The Martian surface as imaged, sampled, and analyzed by the Viking landers

Abstract

The two Viking landers touched down on the surface of Mars in July and September of 1976. Viking Lander 1, renamed the Mutch Memorial Station (MMS), continued to operate for 3 Martian years in Chryse Planitia. The landing site is a rolling, sparsely cratered plain. Impact crater size‐frequency distributions demonstrate that only meters of vertical erosion have occurred over the lifetime of the surface. The site exhibits bedrock exposures, numerous rocks excavated during cratering events, indurated fine‐grained sediment (blocky material) mixed with dust and lithic fragments, and aeolian drifts that overlie the blocky material and which probably accumulated during global dust storms. Lander 2 operated for approximately 2 Martian years in Utopia Planitia, a region subjected to volcanic, impact, aeolian, and periglacial phenomena. Lander 2 is located west of the 100‐km‐diameter crater Mie, on or near the distal end of the continuous ejecta deposits. Polygonal ground suggests that desiccation or freeze‐thaw processes have occurred. The Lander 2 site exhibits numerous rocks excavated from craters and exposed by erosion, set in a matrix of crusty to cloddy sediment. Rare aeolian drifts overlie the crusty to cloddy material. Lander multispectral imaging of sediment exposures at both sites is best matched with iron‐rich, claylike weathering products called palagonites. The spectral characteristics are probably governed by thin deposits of dust from global and local dust storms. Darkest rocks have reflectance values that match iron‐rich igneous rocks, although most rocks are covered with indurated coatings of the palagonitelike products. Chemical compositions of sediments at the two landing sites are strikingly similar, suggesting an aeolian origin. The compositions suggest an iron‐rich igneous source rock and are matched by an assemblage of clays and salts. Samples of blocky materials were found to have about 50% more Cl and SO3 than other samples, implying cementation by salts. The magnetic component in the sediment is silt‐sized or finer and may be maghemite or ultrafine‐grained hematite. No indigenous organic compounds were found. The materials were found to be highly reactive, releasing O2 when H2O was added, oxidizing organic nutrients on board the landers and fixing atmospheric CO2. Clay‐ to silt‐sized particles were inferred for drift and crusty to cloddy materials. No terrestrial samples have been found that replicate all the attributes of the Viking samples. Approximately 10 µm of H2O ice accumulated during both winters at Lander 2 and each time evaporated over approximately 200 Martian days. Thin layers of bright red dust from global and local dust storms accumulated at both sites during the first 2 Martian years. During the third winter, strong winds generally scoured the MMS site, leaving behind a darker aeolian lag deposit. Bedrock at both sites formed billions of years ago, and rocks have been generated and weathered slowly over geologic time, but aeolian sediments accumulate, become cemented, and are subsequently eroded over time spans of millions of years or less. The lifetime of the sedimentary deposits at the landing sites may be governed by the 105‐ to 106‐year variations in climate due to quasi‐periodic changes in obliquity, eccentricity, and spin axis orientation.