Water on Mars
Ted S. Frost
Dendritic Mars channel system. (NASA/JPL)
Is there now, or was there ever, life on Mars? Voluminous data from two Mars rovers and three orbiting satellites has planetary scientists thinking hard. The key, of course, is liquid water—all life as we know it depends upon water.
That is why much current research is aimed at trying to determine if liquid water ever did exist on Mars long enough for life to get started, even if limited to ‘simple’ bacteria.
Although researchers abandoned hope of multi-cellular life on Mars some time ago—Mars is too cold (-55oC), too dry (no liquid water on its surface), has too thin an atmosphere (~1% of atmospheric pressure of Earth), and too subject to cosmic and UV radiation (no magnetic field and no ozone protection)—such constraints do not rule out microbial life, which is capable of existing in extreme environments, including deep underground. The ‘triple point’ of a substance is that temperature and pressure at which it can coexist in three phases (gas, liquid, and solid). The triple point for water is .01oC and .006 atm. The temperature and atmospheric pressure of Mars lie below this point. The temperature averages -55oC. Atmospheric pressure varies with altitude and season, averaging about .007 atm. Water on the surface of Mars exists primarily as ice. If the ground temperature rises above 0oC, the ice evaporates (sublimates) rather than melts.
Underground, overlying rock pressure and geothermal heat might permit liquid water, particularly if salty. But what about liquid surface water in the past? There are tantalizing geological artifacts that indicate water once did flow and pool on Mars, although not necessarily in Earth-like rivers and oceans. Five surface features show evidence of past liquid water: deep, steep valleys and chasms, dendritic (tree-like) channels, sediment layers, the Northern Plains ‘seabed,’ and gullies.
The most prominent deep valley (called a "Valles" in Mars-speak) is the Valles Marineris. It resembles the Grand Canyon, except that it is nearly as long as the United States, with individual troughs 50 to 100 km wide that merge into a central depression as much as 600 km wide and up to 10 km deep—seven times the depth of the Grand Canyon. It seems to have been caused by a flood of astronomical proportions, which has been likened to the ancient catastrophic floods of Eastern Washington.
The cause of channels on Mars is controversial. The most common proposal has been geothermal heating of underground aquifers of water ice.1 However, it’s difficult to account for the apparent volumes. Some researchers hypothesize that these features and others like them are outflow channels carved by catastrophic dewatering of underground evaporate deposits rather than water ice.2 The idea is that large underground sediments of hydrated sulfate deposits such as gypsum (CaSO4.2H2O) and kieserite (MgSO4.7H2O) were heated by upwelling magma, as suggested by nearby volcanoes and topographic bulges.
Subjecting such deposits to geothermal heat from a magma intrusion would separate the water from the sulfate. This could have released enormous quantities of water in a process that expands in volume, erupting in outflows that carved and excavated canyons and valleys. Other channels, less dramatic than Valles Marineris, exist—but these sinuous, deeply incised valleys and channels, created by chaotic and episodic flood events, do not seem like environments that could support life. The dendritic channels are a more intriguing fluvial feature. These are networks of what appear to be outflow stream beds from precipitation collection basins. The existence of dendritic channels suggests water run-off which in turn suggests climatic conditions conducive to liquid water. Although their similarity to Earth stream systems is uncanny, they do not in general have as many tributaries nor are they as extensive. Judging by the number of impact craters, most such systems appear to have been formed 4.5 to 3.5 billion years ago . It is conceivable that Mars’ surface temperature back then might have been higher and its atmosphere denser. This might mean rain was possible and that rain runoff could have formed oceans or large lakes. Or could it? The problem is many of the channel branches are isolated and others have large areas between branches that have no channels. In general, they do not have the tributaries one would expect in an Earth-like tributary system.
