Odyssey counts neutrons and finds buried ice
Neutron counts fell away over both poles. Odyssey read that as ice within a metre of the surface — by one estimate enough to fill Lake Michigan twice, and a target for landers.
In early 2002 Odyssey's detectors reported a sharp shortfall of moderate-energy neutrons across the southern high latitudes, and across the north as well, except the frost-covered ground near the pole. Cosmic rays knock neutrons out of the top metre of soil; hydrogen, whose nucleus is almost exactly a neutron's mass, drains their energy on the way back out, so hydrogen-rich ground returns markedly fewer than bare rock.
2001 Mars Odyssey launched on 7 April 2001, sixteen months after the second of the 1999 losses, entered orbit on 24 October, and aerobraked into a near-polar orbit about 400 kilometres up by January 2002. Its gamma ray spectrometer was three instruments sharing a name: a germanium gamma subsystem under William Boynton at the University of Arizona, a neutron spectrometer under William Feldman at Los Alamos, and a Russian high-energy neutron detector from Igor Mitrofanov's group.
On 28 May 2002 the team published numbers: poleward of roughly 60 degrees in both hemispheres, under a desiccated layer a few tens of centimetres thick, the ground appeared to be 20 to 50 percent water ice by mass, more than half by volume — in the top metre alone, JPL said, enough to fill Lake Michigan twice.
It was an inference, not a sample. Neutrons cannot see much below a metre, and the instruments detect hydrogen rather than water, so hydrated minerals would look similar. The case rested on the layered structure of the signal, on where ice ought to be thermodynamically stable, and on how sharply the effect switched on with latitude. Six years later Phoenix landed at 68 degrees north, scraped down five centimetres, and hit it.
THEMIS mapped surface mineralogy and thermal inertia; the MARIE radiation monitor stopped taking data after a solar storm on 28 October 2003. Odyssey has relayed data from every NASA surface mission since Spirit and Opportunity and is still in orbit, the longest-running spacecraft ever to operate at another planet.