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1. Ancient, Persistent Liquid Water

Achievement: Conclusive proof that liquid water existed for long periods on the ancient Martian surface. Significance: Dramatically increases the probability that life or proto-life could have developed on Mars during its early history, and perhaps did.
Evidence for Persistent Flow and Aqueous Sedimentation on Early Mars Evidence for Persistent Flow and Aqueous Sedimentation on Early Mars
This formation in Holden Crater was originally sand and gravel, which compacted into rock under later deposits and were then exhumed by erosion. It is estimated to be at least 3.5 billion years old. The features now appear in relief because softer rock around them has eroded away. This formation shows a cut-off meander, where a flowing stream changed course and cut off a loop, indicating that water flowed there for a considerable time. These photos were taken by the MOC camera on Mars Global Surveyor.
Fossilized distributary fans indicate that water flowed for geologically significant periods over the ancient Martian surface. At least 40 such fans have been identified, all dating from the earliest (Noachian) Martian geological epoch.
“Evidence for Persistent Flow and Aqueous Sedimentation on Early Mars,” Malin, M.C. et al. Science 302, 1931 (2003); DOI: 10.1126/science.1090544; 18 August 2003; accepted 28 October 2003 Published online 13 November 2003.
Images: NASA/JPL/Malin Space Science Systems
shallow surface channels
Mars experienced rainfall in at least localized areas. These shallow surface channels run up to the tops of ridges (indicating they were not formed by groundwater springs) and are densely packed, unlike networks that form beneath ice. They appear to have been created by rainfall runoff. Based on the number of craters in the area, the network is estimated to be less than 3.4 billion years old, perhaps as recent as 2.9 billion.

Mangold, N. et al., Science 305 (5680), 78-81 (2003), DOI: 10.1126/science.1097549.

Night infrared image IO2119003 by Mars Odyssey THEMIS spectrometer: NASA/JPL/University of Arizona

Gully age and formation mechanisms

Suggestions of at least occasional surface water continue into near modern times. Gully age and formation mechanisms are controversial but (left) this gully, recently photographed by the MRO HiRISE camera, is dated by the impact craters on its depositional fan to about 1.25 million ya.

Gully age and formation mechanismsGully age and formation mechanisms

Gullies in crater walls, possibly formed by occasional melting of subsurface ice or snow and photographed by the Mars Global Surveyor MOC camera. In the closeup, a gully originates under an overhang. Photos and annotations: NASA/JPL/Malin Space Science Systems.

Schon, S.C. et al. Geology 37, 207-210 (2009), doi:10.1130/G25398A.1

Photo and annotations: NASA/JPL/University of Arizona.


A diverse suite of minerals, including hydrated sulfates, phyllosilicates, and silica, produced by the action of water on martian crustal rocks has been identified both from orbit and from the martian surface. The character and concentration of at least some of these minerals systematically change on a global scale over geologic time.

Poulet, F., Bibring, J.-P., Mustard, J.F., Gendrin, A., Mangold, N., Langevin, Y., Arvidson, R.E., Gondet, B., and Gomez, C. (2005) Phyllosilicates on Mars and implications for the early Mars history. Nature 438:632–627.
Poulet, F., Beaty, D.W., Bibring, J.-P., Bish, D., Bishop, J.L., Noe Dobrea, E., Mustard, J.F., Petit, S., and Roach, L.H., (2009) Key Scientific Questions and Key Investigations from the First International Conference on Martian Phyllosilicates. ASTROBIOLOGY, V. 9, p. 257-267, DOI: 10.1089=ast.2009.0335. Squyres, S.W., R.E. Arvidson, et al. (2006a) Overview of the Opportunity Mars Exploration Rover Mission to Meridiani Planum: Eagle Crater to Purgatory Ripple, J. Geophys. Res., 111: E12S12, doi:10.1029/2006JE002771.
Arvidson, R.E., S.W. Ruff, R.V. Morris, D.W. Ming, L. S. Crumpler, A. S. Yen, S. W. Squyres, R. J. Sullivan, J. F. Bell III N. A. Cabrol, B. C. Clark, W. H. Farrand, R. Gellert, R. Greenberger, J. A. Grant, E. A. Guinness, K. E. Herkenhoff, J. A. Hurowitz, J. R. Johnson, G. Klingelhöfer, K. W. Lewis, R. Li, T. J. McCoy, J. Moersch, H. Y. McSween, S. L. Murchie, M. Schmidt, C. Schröder, A. Wang, S. Wiseman, M. B. Madsen, W. Goetz, and S. M. McLennan,. (2008) Spirit Mars rover mission to the Columbia Hills, Gusev Crater: Mission overview and selected results from the Cumberland Ridge to Home Plate, J. Geophys. Res., 113: E12S33, doi:10.1029/2008JE003183.

