Lecture 3: Search for life elsewhere in the solar system

Transcription

1 Lecture 3: Search for life elsewhere in the solar system I. Snellen, Leiden Observatory, February 28, The search for life on Mars 2 The search for life on icy moons 3 Titan: a moon with an atmosphere 1 The search for life on Mars sources: mainly from An introduction to Astrobiology, Gilmore & Sephton History Throughout scientific history, Mars has dominated discussions about possible existence of extraterrestrial life. One of the most influential astronomers was Giovanni Schiaparelli (19th century), who claimed to have observed Martian canali, which was translated to English as canals, assumed to be artificial (note that Schiaparelli remained sceptic). Percival Lowell devoted his life to studying Mars, hypothesizing that the canals were a result of attempts by the Martians to irrigate the planet from the melting polar caps. Even in the 1960s the seasonal changes in Martian colors were thought to be caused by vegetation by some astronomers. The general modern view point is that it is possible that some form of life has at some time existed on Mars, and that it is even possible that some form of life, maybe dormant is still present on the planet. No consensus among the scientific community has been reached. The planet is red because the soil is rich in iron-oxide (rusty). The search for liquid water (either at present, or evidence from the past) is now closely linked to the search for life itself. 1.1 The climate on Mars: past and present The habitable zone: Mars is located at 1.52 AU from the Sun. The exact outer edge of the habitable zone is model dependent. It is somewhere between 1.37 AU (where CO 2 start to condense), and 1.67 AU, the point at which a maximum greenhouse effect would operate. Mars is small: It has only 10% of the Earth s mass, and 40% of its surface gravity. It is therefore less capable of sustaining an atmosphere. The surface pressure is only 5.6 mbar, not enough for a greenhouse effect, resulting in an average surface temperature of 215 K. The Martian atmosphere is dominated by CO 2 (95.3%), N 2 (2.7%), and Ar (1.6%) (also some H 2 O at 0.03%). The Martian atmosphere is complete exposed to UV radiation, which will photo-dissociate CO 2, this is believed to be regenerated through chemical cycles involving photo-dissociated water. 1

2 Its surface temperature shows large variations in time and place, due to: 1. Eccentric orbit (e = 0.093, which accounts for 10% fluctuations in T [20 K]) 2. Large obliquity. Inclination of equator to the orbit is large latitudinal variations due to low thermal inertia (low atmospheric pressure) 4. Huge day and night variations due to the lack of substantial atmosphere ( K.) 5. Dust storms (so large they can engulf half the planet for weeks) can trap heat ( K) Polar caps: The ice caps are prominent surface features on Mars and from the early development of the telescope have been found to grow and shrink on the rhythm of the seasons. The variable part has been shown to be CO 2 ice (it snows dry ice in the winter), while the permanent parts of the caps are water(at least at the North pole). Under current conditions, water can not exist in liquid phase, since the triple point of water is at 0.01 c and 6.1 mbar, frozen water that is subsequently heated will sublimate. Long term variations is the Martian climate: There is very strong geological evidence that there have been periods with abundant liquid water on the surface of Mars (see below). Mars has a strong variation of the axial inclination with periods of 100,000 yrs, and 10 Myr, with the inclination angle varying between 0 and 35 (even up to Myr ago). Such variations also occur on the Earth, but to much lesser extent due to the larger distance to Jupiter and our moon that acts as a stabilizer. Still, on Earth, in combination with variations in the precession and eccentricity, these result in large climate changes and explain the Earth s 100,000 year ice age cycles (Milankovich cycles). These variations must be (are!) much stronger on Mars. (For detailed reading on evidence for the Milankovich effect, see For a mathematical explanation of precession see Planetary Science, Cole & Woolfson) During periods of low obliquity, there is never much sunlight at the poles, resulting in the poles having permanent ice caps. However, during high obliquity, the Sun heats up the ice cap, which is released in the atmosphere. This may have raised the pressure sufficiently for liquid water to be stable for certain periods. In a period of high obliquity, CO 2 and H 2 O would redistribute over the planet, and an annual exchange would take place between the Northern and Southern caps. Layered deposits of dust and ice at the Martian poles provide evidence for such quasi-periodic climate changes. It should also be deposited and stored as permafrost globally over the planet. Although the current Martial climate is too extreme for even Earth s most extreme extremophiles to replicate, they potentially could survive on the planet to wait for better circumstances. 