- Press Release
- Sep 26, 2022
An Exobiological Strategy for Mars Exploration (Part 1)
Prepared by the Exobiology Program Office, NASA HQ
This strategy was formulated at the request of Dr. Michael A. Meyer, Discipline Scientist for the Exobiology Program, NASA Headquarters. The writing group consisted of:
- Dr. Michael H. Carr, USGS
Dr. Benton Clark, Martin-Marietta Aerospace
Dr. David J. DesMarais, NASA Ames Research Center
Dr. Donald L. DeVincenzi, NASA Ames Research Center
Dr. Jack D. Farmer, NASA Ames Research Center
Dr. John M. Hayes, Indiana University
Dr. Heinrich Holland, Harvard University
Dr. Bruce Jakosky, University of Colorado
Dr. Gerald F. Joyce, Scripps Research Institute
Dr. John F. Kerridge, University of California, San Diego & NASA HQ (chair)
Dr. Harold P. Klein, Santa Clara University
Dr. Andrew H. Knoll, Harvard University
Dr. Gene D. McDonald, Cornell University
Dr. Christopher P. McKay, NASA Ames Research Center
Dr. Michael A. Meyer, NASA HQ
Dr. Kenneth H. Nealson, University of Wisconsin
Dr. Everett L. Shock, Washington University
Dr. David M. Ward, Montana State University
In addition, Dr. Carl Sagan, Cornell University, provided helpful advice.
- EXECUTIVE SUMMARY
- Purpose of strategy document
Outline of approach
- THE PRESENT STATE OF KNOWLEDGE
- The geological history of Mars
Evolution of the martian atmosphere
Results of Viking biology experiments
- SEARCH FOR EVIDENCE OF PREBIOTIC CHEMISTRY
The geochemical context for prebiotic chemistry
Objectives for flight experiments
- THE SEARCH FOR EXTINCT LIFE
Experience with Earth’s early fossil record: Relevance for Mars
Key sites of Mars
Objectives for flight experiments
- THE SEARCH FOR EXTANT LIFE
Critical review of Viking biology experiments
Possible habitats for extant life
Objectives for flight experiments
- OBSERVATIONS/MEASUREMENTS REQUIRED FOR EXOBIOLOGY
- Orbital experiments
- PLANNED MISSIONS: ANTICIPATED MEASUREMENTS AND OBSERVATIONS
- The 1996 opportunity
The 1998 opportunity
The 2001 opportunity
The 2003 opportunity
- NECESSARY MEASUREMENTS/OBSERVATIONS MISSING FROM CURRENT PLANS
- SITE SELECTION
- SITE SELECTION
High-resolution orbital imaging
Duration of hydrological systems
High-priority sites for extinct martian life
High-priority sites for extant life
- PLANETARY PROTECTION FOR MARS MISSIONS
- Support of basic R & A
Mobility and sample acquisition
- ADDITIONAL READING
Besides being of immense popular interest, the possibility of life on Mars, either now or in the past, is also a scientific issue of profound importance. This stems from the fact that, although theoretical considerations suggest that prebiotic chemical evolution could commonly lead to the origin of replicating life, we still know of only one planet on which life has emerged. Consequently, the conditions necessary and sufficient for life to originate are still very poorly constrained. The geologic record suggests that the environments of Mars and Earth were quite similar prior to about 3.5 billion years (Gyr) ago, when life was emerging on Earth. In particular, there is abundant evidence for liquid water, in the form of rivers, lakes and possibly even larger bodies of water, on the martian surface at that time. Because liquid water is essential for all known biology, the environment on early Mars may well have been favorable for the emergence of life. Consequently, a determination of how far Mars proceeded along the path towards life would be of fundamental significance, by greatly improving our definition of the “window of opportunity” within which life could originate. It is important to note that this remains true whether or not evidence is found for present or former life on Mars.
From these considerations, it follows that we can divide the scientific issues involved in the exobiological exploration of Mars into three general categories: (1) To what extent did prebiotic chemical evolution proceed on Mars? (2) If chemical evolution occurred, did it lead to synthesis of replicating molecules, i.e., life, which subsequently became extinct? (3) If replicating systems arose on Mars, do they persist anywhere on Mars today? Although these three lines of inquiry frequently involve quite different analytical approaches, particularly in the search for extant life, the broad mission requirements of all three are quite similar, leading to a logical sequence of five explorational phases capable of accomplishing all presently defined exobiological objectives.
The first explorational phase consists of global reconnaissance. In this phase, all three elements of the strategy focus on the role of water, past or present, and on the identification of potentially fruitful sites for landed missions. Thus, global information on the distribution of water (either solid, liquid, chemically combined or physically adsorbed), global mapping of pertinent mineralogical/lithological regimes, thermal mapping, and high-resolution imaging of the martian surface are requisites for this phase of the strategy. For example, the orbital component of a search for extinct life would include the attempted detection of aqueous mineral deposits, by means of the signatures both of near-surface water, in this case chemically bound, and of characteristic mineralogies. Such sites, if found, would also be key targets for evidence of possible prebiotic chemical evolution.
Phase two of the strategy involves landed missions providing in situ descriptions of promising sites identified during phase 1. All aspects of the strategy converge on the need for broad-based geochemical and mineralogical characterization, culminating in elemental, molecular and isotopic analysis of the biogenic elements in a variety of microenvironments at specific sites, including analysis of volatile species. Of particular importance early in this phase is elucidating the extent to which the presence of Mars surface oxidant(s) influences the distribution of organic matter, either living or nonliving. Another key target of this phase would be assessment of the needs for future in situ missions which would deploy critical experiments focused on specific questions within the three categories described above.
