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The Astrobiology of Mars

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1 The Astrobiology of Mars

2 Martian geology: a rapid summary
Mars is geologically distinctive at the large scale. The southern hemisphere is made of ancient cratered terrain and the northern hemisphere flat, more recent (less impacted) terrain that is at lower elevation. This distinctive geology is called the crustal dichotomy. Its origins are not known. Hypotheses include a role for single or multiple large impacts that created the depressed lower elevation of the northern hemisphere. Alternatively, the separation was the result of inhomogeneous mantle convection in an early plate tectonic regime.

3 Martian geology: a rapid summary

4 Martian geology: a rapid summary
Like the Earth, the history of Mars has been split into distinctive geological eons. The eons broadly reflect large scale environmental changes on Mars.

5 The deterioration of Mars
An obvious question is how a planet, that on the face of it seems quite similar to the Earth in its early history (it hosted large quantities of liquid water) became so barren. The answer to this question is by no means simple, but one large scale factor that we can identify as being a contributor is the small size of Mars. Mars cooled quicker than the Earth because of its smaller radius. As a result, the dynamo that generates the magnetic field, and on the Earth protects our atmosphere from being sputtered away by the solar wind, shut down. Magnetic stripes in rocks of the southern hemisphere of Mars have been suggested as evidence for an early tectonic regime where rocks were being exuded from the surface retaining the magnetic field of this early history.

6 The deterioration of Mars
Magnetic stripes in rocks in the ancient cratered surface of Mars give tantalising evidence of the possible record of a changing early magnetic field in rocks being extruded onto the surface of early Mars. Data was collected by the Mars Global Surveyor. Units in Tesla are given underneath the image.

7 The deterioration of Mars
The sputtering away of the atmosphere cannot entirely account for all of the lost atmosphere. Two other processes made a very significant contribution - hydrodynamic escape and impact erosion. The period of early intense bombardment would have lent a helping hand. Giant impacts would have heated atmospheric gases, aiding their escape from the atmosphere compared to a more massive planet. It is thought that this impact erosion may have caused about 90% of the loss of the atmosphere, with the remaining loss accounted for by the other processes described earlier. All of these factors meant that that the atmosphere thinned, water froze and eventually the atmospheric pressure reached values near the triple point, precluding the possibility of surface liquid water. Today, the average surface atmospheric pressure is almost exactly on the triple point (6 mb), meaning that water ice, when heated, sublimes rather than forming standing bodies of liquid water. Even in the deepest part of Mars, the Hellas Basin, the pressure only reaches 11.6 mb, whereas at the highest point, the summit Olympus Mons, the pressure is 0.3 mb.

8 Missions to Mars The early exploration of Mars involved the Mariner spacecraft. There were three fly-by missions by Mariners 4, 6 and 7 and one orbiter mission (Mariner 9). These missions revealed a barren landscape covered in impact craters, not unlike the Moon. Early Mariner 9 image of Mars taken in The towering peaks of the Tharsis volcanoes begin to appear through a global dust storm.

9 Missions to Mars In 1976 the Viking Orbiters and landers further advanced our knowledge of the ancient surface of Mars. They revealed valley networks, tear-drop shaped islands and other indications of water. The two landers carried the first biology experiments sent to another world to seek signs of life in the Martian soils. From 1997 until 2006, the Mars Global Surveyor marked the return of the US to Mars exploration. It was equipped with instrumentation to provide detailed images of Mars, measure its magnetism, surface composition and atmospheric temperatures. Its Mars Orbiter Laser Altimeter (MOLA) provided stunning topographical maps of Mars and the mission took many images that would later be used to decide the landing sites for Phoenix and Curiosity (the Mars Science Laboratory). Mars Pathfinder was a technology demonstration mission in which a static lander was combined with a small robotic rover (Sojourner) that could analyse nearby rocks seen by the lander. It landed in 1997 in the Ares Vallis, in a region called Chryse Planitia. It had X-ray spectrometers and cameras which provided chemical analysis of rocks near the landing site.

