Astrobiology_A Very Short Introduction Read online

  The runaway would also cause Venus’s ocean to vanish. In the upper runaway atmosphere, ultraviolet light would split water vapour into hydrogen and oxygen. Hydrogen is so light that it would escape into space, dragging along some oxygen, while any oxygen left behind would oxidize hot rocks below. Eventually rocks would solidify, but by this time the atmosphere would be loaded with carbon dioxide and nitrogen released from the hot surface. The end result would be today’s hellish Venus.

  Before the runaway, life might have thrived in Venus’s oceans. Recall that the early Sun was less luminous. So, a few hundred million years might have passed before the runaway commenced. Unfortunately, it might be tricky to prove if life ever existed. In the 1990s, radar on NASA’s Magellan spacecraft mapped the Venusian surface. Older surfaces have more craters, so the density of impact craters was used to estimate the age of the surface as a fairly uniform 600–800 Ma. Venus’s surface appears to have been repaved by lava all at once. A possible reason is that whereas Earth’s internal heat is released continuously by creating new seafloor, on Venus internal heat might periodically build up until the interior becomes so hot that lava erupts everywhere. Venus might behave like this because unlike Earth it doesn’t have water to lubricate plate tectonics.

  Resurfacing would destroy any fossils. However, subtle geochemical traces might remain. To investigate if Venus had life, the best option would be to collect samples of Venusian rocks and return them to Earth in a future space mission.

  Watery Mars: an abode for life?

  Many have imagined Mars, unlike Venus, as a potential abode for life. Before the Space Age, basic parameters were known, such as Mars’s 24.66 hour day, a year of 1.9 Earth years, and gravity that is 40 per cent of Earth’s. Not much about the surface was certain until the first successful space mission, the flyby of NASA’s Mariner 4 in 1965, revealed a heavily cratered surface like the Moon. This put a damper on hopes for life. Then, in the early 1970s, NASA’s Mariner 9 orbiter photographed dried-up river valleys and extinct volcanoes, suggesting that Mars had once been quite Earth-like after all. But soon the pendulum swung back in the other direction. The Viking mission, consisting of two identical orbiters and landers, reached Mars in 1976, and failed to find life, as we discuss later.

  After a hiatus in the 1980s, exploration was revived. Mars Global Surveyor, which orbited from 1997 to 2006, mapped Mars and imaged sedimentary layers that implied many geologic cycles of erosion and deposition. In the early 21st century, the Mars Odyssey and Mars Express orbiters, along with Mars Reconnaissance Orbiter, discovered areas of clay minerals and salts, which may have formed in liquid water. Twin Mars Exploration Rovers landed in 2004, and found fossilized ripples from past liquid water and sedimentary rocks, while in 2008, NASA’s Phoenix Lander dug up subsurface ice in a polar region and measured soluble salts in the soil. Finally, in 2012, Curiosity Rover trundled towards a 5-kilometre-high mountain of sedimentary beds within a 150-kilometre-diameter crater named Gale. It found mudstones deposited from water that contain the SPONCH elements (Chapter 1) needed for life.

  Nowadays, astrobiological interest in Mars concerns either biological traces from billions of years ago or microbial-like life that miserably endures underground. Pre-Space Age hopes of an Earth-like Mars today have been replaced with the reality of a cold, windswept, global desert with dust storms, daily dust devils, and no rainfall.

  Mars’s current surface is hostile to life for three reasons. First, while ice exists for sure, no liquid water has been unequivocally identified. The polar caps are water ice, topped with carbon dioxide ‘dry ice’ that grows when about 30 per cent of the atmosphere freezes at the winter pole. Also, above mid latitudes, ice-cemented soil or permafrost lies just beneath the surface. In the tropics, afternoon temperatures in the top centimetre of soil rise above freezing, but there, ice turns to vapour before melting temperatures are reached. A second problem is no ozone layer, allowing harmful ultraviolet sunlight to reach the surface. Third, chemical reactions in the atmosphere make hydrogen peroxide—the same chemical used in hair bleach. Hydrogen peroxide molecules settle to the surface, where they can destroy organics.

