Lessons Learned From Prior Investigations About Mars And Possible Life

There have been two prior detailed investigations into possible martian life. The lessons learned from these investigations have an important bearing on the search for life on another planet. In addition, recent measurements of possible methane in the martian atmosphere are important for the same reason—their potential relevance to searching for martian life.

The Viking Mission

Life detection was one of the major goals of the two Viking spacecraft that successfully landed on the martian surface in 1976. Each spacecraft carried three life-detection experiments designed to detect metabolism and, in addition, a gas chromatograph-mass spectrometer (GC-MS) to detect and identify organic compounds.41 Overall, the results from the experiments were negative. The GC-MS detected no organic matter, and the results from the biology experiment all have plausible abiotic explanations.4243

The GC-MS was, in principle, capable of detecting most organic compounds, except for highly polymerized, kerogen-like matter. Sensitivities were at the parts per billion level for compounds containing three or more carbon atoms and at the parts per million range for compounds containing one or two carbon atoms. Prior to the landings, it was thought that the soils would have detectable levels of organics from meteorite infall alone, and that photo-

FIGURE 2.4 Delta in the Crater Holden at 27°S, 326°E. A stream has cut through the south rim of the crater, just visible at the bottom of the picture, and deposited its sediment load to form a fan within the crater. The branching ridges on the delta surface are former water courses left higher than their surroundings because of greater resistance to erosion. Image from the Thermal Emission Imaging System on the Mars Odyssey spacecraft courtesy of NASA/JPL/Arizona State University.

FIGURE 2.4 Delta in the Crater Holden at 27°S, 326°E. A stream has cut through the south rim of the crater, just visible at the bottom of the picture, and deposited its sediment load to form a fan within the crater. The branching ridges on the delta surface are former water courses left higher than their surroundings because of greater resistance to erosion. Image from the Thermal Emission Imaging System on the Mars Odyssey spacecraft courtesy of NASA/JPL/Arizona State University.

Mangala Vallis

FIGURE 2.5 Mangala Vallis. The source of the outflow channel Mangala Vallis at 18°S, 210°E. The channel starts at a 7-km gap in a graben wall (bottom center) and then extends hundreds of kilometers northward. Faulting appears to have triggered massive release of groundwater. Image from the Thermal Emission Imaging System on the Mars Odyssey spacecraft courtesy of NASA/JPL/Arizona State University.

FIGURE 2.5 Mangala Vallis. The source of the outflow channel Mangala Vallis at 18°S, 210°E. The channel starts at a 7-km gap in a graben wall (bottom center) and then extends hundreds of kilometers northward. Faulting appears to have triggered massive release of groundwater. Image from the Thermal Emission Imaging System on the Mars Odyssey spacecraft courtesy of NASA/JPL/Arizona State University.

FIGURE 2.6 Ice-rich debris flows in the fretted terrain at 40°N, 25°E. At 30° to 50° latitudes in both hemispheres, material shed from slopes commonly shows indications of having flowed like glaciers. Here, what is probably an ice-rich debris flow has been deflected through a gap in an obstructing ridge. Image from the Thermal Emission Imaging System on the Mars Odyssey spacecraft courtesy of NASA/JPL/Arizona State University.

FIGURE 2.7 Gullies on the wall of Nirgal Vallis at 30°S, 321°E. Their origin is still being debated, but they formed recently and liquid water may have been involved in their formation. Image from the Mars Orbiter Camera on the Mars Global Surveyor spacecraft courtesy of NASA/JPL/ Malin Space Science Systems.

FIGURE 2.8 Ceraunius Tholus. Some volcanos, such as this, are densely dissected, possibly the result of melting of surface or subsurface ice by volcanic heat. Formation of the valleys may have been accompanied by hydrothermal activity. Image from the Thermal Emission Imaging System on the Mars Odyssey spacecraft courtesy of NASA/JPL/Arizona State University.

FIGURE 2.8 Ceraunius Tholus. Some volcanos, such as this, are densely dissected, possibly the result of melting of surface or subsurface ice by volcanic heat. Formation of the valleys may have been accompanied by hydrothermal activity. Image from the Thermal Emission Imaging System on the Mars Odyssey spacecraft courtesy of NASA/JPL/Arizona State University.

chemical fixation of CO into organics would be an additional contributor. Failure to detect organics implied that they are destroyed at the martian surface. One possible cause of the absence of organics is that they are destroyed by UV-stimulated reactions with metal oxides in the soils.44 Another possibility is that they are destroyed by reactions with labile oxidants, since the biology experiments showed that in the presence of water, the soil releases tens to hundreds of nanomoles of O2 per cubic centimeter.45 The oxidants may be produced by photodissociation of water in the atmosphere. Another possibility is that organics were present but that oxidants in the soil destroyed them during sample processing by the GC-MS.46