Phase three consists of deployment of such exobiologically focused experiments. In the case of chemical evolution, the goal would be a detailed characterization of any population of organic compounds on Mars. For the issue of extinct life, the task would be a search for biomarkers and for morphological evidence of formerly living organisms. Similar approaches would be involved in the search for extant life; in the event that extant life seemed plausible, experiments to test for metabolism in living systems, similar to those of Viking, but based on a knowledge of conditions and resources at specific sites, would also be needed. Phases two and three are considered together in what follows, under the heading of “landed science”.
The fourth phase, involving robotic return of martian samples to Earth, would greatly improve characterization of the organic inventory at specific martian locations, and furthermore would be essential for verification of any in situ evidence for extinct or extant life obtained in phase three.
Finally, the fifth phase would involve human missions and would lead to establishment of a detailed geological context for any exobiologically significant observations made previously. Also, human presence would aid in the detection of “oases” capable of promoting or supporting life that may have been missed during robotic exploration.
Although the later explorational phases tend to fall outside of the current planning horizon, missions planned for the 1996 and 1998 launch opportunities make a promising start towards implementing the earlier explorational phases outlined above. However, we have identified certain areas in which we make the following recommendations, based on the requirements of explorational exobiology:
First, we believe that it is essential that a gamma-ray/neutron spectrometer be flown on the 1998 Mars Surveyor mission, in order to provide information on the global distribution of near-surface water.
Second, we recommend development aimed at improving the spatial resolution of orbital mid-IR spectrometry to the point where it would be capable of detecting small-scale surface mineral deposits, such as those characteristic of individual hydrothermal vents or springs. Near-term deployment of such an instrument would be required because of the important role it would play in site selection for subsequent missions.
Third, we recommend immediate development of a mineral-identification capability for the earliest landed missions. This would likely take the form of a miniaturized near-to-mid-IR spectrometer plus a combined x-ray-diffraction/x-ray-fluorescence unit.
Fourth, we recommend design, construction, and near-term deployment of techniques capable of acquiring samples from locations protected from the currently harsh surface conditions on Mars. Such techniques would include a drill capable of acquiring a core several m in depth from the martian regolith, and a device capable of extracting a solid sample from beneath the surface of a martian rock..
Fifth, we recommend development of a number of analytical approaches that will be capable of detecting, and then providing detailed information about, any volatile phases, particularly organic compounds, that might be present, possibly sequestered within stable mineral phases, on or near the martian surface.
Sixth, we recommend continued support of several lines of basic R & A which provide much of the intellectual underpinning to the Mars missions.
It should be emphasized that the exobiological evolution of Mars, no matter how truncated it may have been, represents an integral part of martian history and that, as terrestrial experience has shown, such evolution is both influenced by, and in turn influences, the chemical and physical evolution of the planet. Furthermore, the observational objectives, i.e., instrumentation and target selection, of exobiology overlap to a considerable extent those of geology, geochemistry and climatology on Mars. Consequently, the exploration of Mars is best viewed as a joint endeavor in which both exobiology and the disciplines of traditional planetary science work together with interests and approaches that have much in common.
The idea of searching for evidence of life on Mars may strike some as far-fetched, even fanciful. But there is a compelling logic to such a quest, as well as an equally compelling excitement. Early environments were apparently sufficiently similar on Mars and Earth, and life arose so rapidly on Earth once conditions became clement, that emergence of life on both planets at that time is scarcely less plausible than emergence on only one. Furthermore, although a fossil on Mars might seem at first like a proverbial needle in a haystack, experience on Earth tells us that if we know where to look, finding evidence of ancient life is not particularly difficult, especially when one considers that such evidence can be relatively widely disseminated in the form of chemical or isotopic signatures. The key is to recognize that the search for ancient life on Mars will involve a logically designed sequence of missions, each of which will focus on defining ever more closely where and how biosignatures may be found. Although one can never rule out a chance discovery, this quest should not be approached as one that will yield to a single, expeditious mission. (In fact, the proposed strategy lends itself particularly well to the use of a series of relatively small, inexpensive spacecraft, rather than a single flagship-class mission.) The search for life on Mars will take time and commitment, but the reward could be a discovery of inestimable importance, not just to science, but to humanity as a whole.
PURPOSE OF STRATEGY DOCUMENT
Exobiological exploration encompasses many different lines of scientific inquiry, but one of the most prominent of these is the search for evidence of life elsewhere in the universe and specifically within our own solar system. It follows that there is an abiding exobiological interest in the planet Mars, which of all the planets most closely matches the conditions within which terrestrial biota are known to exist. A major goal of the highly successful Viking mission, described further below, was the search for evidence of life on the surface of Mars. Since that mission, whose results are generally interpreted as inconsistent with extant life at the two sites visited by the Viking landers, the scientific focus of planning for future Mars missions has tended towards geological, geophysical and meteorological issues, largely bypassing those of exobiology. The purpose of this document is to attempt to ensure balance in Mars mission planning by showing that (a) exobiology is an integral part of the scientific history of Mars, and (b) pursuit of exobiological goals is generally compatible with the broader-based scientific exploration of Mars.
Establishment of an exobiological strategy for Mars exploration is particularly timely in light of recent developments in several different scientific areas, such as theories of the origin and early evolution of life, Precambrian paleontology, fossilization processes, biochemistry of primitive terrestrial organisms, biology of hydrothermal systems on Earth, geomorphology of Mars, and analysis of SNC meteorites and recognition of their martian origin. These diverse lines of inquiry all combine to generate a powerful scientific justification for an exobiological exploration of Mars that goes beyond the pioneering efforts of the Viking missions.
