Mars, Mars, Mars:
Life on Mars
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The exploration of space is exciting for many reasons, but perhaps paramount among them is the distinct possibility of our discovering some form of life that arose independent of life here on Earth. The consequences and implications of any such discovery would be enormous, for science, sociology, religion . . . indeed, virtually every aspect of human activity. Mars has always loomed large in theorizing about such possibilities--sometimes with reason, sometimes without. This page will give you a rough idea of whether the hope (or fear) of finding life on Mars (or anywhere) is a plausible one.
Overview
"Life" is not an easy term to define in a general way. But if we are satisfied to pass by exotic and improbable possibilities, life--as we understand the term--is built on certain chemical processes based heavily on the element carbon, which is about the only one capable of forming the highly complex molecules cellular life is necessarily built on. Thus, the environmental requirements for a place where life (again, as we now understand it) might arise are remarkably narrow: a goodly amount of carbon and hydrogen for the cell structures, oxygen to fuel the reactions that produce biological energy, a neutral liquid--water or perhaps ammonia--for the reactions to take place in, and a range of temperature not so cold as to make the chemical reactions difficult or impossible and not so hot as to break down the complex organic molecules. Whether such conditions exist on planets circling other stars is currently impossible to say, though a long-standing search for alien interstellar signals continues; but within our own solar system there is only a small handful of candidates, of which Mars is the likeliest.
Before one can think of searching for life beyond Earth, one must have a clear and definite idea of just what constitutes "life". The matter is not as obvious as it might at first seem: even here on Earth, there are borderline cases, such as viruses. Is a virus truly "alive", a living organism? On the one hand, viruses have genes and show inheritance; on the other hand, they cannot reproduce except within a host cell, whose existing enzymes and other molecules they use. Viruses are essentially large, complex molecules; many can be crystallized, and in that form bear little or no resemblance to "living" matter--but, given a suitable environment, they will re-activate. Viruses are commonly said to be "somewhere between being living and non-living." So the definition of "life" is not so simple at all.
Elementary biology as usually taught at the high-school level offers a few simple rules for classifying something as being "alive". Such a thing exhibits:
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| Not alive? |
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| Alive? |
That those criteria--which may look adequate at first glance--are sorely lacking is obvious when we see how easily (even excluding borderline cases such as viruses) some things we would never class as "alive" (such as stars) meet them, and others we certainly would call living (such as mules) fail at least one of them.
Worse, many more "advanced" definitions are clearly based largely or wholly on our experience of life here on Earth. For example, one definition holds that life is a "self-producing, water-based, lipid-protein-bound, carbon-metabolic, nucleic-acid-replicated, protein readout system". That's a mouthful, but all it amounts to saying is that life is that which is common to everything we have already decided is "alive".
Some--though not all--of the more risible problems in the "classic" definitions can be overcome by applying the criteria either more broadly (at the species level, for example) or more narrowly (at the genetic level, for example). But when one speaks in a general way of "life", especially of finding "life" that has arisen elsewhere than on our Earth, one almost always finds that what is being discussed is an organism that, at its most basic level (meaning the cellular) is at least closely analogous to "life as we know it".
It is worth pointing out here that the science of chemistry is classically considered to have two subdivisions: inorganic chemistry and organic chemistry. It turns out, though, that "organic" chemistry is essentially the chemistry of compounds involving the element carbon. That is because the atomic structure of carbon is such that it is virtually unique among the elements in its ability to bond with itself and many other elements to form large, highly complex molecules--the sorts needed by such a complex activity as being alive. For that reason, the presence of carbon in reasonable quantities in a given environment is considered essential to the possibility of "life" arising in that environment.
Certain other related criteria are also typically held to be essential for life to have a chance to arise. One is the presence of ample amounts of hydrogen (though hydrogen is present almost everywhere in the universe). Another is the presence of a suitable "oxidizing" element, with (as the name of the process suggests) oxygen itself being the best suited--oxidization must be easy to achieve, because oxidation reactions are the only method we know of whereby living cells can generate the energy they require to exist. Finally, some suitable large-scale liquid medium is needed in which the chemical reactions can proceed; liquid (as opposed to solid or gaseous) water is the medium most suited, but liquid ammonia, which is not common but not rare, could also suffice.
Attempts to generalize beyond criteria based solely on known life forms are, in the main, based on consideration of what a "living" things do, rather than what they are made of or how they do those things (for example, life is "that which maximizes its range of possible futures").
These many alternative definitions of "life" themselves often have problems of the same sort as the classical: they can include things we wouldn't normally think of as "alive", and they can exclude things that we might consider "alive". Viruses remain one interesting "testing ground" for any suggested definition of "life". Another is computer programs: it is quite possible to write a computer program that will meet the classical criteria for "life"--the classic example is named simply "Life" (at the right is a stable repeating sequence possible in "Life", called a "glider gun", which perpetually launches "colonies"--click here for some yet further demonstrations of "Life" sequences).
