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Life in the Solar System: Mars, Europa & Titan

Continuing on from the previous lecture, this lecture examines the prospects for life on the planet Mars and the satellites Europa and Titan.

  1. Mars
    1. Background
    2. Environment
    3. Prospects for Life
  2. Europa
    1. Background
    2. Environment
    3. Prospects for Life
  3. Titan
    1. Background
    2. Environment
    3. Prospects for Life
  4. Further Reading

Mars

Background

Mars is the outermost of the terrestrial planets, orbiting at an average distance of 1.52 AU from the Sun. Its rotation period is slightly longer than the Earth's, at 24 hours 37 minutes, and its orbital period is 687 days. The rotation axis is tilted at 25 degrees to the orbital axis, which means that Mars experiences significant seasonal variations in its northern and southern hemispheres. Although Mars's radius is around half of Earth's, its low density means that its total mass is only 10% of Earth's. This accounts for a low surface gravity of 0.38 Earth gravities.

Image of Mars taken by the Mars Global Surveyor spacecraft

Image of Mars taken by the Mars Global Surveyor spacecraft

Mars is thought to possess an iron core, but sulphur impurities prevent electrical currents from forming in it to produce a global magnetic field. Although the planet is inactive today, it exhibited significant volcanic activity in the past, which led to the formation of the Tharsis Rise (a cluster of volcanic peaks) and Olympus Mons, the largest volcano in the solar system (rising three times higher than Mount Everest, and with a base diameter of 370 miles!).

Olympus Mons

Olympus Mons

The surface of Mars is strongly differentiated between the northern and southern hemispheres. The northern hemisphere is about 3 miles lower in altitude than the southern, and is relatively smooth and free from impact craters. This suggests that these northern lowlands have been resurfaced by some process which eradicated ancient features.

Elevation of Martian surface from Mars Global Surveyor (white is highest, dark blue is lowest)

Elevation of Martian surface from Mars Global Surveyor (white is highest, dark blue is lowest)

Early observations by Cassini, in 1666, revealed the existence of white caps on Mars's poles. These polar caps vary with season, shrinking during the summer and growing during the winter. The Mariner 9 spacecraft (1972) established that the north-polar cap is probably made from solid deposits of water ice, with a covering of carbon dioxide (dry ice) frost which evaporates in the summer. 20 years later, observations by the Mars Global Surveyor (MGS) mission (which went into orbit around Mars in 1997) revealed that the south-polar cap has a similar composition.

MGS image of the south-polar cap of Mars in summer

MGS image of the south-polar cap of Mars in summer

Environment

Mars has a very thin atmosphere (0.6% of the Earth's atmospheric pressure), comprised primarily of carbon dioxide. Due to the absence of a thick, insulating atmosphere, and also due to its distance from the sun, the surface of Mars is cold. Temperatures range from -140C to 20C, with a mean of -53C.

Given the low surface pressures, water is unable to exist in liquid form for long on the Martian surface; depending on the temperature, it either freezes or evaporates. However, results from MGS and earlier missions have revealed signatures of water which was liquid in Mars's past:

  • Erosion due to flash floods, caused for instance when meteorite impacts melted underground deposits of ice.
  • Erosion due to sustained water flow, presumably formed when the atmosphere was thicker, and water could remain liquid for extended periods of time.
  • Features around the boundaries of the northern lowlands which may be the shores of an early liquid-water ocean or group of lakes. This would explain why the lowlands appear so smooth.
  • Signatures of minerals which can only form in the presence of liquid water, such as sedimentary deposits of crystalline haematite(iron ore).
MGS image of Nanedi Vallis, which is thought to have been caused by sustained water flow

MGS image of Nanedi Vallis, which is thought to have been caused by sustained water flow

More significantly, data taken by MGS, and also the 2001 Mars Odyssey mission have revealed gullies on the sides of craters, which are thought to have been created in the recent past by water erosion. The water is able to remain liquid due to a covering of snow, which prevents its rapid evaporation.