1.2 The Viking space missions The first major mission to Mars was NASA s Viking project, consisting of two identical orbital/lander space crafts ( ), with on board a range of experiments to build up evidence for past or present life. it contained three important experiments to detect possible metabolic activity of potential microbial soil communities. Main assumptions were that life would be carbon based, its chemical composition would be similar to that of life on Earth, and that it would metabolize simple organic compounds. Each experiment was duplicated on heat-sterilized soil as a control sample. The Pyrolytic Release experiment (PR): Also known as the carbon release experiment. Of the three Viking biology experiments, this was the only one to attempt to detect signs of life in the complete absence of water and organic nutrients. It was assumed that any organisms on Mars would have developed the ability to assimilate carbon dioxide and carbon monoxide from the atmosphere and convert these, in the absence of water, to organic matter. Therefore, the PR experiment exposed a small sample of Martian soil to quantities of these two gases for 120 hours of incubation under a warm lamp, with the gases labeled with radioactive carbon-14 for detection purposes. Afterwards the soil was heated to break down any possible organic matter to be able to detect the possibly ingested radioactive carbon. (extracted from 2

3 The Gas Exchange Experiment (GEX): This experiment was designed to test for life under two conditions. In the first mode, it was assumed that organisms that had been dormant for a very long time under dry conditions on Mars would be revived and stimulated back into metabolic activity by the addition of moisture alone. The effect of any subsequent Martian life processes would be to alter the composition of the gases (such as carbon dioxide, nitrogen, and methane) above the sample in a way that would be measurable by the onboard gas chromatograph. During a 10-day incubation period, the gas composition was determined five times. In the second, wet nutrient (or chicken soup ) mode, a rich organic broth (containing 19 amino acids, vitamins, a number of other organic compounds, and a few inorganic salts) was fed to the sample as a further encouragement to induce metabolism. Again, a change in the gas mixture might indicate that some kind of organism was stirring into life. (extracted from The Labeled Release experiment (LR): Designed to detect carbon dioxide released by microorganisms as a result of their metabolic activity. A sample of soil was placed inside a culture chamber and a broth of 7 organic nutrients, labeled with radioactive carbon-14, allowed to drip onto it. If microbes were present in the sample, it was assumed they would metabolize the organic compounds in the nutrient and release radioactive carbon dioxide which would be trapped on a chemically coated film at the window of a Geiger counter. The LR experiment had the virtue of being able to detect growth, as well as metabolism, since the rate of carbon dioxide production would increase exponentially with a growing culture. (extracted from Both the PR and GEX experiments gave a positive result for both the sample and the heat-sterilized control. The LR experiment gave a positive results only for the sample (what in first instance would look like evidence for life). Gas Chromatograph - Mass Spectrometer (GCMS): The GCMS is a device which separates vapor components chemically via a gas chromatograph and then feeds the result into a mass spectrometer, which measures the molecular weight of each chemical. As a result, it can separate, identify, and quantify a large number of different chemicals. The GCMS was used to analyze the components of untreated Martian soil, and particularly those components that are released as the soil is heated to different temperatures. However, the GCMS measured no significant amount of organic molecules in the Martian soil, in fact the strongest organic concentrations it measured were minute trace contaminants brought from Earth, left over