Morris, R.V., G. Klingelhoefer, C. Shroeder, et al. (2008) Iron mineralogy and aqueous alteration from Husband Hill through Home Plate at Gusev Crater, Mars: Results from the Mössbauer instrument on the Spirit Mars Exploration Rover, J. Geophys. Res, 113: E12S42, doi:10.1029/2008JE003201.

Mustard, J.M. et al., Hydrated silicate minerals on Mars observed by the CRISM instrument on Mars Reconnaissance Orbiter, Nature 354, 305-309 (2008).

Squyres, S.W., R.E. Arvidson, S. Ruff, et al. (2008) Detection of silica-rich deposits on Mars, Science, 320:1063-1067. Ehlmann, B.E. et al. Clay minerals in delta deposits and organic preservation potential on Mars, Nature Geoscience, 1, 355 (2008)
Bibring, J-P., Y. Langevin, J. F. Mustard, F. Poulet, R. Arvidson, A. Gendrin, B. Gondet, N. Mangold, and the OMEGA Team, The Mars History defined from the OMEGA/MEx spectra and inferred mineralogy, Science, v312, 400-404, 10.1126/science.1122659 2006.

Meridiani Planum

The Opportunity and Spirit rovers physically verified the orbital data. Opportunity landed at Meridiani Planum, near the Martian equator. The site was chosen because Mars Odyssey had detected crystalline hematite on the surface there; hematite often forms in water. Opportunity landed in a crater, where a meteorite had punched through surface material into the bedrock beneath, and has trekked to successively larger craters, surveying them from the rim and then venturing inside.

exposed subsurface sedimentary

In all the craters, the walls expose subsurface sedimentary rock (fine-grained sandstones) up to ten meters thick. The deeper the crater, the more layers are visible; no underlying rock has yet been seen. The whole of Meridiani Planum, hundreds of kilometers wide, appears underlain by sedimentary rock to a depth that is still unknown.

Pancam photo mosaic, approximately true color: NASA/JPL/Cornell

sediment strata
In the sediment strata, Opportunity has repeatedly photographed centimeter-scale festoon cross-bedding, a sediment ripple pattern produced on the centimeter scale only by flowing water.

Grotzinger, J. et al. (2006) “Sedimentary textures formed by aqueous processes, Erebus crater, Meridiani Planum, Mars.” Geology, 34, 1085-1088.


USGS animation:

Image and annotations: NASA/JPL-Caltech/Cornell

sandstone layering

Much of the sandstone layering appears to be lithified sand dunes, but the grains contain 30-40% (by mass) of sulfate salts, including MgSO4, CaSO4, and the hydrated iron sulfate jarosite, which forms at low pH, indicating exposure to sulfuric acid. Salts are usually residues left by water. This suggests that sandstones initially deposited by wind were later altered by acidic water, which in some areas of Meridiani appears to have been surface water and in other areas subsurface. The sulfuric acid probably originated in volcanic SO3 outgassing.

• Squyres, S.W. et al. (2004) “The Opportunity Rover’s Athena Science Investigation at Meridiani Planum, Mars.” Science 306, 1698-1703 and references cited therein.
• Squyres, S.W. et al (2006) "Two Years at Meridiani Planum: Results from the Opportunity Rover." Science 313, 1403—1407.
• Bibring, J.-P. (2006) "Merging Views on Mars." Science 313, 1899—1901.

blueberries blueberries

Most of the hematite occurs as small (~5mm) spherules (“blueberries”) found all along Opportunity's traverse, littering the surface soil and embedded throughout the sandstone outcrops. They are dispersed through the sediment layers, whereas volcanic or impact origin would concentrate them at layer boundaries, and they do not show elevated Ni levels that would indicate meteoric origin. These spherules appear to be concretions produced after the sediments were deposited, when iron-bearing sediment minerals (such as jarosite) dissolved in groundwater and the iron then precipitated as hematite. The surface scattering is interpreted as a lag deposit, with the hard spherules left behind when softer rock containing them eroded.

• Grotzinger, J.P. et al. “Depositional Model for the Burns Formation, Meridiani Planum.” Paper contributed to the Seventh International Conference on Mars, July 9—13 2007, Pasadena CA. Sponsored by the Lunar and Planetary Institute, Pasadena CA. Retrieved from
• Photos courtesy of Wendy Calvin (University of Nevada, Reno) and Ken Herkenhoff (US Geological Survey). Credit: NASA/JPL/Cornell/USGS/UNR

Mossbauer spectrometer

Halfway around the planet at Gusev Crater, the Spirit rover found extensive rocks that appear water-altered; goethite, an iron oxide hydrate formed only in water, was identified by Mossbauer spectrometry. This proved that water was not isolated at Meridiani, but occurred in both hemispheres of Mars.

Klinglhofer, G. et al. Hyperfine Interact (2005) 166:549–554, DOI 10.1007/s10751-006-9329-y

Image: NASA/JPL/Cornell/USGS/University of Mainz

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