Equipped with rock picks and hand lenses, a team of geoscientists recently converged on the Mojave Desert to investigate something most people would walk right past. Their lead came not from a field survey, but from a NASA sensor cruising at 65,000 feet — one that had already spotted a spectral “fingerprint” pointing to a cache of topaz hiding in plain sight.
Topaz itself isn’t the prize. But its presence, geologists know, can signal something far more strategically significant buried beneath the surface — something that prospectors have roamed this same desert for generations without finding.
A sensor built for other worlds turns its eye on Earth
The tool that spotted the topaz fingerprint is called AVIRIS — Airborne Visible Infrared Imaging Spectrometer. Built at NASA’s Jet Propulsion Laboratory, the technology was pioneered in the early 1980s by a team that included JPL’s Robert Green. It works by analyzing reflected sunlight and matching the result against the unique spectral signatures of chemicals and minerals on the surface below.
Space-hardened versions of AVIRIS have traveled well beyond California, exploring the Moon, Mars, and other rocky bodies. The Earth-based sensor line has kept advancing in parallel. The latest model, AVIRIS-5, recently flew for the first time as part of the NASA-USGS Geologic Earth Mapping Experiment — known as GEMx.
Why topaz is a clue, not the treasure
Porphyry copper deposits form when magma and superheated water from deep underground move through Earth’s crust, chemically transforming the surrounding rock. This process typically occurs at subduction zones — places where one tectonic plate dives beneath another, such as the North American Cordillera stretching from the Canadian Rockies to western Mexico.
Topaz forms under those same volcanic conditions, making it a surface-level indicator — a geological breadcrumb pointing toward something larger underground. That something is copper, the third most used metal in the world after steel and aluminum. Porphyry deposits can also yield molybdenum and tellurium, minerals used in steelmaking and solar panels. At a moment when domestic sources of critical minerals face increasing scrutiny, locating new deposits matters well beyond the geology community.
Geologic CSI: boots on the ground in Barstow
The field team that responded to the AVIRIS signal included three USGS geologists and JPL’s Robert Green, all converging on a site on public land near Barstow, California. The work was physical and methodical — scrambling over steep terrain, splitting open weathered rock, bagging samples for lab analysis.
“What we’re doing out here is geologic CSI,” Green said, cracking open a reddish, weathered rock to reveal a sparkling core. “We’re looking for clues to reconstruct what happened here.”
Lab testing confirmed what the sensor had suggested: topaz was indeed present. Confirming a porphyry copper deposit beneath the surface is a different challenge entirely — one requiring ground-penetrating equipment and much deeper investigation. The topaz find is a promising lead, not a conclusion.
Mapping a million square kilometers — and counting
GEMx is a NASA-USGS initiative with significant scope. Its goal is to identify sources of critical minerals across the American West, including within the waste rock of active and legacy mines. Since 2023, GEMx flights have covered more than 386,000 square miles — over one million square kilometers — of American soil, including most of California.
The platform making that scale possible is the ER-2, one of the highest-flying aircraft in NASA’s fleet. Cruising at roughly 65,000 feet, it collects broad-area, high-resolution spectral measurements in a single pass. In 2025, the ER-2 flew 36 science missions, logged more than 200 flight hours, and collected over 7 billion measurements — the largest airborne surface mineralogy dataset ever gathered in a single NASA-USGS campaign.
Still more to discover in heavily explored terrain
The Mojave Desert is not an overlooked corner of the map. People have worked, hiked, and prospected across it for generations. That history makes the AVIRIS discovery more striking, not less.
“People have been prospecting this area for generations,” said USGS geologist Erik Tharalson. “But there’s a lot more to discover.”
GEMx operates through a broad partnership spanning 45 state geological surveys, federal agencies, tribes, universities, and private industry — all working under the USGS Earth Mapping Resources Initiative, designed to modernize how the nation maps its surface and subsurface for critical and other minerals.
What comes next in the Mojave depends on what ground-penetrating surveys reveal. A confirmed porphyry copper deposit below the topaz signal could represent a meaningful domestic source of materials essential to manufacturing and clean energy. Either way, the finding makes a broader point — that even in familiar landscapes, the right sensor at the right altitude can uncover what generations of boots on the ground have missed.