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I SUMMARY OF VIKING FINDINGS RELEVANT TO MARTIAN BIOLOGY The results of several of the Viking experiments are relevant to questions of current or past life on Mars. We summarize in this section information that seems especially pertinent. A. Elements The discovery of nitrogen in the atmosphere has eliminated a major barrier to the postulation of the existence of Martian biota. There is general agreement that the absence of nitrogen would have constituted a definitive global nega- tive for current life. Calcium, sulfur, magnesium, chlorine, and probably potas- sium and phosphorus have also been detected in soil* samples.3 All six (and especially phosphorus) are likely to be essential to living systems. B. Water (General) 1. Major geographic, topographic, and diurnal variations in the concentra- tions of atmospheric water vapor have been observed, reaching values of 75 precipitable micrometers or higher near the North Pole. The relative humidity at the surface is unknown except in the North Polar region where orbital ob- servations and calculations indicate saturation.4 2. The residual North Polar cap has been shown to be water ice, probably 1 to 1000 m or more thick.4'5 3. Several experiments confirm or strengthen the inference that large amounts of water are locked beneath the surface in the form of a permafrost. Analysis of the composition of the atmosphere suggests that the total volatile inventory on Mars may be much larger than the content of the present at- mosphere. Two independent lines of argument have been proposed.6 One follows from the observed enrichment relative to terrestrial and solar abun- dances of 15 N to 14N. If this enrichment has resulted chiefly from preferen- tial escape of the lighter isotope from the upper atmosphere, it indicates that *Because the word "soil" implies the presence of organic compounds and connotes a material that will support terrestrial plant life, we will chiefly use the more neutral term "regolith," which is defined as unconsolidated planetary surface material, i.e., rocky rubble.

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Mars must have had at least 10 times the present abundance of N2. This same escape process should also have led to an enrichment of 18O, which is not observed. To prevent the enrichment from occurring, one needs a large reser- voir of oxygen, and the most likely reservoir would be water in amounts equivalent to at least 2 bars of vapor. Similar estimates of the size of the water reservoir are arrived at if one assumes that water and nitrogen out- gassed simultaneously. A second proposed line of argument6 follows from the detection of 36Ar, Kr, and Xe in the Martian atmosphere. These gases demonstrate that Mars has outgassed by a factor 100 times less than on Earth. Comparison of the ratios of 36Ar/N2/CO2/H2O on Earth and Mars indicates that the present Martian atmosphere is deficient in N2, CO2, and H2O. Obtaining a match would re- quire that the planet once had at least 10 times more CO2 and N2 than is now seen in the atmosphere. Both lines of evidence suggest Mars once had water equivalent to a layer of 20-30 m over the entire planet. Amounts equivalent up to perhaps 2 m of global coverage are locked up in the residual polar caps; amounts equivalent to perhaps 2 m have probably photolyzed and escaped from the planet. The rest must be buried in the regolith.6 4. The diurnal behavior of water vapor in the atmosphere indicates that most of it is located near the surface and that at least 80 percent of the vapor returns to the solid phase between noon and the following dawn. The rate of reappearance of the vapor at dawn is sufficiently slow to require that this solid phase be beneath the surface.7 The residual North Polar cap appears to represent a region where the permafrost "breaks through" to the surface.4 5. The gas chromatograph-mass spectrometer (GCMS) on Viking has de- tected 0.1 to 1.0 percent (w/w) water in regolith samples heated to 350°C or 500°C, but with one exception much less than 0.1 percent water in samples heated to 200°C.8 The results are consistent with the water being that in mineral hydrates of moderate thermal stability (perhaps hydrates of MgSO43). The one exception is the sample collected by the second lander (VL-2) from beneath a rock. It yielded no water when heated to 50°C but several tenths of a percent when heated to 200°C. It could represent tightly adsorbed water or mineral hydrate water of low thermal stability. C. Water (Liquid) 1. Past liquid water. Evidence from orbital photographs is powerful that massive quantities of liquid water once existed and flowed on the Martian surface. Flowing liquid water means the existence of sediments. However, estimates from crater counts suggest that the channels were formed a billion or more years ago.9 If so, the relevance of the fluvial areas to the existence