Though science fiction of the worse sort throws bizarre--and quite impossible--aliens out at the drop of a hat, science fiction of the better sort has suggested several fascinating, unlikely but possible kinds of non-classical life forms, many of which would, should they exist, be capable of evolving "intelligence"--another amazingly slippery term. (Just as one example, consider beings composed of fluctuations or patterns in energy fields.) Regrettably, while such suggestions are of academic interest, they are--so far as we can see at present (which, as history teaches us, may not be as far as we suppose)--too improbable to be worth expending serious effort on as a search target.
The most interesting (to put it mildly) discovery of extraterrestrial life would be the finding of evidence of an intelligent civilization. For many decades now, it has been accepted that no such thing can exist (other than our own) within our Solar System, so such a search must seek at other stars. There are two chief questions to consider in organizing such a search: what is the likelihood of their being an intelligent civilization to find? and how would we detect one if it existed?
A relatively early answer to the first question was set forth by a by Dr. Frank Drake in the 1960s, and is now generally known as "the Drake Equation". It attempts to estimate the number of intelligent civilizations that might exist in our Milky Way galaxy at any given moment. It involves factors for the probability of planets existing at any randomly selected star, of the probability of an existing planet being able to support life (at least "life as we know it"), the likely duration of an intelligent civilization, and several other like matters. Very obviously, most of those factors are not far, even today, from being pure guesswork, so it is not surprising that different scientists, using the Drake Equation, still come to very different answers about the overall odds. One big boost to believers in good odds has been the recent discovery that planet formation is extremely common--the rule, rather than the exception. In effect, though, you can get any answer you want by choosing estimate terms that will favor your answer, and it is hard to argue for or against any given estimate. Still, a strong argument can be made (and has been made) that however low the chances of finding evidence of an alien civilization, that chance is worth taking because of the stupendous significance such a discovery would have for humankind.
But even given the possibility of there being one (or more) such alien civilizations to be discovered, how should one go about looking? Scientists came to the simple enough conclusion that there would be no point looking for what could not be found--that is, for signals (necessarily as some form of electromagnetic radiation--a term that encompasses such things as light, radio, microwaves, "X-rays", and "gamma rays", among others) that couldn't reasonably be expected to reach Earth from a far star. The problem is that interstellar space is not truly "empty" except by terrestrial standards; in reality, space is full of clouds of gas and dust (see image at left), sometimes quite thick (where gravity is pulling them together), sometimes quite thin. But even a very thin gas, over the monstrous distances between stars, adds up to a lot of blockage; and it turns out that only a relatively narrow spectrum of wavelengths can get through the interstellar medium without much loss. In that spectrum, there is one particular wavelength, associated with a basic natural phenomenon occurring in the base element hydrogen, that suggests itself as a "natural" frequency (but see the comment farther below) on which to send signals across interstellar space.
Coordinated searches, collectively referred to as SETI (Search for Extra-Terrestrial Intelligence), have been under way, in one form or another, since 1960's Project Ozma; and they continue, ever more intensified, today. (An especially interesting note is the development of the "SETI@home" project, whereby anyone anywhere in the word with a computer and an internet connection can "lend" otherwise-idle computer time to the SETI effort. (At right: a radio-telescope antenna, part of the SETI effort.)
Hope and optimism notwithstanding, there are strong arguments against our finding intelligent life at other stars; the strongest is the so-called "Fermi Paradox", sometimes epitomized as "Where are they?" More fully, it runs like this:
While superluminal (faster-than-light) travel is not considered possible within our present understanding of the laws of physics, and even close-to-light speeds would be very difficult to achieve, interstellar travel at some modest fraction of the speed of light--say ten percent--are perfectly practicable, and indeed not far beyond our own present abilities. If one assumes that: a) the stars in our galaxy are, on average, ten light-years apart; b) that an interstellar mission could travel at an average of 10 percent of the speed of light; and c) that such a mission, arrived at its target star, could itself send out a new mission within four centuries of its arrival; then the "doubling time" for the number of the interstellar colonies of such a civilization would be 500 years. That, in turn, means that such a civilization could colonize the entire galaxy within five million years.
Indeed, even if one takes more pessimistic numbers--travel at only 1 percent of the speed of light, and thousand years before a colony could send out a new mission--the galaxy would still be completely populated in 20 million years. Twenty million years may sound a long time, but on an astronomical or cosmic scale it isn't very much (it's less than 0.15% of the life of the universe to date).
And even if we discard the idea of some one civilization relentlessly colonizing the galaxy for millions of years, there still should be, if technological civilizations exist at all elsewhere in the galaxy, quite a substantial penetration of settlements.
So, since we have yet seen no evidence at all of the presence of any such civilization, Fermi's argument is that there are no such interstellar civilizations.
Now that may seem persuasive, but it has many logical holes in it. For one, it assumes that civilizations technologically capable of developing interstellar travel necessarily would choose to do so--and, further, that if they did, that they would relentlessly seek to colonize every place they could as quickly as they could; those assumptions reflect only human nature, and are by no means a requirement for an advanced non-human civilization. Also, the assumption of a continuity over millions of years in a particular civilization is shaky at best. Even assuming that humankind is unusually self-destructive for an advanced civilization, a continuity of millions of years seems questionable; but that does not at all mean that there might not be many such civilizations around in the galaxy at a given moment. (Refer back to the Drake Equation comments.)