10 Missions to Mars Following the success of the Sojourner rover and the scientific questions that arose from data sent back from Mars Global Surveyor, two identical rovers, the Mars Exploration Rovers (MER), ‘Spirit’ and ‘Opportunity’, were sent to two different locations to explore the geology of Mars, beginning in 2004. To achieve this, they were equipped with instruments designed to analyse the fine-scale textures and chemical composition of rocks and soils including a microscopic imager, a miniature thermal emission spectrometer (Mini-TES), X-ray spectrometry (APXS) and a rock abrasion tool (RAT) that allowed for investigations just below the surface of rocks. Opportunity was sent to Meridiani Planum to investigate hematite-rich sedimentary outcrops that formed through the result of acidic fluids leaching through the rocks, leaving this iron oxide mineral (hematite has the formula, Fe2O3). Spirit was sent to Gusev Crater, and explored the nearby feature ‘Home Plate’. This area was found to be volcanic, with evidence for hydrothermal activity in the form of 90% pure silica deposits suggesting an ancient hydrothermal system (Figure 6). Both these findings suggested liquid water environments that might once have been habitable. The rovers significantly outlasted their 90 sol nominal mission and were operating ten Earth years after their arrival. .

11 Missions to Mars Pale silica deposits on Mars revealed by the wheels of the Spirit rover. The groove is ~20 cm across. The deposits are evidence for water-rock interactions.

12 Missions to Mars Alongside the MER rovers came the European Space Agency’s Mars Express Orbiter Mission, which flew instruments to map the subsurface (MARSIS; Sub-Surface Sounding Radar Altimeter) and map minerals on the surface (OMEGA; Visible and Infrared Mineralogical Mapping Spectrometer). It also had a high resolution camera that gathered spectacular images used by geologists – the HRSC (High Resolution Stereo Camera). The craft also had a landing element, the Beagle 2 lander, which successfully landed on Mars, but did not fully deploy. It would have carried out surface geology and astrobiology experiments including the search for carbon. A few years after Spirit and Opportunity, another orbiter (the Mars Reconnaissance Orbiter) was sent to Mars equipped with instruments to image the surface at high resolution. These included HiRISE (High Resolution Science Experiment), which could resolve images ~1 m in size, CTX – a context camera to provide three dimensional images of the Martian surface, and CRISM (Compact Reconnaissance Spectrometer for Mars) – a spectrometer to image the surface at wavelengths spanning the visible to infrared, revealing the fine-scale mineralogy of Mars. CRISM in particular found the first extensive deposits of ‘hydrated minerals’ – minerals including clays (phyllosilicates), sulfates (e.g. gypsum) and carbonates that could only form in the presence of liquid water.

13 Missions to Mars False colour image of the edge of Jezero crater (49 km in diameter) in the northern hemisphere of Mars (at 18.4°N 282.4°W) in the Nili Fossae region showing delta-like features near the end of a putative ancient water channel (coming in from the left). Phyllosilicate-bearing materials are green, olivine-bearing materials are yellow, low-calcium pyroxene-bearing materials are blue and purple–brown surfaces have no distinct spectral features. The image shows an area approximately 10 km across (image: NASA/JPL/JHUAPL/MSSS/Brown University).

14 Missions to Mars In 2008, the NASA Phoenix lander was sent to the north polar region of Mars to study water-ice deposits. It landed at °N °E in the Vastitas Borealis (Figure 8). Perhaps one of the most startling discoveries of the lander was near-surface ice, which was imaged by the lander cameras and was also observed in regions where the rocket exhaust had blasted away the surface dust. The Phoenix lander had a wet-chemistry lab on board, which showed the presence of perchlorates (compounds containing the perchlorate ion; ClO4-) within the Martian soil. This was an unexpected finding. The presence of perchlorates may provide an explanation of the lack of detection of organics in earlier missions, such as Viking. Perchlorates would have oxidised organic molecules when heated to high temperatures, such as during pyrolysis (the heating step) during mass spectrometry analysis of Mars soils by the Viking landers. The wet chemistry lab showed the surface soil was moderately alkaline (between pH 8 and 9). Magnesium, potassium, sodium and chloride ions were detected and the salinity was low, suggesting a very benign environment from the point of view of life.

