Search for signs of ancient life on Mars: expectations from hydromagnesitemicrobialites, Salda Lake, Turkey

The 'White Rock', constituting a portion of what may be a lacustrine sedime ntary sequence near the margin of a crater c. 90 km across in Sabaea Terra, Mars, measures 18 x 15 km x 180-540 m high. It is re-interpreted as a lens of magnesium carbonate precipitated where ground waters seeped into an anc ient evaporating crater lake. Were life to have emerged on Mars, as seems f easible, then the 'White Rock' might be expected to comprise a complex of s tromatolitic mounds. Salda Golu (Lake) in Turkey, is taken as an analogue. This enclosed lake is nearly surrounded and underlain by partially serpenti nized harzburgite. Hydromagnesite stromatolites (microbialites) up to 7 m h igh coalesce to form a group of small islands 200 m across. The microbialit es are seen to be growing near the mouth of the usually dry Salda River in the southwestern sector. Smaller developments of hydromagnesite encircle th e lake and image processing of satellite data reveals a second extensive zo ne beneath the lake surface over a delta in the southeast. Individual colum ns a few centimetres high constitute bulbous mounds which are about 2 m in diameter. These columns terminate in domes a centimetre or so across. The d omes are often annulated and are covered with a green biofilm a few millime tres thick comprised of cyanobactrial filaments. The columns consist of alt ernating fine and coarse hydromagnesite layers differentiated on a millimet ric scale. The coarser layers near the surface still contain traces of the biofilm. Fossil microbialites were also discovered in the friable hydromagnesite cli ffs shoreward of the main developments, though the structures of the indivi dual microbes have not survived. Instead the vestiges of microbialites are easily recognized and delineated by their coarse grain size and high porosi ty. Annular structures on their upper surfaces can be seen in places. The i ntervening and overlying material, also comprised of hydromagnesite, is a s emi-lithified mud. Bulbous megascopic structures, separated by finer grained magnesium carbona te mudstone, within strata in the 'White Rock', would be strong evidence of a photosynthetic microbial genesis. Another deposit of white rock on the w estern margin of Juventae Chasma could have a similar origin.