A recent article in Nature Geoscience suggests that at least one large, deep crater on Mars may once have supported an alkaline lake that was fed by water from kilometres below the planet’s surface.
Alkalinity is a measure of the ability of solutions, including lake water and groundwater, to neutralise acids. Alkaline waters on Earth may be very basic (like drain cleaner!) or closer to neutral pH but with relatively high amounts of dissolved CO2.
The study of Mars – by Joseph Michalski and co-workers – found convincing evidence McLaughlin Crater once contained water sourced from within the ground. This gives us a promising new place to look for evidence of ancient life on Mars.
Channels, now dry, appear to flow down the walls of McLaughlin Crater and stop well above the crater floor, which indicates they once provided water to an ancient lake.
Hummocky features at the base of the crater are also reminiscent of landscapes produced by underwater landslides. The authors of the study also used spectroscopic capabilities (i.e. analysing light coming from the surface of Mars) on board the Mars Reconnaissance Orbiter to identify clay minerals and carbonate minerals within material on the crater floor.
On both Earth and Mars, minerals and rocks (which are made of one or more minerals) form from aqueous (i.e. dissolved in water) solutions or other fluids.
Specific minerals can only form under certain conditions of temperature, pressure and solution chemistry. This is extremely useful because it allows Earth and planetary scientists to infer information about ancient water bodies, climate and the depth of rock formation by identifying the minerals involved.
Clay minerals are relatively common at the surface of Mars and magnesium- and iron-rich smectite – a type of clay mineral – found in McLaughlin Crater is thought to have formed in association with water at or near the planet’s surface during Mars’ warmer, wetter past.
Serpentine, another type of clay mineral identified by the study, typically forms by a process called serpentinisation, which can occur at relatively shallow depth (a few kilometres) beneath the surface of the Earth – and probably Mars. Serpentine can form when surface water migrates underground to become groundwater, is heated at depth, and reacts with magnesium rich rocks.
The presence of serpentine suggests that groundwater may have been causing reactions to depths greater than 5km beneath the surface surrounding McLaughlin Crater.
Of interest to astrobiological exploration, deep groundwater activity at McLaughlin Crater could have delivered essential water and carbon to any micro-organisms that may have been living in the deep subsurface.
Michalski and co-workers speculate that these deep, sheltered environments may have been the cradle and refuge of microbial life on Mars and possibly early Earth.
Deep craters such as McLaughlin Crater provide an amazing glimpse into the past of Earth’s sister planet. This is because the impacts that produce craters blast away patches of Mars’ surface, leaving behind a cross-section through layers of rock produced over millions or billions of years.
Mars and Earth are thought to have shared similar early histories before the red planet became cold and hyper-arid. Unlike Mars, Earth is a dynamic planet, shaped by the slow but powerful movement of tectonic plates and pervasive colonisation by life.
The consequence of living on an energetic planet is that most of Earth’s early history has been erased by the transformation and recycling of ancient rock.
Life on Mars?
Michalski and co-workers suggest that glimpses into Mars’ deep past – at sites such as McLaughlin Crater – may give us an opportunity to glimpse signs of ancient life. Such discoveries could also shed light on the early evolution of life on Earth.
Finally, identification of calcium- and magnesium-rich carbonate minerals at McLaughlin Crater provides further evidence that groundwater re-emerged to form an alkaline lake.
Magnesium and calcium carbonate minerals commonly form in alkaline lakes and springs on Earth. Mineral deposits of this sort occur in Oman, California and Italy in association with groundwater springs near serpentinites (rocks that are composed almost entirely of serpentine).
The formation of carbonate minerals in lakes can also be mediated by photosynthetic bacteria such as cyanobacteria, which are common in alkaline lakes on Earth.
Sometimes these bacteria (and their unfortunate neighbours) actually become entombed in carbonate minerals, forming microscopic fossils.
If similar conditions once existed in an alkaline lake at McLaughlin Crater, fossil micro-organisms may be waiting there for us to discover.