Day 11
Dark Abodes for Life
May never glorious sun reflex his beams
Upon the country where you make abode;
But darkness and the gloomy shade of death
Environ you
First Part of King Henry the Sixth, Act V. Scene IV.
William Shakespeare (1564–1616)
The Oxford Shakespeare, 1914
Shakespeare could not imagine life in the dark. Neither could most biologists until the discovery that the tree of life begins with non-photosynthetic organisms. Our distant ancestors lived in the dark, probably as microbes in cracks in rocks. We know some about this environment on the Earth from extant organisms. We know very little about its fossil life. Traditional geology considered the deposits within cracks in rocks to be unimportant, unless they contained economic minerals. Paleontologists eschewed these cracks in their search for fossils.
Astrobiologists need to stretch the envelope of terrestrial life to proceed. We do not have another Earth in our solar system where looking is easy. I describe places where there is some possibility of looking in our solar system. They need to check out the habitability around faint stars and even the clouds of Venus. I discuss that during the next Day.
Mars and magnets. Kevin Zahnle and I were peaceably preparing a journal paper that Mars was a safer place for microbes than the early Earth. There was no need for haste because the next Mars mission was years away. While I was driving to a weekend in the Sierras with my family, the radio blared that NASA scientists had found life in a rock from Mars. I knew it had to be ALH-84001. (“ALH” stands for Allan Hills in Antarctica where the rock was found. It is relatively easy to find dark meteorites in places where the ice comes to the surface and evaporates. “84” stands for the December 1984-January 1985 field season when it was found. “001” means it was the first one of that year's collection from the Allan Hills catalogued upon return to the Johnson NASA center.) This rock is 4.5 billion years old. It looks so much like an Earth rock that the scientists had to argue with the technicians to bag it. It contains cracks filled with mineral deposits. These resemble mineral deposits from low-temperature water. This environment teems with life on the Earth.
The press focussed on tiny worm-shaped structures in the rock. These were smaller than extant organisms. This bothered biologists in that there is a minimum size for an organism resulting from the fact that there are a finite number of atoms in its cell. Paleontologists were wary. They had been burned by interesting-looking pseudo-fossil structures in ancient Earth rocks.
Figure 1: Kathie Thomas-Keprta imaged magnetite grains in the Mars rock ALH 84001. These 3.9-billion-year-old grains resemble modern biological magnetite and magnetite fossils on the Earth. This is evidence that live existed on Mars as physics determines the optimal shape of biological magnetite. . Figure 4 in Applied and Environmental Microbiology, Aug. 2002, p. 3663–3672 Vol. 68, No. 8, 0099-2240/02/$04.00_0 DOI: 10.1128/AEM.68.8.3663–3672.2002
Scientific attention gravitated toward tiny (less than 1 millionth of a meter across) magnetite grains in the rocks (Figure 1). Terrestrial organisms use such grains for magnetic compasses. We have them in our brains, but they have not been shown to form a useful sense. Maybe they help will balance. The birds and the bees use them to find their way around.
Figure 2: Magnetic microbes follow magnetic field lines to find conditions to their liking. Here the magnetic lines are down and north in the Northern Hemisphere. Typically the sediments are oxidizing at the surface and reducing at depth. This microbe moves along a field line to find more reducing conditions at depth. It is inefficient to move downward at an angle, but this is a lot better than wandering around lost. This habitation is useful wherever gradients between oxidizing and reducing exist. This includes sediments and water bodies like the Black Sea. This situation exists now in the subsurface of Mars.
Microbes use them to find up and down (Figure 2). In the Northern Hemisphere, the magnetic field (with the standard sign convention) points north and down. We can measure the downward component with equipment dating from William Gilbert’s work around 1600. In the Southern Hemisphere, the field points north and up. At the magnetic equator it is horizontal. The force of gravity on a microbe is too small to aid it in finding up and down. The force on the magnetite grain from the Earth’s magnetic field is much larger than the gravitational force. The organism finds up and down using the magnetic field. The field is not vertical except near the poles but it is more efficient for the microbe to go up and down at an angle than it is for it to wander aimlessly.
There is an optimum size and shape for a magnetite grain to function as a compass. It needs to be elongate in a particular crystallographic direction. Magnetite does not grow this way on its own. It, like salt and garnet, tends to grow equidimensionally. Magnetic microbes have highly evolved mechanisms to grow the grains that they want.
Careful comparison of the magnetite in the Mars rock and magnetite from terrestrial microbes by a group led by Joe Kirshvink showed that the Mars magnetite resembles that from terrestrial organisms. This is important because the need for microbes to move up and down occurs on Mars. As on the Earth, the surface is more oxidized on average than the subsurface. The optimal shape for the grains is physics. Evolution will converge on it.
