Astrobiology_A Very Short Introduction Read online

  In 1996, possible signs of past life in ALH84001 were reported. This Martian meteorite was the first one (the ‘001’) collected in the Allan Hills, Antarctica (the ‘ALH’), in a 1984 expedition (the ‘84’). ALH84001 crystallized as an igneous rock on Mars at 4.1 Ga. Inside the rock are some carbonate globules about 0.1 mm across that formed at 4.0–3.9 Ga, and within them are four possible traces of life: alleged microfossils; carbonates said to be precipitated by microbes; traces of organics called polycyclic aromatic hydrocarbons (PAHs), which are made of hexagonal rings of carbon atoms; and crystals of the mineral magnetite (Fe3O4) said to be similar to those within certain bacteria. On Earth, magnetotactic bacteria make magnetite crystals inside their cells with shapes that evolution has honed into magnets. The bacteria use magnetite compasses to move along the up–down component of the Earth’s magnetic field in order to find a boundary between a lower oxygen-poor zone below and an upper oxygenated zone, which optimizes their metabolism.

  In subsequent years, research has cast doubt on all four claims. The alleged microfossils are rod-shaped structures that merely look like microbes. Scientists have since found inorganic mineral surfaces with similar shapes. Also, the structures in ALH84001 are about ten times smaller than terrestrial microbes and probably beyond the minimum volume needed for essential biochemistry. An evaporating fluid can produce the carbonates in the globules and so there’s little reason to invoke biogenic salts, proposed as the second feature. Regarding the third line of evidence, analyses showed that most PAHs got into the meteorite while it was sitting in Antarctica for 13,000 years. PAHs are ubiquitous atmospheric pollutants produced from burnt organic material. Meteorites are dark and absorb sunlight well, so they can sit in puddles in Antarctic ice during summertime, allowing water and chemicals to infiltrate cracks. If there are Martian PAHs in the interior of ALH8401, these could have been made inorganically. The rock was heated by impacts that occurred before the ejection impact. Heating should have released carbon-bearing gas from the carbonates, which can react with water to make organic matter without life. The fourth argument concerned magnetite. It turns out that only a tiny fraction of magnetites in ALH84001 have biogenic-like shapes. Some scientists argue that magnetite of many shapes, including bacteria-like ones, formed during heat shocks prior to ejection when iron carbonate decomposed into magnetite.

  The controversy about ALH84001 shows how difficult it is to prove that microscopic life exists in old rocks. But since most terrestrial rocks lack fossils, absence of life in ALH84001 doesn’t mean that life on Mars didn’t exist. We need to keep looking.

  Ceres: a serious candidate for habitability?

  Beyond Mars and within Jupiter’s orbit is an asteroid belt of millions of small rocky bodies, including the largest, Ceres, some 950 km in diameter, at 2.8 AU from the Sun. Ceres’s surface contains clay minerals and water ice. As on Mars, clays suggest past liquid water. A leading model for Ceres’s internal structure is a rocky core that’s surrounded by a 100-km-thick shell of ice. Just above the core, there may be a subsurface ocean. This could be very salty, allowing its persistence at sub-zero temperatures, and perhaps it once flowed to the surface.

  Could life exist in hydrothermal vents at the bottom of Ceres’s ocean? Ceres is so small that there’s probably little internal heat available today, so any biomass would be extremely meagre. But perhaps in the past, Ceres was more habitable. NASA’s Dawn mission will arrive at Ceres in 2015. Spectra of infrared emission and reflected sunlight should give us more detail about the surface composition and habitability.

  The icy Galilean moons of Jupiter

  Beyond the asteroid belt is Jupiter, which is probably not a realistic target for astrobiology for reasons given in Chapter 5. But Jupiter has sixty-seven moons and three might be habitable. Galileo discovered the four largest moons in 1610, which are Io, Europa, Ganymede, and Callisto, going outwards from Jupiter (Fig. 9). Io and Europa are similar in size to the Moon, while Ganymede and Callisto have proportions comparable to Mercury. The outer three satellites have rocky interiors covered with ice, while Io’s exterior is just rocky. Io, Europa, and Ganymede probably also have iron cores.