In addition to a positive scientific context, there are good programmatic reasons for a resumption of the exobiological exploration of Mars. Those reasons may be characterized as follows.
First, the tragic loss of Mars Observer has resulted in a delay of at least four years for the global reconnaissance data needed for detailed planning of later landed missions. Following the recommendations of the Elachi Committee for recovery of Mars Observer data, those global data will now be acquired using two Mars Surveyor spacecraft scheduled for launch in 1996 and 1998, respectively.
Second, as a result of the problems plaguing the Russian economy, the Mars ’94 and ’96 missions have been delayed until 1996 and 1998 (or later), respectively. The Chief Scientist for these missions has indicated an interest in establishing an interdisciplinary science team which will “create recommendations…for small modification of the [spacecraft] instruments (if…possible) and recommendations for program of operation of the instruments expedient from the exobiology point of view.” The earlier mission already carries a U.S.experiment designed to study the oxidant(s) believed to be responsible for elimination of organic molecules from martian soil at the two Viking landing sites.
Third, Mars Pathfinder, NASA’s technical trial of a direct-landing approach, is scheduled for launch in 1996, with deployment of a mini-rover on the martian surface in 1997.
Fourth, NASA’s plan to establish a geophysical/meteorological network on the martian surface, MESUR Network, has been postponed indefinitely because the estimated cost of the mission is considered prohibitive under present funding constraints. It seems likely that deployment of such a network would be feasible only within the context of a joint international project that is unlikely to occur before the 2003 launch opportunity.
Fifth, with the shelving of MESUR Network, the focus of NASA’s near-term Mars strategy has shifted towards a combination of surface geology, geochemistry and climatology. Within this context, the Mars Science Working Group has recommended that the study of volatiles and climate history should constitute the near-term goal for Mars exploration.
Finally, NASA’s Mission From Planet Earth Study Office (Code SX) has formally decided to make the search for life on Mars one of the overarching goals of long-term solar-system exploration.
From these considerations, it is clear that, not only is planning for Mars exploration in a very active phase at this time, but exobiology is well placed to make a major contribution to that exploration. The aim of this document is therefore to define the exobiological issues which will serve as the scientific foundation to that contribution, and to provide a scientifically sound exobiological context within which scientists, engineers and managers will be able to optimize science return from future missions. Such optimization could involve design and development of instrumentation, instrument selection or modification, definition of spacecraft operations, and/or selection of landing sites or targets for remote sensing.
OUTLINE OF APPROACH
The approach adopted in this document largely follows that employed by Klein and DeVincenzi in their report on a NASA Ames workshop on exobiological exploration of Mars held in 1992. After a brief summary of the present state of knowledge about Mars, its geology and atmosphere, and the results from the Viking biology experiments, the strategy for further exobiological exploration of Mars is discussed from the perspective of three distinct scientific aspects: The search for evidence of prebiotic chemical evolution; the search for evidence of an ancient biota that is now extinct; and the search for life extant on Mars today. Then, the extent to which missions that are currently either proposed or planned will fulfill those strategic elements is discussed. Finally, a brief discussion of planetary protection issues, more completely covered in the recent National Academy of Sciences report [see Appendix], will be followed by a set of recommendations in the areas of basic research support, development of instruments and spacecraft, and mission planning.
THE PRESENT STATE OF KNOWLEDGE
THE GEOLOGICAL HISTORY OF MARS
Mars is a geologically diverse planet with heavily cratered terrains, huge volcanoes, enormous canyons, extensive dune fields, and numerous different kinds of channels seemingly cut by running water. The geologic record preserved at the surface includes examples from the period of heavy bombardment, that ended around 3.8 Gyr ago, up to the present. Like the Earth, the surface has been affected by volcanism, tectonic activity, and the action of wind, water and ice. The geologic and climatological evolution of the two planets has, however, been very different.
The abundant evidence for liquid-water erosion on Mars is particularly intriguing since present atmospheric conditions are such that liquid water cannot exist at the surface. Surface temperatures range from 150 K at the winter pole to a daily average of 215 K at the equator. At low latitudes the diurnal temperatures range from about 170 K to 290 K. Under these conditions, and with the present low-pressure atmosphere, liquid water is unstable everywhere, and the planet has a zone compatible with buried permafrost, several hundred meters thick at the equator and kilometers thick at the poles. At low latitudes (<40 deg. ), water ice will sublime into the atmosphere at rates dependent on the permeability of the overlying lithologies. Near-surface materials at low latitudes should, therefore, have lost all their unbound water. At latitudes 40-80 deg. ice is stable at depths greater than about a meter below the surface. At the poles, water ice has been detected at the north pole, where it is exposed when the overlying seasonal CO2 cap sublimes in summer.
Highlands and plains
The surface of the planet can be divided into two main components: an ancient cratered highlands, covering most of the southern hemisphere, and low-lying plains that are mostly at high northern latitudes. Superimposed on this two-fold division are the high-standing volcanic provinces of Tharsis and Elysium. The cratered highlands cover almost two thirds of the planet. They are mostly at elevations of 1-4 km above the datum, in contrast to the northern plains which are mostly 1-2 km below the datum. The cause of the division between the highlands and plains is unclear but it may be the result of a very large impact at the end of accretion. The density of impact craters in the martian highlands is comparable to the lunar highlands. The surface clearly dates back to the very earliest history of the planet when impact rates were high. On the Moon, the transition from very high impact rates to rates comparable to the present took place around 3.8 Gyr ago, and the transition probably took place at the same time on Mars. The martian highlands differ from the lunar highlands in three main ways: most of the martian craters are highly degraded, the ejecta around craters 5-100 km in diameter is commonly arrayed in discrete lobes, each lobe being outlined by a low ridge, and in the martian highlands are numerous branching valley networks that superficially resemble terrestrial river valleys. The highly degraded nature of many of the craters has been attributed to high erosion rates on early Mars, possibly a result of warmer climatic conditions; the lobed ejecta patterns have been attributed to the pervasive presence of ground ice; and the channel networks have been attributed to fluvial activity during warmer climatic conditions.
