Cite as: Benner, S. A. (2023) “What Do Martians Do at Night?”. Primordial Scoop, e20231110. https://doi.org/10.52400/ECUK9571
Last month, I gave a talk at the Mars Society about the Viking 1976 life detection experiments on Mars. The audience included some 6th graders. This forced me to ask something that I rarely ask: “Am I smarter than a 6th grader?”
This required also asking “Why?” questions at a 6th grade level. Questions that do not over-think things. Just the basics.
We start by noting that Viking generated three sets of reliable results, more or less duplicated at two sites on Mars. Here is how I presented these results to the students:
Result (1). The Martian surface “respires” when radioactive food is added in water
When water containing radioactive “food” was delivered to the Martian surface by Viking, radioactive 14CO2 was released. Since we teach 6th graders that science is best done quantitatively, I noted in my talk that 2 x 10-7 moles of releasable radioactive carbon was in the food, and about 15% of that was released as 14CO2 over 10 days. That is, 3.0 x 10-8 moles, 1.3 x 10-6 grams, of radioactive carbon came out of the soil.
Similar results are seen with samples from Earth. Here, this process is called “respiration”. You, as a human, respire. When you are fed radioactive food made out of radioactive 14C, you exhale, after you inhale dioxygen (O2) to serve as the bio-oxidant. For the radioactive formic acid (H14COOH), one of the seven items of “food” fed to the Martian surface by Viking, the chemical equation for respiration is:
Equation (1) 2 H14COOH + O2 –> 2 14CO2 + 2 H2O + Energy
Respiration on Mars disappeared when the sample was heated at 160 °C, and partly disappeared when the soil was heated to 50°C. You too would cease respiration if you were heated to 160 °C,
The designers of this experiment, Gil Levin and Patricia Straat, interpreted these results as indicating that the soil sample contained respiring Martian microbes. They interpreted the results after heating as steps that killed those microbes, partly at 50 °C and completely at 160 °C.
So what “hidden assumptions” stand behind these interpretations? In technical terms, biological respiration implies a “bio-catalyst”, since O2 does not react with most organic chemicals at low temperatures at any easily measured rate. On Earth, the bio-catalysts would be enzymes that catalyze a process by which the food is metabolized with O2 to yield energy at a useful rate. Without catalysis, none of the foods presented by Viking react with O2 at the rate observed by Viking. The “logic” of seeing respiration as a “biosignature” holds that only Darwinian evolution can generate bio-catalysts that release 14CO2 from 14C-labeled food at the Viking-observed rate.
So what are some 6th grader questions? Well, try this one:
Question (1). “Since the Martian atmosphere does not contain O2, what did the soil use as a bio-oxidant to generate CO2 from the Viking-fed food? How did it balance Equation (1)?
Good question. Interestingly, Gil and Patricia never quite answered this question, and answering it turned out to be a large part of my talk to the Mars Society. Let us ask the next 6th grader question that might fill the gap:
Question (2). “Does the Martian soil itself contain any bio-oxidant that might make up for the fact that the atmosphere does not contain O2?”
Here, the answer was later found to be “Yes”. The Mars Polar Lander observed perchlorate in the Martian soil. Perchlorate is the negatively charged part of a “salt”. For example, sodium perchlorate has the formula Na+ ClO4 –. We can re-write Equation (1) to use ClO4 – instead of O2 as the oxidant:
Equation (2) 4 H14COOH + ClO4– –> 4 14CO2 + 4 H2O + Cl– + Energy
Now, must perchlorate be a “bio-oxidant”, or can it simply be an oxidant? The difference relates to the question: Can perchlorate react with 14C-labeled food to form 14CO2 at the rate seen by Viking without a catalyst? The answer here is “No”. Perchlorate does not react with any of the foods presented by Viking at Viking temperatures. If perchlorate is used to balance Equation (1), it must be a bio-oxidant if it is to account for Result (1). We will discuss in the next post in this series the “in the alternative” question: “Are there non-biological catalysts that might speed the rate of the reaction described in Equation (2)?”
But we can ask another 6th grader question:
Question (3). “Could the Martian soil contain absorbed O2? Could that bio-oxidant balance Equation (1)?