Could the channels be better explained as melting glacier run-off or groundwater seepage? David Catling and Conway Leovy suggest flash heating from large asteroid and comet impacts that bombarded Mars some 4.5 to 3.5 billion years ago.3 The largest of these impacts would have vaporized substantial quantities of rock. Upon condensing and falling, the rock would flash heat the surface of Mars, melting surface ice, and creating water vapor, the resulting rain causing rapid run-offs and floods. As long as the surface remained hot, which Catling and Leovy suggest could be from a few weeks to as long as thousands of years, the cycle of water evaporating and precipitating could produce dendritic networks. Although this could not create a long-term warm climate, it could have produced repeated short-term warm climate events and short-term liquid water. Volcanism could also trigger short-term warming. Major eruptions may have spewed forth enough water and volatiles to influence the climate. The Mars volcano Olympus Mons is three times as high as Mt. Everest and as wide as the entire Hawaiian Island chain, on a planet whose surface area is only 28% that of Earth. Mars rover Opportunity observed distinct sediment layers on a bedrock outcrop on the side of its Eagle Crater landing site. These sediments show the discordant truncating and cross-bedding associated with rippled sediments deposited in currents of water. Because of their shallow nature, the initial interpretation was that these sediment beds represented evaporate sediment laid down during the alternate flooding and drying of a small shallow sea. However, this interpretation has recently been challenged by other scientists who say that the sedimentation was probably due to volcanic activity and/or meteorite impacts.4 Along with sediments, Opportunity found the famous Eagle Crater "blueberries," BB-sized accretions of hematite (Fe2O3) that typically form as groundwater moves through buried rocks. So far, however, there has been no evidence of larger flakes of gray hematite precipitate that would be a sure indicator of liquid surface water.
Mars orbiters have photographed features that show evidence of sedimentary depositions that may have formed in standing pools of water. These include sediment layers in meter craters as well as light layered outcrops (LLO) that appear on chasm rock walls. The current speculation is that these features represent sediments laid down by multiple occurrences of either evaporating lakes, or, sulfate rich snow/ice melts during planetary obliquity cycles.5 These sediments appear to be ancient, dating back to Mar’s bolide impact era, some four billion years ago. The Gusev crater chosen as the landing site for the Mars rover Spirit was selected specifically because orbiter observations indicated it might be the location of an ancient lake, especially since a dry river bed appears to enter one end of the crater. Unfortunately, Spirit found the floor of Gusev crater littered with volcanic debris and dust rather than lake sediments. If sediments do exist, they must be buried.
Cross-bedded sediment layers on rock out-croppings in Eagle Crater, the landing site of rover Opportunity. (NASA).
The top one-third of Mars is ‘bald.’ The northern hemisphere, called the Northern Plains, is smooth, flat, and lower than the rest of the planet. In addition, its underlying crust is thinner and there is little evidence of volcanic activity. Around the perimeter are prominent cliffs that can be as high as 2-3 km. In contrast, the southern regions are scarred and cratered with channels, volcanoes, highlands, cliffs, valleys, and chasms. Could the smooth features of the Northern Plains be the dried-up basin of a primitive ocean and the surrounding cliffs the remains of its seashore, the site of long-standing liquid water and a cradle for life? It’s tempting to think so. Much of the Northern Plains consists of an area known as the Vastitas Borealis Formation (VBF), which seems to be a thin veneer of layered sedimentation material covering underlying lava plains and craters. Outflow channels from various catastrophic flood events lead into the VBF. Other surface features appear to represent melt-water associated with glaciated ice sheets.
Some researchers such as Michael Carr and James Head hypothesize that the VBF represents deposits left behind from an ancient ocean.6 But if the VBF once was an ocean, what happened to all the water? If it froze, where is the ice? There is an icecap on the north pole of Mars, but it is quite small in relation to the area of the Northern Plains.
Carr and Head suggest 20% of any ancient ocean could be present in the polar caps, 30% lost in space by sublimation, and the rest could be trapped in volatilerich surface deposits or redistributed in the groundwater system. Spectrometer analysis indicates considerable ice in the upper surface of the Northern Plains. Despite an extensive search, no carbonate sediments have been found on Mars. Carbonate sedimentation on Earth comes from weathering of rocks by carbonic acid (H2CO3), which is created when carbon dioxide combines with liquid water. Carbonic acid dissolves rocks over time resulting in carbonated mineral deposits on the bottom of oceans and lakes.
The atmosphere of Mars is 95% carbon dioxide. If longstanding liquid water did once exist there, wouldn’t carbonate sediments have formed as they have on Earth? Compounding that disappointment are extensive deposits of the mineral olivine, a mineral that is chemically altered in the presence of water. Mars Express orbiter7 has detected patchy deposits of water-altered minerals (phyllosilicates, i.e. ‘clays’) in the Southern Highlands region These types of deposits are created by standing water. However, they are ancient, dating back to the Noachian Period some four billion years ago, when Mars was being clobbered by bolides. So far, there is no evidence of similar deposits during periods more suitable for life. Roger Buick points out that clays can form quickly, so they do not necessarily indicate long-standing liquid water.
