Although liquid media other than water are possible sites for life, a well-constrained search for solar system environments in which life might have arisen should focus on liquid water – because we know that life arose at least once in such a medium!
In our own solar system, there are four environments within which liquid water is known to be stable at or near the surface, or was likely stable for long periods in the past: Earth, Mars, the interior of Europa, and the water clouds of the giant planets.
Saturn’s moon, Titan, although too cold for liquid water to be stable, is rich in organic molecules and has vast lakes and seas of liquid hydrocarbons. Also, large impacts on Titan may have provided energy to melt its ice crust and provide liquid water for relatively short periods of time.
12.6.1 Atmospheres of the giant planets
The atmospheres of the giant planets represent the most specu- lative site for life, one proposed by Carl Sagan and others some decades ago. As discussed in Chapter 11, no solid surface exists except at enormous depths in the interiors of these gaseous bod- ies. The water clouds lie below a layer of ammonia clouds, which in turn lie below methane clouds in the cases of cold Uranus and Neptune. Living organisms there might be composed of struc- tures that allow them to cycle in depth from the relatively warm water clouds up to the ammonia clouds, absorbing sunlight at the higher levels and various trace organic constituents at a vari- ety of altitudes. Although one cannot rule out such a biota, the initial evolution of such organisms to a point of sophistication such that they could safely cycle in altitude without sinking into excessively hot depths is an open issue.
12.6.2 Interior of Europa
The shallow interior of Europa, a satellite of Jupiter just 10%
smaller than our own Moon, contains liquid water and hence is a potential venue for life. Spectroscopic studies of its surface
(b) (a)
Figure 12.7Galileoviews of Europa. (a) Large-scale view.
(b) Close-up image showing details around one of the crack systems.
Courtesy of NASA Jet Propulsion Laboratory.
from Earth reveal that water ice is an important or predominant component. Radar signals bounced off of Europa from Earth are reflected back with very high intensity, like a mirror, again indicating the predominance of water ice. Photographs taken by Voyager 2 in 1979 reveal a bright surface covered in cracks, which themselves are only slightly darker than the surrounding surface. So bright is Europa’s surface that most of the sunlight hitting it is reflected, and the average surface temperature is below 110 K, colder than the darker surfaces of Ganymede and Callisto.
From 1996 to 2003, the sensitive electronic camera aboard the Galileo Jupiter orbiter imaged the surface of Europa in much greater detail than couldVoyager. Preliminary study of the images yields additional circumstantial evidence that a liq- uid water layer exists beneath the ice. These include an enormous variety of cracks of different degrees of freshness, areas where cracks have been cut or buried by flows of liquid or warm ice, pieces of crust that have tilted upward as if floating on a layer of liquid water beneath, and craters in the ice displaying softened edges consistent with a thin ice crust (Figure 12.7).
The magnetometer aboard theGalileoorbiter recorded sig- natures of Jupiter’s magnetic field as it passed by each of the large moons of Jupiter, and provided the best evidence for a
subsurface liquid water ocean on Europa. In contrast to Ganymede, which possesses its own magnetic field, Europa simply distorted the shape of Jupiter’s field as it moved through it on its orbit. The amount of distortion was strong and eas- ily measurable from different angles on different flybys of the Galileoorbiter, and could best be explained if an electrically conducting layer existed inside Europa but close to its sur- face. To fit theGalileodata, the layer must be so electrically conducting that only a salty, liquid water ocean is plausible (one could invent other possibilities like molten metals and so forth, but these could be neither close to the surface nor molten.)
Europa’s surface layers may be cracked water ice at very low temperatures, but the density of this moon tells an intriguing story about the interior that is also important to the possibility of life there. At 3.0 g/cm3, Europa cannot be composed entirely of water ice, which has a density close to 1 g/cm3 (varying somewhat with pressure). To match the density with water ice and rock requires a moon composed of 80% rock and only 20%
ice by mass. The rocky component has embedded within it the radioactive isotopes of potassium, uranium, and thorium and, as described in Chapter 10 for Earth, the decay of such isotopes produces heat.
Add to this source of heat one other: tidal heating. Both Io and Europa have orbits that are slightly noncircular and are main- tained that way by the mutual gravitational pulls of Io, Europa, and Ganymede against each other. These pulling motions are effective because the orbital periods of the three satellites are simple multiples of each other – the period of Europa is twice that of Io, and that of Ganymede twice Europa’s. So, like a child on a swing pumping his or her legs in synchroneity with the period of the swing, these satellites tug gravitationally on each other and keep their orbits noncircular. This, in turn, means that even though each moon keeps one face approximately toward Jupiter all the time – as does our Moon to Earth – there is a small amount of twist as each moon varies in its orbital speed.