Moreover, while the SETI efforts have found nothing yet, it would be remarkable if they had, even on optimistic assumptions, in that there is so much to search, in terms of both actual stars and of possible frequencies of communication.
Finally, the "highly logical" and "natural" methods we postulate that intelligent aliens would use are extremely questionable; scarcely a century ago, our best guess was that aliens would build giant bonfires across their planet to signal their existence--our assumptions of today may look as silly as that to the human science of a century from now. In short, we may be looking in all the wrong places. (Much more discussion can be found at the web site of the SETI Institute).
If searches of the galaxy for intelligent life are fraught with difficulties, searches for life of any sort, however primitive, are substantially easier (not easy, just easier) to make, because we can look within, astronomically speaking, our own back yard: the Solar System. Such looking logically divides into two parts: seeking places with conditions in which "life as we know it" might plausibly have arisen; and then actually seeking evidence (or the lack of it) that life is present at such a place.
Planetary science has made huge strides in recent years as to methods and tools for making observations and obtaining data--including not only ground-based tools such as optical telescopes, radio telescopes, and even X-ray and gamma-ray telescopes, but also orbiting observatories (such as the wonderful Hubble observatory pictured at the left) and deep-space probes (such as the Cassini-Huygens mission to Saturn--artist's impression at right). As a result, we are now fairly sure that extraterrestrial life --but again, and always, "life as we know it"--is highly unlikely ever to have arisen on any planets of our Solar System other than our own Earth and Mars; but we also now believe that there is at least a possibility that life might have arisen on one (or more!) of several of the many moons of two of the system's gas giants: Jupiter and Saturn:
Each of those moons is thought to have one or more substantial bodies of underground liquid, as well as the other life requirements mentioned before, so that life might have arisen on any of them, even given their somewhat harsh conditions, in a manner not unlike the evolution of life in Earth's oceans near deep-sea vents.
The moons mentioned above are the leading, not the sole, candidates for extraterrestrial life within our Solar System. But it remains true that of all the candidates, Mars is, by our current understanding, the likeliest. So, though we have actually dropped a spacecraft onto the surface of Titan--one of the greatest achievements yet for the human race--our chief search focus, owing both to probability and accessibility, remains Mars.
(We will not here further discuss the actual exploration of Mars, because there is a whole separate page of this site on that topic.)
Present conditions on Mars are by no means implacably hostile to life as we know it, but neither are they inviting:
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Year: |
687 Earth days |
The surface of Mars today. The most notable statistics from the Table at the left are the surface temperature, the atmospheric pressure, and the atmospheric make-up. The present-day atmosphere of Mars is plainly very thin--its pressure is less than 1% of Earth normal--and it is almost entirely composed of carbon dioxide; the oxygen content is nearly negligible. The Martian temperatures largely speak for themselves. They are not such as to make life impossible--especially at the upper end--but they are not "life-friendly". But remember that those are surface temperatures: temperatures even a small ways down into the soil are likely to be much less drastic. |
| Equatorial diameter: | 4,230 miles (= 53% of Earth's) | |
| Surface area: |
c. 56 million square miles (= 28% of Earth)
(roughly equals Earth's land area) |
|
| Equatorial gravity: | 37.6% of Earth normal | |
| Day: | 1.026 Earth days (24.623 Earth hours) | |
| Axial tilt: | 25.19° (Earth's tilt is 23.45°) | |
| Albedo (reflectivity): | 15% (Earth's is circa 38%) | |
|
Surface
temperatures
- minimum - average - maximum |
-220° F. -82° F. +68° F. |
|
| Atmospheric pressure: | 0.7-0.9 kPa (less than 1% of Earth normal) | |
|
Atmospheric composition:
Carbon dioxide: Nitrogen: Argon: minor amounts: Oxygen: Carbon monoxide: Water vapor: Nitric oxide: Neon: Krypton: Xenon: Ozone: Methane: |
95.32% 2.7% 1.6% 0.13% 0.07% 0.03% 0.01% 2.5 ppm 300 ppb 80 ppb 30 ppb 10.5 ppb |
Nonetheless, it would be wrong to think of Mars, even today, as a "dead" planet, sterile and unchanging. The fascinating animated image to the right--a composite of actual photographs
from the surface of Mars--shows a Martian
"dust
devil", literally graphic evidence of an active weather system.
Besides winds, there are other phenomena of true weather: while there is at present little or no liquid water on the surface of Mars, It does have polar ice caps that are a mix of frozen carbon dioxide and ice; in the Martian summer, those caps largely or wholly melt, and the water vapor enters the atmosphere, forming cirrus clouds (see photo at left) and, occasionally, light frosts (see photo at right).
But the biggest news by far concerning present-day Mars has only quite recently--early 2005--come to light: there is at least one frozen sea on Mars.