  While the surface is unpromising, geothermal heat underground might allow liquid water and life to exist. In fact, from 2004, reports of atmospheric methane averaging ten parts per billion by volume led to excitement that subterranean methanogens might be present. Sunlight reflected from Mars gathered by telescopes and Mars Express appeared to show absorption by atmospheric methane. However, the methane signal was barely distinguishable and some sceptical scientists (including me) doubted whether methane was really present. Subsequently, the Curiosity Rover has failed to detect methane down to levels of one part per billion.

  In discussing past life, astrobiologists refer to the Martian geological timescale, which is divided into aeons called the Pre-Noachian (before 4.1 Ga), Noachian (4.1 to about 3.7 Ga), Hesperian (3.7 to 3.0 Ga), and Amazonian (since 3.0 Ga). Surfaces on Mars are placed into each aeon according to impact craters. Older surfaces have accumulated bigger and more numerous craters. In fact, the dates bounding each aeon actually come from the Moon. The ages of rocks brought back by the Apollo astronauts are known from radioisotopes and these correlate lunar cratering densities with time. Astronomical calculations that account for more impactors on Mars than the Moon allow the lunar correlation to be extended to Mars.

  Evidence that liquid water used to be present suggests that Mars was once more habitable than today. Images show fluvial (stream-related) features in the landscape, including gullies, dried-up river valleys, deltas, and enormous channels. Also, the soil and rocks contain minerals that form in the presence of liquid water.

  Gullies are incisions of tens to hundreds of metres length on the walls of craters and mesas between 30 and 70° latitudes in both hemispheres. Because gullies lack superimposed craters and sometimes flow over sand dunes, they must be very recent. Initially, they were thought to form when ice melted. However, images show them forming when carbon dioxide frost vapourizes, presumably releasing a dry flow of soil and rocks.

  Far more ancient features are valley networks, which are dried-up river-like depressions that spread out in tree-like branches with tributaries (Fig. 8). Most incise heavily cratered Noachian terrain and they’re 1–4 km wide and 50–300 m in depth. The density of tributaries is far less than for most terrestrial rivers, but sometimes enough to infer the drainage of rain or meltwaters. In other cases, valleys with stubby tributaries were probably formed by sapping, when underground (melt)water caused erosion and collapse of the overlying ground. A few valleys end in deltas.

  The Noachian landscape consists of craters with degraded rims and shallow floors. What caused this erosion is unclear. Valley networks are incised on top and so weren’t responsible. Once the Hesperian started at 3.7 Ga, erosion rates dropped dramatically and valley networks became rare.

  Towards the end of the Hesperian (around 3 Ga), outflow channels appeared (Fig. 8). Channels form from fluid flow confined between banks, lacking tributaries and emerging from a single source, unlike river valleys. The outflow channels are huge: 10–400 km width, up to 1,000 kilometres or so long, and up to several kilometres deep. Most begin in chaotic terrain where the ground collapsed, sometimes in canyons or chasms where there are mountain-sized heaps of sulphate salts, which might be evaporation residues from salty water.

  8. a) Valley networks on Mars. On the left is the eroded rim of 456-km-diameter Huygens Crater. The image is centred at 14°S, 61°E, north upwards. Scale bar = 20 km; b) Outflow channel Ravi Vallis (0.5°S, 318°E) about 205 km long

  The leading explanation for outflow channels is that they formed from floodwaters when underground ice melted or aquifers burst. However, they require 10–100 times more flow than the best-known terrestrial analogue: flood-carved land across eastern Washington State, USA, which early settlers called scablands. The scablands formed at the end of the last ice age when ice damming of a large lake periodically r
uptured.

  It’s not easy to explain the water needed to erode Mars’s outflow channels. Estimates suggest the equivalent of a global ocean several hundred metres deep, which is far more water than exists as ice today. Since the channels flow to the northern lowlands, some scientists speculate that an ocean formed there. In contrast, a minority thinks that the outflow channels were not carved by water but by lavas. Chemistry suggests that Martian lavas should have been runny, turbulent, and erosive.