The twisting is enough to cause frictional rubbing of rocks against each other in Io, leading to extraordinary heating that has melted its rocky interior and produced spectacular volcanic eruptions viewed byVoyager. Europa, 50% farther from Jupiter than Io is, suffers some tidal heating, but much less than Io experiences.
The preponderance of silicates in Europa is important to the possibility of life not only for the heating they provide, but also for access to elements that tend to be present in silicates and are important to life. Models of the interior of Europa indicate that, almost certainly, the subsurface ocean is in direct contact with the silicates beneath; that is, the base of the ocean is rocky as is the case for the other. Leaching of phosphorus, magnesium, and other elements important for life, as well as the potential main- tenance of chemical gradients providing a source of available energy for life, may depend on this property. In contrast, while other giant moons like Ganymede’s Titan and even Callisto may have deep-seated oceans, the base of these oceans is a thick stratum of high pressure water ice perched above the silicate core.
Europa, then, almost certainly has a liquid water ocean lying beneath a frigid surface, and a source of heating that maintains the liquid state. What is uncertain is whether Europa acquired
enough carbon-bearing, nitrogen-bearing, and other compounds during its formation to allow for carbon-based life in the ocean.
It is also not known just how thick is the intervening ice crust.
To determine these important characteristics for life will require a mission to orbit Europa, equipped with radar to probe the ice, precision laser altimeters to measure the shape of Europa as Jupiter distorts it along its orbit, and spectrometers to search for organic molecules that may have seeped to the surface along fractures. Such a mission is challenging because of the intense radiation associated with Jupiter’s magnetic field, radi- ation which bombards spacecraft electronics and sterilizes very quickly the surface of Europa itself.
12.6.3 Titan
Saturn’s largest moon, Titan, is bigger than the planet Mer- cury. It has an atmosphere that has a surface pressure of 1.5 atmospheres and is mostly nitrogen. Methane is the next most abundant gas in the atmosphere. Titan is so far from the Sun that the surface temperature is 95 K. This is so cold that methane and similar molecules exist on Titan’s surface as rivers and seas.
Most of what we know about Titan comes from theCassini–
Huygensmission, which dropped a lander to the surface in 2005 and continues to observe Titan from a complex spacecraft that orbits Saturn and makes repeated flybys of the giant moon (Figure 12.8). Titan’s surface and atmosphere appear to be an intriguing mimic of Earth, with methane substituted for water. The action of solar ultraviolet photons creates a com- plex organic chemistry in Titan’s atmosphere, forming a globe- encircling haze that obscured the surface, requiring instruments such as radar and infrared cameras to probe the surface. Some products of the methane chemistry are stable as liquids on Titan’s surface, mixing with methane to make the polar lakes and seas. Others are solids and both coat the surface over large areas as well as form fields of dunes in the equatorial regions.
Titan is a complex, planet-sized chemical factory that almost assuredly has no life like that of the Earth because of the low temperatures. However, arguments have been made that Titan is in fact a good place to go to test whether life, in its most generally defined way, is a natural outcome of the availability of suitable molecules, liquids, and sources of energy. Should a form of organized, self-replicating chemistry, based on organic molecules but with methane rather than water as a liquid, be found in the lakes and seas of Titan, it would be a momentous discovery. Exploration of this world is therefore of high inter- est. However, because much of the chemistry going on in the atmosphere and on the surface may give us insight into organic chemistry on the earliest Earth, Titan is an important place to explore even in the absence of an exotic form of life. Although Titan’s atmosphere is probably chemically much more reduc- ing (that is, rich in hydrogen) than the early Earth’s, it is likely to be a better analogue to the Hadean Earth than is our present, oxygen-rich, biologically dominated planet. Occasional impacts or volcanic eruptions might melt the crust and produce pools of liquid water and ammonia for some thousands of years or more;
within these transient pools interesting prebiotic chemistry akin to that which occurred on the Earth might occur.
12.6.4 The Mars of today and yesterday
The AmericanViking 1andViking 2robotic landers operated on the surface of Mars beginning in 1976 and extending into the early 1980s. Among the experiments were four designed to determine whether life existed in the upper few centimeters of soil collected by each lander. Each of the experiments was designed to test for a particular kind of energy-generating mech- anism, such as photosynthesis, fermentation, and chemisynthe- sis. Soil was activated through the addition of potential nutrients, and then the experiments monitored release of gases that might indicate biological activity.