Even more recently--autumn 2005--evidence surfaced that there were almost certainly substantial glaciers on Mars from a relatively recent (a few million years ago) period, not from the generally acknowledged early "wet" period of about 3.8 billion years ago (we'll return to that in the discussion below on early Mars); it is even possible that some glaciers may yet exist, buried under a light covering of Martian surface soil. These discoveries of major climatic activity on relatively "modern" Mars have large implications for the question of possible life on that planet.
Especially fascinating is the relatively recent discovery of traces of methane in the Martian atmosphere. Even though that presence is minute, as the Table above shows, it is important, because methane is a quite unstable gas, and would almost certainly disappear completely from the present-day Martian surface within a few centuries--a geological eyeblink. Thus, its presence today indubitably signifies that there is (or must till quite recently have been) a source releasing methane into the Martian atmosphere. There are several possible sources for methane release, including volcanic action; but one common source of methane on Earth is microbial action--life. Mars has a good number of huge volcanoes, but all are now extinct; but it is not impossible that there are some minor seepages of gases still taking place. We will have a better idea of what's what when later missions look for other gases in the patches where the methane has been detected: sulfur dioxide would suggest vulcanism, while ethane would suggest biological processes.
Many observers believe that there is a great significance to the fact that the methane is apparently being detected most notably over the newly discovered frozen sea (photo at right). If that turns out to be definitely so, some Mars scientists feel it could signify that primitive micro-organisms might have survived right up to the present. The discussion, and the hunt for more and better data, continues.
Complementing the methane findings has been the apparent discovery of ammonia in same locations. That, if verified, must add substantially to the estimated likelihood of life even on present-day Mars, because ammonia exposed to the surface of Mars would have a lifetime of mere hours--so necessarily something would have to be continuously and actively emitting ammonia right now. Even confirmation, however, while it adds to the evidence, is by no means definite proof: while ammonia gas is a common by-product of microbial life, there also remains, for one example, the possibility of a volcanic origin for the ammonia gas.
One last point that deserves mention here is that continuing researches here on Earth have recently yielded relevant information: while we have long known that there are elementary life forms that can survive in quite extreme conditions (and are thus referred to as "extremophiles", the meaning of "extreme" in that context has been significantly expanded. Obviously, the known abilities of terrestrial extremophilic life yet further raise hopes that if life ever arose on Mars at all (or on any one or more of the other candidate bodies in the Solar System), it may well still survive.
(It should be noted that as recently as 2002 a few reputable planetary scientists were strongly suggesting that there might even now be something more than microbes present: primitive vegetation--algae or lichen--they think, might exist on the surface of Mars; most others disagree, but the door is not completely shut.)
In short, while there are tantalizing evidences, which seem to be accumulating as we learn more, that there might even today be some sort of life on Mars, if such life exists it will almost certainly be very "primitive"--microbial in nature--and will have adapted, over geologic ages, from life that would have first arisen long ago during an era when surface conditions on Mars were very different from what they are today. Such life would most likely not be on the actual surface of Mars, but would likely exist within the soil of Mars; on Mars as on Earth, soil temperatures at even small depths are much more stable than they are at the surface.
"The fact that there have been warm and wet places beneath the surface of Mars since before life began on Earth, and that some are probably still there, means that there is a possibility that primitive micro-organisms survive on Mars today. This mission [the European Space Agency's Mars Express orbiter] has changed many of my long-held opinions about Mars--we now have to go there and check it out."
--Dr. John Murray, leading planetary scientist
(The complete story of searches for evidences of life, current or past, is discussed on the separate page of this site on the exploration of Mars.)
If there is or ever was life on Mars, that life surely arose there--as on Earth--far back in planetary history, billions of years back. At that time, conditions on Mars were strikingly different from what they are today. We will now take a closer look at those conditions.
The early Mars was likely not drastically different from the early Earth. (How the planets formed is discussed on a separate page of this site.) Looking back at the table above, we see that the length of it's day is almost identical; and its axial tilt--which determines the intensity of seasons--is also highly similar.
Although the weakness of Mars's current magnetic field once suggested that its core, originally as liquid as Earth's still is, had by now solidified, newer work shows that it is still at least partially molten, and possibly entirely so. The state of a planet's core is important because that core generates the planetary magnetic field; that field, in turn, is important when considering possible life on a planet, as we will see momentarily.
How and why Mars's magnetic field today is relatively weak even with a liquid core is a question yet to be answered (one good guess is that the core is not circulating fast enough to be much of an electromagnetic dynamo). But much more important is that current data demonstrate clearly that in earlier geological times Mars did have a substantial planetary magnetic field. The estimates are that Mars's planetary field did not seriously weaken till about 3.5 to 4.0 billion years ago; that sounds like a long time, but it means that Mars did have a "magnetic shield" in place for its first 1.0 to 1.5 billion years, definitely a nontrivial duration.