MGS image of gullies on the walls of Nirgal Vallis

MGS image of gullies on the walls of Nirgal Vallis

Prospects for Life

Prospects for life having existed on Mars in the past are reasonable. The prime piece of evidence which supports this conclusion is the meteorite ALH84001, which was discovered in 1984 by a geological expedition in Antarctica. By analyzing the composition of trapped gasses within ALH84001, researchers were able to conclude that the meteorite was ejected from Mars around 16 million years ago, and landed on Earth around 13,000 years ago.

Martian meteorite ALH84001; the cube to the right has sides 1cm in length

Martian meteorite ALH84001; the cube to the right has sides 1cm in length

Other Mars-originated meteorites have been found on Earth, but the interesting aspect of ALH84001 is that it is old enough (4.5 billion years) to have been on Mars when liquid water was stable. Furthermore, within the meteorite were found grains of carbonate minerals, which require liquid water to form.

Carbonate grains within ALH84001

Carbonate grains within ALH84001

Most excitingly, within these carbonate grains were discovered three pieces of evidence for ancient Martian life:

  • tiny, elongated tubular structures around 100 nanometres long, which resemble fossilized microbial life.
  • pure crystals of iron sulphide and magnetite, which are rarely found together, but can be formed by certain types of bacteria.
  • polycyclic aromatic hydrocarbons (PAHs), organic molecules which result from the decay of microorganisms.
Tubular structures within the ALH84001 carbonate grains

Tubular structures within the ALH84001 carbonate grains

Each piece of evidence on its own proves nothing, but together they support the conclusion that primitive life may have existed in Mars's distant past, when it was warmer and liquid water was stable. However, some scientists dispute this conclusion, claiming that the tubular structures are too small to be fossilized microorganisms.

To resolve the controversy fully requires a sample-and-return mission to Mars, to bring back samples of the Martian surface for analysis on Earth. In the meantime, however, we must rely on the past and future robotic missions to the planet. The first of these were the two Viking Lander probes, which touched down in 1976 on different parts of the northern lowlands and performed basic analyses of the Martian soil. Although the probes looked for evidence of life, the results were inconclusive.

View of the Martian surface from Viking Lander 1

View of the Martian surface from Viking Lander 1

More recently (1997), the Pathfinder mission touched down 800km from the Viking Lander 1 site, on the bed of an ancient flood channel. Pathfinder was equipped with the Sojourner, a rover able to move freely over the surface and analyze rock samples. Although it provided valuable geological data, Sojourner was primarily a proof-of-concept, and didn't conduct any searches for life.

The Sojourner from the Mars Pathfinder lander

The Sojourner from the Mars Pathfinder lander

The next Mars-bound mission specifically targeted at searches for life is the British Beagle 2 robotic probe, which will be carried on the Mars Express mission of the European Space Agency. Mars Express is due for launch in June 2003, and will arrive at Mars in December 2003, when the Beagle 2 probe will immediately descend to the surface. It will land on Isidis Planitia, a large sedimentary basin situated between the Martian highlands and lowlands.

Simulation of Beagle 2 on the Martian surface

Simulation of Beagle 2 on the Martian surface

The other important future Mars missions are NASA's two Mars Exploration Rovers, which are due for launch in May/July 2003, and will land in January 2004 (the landing site has not yet been finalized). Unlike Beagle 2, these robots are fully mobile; however, they are primarily targeted at investigating Mars's geology, rather than looking for signs of early life.

Possible landing sites for the Mars Exploration Rovers

Possible landing sites for the Mars Exploration Rovers

Europa

Background

Europa is one of the four Galilean satellites of Jupiter, the other three being Io, Callisto and Ganymede. It orbits Jupiter once every 3.5 days, at an average distance of 670,000km. Its density of about 60% of Earth's means that Europa is mostly comprised of rocky materials; with a radius of 1,500km, it has a surface gravity of 0.14 Earth gravities.

Image of Europa taken by the Galileo spacecraft

Image of Europa taken by the Galileo spacecraft

The most striking aspect of Europa is its very flat surface, the smoothest in the solar system. It is devoid of any surface features greater than a few hundred metres high, and there are almost no impact craters, indicating that the surface is very young. Early spectroscopic observations of Europa suggested that the surface is comprised of water ice, a fact confirmed by instruments onboard the Galileo spacecraft.