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of current life is dubious, although their relevance to the possible existence of past life and to organic chemical evolution may be profound. The topo- graphical evidence for surface liquid water in the past is consistent with the conclusions derived from analyses of isotopic ratios of atmospheric nitrogen and argon. These analyses indicate that the total surface pressures on Mars could easily have been high enough to have permitted the existence of surface liquid water. 2. Present liquid water. Liquid water is generally agreed to be essential for the functioning of living forms. But, unfortunately, Viking was not designed to detect free liquid water and has not done so. Liquid water adsorbed to the soil ought to have been detectable in the 200°C pyrolysis in the GCMS, but less than 0.1 percent was detected (several tenths percent in the subrock sample), and that could represent mineral hydrate water of moderate or low thermal stability.8 The detailed measurements of surface temperatures and atmospheric pres- sures continue to preclude the existence of pure bulk water under equilibrium conditions. The three possibilities for liquid water proposed prior to Viking still remain remote possibilities: (1) liquid water adsorbed to subsoil; (2) water that is liquid by virtue of kinetic factors slowing the approach to equilibrium (i.e., conditions under which diffusion of water is slower than diffusion of heat10); (3) water that has its chemical potential (and hence freezing point) lowered by the presence of dissolved solutes. Possibility (3) has been enhanced by the X-ray fluorescence detection of elements like Ca, Mg, Cl, and S that could give rise to water-soluble ions.3 (In fact the existence of MgSO4 at the two landing sites is now considered likely.) Salts like CaCl2, MgCl2, and K2CO3 have eutectic points below -30°C, and their presence would permit stable liquid water down to these temperatures. The electro- lyte concentrations, however, would be multimolar. Even though data from the Viking landers have neither lessened nor espe- cially enhanced the possibility of stable or metastable liquid water in the regolith, the surface and subsurface temperatures that have been estimated from orbital infrared measurements, in conjunction with the low atmospheric pressures, continue to make that likelihood remote. Summer surface tem- peratures at the landing sites vary diurnally between -88°C and -3°C. Some 24 cm below the surface the summer temperatures are expected to be a steadier -51°C to -56°C. In the winter, the surface temperatures at VL-1 are expected to vary diurnally between -95°C and -22°C, and those at VL-2 be- tween -124°C and -82°C. Some 24 cm below the surface the winter tempera- tures at VL-1 and VL-2 will be -69°C and -108°C, respectively.11 Mechanisms for providing liquid water below -50°C become increasingly limited; no mechanisms are known to provide liquid water below about -70°C. To give some feeling as to the harshness of these temperatures to terrestrial biota, the

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lowest confirmed minimum growth temperature for Earth organisms is -15°C, but very few can grow below 0°C. Interlamellar layers of water adsorbed on soils remain unfrozen to at least -30°C,12 but the very forces that keep the water unfrozen make it a difficult source for organisms. D. Reduced Carbon and Organics 1. With the possible exception of data from the pyrolytic release experi- ment, there continues to be no evidence for the existence of carbon reduced below the state of CO and no clear evidence of any form of carbon save in the atmosphere and in the winter polar caps. 2. Organic compounds. No organic compounds, other than traces attribu- table to terrestrial contaminants, have been detected in regolith samples ana- lyzed by the GCMS. If volatizable organic compounds were present in the samples, they were either present in concentrations below the parts per billion range (the detection limit of the instrument) or they were totally restricted to substances like methane with molecular weights of less than 18, which are undetectable or detectable only at reduced sensitivities. (A third possibility, the complete oxidation of organics during heating in the sample chambers, is considered by the molecular analysis team to be very unlikely.8 One argument presented is that known terrestrial organic contaminants, methyl chloride, acetone, toluene, and benzene, were detected in expected amounts during the experimental runs on the Martian samples.) Instruments with the same characteristics as the flight instrument have invariably detected organic compounds in all terrestrial soil samples tested, including antarctic soils with few living organisms. The concentrations of organics, if present in Martian samples, appear less than the concentration of organics in lunar samples.13 The concentrations are less than those expected from the influx of carbonaceous chondrites (assuming the regolith is mixed to a depth of 100 m or less8). This latter conclusion, combined with two lines of evidence for the existence of strong oxidants in the regolith, leads to the view, that in at least the top