A panel of scientists examined rover and orbiter data and concluded that arid, acidic, and oxidizing conditions were wide-spread in the early environment of Mars. While some Earth microbes have adapted to these kind of constraints, these conditions challenge the chemical reactions thought necessary for the creation of life.8 So, where are we? There has been flowing surface water on Mars but most likely it was from catastrophic geothermal floods, heat-flash bolide impacts, and cyclically wet and dry shallow pools. The preponderance of data argues against any long-standing ‘life-friendly’ bodies of liquid water.
Could Mars ever have been warm? Mars is half again the distance from the Sun as Earth (1.52 AU’s). Solar radiation flux varies by the inverse square of the distance, so sunlight on Mars is only 43% as bright as on Earth. In addition, 4 billion years ago, the presumed time warm-wetness would be most likely, the Sun was only 70% as bright as it is today.
A large greenhouse gas effect would be needed. According to Catling and Leovy, the required density would cause carbon dioxide to condense into clouds whose albedo would counteract the greenhouse effect. This accords with analysis of the famous four-billion year-old Mars meteorite ALH840019. Researchers have concluded that the site on Mars where ALH84001 originated shows no evidence of having been above freezing during the four billion years of its existence.10
Still, there are all those intriguing geomorphic features that hint liquid water once did accumulate and flow. And despite the current lack of liquid surface water, there is abundant evidence of water in the form of subsurface water ice and polar ice caps
Spectrometer analyses of gamma ray and neutron emissions generated by cosmic rays striking the surface of Mars indicate extensive water in the upper meter of the surface of Mars11 (if one assumes neutron emissions are a reliable proxy for water). The Mars Reconnaissance Orbiter scheduled to be in place March of 2006 should provide verification. It contains shallow subsurface radar (SHARAD) which will beam 15-25 MHz radio waves that will map subsurface water deposits to a depth of 1 km.
It is clear the upper crust of Mars is frozen and probably has been frozen for some time. However, the intriguing possibility does exist that liquid water may actually be present currently on Mars in the form of underground aquifers. Researchers think gullies are caused by water saturated debris flows triggered by the rapid release of water from ice barriers on the surfaces of slopes. The water fueling the gullies is presumed to be outflows from underground aquifers liquefied by geothermal heating. The following images show current evidence for such a process.
Evidence of recent flow of fluids. Creation of a gully sometime between 7/17/ 02 and 4/27/05. (NASA/JPL/ASU)
If these aquifers exist, as gullies and other features12 seem to indicate, could they be a current refuge for microbes? They seem our best hope for life on Mars.
References & Notes:
1. “Martian floods at Cerberus Fossae can be produced by groundwater discharge,” M. Manga, Geophysical Research Letters, Vol. 31, L02702, Oct, 29, 2004.
2. “Formation of Martian outflow channels by catastrophic dewatering of evaporate deposits,” D. R. Montgomery et al., Geology, Vol. 37, Aug. 2005, p. 625-658.
3. “Mars Atmosphere and Volatile history,” D. C.Catling & C. Leovy, Encyclopedia of the Solar System, 2005 (submitted).
4. “A volcanic environment for bedrock diagenesis at Meridiani Planum on Mars,” T. M. McCollom et al., Nature, Vol. 438, Dec. 22, 2005, p. 1129- 1131, and “Impact origin of sediments at the Opportunity landing site on Mars,” L. P. Knauth et al., Nature, Vol. 438, Dec. 22, 2005, p. 1123-1128.
5. “Light-toned layered deposits in Juventae Chasma, Mars,” D. C. Catling et al., Icarus, Dec. 22, 2005.
6. “Oceans of Mars: An assessment of the observational evidence and possible fate,” M. H. Carr & J. W. Head III, Journal of Geophysical Research, Vol. 108, 2003.
7. “Phyllosilicates on Mars and implications for early Martian climate,” F. Poulet et al., Nature, Vol. 438, Dec. 22, 2005, p. 623-627.
8. “An astrobiological perspective on Meridiani Planum,” A. Knoll et al., Earth and Planetary Science Letters, Vol. 240, Nov. 20, 2005; p 179-189.
9. ALH84001 became famous when astronomers claimed in 1996 it showed evidence of life on Mars in the form of microbial fossils and biomarkers. The resulting publicity included a White House press conference. Subsequently, biologists (E. I. Friedmann & others) have discredited the evidence and, to the extent any exists (such as magnetite crystals), maintain it represents Earth contamination.
