagues used ground-penetrating radar to spot these so-called ghost tracks. The radar images also revealed layers of compressed sediment beneath mammoth tracks, which could reveal information about how the now-extinct woolly giants strode across the Pleistocene world.

Invisible ink

To test the method, Urban and his colleagues pulled a radar antenna across the pale gypsum sands of the former lake shore, pacing out a grid pattern over a site where, 12,000 years ago, a human and a mammoth crossed paths. Excavations at the site had already revealed ghost prints left by a person who walked north, and then back south, for about 800 meters (2,625 feet).

Sometime in the past, the prints filled with sediment and then got covered by a layer of fine gypsum sand, so theyre usually invisible from the surface. But the sediment filling the tracks holds more moisture than the sediment around them, so when theres just the right amount of water present, the tracks stand out dark on the pale ground. They appear and vanish again like a message written in invisible ink.

Archaeologists already knewone location along the trackway had about 27 ghost prints. When Urban and his colleagues put their radar to the test, the images revealed 26 of the printsand the images were detailed enough to calculate the length of the persons stride and estimate their stature. It turns out that the sediment filling the tracks also reflects radar signals differently than the surrounding material, making it possible to detect otherwise invisible tracks.

Farther north along the same trail of human footprints, Urban and his colleagues noticed a set of anomalies that looked like animal tracks. These were in a place where the playas surface looked blank and archaeologists hadnt documented any tracks. After a rain, the anomalies turned out to be sloth tracks. Urban and his colleagues say their results suggest that the ground-penetrating radar could help archaeologists search for other ghost tracks. That could help researchers avoid having to wait for specific weather conditions to find tracks and could boost efforts to preserve the ancient stories written across the desert in invisible ink.

  • Ghost prints are a common phenomenon in playa landscapes like White Sands.
  • Urban and his colleagues say they avoid visiting possible track sites in wet conditions, to avoid damaging them.
  • In this close-up radar image, you can see a mammoth print, a human print, and a sloth print.
  • These human and mammoth tracks have been excavated and date to around 12,000 years ago.

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Shortly after the Ice Age pedestrian passed by on their way north, a mammoth lumbered west across the fresh tracks. Its massive forefoot distorted two of the human tracks as it passed. Sometime later, the same person crossed the mammoth’s path on the way south again, stepping right in the middle of a massive mammoth print. Beneath the mammoth tracks, radar revealed a complex 3D structure of compressed and deformed sediment—a record of the biomechanics of a long-extinct mammoth.

When an African elephant—the closest thing we now have to a mammoth—takes a step, most of its weight is on the front part of the foot at first. But as the elephant pushes off the ground with its toes, it shifts the pressure backward. And as the foot lifts off the ground, the pressure on the soil suddenly releases. Each of those phases pushes on the ground beneath in a different direction, and studying the patterns left behind can reveal a lot about the mechanical aspects of how an animal walked.

“It turns out that the sediment itself has a memory that records the effects of the animal’s weight and momentum in a really beautiful way,” said Urban in a recent press release. “It gives us a way to understand the biomechanics of extinct fauna in a way that we never had before.” Unlike the tracks themselves, these structures, called plantar pressure records, are hard to get at by excavating.

Ground-penetrating radar seems to be able to detect these plantar pressure records from mammoth tracks, which gives paleontologists a non-destructive way to access important information about how mammoths moved. “If you want to really study the detailed print morphology you still need to excavate it, but we are getting additional information which is rather exciting below the track which you would not get if you excavated,” co-author Matthew Bennett of Bournemouth University told Ars.

The next steps

If radar can reveal plantar pressure records beneath mammoth tracks, Urban and his colleagues suggest that under the right conditions, it might also shed some light on the gait of much older animals: dinosaurs.

“Our next task is to take the technique somewhere with a different type of sedimentary set up and try it. Namibia is one possibility (there are great tracks there which I have worked on in the past),” Bennett told Ars. “Then it is about trying it on a lithified dinosaur track and experimenting with the set up to see what we can see. There are a few local sauropod tracks here on the Jurassic Coast in the UK which I have my eye on.”

Meanwhile, at White Sands, radar surveys and excavation have revealed previously unknown trackways crossing the ancient playa. "We have done further investigation and we do know more, but that will be part of a future paper," Urban told Ars. "The work has been ongoing and will continue in 2020."

Scientific Reports, 2019. DOI: s41598-019-52996-8 (About DOIs).

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