  9. The Galilean moons of Jupiter: Io, Europa, Ganymede, and Callisto

  Io is the most volcanic body in the Solar System, with volcanoes spewing sulphur dioxide, which freezes onto Io’s surface. But Io is devoid of liquid water and surely lifeless. Io is so volcanic because gravitational forces from Jupiter vary as Io goes round in an elliptical orbit, squeezing Io like a stress ball. This tidal heating of Io is caused by friction when solid material moves up and down, similar to the way that water sloshes in Earth’s oceanic tides. Io’s orbit is forced to be elliptical because of periodic gravitational prodding from Europa and Ganymede. The times that Ganymede, Europa, and Io take to make orbits have a ratio of 4:2:1, which causes the moons to line up cyclically and nudge each other gravitationally. This relationship is the called the Laplace resonance, after Pierre-Simon Laplace, the French mathematician mentioned in Chapter 2.

  Europa also has an orbit that’s forced to be elliptical and so it has tidal heating too. The heating is smaller than that of Io because Europa is further from Jupiter. The average temperature of Europa’s icy surface is –155°C, but tidal heat can maintain liquid water deep under the ice. Impact cratering densities suggest a surface age of only 20–180 Ma consistent with slush from below repaving the surface. Images show chaotic terrain, i.e. disrupted surfaces with blocks that have shifted, which suggest that subsurface ice might convect. A global network of stripes and ridges includes bands where the surface has ripped apart and ice appears to have been extruded. Magnesium sulphate salt is present on the surface, perhaps originating from waters within, while unknown materials cause reddish streaks.

  The key evidence for Europa’s subsurface ocean comes from magnetic measurements by NASA’s Galileo spacecraft, which orbited Jupiter from 1995 to 2003. Unlike Earth or Jupiter, Europa’s interior doesn’t create its own, intrinsic magnetic field. But in passing through Jupiter’s large magnetic field, electrical currents are induced inside Europa. In turn, these currents generate a weak and varying induced magnetic field. The strength of this field requires that an electrically conducting fluid exist within 200 km of Europa’s surface. The most likely explanation is a salty ocean up to twice the volume of Earth’s ocean. The largest craters suggest that the ice cover above the ocean is at least 25 km thick. However, some terrain that pokes up might be produced by upwelling ice that melts a few kilometres below the surface and then refreezes. So shallow lens-shaped lakes might exist.

  Besides liquid water, the possibility of life depends on energy sources, interfaces, and the availability of SPONCH elements. Europa probably has a good supply of biogenic elements if it was made of material similar to carbon-rich meteorites or comets. Moreover, heat-releasing reactions of water and a rock seafloor could supply energy as well as nutrients. Radiogenic heat, which is produced by the decay of radioactive isotopes within the rock (such as potassium, uranium, and thorium), might produce seafloor vents that supply carbon dioxide and hydrogen for chemoautotrophs.

  The availability of oxygen in the ocean would also permit biological oxidation of iron and hydrogen on the seafloor. There are two small sources of oxygen. Charged particles trapped in Jupiter’s magnetic field slam into Europa’s icy surface and break water molecules, releasing oxygen. If the ice churns, some of the oxygen could be carried down to the ocean. Other oxygen comes from water molecules split by radiation from radioactive elements. Estimates of Europa’s biomass vary from around a thousand to a billion times less than on Earth. Some optimists even speculate that there might be enough oxygen for animal-like creatures!

  Measurements of induced magnetic fields on Ganymede and Callisto also imply subsurface oceans but deeper, below roughly 200 km and 300 km depth, respectively. Ganymede’s average surface age is about 0.5 Ga, so its ocean has probably not gushed to the surface recently. Because Ganymede is Ju
piter’s largest moon, its interior supports high-pressure forms of ice that would lie beneath the ocean. Lack of a rock–water interface might be less favourable for life than on Europa. The lower tidal heating on Ganymede also implies a smaller biomass.

  Inference of a subsurface ocean on Callisto is surprising because Callisto lacks tidal heating. The warmth must be radiogenic and the ocean might be loaded with salts that lower the freezing point. Callisto’s surface age is 4 Ga, so the prospect of finding evidence for oceanic life there might be slim. Also, with less energy available, any biomass should be even smaller than that on Ganymede.

  Enceladus and Titan: icy moons of Saturn

  Saturn and its sixty-two moons lie at about twice the distance of Jupiter from the Sun. The largest moons, discovered before the Space Age, are (going outwards): Mimas, Enceladus, Tethys, Dione, Rhea, Titan, Hyperion, Iapetus, and Phoebe.