The plains are located mostly in the northern hemisphere. The number of superimposed craters on them varies substantially, indicating that they continued to form throughout the history of the planet. The plains are diverse in origin. The most unambiguous in origin are those on which numerous flow fronts are visible. They are clearly formed from lava flows superimposed one on another, and are most common around the volcanic centers of Tharsis and Elysium. On other plains, such as Lunae Planum, flows are rare but wrinkle ridges like those on the Moon are common. These are also assumed to be volcanic. But the vast majority of the low-lying northern plains lack obvious volcanic features. Instead they are curiously textured and fractured. Many of their characteristics have been attributed to the action of ground ice, or to their location at the ends of large flood features, where lakes must have formed and sediments been deposited. In some areas, particularly around the north pole, dune fields are visible. In yet other areas are features that have been attributed to the interaction of volcanism and ground-ice. Thus, the plains appear to be complex in origin, having variously formed by volcanism and different forms of sedimentation, and then subsequently been modified by tectonism and by wind, water and ice.
The most prominent volcanoes are in two regions, Tharsis and Elysium. Tharsis is at the center of a bulge in the planet’s surface, the deformation being over 4,000 km across and 10 km high at the center. A similar bulge centered on Elysium is around 2,000 km across and 5 km high. Three large volcanoes are close to the summit of the Tharsis bulge, and Olympus Mons, the tallest volcano on the planet, is on the northwest flank. All these volcanoes are enormous by terrestrial standards. Olympus Mons is 550 km across and 27 km high, and the three others have comparable dimensions. They all appear to have formed by a series of eruptions of fluid lava with very little pyroclastic activity. The large size of the volcanoes has been attributed to the lack of plate tectonics on Mars. The small number of superimposed impact craters on their flanks indicates that surface flows are relatively young, although the volcanoes may have been growing throughout much of Mars’ history.
To the north of Tharsis is Alba Patera, the largest volcano on the planet in areal extent. It is roughly 1,500 km across but only a few km high. Flows are visible on parts of its flanks, but elsewhere the flanks of the volcano are dissected by numerous branching channels. The easily eroded, channeled deposits have been interpreted as ash. Densely dissected deposits on other volcanoes such as Ceraunius Tholus in Tharsis, Hecates Tholus in Elysium, and Tyrrhena Patera in the southern highlands have also been interpreted as formed of ash.
Thus, Mars appears to have experienced both the Hawaiian style of volcanism, involving mostly quiet effusion of fluid lava, and more violent, pyroclastic eruptions, that result in deposition of extensive ash deposits. Abundant volcanism, and evidence of water and ice suggests that hydrothermal activity has been common.
The most widespread indicators of surface deformation are normal faults, indicating extension, and wrinkle ridges indicating compression. The most obvious deformational features are those associated with the Tharsis bulge. Around the bulge is a vast system of radial grabens that affects about a third of the planet’s surface. Circumferential wrinkle ridges are also present in places, particularly on the east side of the bulge in Lunae Planum. Both the fractures and the compressional ridges are believed to be the result of stresses in the lithosphere caused by the presence of the Tharsis bulge. Fractures also occur in other places where the crust has been differentially loaded, as around large impact basins, such as Hellas and Isidis, or around large volcanoes, such as Elysium Mons and Pavonis Mons. There is no evidence of plate movement as on the Earth
The vast canyons on the eastern flanks of the Tharsis bulge are the most spectacular result of crustal deformation. The canyons extend from the summit of the Tharsis bulge eastward for 4,000 km until they merge with chaotic terrain and large channels south of the Chryse basin. In the central section, where several canyons merge, they form a depression 600 km across and several kilometers deep. Although the origin of the canyons is poorly understood, faulting clearly played a major role. The canyons are aligned along the Tharsis radial faults, and many of the canyon walls are straight cliffs, or have triangular faceted spurs, clearly indicating faulting. Other processes were also involved in shaping the canyons. Parts of the walls have collapsed in huge landslides, other sections of the walls are deeply gullied. Fluvial sculpture is particularly common in the eastern sections. Faulting may have created most of the initial relief, that relief then enabling other processes such as mass wasting, and fluvial action to occur. Creation of massive fault scarps may also have exposed aquifers in the canyon walls and allowed groundwater to leak into the canyons, thereby creating temporary lakes.
One of the most puzzling aspects of martian geology is the role that water has played in the evolution of the planet. Although liquid water is unstable at the surface under present conditions, we see abundant evidence of water erosion. The most intriguing features are large dry valleys, interpreted as having formed by large floods. Many of the valleys start in areas of what has been termed chaotic terrain in which the ground has seemingly collapsed to form a surface of jostled and tilted blocks, 1-2 km below the surrounding terrain. The largest areas of chaotic terrain are in the Margaritifer Sinus region east of the canyons and south of the Chryse basin. Large dry valleys emerge from the chaotic terrain and extend northward down the regional slope for several hundred kilometers. Several large channels to the north and east of the canyons converge on the Chryse basin and then continue further north, where they merge into the low-lying northern plains. The valleys emerge full-size and have few if any tributaries. They have streamlined walls, scoured floors, and commonly contain tear-drop-shaped islands. All these characteristics suggest that they are the result of large floods, rather than the slow erosion of running water. Although most of the floods are around the Chryse basin, they are found elsewhere. Those near Elysium and Hellas have already been mentioned. Others occur in Memnonia and western Amazonis. Impact craters superimposed on the flood channels suggest that they have a wide range of ages.
