Mawrth Vallis (Welsh: [maurθ]) (Mawrth means "Mars" in Welsh) is a valley on Mars, located in the Oxia Palus quadrangle at 22.3°N, 343.5°E with an elevation approximately two kilometers below datum. Situated between the southern highlands and northern lowlands, the valley is a channel formed by massive flooding which occurred in Mars's ancient past. It is an ancient water outflow channel with light-colored clay-rich rocks.

Prior to the selection of Gale Crater for the Mars Science Laboratory (MSL) Curiosity rover mission, Mawrth Vallis was considered as a potential landing site because of the detection of a stratigraphic section rich in clay minerals. Clay minerals have implications for past aqueous environments as well as the potential to preserve biosignatures, making them ideal targets for the search for life on Mars. Although Mawrth Vallis was not chosen as a landing target, there is still interest in understanding the mineralogy and stratigraphy of the area. Until a rover mission is committed to exploring Mawrth Vallis, orbiters remain the only source of information. These orbiters consist of a number of spectrometers that contribute to our knowledge of Mawrth Vallis and the rest of the Martian surface.

One of the oldest valleys on Mars, Mawrth Vallis holds special interest because of the presence of phyllosilicate (clay) minerals which form only if water is available, first identified in data from the OMEGA spectrometer on the European Space Agency's Mars Express orbiter. Mars Reconnaissance Orbiter's Compact Reconnaissance Imaging Spectrometer for Mars has identified aluminium-rich and iron-rich clays, each with a unique distribution. Some of the clays recently discovered by the Mars Reconnaissance Orbiter are montmorillonite and kaolinite, alunite, and nontronite. Since some clays seem to drape over high and low areas, it is possible that volcanic ash landed in an open body of water. On Earth such clays occur in (among other environments) weathered volcanic rocks and hydrothermal systems, where volcanic activity and water interact. Mawrth Vallis was at one point considered as a landing site for the Mars Science Laboratory, which ultimately landed at Gale Crater. Clay minerals easily preserve microscopic life on Earth, so perhaps traces of ancient life may be found at Mawrth. It was considered a potential landing site for the Mars 2020 rover, but did not make the final cut.

The region is well studied with more than 40 papers published in peer-reviewed publications. Near the Mawrth channel is a 200 meter high plateau with many exposed layers. Spectral studies have detected clay minerals that present as a sequence of layers. Clay minerals were probably deposited in the Early to Middle Noachian period. Later weathering exposed a variety of minerals such as kaolin, alunite, and jarosite. Later, volcanic material covered the region. This volcanic material would have protected any possible organic materials from radiation.

Mars Global Surveyor was the first orbiter launched by the US since 1976 when the Viking lander was sent to Mars. The purpose of Global Surveyor was to map the surface of Mars using the Mars Orbiter Camera (MOC), the Mars Orbiter Laser Altimeter (MOLA), the Thermal Emission Spectrometer (TES), and a Magnetometer. MOC could capture high resolution images ranging from 1.5 to 12 m per pixel. MOLA was used to provide topographic maps of Mars. TES uses six detectors to measure both thermal infrared and visible near infrared data which is used to identify the composition of the surface of Mars. TES has a resolution of 3 x 6 km, this is a much larger field of view when compared to future orbiters. This resolution does not provide detailed composition maps but does serve as a good baseline for understanding the makeup of martian rocks.

TES was able to provide thermal inertia information at Mawrth Vallis despite the low resolution. Thermal inertia compares daytime and nighttime infrared data to determine how well a surface holds heat. Objects with high thermal inertia (hold more heat) are either highly indurated, very dense, or have a large particle size while low thermal inertia represents fine grain particles such as dust. Mawrth Vallis has a thermal inertia that indicates the size of surface particles range from dusty to larger rocks.

Odyssey is currently NASA's longest serving spacecraft at Mars and has been orbiting Mars since October 2001. Odyssey's primary purpose is to map the mineralogy of Martian surface but is also used to assess potential landing sites for rovers and landers. Odyssey consists of three instruments for measuring the surface of Mars; a Thermal Emission Imaging System (THEMIS), a Gamma Ray Spectrometer (GRS), and Mars Radiation Environment Experiment (MARIE). MARIE was damaged in 2003, most likely by a solar particle, and GRS is not in use after Odyssey changed its orbit in 2008 to increase THEMIS sensitivity. In addition to the onboard spectrometers, Odyssey serves as a communication relay between Earth and the rovers and landers on Mars's surface.

THEMIS detects infrared reflectance of ten spectral bands which are used for identifying the composition of the Martian surface. Using multiple-spectra allows Odyssey to better characterize the minerals found on Mars. THEMIS is similar to TES on the Mars Global Surveyor but has a lower spectral resolution (10 bands compared to TES's 143 bands) but has an increased spatial resolution (100 m compared to 3 x 6 km on TES) Odyssey is searching for terrains that represent past water, so the increased special resolution and narrow spectral resolution are targeting hydrated minerals.