Building a lake on Mars
This image and its associated caption give a geological timeline for the Terra Sirenum region of Mars, – a region which once held one of the planet's largest lakes – as well as expanding upon the climate and other mechanisms that likely led to the evolution. This information is from a study using data from the High Resolution Stereo Camera (HRSC) on ESA's Mars Express, and the High Resolution Imaging Science Experiment (HiRISE), Context Camera (CTX) and mineral-finding Compact Reconnaissance Imaging Spectrometer (CRISM) on NASA's Mars Reconnaissance Orbiter (MRO). The study focusses on four basins – Atlantis Chaos, Simois Colles, Caralis Chaos and an unnamed basin referred to in the study as the Northern Basin – each of which hosted individual lakes following the fragmentation of the much larger Eridania Lake. Details of the study can be found in Geologic evolution of the eastern Eridania basin: Implications for aqueous processes in the southern highlands of Mars, by Solmaz Adeli, Ernst Hauber, Laetitia Le Deit, and Ralf Jaumann in the Journal of Geophysical Research, Volume 120, November 2015; doi: 10.1002/2015JE004898.
A wet and cratered world – the Noachian period
The Noachian period on Mars began around 4 billion years ago in a time when meteorite and asteroid impacts in the inner Solar System were around 500 times more frequent than they are today. For this reason, the Noachian-age surfaces visible on Mars now are densely peppered by impact craters and rough inter-crater plains.
These, now highly degraded, surfaces are not within basins but on the highlands of the region studied, at elevations of around one to three kilometres, and they are collectively known as the Noachian Cratered Plateau. In this study, researchers have found material which would once have formed part of this plateau along the rims of their studied basins. This leads the team to suggest that the material was eroded from the highlands late in the Noachian period by the flow of water. Even though this erosion of the highlands might imply the lake itself was formed from surface run-off the team's map of the area does not find enough valleys in the area for this to be the case. They, and others before them, instead propose that the lake was originally created primarily through rising groundwater.
A dramatic shift – the transition from the Noachian period to Hesperian period
As Mars transitioned into the Hesperian period the geology and climate of the planet began to change. It was during the transition between these two periods that the Eridania Lake is thought to have undergone extreme flooding and formed Ma'adim Vallis, one of the largest valleys on Mars, through which the lake would eventually drain leaving smaller pockets of water behind. Each basin studied by Adeli and her team would eventually hold an individual lake following this decrease in water levels.
Studying the morphology of material gives valuable clues to the changes to the Eridania Lake at this time. In particular the team studied the morphology and composition of the light-toned materials found on the plateaus, floors and rims of the basins, directly on top of the older terrains. During the reign of the lake this material would have been deposited by airfall in the form of ash or dust – a process that took place either before or during the transition to the Hesperian period – before being shaped and eroded by the splitting of the Eridania Lake into smaller lakes and its ultimate demise. (Step 1: this is depicted by the lower panel.)
The light-toned material was originally deposited on the plateaus as well as the floors and rims of the basins. As the water level in the Eridania Lake dropped (Step 2: depicted in the second frame from below), leaving only the smaller lakes behind, the material that had found itself on the plateaus was eroded by wind and water and carried into the basins. This erosion shows that at the end of the lifetime of Eridania Lake there was still some liquid water entering the basins. But there was more water evaporating from the lake – or, in the possible case that the lake was frozen, an equivalent process known as sublimating – than was entering and so the lake was never to return to its former glory.
What remains of the light-toned material on higher ground are flat-topped and steep sided areas of land known as Mesas – the Table Mountains of Mars.
The lives of four lakes – firmly in the Hesperian period
Within the individual basins, which during this period hosted individual lakes due to the significant fall in water levels, the researchers in this study found that the light-toned material contains important minerals. These minerals, known as phyllosilicates, are a group of minerals that include mica, talc and clay.
The team confirmed that the light-toned material in the basins is particularly rich in minerals known as Iron (Fe) and Magnesium (Mg) Phyllosilicates; clear indicators of a body of water. These are present both in the smooth light-toned materials observed on the floors and rims of the basins and in the dramatic knobs of light-toned material which jut up to 600 metres in height from the floors of the Atlantis, Simois and Caralis basins, giving them their chaotic appearance. On the rim of the Simois and the Northern Basin a sequence of Aluminium (Al) Phyllosilicates were also discovered within the smooth light-toned materials lying on top of the other phyllosilicates.
The knobs of light-toned material are perhaps the most telling of the region's geological features. Like the smooth light-toned material this sediment was originally deposited from the air onto the floors and rims of the basins, or carried from the exposed plateaus. As water levels on Mars declined further (Step 3: illustrated in the middle frame) towards the end of the Hesperian period the material in each basin began to crack as it dried and was eventually eroded by the moving waters of the period into these often gigantic monoliths.
The demise of the Martian lakes – the Amazonian period begins
In the Northern Basin, on top of the phyllosilicates, the study confirmed the presence of chloride salts within the smooth light-toned materials that lie in shallow depressions ranging from a few tens of metres to a few kilometres across. The presence of these salts suggest that the individual lakes entered a process of extreme drying at the end of their existence, or even later with the reoccurrence of liquid water in the area. These areas were partly encrusted with salt as water – from rain, snow or ice melt – filled and then evaporated from them. This supports the team's deduction that the isolated lakes were eventually lost via evaporation or infiltration into the ground rather than run off, as outlets through which surface water could have escaped to lower ground have not been found.
Above the light-toned material in its various forms, Solmaz and her team have found two layers of material (Step 4: llustrated in the second frame from the top) thought to have formed early in the Amazonian period, the geological time-frame in which Mars currently finds itself. Both were deposited when the basins were completely dry and provide strong evidence that the smaller lakes that followed the demise of Eridania Lake were wiped away before the Amazonian period began.
One layer is a much more abundant thin layer of dark material which covers the floor of all four basins and caps the knobs of material in the three knobbly-floored basins. The other (Step 5: shown in the top frame) is a thin smooth material which filled the gaps between the knobs in the Atlantis and Caralis basins. The material has cracked as a result either of the rapidly drying landscape or due to potential lava flow cooling, indicating that there was volcanic activity in this period.
The nature of the last units of material mean that they formed after the decline of the Eridania Lake and the smaller lakes that followed. Combining this with knowledge that the light-toned material was shaped by the Eridania Lake’s evolution has led the team to propose that the lakes had disappeared by the time the Amazonian period began.