In 1849 the Italian chemist J. Usiglio performed a set of evaporation experiments with seawater along the French Riviera and established the order in which evaporite minerals precipitate from an aqueous environment. On earth these minerals are mostly gypsum and halite, associated with borates, potassium and magnesium salts. These minerals (not surprisingly) if found in a stratigraphic column are compelling evidence for the former presence of water.
Fig.1. The Groeden sandstone was deposited in a landscape consisting of a succession of perennial and periodic rivers, alluvial fans, lakes and coastal plains in a semiarid climate (hot and dry with seasonal excessive rainfalls). Pedogenetic horizons with folded gypsum layers record strong evaporation and water level fluctuations. The upper cross-bedded sandstone records periods when meandering fluvial channels alternated with sand dunes of a desert environment.
Already from the orbit the Mars Global Surveyor identified terrains on Mars composed of a stratified material, covering a more rugged and cratered relief. The interpretation for this terrain ranged from volcanic lava sheets to aeolian sediment to deposits formed in water. To clarify the origin of these layers a field-investigation was necessary. In 2004 the "Opportunity" rover landed in the "Eagle-Crater" located on the "Meridiani-Planum" - a flat, uniform plain with few impact craters and delimitated by cliffs with recognizable stratified structure. Opportunity spotted some outcrops in the small crater, however the exposed stratigraphic column was very short. The rover was therefore directed to the larger "Endurance-Crater" (with a diameter of 200m).
The outer rim of this crater provided an unique outcrop - soon named Burns-Cliff, after Roger Burns, who predicted the mineralogy of the Martian rocks (composed mainly of ultrabasic minerals, like Olivine, and ferric sulfate minerals) based on the preliminary results obtained by the Viking missions.
Along the slope of the cliff geologists recognized a succession of rock types, or facies, named informally "Burns-Formation", the only extraterrestrial geologic formation at the time. The Burns-Formation consists almost entirely of sandstone with grains of basalt, oxides, silicates and evaporite minerals (Calcium and Magnesium- sulfates, chlorides and phosphates).
The Burns-Formation can be subdivided in three members:
- A lower unit of cross-bedded sandstone, probably sand dunes formed on a dry lakebed
- A middle unit with finely laminated sandstone, interpreted as ancient sand sheet deposits, overlies the lower unit with an Rare cross-laminated layers maybe represents sporadic flood events. This units displays various effects of groundwater infiltration, like dissolution of minerals and precipitation of new ones. Convoluted layers formed probably when minerals expanded due chemical reactions with the groundwater.
- The upper unit consists of finely laminated sandstone, however layers of cross-laminated sandstone are more frequent. These layers show also a particular sedimentary feature named "festoon cross-bedding" - concave, intersecting sets of thin layers, found on earth only in cross-sections of subaqueous ripples.
Fig.2. The Burns-Formation at Burns-Cliff, southeast rim of Endurance crater (after GROTZINGER et al. 2005), images acquired by "Opportunity" - NASA (image in public domain).
The Burns-Formation records the transition from a dry dune-field to a wet "playa" environment. The infiltrated water stabilized the grains against erosion by Martian wind and flat sheets of sandstone formed. Sporadic flood events formed ripples, during dry phases evaporite minerals precipitated from the evaporating water. In later times Meridiani-Planum was covered for longer periods with water - maybe a shallow lake formed. This transition from dry to wet conditions occurred probably many times in Mars' past , as the dunes of the lower unit are already formed by reworked evaporite minerals from older playa sediments.
The age of Burns-Formation is unknown. The lack of big impact craters and the few small craters suggest a relatively "young" age of Noachian (4,1- 3,7 billion years) to Lower Hesperian (3,7- 3,0 billion years).
The landing site of "Curiosity" was selected for similar criteria as the Opportunity site. "Gale Crater" is a 3,7 billion year old "Ghost crater", once covered by layers of sediments, erosion later exhumated the crater. "Aeolis Mons", the central elevation inside the crater, is a mesa with stratified structure - the first picture send to earth seems promising…
CARR,M. H. (2006): The Surface of Mars. Cambridge Planetary Science Series - Cambridge University Press - New York: 307
GROTZINGER, J.P., ARVIDSON, R.E., BELL III, J.F.; CALVIN, W.; CLARK, B.C.; FIKE, D.A.; GOLOMBEK, M.; GREELEY, R.; HALDEMANN, A.; HERKENHOFF, K.E.; JOLLIFF, B.L.; KNOLL, A.H.; MALIN, M.; McLENNAN, S.M.; PARKER, T.; SODERBLOM, L.; SOHL-DICKSTEIN, J.N.; SQUYRES, S.W.; TOSCA, N.J. & WATTERS, W.A. (2005): Stratigraphy and sedimentology of a dry to wet eolian depositional system, Burns formation, Meridiani Planum, Mars. Earth and Planetary Science Letters 240: 11-72