15 Missions to Mars In 2008, the NASA Phoenix lander was sent to the north polar region of Mars to study water-ice deposits. It landed at °N °E in the Vastitas Borealis (Figure 8). Perhaps one of the most startling discoveries of the lander was near-surface ice, which was imaged by the lander cameras and was also observed in regions where the rocket exhaust had blasted away the surface dust. The Phoenix lander had a wet-chemistry lab on board, which showed the presence of perchlorates (compounds containing the perchlorate ion; ClO4-) within the Martian soil. This was an unexpected finding. The wet chemistry lab showed the surface soil was moderately alkaline (between pH 8 and 9). Magnesium, potassium, sodium and chloride ions were detected and the salinity was low, suggesting a very benign environment from the point of view of life.

16 Mars and water Liquid water and Mars
Liquid water is the essential solvent for life and its history on Mars underpins our understanding of the habitability of the planet. Generally speaking, Mars has had a history of declining liquid water availability over time. In the earliest history of the planet there is a great deal of evidence for liquid water. The presence of liquid water on ancient Mars is supported by observations of clay compounds produced when water reacts with volcanic rocks - the phyllosilicates. Phyllosilicates are found in many Noachian terrains. In some regions magnesium carbonates are found associated with the clays, also evidence of liquid water.

17 Mars and water Geological diversity in the Nili Fossae, Mars. Image of part of a fracture in the Nili Fossae region near 21.9°N, 78.2°E; Compact Reconnaissance Imaging Spectrometer for Mars (CRISM). The top left shows the Isidis basin image (small square) superposed on a Mars Orbiter Laser Altimeter (red higher elevations, blue lower). The fracture shown, which is 11 km at its narrowest point (top right) is overlain on a Viking orbiter digital image (lower left) to show topography. The top right image also shows the region in the infrared channels, false coloured. Bright green is phyllosilicates, yellow-brown are olivines, purple are pyroxenes. The CRISM data is superposed on High-Resolution Imaging Science Experiment (HiRISE) (lower right) showing that phyllosilicates are in small eroded outcrops of rock and olivines in sand dunes. (image credit NASA/JPL/JHUAPL/Brown University).

18 Mars and water Like the Earth, early Mars too confronts us with a Faint young Sun paradox. How was the liquid water sustained on the surface when the Sun was less luminous? Various proposals have been advanced, from a greenhouse effect caused by CO2 ice clouds to high concentrations, or episodic production, of volcanic SO2. It has also been proposed that much of the water could have been under glacial ice sheets rather than directly exposed at the surface, invoking ideas of a cold, wet Mars rather than a warm, wet Mars. As the Noachian transitioned into the Hesperian, water bodies became less abundant, but nevertheless there is still evidence for groundwater activity during this time. For example, layered terrains in the Burns Formation, Meridiani Planum are interpreted to be sandstones formed in shallow water systems and have been the subject of intense geochemical and geological discussion. The widespread presence of sulfate salts and observations of hematite concretions support a model of water in the ground seeping up and altering the surface, leaving iron oxides and sulfate salts behind. During the Hesperian, the hydrological cycle was generally characterised by low water-rock ratio interactions in the near-surface environment.

19 Mars and water Catastrophic outflow channels provide particularly compelling evidence for subsurface and surface water since the Noachian. These features begin from a fracture or region of chaotic terrain and consist of broad depressions tens to thousands of kilometres long with streamlined islands and deposits around craters along their beds. Catastrophic outflow channels on Mars. A) Kasei Vallis (25° N, 300° E), the largest outflow channel on the planet, emerges from a shallow north–south canyon to the west. Scale bar 100 km. B) Streamlined islands near the mouth of the Ares Vallis outflow channel in Chryse Planitia. This image shows two teardrop-shaped scarps with heights of 400 m (upper island) and 600 m (lower island) formed by the erosive power of the flood that passed through this region early in Martian history. The lower crater is 10 km in diameter (image credit NASA).

20 Mars and water Mars today hosts a large body of frozen water. The near-surface (to a depth of tens of centimetres) of Mars is thought to harbour water ice deposits that vary from 2% weight at the equator to pure ice at the polar regions mixed with surface volcanic regolith. These ice deposits have been detected with the Gamma Ray Spectrometer on the Mars Odyssey spacecraft. Hydrogen in the surface of Mars detected by Gamma Ray Spectroscopy, interpreted as the lower limit of water in the near surface of Mars (mass fraction shown in the colour scale at the top of the image) (image: NASA/JPL-Caltech/Los Alamos National Laboratory).