Environment

Although Europa's surface is very flat, it is criss-crossed by a multitude of cracks, ridges and ice rafts. These resemble the features found on terrestrial sea ice, suggesting that beneath Europa's frozen surface is an ocean of liquid water. If surface ice plates move apart, this liquid water wells up in the crack and freezes. Conversely, if plates move together, they pile up to form a ridge, or ride over one another to create an ice raft.

Ice rafts on the surface of Europa

Ice rafts on the surface of Europa

Further support for the existence of a sub-surface liquid ocean on Europa comes from magnetic measurements by the Galileo spacecraft. Europa is too small to generate its own magnetic field, but a weak field was detected by Galileo. The interpretation of this result is that Europa has a conducting fluid beneath its surface, which generates its own field as it moves through Jupiter's strong magnetic field. Liquid water containing dissolved minerals is a strong candidate for this conducting fluid.

Since Europa is so small, it is difficult to see how the satellite's own internal heat is sufficient to keep an ocean in liquid form. However, the situation with Io, the inner-most Galilean satellite, may provide a clue to the mechanism. As Io moves through Jupiter's gravitational field, tidal forces keep its interior warm enough for significant volcanic activity to occur. The same tidal heating can explain how Europa's interior is warm enough for water to remain liquid beneath its frozen surface.

A volcano erupting on Io

A volcano erupting on Io

Prospects for Life

Galileo observations of neighbouring Callisto and Ganymede indicate abundant quantities of organic compounds containing oxygen, carbon, sulphur, hydrogen and nitrogen. If similar abundances are found on Europa (which is likely), and if Europa does indeed possess a liquid ocean (which is likely), then the prospects are good for life to exist today on this icy moon. This life might be found around geothermal vents on the ocean floor, much like the extremophiles which live on black smokers in Earth's oceans (see Lecture 6).

Coloured cracks in Europa's surface ice

Coloured cracks in Europa's surface ice

Interestingly, some scientists have claimed that evidence for life on Europa already exists. The reason why cracks in its surface ice are visible, is that they are somewhat darker than the surrounding ice, and reddish-brown in colour. Infra-red spectra taken by Galileo have been used to investigate the reasons for this different colouration. The general consensus is that it arises from salt minerals contained within the ice, but attempts to find the precise composition of these minerals have proven difficult.

An alternative interpretation is that the colouration is due to microorganisms trapped within Europa's surface ice. The infrared spectra of the dark cracks share a number of features with spectra of terrestrial bacteria under the same conditions as those on Europa's surface.

To discover the true reason behind the colouration of the cracks, and also to establish the nature (and possible inhabitants) of Europa's hypothetical oceans, new exploration missions are required. As part of NASA's Outer Planets/Solar Probe Project, preliminary development began in 1998 on a mission to send a spacecraft to Europa. The Europa Orbiter was designed to measure the thickness of the surface ice, and look for an underlying liquid ocean. Unfortunately, the project was canceled in 2002 due to budgetary constraints.

However, NASA are also considering the Jupiter Icy Moons Orbiter. (JIMO) mission, to examine the three outer Galilean satellites (Europa, Callisto and Ganymede). This mission is scheduled for launch no earlier than 2011. If approved, it will be the first to use a new nuclear-based propulsion system, which may form the basis for all future space missions.

Artist's impression of JIMO approaching Europa

Artist's impression of JIMO approaching Europa

Ultimately, a thorough investigation of Europa will require the use of a cryobot, to melt through the surface ice, and a hydrobot, to explore the underlying ocean and look for signs of life. NASA's Jet Propulsion Laboratory (JPL) is currently working on a cryobot to explore Lake Vostok, the world's fourth-largest freshwater lake, which is situated deep under the Antarctic ice sheet. It is thought that prehistoric organisms may still thrive within the lake. The technology developed in exploring Lake Vostok will provide a valuable starting point when robotic exploration is eventually used to unlock Europa's secrets.