As the word "shield" might suggest, a magnetic field is important for questions of life on Mars. The reason is twofold:
First, such fields, if present, greatly reduce the amount of high-energy "cosmic radiation" that reaches the planet's surface from surrounding space; without such a magnetic "shield" to deflect that radiation, life--while it can readily survive, and has (on Earth) survived such high radiation levels--is perhaps less likely to have formed: the molecules that constitute living things are large and highly complex, which makes them especially vulnerable to damage from high-energy radiation (which is why "radiation poisoning" is so dangerous).
We know today that the Earth's magnetic field has reversed polarity many times, the north and south magnetic poles exchanging places. In such a changeover--which happens in a remarkably brief time, perhaps 5,000 years or so--the magnetic field drops to essentially zero strength, then builds back up with the reversed polarity; during much of that changeover time, the reduced field results in a substantial increase in the levels of radiation reaching Earth's surface. While such periods in the past do not appear to have done any major damage to the life then existing on Earth--early humanoids certainly survived some--there is some tentative evidence that mutation rates may have increased significantly during those periods, with a corresponding increase in the rate of species evolution. The average time between reversals is about 250,000 years, but that can vary immensely--at least once it was 30 million years. Right now, we appear headed for another reversal: for the last 150 years or so, the Earth's magnetic field has been weakening at a rate that, if it continues, would cause the Earth's magnetic field to drop to zero or near-zero sometime between 3000 A.D. and 4000 A.D.
Second, and even more critical, the planetary "magnetic shield" acts to keep the "solar wind"--a stream of high-energy particles originating in the Sun--from acting to strip away the planet's atmosphere. That stream is not called a "wind" for nothing: when it hits a planet full force (or nearly so), it literally blows away the outer parts of that planet's atmosphere. Over sufficient time, it will blow away most of that atmosphere. That is why Mars today has only a very thin atmosphere remaining: for between three and four billion years, the solar wind has been blowing that atmosphere away into deep space. But the other face of the coin is this: for a geologically significant span--perhaps 1.0 to 1.5 billion years--the early Mars did have a significant atmosphere, owing to its early magnetic field mostly blocking that solar wind. (The "wind" consists of ions, which are electrically charged particles; electrically charged particles moving in a magnetic field are "pushed" sideways by that field, so a planetary magnetic field literally deflects incoming ions off away from the planet.) We will return to discussing the atmosphere of early Mars in a moment.
Other crucial information has come from the careful recording of the present-day magnetic field of Mars: those data show clearly that large parts of the Martian crust--perhaps most of the surface--are magnetized in alternating stripes roughly averaging 100 by 600 miles; that is a pattern notably similar to what one sees on ocean floors here on Earth, and scientists feel that those data thus strongly suggest that Mars once had surface oceans. Further evidence for extensive Martian surface water in the geological past comes from the presence of various types of rock and mineral deposit (notably hematite, jarosite, and goethite that, in our Earthly experience, characteristically form in a watery environment.
The existence of large bodies of surface water for geological lengths of time is critical to our belief in the possibility of life having arisen on Mars: unless such oceans existed, the probabilities are very low. The evidence we already have to hand now seems so strongly in favor of such conditions that in the 9 December 2004 edition of the respected scientific journal Science, a group of some fifty scientists co-wrote an article remarking in part:
"Liquid water was once intermittently present at the Martian surface at Meridiani, and at times it saturated the subsurface. Because liquid water is a key prerequisite for life, we infer conditions at Meridiani may have been habitable for some period of time in Martian history."
While that is seasoned with the usual cautious qualifiers--"may have been", not "was"--it is a remarkably clear and, as such things go, strong assertion. And, while it applies to one particular region of Mars, there is now also perhaps even more suggestive evidence from other regions (the "Columbia Hills") as well. Moreover, there is the evidence of the physical surface, which shows many features that strongly suggest that they result from erosion owing to flowing surface water (see photos at left and right). All taken with all, at least for right now it looks highly likely--many would say near-certain--that there was ample surface water on Mars for long enough periods for it to have been suitable as a starting place for life; that doesn't necessarily imply that it was such a cradle, but one major necessity for life seems to be firmly in place.
(The same magnetic data also strongly imply that in its youth the geology of Mars was driven by plate tectonics, just as Earth's was and still is; plate tectonics signifies that a planet's surface consists of a number of separate plates floating on a semi-fluid planet-wide underlayer--the asthenosphere--and slowly moving east to west, owing to planetary revolution; where such plates "bump up" against one another, as where the North American plate meets the Pacific plate or the Indian plate meets the Eurasian plate, mountain ranges tend to form, and earthquakes and volcanos are common. Plates moving west tend to "dive under" the leading edge of the plate they are meeting, and so plates very slowly enter and emerge from deep in the planet's crust, and this geologically slow "churning" over time brings new supplies of some elements and compounds from the inner planet up to the surface.)
Now let's get back to the early Martian atmosphere.
We said above that the early Mars was likely not drastically different from the early Earth. That is so because as the planets of our Solar System coalesced from dust and gas clouds (as we describe elsewhere on this site), the inner planets each formed a rocky ball, typically with a core of molten metal (heated partially by the decay of radioactive elements and partly by the pressure of the material above that core) and a surrounding atmosphere.