few centimeters of the surface, carbon—carbon bonds are dis- rupted faster than they are deposited or synthesized. The first line of evi- dence comes from orbital measurements of the atmosphere and modeling. One model predicts the existence of active strongly oxidizing species, espe- cially hydrogen peroxide.14 Second, the gas exchange experiment (GEX) on the Viking landers showed the release of up to nearly a micromole of oxygen when the ~l-cc soil samples were humidified with water and warmed to ~10°C15'15a (see below). The inability to detect organic compounds in the regolith samples does not itself exclude the possible existence of a microbial population. Fewer than 10s to 106 representative terrestrial bacterial cells would not contain suffi- cient organics to be detectable with the GCMS.S However, postulating the 6

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existence of a viable microbial population under such conditions requires speculative scenarios. On Earth the great bulk of the organic compounds in soils represents the transformed remnants and metabolic products of the organisms and not the organic content of the living organisms themselves. Thus, the ratio of organic carbon in living microorganisms to that in humus is about 1 percent, and the ratio of organic carbon in living microorganisms to the total organic and elemental carbon in oil, gas, coal, oil shale, humus, and in the oceans is estimated to be 0.0001 percent to 0.001 percent.2''6 If Martian surface samples in fact contain living microbes, one must assume that mechanisms exist which permit their existence, while at the same time preventing the buildup of their organic detritus to levels detectable in the GCMS. There are possibilities such as (a) the transport of living organisms from other, more hospitable areas at a rate sufficient to balance the destruc- tive processes, and thereby provide a steady-state population of viable cells; (b) efficient recycling of organic detritus by the microbial population; or (c) the organisms possess biologically driven devices or mechanisms to protect their organic matter, and these devices and mechanisms disappear upon their death. The best that can be said for items (a) and (c) is that there are partial Earth analogues. However, on Earth we know that hospitable areas exist; and on Earth samples from even harsh environments contain detectable organic compounds, and they possess ratios of total organics to the organics in living cells that far exceed unity. Possibility (b) is not especially helpful unless the recycling efficiencies approach 100 percent. E. Biology Experiments The biology package in each Viking lander contains three separate experi- ments: gas exchange (GEX), labeled release (LR), and pyrolytic release (PR). The first two provide the Martian samples with water vapor at high activity or with liquid water containing organic substrates commonly used by ter- restrial microorganisms. The third provides only two gases known to be con- stituents of the Martian atmosphere (CO and CO2), light (optional), and small amounts of water vapor (optional).* In all three experiments, the samples have so far been incubated at 8°C to 26°C. The significant measurements are the quantity of gas(es) evolved (GEX, LR, and PR), the type of gas (GEX), or *In the GEX experiment, the amount of aqueous nutrient medium (0.6 cm3) added initially to the bottom of the test cell is such that the regolith sample is contacted by water vapor only and not by the liquid medium. Subsequent additions of medium actually wet the sample. In the LR experiment, 0.115 ml of liquid medium is added initially to 0.5 cm of sample, and the liquid contacts only the central core of the sample. Subsequent additions of medium wet the entire sample. The PR experiment is run either without the addition of any water or with the addition of about 80 jug of water vapor to 0.25 cm of soil in the 4 cm test cell. 7

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the kinetics of its evolution (GEX and LR). In PR the samples are heated in such a way after incubation and the volatiles passed through a trap of such characteristics that the detection of 14CO2 in the so-called "second peak" is presumptive evidence for the synthesis of organic compounds during the incubation of the sample. The experiment is designed to test the samples "for the presence of microorganisms by measuring the incorporation of radioactive COa and CO into the organic fraction of a soil sample."15 In GEX and LR the evolution of gases indicates that reactants in the sam- ples or added reactants have undergone chemical reactions. The assumption is that microbial activity could be diagnosed from the amounts, or types, of evolved gases and from the kinetics of their appearance. All three experiments have yielded signals that clearly indicate chemical activity.15 What is less clear is the interpretation of the signals. Some aspects of the data are consistent with those expected from biological activity com- parable to that observed on Earth, but other aspects are