15.3.1 Limits to a carbon dioxide greenhouse
The picture of a warm early Mars is drawn by analogy with the early Earth – a thick carbon dioxide atmosphere sustaining a greenhouse effect in the face of a faint early Sun. Because of Mars’ greater distance from the Sun compared to the Earth’s –
yielding only half the sunshine that Earth receives – a higher carbon dioxide pressure is required to sustain a certain temper- ature at any given epoch in the Sun’s history. At least several atmospheres worth of carbon dioxide, or more, were required for Martian surface temperatures to be above the freezing point of water early in its history.
As shown in Figure 15.7, a potentially serious flaw arises for such a CO2-only Martian greenhouse. For progressively smaller
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Figure 15.7Greenhouse problem for Mars. The amount of carbon dioxide in the Martian atmosphere is shown for various values of the carbon dioxide surface pressure. Each profile is the ratio of the saturation pressure of carbon dioxide to the actual pressure as a function of altitude; where this ratio equals one (vertical dashed line), cloud formation occurs. The present-day carbon dioxide pressure near the surface (0.006 bar) leads to an atmosphere that does not produce carbon dioxide clouds (except near the poles). To get a Martian surface warm enough for liquid water, over 2 bars of pressure is needed at the surface (5 bars during the faint-Sun epoch). However, for any surface pressure above 0.35 bar, according to the figure, cloud formation will occur. Reproduced from Kasting (1991) by permission of Academic Press.
amounts of sunlight, one requires a higher carbon dioxide pres- sure to sustain a given atmospheric temperature. Carbon dioxide, like water, can form clouds, though much lower temperatures are required for a given amount of carbon dioxide to condense than for the same amount of water. It is possible to plot the car- bon dioxide pressure for various temperatures at which carbon dioxide cloud formation will occur – the lower the tempera- ture, the lower the pressure at which such clouds will form. An equivalent curve for water determines the altitude at which water clouds form in Earth’s atmosphere for particular conditions on any given day. Figure 15.7 shows that CO2 cloud formation occurs on Mars, for present solar luminosities, when the surface pressure of carbon dioxide exceeds 0.35 bar. This is many times less than the pressure of carbon dioxide needed to warm the surface to the water melting point, and is a direct consequence of Mars’ greater distance from the Sun compared with Earth’s.
The carbon dioxide pressure needed to keep liquid water stable onearlyMars is higher still (since the Sun was fainter), ensur- ing that CO2cloud formation must be considered in models of a warm early atmosphere.
The effect of carbon dioxide clouds on the early Martian greenhouse is not fully understood, because both scattering of sunlight and absorption of infrared energy (heat) may occur within such clouds. Most simple models suggest that the net effect of the clouds is to cool the atmosphere. This, in turn, requires higher carbon dioxide pressures to achieve a given surface temperature, which cannot be obtained because the gas simply condenses into thicker and thicker clouds, and
eventually to carbon dioxide snow or rain. More elaborate mod- els allow for both warming and cooling effects of clouds, so that cloud formation no longer is a hard limit to a CO2greenhouse effect. Francois Forget of Universit´e Pie et Marie Curie, Paris and Ray Pierrehumbert of the University of Chicago found that clouds made of large particles of CO2ice can actually warm the Martian surface. Predicting particle size in clouds is difficult, but it is at least possible that a greenhouse atmosphere on early Mars was supported by CO2clouds with this property. Carbon dioxide pressures up to 2 bars can be contemplated, at which point the gas in the atmosphere may begin to reflect so much sunlight back into space that the greenhouse heating becomes inefficient.
One plausible way to circumvent the problem of warming early Mars is to posit other greenhouse gases that enhance the effect of the carbon dioxide. Water vapor is not a candidate, because the low temperatures at which the carbon dioxide cloud formation occurs are such as to keep the atmosphere extremely dry (water condenses out at much lower pressure, for a given temperature, than does carbon dioxide). What is required is a gas that condenses out at much higher pressure, and is a good infrared absorber. Methane (CH4), ammonia (NH3), and various compounds of sulfur – particularly sulfur dioxide (SO2) have been proposed. The first two could plausibly be present only in the very earliest history of Mars, perhaps the first few million years, because they would quickly be broken apart by sunlight or surface reactions to form other species. The sulfur compounds might be more stable, and would be consistent with the large amount of sulfate deposit seen on the Martian surface today.