Before local conditions operated to effect changes, that atmosphere would initially have been pretty similar from planet to planet, simply reflecting the chemical makeup of the solar disk from which they all formed. We have already seen that a planetary atmosphere not protected by a planetary magnetic field will, in time, be blown away off into space by the Solar wind; but there are other processes at work even on a "shielded" planet.
The gases in a planet's atmosphere are forever liable to being literally "boiled off" into space--after all, the only thing that holds any gas molecule to a planet is that planet's gravitational pull. An atmosphere is just a mixture of various gases (which by and large act independent of one another, because they are a mixture, not a chemical compound). Now a gas, in turn, is just a collection of atoms or molecules floating about under some constraint (for example, physical barriers or gravity--see illustration at left); and the temperature of a gas is simply a measure of the average kinetic energy (energy of movement) of its individual molecules--the hotter a gas is, the more energy, on average, each molecule of it has, which is just to say that the hotter a gas is, the faster (on average) each of its molecules is moving. (Consider: a car crashing into a brick wall at 60 miles an hour will deliver a lot more energy to that wall than the same car moving at 20 miles an hour--that is, the faster a thing is moving, the more "kinetic" energy it has.)
The direction of motion of the molecules of a gas is random: at any instant, each is moving in some arbitrary direction (they change direction by randomly colliding with and "bouncing off" other molecules, like the balls on a billiards table). In a planetary atmosphere, a highly energetic molecule that happens to be moving more or less upward can--if it is moving fast enough--escape the planet's gravitational pull (just like a space rocket does by moving fast enough).
If the planet (or moon or asteroid or whatever) is small, its gravity is weak and it cannot hang onto even fairly low-energy (cool) gases; even if the planet is of moderate size, it may have trouble hanging onto highly energetic (hot) gases. The very earliest planetary atmospheres--mostly hydrogen and helium--probably boiled off fairly quickly on all the four inner planets, but they were constantly being replaced by heavier gases (nitrogen, carbon dioxide, the "noble gases", and others) emitted by the planet itself through volcanic activity and plate-tectonic activity.
As the early Solar System evolved, each planet's atmosphere changed in makeup as those inevitable processes went to work. Mercury, fairly small and very close to the Sun, soon lost its atmosphere; giant Jupiter, relatively far from the Sun, held onto (and still holds onto) even the lightest of gases, hydrogen and helium. Mars--which, like the other Solar planets, formed roughly 4.6 billion years ago--probably held onto a reasonably dense atmosphere for a good billion or more years. And that's a long time.
The geological history of Mars is conventionally divided into three eras (or Epochs), which are though to roughly mark stages in the planet's development. The earliest is referred to as the Noachian Epoch, the middle one as the Hesperian Epoch, and the latest (and current) one as the Amazonian Epoch; all of those names are taken from particular regions on Mars. The Noachian Epoch lasted till about 3.5 to 3.8 billion years ago, and thus was--assuming the 4.6-billion-year age usually reckoned for Mars--from 0.8 to 1.1 billion years long, which we can conveniently call a billion years (what's a few million years among friends?); it is during the Noachian Epoch that Mars had a reasonably dense atmosphere and liquid surface water. (Do understand that these Epochs are convenient discussion terms, and that there was not any sharp transition from one to the next.)
To understand the Martian atmosphere in the Noachian Epoch, and why and how it changed into something so different from our own, we have to keep in mind the remarkable extent to which various processes that may appear at first unrelated actually interact in important ways. On Mars, a critical factor was the transition from a planetary surface activated by plate tectonics (as we discussed briefly above) to a cooled-down, essentially solid one. On Mars as on Earth, the carbon dioxide in the early atmosphere reacted with material in the Martian surface and got "locked away" by those chemical reactions into carbonate rocks. On Earth, and on early Mars, tectonic activity recycled that trapped CO2 out of the rocks and back into the atmosphere. But the Earth is notably larger than Mars, so the early Mars cooled more and more rapidly than did the early Earth (because the ratio of its surface area (which is where heat is radiated away) to its mass was much higher--roughly double Earth's--as simple geometry shows).
And, as the Martian surface cooled--and the plates joined into a solid surface--such recycling could no longer take place. That created a vicious cycle: cooling slowed tectonic activity, which slowed the return of CO2 to the atmosphere, which reduced the greenhouse effect (discussion and illustration farther below), which increased the cooling. Today, though Mars's atmosphere is almost entirely carbon dioxide as a percentage, the actual amount of CO2 is not great because that atmosphere is so thin.
Meanwhile, the planet-wide cooling and crustal solidification was also reducing the Martian "magnetic shield", the planetary magnetic field, by reducing movement in the planetary core and so slowing that core's "dynamo effect", which produces the field. That, as we have already seen, accelerated the escape of much of Mars's atmosphere into space. One notable component of that loss was the atmospheric water vapor: on Earth, with its moderate temperature range, the early-atmosphere water vapor eventually condensed out into the oceans that cover so much of our planet today; on Mars, the water vapor was mostly driven off into space before it could condense. And on Earth, most of the CO2 remaining in our atmosphere dissolved into those new oceans, leaving our atmosphere as mainly nitrogen (oxygen was to come later, from processes we'll explain in a moment). Another notable Martian loss was the lightweight gas nitrogen.