inconsistent. In the LR experiment, the production of 14CO2 when regolith samples were initially moistened with a nutrient medium is consistent with biological activity.17 So also is the synthesis in the PR experiment of picomole quantities of organic matter during the 120-h incubation of samples in the light.15 (However, al- though statistically significant, the amount synthesized in the PR experiment is only about one-tenth that synthesized by terrestrial soils that give the minimal observed response, i.e., antarctic soils.18'19) In both the LR and PR experiments, the activity was abolished or appreciably reduced when the regolith samples were preheated to 170°C to 180°C for 3 h. Heating to such temperatures abolishes biological activity in terrestrial samples. A major difficulty with a biological interpretation of the data is the re- sponse of samples to water. Rather than enhancing the signal as expected for biological activity, the addition of water vapor in the PR experiment totally prevented the reaction from occurring.18* In the LR experiment, the initial "This conclusion has now been contradicted by the results of the final PR experiment, results received after completion of this report. The last three runs (C-4, C-5, and C-) were performed on aliquots of a sample collected by the VL-1 lander and stored at 5°C to 24°C for 0, 71, and 143 (Earth) days, respectively. All three gave statistically identical values for the "second peak," in spite of wide differences in their thermal and water treat- ment. Sample C-4 was run dry at 16°C in the usual fashion; sample C-5 received water vapor, and was then vented, heated to 90°C for 112 min, and finally incubated at 17°C; sample C-6 received water vapor, and was then incubated at 15°C. The response of this last sam- ple differed dramatically from the results of the previous sample (U-2) run in the pres- ence of water, a run that had given a second peak of 0. The discrepancy remains unex- plained. Horowitz, Hobby, and Hubbard (personal communication, 1977) conclude, however, that "a biological interpretation of the [PR] results is unlikely in view of the thermostability of the reaction." Even after a sample was heated to 175°C for 3 h, it yielded a second peak count that was 12 to 60 percent of that in unheated samples. 8

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addition of medium, which wet only a portion of the sample (footnote, page 7), appeared to exhaust the reactants in the entire sample.15'17 Fur- ther inconsistencies emerge from the results of the GEX . It showed the un- expected release of as much as 0.7 ;umoles of molecular oxygen, indicating the presence of strong oxidants in the samples (probably peroxides or super- oxides—see below). Here too the initial introduction of water vapor exhausted the reactants that were the source of the oxygen, i.e., further additions of liquid aqueous medium produced no further evolution of oxygen.15'15a Intensive attempts are now being made to simulate the results of the Viking biology experiments abiologically. Although the information is pre- liminary, major features of the LR and GEX experiments have been mimicked at least qualitatively by nonbiological reactions. The major feature of the LR experimental results is the decarboxylation of the organic substrates to yield CO2. A number of strong oxidants like hydrogen peroxide and metal per- oxides and superoxides are known to drive that reaction.193'20 Carbon di- oxide is also evolved when either formate or the nutrient mixture used in the LR experiment is subjected to intense uv radiation in the presence of ferric oxide.20 The major feature of the GEX experimental results is the release of oxygen when the Martian samples are humidified. Once again, a number of metal peroxides and superoxides evolve oxygen when placed in contact with water.20'20* Wydeven and his colleagues203 have also obtained oxygen evolu- tion in amounts and at a rate comparable to that observed in GEX on Mars by exposing soil samples in a GEX experiment on Earth to a gas mixture obtained by passage of oxygen through a radio frequency glow discharge. The RF treat- ment produces active species of oxygen similar to those expected to be gen- erated at the Martian surface by solar uv radiation. The latter process has been modeled in some detail.14 Splitting of H2O gives H and OH, which leads by well-characterized pathways to the production of H2 O2. To date, the results of the PR experiment have not been simulated abio- logically, and possible abiological explanations are speculative. It seems clear that picomole quantities of organic compounds were synthesized in several of the PR experiments. The specific observations were that after the Martian samples (no H2O added) were irradiated at > 320 nm (0.5 percent <320) in the presence of 14CO2 and 14CO, and then pyrolyzed at 625°C, significant quantities of 14C-containing material were retained on the organic vapor trap (OVT) at 120°C. Heating of the OVT to 650°C released this material and oxidized it to 14CO2 where it was detected as the "2nd" peak. Experiments with terrestrial soils have shown: (1) the ability of the OVT to retain organics other than gaseous forms like methane or ethane; (2) high efficiency of the OVT in passing through unreacted 14CO2 and 14CO (less than 0.01 percent is retained); (3) the PR experiment yields positive results in all terrestrial soils