The floods were enormous, some having discharges one hundred times the annual outflow of the Mississippi river. The cause of the large floods is unclear, and they may not all be of the same origin. One possibility is that Mars has an extensive groundwater system and that in low areas the large floods are the result of extreme artesian pressures. Another possibility is catastrophic release of water dammed in lakes. Sediments within the large equatorial canyons suggest that the canyons at one time contained lakes, probably as a result of groundwater flow out from under the surrounding plateau. Catastrophic release of water from these lakes may have caused some of the large channels that connect with the canyons to the east. After the floods were over, large lakes must have been left at the ends of the channels, and several linear features at the ends of the channels have been interpreted as shorelines of former lakes.
Other fluvial features appear to be the result of slow erosion of running water. Branching valley networks are found throughout the heavily cratered terrain, and occasionally on younger surfaces. They resemble terrestrial river valleys in that they have tributaries and increase in size downstream, although only rarely can a channel be observed within the valley. The valleys are generally short compared with terrestrial river systems, most being less than a few hundred kilometers in length, so rarely does one valley system dominate drainage over a large area. The most plausible explanation for the valleys is that they formed by erosion of running water. The open nature of some networks, alcove-like terminations of tributaries, and the range of junction angles between branches are suggestive of groundwater sapping. Other networks, however, lack these characteristics, and more resemble valleys formed by surface runoff.
Although ice is unstable at low latitudes, it may be present at depths of a few to several hundred meters because of the slow rate of diffusion of water vapor away from the ice, through the overlying materials, into the atmosphere. The almost universal presence of lobate flows around craters larger than about 5 km suggests the presence of ground ice or groundwater everywhere at depths greater than a few hundred meters. There is evidence for near-surface ice at high latitudes. In the 35-50 deg. latitude band in both hemispheres, debris flows commonly occur at the base of cliffs. These are convex-upward flows that extend about 20 km away from the base of the cliff. Such features are rare, if present at all, at low latitudes. The simplest explanation is that at low latitudes, when cliffs form, talus simply accumulates on the cliff slope, and so inhibits further erosion. At high latitudes, however, because of the presence of ground ice, ice could become incorporated into talus, thereby facilitating its flow away from the cliff, exposing the slope to further erosion. Debris flows are particularly common in regions of what has been termed fretted terrain, in which flat-floored valleys, filled with debris flows, extend from low-lying plains far into the cratered uplands. Formation of these valleys appears to be connected in some way with the formation of the debris flows. A general “softening” of terrain features at high latitudes has also been attributed to ground ice. At low latitudes many features, such as crater rim crests, are crisply preserved, whereas poleward of about 40 deg. latitude similar features are rounded or muted in appearance. This softening at high latitudes is generally attributed to ice-abetted creep of the local lithic materials.
At each pole, and extending outward to about the 80 deg. latitude circle, is a stack of layered sediments. They are at least 4-6 km thick in the north and at least 1-2 km thick in the south. Incised into the smooth upper surface of the deposits are numerous valleys and low escarpments. These curl out from the pole in a counterclockwise direction in the north and a predominantly clockwise direction in the south. Between the valleys, which are roughly equally spaced and 50 km apart, the surface of the deposits is very smooth and almost crater-free. For most of the year the layered deposits are covered with CO2 frost, but in summer they become partly defrosted. The layering is best seen as a fine horizontal banding on defrosted slopes. The deposits are believed to be composed of dust and ice, with the layering caused by different proportions of the two components. The scarcity of impact craters indicates relatively recent activity, although the age of the deposits could still be on the order of hundreds of Myr.
The layering suggests cyclic sedimentation, and the origin of the deposits may be connected in some way with the obliquity cycle. The obliquity is the angle between the equatorial plane of a planet and the orbital plane, and in the case of Mars is thought to oscillate chaotically over a wide range of values. At the highest obliquity twice as much insolation falls on the poles as at the lowest obliquity. As a consequence, the capacity of the high latitude regolith to hold adsorbed CO2, the size of the CO2 cap, and the atmospheric pressure are expected to change with the obliquity cycle. These changes could affect global wind regimes, dust-storm activity and sedimentation rates at the poles, thereby causing episodic sedimentation. Layering would also be caused by any event that resulted in large amounts of water vapor being introduced into the atmosphere. Possibilities are volcanic eruptions, large floods, and cometary impacts. These would all result in deposition of an ice-rich layer at the poles.
Mars, like the Earth, has had a diverse geologic history. An ancient heavily cratered surface preserves evidence of events of the planet’s earliest history. Volcanic activity continued throughout the planet’s history, possibly to the present, and resulted in the formation of extensive lava plains and large shield volcanoes. Much of the volcanic activity has been in two provinces, Tharsis and Elysium, which are at the centers of large bulges in the planet’s surface. The Tharsis bulge caused fracturing over about one third of the planet’s surface, and this fracturing may underlie the formation of enormous canyons down the east flank of the bulge. Massive floods of water have periodically flowed across the surface, some being triggered in some way by volcanism. In addition, dissection of some surfaces, particularly in the ancient cratered highlands, by small branching valley networks indicates slow erosion by liquid water and possibly massive global climate changes. The morphology of impact craters, and presence of numerous features suggestive of ground ice, indicate that ice is abundant at shallow depths at high latitudes, and at greater depths at low latitudes. Thus Mars is a planet on which most of the geologic processes familiar to us here on Earth have operated. The two planets are, however, also very different. The lack of plate tectonics on Mars has led to greater stability of the surface and to accumulated development of enormous volcanoes and canyons. In addition, the ineffectiveness of water erosion in eliminating relief has led to almost perfect preservation of features widely ranging in age. Despite the excellent photographic coverage and near-perfect preservation, the origin of many of the features remains obscure.
