“…The global map was created using an instrument on NASA's Dawn probe, which is currently orbiting the dwarf planet, called the Gamma Ray and Neutron Detector (GRaND).
This instrument detects two kinds of particles:
1) neutrons, one of the particles that make up atoms, and
2) gamma rays, very high-energy light.
When cosmic rays (very high-energy particles from space) crash into the surface of the dwarf planet, the collision can create a spray of debris particles, including neutrons and gamma rays. But the debris isn't random; the characteristics of some of those gamma rays and neutrons can provide information about the chemical composition of the surface of Ceres and to certain depths below the surface. So scientists looking at data from GRaND can learn about the abundance of elements, including potassium, iron and hydrogen on the surface of Ceres, and to a depth of about 3 feet (1 meter).
The instrument cannot directly detect water molecules, but that can be inferred from the data, according to the authors. One way this is done is with computer models, which can recreate the evolution of Ceres, producing various possible outcomes that show how those elements (and water) would be distributed today.
Comparing the models with the new map shows that water ice on Ceres is concentrated near the poles: At high latitudes (past about 40 degrees in both hemispheres), water ice on the surface of Ceres and in the layers just under the surface may compose up to 27 percent of Ceres' mass, according to the new research. Near the equator, the water ice concentration is much lower….”