An alternative view is that the Martian surface was never sta- bly warm for long periods of time, and that frequent impacts of asteroidal and cometary fragments released water and carbon dioxide into the atmosphere, allowing rainfall and formation of valley networks. A recent reanalysis by Brian Toon and col- leagues of the timing of the formation of impact basins – large craters – on Mars, relative to the valley networks, suggests that they could have been causally related, and that impact-generated atmospheres thick enough to produce a greenhouse warming – while geologically brief in duration – would have allowed mul- tiple flooding events occurring over centuries. The total number of such impact-driven clement episodes might have been suffi- cient to carve the channels. Only impacts early in Mars’ history would have been effective in raising greenhouse atmospheres, because eventually the CO2required would have been depleted by loss to space and formation of crustal carbonates. Whether such a model is consistent with the evidence for long periods of standing water suggested by some of the Mars rover results remains an open issue, in part because the environment within which the liquid water was present remains poorly understood.
15.3.2 Abodes for life on Mars
Where might life have begun on early Mars? In the presence of a dense atmosphere, running or standing water at the surface might have provided environments suitable to the formation and maintenance of life. However, if standing or running water were transient or episodic, conditions obtained for a declining or marginally thin atmosphere, the environment would have been too unstable for prebiological chemical reactions to build in complexity and generate sustained, biochemical systems. The
presence of phyllosilicates, as noted earlier, does suggest an extended period of standing liquid water on or within the surface of Mars, enough for life to have formed based on when the earliest chemical traces of life appeared on Earth.
A stable liquid water environment need not mean that the liquid reached all the way to the Martian surface. NASA Ames scientist Chris McKay and others have studied lakes in Earth’s Antarctic continent that are covered by a layer of ice year round.
They sustain photosynthesizing algae. The liquid region below the ice is maintained in large measure by the warming effects of sunlight, transmitted through the clear ice into the liquid water below. In this respect, the ice-covered lake is analogous to a greenhouse atmosphere. However, the lake liquid also is stabilized by the warming effect of freezing itself, which releases heat. (This is the converse process to the cooling of a drink by the melting of ice cubes.) Calculations and field observations in the Antarctic suggest that such lakes are stable for temperatures as low as 240 K, fully 33 K below the freezing point of water.
Another possible birthplace of life is hydrothermal systems in the early Martian crust, regions where liquid water circulates in the rock and is warmed both by heat flowing from the interior and by the insulating effects of being underground. At Earth’s mid- ocean ridges, hydrothermal systems are rich in the chemical and thermal energy needed to support an array of living organisms in the complete absence of sunlight. Such could have been the case on early Mars.
It is difficult to estimate how long either of these two types of ecosystems – surface bodies of liquid and hydrothermal systems – might have lasted on Mars. The groundwater hydrothermal systems are particularly problematic in this regard;
we simply do not understand the details of Martian geologic his- tory sufficiently well to predict where such systems could have been most long lived.
With regard to the ice-covered lakes, McKay and colleagues argue that they could have been maintained for some 700 million years after mean annual surface temperatures fell below the freezing point on Mars. If, for argument’s sake, Mars had a warm climate continuously or episodically up through 3.5 billion years ago, then ice-covered lakes might have contained liquid water as late as 2.8 billion years ago. At this time on Earth, life was still solely in the form of single-celled prokaryotes, and the dramatic changes in our atmospheric composition wrought by such organisms (Chapter 17) had yet to take place.
We thus expect that any life on Mars would have remained at the single-celled stage at the time of its extinction. Life might have been sustained to the present day if the life formed in sur- face lakes or hydrothermal systems transitioned to deep environ- ments in the crust, where liquid water may still exist today. The recent discoveries of bacteria living several kilometers beneath the surface of the Earth, in rocks that allow access to water and nutrients, hint at possible environments for life on present-day Mars. Perhaps beneath the surface of Mars, warmed by deep interior, simple Martian biota carry on.
