A gravimeter hauled up Mount Timpanogos mapped 1.55 million cubic meters of buried ice

The measurement rests on a density contrast. Ice is less dense than the bedrock beneath it, and a gravimeter picks up the tiny difference in gravitational pull that results, which is what lets it distinguish rock from ice underground ^[9]. "As we get to areas where the ice is thicker and thicker, the gravitational acceleration is just lower and lower," Thorne said ^[10].

The idea is not new. Gravimeters were used on glaciers before ground-penetrating radar arrived, then fell out of favor ^[11]. Leif Andersson, a glaciologist at the University of Utah and a coauthor, said modern instruments resolve "really small deviations in the gravitational field" ^[12]. "The reason why the study was possible was the technology," he said ^[13].

Thorne framed the project as a methods question: "Part of our motivation has been, well, 'Can we come up with techniques for imaging the interior of these rock glaciers that don't only rely on ground-penetrating radar?'" ^[7]. The 232 readings were fitted with a Bayesian inversion, which compares thousands of simulated distributions of subsurface ice against the measurements and scores how well each one fits ^[15].

The transferable output is a relationship the team derived between a rock glacier's surface area and the typical ice volume beneath it, built from Timpanogos plus some measurements of other rock glaciers ^[16]. Timpanogos holds 1,550,000 cubic meters of ice, which is 0.00155 cubic kilometer, against the 1.08 cubic kilometers the team assigns to Utah's rock glaciers as a group: about 0.14 percent of the state total ^[22]. The larger figures come from the same relationship, with the western United States at up to 13 cubic kilometers, roughly a quarter of the 52.38 cubic kilometers credited to all known rock glaciers on Earth ^[17][23]. The Eos account does not report how many other rock glaciers went into the relationship, or the uncertainty on those totals ^[21].

A gravity survey resolves where mass sits at the moment of measurement ^[9]. Melt rate and whether that water reaches a stream are separate measurements, so these numbers are a stock estimate and not a yield forecast.

Doug Clark, a geologist at Western Washington University who researches rock glaciers in the western United States and New Zealand and was not involved in the work, called it "an elegant approach to address one of the more vexing problems regarding rock glaciers: What is the internal ice content of these things?" ^[18][19]. He said the modeling and the use of the gravimeter marked a "significant improvement over past efforts of estimating ice volume" in rock glaciers ^[20].