from the assembly and cleaning of the sample chambers and instruments. This result was difficult to explain if Martian bacterial metabolism was responsible for the positive results seen by LR. (extracted from biological experiments). It is now generally accepted that the results of the Viking biology experiments were caused by chemical and not biological activity, due to photochemical processes. However the exact details remain unclear. Still doubts?: Possible new insights in the Viking results (08/01/2007). A recent press release argues that the Viking experimental results could be explained by Martian life based on H 2 O 2 (hydrogen peroxide), and that the instrumental setup subsequently killed off these organisms, giving the strange results. (difficult to judge this theory) 1.3 Meteorites from Mars: ALH84001 How do we know that some meteorites come from mars? The formation ages of SNC meteorites (shergottites, nakhlites, and chassignites) are typically 200 Myr. This while the ages of most meteorites are 4.5 Gyr. Only a body of planetary dimensions could have produced sufficient heat 200 Myr ago, meaning that the only candidates for SNC meteorites are Mercury, Venus, Mars, and the moon. Some 40 meteorites have been found originating from the moon, but the SNC meteorites do not look like anything returned from the moon. Removal of material from Mercury, Venus, and Io which is subsequently caught by Earth is gravitationally very unlikely. The SNC meteorites are formed from magma, but are ejected from the surface through an projectile impact and show the effects of shock. it resulted in a shock-produced glass that trapped some of the atmosphere of the planetary body. The chemical abundance ratios match perfectly with that of the Martian atmosphere. 3

4 Meteorite ALH840001: Evidence for life? (1996): This meteorite was found on Antarctica in 1984, but only recognized as a meteorite from Mars in Geology tells us that it was formed 4.5 Gyr ago, and blasted of the planet 15 Myr ago, and arrived on Earth a few thousand years back. In 1996 NASA scientists announced of ALH84001 containing possible extraterrestrial life. Under the scanning electron microscope structures were revealed that for some time were considered to be the remainsin the form of fossilsof bacteria-like lifeforms. The structures found on ALH are nanometres in diameter, similar in size to the theoretical nanobacteria, but smaller than any proven cellular life[citation needed]. If the structures are really fossilized lifeforms, they would be the first solid evidence of the existence of extraterrestrial life, aside from the chance of their origin being terrestrial contamination (wikipedia). The announcement was reinforced by the then US president, Bill Clinton. Several tests for organic material have been performed on the meteorite and amino acids and polycyclic aromatic hydrocarbons (PAH) have been found. The debate if the organic molecules were created by nonbiological processes or are due to contamination from the contact with Antarctic ice is still on going. As of 2006 however, most experts agree that the microfossils are not indicative of life, but of contamination by earthly biofilms. It has not yet conclusively been shown how the features were formed, but similar features have been recreated in labs without biological inputs. According to the Associated Press, skeptics have found non-biological explanations for every piece of evidence that was presented on Aug. 6, And though they still vigorously defend their claim, the NASA scientists who advanced it now stand alone in their belief. (wikipedia) 1.4 Recent/current Mars space missions and the search for water In the 20th century there have been 32 missions to Mars, of which only 12 were in some way successful. Particularly landing on Mars is difficult. Currently there are 4 missions active on and in orbit around Mars. Their main goals are a better understanding of the geological history of mars, and the past and current role of water (for obvious reasons). Some highlights: Mars Global Surveyor (NASA, now) - gully, debris flows, gully formation - evidence for occasional sources of liquid water - sedimental layers on the pole - repeatable weather patterns Mars Explorer Rovers (NASA, now) - Range of geological evidence for water Mars Odyssey(NASA), Mars express (ESA) (2001; 2003, now) - identified subsurface water 1.5 Conclusions: possible habitats on Mars The surface of Mars is clearly very hostile to life: High level of short-wave UV radiation, and an dry, oxidized, dusty soil. However, there is strong evidence that there have been periods in the history of Mars, that its atmosphere was more substantial, and that liquid water did occur on its surface. Water that is currently locked in the polar caps, and subsurface, for which there is strong evidence for ice-rich layers. It is therefore possible that life once existed on Mars, or that life is still present (but dormant), surviving the harsh surface conditions underground, waiting for better... One mission to look out for is NASA s Phoenix lander to be launched this year. It is planned to landed in the polar region and dig for water ice to analyze. 4