Again we have a vicious cycle at work: as the atmosphere thinned out, it provided less and less shielding against the Sun's ultraviolet rays. Those rays, being quite energetic compared to visible light (think beach and UV sun-block lotion), were more and more able to break down such molecules of water vapor as were still present in the atmosphere into their constituent gases--hydrogen and oxygen--which, as separated gases, were more easily lost into space than the heavier water vapor itself. And so the atmosphere got thinner yet, and so things went (literally "went").
What water vapor did remain on Mars as it got colder and colder eventually either combined with chemicals in surface rocks to be trapped there, or froze out into such ice as remains today (which we now find may be considerable in bulk). On our Earth, in the early stages of its planetary life, while it was still largely or wholly molten, most of its iron sank into its core, which is why Earth has a large core of molten iron (and a healthy magnetic field); on Mars--it being much smaller and, as we have seen, quicker to cool--not all of its iron had time to sink to its core, and its surface retained much more iron than did Earth's. Thus, much of the water--liquid or vapor--on Mars, as well as such little free oxygen as it may have had, chemically combined with that iron to form iron oxide--rust--whence the planet's characteristic reddish color.
But though we now see how the early Martian atmosphere became what we find today, we must not lose sight of the reality that for most or all of the Noachian Epoch, Mars did have a substantial atmosphere, including carbon dioxide and some nitrogen. That is critical for the possible development of life there.
We should also say another few words about surface temperatures doing the Noachian Epoch. Obviously, the very existence of liquid water in quantities sufficient to etch visible features in the surface tells us that temperatures, at least at one time, must have long been within a range comfortable for life processes--else that water would have been ice or steam. One suggestion for planetary warming is the "greenhouse effect" (see illustration at right), and the large amount of carbon dioxide that there must have been in the early Martian atmosphere makes that quite plausible; but whether the greenhouse effect by itself was sufficient to maintain mild temperatures on Mars is still being debated. Another possibility suggested is energy released from bombardment of the planet by meteorites, though there are problems with that (the visible craters seem younger than the presumed water-erosion channels). Yet another suggestion recently made is that small amounts of sulfur dioxide in the early Martian atmosphere--possibly from volcanic outgassing--could, by absorbing ultraviolet energy from the Sun, have warmed the middle atmosphere (as the Earth's ozone layer does for us today), which would have allowed the Martian greenhouse effect to operate more strongly. But, regardless of mechanism, the evidence is that for geological ages the planet was far from being the icebox it is today.
The first life forms on Earth--and, we would suppose, on Mars if it had any--were "prokaryotes": extremely simple microbes, lacking a cellular nucleus, or, really, any true cell structure at all. (It is usual to equate prokaryotes with bacteria, though the correspondence may not be exact.) The chief feature of interest of these earliest life forms to us here is their chemistry: they rely on photosynthesis for their energy supply. In photosynthesis, which current terrestrial plants rely on to this hour, the organism takes in water and carbon dioxide gas and--using the energy of sunlight--chemically converts them to certain sugars and free oxygen; so such organisms could and would (and on Earth did) thrive in the early atmosphere, which was rich in carbon dioxide but almost wholly lacking in oxygen. Moreover, most prokaryotes are extremophiles--the sorts of life we mentioned earlier, which thrive in strikingly inhospitable environments, such as extreme temperature ranges.
On Earth, the early prokaryotes proliferated so richly that over geological time they wholly transformed Earth's atmosphere by their release of oxygen into it. In time, that oxygen was sufficient to make an ozone layer--ozone is simply an energy-pumped form of oxygen, O3 instead of the usual O2--which layer acted as a shield against the Sun's ultraviolet rays and made possible the existence of life on the surface of the Earth (as opposed to only in its oceans). That UV-shielding effect not only was but still is essential for dry-land life on Earth, which is why there is so very much concern about the damage our current industrial society is doing to the ozone layer with the gases we release into the atmosphere. Moreoever, when the fraction of our atmosphere that is oxygen rose high enough, it became possible for animal life as we know it to develop, using that newly made atmospheric oxygen (which was also present in solution in ocean water) directly to make cellular energy; in animal bodies, including our own, every cell is washed in a supply of blood, which carries oxygen to those cells in the red blood cells, which are red because of their hemoglobin, which can be described as an exotic form of rust--iron oxide. (In effect, when animal life moved up out of the ocean, it carried a bit of that ocean around with it: blood is in many ways similar to sea water.)
That we do not today find any nontrivial amount of oxygen in Mars's atmosphere does not tell us much about whether life ever arose there, though it does strongly suggest that if life did arise, it never got beyond the stage of simple microbes. Such early microbial life would not have transformed Mars's atmosphere as it did Earth's because any oxygen liberated by those possible microbes would have boiled off or been blown off just as most of the planet's atmosphere was. Thus, such hypothetical microbes might have existed, and might even exist to this hour--the water, the carbon dioxide, and the sunlight are still there, to a degree--but life's next stage, oxygen-breathers, could never have arisen.
