An algorithmic image-processing technique originally engineered by NASA’s Jet Propulsion Laboratory to analyze Martian surface photography from space has uncovered faded, nearly invisible ancient paintings high within the central tower of Cambodia’s Angkor Wat temple complex.
From the Martian Surface to Angkor Wat
Thousands of tourists walk past historical artwork daily without realizing it. In the central tower of Angkor Wat, paintings of horseback riders and traditional musical ensembles have faded to the point of invisibility. Yet, an archaeologist successfully exposed them between 2010 and 2012 by utilizing decorrelation stretch—an image enhancement algorithm first developed by NASA.
The method works by heightening contrasts in digital imagery, making faint, obscured features pop out against natural backgrounds. While it sounds like a modern software trick, its roots stretch back decades into planetary science and remote sensing infrastructure.
Decoding the Astronomy Roots of Decorrelation Stretch
The algorithm traces its lineage back to 1996, when Ronald Alley—an employee at NASA’s Jet Propulsion Laboratory (JPL)—authored a technical paper outlining applications for the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), a Japanese imaging instrument mounted on NASA’s Terra satellite. One of Alley’s former JPL supervisors had coinvented the decorrelation stretch method to extract actionable data from ASTER imagery.
Enter Jon Harman, a retired rock art enthusiast and medical imaging professional living in Pacifica, California. Around 2005, Harman observed NASA images showcasing Martian terrain comparisons rendered with and without decorrelation stretch. Recognizing the immense potential for earthly archaeology, Harman tracked down the 1996 NASA paper.
“I Googled it and found a NASA paper that explained how to do the algorithm,” Harman stated. Drawing from his professional background in medical imaging, he built a dedicated plug-in for ImageJ, an open-source image-processing program developed by the National Institutes of Health.
Global Archaeological Impact Beyond Mars
Today, Harman fields roughly 200 direct requests for his Dstretch plug-in annually. Around 2010, he also launched streamlined smartphone applications mimicking the decorrelation stretch effect, which have since been downloaded thousands of times.
The unexpected cross-pollination between planetary geology and ancient history has rippled across international research projects. Academic papers detailing Dstretch’s utility have documented successful deployments across diverse global sites:
- Clarifying eroded rock art beneath cliff faces in Norway.
- Uncovering hidden details inside Egyptian tombs.
- Analyzing petroglyphs at a park in Canada.
- Exposing buried foundations of ancient Greek architecture.
- Examining intricate tattoos preserved on mummified human remains.
Reflecting on the widespread adoption of his hobbyist tool, Harman noted that while he anticipated traction within rock art communities, the breadth of secondary applications caught him by surprise. “But I’ve been surprised by a lot of the different applications people have found. So that’s been cool,” he said.