5 2 The search for life on icy moons As mentioned in lecture 2, the Sun does not have to be the main source of energy for a solar system body. What is particularly interesting in the case of moons of Jupiter or Saturn is tidal heating, which warms up the inner moons of Jupiter, and in particular makes Europa interesting as a potential habitat for life. 2.1 Jupiter, Saturn, and their satellites Jupiter and Saturn are the two largest gas giants in the solar system, at 5.2 and 9.5 AU from the Sun (so well outside the circum-stellar habitable zone). Both have a rich system of moons and rings. More than 60 moons have been identified around Jupiter, and > 50 around Saturn. Much of our knowledge of these moons come in first instance from the Voyager I & II space missions. The largest moons are: Name Planet radius orbital distance Eccentricity Mass Grav surface orbiting Jupiter Io 1815km 422,000km e25kg g Europa 1565km 671,000km e25kg g Ganymede 2634km 1,070,000km e26kg g Callisto 2403km 1,880,000km e26kg g orbiting Saturn Rhea 764km 527,000km e24kg g Titan 2575km 1,221,000km e26kg g Iapetus 718km 3,561,000km e24kg g 2.2 Tidal heating of satellites Tidal heating is an important energy source for some of Jupiter/Saturn s moons. Most obvious in this respect is Io, which almost acts like one big volcano. Tidal force is caused by the fact that a gravitational force is not constant across a body of a certain size. Its strength is proportional to the third power of the distance and the radius of the body. F t = 2GMmr R 3 The energy generated in the planet caused by the tidal forces is extracted from the orbital energy and the spin energy of the planet. It will circularize the planet, and will lock the planet such that the rotational and orbital period are equal (like for our moon). Since the object is facing with the same side towards the planet, the tidal bulges will be locked and not generate any more heat. However, if the orbit is not exactly circular, the tidal force will vary with time, and although the rotational and orbital periods will be identical, the planet will move slower and faster through its orbit at certain times, making the tidal bulges move about a bit. This would still generate heat. There has to be a process that keeps the eccentricity non-zero to make this happen. This is the gravitational influence of other moons orbiting the planet. This is the strongest if moons are in resonance (orbital periods of adjacent moons are simple ratios). Europa s orbital is twice that of Io, and Ganymede has twice the orbital period of Europa. Of the 4 galileian moons, Io is the closest to Jupiter. It gets so much heated up by tidal heating that it has huge volcanos, and its surface is dominated by lava flows. The tidal heating on Ganymede and Callisto is probably too insignificant, but Europa is an interesting case 5

6 2.3 Jupiter s satellite Europa Jupiter s satellite Europa is a fascinating object. It has an albedo of 0.7 (70% of the light is reflected, see later in the course), due to its surface of relatively young ice (the younger the ice, the brighter). Midday temperatures are about -140 C at the equator and -190 C at the poles. It experiences virtually no seasonal changes during its orbit around Jupiter and Jupiter s orbit around the Sun. A sea underneath the ice? It is well possible that beneath the ice is an ocean, kept warm through the processes of tidal heating. Where there is liquid water, there may be life, in particular because life on Earth may have evolved initially from around volcanic activity on the sea floor. Near infrared spectroscopy shows that its surface is mostly water ice, which turns out on some places to be salty (magnesium and sodium salts). The distribution of salt is highly non-uniform. In places where the surface seems to be disrupted most recently, the salt concentration can reach 99%. Geological evidence for an ocean So far, evidence is insufficient to rule out that Europa consists of solid ice down