The magnetite in many living microbes comes in chains of grains, like a line of bar magnets. Reliable detection of such chains in the Mars rock would elevate the potential biomarker to a fossil of past Martian life. To date, scientists have found no convincing chains in the Mars rock.
Note here that the level of evidence to establish a major claim, that life existed on Mars, is higher than that for a routine claim. Less evidence suffices to trigger further work. This is generally true in science as it is in ordinary affairs. For example, you would not have much trouble convincing people that you saw a pigeon in the park. You would have a lot of difficulty if you claimed to see a pterodactyl, but you might get someone to come with a good camera. There is nothing illogical or mystical about life on Mars. It is perfectly consistent with what we know. We just want to be sure before we say we are. The level of evidence for really outlandish claims is much higher.
Cereal seed. Ceres is the largest asteroid in the solar system, about 950 km in diameter (Figure 3). Telescopic observations indicate that its surface is made of the same material as the carbonaceous meteorites that contain complex organic commands, including amino acids. We do not know which of these meteorites come from Ceres as there are numerous smaller asteroids with similar surface characteristics.
Ceres is now a dead body with no tectonics and freezing temperatures all the way to the center. However, its insides were clement early in the history of the solar system. Liquid water flowed in its interior. Hot water coming from its interior was out of chemical equilibrium with cooler surface rocks. It had the basic ingredients to start life, including several percent by mass of organic matter. It was large enough that it stayed clement for hundreds of million years, long enough for evolution to occur.
Figure 3: The Hubble telescope imaged Ceres. The dark feature Piazzi is at the center of the image. We know from spectra that carbonaceous material exists at the surface, but available images reveal very little. http://www.jpl.nasa.gov/news/features.cfm?feature=545
Ceres is an excellent astrobiology target to catch prebiotic evolution in the act. It may have even provided the seed of life to the inner solar system. It was a clement solid body before the Earth-Moon system formed and probably before the surface of Mars was habitable. Geochemists know this from analyzing the products of radioactive decay of short-lived radioactive isotopes that nearby supernovae added to the solar nebula. The low gravity of Ceres made it easy for impacts to eject rocks. Some of these ejecta hit the Earth and Mars just like the carbonaceous meteorites do today.
Ceres is an easy exploration target. It has some gravity to keep a lander in place but not enough to make landing difficult. There is no need to drill deeply as craters have exhumed the subsurface over geological time. It is not currently on a NASA missions list.
Ice worlds. Galileo viewed Jupiter and its four large moons as a miniature solar system. This analogy is far more applicable today than Galileo imagined. The moons condensed out of a dust and gas nebula surrounding Jupiter, like the inner planets formed from the solar nebula. At the time of moon formation, Jupiter was hot from the gravitational energy of accretion. Io accreted nearest Jupiter. It retained rock, iron, and sulfur. It is very volcanically active. Any water it had has been since lost. It is likely to be sterile. Europa accreted out next (Figure 4). It retained a 100 to 200-km thick layer of water over rock. Its interior likely melted, forming an iron core like the Earth’s. It is at the goldilocks distance for life from Jupiter so I return to it after discussing the other moons. Ganymede is next out. It got hot enough to differentiate ice and rock. The ice layer is hundreds of kilometers thick. There is evidence that the liquid water has erupted to the surface. If there is still liquid water at great depths the situation is similar to Europa, except the logistics are not good. The Neptune moon Triton and Pluto are similar in that liquid water may exist deep in the subsurface. Callisto, the farthest out, has never melted. It is a sterile mixture of ice and rock.
Figure 4: Photo from the Galileo spacecraft shows long linear features on the surface of Europa. The long dimension is 2000 kilometers. We have good images of the surface of this Moon but no samples other than from spectra of remote images. http://photojournal.jpl.nasa.gov/catalog/PIA00874
Returning to Europa, photos from the Galileo probe show a complex active surface resembling frozen pack ice (Figure 5). The lack of craters, as on the Earth, indicates the surface is youthful. It appears that water or slush erupts to the surface. Large blocks of ice have turned over like unstable icebergs. The ice layer may be about 10 km thick. The gravity of Europa is about 1/8 of the Earth’s. The pressures at the bottom of the ocean are like those at 12- to 24-km depth on the Earth. The deepest Earth ocean is not quite 11-km deep. It teems with life. Pressure is not a serious restriction to life on the Earth. Europa is habitable by microbes if an energy source exists.
