[/caption]
One of the most controversial and long-debated aspects of Mars exploration has been the results of the Viking landers’ life-detection experiments back in the 1970s. While the preliminary findings were consistent with the presence of bacteria (or something similar) in the soil samples, the lack of organics found by other instruments forced most scientists to conclude that the life-like responses were most likely the result of unknown chemical reactions, not life. Gilbert V. Levin, however, one of the primary scientists involved with the Viking experiments, has continued to maintain that the Viking landers did indeed find life in the Martian soil. He also now thinks that the just-launched Curiosity rover might be able to confirm this when it lands on Mars next summer.
Curiosity is not specifically a life-detection mission. Rather, it continues the search for evidence of habitability, both now and in the past. But is it possible that it could find evidence for life anyway? Levin believes it could, between its organics detection capability and its high-resolution cameras.
The major argument against the life-detection claims was the lack of organics found in the soil. How could there be life with no organic building blocks? It has since been thought that any organics were destroyed by the harsh ultraviolet radiation or other chemical compounds in the soil itself. Perchlorates could do that, and were later found in the soil by the Phoenix mission a few years ago, closer to the north pole of Mars. The experiments themselves, which included baking the soil at high heat, may have destroyed any organics present (part of the studies involved heating the soil to kill any organisms and then study the residual gases released as a result, as well as feeding nutrients to any putative organisms and analyzing the gases released from the soil). If Curiosity can find organics, either in the soil or by drilling into rocks, Levin argues, that would bolster the case for life being found in the original Viking experiments, as they were the “missing piece” to the puzzle.
So what about the cameras? Any life would have to be macro, of visible size, to be detected. Levin and his team had also found “greenish coloured patches” on some of the nearby rocks. (I still have a little booklet published by Levin at the time, “Color and Feature Changes at Mars Viking Lander Site” which describes these in more detail). When as a test, lichen-bearing rocks on Earth were viewed with the same camera system using visible and infrared spectral analysis, the results were remarkably similar to what was seen on Mars. Again, since then though, those results have been widely disputed, with most scientists thinking the patches were mineral coatings similar to others seen since then. Of course, there is also the microscopic imager, similar to that on the Spirit and Opportunity rovers, although microorganisms would still be too small to be seen directly.
Regardless, Levin feels that Curiosity just might be able to vindicate his earlier findings, stating “This is a very exciting time, something for which I have been waiting for years. At the very least, the Curiosity results may bring about my long-requested re-evaluation of the Viking LR results. The Viking LR life detection data are the only data that will ever be available from a pristine Mars. They are priceless, and should be thoroughly studied.”
As an overall reflection, the eager pattern search of Levin (colors, “pristine” environment) is not engaging towards an actual test of the prediction of the labeled release experiment.
I would imagine that the same oxidizing perchlorates that would make the pyrolytic release experiment outcome compatible with a (inconclusive) test for organics would explain the release of CO2 from added organic nutrients in early test of samples and the absence of it in later tests.
So perhaps perchlorates predicts the results, and organics (or life) would go undetected.
As an overall reflection, the eager pattern search of Levin (colors, “pristine” environment) is not engaging towards an actual test of the prediction of the labeled release experiment.
I would imagine that the same oxidizing perchlorates that would make the pyrolytic release experiment outcome compatible with a (inconclusive) test for organics would explain the release of CO2 from added organic nutrients in early test of samples and the absence of it in later tests.
So perhaps perchlorates predicts the results, and organics (or life) would go undetected.
I remember reading that the gasses released in the Viking experiment had a diurnal cycle. If the temperature in the vessel where this was performed was kept constant this strikes me as a curious observation.
LC
It looks like noisy data (fig 1 & 2) and no test for diurnality seems to have taken place. Interestingly Levin dismissed the strong oxidant hypothesis, which with Phoenix seems reasonable. (Activated by moisture foremost, ClO4 being hygroscopic, and heat.)
The temperature was allowed to vary widely (10 +/- 5 degC) and no test for diurnality seems to have taken place. Reasonably the crafts and their inner environments cycled diurnally, so these experiment planchets did and their temperature controllers did too.
Levin dismissed the strong oxidant theory because he could not explain what oxidant would have been inactivated by merely heating the active sample to the control run temperature of pre mission standard 160°C and adjusted tests of 46-52°C respectively control runs which resulted in a nil response (or reduced repsonse in the 46° control run) as was expected for the control runs in case the active run would yield positive results (which it did at both VL1 and VL2) due to microbes.
Still the question remains open: which chemical oxidant will cease its oxidative behavior after being heated to max. 160°C (without applying water or anything else to it)? Remember perchlorates (~ 0.6 wt % perchlorate have been found in the soil around Phoenix) are stable up to 400°C and shouldn’t have been inactivated by the control run temperatures of the Viking LR.