The short answer is “yes”. Martian soil could contain absorbed O2. Again, that O2 must be a bio-oxidant because O2 is not sufficiently reactive to generate CO2 under Viking conditions without a catalytic system, presumably a few enzymes. Again, we reserve for the next post in this series the question: “Are there non-biological catalysts that speed the rate of the reaction described in Equation (2)?” This brings us to Result (2), also duplicated at two sites on the Martian surface.
Result (2). The Martian soil releases O2 when water is added without food.
And here we have the 6th grader Question (3). At first glance, it appears that Viking showed that the Martian soil contains O2, or something that generates O2 when treated with water without food. Let us call it “O2” in quotation marks.
Private comments by Chris McKay are inspiring me to prepare a follow-up piece to elaborate on this point. For now, let me say that our Martian friends need not store O2 as a gas in bubbles. They can store it as we do, complexed to iron. We use iron in hemoglobin, which has the 2+ or 3+ oxidation state, depending on how you place electrons. They may use something else. For now, let me simply note that “O2” cannot be an energy “sink”, so stable that respiration using it does not retrieve energy.
Again being quantitative, the amount of released O2 was comparable to the amount of 14CO2 released in the experiment with food. Per milliliter of soil, 7.7 x 10-7 moles of O2 were released at the Viking 1 site at 22° N latitude (on Chryse Planitia), 1.7 x 10-7 moles at the Viking 2 site at a higher latitude (48° N, on Utopia Planitia) and, for a sample collected from under a rock on Chryse Planitia, 7 x 10-8 moles.
That is, the soil evidently held enough O2 to account for the amount of 14CO2 released in Result (1). But not in large excess. The available released O2 was twenty times more than needed to explain Result (1) on Chryse Planitia, about six times more than needed to explain Result (1) on Utopia Planitia, and about twice what would have been needed if the same experiment had been done under a rock.
So ask the next 6th grader question:
Question (4). “Why might an organism living in the Martian soil store O2? And what is this ‘under a rock’ stuff?”
An excellent question, as it forces us to admit that we have already let slip an important 6th grader question regarding Result (1).
Question (5). “Why would an organism living in the Martian soil be prepared to oxidize the food that Gil and Patricia dumped on its head? Surely this is not part of its normal life cycle.”
A damn good question. In fact, Norm Horowitz, whom we will meet in a moment, noted that Gil and Patricia were treating the putative Martian life in a way likely to be unnatural to it.
But perhaps not entirely. The Martian soil at the Viking sites has access to dust from around the planet, through dust storms. They could contain “food” in the form of reduced carbon compounds. After all, such compounds come to Mars all the time via meteorite. This model for the Martian biosphere at the Viking sites is called “heterotrophy”.
But there is also an “autotrophy” life style available to the Martians living at the Viking site. It is based on a chemical equation that is the reverse of the equation used to analyze Result (1).
Equation (3) Energy + 2 14CO2 + 2 H2O –> 2 H14COOH + O2
Note that I need make no statement about the source of the energy that is needed to drive this reaction. It is simply a question of atoms. Or, as we chemists like to say, “stoichiometry”.
How so? Well, life requires organic molecules that contain reduced carbon atoms. These are molecules containing C-H bonds (like HCOOH) and C-C bonds (the other foods in Result (1)). If the source of carbon atoms is CO2, getting reduced carbon atoms requires the formation of O2. As a byproduct. To balance the equation.
Now, on Earth’s surface, primary producers generally use the energy from the Sun to make organic molecules out of CO2 and H2O. This process is called photosynthesis.
This is why an “under the rock” sample of Martian soil was tested. Photosynthetic Martians living under a rock would presumably have less Solar energy to create reduced carbon and, atom for atom, produce less O2 to be stored in the soil. And indeed, less O2 was released from the soil sample taken from beneath the rock.
We can construct many alternative energy sources for Martian life at the Viking sites to run the reaction in Equation (3). Let us assume, just for simpilicity, that the energy for Equation (3) comes, directly or indirectly, from the Sun. What is the next 6th grader question? Try this out:
Question (6). “We get it. During the day, Martians make reduced carbon from CO2, generating O2 as a byproduct. But what do Martians do at night?”
Well, what do Terran autotrophs do at night? The answer is simple enough: They respire. Plants on Earth at night do the opposite of photosynthesis. At night, they create the energy to stay alive by using the reverse reaction: Creduced + O2 –> CO2 + energy. Terran plants consume food they made during the day by oxidizing it with biological catalysis at night. Here, the bio-oxidant is O2.
And the next question?