Figure 5: Photo from the Galileo spacecraft shows chaotic terrain on Europa, resembling repeatedly cracked pack ice. Liquid water may have reached the surface numerous times. The photo is 42 km across. http://photojournal.jpl.nasa.gov/catalog/PIA00591
Europa is too small for radioactive heat to drive tectonics. It would be more inactive than the Moon. However, tides heat the water and the interior of Europa. Io gets too much tidal heating and is a mass of partially molten rock. Ganymede gets too little and liquid water exists only at great depths. Europa gets the right amount. Enough to keep the Moon and the ice active, but not enough to drive off the ice and water. Tidal heating may drive rock volcanism and hydrothermal vents beneath the Europan ocean, just what nourished early life on the Earth.
Much of the interest in Europa arises because we have an independent place from the Earth for life to evolve. Exchange of life with the inner solar system is unlikely. Ice knocked off Europa vaporizes like a comet before it can hit within the inner solar system. Earth and Mars rocks can hit Europa, but the current rate is well below 1 rock per million years. When they do hit, they strike the icy surface with cosmic velocity. The hard shock is not good for microbes.
Physically rocks in solar orbit that come within the region of the large moons tend to get ejected from the solar system by the gravity of Jupiter. If they do not hit a moon on their first couple of passes they get no more tries. NASA engineers use the gravity of Jupiter to eject space probes from the solar system.
There is hope of both catching prebiotic chemistry that did not form life in the act and finding dead evidence of extant life on or near the surface. Atomic particles trapped in Jupiter’s magnetic field sterilize any exposed frozen life. We will probably have to drill into the ocean or meters down into recently vented frozen water to get anything live.
An airy moon. Christiaan Huygens discovered the large moon Titan orbiting Saturn in 1655. Its thick atmosphere is visible from the Earth. I remember the pre-space exploration question trivia question “What planetary satellite has an atmosphere?”
The Voyager mission revealed that the atmosphere is nitrogen with a little methane, ethane, carbon dioxide, and hydrogen cyanide. The latter is very poisonous to us. It was used in gas chambers. However, “toxic” is in the eye of the beholder. Hydrogen cyanide is one of the basic building blocks of life.
Titan’s surface is 95 K, far too cold for terrestrial life. There are methane-ethane lakes or seas on the surface. This environment does not occur on the Earth and would require a different chemistry than water-based life. We would not expect to find life on the Earth fit for conditions that have never occurred here.
There are certainly chemical disequilibria that might support methane-based life. Ultraviolet light from the Sun continually produces complex organic compounds in the upper atmosphere. Ammonia-water solution may exist within the icy shell and, as with Europa, the liquid ocean at depth may be inhabited. More relevant to feasible exploration, the surface of the moon is quite active geology. Similar to the Earth, there are few impact craters. The water-ammonia solution erupts to the surface and then freezes. Disequilibria within the ice may feed methane-based microbes. The ice itself may be food. Where it freezes quickly, it forms an amorphous glass. Microbes potentially may get energy by turning the glass into crystals.
Figure 6: Photo from the Huygens probe of the surface to Titan. Liquid methane rives flow into a methane sea. The hills are probably composed of water ice with some ammonia. Scientists have processed the image so that it appears to be viewed directly from above. The imaged region is 5 km across at the bottom. http://photojournal.jpl.nasa.gov/catalog/PIA07236
The Huygens probe descended to the surface of Titan and provided our first look at this strange (to us) environment in 2005 (Figure 6). It analyzed the chemistry of the atmosphere and the landing site. The moon is geologically active. The cameras showed valleys cut into water-ammonia ice by methane rivers and possible coastlines of a methane sea. The lander came down in a delta. The surface analyses are of particularly biological interest. The science team is searching for any signs of pre-biotic chemical evolution.
Checklist. If we find independently evolved life anywhere in our solar system, we will know life is common in the universe. We are able to search environments where the Sun does not shine. Mars is promising and close by. It had earthlike conditions early in its history. Any extant life has retreated to the subsurface. It may occasionally stick its head up when water erupts to the surface. There are plenty of rocks over 4 billion years old that may contain fossils.
The large icy moons and Pluto may have liquid water in the subsurface. Europa is easiest to sample. Unlike Mars, life if any will be independent of the Earth.
Ceres presents a dead environment but a possible seed for inner solar system life. It is easy to sample. We have already landed on an asteroid.
Titan may has deep hidden water ocean, but its surface is of interest. It presents a stage for biochemistry far different than water next to rock. We now have chemical analyses of its surface. We do not have any evidence for biology.
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