The Viking LR is really an exciting topic of research if you go deeper into it!
Levin dismissed the strong oxidant theory because he could not explain what oxidant would have been inactivated by merely heating the active sample to the control run temperature of pre mission standard 160°C and adjusted tests of 46-52°C respectively control runs which resulted in a nil response (or reduced repsonse in the 46° control run) as was expected for the control runs in case the active run would yield positive results (which it did at both VL1 and VL2) due to microbes.
Still the question remains open: which chemical oxidant will cease its oxidative behavior after being heated to max. 160°C (without applying water or anything else to it)? Remember perchlorates (~ 0.6 wt % perchlorate have been found in the soil around Phoenix) are stable up to 400°C and shouldn’t have been inactivated by the control run temperatures of the Viking LR.
The Viking LR is really an exciting topic of research if you go deeper into it!
It looks like noisy data (fig 1 & 2) and no test for diurnality seems to have taken place. Interestingly Levin dismissed the strong oxidant hypothesis, which with Phoenix seems reasonable. (Activated by moisture foremost, ClO4 being hygroscopic, and heat.)
The temperature was allowed to vary widely (10 +/- 5 degC) and no test for diurnality seems to have taken place. Reasonably the crafts and their inner environments cycled diurnally, so these experiment planchets did and their temperature controllers did too.
I remember reading that the gasses released in the Viking experiment had a diurnal cycle. If the temperature in the vessel where this was performed was kept constant this strikes me as a curious observation.
LC
Thanks Paul for that really good article about a topic which is has been mostly forgotten or ignored through all the fuzz with MSL!
One more thing to consider though: MSL also got the ability of measuring the isotopic rates of trace gases in the Martian atmosphere. And could yield interesting data about the Martian methane issue. By comparing these ratios with Earth biologic produced methane it can also establish a biogenic origin of Martian methane.
So much for MSL being an absolutely none life detection mission as was repeated by NASA over and over again on any occasion suitable 🙂
Like the Labeled Release experiment this is unfortunately not a very good life detection experiment. Here the confounds are even worse because serpentinization creates the same light isotope ratios as some of our core metabolic pathways do:
“For CH4 formed by serpentinization from crustal C (and assuming isotopic equilibrium) we predict methane to have ?13C between –31 ‰, –21 ‰ and –14 ‰ relative to crustal carbonate. If carbonates in ALH84001 and Nakhla, with ?13CPDB ~ + 15 to + 55 ‰ [Romanek et al., 1994; Jull et al., 1997], are representative of crustal carbonates, Mars CH4 produced by serpentinization should be ?13CPDB ~ – 15 to + 40 ‰, assuming isotopic equilibrium between 200 and 400 °C. Isotopically light CH4 (~ -30 to -15 ‰ vs. PDB) may suggest serpentinization and a crustal C source, a magmatic C source (~ -24 ‰ vs. PDB), or a biogenic source. Isotopically very light CH4 (<< -30 ‰ vs. PDB) may be evidence for biogenic production. Kinetic effects in serpentinization experiments at 200 °C yielded CH4 with ?13C ~ -50 ‰ relative to starting carbonate [Horita and Berndt 1999], ~ 20 ‰ lighter than expected in equilibrium, and so must be considered another possible source of light CH4. However, carbon isotope ratios of CH4 in terrestrial MOR vent fluids are not consistent with CH4 formation by serpentinization at low temperatures (~100 – 200 °C) [Kelley at al. 1996; Charlou et al. 2002]. A tighter constraint on the crustal carbonate carbon isotope ratio would greatly improve our chances of distinguishing between a crustal and magmatic C source for martian CH4.” [My bold.]
Similar light carbon effects comes from organics producing Fischer-Tropsch processes of hydrothermal vents of the kind observed, at least once active, on Mars.
There is a small window of ?13C ~ – 55 – 70 ‰ which seems difficult to predict from geological processes. This can be produced in our glycolysation IIRC. (Interestingly, the ~ 4.4 Ga bp Jack Hill diamonds tops around here.)
But besides that, the presence of methane on Mars is contested. You can dismiss both terrestrial (own methane) and satellite (weak signals, IIRC) signals.
[Now it looks like I dump on all experiments.
– I would be happy with O2 and NOx imbalances of biospheres under some temperatures, but we won’t get that on Mars I think.
– Better yet, instead of quick-and-dirty experiments looking for perhaps peculiar individuals in peculiar environments, I would like to look for population effects. Reproduction and change of traits, extant or extinct (fossils).]