10. “Martian surface paleotemperatures from thermochronology of Meteorites,”B. Weiss et al, Science, Vol. 309, Jul. 7, 2005; p. 594-597.
11. “Distribution of Hydrogen in the Near Surface of Mars: Evidence for Subsurface Ice Deposits,” W. V. Boynton et al., Science, Vol. 297, Jul 5, 2002, p. 81-85.
12. Trace amounts of methane (CH4) have been detected in Mars’ atmosphere. Because methane can exist for only a few hundred years on Mars, something must be emitting it. Some (V. A. Krasnopolsky et al) hypothesize it is being created by micro organisms, ala Earth’s methanogenic archea. Others (C. Oze et al) postulate that it is from nonbiological geothermal processes like the serpentination of olivine that occurs on Earth. In either case, liquid underground water is required.
Armchair Exploration of Mars
Bill O’Neill
Scientists are accumulating masses of data on the red planet. Last Fall, I audited a course on Mars, part of the graduate Astrobiology program (AB 502) at the University of Washington. We concentrated upon evidence about Mars’ suitability for life, now or in its past, guided by faculty from several disciplines. There was no textbook: many of our sources would interest general readers seeking to satisfy their curiosity about Mars.
The best single reference I know of, besides the Internet, is A Traveler’s Guide to Mars (Workman Publishing, 2003), available for $20 or less. The author, William Hartmann, earned a Ph.D. under Gerard Kuiper, pioneered crater counting as a method for estimating the age of planetary surfaces, and has been involved in Mars exploration from Mariner 9 (1972) through contemporary Mars Global Surveyor (MGS) and Mars Orbiter Laser Altimeter (MOLA). The text is clear and easy to read, organized by the regions of Mars which first or best characterized aspects of the planet’s nature. The number and quality of the illustrations are what you’d expect in a much more expensive book.
For larger format color pictures, you can’t yet beat Magnificent Mars by Ken Croswell (Free Press, 2003, $60). But you can access all images from US orbiters at www.msss.com and from the surface at marsrovers.jpl.nasa.gov/home/. For more recent views of Mars there can be no better source than Steven Squyres, the Principal Investigator and spokesman for The Mars Exploration Rovers: those marvelous robots which have persevered over two years (>750 days!). Squyres’ book, Roving Mars (Hyperion, 2005), provides an engaging and accessible description of development of the rovers from the standpoint of the technicians directly responsible for them. For anyone interested in engineering at the brink of know-how, under time-pressure and public scrutiny, it is a thrilling read. Non-geeks might find the first twothirds of the book tedious. An excellent audio version is available, which I found delightful, and Disney has just released an IMAX movie based upon the book.
For some reason, Disney decided not to allow Seattle’s Pacific Science Center to show “Roving Mars.” So I plan a pilgrimage to Vancouver, B.C. in March to see it even though the reviews have been mixed, at best. With respect to scientific results, Squyres’ book summarizes what appeared to have been gleaned from the first 400 days of the rovers’ explorations. Many of the details have been published: in Nature 436:44-69 (7 July, 2005), an entire issue of Earth & Planetary Science 240 (30 Nov, 2005) and elsewhere. Squyres has written occasional synopses in a casual log on the NASA web site. He gave a general public lecture at UW on February 28th and will present a more technical seminar, probably also open to the public, in the UW Astronomy Annex Auditorium at 4 p.m. on March 2.
Finally, I can’t think of a more interesting context for appreciating what is being learned about Mars than the epic, 3-part novel, beginning with Red Mars (Bantam, 1993) created by Kim Stanley Robinson. Employing USGS maps of Mars that depicted most of its actual topography, first revealed by 7,000 images from the Mariner 9 orbiter, Robinson moved his characters around realistic landscapes all over Mars through three centuries of exploration and transformation. Terra forming, modifying Mars’ environment to make it more suitable for life derived from Earth, underlies a plot which raises questions about both the feasibility of changing the planet and philosophical issues of whether it would be appropriate to do so. Readers perceive developments through the eyes of key members of the “First 100”, the initial colonists launched to Mars in the 3rd decade of this 21st century. It is most intriguing that the author provides the means to traverse the planet with his characters pole to pole, experiencing pristine Martian landscape and sites for communities, exploration and exploitation. You needn’t take my word about it; Robinson’s Mars trilogy and its author are described at length within the admirable non-fiction text, Mapping Mars, by Oliver Morton (Picador, 2002, pages 173-183). For my time and money, Red Mars far surpasses Robert Zubrin’s novel, First Landing (Ace, 2001).