15.3.3 Searching for evidence of life, and the early climate
It is a daunting task to explore other worlds, and even more so to search for evidence of microscopic life-forms that might be rare or hidden in inaccessible places. The search for evidence
of past Martian life would involve looking for physical evi- dence (for example, stromatolites in ancient sediments), chemi- cal evidence (ratios of carbon or other isotopes in rocks that are unusual except in biological processes), and mineralogical evi- dence (types of minerals that are not normally found together, except when formed through the mediation of biological activ- ity). The search for extant life requires techniques to stimulate and detect metabolic activity or (more difficult) to isolate, repro- duce, and then study the genetic materials of living organisms.
The more exotic a Martian organism is to Earth life, the more challenging is its detection. Such searches must be conducted in regions of Mars where life was most likely to have formed and been sustained, and these are invariably the most difficult and dangerous sites to reach. For cost reasons, the search will continue to be conducted using robotic vehicles, most likely rovers as has been done withSpiritandOpportunity, but also with sensitive atmospheric sensors on orbiters – an approach that has already led to an intriguing result in the discovery of CH4(methane) in the Martian atmosphere.
Observed both by the European Mars Expressorbiter and large Earth-based telescopic observations, methane was released into the Martian atmosphere in 2003 at levels of about 20 to 30 parts per billion. The methane seemed to be clustered over three different areas, including Nili Fossae, which is found to contain phyllosilicates (water-bearing minerals). By 2006 the abundance of methane had declined by a factor of two or more, and it has not been observed since. Although methane is destroyed by ultraviolet light in the Martian atmosphere, this process takes hundreds of years, and the rapid decline of the methane is a mystery (some have even suggested that the original obser- vations were mistaken). If methane really was present in the Martian atmosphere, it could be the signature of biological pro- cesses, plausibly deep in the Martian crust where water and minerals might be present to sustain metabolisms. However, methane can also be produced by the reaction of carbon diox- ide with water, in the presence of certain types of minerals, in a process called “serpentinization”. Methane produced in this way could be a food source for life in the Martian crust, but would not be evidence of such life. A possible discriminant between biologically and nonbiologically produced methane is the ratio of two stable isotopes of carbon13C/12C, which differ in organic molecules processed by living versus nonliving sys- tems (see Chapter 6), but the measurements fromMars Express and Earth-based telescopes are not sensitive enough to mea- sure the isotopes. A future mission to Mars will be required to do this.
Although the discovery of evidence for past life on Mars would be a profoundly moving moment in human history, we must be cautious in how such a discovery would be interpreted.
The existence on Earth of rocks blasted off of Mars in impact events shows that Earth and Mars have exchanged material over their histories. If life first formed on Mars, it might have seeded the Earth with life by this process of “impact exchange”, or (less likely because of Earth’s higher gravity) vice versa. To determine whether life on Earth and Mars had separate origins – a truly momentous discovery – would likely require analyzing preserved DNA from Martian organisms in terrestrial laborato- ries. Faint chemical signatures of past life on Mars could not establish whether or not it had a separate origin from life on Earth (Figure 15.8).
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Figure 15.8Conceptual exchange of genes between life on Earth and putative life on Mars by hypervelocity impact. Because the rate of impacts was much larger early in the history of these planets, the rate at which one planet’s biota was contaminated by the other was larger than it is today. The left hand graph shows the rate of impacts with time normalized to a value of 1 for today. The right hand graph shows in a cartoon form the result: since a small amount of material deep inside a meteorite may not be shocked or heated to lethal values, organisms transported off one surface may survive landing on the destination planet. Thus Mars and the Earth have contaminated each other numerous times over their history, more frequently near the beginning.
Life on Earth, and life on Mars if it exists, may have had a common origin. Figure provided by C. H. Lineweaver, from Lineweaver (2004).
Whether we find life on Mars – extinct or extant – or fail to do so, exploration of Mars is important for other reasons. A vigorous campaign to do so will pay off because the nature and demise of putative warm conditions on early Mars remains an unsolved problem. To solve it would be to gain a deep insight into the remarkable stability of our own planet’s climate over four billion years.