Like the Labeled Release experiment this is unfortunately not a very good life detection experiment. Here the confounds are even worse because serpentinization creates the same light isotope ratios as some of our core metabolic pathways do:
“For CH4 formed by serpentinization from crustal C (and assuming isotopic equilibrium) we predict methane to have ?13C between –31 ‰, –21 ‰ and –14 ‰ relative to crustal carbonate. If carbonates in ALH84001 and Nakhla, with ?13CPDB ~ + 15 to + 55 ‰ [Romanek et al., 1994; Jull et al., 1997], are representative of crustal carbonates, Mars CH4 produced by serpentinization should be ?13CPDB ~ – 15 to + 40 ‰, assuming isotopic equilibrium between 200 and 400 °C. Isotopically light CH4 (~ -30 to -15 ‰ vs. PDB) may suggest serpentinization and a crustal C source, a magmatic C source (~ -24 ‰ vs. PDB), or a biogenic source. Isotopically very light CH4 (<< -30 ‰ vs. PDB) may be evidence for biogenic production. Kinetic effects in serpentinization experiments at 200 °C yielded CH4 with ?13C ~ -50 ‰ relative to starting carbonate [Horita and Berndt 1999], ~ 20 ‰ lighter than expected in equilibrium, and so must be considered another possible source of light CH4. However, carbon isotope ratios of CH4 in terrestrial MOR vent fluids are not consistent with CH4 formation by serpentinization at low temperatures (~100 – 200 °C) [Kelley at al. 1996; Charlou et al. 2002]. A tighter constraint on the crustal carbonate carbon isotope ratio would greatly improve our chances of distinguishing between a crustal and magmatic C source for martian CH4.” [My bold.]
Similar light carbon effects comes from organics producing Fischer-Tropsch processes of hydrothermal vents of the kind observed, at least once active, on Mars.
There is a small window of ?13C ~ – 55 – 70 ‰ which seems difficult to predict from geological processes. This can be produced in our glycolysation IIRC. (Interestingly, the ~ 4.4 Ga bp Jack Hill diamonds tops around here.)
But besides that, the presence of methane on Mars is contested. You can dismiss both terrestrial (own methane) and satellite (weak signals, IIRC) signals.
[Now it looks like I dump on all experiments.
– I would be happy with O2 and NOx imbalances of biospheres under some temperatures, but we won’t get that on Mars I think.
– Better yet, instead of quick-and-dirty experiments looking for perhaps peculiar individuals in peculiar environments, I would like to look for population effects. Reproduction and change of traits, extant or extinct (fossils).]
I agree on the difficulties of distinguishing pure geologic and biogenic methane by those isotopic ratios but research on this is ongoing and might yield a sufficient conclusion while the mission is running or even after the results are in. Actually MSL needs to get a little Martian methane in the first place which is not sure as it wont land in one of the areas with higher methane concentration (Mawrth Vallis at the northern edge of Arabia Terra would have been the better choice on this behalf too).
I disagree on the Viking LR not being a good life detection experiment after studying it in detail though. Did you know that it has been also tested on UV irradiated moon dust (from the Apollo missions)? Of course it yielded a negative result and confirmed that sample being sterile. Remember it never yielded either false positives nor false negatives in about 20 years of testing of various soils including antarctic samples. sterile Mars analog soils and of course the Mars experiments itself – why should of all things the Martian runs be the only false positive case(s)?
I agree on the difficulties of distinguishing pure geologic and biogenic methane by those isotopic ratios but research on this is ongoing and might yield a sufficient conclusion while the mission is running or even after the results are in. Actually MSL needs to get a little Martian methane in the first place which is not sure as it wont land in one of the areas with higher methane concentration (Mawrth Vallis at the northern edge of Arabia Terra would have been the better choice on this behalf too).
I disagree on the Viking LR not being a good life detection experiment after studying it in detail though. Did you know that it has been also tested on UV irradiated moon dust (from the Apollo missions)? Of course it yielded a negative result and confirmed that sample being sterile. Remember it never yielded either false positives nor false negatives in about 20 years of testing of various soils including antarctic samples. sterile Mars analog soils and of course the Mars experiments itself – why should of all things the Martian runs be the only false positive case(s)?
Thanks Paul for that really good article about a topic which is has been mostly forgotten or ignored through all the fuzz with MSL!
One more thing to consider though: MSL also got the ability of measuring the isotopic rates of trace gases in the Martian atmosphere. And could yield interesting data about the Martian methane issue. By comparing these ratios with Earth biologic produced methane it can also establish a biogenic origin of Martian methane.
So much for MSL being an absolutely none life detection mission as was repeated by NASA over and over again on any occasion suitable 🙂