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In sum, no men--little, green, or otherwise!--but very possibly (and, some think, probably) microbial life, once and perhaps yet. But we can't wind up this discussion without some comment on the controversial "life on Mars" meteorite.
It is not as widely known as perhaps it should be that the Earth has already been invaded by Mars. Not, mind, by Martians, but by Mars.
Planets and moons throughout the Solar System can be, and are, continuously bombarded by bits of astronomical junk--meteoroids, for example. If the planet has a reasonably substantial atmosphere, most of those incoming bits of junk--owing to the high speeds at which they hit that atmosphere--are burned up before ever they reach the ground, and most of the rest arrive as only a small chunk, perhaps pebble-sized. But if the planet, or moon or whatever, has little or no atmosphere, two consequences follow: one, the incoming body is neither slowed down nor pared away, and arrives with an impact often equalling a nuclear bomb's force; and two, if that force is great enough to blast out flying chunks of the target's surface, some of those chunks might actually escape into space themselves. And, over cosmic time, some of those blasted-off pieces of a world can arrive at another world--such as our own. And if they are large enough they at least a portion of them can survive the atmospheric burn and arrive at the surface of the Earth. That is not a wild theory: it is a fact that is known to have been repeated many a time. Such things are not even rarities: almost any decent astronomical musem can display one or more chunks of matter that were once part of the Moon or, yes, Mars.
On 27 December 1984, a team of American meteorite hunters from the Antarctic Meteorite Location and Mapping Project was working in the Allen Hills region of Antarctica. The team recovered roughly 7,000 samples; one of them, identified as ALH84001 (from Allen Hills, 1984, sample #001), weighing in as discovered at about 4¼ pounds, was found on later examination to be--with near certainty--from Mars. Its internal structure shows that it was initially shocked and fragmented off by one (or more) meteorite impacts some 3.6 billion years ago, but not blasted off Mars till a much later impact, occurring about 15 million years ago; finally, about 13,000 years ago, ALH84001 arrived on Earth. So far, nothing particularly exciting or dramatic.
That changed in August of 1996. Examination of ALH84001 had shown that it was unique among Martian meteorites so far known, differing from the others in a number of chemical and geological ways. But the stunning news was in an article published in the prestigious Science magazine by NASA scientist Dr. David McKay: in that article, he announced that ALH84001 contained evidence for primitive bacterial life on Mars. (At left: possible micro-organism fossil from AHL84001.)
As you can imagine, the article caused quite an uproar--one that has not fully died down yet. The problem is that the evidence, such as it is, is indicative, but teasingly so: there is nothing that anyone, not even the original authors of the article, can or does say is proof positive. Opinions today vary. Many experts feel that the evidence is so far from conclusive as to not be evidence at all; others feel that there remains a definite possibility that the indicators accurately point to microbial life. It seems that scarcely a month goes by without publication of another paper that claims to support or to refute the evidence, and we get no closer to any certainty. We are thus effectively little or no better off for knowledge than we were before the discovery. Life on Mars, albeit primitive microbial life, at least at some time in its past but possibly even today, remains a good probability but neither proven nor anything close to proven.
Later add: in February of 2006, analysis of pieces of the "Nakhla" meteorite--which fell in Egypt, in 1911--found new evidence strongly suggestive of early primitive life on Mars: a finely veined carbonaceous substance very much like that found in similar vein-like fractures in volcanic glass from the Earth's ocean floor and believed to be etched into such materials by microbes. The meteorite material was verified by cracking open, under sterile conditions, a piece of the meteorite (it broke into eleven pieces when it fell), so that there was no question of the carbonaceous material being terrestrial contamination of the sample. This is still by no means "proof" of early microbial life on Mars, but it's another suggestive datum in what is slowly but surely piling up as an impressive body of suggestive data. (A lengthy technical paper on the meteorite analysis work is available on line in two PDF files, Part 1 and Part II.)
Time will tell.
To see how we have learned what we already know about Mars, and how we are planning to go about finding out more, visit the page of this site on the exploration of Mars.
(We also discuss, on yet another page of this site, the possible significance and consequences of a verified discovery of independently evolved life.)
(Why not look in at Is it a blog yet?)
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Site Directory:
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(introducing this site) |
Introductory Material: | ||
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Please do take a moment to look over the Introduction page--it's helpful. |
Front Page:
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(the hard-core Mars information) |
Scientific Mars: | ||
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Planets:
· what they are and where they come from |
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Life on Mars:
· the possibilities there, and elsewhere |
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Exploring Mars:
· who did what when, and what's next |
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(Mars in human perspective) |
Human Mars:
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No technical information about Mars here, but lots of interesting Mars-related things. |
Mars in History and Culture:
· a history of beliefs about the heavens and the objects in them |
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What If We Find Life on Mars?:
· speculations on the consequences of proven extraterrestrial life |
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Science and Religion:
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Further Mars Resources:
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NASA Online Resources:
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