  Enceladus is special because it has active geology. It’s Saturn’s sixth largest moon, but its 500 km diameter could almost fit within Great Britain. Enceladus orbits Saturn twice for every orbit of Dione. As a result, periodic gravitational nudges from Dione force Enceladus’s orbit to be elliptical, resulting in varying gravitational forces from Saturn that flex and heat Enceladus. For reasons that are not fully understood, the heating is concentrated under Enceladus’s south pole. There, icy particles and gas spray out of parallel fractures dubbed tiger stripes, which are about 130 km long, 2 km wide, and flanked by ridges. The jets contain traces of methane, ammonia, and organic compounds, along with salt. In fact, the jets supply particles to the ‘E-ring’ of Saturn within which Enceladus orbits.

  Enceladus has a rocky core, an icy shell, and an underground sea beneath the area of the jets. The jackpot combination of organic molecules, energy, and liquid water implies that life might exist inside Enceladus. Such life could be chemoautotrophic, feeding off hydrogen produced by water–rock reactions or hydrogen and oxygen from water that’s split by radioactivity. If life does exist there, methane or organic matter in the jets could be biological.

  Titan, the largest moon of Saturn, is also of astrobiological interest. It’s similar in size to Ganymede and Mercury and is the only satellite in the Solar System to have a thick atmosphere. In fact, the air pressure at Titan’s surface is 1.5 bar, which is 50 per cent larger than that at the Earth’s surface. The atmosphere of 95 per cent nitrogen and 5 per cent methane provides a 10°C greenhouse effect, but the sunlight at Saturn’s distance is one hundred times less intense than at Earth, so Titan’s surface is incredibly cold at –179°C.

  Titan’s atmosphere contains a smoggy haze of hydrocarbons. At high altitude, ultraviolet sunlight breaks up methane molecules (CH4) and subsequent reactions build up hydrocarbons including ethane (C2H6), acetylene (C2H2), propane (C3H8), benzene (C6H6), and reddish-brown particles containing polyaromatic hydrocarbons. The particles are called tholins from the Greek tholos for ‘muddy.’

  The products derived from methane sediment out of the atmosphere. In fact, about 20 per cent of Titan’s surface is covered in tropical dunes that are made of sand-sized particles, which are at least coated with organics if not made of them. At the poles, over 400 lakes are mixtures of liquid propane, ethane, and methane, with extraordinary beaches made of particles of benzene and acetylene.

  Methane on Titan behaves like water on Earth and forms clouds, rain, and rivers. Most of our knowledge about Titan comes from the Cassini–Huygens mission, which arrived in 2004. Huygens landed on Titan in 2005, while Cassini is a Saturn orbiter. Near the landing site of Huygens, channels were eroded by liquid hydrocarbons (Fig. 10). At the landing site itself, cobbles (presumably made of water or carbon dioxide ice) have been rounded as if by transport in rivers of liquid methane. Methane rain should be very infrequent, but when it rains, it pours.

  Because methane is destroyed by sunlight, Titan’s atmosphere would run out of methane in about 30–100 million years if it were not resupplied. How methane is replenished is a mystery. The leading hypothesis is that methane leaks out of Titan’s interior, which suggests geological movement inside Titan.

  Titan flexes too much to be entirely solid, which is evidence that Titan has a subsurface ocean. The eccentricity of Titan’s orbit ensures that Titan is squeezed by Saturn’s gravity. The ocean exists below an icy crust of less than 100 km thickness. But as with Ganymede, the seafloor should be dense ice rather than rock because Titan’s mass allows such high-pressure forms of ice.

  10. a) A network of channels that appear to flow into a plain near the Huygens landing site; b) Image of the surface at the Huygens landing site. Stones in the foreground are 10–15 cm in size and sit on a darker, fine-grained substrate

  Two types of life might exist on Titan: Earth-like life in the subsurface ocean; and weird life in the hydrocarbon lakes. Regarding the first possibility, when Titan formed, the heat released from the capture of smaller bodies should have created liquid water on the surface temporarily. Perhaps water-based life evolved and survived as an ocean retreated underground.