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shown to contain viable cells (provided that water is present); and (4) it yields negative results in sterilized soils.19'21 It is known, however, from the work of Hubbard et a/.21 that nonbiologi- cal organic synthesis can occur under conditions analogous in several respects to those that prevailed in the PR experiment. They have found that, in the presence of solids of high surface area, formic acid and other organic com- pounds are synthesized when CO, CO2, and small amounts of H2O vapor are irradiated at wavelengths of 250-280 nm. Very little abiogenic synthesis occurs at longer wavelengths, and it was for this reason that only wavelengths longer than 320 nm were allowed to reach the samples in the Viking PR ex- periment. But perhaps on Mars, these longer wavelengths can drive the reac- tion. For instance, as mentioned, hydrogen peroxide is likely to be present in the Martian regolith.14 Hydrogen peroxide dissociates into hydroxyl radi- cals when irradiated with even visible light (quantum yield 0.3 at 313 nm and measurable reactivity at 365 nm23). And hydroxyl radicals have been implicated22 in the abiogenic syntheses observed by Hubbard etal. Possibly then the reaction occurring in the PR experiment on Mars is some- thing akin to H2O2 + COh> formic acid. (This illustrative reaction is at least thermodynamically feasible, for it has a free energy of -20 Kcal/mol.24) Reacting along this pathway, however, would require special conditions, for the oxidative pathway, which yields CO2 and H2O, has a free energy of -90 Kcal/mol.24 We are not proposing that this is necessarily the actual reaction that oc- curred in the PR experiment. We cite it to illustrate two points. One point is that abiogenic explanations of the PR results are conceivable. The second point (applicable to LR and GEX as well) is to emphasize that conditions at the regolith-atmosphere interface on Mars are vastly different from those at the soil-atmosphere interface on Earth. This vast and incompletely charac- terized difference makes it inordinately difficult to conclude that experi- mental results, which are unambiguously ascribable to biological activity on Earth, are unambiguously ascribable to biological activity on Mars. The con- verse is also true. The incompletely characterized differences will make it difficult to determine rigorously whether the Viking biology results have been generated abiologically. The ambiguities are likely to remain despite continued experimentation on the Viking landers and despite further efforts at terrestrial simulation. Nevertheless, we consider further efforts on simula- tion to be vital, for they will permit the ambiguities to be reduced to a minimum. Despite current (and possibly future) inability to reach rigorous interpre- tations of the results of the Viking biology experiments, our Committee is charged with recommending strategy for the future biological exploration of Mars. Our judgment is that the evidence at hand is sufficiently persuasive 10

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to require that that strategy be predicated on the assumption that the positive signals from Mars are not biological in origin. That judgment rests chiefly on six points: (1) the evidence from GEX for the presence of strong oxidants; (2) the inhibitory or dissipative effects of the presence of added water (but see footnote, p. 8); (3) the lack of detected organic compounds; (4) the ability to account, at least qualitatively, for the results of GEX and LR by nonbiological reactions; (5) the prior demonstration that abiological organic synthesis can oc- cur under conditions analogous to those in the p R experiment except for the wavelength of the incident radiation; and (6) the existence of at least con- ceivable mechanisms for a different wavelength dependence at the Martian surface. We wish to emphasize that we cannot draw conclusions as to whether life currently does or does not exist on Mars. Although increasingly unlikely, the positive signals from one or more of the experiments could be biological in origin; a second possibility (also remote in our view) is that, although the signals are abiogenic, life in fact exists at the landing sites but was undetected. A third possibility is that, although the samples are lifeless, life exists else- where on the planet's surface or beneath its surface. A fourth possibility is that life evolved but no longer exists. Finally, the fifth possibility is that it never evolved. As mentioned in the introduction, the last three possibilities are all questions of fundamental biological importance, and it is they that form the basis of the ensuing discussion of post-Viking strategies. 11