Artist's impression of a cryobot/hydrobot combination exploring Europa's ocean

Artist's impression of a cryobot/hydrobot combination exploring Europa's ocean

Titan

Background

Titan is the largest of Saturn's satellites, and the only moon in the Solar System which possesses a significant atmosphere. At a distance of 1.2 million kilometres, it orbits Saturn once every 20.3 days. It is made primarily from a mixture of ices and rocky compounds, accounting for its lowish density of about one third of the Earth's. Titan's diameter is over 5,000 kilometres, meaning that it is larger than Mercury.

Image of Titan taken by Voyager 1

Image of Titan taken by Voyager 1

Titan was revealed to have an atmosphere in 1944, when the signature of methane was discovered in its reflected-sunlight spectrum. The first close-up observations of the moon were taken by the two Voyager] spacecraft, which flew past in 1980 and 1981. They found Titan's atmosphere to be very thick (even more so than the Earth's), and to be made from a mixture of nitrogen, ammonia, methane and hydrocarbons such as ethane and propane. This mixture takes the form of an aerosol, similar to smog on Earth.

Environment

Surface temperatures on Titan are very low, around -180C. Therefore, any water will be frozen as ice. However, infra-red images of Titan reveal that the surface is not uniform, and has large dark areas which absorb infra-red radiation from the Sun. These areas have been interpreted by some as seas of liquid hydrocarbons, probably ethane, surrounding continents of rock and water ice.

Infra-red image of Titan, taken by the Hubble Space Telescope

Infra-red image of Titan, taken by the Hubble Space Telescope

The methane in Titan's atmosphere is continually being destroyed by incoming sunlight, leading to the formation of more-complicated molecules, of the sort which were found in Earth's early history. Atmospheric lighting may also play a part in this process, reminiscent of the Miller-Urey experiment (see [link:diploma-4|Lecture 4]). Although the temperatures on Titan are very low, it is widely accepted that complex organic chemistry occurs on this moon.

Prospects for Life

The present-day prospects for life on Titan are poor, due to the low temperatures. Nevertheless, Titan is very interesting from a biological standpoint, since its atmosphere and surface are expected to be rich in the prebiotic compounds which eventually produced life on Earth.

Our knowledge of Titan's atmosphere and surface will be greatly increased in 2004, when NASA's Cassini spacecraft arrives at Saturn. Its primary mission is to analyze the rings of Saturn; however, Cassini is also carrying the Huygens probe, manufactured by the European Space Agency. Huygens will be dropped into Titan's atmosphere, where it will descend slowly with the aid of a parachute, taking samples of the atmospheric aerosols. When it reaches the surface, Huygens will determine whether it is liquid or solid, and attempt to measure physical properties such as density and temperature.

Artist's impression of Huygens descending through Titan's atmosphere

Artist's impression of Huygens descending through Titan's atmosphere

Although Titan is almost certainly lifeless today, there is a chance that life may develop there in the distant future. When the Sun exhausts its supply of hydrogen fuel in its core, it will swell up into a red giant, and its luminosity may increase greatly. With the greater amounts of radiation reaching Titan, temperatures may rise high enough for the ice to melt, producing liquid water on the moon's surface. Since the necessary chemical building blocks would be in abundance, it is possible that life would then develop. However, one problem with this scenario is that, if temperatures were high enough on Titan for water to remain liquid, the low surface gravity of the moon would mean that the water would evaporate and escape into space.

Further Reading

For some entertaining reading regarding life in the solar system, I recommend the following books:

  • Mars, by Ben Bova. This book, and its sequel Return to Mars, tells the story of a manned expedition to Mars, and the discovery of ancient life there.
  • 2010 : Odyssey Two, by Arthur C. Clarke. This book, which is a sequel to the famous 2001: A Space Odyssey, includes a scenario where life develops on Europa.
  • Titan, by Stephen Baxter. This book tells the tale of a manned expedition to Titan. No life is discovered, but (without spoiling it!) that is not the end of the story!

All of these are science fiction, so be warned that any scientifically-accurate facts they contain are well-mixed with conjecture and artistic license. However, they are all well-written, and worth a read. Enjoy!


Updated 2009-10-13 13:21:36