EVOLUTION OF THE MARTIAN ATMOSPHERE
Mars shows clear geological evidence for liquid water having existed on the surface, yet present-day conditions are such that liquid water could exist only under transient, metastable conditions. The martian atmosphere and volatile systems are key components to issues such as the possible origin of life at some time during Mars’ history and the evolution of the martian climate. Our understanding of climate change is clearly driven by the geological evidence, as discussed in the previous section. Degradation rates were much higher prior to about 3.5 Gyr ago, and degradation styles are suggestive of the presence of liquid water on the surface as an erosive agent. In addition, the valley networks on the ancient terrain have been interpreted as being indicative of liquid water flowing either on or beneath the surface; the features were eroded either by runoff of precipitation or by sapping. All of the evidence suggests that the climate during the earliest epochs was fundamentally different from that at the present, although quantifying this difference is not easy.
In addition, enrichment in the ratio of 15N/14N in atmospheric nitrogen is usually interpreted as being due to the preferential loss of the lighter isotope to space via non-thermal escape. As much as 99% of the original nitrogen inventory could have been lost. Similarly, enrichment in the ratio of D/H in atmospheric water also is interpreted as being indicative of loss to space. The D/H ratio is enriched by about five times, compared to its initial value (based on terrestrial and meteoritic values); preferential loss to space of the lighter isotope is the only mechanism which can produce this large a fractionation. Up to ~99% of the water must have been lost to space. Spacecraft measurements demonstrate that loss of hydrogen and oxygen occurs at the present.
There are three common approaches to understanding the history of the martian atmosphere and of water in particular:
First, from the geological perspective, the issues around the early atmosphere involve (i) understanding what climatic conditions were required to produce the early extensive erosion of Mars’ surface and to allow the valley networks to form, and then (ii) what atmospheric composition can produce such conditions.
The degradation of craters on the older surfaces is the most compelling evidence for climate change. Craters smaller than about 15 km diameter have been removed almost completely, and the remaining craters have been eroded substantially. Erosion by a surficial process is the best explanation for these features. The cratering age determinations indicate that the erosion ceased at about 3.5 Gyr ago.
The valley networks appear to be dendritic drainage patterns, and occur predominantly on the older surfaces. The mechanism of formation could be either runoff of precipitation or sapping by subsurface water, and each mechanism is favored by some surface features.
That these climate-related features appear only on the older surfaces indicates that they cannot form under the present climatic conditions. However, the ambient conditions during those epochs are still extremely uncertain. Exactly what surface temperature is required will depend on what the mechanism(s) are for formation of the geologic features. Compared to the average surface temperature today, about 220 K, temperatures of 245 K are often cited as the minimum average required for occasional liquid water, and 273 K, of course, for ubiquitous liquid water.
The simplest way to raise the average temperature is with an atmospheric greenhouse, such as by CO2 in the atmosphere. However, a thick CO2 atmosphere during early epochs would saturate and condense at high altitudes, thereby limiting the amount of CO2 that could reside in the atmosphere. The 1 to 2 bars CO2 that can reside in the atmosphere during those epochs is insufficient to raise the temperatures adequately. Additional heating could come from other greenhouse gases, such as methane or ammonia, but their presence and stability is uncertain.
Geological estimates of the total inventory of water are based on the amounts of water released from volcanic activity (both extrusive and intrusive) and from catastrophic flooding. The crust is estimated to hold perhaps the equivalent of a layer of water 0.5 km thick, and 10 to 20% of this is estimated to have been released to the surface.
Second, from the geochemical perspective, estimates of the volatile inventory of the whole planet come from our understanding of the volatile abundance in the material from which Mars accreted and from the geochemistry of the materials sampled in the martian regolith and in the SNC meteorites. As such, they are very uncertain. Estimates can be made of the amount of water, for example, incorporated into the accreting Mars, based on the water content of the materials from which it accreted. These estimates vary, up to the equivalent of a couple hundred meters of water, assuming that all of the water outgassed to the surface. Clearly, this is insufficient to account for the crustal water as inferred from the geology. One solution to this problem allows for a late-accreting veneer of volatile-rich materials (such as comets) to supply the martian volatiles. Of course, in this case, the total size of this veneer is uncertain, and there can be no first-principles estimate of the initial water content of Mars.
Estimates of the escape rates of atmospheric species are also uncertain when integrated over geologic time, but they can be constrained by present-day ratios of isotopes of stable elements in the atmosphere. Loss to space can occur by Jeans’ escape for hydrogen and by various non-thermal escape processes for heavier elements. Non-thermal escape includes photochemical processes such as dissociative recombination (of nitrogen, for example), or sputtering ejection by solar-wind pick-up ions. The latter process can be responsible for loss of a substantial fraction of a bar of CO2; it can also be responsible for loss of the light noble gases such as argon or neon.