  Apart from ancient valley networks and outflow channels (if water eroded), minerals provide other evidence for a wetter early Mars. Everyone knows how lettering on old gravestones disappears because of chemical reactions with water. Such reactions change or dissolve minerals in chemical weathering. Much of the Martian surface, like that on Venus and the Earth’s seafloor, is made of basalt, a dark-coloured igneous rock rich in iron and magnesium silicate minerals. When basalt is chemically weathered, alteration minerals are produced, such as clays. So the presence of alteration minerals means that liquid water was present, sometimes with a specific pH. For example, alkaline waters tend to produce clay minerals from basalt.

  Hydrous (water-containing) alteration minerals have been identified from analysis of infrared radiation emitted and reflected by Mars’s surface. But only about 3 per cent of Noachian surfaces have hydrous clays and carbonates. Sulphate minerals dominate late Noachian or Hesperian areas, while reddish, dry iron oxides are common on younger Amazonian surfaces. This pattern might imply three environmental epochs. In the first epoch, alkaline or neutral pH waters weathered basalt and made clays. During the second, sulphuric acid was derived from volcanic sulphur gases and made the sulphates. The third epoch continues today with a cold, dry environment and rust-coloured surfaces.

  One of the twin Mars Exploration Rovers, named Opportunity, actually landed near Noachian sulphates. Millimetre-sized spheres of the iron oxide hematite (Fe2O3) were embedded in sulphate layers, like blueberries in a muffin. The hematite precipitated from minerals carried in water percolating underground about 3.7 billion years ago. Also, ankle-deep water appears to have ponded on the surface, leaving behind ripples in the sediment. The other Rover, named Spirit, found evidence of ancient hot springs on the opposite side of the planet.

  The early atmosphere and climate of Mars

  Evidence of liquid water leaves us wondering whether the climate on early Mars was warm and wet. Scientists disagree and fall into two camps. One group argues that Mars had a warm, wet climate for tens or hundreds of millions of years. The other maintains that transient melting of ice in a cold climate could account for what is seen.

  The first scenario is obviously more favourable for life. But unfortunately, no one has yet explained how early Mars was kept warm for millions of years. Some 3.7 billion years ago, the Sun was 25 per cent fainter. A greenhouse effect of about 80°C would have been needed to keep early Mars just above freezing, compared to Earth’s modern 33°C greenhouse. It’s generally thought that any atmospheric hydrogen should have escaped into space rapidly when Mars formed, leaving the atmosphere oxidized and full of carbon dioxide and nitrogen. Mars couldn’t ever have had an extremely thick Venus-like atmosphere because carbon dioxide would condense into ice and form clouds at Mars’s distance from the Sun. A thick carbon dioxide atmosphere is also good at scattering sunlight back to space, which cools the surface. So carbon dioxide can’t provide an early warm climate. An alternative suggestion is that volcanic sulphur dioxide was the key greenhouse gas. However, sulphur dioxide dissolves in rain and would be flushed from the atmosphere if Mars became wet. Also, atmospheric reactions at high altitudes make a fine suspension of sulphate particles from sulphur dioxide, which reflects sunlight and cools the planet. Such cooling happened on Earth during 1991–3 as a result of volcanic emissions from Mt Pinatubo in the Philippines.