Some of the experiments showed positive results, but in com- bination, the experiments weighed against biological activity in the Martian soils. The positive activity was best explained as reaction of a nonbiological, highly oxidized component in the soil reacting with the gases and liquid nutrients supplied by the experiments. The death-knell for a biological explanation came from thegas chromatograph/mass spectrometerexperiment, a device that can detect very small amounts of organic molecules.
The amount of organic molecules at the twoVikinglanding sites, in the soil, was less than one part per million, and probably at the part-per-billion level for more complex organics. With so few organics, carbon-based life evidently was not present, and speculation about, for example, silicon life was not profitable because no evidence for such life could be obtained from the experiments.
The twoVikinglander sites, on high plains, were particularly poor candidates for sustaining living organisms, and areas near canyon bottoms, where the thin atmosphere has higher pressure, might be better. Nonetheless, current Martian conditions – an atmosphere of carbon dioxide with a total pressure less than a hundredth that at the surface of Earth, and surface tempera- tures generally well below the water freezing point – are not promising for life. The search for life in protected enclaves will be extremely challenging by robotic means, and piloted expedi- tions are decades away. Hence, the focus of research into life on Mars has shifted to finding evidence of past life.
Evidence for ancient clement conditions on Mars is described in Chapter 15, and consist of now-dry valley networks and out- flow channels that appear to have been carved by liquid water (or debris carried by water) and more-controversial evidence in the form of possible glacial features and dried lake beds.
Clearly something different happened on Mars in the past than today, and ancient conditions appear more promising than those at present. What further buoys the hopes of searchers for evi- dence of past Martian life is the presence of living organisms on Earth in extraordinary places. Submarine hot springs at mid- ocean ridges are, in the absence of sunlight, a rich abode of life. The dry valleys of Antarctica sport lakes whose surfaces are frozen over year-round, but in which a variety of microbial organisms thrive. Bacteria have been found living in rocks kilo- meters under the surface of Earth. All these sites could plausibly have had their analogue on early Mars, and in Chapter 15, we place possible life in the context of the history of Mars and the times when such environments might have existed.
To conduct the search for fossil evidence of life on Mars will be a daunting challenge, because such life very likely was microbial and not large, complex eukaryotic organisms (see
(a)
(b) (c)
Figure 12.8Cassini-Huygensviews of Titan (a) Large-scale view, showing details of surface features from the radar and the VIMS, a
near-infrared mapping camera. (b) Radar image of one of Titan’s large hydrocarbon seas, called Ligeia Mare, about 800 km across. (c) Image from the Huygens probe under the parachute of a system of dendritic channels stretching over 20 square kilometers; features as small as 30 meters are visible. Images courtesy NASA/ESA/JPL/Space Science Institute/Univ. Arizona. See color version in plates section.
Chapters 15 and 18). However, even microbial life leaves behind signatures. The isotopic ratio of carbon-12 to carbon-13 in organic molecules may be increased from the baseline value by cycling through biological systems, and hence looking for evi- dence of unusual isotopic patterns in trapped gases in Martian
rocks is one avenue. Researchers at the University of Califor- nia have found that relatively primitive life-forms can have an effect on the kinds of mineral structures that precipitate from seawater on Earth, and the same effect might be preserved in rocks on Mars. Finally, the macroscopic evidence of earliest
abundant Earth life is the lithified remains of bacterial colonies, the stromatolites of Chapter 10. Because evidence for such organisms stretches back to the oldest fossil record, examination of sedimentary Martian rocks for such patterns also should be undertaken.
In August 1996, NASA scientists David McKay and col- leagues brought the search for Martian life to the attention of mil- lions of people around the world with an astonishing assertion:
that they found evidence for relict biological activity in a mete- orite thought to have come from Mars. Meteorite ALH84001, found in Antarctica in 1984, is one of 12 SNC meteorites thought to have been from material blown off of Mars by an impact, and onto a collision course with Earth. These samples contain trapped gases, which, in their chemical and isotopic signatures, are identical to the present atmosphere of Mars as sampled by the Viking mission. Work over the past several decades by a number of researchers has shown that it is plausible for a large impact to gouge out a portion of the Martian crust and send some of it toward Earth.
ALH84001 is an old igneous rock, with a radioisotopic age of 4.5 billion years. It is therefore from some of the earliest Martian crust. The meteorite was heated again about 4.0 billion years ago by a strong shock, possibly a nearby impact. Globules of carbon- bearing minerals calledcarbonates, described in more detail in Chapter 14, were found in the rock. These formed later than the rock itself and are tentative evidence for the presence of liquid water flowing through the rock, tentative because carbonates can under some circumstances form in the absence of liquid water.