to its rocky shell. However: 1. The shape of impact craters; There are few two obvious (a few million years old) impact craters visible on the surface, e.g. Pwyll. The rim of Pwyll is only 200m high, and the crater floor is barely lower than the terrain outside. This is all consistent with an impact into relatively thin (20 km) and weak ice. 2 Fracturing and motion of the ice shell: There is evidence of fracturing and motion of the ice shell on Europa (see lecture slides). Complicated geological structures are seen, reminiscent of balls of string, probably the result of some form of cryovolcanic eruption along a crack. Tidal heat and life: It is not clear ow the tidal heat is dissipated. At one extreme all could be dissipated in the icy shell, keeping the ocean warm from heating from above. This would not give much room to hydrothermal vents. On the other hand, if the tidal heating would occur deeply in its interior, possibly melting part of its rocky crust, hydrothermal vents could be abundant, allowing more complex chemical processes. We could find out whether Europa has an ocean of liquid water under its icy shell by measuring the hight of the tidal bulge. It should be about 1 m high if it is made of solid ice, and about 30 m high if it has a 10 km deep ocean. This possibly could be measured with a future space mission (NASA s JIMO mission). Only the moons Triton (Neptune) and Enceladus (Saturn) have a higher albedo than Europa, and seem to have an even younger ice-sheet surface. Both moons show cryovulcanic activity and spit out water, indicative of a liquid ocean below the surface. 2.4 Earth s equivalent of icy moons?: Lake Vostok In the 1960s Russian scientist hypothesized that below the ice sheet on Lake Vostok in the antarctics could be a sea of liquid water, based on results of seismic soundings. This was confirmed through satellite ice sheet mapping. The oldest ice overlaying the water is less than a million years old, but it is slowly moving across the lake, which seems to have been sealed off from the surface for more than 10 million years. The Russians started drilling the 3km thick ice sheet in 1974, to study samples of the ice and gas and trace materials in it that provide a continuous record of climate changes and large volcanic eruptions over the last hundred thousands of years. Drilling was stopped about 100m before reaching the bottom of the ice sheet in fear of contamination. The bottom few hundred meter of the ice sheet is actually (re)frozen water from the underlying lake, which contained micro-organisms possible from an indigenous ecosystem. No-one knows what kind of organisms could survive the cold, darkness and pressure of 360,000 mbar, but it are environmental circumstances that are reminiscent of that of Jupiter s moon Europa. The problem of using the Russian bore hole is that to keep it from freezing, they pumped about 60 tonnes of toxic chemicals into it, which could leak into the lake if drilling would continue. 6

7 3 Titan: a moon with an atmosphere 3.1 History Titan, the only moon with a significant atmosphere, is one of the most enigmatic bodies in the solar system. It was discovered by Huygens. Infrared spectroscopy by Kuiper in the 1940s showed its atmosphere to consist methane. The Voyager II mission showed that its atmosphere consists for 97% of nitrogen, 3% of methane, and further traces of other carbohydrates. Unfortunately, Titan further remained a mystery because the atmosphere is too thick to peer through. 3.2 The Cassini-Huygens mission The rich inventory of hydrocarbon gases makes Titan a vast organic chemical laboratory, allowing the study of organic chemical evolution under total natural circumstances over long time spans. In particularly is the link with prebiotic chemistry which is believed to have happened on the young Earth (see lecture 1). The surface temperature of Titan is about 94 K, and surface pressure is between 1000 and 1500 mbar. The Cassini/Huygens mission is a collaboration between NASA and ESA, and reached Saturn and Titan in It released its Huygens probe into the atmosphere of Titan. For the first time the secrets of Titan were revealed, which is the topic of the guest lecture of Lebreton (project scientist of Huygens). Some of the highlights are summarized here: The probe made a soft landing on the surface of Titan, making thousands of pictures on the way it showed a varied landscape, mountain ranges and channels indicative of surface liquids. Evidence for methane rain The first body outside Earth on which lakes are present, filled with liquid methane. 7