On Earth, lakes are exquisite recorders of climate history because the geochemistry, mineralogy, textures and paleobiology of their sedimentary deposits trace the balance of atmosphere-hydrosphere-lithosphere-biosphere interaction through time. Analysis of the rock types, mineral assemblages, isotopic geochemistry and fossils or microfossils in finely laminated and bedded lake deposits reveal important evidence of surface temperature, atmospheric conditions and water chemistry. In addition, lakes on Earth are generally teeming with life because they lakes contain abundant water, chemical nutrients and energy sources – especially photosynthetic energy from sunlight that penetrates into the photic zone.
For all these reasons, the idea of exploring lake deposits on Mars has long been a “Holy Grail” of planetary exploration. Would lacustrine deposits on Mars be the best place to search for evidence of microbial activity trapped in the sediments? Many planetary scientists would argue yes, but I would say suggest that the answer is not so obvious…or at a minimum, it is highly nuanced.
How can we be sure that ancient Mars had lakes?
A characteristic of the ancient (>3.6 billion years ago) martian crust is that it contains dry river channels. This period of time is called the Noachian, as in the age of floods after the biblical patriarch Noah. In some cases, a channel can be observed to have breached the rim of and flowed into an impact crater, indicating that the aqueous flow in the channel would have unavoidably produced some standing body of water in the topographic low of the crater basin. This is what we refer to as a “closed basin lake” meaning that the terminal basin had a lake of some depth for some time. A better constrained scenario is where a channel flowed into a crater, filled the crater, eroded an outlet channel and flowed out of the basin. This is what we refer to as an “open basin lake” (Figure 1).
Figure 1: Mars Orbiter Laser Altimeter data draped over infrared (THEMIS) image mosaics show examples of an open-basin lake (a) and a closed-basin lake (b).
Lakes can be fed by groundwater in addition to overland flow. One thoroughly studied example in McLaughlin crater, which once sustained a deep, voluminous lake fed nearly exclusively by groundwater flow (Figure 2). No channels feed into the lake basin from distant sources and no outlet channels are observed. These types of lakes are very difficult to detect and their geologic histories can only be established through detailed integrated studies, particularly focused on the mineralogy of sedimentary rocks in the basin.
Figure 2: HRSC elevation data have been colorized and draped onto image data for McLaughlin crater lake, which was approximately 500 metres keep, around 3.8 billion years ago. There are small channels inside the basin possibly formed by groundwater springs or snow melt, which all terminate at approximately the same elevation - which was the level of the lake. Note however, that there are no large external channels feeding the lake, indicating that this type of basin was fed by groundwater.
In our recent paper published in Nature Astronomy “Geological Diversity and Microbiological Potential of Lakes on Mars,” we report that ~500 known closed basin and open basin ancient lakes exist on Mars (Figure 3).
Figure 3: Compiled global distribution of lake basins on Mars. Most lakes occur in Noachian terrain, although some occur in Hesperian terrain. Thermokarst lakes occur in Amazonian terrain. Landing sites targeting lake deposits described in the text are also labelled.
Standing on the shore of a martian lake
If you were to stand on the shore of a lake on Mars, 3.5 billion years ago, what would you see? Would it be comparable to our familiar experience in such settings on Earth? In short, probably not because lakes and seas on ancient Mars would have had some fundamental differences.
Ancient Mars had a reduced, methane-bearing atmosphere similar to the Archean Earth and different to our modern environment. This would have imparted a pink-orange hue to the sky. More importantly the reduced surface waters would have had large amounts of dissolved ferrous iron, likely giving the lakes colors between red-brown and green-brown. Standing on the edge of an ancient martian lake, we would have seen colorful, sediment-rich water, though dissolved and suspended sulphides could have rendered the lakes black at times.
On Earth, clear cutting or burning of vegetation causes problems of soil erosion because without the roots to hold the soil down, it is easily mobilized by runoff. On Mars, there would not have been such surface vegetation, and therefore the erosion and transport of soil particles would have been efficient, meaning that martian lakes would have received significant loads of suspended sediment. In the lower gravity environment on Mars, clay and silt would remain suspended for longer periods, making the lakes murky.
