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• 1 media/viking_first image_cropped.jpg 2016-04-05T10:11:26+00:00 From habitability to searching for signs of life on Mars 68 image_header 2022-06-14T15:14:04+00:00 The overview given above has clearly illustrated that life is capable of existing in very inhospitable, well, to us, environments. Let us now look at some specific locations within the solar system and speculate on what we might find.  We’ll start with….
  
  Life On Mars?
  We had already pointed out that Percival Lowell thought Mars to be inhabited, but his assertions were not correct.  There is no Martian civilization and reports of their invasion turned out to be wrong first in 1898,  in 1938, in 1953 and again in 1996 and again in 2005.  Even when set to music it was wrong. 
   
  But there have been some very interesting developments:

  Viking

  In 1976, 2 separate Viking landers touched down on Mars. The image at the top of the page is part of the first clear image transmitted by Viking 1 on July 20, 1976.  Unlike the current rovers, these landers were stationary, gathering samples by means of their sampler arm. Of the four experiments designed to search for life, one, the Labeled Release (LR) experiment, returned a positive result.  It has generally been accepted that these results are the result of unusual Martian soil chemistry, namely that a superoxide in the Martian soil oxidized the organic molecules provided in the LR nutrient. 
  
  Nevertheless, discussions about the results continue periodically.  For example in 2012 a team of researchers argued on the basis of statistical complexity analysis of the original data that the data does suggest a finding of life.  The consensus opinion remains that the Viking experiments did not find life.

  Mars meteorites

  After Viking, there was a hiatus in Mars exploration until the 1990s. But importantly, over that time, the Martian meteorites were discovered to originate from Mars (see Mars volcanism section).  In particular interest for life, an unusual Martian meteorite discovered in Antarctica, called Allan Hills 84001 (ALH84001). Dubbed the “Life on Mars” meteorite by some, ALH84001 contains mineral globules and chain structures that some researchers interpreted to be of possible biogenic origin. The discovery (interpretation, actually) was considered so significant that it was announced by President Clinton​ in 1996, making global headlines.
   
  While the potential biosignatures in ALH84001 captured the nation’s attention, it is now considered unlikely by the scientific community. Other hypotheses for their formation include high temperature and low temperature water-rock interaction and alteration.  (You can, however still buy a plushie of the ALH84001.) Other meteorites have since been examined as possibly including evidence of past life, but nothing definitive is widely accepted.
   
  Importantly, the discovery of potential traces of life in ALH84001 blasted open the doors for Mars exploration.  Funding allocated by the US congress for the Mars Exploration Rovers (Spirit and Opportunity rovers in 2004) was a direct result of the excitement generated by ALH84001. And the Mars Science Laboratory (MSL) Curiosity mission was in turn a direct result of the success of Spirit and Opportunity. We actually owe a lot to one little stone from Mars.

  Opportunity

  The Mars Exploration Rover Opportunity operated on the surface of Mars from 2004 to the middle of 2018 - over 14 years of exploration! Opportunity was sent to Meridiani Planum because orbital observations suggested the occurrence of an oxidized iron mineral (hematite) that is likely to have formed in the presence of liquid water.
   
  Rocks at Meridiani are layered and sedimentary in origin.  In addition to hematite, there is an abundance of sulfate minerals. Because the rock layers are flat-lying, Opportunity focused its attention on cliff outcrops exposed in impact craters.  The first big crater examined was Endurance Crater.
  An excerpt from a 2005 press release presents an optimistic possibility for life in the Meridiani sediments.

  The stack of layers in Endurance Crater resulted from a changeable environment perhaps 3.5 to 4 billion years ago. The area may have looked like salt flats occasionally holding water, surrounded by dunes. The White Sands region in New Mexico bears a similar physical resemblance. For the chemistry and mineralogy of the environment, an acidic river basin named Rio Tinto, in Spain, provides useful similarities, said Dr. David Fernandez-Remolar of Spain's Centro de Astrobiologia and co-authors.
   
