Mass: 3.4 kilograms (7.5 pounds)
Power: 6.9 watts
Development: Johns Hopkins University Applied Physics Laboratory
XRS maps the elements in the top millimeter of Mercury’s crust using three gas-filled detectors (Mercury X-Ray Unit, or MXU) pointing at the planet, one silicon solid-state detector pointing at the Sun (Solar Assembly for X-rays, or SAX), and the associated electronics (Main Electronics for X-rays, or MEX). The planet-pointing detectors measure fluorescence, the X-ray emissions coming from Mercury’s surface after solar X-rays hit the planet. The Sun-pointing detector tracks the X-rays bombarding the planet.
XRS detects emissions from elements in the 1-10 kiloelectron-volt (keV) range – specifically, magnesium, aluminum, silicon, sulfur, calcium, titanium, and iron. Two detectors have thin absorption filters that help distinguish among the lower-energy X-ray lines of magnesium, aluminum, and silicon.
Beryllium-copper honeycomb collimators give XRS a 12° field of view, which is narrow enough to eliminate X-rays from the star background even when MESSENGER is at its farthest orbital distance from Mercury. The small, thermally protected, solar-flux monitor is mounted on MESSENGER’s sunshade.