On the 18^th February 2021, the NASA Perseverance rover successfully landed on Mars to search for signs of life. This search dates back to the 1600’s, when man first used telescopes to observe Mars. Unsurprisingly, no signs of existing life have been found on the harsh surface, on which life-harming radiation floods in through the thin atmosphere. The air is a dusty red fume and in appearance it is a barren wasteland filled with volcanoes and craters. Trip to Mordor anyone?

 

Mars is geologically dead at the moment, and apart from giant dust storms there isn’t much activity. Dust aside, compared to Earth, erosion rates on Mars are fairly low. Thus, different features are preserved, which are visualised in a height map of Mars taken by NASA’s Mars Global Surveyor Aircraft (Figure 1A). Based on these features, scientists have interpreted the geological history of Mars, shown in Figure 1B, which contrary to Mars in its present state was fairly eventful!

 

Broadly speaking, the majority of the planet’s surface is covered by basalt volcanic rocks, which is also found on Earth. These volcanic rocks fall into groups that can be split along the northern and southern hemisphere based on height, composition and age. Overall, Mars is 4.5 billion years old, and its oldest rocks are found in the southern highlands. This region is characterised by highlands, craters and volcanoes. It shows the first 1.5 billion years of Mars’ history, including the Noachian and Hesperian eras. The northern lowlands are characterised by flat volcanic plains containing young silica rocks and shows what transpired between 3 billion years ago and the present.^[1]

 

… there may be signs of previous life trapped in these rocks,
preserved from the debilitating effects of the harsh surface.

Mars history – volcanoes and water

Early Mars was a volatile place. We know this because in the southern highlands, Noachian age craters have been preserved. At this time, Mars and Earth were simultaneously bombarded by meteorites and experienced mass volcanic eruptions. Volcanoes and craters don’t sound promising for life, do they? Thankfully, we have evidence that things got better shortly afterwards, in the late Noachian age, when water seems to have appeared. The evidence for the appearance of water are altered volcanic rocks forming clays and cutting through craters in the form of channels. And water, as we know, is a key ingredient for life!

 

Water remained present in the subsequent Hesperian age. This is evident due to salt rock deposits which formed in salty lakes, and volcanic activity continuing in smooth flows rather than eruptions. Salt rocks are intriguing, because the first signs of fossilised life on Earth also formed in salty lakes and oceans in rocks known as stromatolites. Stromatolites form when bacteria trap sediment and mineralise, forming a ‘layered rock’ preserving signs (biosignatures) of these bacteria. Future rovers, such as ESA’s Franklin Rosalind, will be searching for biosignatures on the Martian surface, in places where the clays of the Noachian and the salty rocks of the Hesperian can be found. This is promising, because there may be signs of previous life trapped in these rocks, preserved from the debilitating effects of the harsh surface.^[2]

 

All good things come to an end, however, and by the ongoing Amazonian age, the ground froze, lakes disappeared and to top it all off, volcanoes erupted. This produced Olympus Mons (Figure 1A), the solar system’s largest volcano, located in the Tharsis bulge area. Olympus Mons erupting is considered to be the lava source for the silica-rich northern lowlands basalts, which are mineralogically different to the iron-rich southern highland basalts. This difference could be because of the depth the magmas erupted from. Alternatively, it could be caused by hydration of the lavas. Lava flowing and interacting with frozen ground could have caused melting which altered and hydrated the lava. In any case, at some point the eruptions stopped and Mars took on its current quiet form.

 

Underground water on Mars?

So far we have considered the possibility of potential life to be linked to surface water flows, but perhaps the underground can be life-nurturing as well, if the ground ice melts and water flows as mentioned above. Based on radio wave measurements scientists hypothesise that there are large bodies of water on Mars, making water flows likely. Satellite measurements into the openings of features on Mars’ surface, known as lava tubes (Figure 1C), present temperature data which shows a favourable environment for potential life.^[3]

 

Lava tubes are formed when lava flows down volcanic slopes. When this happens, the outer layer rapidly cools forming a shell through which the rest of the lava drains, leaving behind an empty tube. Occasionally, the roof of this structure collapses giving us a window to the underground. Heat measurements of these ‘windows’ have found warmer temperatures within them compared to the frozen surface. To paraphrase Ian Malcolm of Jurassic Park: did life find a way? Warmth, flowing water and underground refuge from a surface bombarded with high levels of radiation definitely make it a possibility worth considering!

 

 

Bacterial mats in Mars tubes, fact or fiction?