  Weird life would be limited in biochemical complexity because oxygen, which is scarce, is required for sugars, amino acids, and nucleotides. Titan captures some oxygen-containing molecules from space, but the supply is tiny. Hypothetical life in cold hydrocarbon solvents might use acetylene for energy. On Earth, acetylene burns with oxygen in welding torches. But Titanians could metabolize acetylene with hydrogen from Titan’s air. The chemical reaction,

  C2H2 (acetylene) + 3H2 (hydrogen) = 2CH4 (methane)

  produces energy, but the idea is speculative. The main problem for weird life on Titan, I think, is that liquid hydrocarbons are poor at dissolving large molecules, which are essential for genomes. Larger molecules are less soluble than smaller ones in liquid hydrocarbons. Furthermore, solubility declines at lower temperatures.

  Some exoplanets might have Titan-like liquid hydrocarbons. The most common stars are red dwarfs, which are smaller and colder than the Sun. Exoplanets near 1 AU distance around such dwarfs would have surface temperatures in the range of liquid hydrocarbons, not liquid water. Thus, if we were to discover weird life on Titan, we might imagine a universe teeming with life utterly different from our own. Ironically, we would then be the weird life.

  Triton: a captured Kuiper Belt object around Neptune

  At and beyond the orbit of Neptune at 30 AU, objects are covered in nitrogen ice (N2). Triton, the largest of thirteen moons of Neptune, and Pluto are both really Kuiper Belt objects (KBOs). The Kuiper Belt is a region of icy bodies within the plane of the Solar System at 30–50 AU left over from Solar System formation. There are also KBOs scattered out to 1,000 AU. Triton was captured by Neptune because two features of its orbit suggest so: Triton orbits in the opposite direction to Neptune’s rotation, and its orbital plane is tipped up 157° with respect to Neptune’s equator.

  Triton’s surface is tremendously cold, around –235°C, because it reflects 85 per cent of sunlight. An extremely thin nitrogen atmosphere exerts 20 millionths of a bar surface pressure. This ‘air’ is simply the nitrogen vapour that will sit over nitrogen ice at Triton’s prevailing temperature. There’s also a little methane (about 0.03 per cent of the nitrogen concentration), which is destroyed by ultraviolet sunlight, creating a thin smog of hydrocarbon particles. If methane destruction has been operating at the same rate for 4.5 billion years, about a metre’s depth of organic material should have accumulated. However, mobile frosts would cover this up. A reddish tint to some of the ice might be the organics. In fact, sunlight sometimes vaporizes nitrogen ice into geyser-like plumes, which carry dark particles that are perhaps organic. Apart from nitrogen ice, water and carbon dioxide ice make up some of the surface.

  Triton is geologically active because crater counts suggest that resurfacing is only 10 Ma and there are icy structures like vents, fissures, and lavas. We call these cryovolcanic, meaning a form of volcanism in which slushy ice comprises the equivalent of lava and magma. Triton’s density sugges
ts a large rocky core that supplies sufficient radiogenic heat for cryovolcanism and potentially a subsurface ammonia-water ocean.

  Early Triton may have been more habitable. Just after capture, Triton’s orbit would have been highly elliptical, producing huge tidal heating from Neptune that should have melted an extensive subsurface ocean. Tides tend to circularize orbits and today Triton’s orbit is close to circular. Thus, after capture, the ice shell gradually thickened as tidal heating declined. Today, the ocean might be below hundreds of kilometres of ice, and would be underlain by high-pressure forms of ice. But although oceanic life might be deeply hidden, the presence of cryovolcanism suggests that if we sample organic matter on the surface, there’s a chance of finding traces of life.

  Does Pluto have an underworld ocean?

  Pluto, named after the god of the underworld, has a surface mainly of nitrogen ice, and a thin nitrogen atmosphere of 8–15 microbar surface pressure. There’s also a little atmospheric methane that sunlight destroys just as on Triton and Titan. Again, this produces hydrocarbon particles, which settle out, possibly accounting for dark areas on Pluto’s surface.

  A collision between Pluto and another KBO is believed to have created Charon, the largest of Pluto’s five moons, in an event analogous to the impact that formed the Earth’s Moon. Thus, Pluto is really a double KBO: Charon is half Pluto’s size and one-ninth its mass. Every 6.4 days they orbit around a point that lies between them. After the Charon-forming impact, tidal heating should have melted a subsurface ocean on Pluto. Pluto probably has a rocky core that might supply enough radiogenic heat to maintain a subsurface ocean of ammonia-rich water today.