Evidence of water on Mars from Mars Odyssey

Lore & Background

The Mars Odyssey orbiter used its Gamma Ray Spectrometer to detect water ice just under the surface across both hemispheres from 55 degrees latitude to the poles. In these regions, one kilogram of soil contains about 500 g of water ice. Near the equator, water content drops to only 2 to 10%, much of it locked in minerals like clay and sulfates. The Viking landers had previously detected low levels of chemically bound water in the soil, and scientists believe ice may lie just a few feet deeper even where the upper surface contains only a percent or so of water.

The Phoenix lander later confirmed these findings, discovering ice a few inches below the surface, about 2 to 4 inches deep.

Reader's Guide

The 2001 Mars Odyssey mission provided the first definitive proof of abundant water ice just beneath the Martian surface, fundamentally changing our understanding of Mars as a potentially habitable world. Its Gamma Ray Spectrometer revealed that from 55 degrees latitude to the poles, the soil contains roughly 50% water ice by weight—enough to fill Lake Michigan twice. This discovery, announced at a conference in July 2003, resolved decades of speculation and directly influenced subsequent missions like Phoenix, which confirmed the ice's presence and behavior. The finding also explained many surface features seen in Odyssey's THEMIS images, such as branching valleys, layered deposits, and lineated valley fill, which indicate past liquid water and possible glacial activity. By showing that water ice is widespread and accessible, Mars Odyssey laid the groundwork for future human exploration and the search for past life, while also raising questions about the total water inventory, as the instruments could only probe the top meter of soil.

Did You Know?

• Mars has enough ice just beneath the surface to fill Lake Michigan twice.
• The Phoenix lander confirmed the Mars Odyssey findings, finding ice a few inches below the surface.
• Some ice was exposed by the landing rockets of the Phoenix craft.

The Hydrogen Revelation and Subsurface Ice Mapping

On May 28, 2002, during what was logged as sol 210 of the mission, NASA announced a finding that would reshape understanding of Martian hydrology. The Gamma Ray Spectrometer aboard the orbiter had registered substantial quantities of hydrogen in the planet's shallow subsurface. Because hydrogen on Mars is most plausibly bound within water molecules, the detection implied that ice lay no more than a meter beneath the surface in the regions being scanned. Rather than treating this as a one-off observation, the mission team set about systematically mapping where that hydrogen, and therefore that water, was distributed across the Martian terrain. The work did not stop at the polar regions. The orbiter went on to identify vast reservoirs of bulk water ice lurking just beneath the surface in equatorial zones, a finding that expanded the picture of where liquid or frozen water might have been accessible to any past biology. Together, these observations gave astrobiologists a concrete, spatially resolved map of a resource that is a prerequisite for life as we know it.

From ARES to Odyssey: The Naming Controversy

In August 2000, NASA opened a public call for candidate names for the upcoming Mars mission. Two hundred entries poured in, and the selection committee ultimately settled on Astrobiological Reconnaissance and Elemental Surveyor, shortened to ARES, a nod to the Greek god of war. The choice quickly drew fire from critics who found the name both uninspiring and unnecessarily combative for a scientific endeavor. The committee reconvened to reconsider. A candidate that had been set aside earlier, 2001 Mars Odyssey, resurfaced. The original hesitation had been purely legal, rooted in copyright and trademark concerns tied to Arthur C. Clarke's novel and the 1968 Stanley Kubrick film. NASA reached out to Clarke, who was living in Sri Lanka at the time. His reply was enthusiastic; he expressed delight at the idea and raised no objections. On September 20, Associate Administrator Ed Weiler formally recommended the switch, and Peggy Wilhide signed off on the change. The spacecraft that launched in April 2001 thus carried a name linking the mission to a beloved work of speculative fiction.

The Instrument Suite and Its Collaborative Origins

The orbiter's ability to find water and characterize the Martian surface rests on three principal instruments. The Thermal Emission Imaging System, or THEMIS, serves as the spacecraft's eye, capturing both visible-light and infrared imagery that reveals how different minerals are spread across the terrain. The Gamma Ray Spectrometer, which includes the High Energy Neutron Detector, is a joint effort involving the University of Arizona's Lunar and Planetary Laboratory, Los Alamos National Laboratory, and Russia's Space Research Institute. It scans the gamma-ray portion of the spectrum to identify elements such as carbon, silicon, iron, and magnesium in the atmosphere and near-surface material. The third instrument, the Mars Radiation Environment Experiment, is an energetic particle spectrometer dedicated to measuring radiation levels in the space around Mars, data that directly feeds into risk assessments for future human crews. Together these tools let the mission pursue its core objectives: mapping elemental abundance across the entire surface, quantifying hydrogen in the shallow subsurface, building a high-resolution mineral library, documenting surface morphology, and characterizing the near-space radiation environment that astronauts would face.

A Relay, a Record, and a Long Endurance

Beyond its science payload, the orbiter has served as an indispensable communications link between Mars surface missions and Mission Control on Earth. Roughly 85 percent of the images and data returned by the twin rovers Spirit and Opportunity traveled home through this relay. The spacecraft also helped engineers evaluate candidate landing sites for those rovers and for the Phoenix lander that touched down in May 2008, and it monitored atmospheric conditions while the Mars Reconnaissance Orbiter performed its own aerobraking maneuvers in 2006. The mission's endurance is equally remarkable. Launched on a Delta II 7925 from Cape Canaveral on April 7, 2001, the spacecraft arrived at Mars in late October and then spent about 76 days using atmospheric drag to circularize its orbit, a strategy that saved roughly 200 kilograms of propellant and allowed the smaller, less expensive launcher. By December 15, 2010, it had surpassed 3,340 days of continuous operation, breaking the previous record held by the Pioneer Venus Orbiter. As of April 2026 it remains active, holding the title of the longest-surviving spacecraft in orbit around a planet other than Earth.