During the Noachian, when most of the known lakes existed, the Faint Young Sun was dimmer, and at Mars greater distance, the surface would have received only about 30% of the sunlight we are accustomed to on modern Earth. Given that there was less radiation available, the muddy lakes may have had significantly lower photic zones in which photosynthetic life could evolve and sustain.
Millions or lakes are known to exist on Earth. On this planet, small lakes (<100 km2) make up ~70% of the known lacustrine systems. Most of these occur in periglacial settings at high latitudes, not unlike cold, icy environments thought to have been common on Mars. But, a critical point is that we know relatively little of the small size fraction of lakes on Mars. This is a bias arising from the remote sensing methods we use to identify lake basins and deposits, and from the fact that the science is still evolving; it is easier and more reliable to focus on the larger features. We also do not have our highest resolution data for much of the planet. All of this adds up to a reality that we simply have not yet discovered what are likely thousands of small lake systems on Mars.
The largest lake
What is the difference between a lake and a sea? It is generally a sense of scale (seas are larger), of salinity (seas are somewhat saline) and connectivity (on Earth, as sea is partially or largely cut off from the rest of the ocean). It is not clear that Mars ever had the long sought global ocean. But it is clear that it had at least one and probably more large inland seas, such as the Eridania sea. This suite of connected basins contained >1 km-deep water ~3.8 billion years ago (Figure 4). This site is would be an amazing location for in situ exploration because it contains shallow and deep water sediments, salty evaporites, submarine and marginal marine type hydrothermal systems, and many other environments for sampling.
Figure 4: Elevation data draped over hillshade data from the MOLA and HRSC instruments show approximate depth of an ancient sea in Eridania basin. The image is about 900 km across
Lakes, looking forward
What’s next in martian limnology (the study of lakes)? Data returned from Mars by orbital spacecraft and surface rovers are of sufficient scope, quality and detail to allow geologists to ask and answer some sophisticated questions about lake environments. One of the obvious things to do is to search for the "missing" ~70% of martian lakes. These are likely small basins that are not so obvious to identify...but could be discovered. One way to make progress is to direct machine learning algorithms to identify candidate sites in global datasets, which can then be scrutinized more closely by human brains. Another is to use similar AI to help extract more information from existing spectral data in order to detect key tracer minerals that might provide more insight into environmental conditions. So far, we have always focused on major mineral occurrences but geologists on this planet are often interested in minor mineral occurrences that are hugely important for reconstructing geologic history - and that bit is largely missing in Mars science so far.
Ancient lakes will continue to be targets for future landing missions, but mission priorities will evolve over time. NASA will feel some pressure to return to Jezero crater lake in order to collect and return samples from that site, but in some sense this exploration strategy is limiting. Jezero is a good landing site with interesting rocks. But as discussed by some members of the community the lake environment might have only lasted for thousands to10s of thousands of years. Even if the lake lasted an order of magnitude longer, would it have been enough time to allow for life to form and take hold? Maybe yes. Maybe we are asking the wrong question. But in regardless of the ambiguities of this site along, here is the key point: If our exploration strategy requires that we follow on to chase samples selected from a site, which itself was selected decades before, it provides no latitude to update our knowledge. Exploration needs to be more nimble in order to take on board new discoveries.
China will launch a sample return mission to Mars late this decade, but they have not announced any target site. It is possible that CNSA will have the flexibility to explore the most exciting sites available at that decision point. Space X will also plan Mars landings in the coming years. But, these missions will likely target natural resources, specifically water ice for extraction and consumption and the sites where H2O is potentially available are not necessarily the best sites for scientific exploration. This story has yet to unfold...but it will be interesting to see how strategic exploration of Mars by humans will create synergy or not with robotic scientific exploration.
My personal opinion is that we should build up a long term exploration outpost in a place such as the Eridania basin, which contains a range of science targets. The wealth of information contained there would keep scientists on Earth busy for many years. It is not about trying to find life itself, in my opinion. More importantly, we should be thinking about understandinf the conditions that lead to the formation of life. The rocks of early Mars would shed light on the geology of the Archean Earth, which is not well represented in our geologic record here. In this way, Mars is the window into our own history.