21 Mars and water Quite apart from massive ice deposits at the poles, with an estimated volume of x 106 km3, and buried ice just discussed, there is a large literature on other glacial and periglacial features on Mars. Evidence for subsurface ice includes distinctive ordered features in the ground such as polygonal structures (which form when ground freezes and thaws), parallel sorted stone stripes (which also form from freezing and thawing of the ground), gullies and ice-sublimation related features among others. Pingos, which are mounds produced by liquid water being injected into the subsurface that then freezes and expands, causing upheaval, have been suggested. They are of particular interest as their formation mechanism involves bulk liquid water movement. Observations from orbit include recently excavated small craters which reveal water ice. The ice is observed to sublimate away after several months

22 Mars and water Recent small craters on Mars reveal subsurface ice which is observed to sublimate after a few months. This is a fresh, 6-meter-wide, 1.33-meter-deep crater on Mars photographed on Oct. 18, 2008, and again on Jan. 14, 2009, by Mars Reconnaissance Orbiter's HiRISE camera.

23 Mars and water Liquid water at the surface of Mars today is rendered unstable, partly because much of the surface is at the triple point and partly because the low humidity means that when liquid water is formed, it will rapidly evaporate, even if it does not boil. However, evidence has been suggested for near-surface present-day liquid water. Other evidence for present-day water has been suggested. Dark slope streaks which are observed on Mars and change seasonally could be present-day near-surface salty water. They are called Recurring Slope Lineae (RSL).

24 Mars and water Support for the stability of brines on present-day Mars? Top. Palikir Crater, which is inside the much large Newton crater (41.6°S; 202.3°E), contains thousands of individual flows called Recurring Slope Lineae, or RSL. In the Southern middle latitudes, RSL form and grow every summer in certain places, fading in late summer and fall (examples are shown with red arrows). This image shows a comparison of RSL from 2011 to 2013 over a small piece of Palikir Crater's steep northwest-facing slopes. The new image shows RSL are slightly more extensive and longer than at nearly the same time the year before (image: NASA/JPL-Caltech/Univ. of Arizona). Below. Putative drops of brine on the legs of the Phoenix lander that change positions over many days (LMST is Local Mean Solar Time). (image: NASA).

25 Mars and life Basic elements for life on Mars
Life requires six basic elements to construct macromolecules (C, H, N, O, P, S). Carbon atoms are likely to have been, and continue to be, present in the surface and subsurface of Mars as a consequence of atmospheric exchange (in the present day atmosphere the composition is 95.32% CO2 and includes 800 ppm CO) and could be acquired by life through autotrophy. The detection of carbonates suggests that aqueous interactions with these rocks could generate a source of inorganic autotrophically available carbon throughout Martian history as bicarbonate ions. The concentration of organic carbon on Mars, a potential source of carbon for heterotrophs, in different regions and depths on Mars, is unknown. The in-fall of carbonaceous chondrites and other organic carbon-bearing material is expected, but organics are likely to be destroyed by reactive oxygen species, ultraviolet radiation and ionising radiation in the near-surface environment.

26 Mars and life Basic elements for life on Mars
Hydrogen atoms are available from water throughout the Martian depth profile, which could be split radiolytically (by radioactivity) in the subsurface to produce hydrogen. Hydrogen could also be generated in chemical reactions. The presence of serpentine in impact craters suggests the possibility of hydrogen production through serpentinisation reactions, particularly when water flow was more extensive in the Noachian. Nitrogen gas is present in the modern atmosphere at 2.7%. Fixed nitrogen compounds have been reported in Martian meteorites and confirmed on the surface of Mars. They have been predicted to include nitrate and ammonium based on terrestrial analogues. The global distribution, however, of biologically accessible nitrogen is not known. Oxygen atoms could be provided by CO2, H2O, sulfates, perchlorates, ferric oxides and reactive oxygen species. Oxygen atoms are bound to many of the biologically accessible compounds discussed here in association with other elements (C, H, N, P, S).