Question (7) “Where do Terran plants get the bio-oxidant O2 to stay alive by respiring at night?”
The answer is equally easy: “From the Terran atmosphere, of course.”
But the 6th graders are on to us. If a photosynthetic organism on Earth can access Solar energy, then it always and at all times can access O2, day or night. Thus, Earth plants “know” that they can recover from the atmosphere the O2 molecules that they discarded in the day when they need them at night.
But this does not apply to photosynthetic life living on Mars. Think like a Martian autotroph:
• I, a Martian autotroph, use energy from the Sun in the day to make carbon according to the reaction CO2 –> Creduced + O2
• On Mars, my atmosphere has ~no O2 to serve as a bio-oxidant.
• This means that if I discard the O2 that I generate in the day into the Mars atmosphere, I have no bio-oxidant to use at night to stay alive by respiration.
• So, I must store O2 that I generate during the day to be able to respire at night, and stay alive.
Thus, Result (2), the release of O2 upon handling the soil, is exactly what we might expect from a Martian autotroph generating O2 as a byproduct of the biosynthesis of reduced carbon. We expect that Martian to store the O2 to serve as the bio-oxidant for respiration of Martian bio-organics at night. Viking results let us estimate the amount of O2 bio-oxidant stored; it is ~10-7 moles per milliliter of soil. This O2 bio-oxidant was released when Viking dumped on to the biosphere water without food. However, when water with food was dumped on to the biosphere, about 30% of the O2 bio-oxidant was used to oxidize the food. The rest of it was likely released as O2.
We can now we can build one (of several) model for the lifestyle of the Martian microbes that Viking encountered. It is coherent. At the bottom of the food chain, the microbes are autotrophs that generate reduced carbon by day, and store the O2 byproduct for respiration at night. The respiration enzyme systems are less important than the formation of reduced carbon (“fixation”) during the day, as they are in Terran plants.
But when they have food dumped on them, they respire about 15% of the available carbon in the food to create 14CO2. Perhaps they interpret this bounty as good fortune, lots of food arriving by dust storm. Perhaps the Viking induced gluttony harms them. Either way, this is a biosignature, again deferring for the next post the possibility that non0biological ways exist to both store O2 and oxidize the Viking food.
And so we can ask the next 6th grader question.
Question (8). “Can we eat Martians?”
It is interesting to compare our model for Martian microbes with the Terran microbes. As respiring life forms, we can eat Terran microbes, of course. We can do this because we have lungs that collect from our atmosphere the bio-oxidant, O2, needed for the respiration. Thus, we can do “reverse photosynthesis”, C + O2 –> CO2 + energy by breathing.
Because their atmosphere contains no O2, Martians do not have this option; they must store O2. Otherwise, they could not respire, even if Martians had lungs. Thus, Martians are life forms that contain both the reduced fuel and its oxidant.
Indeed, comparing Terran microbes to Martian microbes is like comparing jet fuel to rocket fuel. Jet fuel contains only the reduced component of the propulsion chemistry; it must get the oxidizer from the air.
Rockets cannot, of course, get any oxidant from any atmosphere. They fly in outer space. Thus, just like Martians who wish to survive the night, rocket fuel must contain both the oxidant and the reductant.
But we are not yet done. Viking delivered one last result from a third experiment, one intended to find evidence for photosynthesis on Mars. This generated Result (3).
Result (3) Radioactive 14C was “fixed” in soil samples exposed to gaseous 14CO2 and 14CO.
This radioactivity was released from the soil by heating. This was expected for life that uses atmospheric CO2/CO as a source of carbon to synthesize Creduced. Indeed, that is why this experiment was designed by Norm Horowitz. Norm argued that his life detection experiment was the best because it perturbed the Martian soil the least away from what it habitually experienced before Viking started dumping things on it.
Now, the fact that 14C was fixed in the soil could be taken as a third Result confirming the presence of Martian life with the lifestyle that we are modeling. However, fixation showed no simple dependence on the presence or absence of light. The experiment was designed to test for photosynthesis where the energy in Equation (3) comes from the Sun. Thus, it was not regarded as evidence for life, or evidence against life.
However, the Viking scientists had not gone this far in their effort to ask 6th grader questions. Therefore, they did not ask this 6th grader question:
Question (9). “If the Martian surface contained, for example, a photosynthetic autotroph that pulls CO2 and CO from the atmosphere and makes reduced carbon from it, and stores the O2 byproduct for later use, how might it manage atmospheric CO2/CO?”