  Many types of microbes live in the Rio Tinto environment, one of the reasons for concluding that ancient Meridiani could have been habitable. However, the organisms at Rio Tinto are descended from populations that live in less acidic and stressful habitats. If Meridiani had any life, it might have had to originate in a different habitat.
   
  "You need to be very careful when you are talking about the prospect for life on Mars," Knoll said. "We've looked at only a very small parcel of Martian real estate. The geological record Opportunity has examined comes from a relatively short period out of Mars' long history."

  It is now considered unlikely, however that the environment recorded by the rocks of Meridiani Planum was habitable.  A 2008 paper by Tosca and others discusses how salinity indicated by the geochemistry and mineralogy would have been much too high and exceeds levels tolerated by known terrestrial organisms. So while Meridiani sediments have evidence for water, the conditions were not favorable to life as we know it.
  
  The Opportunity rover continued to make important discoveries until its end of mission (last contact in June 2018).  In 2011, it reached a large crater called Endeavour, where more ancient rocks are exposed in the crater walls.  In 2013, the mission announced the discovery of clay minerals (phyllosilicates) that point to more neutral, water-rich environments that would have been more potentially habitable for life.  (This announcement came around the same time that Curiosity published finding phyllosilicates as was described in the previous module and demonstrates a convergence of evidence that Mars was once habitable.)
  

  Curiosity’s search for organic molecules

   
  As was described by the previous module, the identification of clay minerals in powdered mudstone samples by the MSL Curiosity rover indicate that the rocks formed in a lake setting when Mars was once potentially habitable. As Curiosity continues to explore Gale crater, it will encounter new types that may aid in constraining conditions of other past habitable environments. At the same time, the focus of the MSL mission has shifted a bit.  Now, there is a push to identify organic matter preserved in Gale rocks and to learn more about optimal environments for organic preservation. 
   
  Central to the study of organics at Gale crater is the SAM instrument suite, which stands for Samples Analysis at Mars. SAM examines atmospheric gasses and powdered rock samples to measure elemental and molecular chemistry. The atmospheric gas analyses tell us about the Mars environment today, whereas the powdered rock samples give us information about ancient Mars.

  Methane

  The study of atmospheric gasses by the SAM instrument has really been focused on measuring the concentration of methane, which is a gas molecule containing one carbon and four hydrogen atoms (CH4). Trace amounts of methane (parts per billion level) were claimed to be present in the Mars atmosphere by several studies prior to MSL. Break up of methane by ultraviolet (UV) radiation should only take ~350 years and so geologic sources of methane must be replenishing the gas. Microbes represent only one possible methane source, along with water-rock interactions and break down of surface organics.
   
  When Curiosity landed in 2012, SAM did not detect any measurable methane in the Mars atmosphere.  Then in 2014, SAM identified a “tenfold spike” in methane, which suggests a point source.  It is unknown what caused the spike, but it suggests an active methane cycle on Mars. The Curiosity rover has since detected spikes in methane abundance in the atmosphere many times over the course of the mission and its occurrence appears to be seasonal, although the source is unknown.

  Organics in rocks

  The SAM instrument also looks for larger organic molecules in powdered rock samples.  So far, only one powdered sample has measurable organics (chlorobenzene), so organics are not preserved everywhere. In fact, of two samples collected 1 meter apart and from the same rock outcrop (dubbed John Klein and Cumberland), only one had measurable concentrations of organics.
   
  This finding points out the real difficulties in finding organics on Mars. Sediment depositional environment, groundwater chemistry, and surface radiation all play a role in degrading organic molecules. But even though it is hard, it doesn’t mean it is not worth doing, particularly if we eventually are able to find more complex organics (like amino acids and lipids) that would hint at life.