Interpretation of isotopic ratios is confusing for hydrogen, oxygen, and carbon, because the atmospheric species can exchange with non-atmospheric reservoirs (such as the polar caps or the deep crust) of uncertain size. The noble-gas isotopic ratios are much easier to interpret, however, because the nonexchangeable noble-gas reservoir is quite small, relative to the atmospheric reservoir. For example, the observed ratio of 38Ar/36Ar implies loss of between 50 and >90 % of the atmospheric argon. Therefore, the non-radiogenic argon abundance in the present atmosphere cannot be used as an indicator of volatile inventory or outgassing efficiency without augmenting it to account for sputtering loss. Sputtering loss also must be accounted for in assessing the inventory of climatic volatiles such as water and carbon dioxide. A synthesis of the data suggests that substantial volatiles have been lost to space, with most of the loss occurring at about the same time that the geology suggests that the climate was changing.
Of additional relevance, the abundances of helium and neon in the atmosphere and the ratio of 22Ne/20Ne, in the presence of rapid loss by sputtering, requires that juvenile volatiles continue to be outgassed to the atmosphere during recent epochs.
The geochemical and geological inferences overlap in one important area for understanding the evolution of martian volatiles, namely hydrothermal systems. Volcanism and impacts provide abundant sources of heat, and there is considerable evidence for the presence of water, as discussed above. The effects of hydrothermal alteration are seen in SNC meteorites, and there is some evidence that hydrothermal systems may have played a role in the formation of the valley networks and other surface features. In terms of atmospheric evolution, however, the key issue is the recent realization that water in such systems exchanges between the crust and the atmosphere. This evidence lies in the D/H ratios measured in water-bearing minerals in the some SNC meteorites, which can have values as high as the martian atmospheric value (five times the terrestrial ratio). As such a large enrichment can only occur by escape to space, the high crustal values require exchange between the atmosphere and crust. This exchange has important implications for the abundance and history of water, and possibly for other volatiles as well.
Finally, there are possible variations in the atmosphere on shorter timescales. Geological evidence for changes in the atmosphere come from the polar caps and from putative shoreline features seen in various locations. Layers within the polar caps appear to consist of alternating laminae of water-ice-rich and dust-rich materials. The layers are presumed to result from quasi-periodic variations in the martian climate. The large swings in the axial obliquity, which occur on timescales of about 105 and 106 years and longer, are thought to be responsible for the climate variations. The obliquity varies chaotically with a timescale of about 107 years, and values as large as 60 deg. may have occurred in the recent geological past. The polar deposits contain a significant amount of CO2 and/or H2O, and these may be released into the atmosphere at high obliquity.
Geologically, observations of benches seen in certain locations in the northern lowlands have been interpreted as having been carved by wave action from a putative northern ocean. Ages of these features would require the ocean to have been there during parts of the last 3.5 Gyr. Although the outflow channels certainly debouched into the lowlands and may have created standing bodies of water throughout this period, it is not clear that these could have remained liquid long enough for waves to have cut such features. The possible release of CO2 and water from the polar caps during periods of high obliquity, on the other hand, may have provided a mechanism for stabilizing water temporarily.
In summary, all of the available evidence related to the evolution of the atmosphere and volatile system suggests the following:
(i) The climate on Mars was substantially different during the earliest epochs, prior to about 3.5 Gyr ago.
(ii) Although there is still debate on this topic, it appears likely that the early atmosphere was warmer than the present one and may have allowed at least occasional occurrences of precipitation and surface runoff.
(iii) The change in climate around 3.5 Gyr ago may have been caused at least in part by loss of volatiles to space by non-thermal escape and by the sequestration of volatiles into the polar caps.
(iv) The present-day climate contains a thin, dry atmosphere, in which water can be present as a liquid only under special conditions and then only as an unstable, transient phase.
(v) There is evidence for active and abundant hydrothermal systems, existing throughout much of martian history, that contain substantial amounts of water that may exchange with the atmosphere.
(vi) Although very speculative, it is possible that there may have been periods in the last few Gyr during which volatiles were released into the atmosphere, possibly from the polar caps during periods of high obliquity, resulting in occasional times of a more-clement climate.
RESULTS OF VIKING BIOLOGY EXPERIMENTS
From a biological point of view, the Viking mission to Mars in the late 1970’s can be interpreted as a test of the Oparin-Haldane hypothesis of chemical evolution. Taking into account that the early histories of Mars and Earth were probably similar, and that terrestrial life appears to have originated on Earth very early – from materials that could well have been present also on Mars – it is not unreasonable to assume that chemical evolution, leading to complex organic compounds capable of replication, could have also occurred on Mars. Further, if replicating systems did appear on Mars in an earlier, more benign, environment than exists today, the question is whether these ancient organisms were able to adapt to worsening conditions on that planet, as it lost its surface water and much of its atmosphere, and cooled to its present cold, arid, seemingly hostile condition. The biological experiments aboard the Viking spacecraft were intended to probe this possibility.
During the 1960’s numerous approaches were proposed for the detection of a martian biota ranging from physical to chemical and biological measurements of martian surface samples. For the Viking mission, the general approach selected was to test samples for possible metabolic activity of indigenous organisms, predicated on the assumption that using such an approach would allow the amplification of possibly weak biological signals by prolonged incubations.
Relevant nonbiological Viking instrumentation
In addition to the Viking Biology Instruments discussed below, it is important to emphasize that several other instruments aboard the Viking landers ultimately made substantial contributions to our understanding of the status of extant biology on that planet. These included an extremely sensitive gas chromatograph-mass spectrometer for elucidating the nature of organic compounds that might be present in martian surface material, and also for determining the composition of the martian atmosphere; an X-ray fluorescence instrument for analyzing the elemental composition of surface samples; and an imaging system capable of surveying the local surroundings in black and white and color, over the course of the seasons.