In the pyrolytic-release experiment, isotopically labeled CO and CO2 were added to the soil.47 After a suitable incubation time, the gases were flushed out, and the soil was heated to see if any of the labeled carbon had been incorporated into less volatile species. Small amounts of carbon were fixed into organics, which the experimenters attributed to inorganic synthesis catalyzed by the martian soil. In a second gas-exchange experiment, the soil was humidified and nutrients added.48 The resulting rapid release of oxygen was attributed to the presence of oxidants in the soil. In the third experiment, simple organic compounds labeled with radioactive tracers were added to the sample.49 This resulted in rapid release of labeled gas followed by slow release. Subsequent experiments show that addition of the nutrients to soil containing Fe2O3 and H2O2 simulated the Viking results. The consensus is that all the results from the biology experiments have plausible inorganic explanations. A few researchers, however, maintain a contrary view.50

In retrospect, the Viking mission could be criticized as reaching too far too soon. In the late 1960s when the mission was conceived, knowledge of conditions on the martian surface was rudimentary. Speculations on the prospects for life were based largely on telescopic observations. Little information was available on the surface conditions and on where best to land in order to look for life. It could also be argued that the experiments were narrowly designed to detect a limited spectrum of terran life. Despite the negative results from the life-detection investigations, the Viking mission returned invaluable information for future biological experiments, such as data on the oxidizing nature of the surface and the possible scarcity of organics. In hindsight, the Viking missions constitute a compelling argument in favor of the kind of systematic approach advocated in this report and in NASA's 1995 report An Exobiological Strategy for Mars Exploration.51

The Search for Life on Early Earth and in the Martian Meteorite ALH 84001

The lack of a conclusive set of criteria for life detection and preservation has been illustrated recently by two debates: the search for the oldest evidence of life on Earth and the raging debate on the claims for life in ALH 84001. The scientific controversies over the former debate, that of the earliest evidence of life on Earth, have recently intensified but are still unresolved.52-60 The common denominator in both of these debates is the underlying difficulty, or inability, to demonstrate conclusively the biological origin of the respective evidence.

The Earliest Life on Earth

Various claims and counterclaims have been published in the scientific literature in recent years concerning the earliest evidence for life on Earth.6162 The conflicting results of these studies illustrate the potential pitfalls in collecting and interpreting data from the ancient geological record.

Our planet's earliest known microfossils have been ascribed to 3.5-billion-year-old cyanobacteria identified by Schopf in samples of the Apex chert of Western Australia.63 The morphological identification of these tiny dark clumps was always controversial, given that the range of bacterial morphologies at their simplest are ambiguous to interpret; bacteria have little morphology to begin with. Furthermore, the earliest evidence of biomarkers specific to cyanobacteria is 2.7 billion years ago, roughly consistent with molecular clock estimations of the emergence of cyanobacteria.64 Such organisms would have produced photosynthetic oxygen, which does not appear as a significant atmospheric component until much later.65 66

The identification of these "microfossils" has been called into question by Brasier at al.,67 based on reexamination of the chert sections of the original study. Many of the "microfossils"were observed to have branched morphologies inconsistent with filamentous bacteria. Schopf countered that the specimens are not branched, but folded by later deformations. In revisiting the collection site, Brasier et al. also determined that the Apex chert itself was not a sedimentary deposit, but instead was a vein formed by hydrothermal activity. Brasier et al. argued that the "microfossils" are merely bits of carbonaceous matter, unrelated to life, and squeezed out of forming quartz crystals and wrapped around them to resemble microfossils. To date no studies have satisfactorily determined the abiogenic or biogenic nature of the carbon forming the "pseudofossils."

To support his original claim, Schopf teamed with other scientists utilizing laser-Raman spectra to determine that carbon is present within the "microfossils."68 However, other experts in this technique have criticized this work,69 noting that there is nothing diagnostic in the spectra that indicates that the analyzed carbon-bearing clumps are the remains of organisms rather than abiotic organic matter.

The isotopic composition of carbon has been used as a biomarker, because photosynthetic organisms pref erentially incorporate the lighter isotopes. Tiny bits of carbon (now graphite) in a 3.85-billion-year-old gneiss in Greenland have been determined to be depleted in i3c.7071 The authors of this study suggested that the host rock was a sedimentary, banded iron formation. They hypothesized that biogenic matter collected at the bottom of the ocean and was incorporated into sediments; later metamorphism transformed the organic matter into graphite, but its carbon isotopic composition was preserved. They interpreted this finding as strong evidence for life, some 400 million years earlier than previously thought. In mapping the outcrop from which the samples were collected, other scientists have found that it is not a banded iron formation, but instead represents a volcanic rock into which metamorphic fluids were injected.72 These fluids precipitated quartz to form the observed banding. This appears to be a highly unlikely site for the preservation of organisms. Moreover, it has now been shown that isotopically light hydrocarbons can be produced via several abiotic pathways.73

Although argument continues on both these controversies, most scientists appear to have sided with the skeptics. Several lessons can be drawn from these controversies about Earth's oldest life. Morphology alone is not a sufficient criterion for the identification of simple life forms. Understanding of the geological context of the sample is of prime importance, because it provides information on the environment in which the putative organism lived or was preserved. And, finally, the interpretation of geochemical analyses of extremely small samples is fraught with difficulty and sometimes ambiguity. In short, it is not enough to show that some chemical property is consistent with life, but it must also be inconsistent with abiotic formation.

Was this article helpful?

0 0

Post a comment