The age of the carbonate formation in the rock is highly uncertain, with different groups estimating ages from 3.6 bil- lion years (from potassium–argon dating) to 1.4 billion years (from rubidium–strontium isotopes). In either case, these ages are much older than the time when the rock was blown off of Mars. This time is estimated by examining tracks made by cos- mic rays on the surface of the rock exposed to space. The abun- dance of unusual isotopes of noble gases made by cosmic-ray collisions give a residence time in space of between 10 million and 20 million years. Once on Earth, isotopes such as carbon-14 and others of boron and chlorine, also made by cosmic-ray hits, begin to decay; their abundances indicate that ALH84001 has been on Earth only 13,000 years. So, the impact and delivery of this rock to Earth were much more recent than the formation of the carbonates, indicating that the carbonates were formed when the rock was in the Martian crust. Supporting this is the fact that the organic (carbon-bearing) content of the meteorite increases toward the center of the rock, suggesting that at least some of the carbon-bearing material is from Mars.
McKay and colleagues went a step further to propose three lines of evidence that are consistent with biological activity.
First, ring-shaped carbon-bearing molecules calledpolycyclic aromatic hydrocarbons, or PAHs, were found near the carbonate globules. Although PAHs can be formed in nonbiological envi- ronments, including interstellar space, from which their spectra are detected by sensitive Earth-based telescopes, the structure of the Martian PAH molecules differs from those seen to date from nonbiological sources. However, McKay and colleagues have not shown that biological activity necessarily forms such PAH structures either, and so, by themselves, the PAHs do not argue strongly for biological activity.
The second line of evidence comes from the presence of microscopic crystals of magnetite (Fe3O4) in the meteorite.
Crystals of similar chemical composition and size are made by so-called “magnetotactic” bacteria on the Earth that can orient themselves according to the direction of the Earth’s magnetic field. Such crystals, it was claimed, are not normally formed by abiotic processes expected in the Martian environment; however, a series of experiments performed at NASA’s Johnson Space Center showed that impact processes could product magnetite crystals of the right size and composition. The mineral siderite, an iron carbonate with the formula FeCO3, will decompose into magnetite and carbon dioxide when heated, either by the shock of impact or other processes. It therefore seems that the presence of such crystals does not require a biological explanation.
Most controversial are images, constructed by bombarding electrons into the sample, of very small structures near the globules – about 10 to 100 times smaller than terrestrial bacte- rial. They look like microbial forms, and McKay and colleagues argue that they could be evidence of life. Biologists and geol- ogists today argue about the possible existence of simple cells some 10 times smaller than bacteria that are indirectly inferred to be present in Earth rocks. Thesenannobacteriaare a spec- ulative and as yet unproven form of terrestrial life, and their invocation as support for the Martian microbe interpretation has raised more controversy. To show that the forms in ALH84001 are cells requires finding cell walls, which as yet cannot be seen in the images.
The reader should be skeptical of the interpretation of the ALH84001 data in terms of biology. McKay and colleagues argue that the timescales are right – if the carbonates are 3.6 billion years old, then the evidence in theVikingimages of a more clement Mars at the time are consistent with life beginning then. But the carbonates could be younger.
Further evidence against Martian life in ALH84001 came in 1998 when scientists from the University of California and University of Arizona analyzed the “organic” carbon (that is, the carbon not in the carbonates) from ALH84001 in more detail.
From several lines of investigation, including measuring the carbon-14 to carbon-12 ratio, they concluded that most or all of this carbon is terrestrial, not Martian – though they confirm that most of the carbonate phase is likely from Mars. The ambiguities of interpretation associated with the claim of evidence for life in ALH84001 illustrate the great challenge of finding evidence for microbial life from subtle chemical clues and images at very small scales. It will be even more difficult to perform such searches on the surface of Mars; perhaps the best strategy is to locate promising sites on Mars where life may have existed, and then return samples back to Earth.
More recently the discovery of methane in the Martian atmo- sphere has rekindled discussion over the possible existence, not of fossilized life, but of extant life within the crust of Mars.
Detections by an instrument aboard the EuropeanMars Express orbiting spacecraft built by V. Formisano of INAF in Italy, fol- lowed by ground-based observations by M. Mumma and col- leagues in the US, which documented sources in several places on Mars (Figure 12.9), firmly established the existence of this simplest organic molecule. In a CO2-rich atmosphere bathed in ultraviolet radiation, overlying a reactive surface in which enough free oxygen is available to oxidize quickly any organic