Major findings of interest in connection with the subject at hand were: a) that no organic compounds were detected in surface samples; b) that the inorganic elemental composition of surface samples were consistent with a mixture of iron-rich (smectite) clays, magnesium sulfate, and iron oxides; c) that, in addition to carbon dioxide and carbon monoxide, the surface atmosphere contained about 2.5% nitrogen and 0.15% oxygen; and d) that no structures uniquely attributable to biological entities were present in more than 4500 images obtained from the two landers.
The Viking biology experiments
For the metabolic experiments conducted on the two Viking landers, three different instruments were included, based on four different assumptions about the nature of martian biota. Using these instruments, a variety of environmental conditions could be provided for incubating samples. In addition, in each case, provision was also made to heat surface samples to temperatures of 140 to 180 deg. C prior to incubation to serve as controls for provisionally positive results. In all, a total of 26 separate incubations were made at the two lander sites.
The Pyrolytic Release Experiment
This experiment, which was performed 9 times, had as its underlying basis the assumption that some photosynthetic species, capable of assimilating either CO2 or CO, might be present in the martian surface. Accordingly, samples were first incubated with 14C-labeled CO2 and CO in the presence of a xenon arc lamp to simulate solar illumination. After several days, the samples were subjected to high temperatures to drive off and trap any organics on a column and, during this process, to free any trapped organics from initial radioactive gases. After this, the organics were released from the column by heating to pyrolyzing temperatures, the resulting pyrolysis products being monitored by a 14C detector. Later modifications of this basic protocol included incubations in the dark and with the addition of traces of water vapor during incubation.
Extremely small, but statistically significant, amounts of incorporation of the initial gas mixture (presumably into organic compounds) were seen in these experiments. However, the results have been interpreted as being non-biological on the following grounds: heating one sample to “sterilizing” temperature (90 deg. C) for 2 hr did not reduce the level of incorporation; addition of water vapor (to supply reducing power for the reaction) totally abolished the reaction; and the reaction proceeded in the dark. To date, no satisfactory explanation has been given to account for these results.
The Labeled Release Experiment
This experiment was based on the assumption that organisms might be present in the surface of Mars that were capable of metabolizing simple carbon compounds similar to those readily formed in laboratory simulations of early organic chemical evolution. For this experiment, which was carried out 10 times, a dilute aqueous solution of 1, 2, and 3-carbon compounds, labeled with 14C in all carbon atoms, was injected onto samples and the system was incubated for periods of up to several months. During incubation, the head-space was monitored by a 14C detector for evidence of degradation of the substrate molecules.
The major findings from this experiment were that each time a fresh surface sample was incubated, there was a rapid evolution of radioactive gas, amounting to about 10-15% of the initial radioactivity supplied and that samples heated to 160 deg. C were completely inhibited, and samples heated to 50 deg. C partially inhibited, in the evolution of radioactive gas. These results are consistent with pre-Viking criteria of a positive response. Nevertheless, there are several considerations that argue against assigning such a conclusion to these experiments, the most significant of which are the data from the GC-MS and Gas Exchange experiments which point to highly reactive substances in the martian surface that could readily oxidize the low-molecular-weight substrates used in this experiment. To resolve this issue, further experiments with martian material and with simulated martian environments are clearly indicated.
The Gas Exchange Experiment (“wet” mode)
Here the assumption was made that surface samples contained heterotrophic micro-organisms requiring organic substrates and possibly various organic growth factors for their metabolism, and that metabolism would involve the uptake or release of metabolic gases. To elicit the growth of such organisms, a complex aqueous solution, containing 19 amino acids and over a dozen growth factors, plus numerous inorganic salts, was added to surface samples. Metabolism was monitored with a gas chromatograph capable of measuring changes for a number of metabolically prominent gases, such as H2, N2, O2, CO2, and CH4. Three experiments were conducted in this mode, with incubation times of up to several months. The data gave no indication of metabolic activity; the main gas change observed under these conditions was a slow, steady release of CO2 over the course of incubation, which has been interpreted as resulting from a slow oxidation of one or more of the organic constituents.
The Gas Exchange Experiment (“humid” mode)
This mode of conducting the gas exchange experiment was based on an assumption that indigenous organisms only required the presence of moisture in order to elicit their metabolism. Accordingly, for these incubations, after surface samples were dispensed into a porous cup in the incubation chamber, nutrient solution (described above) was allowed to enter the incubation chamber without touching the samples, and the chamber was sealed. Under these conditions, the samples became humidified without coming into contact with added nutrients. In each of the three experiments carried out under these conditions, there was an unanticipated, rapid, evolution of O2 into the headspace. That this reaction was non-biological is very likely for a number of reasons: The reaction was extremely rapid; release of O2 took place in the dark; and, finally, after prior heating of the samples at 145 deg. C for 3.5 hr the reaction still took place. While many explanations have been offered for the reactivity seen in these experiments, the prevalence of one or more reactive oxidants in the surface material seems to hold the most credence at present. However, the actual mechanism underlying this reactivity remains to be ascertained in future experimentation with martian samples. Of interest is the observation that the data from the three samples tested in this manner suggests an inverse relationship between the active agent in the surface and the prior water exposure of the samples.
Taking into account all of the data summarized above, it is fair to state that the Viking experiments ruled out a number of possible scenarios for biology on the surface of Mars, but left open the possibility that organisms might still exist in some cryptic environment – yet to be discovered. Viking also raised a number of important questions: Are organic compounds accessible anywhere on the planet? What is the nature of the reactive oxidant in surface material, and how is it distributed on the planet? Are there regions on or near the surface of Mars that sustain indigenous organisms?