  The other camp proposes many mechanisms that allow liquid water in a cold climate. They note that impacts would have vaporized ice into steam, which, in turn, would have produced rainfall that eroded river valleys. Also, erosion might have been produced by local snowmelt as a response to past fortuitous combinations of Mars’s axial tilt and orbital shape. These characteristics are perturbed over time by the gravitational influence of other planets. The tilt of a planet’s axis with respect to the planet’s orbital plane, e.g. 23.5° for Earth, causes the seasons because one hemisphere gets more sunlight at one point in the orbit than the other. Unlike Earth, Mars has no big moon to stabilize its axis (its two tiny moons have negligible effect), so Mars’s tilt has varied between 0° and 80° over the last 4.5 billion years. At high tilts, summertime polar ice faces the Sun and vapourizes. Air currents transport the vapour to the cold tropics where it snows. Sunlight during other seasons or at lower tilts could then produce meltwaters and fluvial erosion. Moderate tilts in the last few million years might explain relict midlatitude patches of dust that were once ice-cemented. Finally, salty water on Mars can remain liquid far below 0°C. On Earth, we spread sodium chloride on icy roads in winter because it melts ice down to –21°C. Another salt, perchlorate, which was detected in Martian soil by the Phoenix lander in magnesium or calcium form, can depress the freezing point of water below –60°C.

  Whatever the truth about early Mars, Mars’s small size ultimately spoiled its habitability. Because small objects cool faster than large ones, internal heat was lost rapidly, so that widespread volcanism ceased long ago. Without volcanism, Mars couldn’t recycle carbon dioxide, leaving the gas to be converted into carbonates, which are present but not in the abundance expected if a thick carbon dioxide atmosphere had all been transformed. So, in addition, part of the early Martian atmosphere was probably blasted away to space by large comet and asteroid impacts in so-called impact erosion. The atmosphere was vulnerable because of Mars’s low gravity. Along with more gradual escape of gases to space later, Mars was ultimately left with its thin atmosphere.

  Looking for life on Mars

  We still don’t know if Mars has life or ever had life, despite attempts to find it. The Viking landers tried to detect life in 1976, and, since the 1990s, scientists have looked for signs of past life in Martian meteorites.

  Each Viking lander had three experiments to detect metabolism and a fourth to find organic molecules. The first, the carbon assimilation experiment, examined if Martian microbes obtained carbon from air. Martian soil (sometimes mixed with water) was exposed to carbon dioxide (CO2) and carbon monoxide (CO) gases brought from Earth, with carbon-14, a radioactive isotope, in each gas. Afterwards, the soil was found to have incorporated carbon-14. When a ‘control experiment’ was done where the soil was first sterilized at 160°C, the soil still took up carbon-14, which suggested that inorganic chemistry was responsible, not Martian microbes. A second test was the gas exchange experiment, which monitored Martian soil and a solution of organics brought from Earth to see if gases were generated from metabolism. Strangely, O2 was released, but it was also released from sterilized soil. Evidently, chemicals in the soil decomposed into O2 with water or heat. The third investigation, the labelled release experiment, added organics containing carbon-14 to soil. If Martians metabolized the organics, they would give off carbon-14-containing CO2. Radioactive gas was emitted, while sterilized soil released no radioactive gas. At face value, this was a positive detection of life. But most scientists think that soil oxidants reacted with organics to make CO2 and were inactivated by heat. A reason to prefer this explanation came from the fourth experiment, in which a sort of electronic nose, called a gas chromatograph mass spectrometer, identified molecules wafting off heated soil. It found no organic material in the soil to a detection limit of a few parts per billion.

  Overall, the moral is that before looking for extraterrestrials, you need to understand the inorganic chemistry of the environment to avoid false positives. In fact, Harold ‘Chuck’ Klein (1921–2001), who led the Viking biology experiments, told me that he wanted to do this when the Viking mission was conceived, but ma
nagers at NASA who controlled finances instead insisted on looking for life directly.

  While the Viking results were being pondered in the 1980s, a remarkable discovery was made: there were already rocks from Mars on Earth—Martian meteorites! Impacts knock rocks off Mars and some of these fall on the Earth. In fact, about 50 kg lands every year, mostly in the ocean. Gas sealed within some meteorites during minor melting associated with impact ejection proves a Martian origin because it matches the atmosphere measured by the Viking landers. Other Martian meteorites without trapped gas have a triple oxygen isotope (16O, 17 O, 18O) composition in their silicate minerals that is unique to all Martian meteorites and identifies them like a fingerprint. By 2013, sixty-seven Mars meteorites were known but the list keeps growing.