The current rovers on Mars have to stick to flat terrain and well-lit areas for access to solar energy. Therefore, sending them into dark tubes to find potential life is currently unpractical. Hence, we can only speculate about potential subsurface life using Earths lava tubes (figure 2A). Lava tubes on Earth ubiquitously contain life, in the form of colourful bacterial mats. These are distinguishable from the adjacent rocks and similar in appearance to stromatolites, as seen infigure 2B. This makes identifying them by eye easy, and scientists can collect mat samples for DNA testing. The environment the bacteria are found in influences the type of bacteria found in different lava tubes, which can result in different mat types. Bacteria react to environments based on how many nutrients are available, as well as the rock types, since different rocks contain different energy sources. Hence, scientists compare samples from different lava tubes on Earth to explore these environmental effects.^[4]

 

If life still exists in the Martian subsurface, finding colourful bacterial mats would be unequivocal evidence. However, if the mats didn’t grow to a size by which they are able to be perceived by eyesight or were preserved in some way, it would be important to seek alternative signs which are usually microscopic in size and more subtle. For example, potential biosignatures can be found within alteration features that are formed on lava tube walls. Whilst alteration is usually caused by running water, in many cases bacteria also play a role in producing secondary minerals such as manganese oxide (figure 2C). However, to distinguish the difference between the two, it’s necessary to look at the altered rock under a microscope. Bacterial cells or features are usually on the order of 10 microns, which is 100x smaller than the naked eye can see.

 

 

Biosignatures under a microscope – a hidden world the eye can’t see

Signs of bacteria in rocks can be destructive or constructive in nature, and can be either observed visually or as chemical signatures within the rock. For example, rocks contain many natural cracks but research has shown that bacteria can also create well-ordered cracks by living and tunnelling in volcanic glassas seen in figure 3A.^[5] Definitive evidence of bacterial tunnels is usually accompanied by chemical signatures of carbon, nitrogen or phosphate.^[6]

 

Figure 2C shows the secondary mineral manganese oxide growing on a lava tube wall. The researchers investigating this rock material found layered rings within the manganese oxide as seen in image 3B, which are similar in appearance to bacterial mats, which in turn indicates a biogenic origin!^[7] A sure way of checking for the presence of bacteria is to look for stringlike tubes growing on the rock surface using a microscope. An example is seen in figure 3C, where the linear feature is evidence of bacterial origins amongst a sample of manganese oxide.^[8] Research shows that bacteria grow preferentially over different rock surfaces based on the minerals.^[9] This could be interesting in the search for life on Mars, since the north and south are mineralogically different.

 

Bacteria don’t only tunnel into or grow on top of rock surfaces, they are actually involved in the process of producing secondary minerals as well. Odd shapes and distinct chemical signatures help distinguish minerals formed by water or life. This happens because bacterial cell walls are made of exopolysaccharide (material you find in sugars and carbohydrates), which is slightly electrically charged. The charge in these cell walls attracts ions, and buffers them from the surrounding cave environment.^[10] This leads to minerals forming in environments they wouldn’t be able to form in otherwise. For example, carbonate minerals don’t always form in the acidic environment of a lava tube. Again, the best way to check the origin of the mineral is using a microscope such as in figures 3D and 3E which shows carbonates growing in a pattern formed by life compared to block-shaped carbonate grown in a lab.

 

 

Applications of microscope signs to future Mars missions

Visual signs similar to those shown above could be sought in Martian lava tubes or within the past surface rocks of Mars (within clays or salt rocks). However, this would require sample preparation and a vacuum which the rovers are currently unequipped to carry (due to weight). Instead, the rovers are fitted with lightweight spectroscopy tools which fire beams of light at rocks and collect chemical measurements on-site. Whilst the lava tubes are currently difficult to navigate, the surface outcrops could be explored for signs of alteration and biologically formed chemical signs. Research shows that some minerals – when hydrated – are more likely to be formed biologically, since life can form minerals in the presence of water due to thermodynamics (but we won’t cover that here). Rocks with interesting chemical signatures could be sent back to Earth for review under a microscope or checked on future manned missions to Mars. In both cases, the samples could be compared to biosignatures found on Earth. Due to this we require ongoing research in this field.^[11] If we’ve learnt anything so far, it’s that there is far more to lava tubes than meets the eye and that the search for life on Mars will take place on both the macro- and microscopic scales before we can definitively come to a conclusion.

 

Redacteur: Tom Verhoeve