  Mars 2020 Perseverance Rover

  After finding evidence of habitable environments at both the Curiosity and Opportunity rover sites, the goal of the Mars 2020 Perseverance Rover mission is to find evidence of past microbial life. This rover mission built on the success of Curiosity and used the same sky crane landing technology in addition to Artificial Intelligence to successfully find a flat region to land in Jezero crater.  Check out this incredible landing video that was shot in part by cameras onboard the rover to experience the thrill of landing on the Mars surface!
  Also, the Perseverance rover looks a lot like Curiosity; its size (~3 m long, 2.2 m tall) and configuration are similar. The instrumentation is different, however and is geared toward its stated science objectives:
  
  Studying Mars' Habitability, Seeking Signs of Past Microbial Life, Collecting and Caching Samples, and Preparing for Future Human Missions.

  Looking for Habitability:	Identify past environments capable of supporting microbial life.
  Seeking Biosignatures:	Seek signs of possible past microbial life in those habitable environments, particularly in special rocks known to preserve signs of life over time.
  Caching Samples:	Collect core rock and "soil" samples and store them on the Martian surface.
  Preparing for Humans:	Test oxygen production from the Martian atmosphere.

  
  
  You can learn details about the individual instruments onboard Perseverance here.
  
  While roving robots are amazing, there is a limit to what they can accomplish on Mars without laboratory equipment, such as in research institutions on Earth. In addition, lab technology continues to advance and a well curated sample returned from Mars may allow us to make discoveries that are unfathomable at present. For this reason, the Perseverance rover will also have a system of sampling rocks that will eventually be returned to Earth.  The strategy is described here and will be to drill into rocks and drop pencil-length core samples on the surface for eventual retrieval by a later rover mission. Mars sample return to Earth may not occur for some time (earliest date quoted is 2034), but the investment to produce a well characterized sample suite by the Mars 2020 mission should pay off.
  
  The landing site of Perseverance, Jezero crater at the western edge of Isidis Planitia is shown below.  Perseverance was sent there because geomorphic evidence supports the interpretation that Jezero once held a lake.  This evidence includes inlet and outlet channel systems as well as a in the form of a platform of sediments at the mouth of the inlet channel that is interpreted to be a river delta.  Also, mineralogical evidence, including clay and carbonate minerals detected from orbit suggest neutral to alkaline waters were once present.  The geomorphic and mineralogical evidence together supports the possibility that Jezero once held a lake and was once habitable. This is an ideal sort of environment to look for evidence of past life, if it once existed on Mars.
  
  Here is a map (via the kind folks at NASA) which shows where Perseverance is right now:
  
  
  It should be noted that the Mars 2020 rover mission is only the next stage in the exploration of Mars. The idiom of “follow the water” has now completely morphed into seeking signs of life and preparing for human exploration. The timeline below puts the history and future of Mars exploration into perspective.
  
  
  For a really good summary of how and why we search for life on Mars, watch this TED talk by Dr. Natalie Cabrol.  (She is also a really kick ass scientist).
  
  
  (September 2018 update)
  According to a recent study, ancient Mars had the right conditions for underground life about 4 billion years ago.  As pointed out in this press release:

  While evidence of past water activity (on Mars) is unmistakable, it's not clear for how much of Martian history water actually flowed. State-of-the-art climate models for early Mars produce temperatures that rarely peak above freezing, which suggests that the planet's early wet periods may have been fleeting events. That's not the best scenario for sustaining life at the surface over the long term, and it has some scientists thinking that the subsurface might be a better bet for past Martian life.

  "The question then becomes: What was the nature of that subsurface life, if it existed, and where did it get its energy?" said Jack Mustard, a professor in Brown's Department of Earth, Environmental and Planetary Sciences and a study coauthor. "We know that radiolysis helps to provide energy for underground microbes on Earth, so what Jesse did here was to pursue the radiolysis story on Mars."

  Based on their research,
   

  the researchers conclude that Mars likely had a global subsurface habitable zone several kilometers in thickness. In that zone, hydrogen production via radiolysis would have generated more than enough chemical energy to support microbial life, based on what's known about such communities on Earth. And that zone would have persisted for hundreds of millions of years.