Again, we must think like a Martian autotroph. Its environment is not limited by CO2 or CO. It is not limited by N2. It is not limited by energy. It is not limited by minerals.
Rather, only water and its constituent hydrogen atoms (with their electrons) are scarce in our Martian environment. Thus, we must be ready to act the instant that this scarce resource comes along. Water vapor or, more rarely, water ice or, perhaps still more rarely, liquid water.
Specifically, we never want to be in the position where we encounter water and not have CO2 (or CO) ready to promptly be reduced to create biomass (Creduced ). We Martian microbes will store CO2 (or CO).
Now, Terran biochemistry illustrates many ways to store CO2 (or CO). Many of these involve iron. A well-known example is the iron coordinated in hemoglobin, which binds CO2. It also binds CO (better) , although in human physiology, this is not desired.
Thus, to a sensible Martian microbe, Result 3 is simply the exchange of radioactive 14CO2 (or 14CO) presented to the surface by Norm for the cold CO2 (or CO) that is already bio-bound by the microbes to be ready when the scarce resource comes along.
Can alternative non-biological processes generate CO2, generate O2, and fix CO2?
As we will discuss in the next post, the Mars “community” turned after Viking to find non-biological oxidants that might generate the Results without biocatalysis. Peroxides, supervalent iron, and “oxychlorine species” such as bleach and chlorine oxide generated by cosmic rays, all have been proposed to be species that, without biocatalysis, upon addition of water, release O2, oxidize organics to generate CO2 and, somewhat paradoxically, allow the fixation of CO2, perhaps as Creduced. Some authors have proposed that these powerful oxidants are so pervasive across the Martian surface, the soil is “self-sterilizing”.
And indeed, they must be powerful oxidants if they are to explain Result (1). So powerful as to not allow organic species to survive for any length of time in the Martian soil.
So ask the 6th grader question:
Question (9). “Organic molecules have been found in the Martian soil over the past 20 years by Martian rovers. How could those organics possibly be there if the Martian soil contains oxidants so powerful as to produce Result (1)?
A very good question. But one that does not seem to have been asked by those who continue to argue that the Viking results can be explained non-biologically. Or answered.
Conclusion
All of the Viking Results are expected from our model for the lifestyle of Martian microbes in the biospheres that Viking encountered. We would expect those Martian microbes to create Creduced (biomass) by reducing CO2 (or CO). We would expect them to store the stoichiometrically necessary O2 byproduct for later respiration. We would expect them to bio-trap CO2 (or CO) to have it ready when the scarce resource (water) comes along.
In this model, Result (3) is the result of the trapping of Viking-presented radioactive 14CO2 (or 14CO) in exchange equilibrium with earlier bio-trapped CO2 (or CO). Result (2) is the release of the bio-stored O2 resulting from carbon bio-fixation from CO2. The release of radioactive 14CO2 in Result (1) is the result of respiring enzyme systems using stored bio-oxidant O2 to oxidize a dose of 14C-labeled food in water, an event that might have created a Mars-topia, or Martian gluttony.
At the October Mars Society meeting, even the 6th graders understood the coherency of this model. Indeed, they readily generated alternatives to this model, including those who got their primary energy from sources other than direct sunlight.
Interestingly, the Mars “community” has not, it seems, considered this model. For the most part, the “community” seems unaware that this model exists. Even today, the “community” seems to think that the Viking mission “proved” the absence of life at the two Viking landing sites.
Why do I say? For example, the 2022 Meadows-Graham “Standards” report, commissioned by NASA to constrain speculation about life detection, states that Viking results “support a non-biological interpretation [1]. NASA committees, including the last Decadal Survey, continue to see that missions to detect extant life on Mars should “not have a high priority” [2].
In the next installment, we will discuss why this is the case. It offers a fascinating look into NASA culture, and the failure of NASA to meet its intergenerational educational mission.
References
[1] Meadows, V., Graham, H., Abrahamsson, V., Adam, Z., Amador-French, E., Arney, G., et al. (2022). Community Report from the Biosignatures Standards of Evidence Workshop. arXiv preprint arXiv:2210.14293.
[2] Rummel, J. D., Conley, C. A. (2017) Four fallacies and an oversight: Searching for Martian life. Astrobiology, 17, 971-974.