27 Mars and life Basic elements for life on Mars
Phosphate has been reported in Martian meteorites and on the surface of Mars in a number of missions. For example, the Mars Exploration Rovers found rocks containing apatite (a group of phosphate minerals) at between 0.1 and 2.4 % weight. Some rocks with phosphorus abundances of over 5% were observed in Gusev crater by the Spirit rover. Phosphorus was observed in alkaline basalts studied in Gale Crater by the Curiosity Rover at <1% weight abundances. Sulfur has been detected on Mars in meteorites and on the surface of Mars in the form of sulfate salts including gypsum, ferric sulfates, jarosite and other S-bearing species in different oxidation states, including sulfides. The extent of these compounds in the subsurface is not known, but the dominance of the sulfur cycle on Mars suggests that sulfur species would have been distributed from the mantle to the surface throughout Martian history, potentially including sulfur in microbially accessible gaseous phases such as H2S and SO2.

28 Mars and life Rocks on Mars have many of the trace elements required for life. This is a vintage image from the Viking 2 landing site with a landing leg in the bottom right and a protective canister from one its instruments on the surface. The landing pad diameter is approximately 30 cm (image: NASA).

29 Energy and redox couples for life on Mars
Photosynthesis would be a plausible mode of metabolism if surface water was available. There are depths on the order of millimetres or less in the near-surface where the ultraviolet (UV) biologically effective irradiances are no worse than on Earth today, but where photosynthetically active radiation (PAR) is sufficient for phototrophy, such as anoxygenic photosynthesis using ferrous iron or reduced sulfur species. The lack of liquid water on the surface today precludes a productive surface photosynthetic biosphere. As for subsurface life on Earth, photosynthesis is eliminated at depth. Chemoautotrophic redox couples are an alternative energy source. Ferric and sulfate ions as electron acceptors, both detected on Mars, can be reduced with hydrogen (suggested from the presence of olivine, serpentine and other substrates or products of hydrogen-evolving mineral weathering such as serpentinisation). On the Earth, hydrogen can act as the electron donor in the subsurface for microbial redox reactions with sulfate and ferric iron. Large resources of ferrous (Fe2+)-bearing minerals such as olivines are available for chemoautotrophic iron oxidation.

30 Energy and redox couples for life on Mars
The presence of reduced sulfur species such as sulfides, found in Martian meteorites and on the surface of Mars, suggest the possibility of sulfur species oxidation. However, anaerobic conditions prevent chemoautotrophic sulfur species oxidation using oxygen as the terminal electron acceptor. Sulfur can be oxidised using ferric iron as the electron acceptor. Other chemoautotrophic redox couples could include methanogenesis and acetogenesis, both using CO2 from the atmosphere or from dissolved carbonates as the electron acceptor and H2 from serpentinisation reactions, as observed in the subsurface of the Earth, as the electron donor. Methane itself can be oxidised by microorganisms as a source of energy and could be produced abiotically. Serpentinised ultra-mafic rocks are known to host thriving microbial communities in the subsurface of the Earth and could provide analogies to potential water-rock-microbial interactions for the Martian subsurface.

31 Physical limits to life: Radiation
Mars lacks any significant amount of ozone or other gaseous absorbers in the atmosphere that can absorb ultraviolet (UV) radiation above 200 nm. Like the early Earth, the surface of Mars therefore experiences UV radiation down to 200 nm (CO2 absorbs most of the radiation below 200 nm). However, UV radiation is rapidly attenuated in the subsurface, so although the surface flux generates biologically-effective DNA damage about three orders of magnitude higher than on the surface of the Earth, within a depth of a just few tens of microns to millimetres, depending on soil particle size, UV radiation is extinguished. Today, the surface of Mars is uninhabitable because of desiccation. However, UV radiation would not, in itself, create uninhabitable conditions if other conditions for life were met. Ionizing radiations of solar energetic particles (SEP) and galactic cosmic rays (GCR) are more penetrating. The total dose of ionizing radiation experienced on the Martian surface has been measured as 76 mGray(Gy)/yr, much lower than the fluxes that can be tolerated by radioresistant organisms such as Deinococcus radiodurans, which can withstand doses in excess of 5 kGy without appreciable loss of viability. However, inactivity would result in accumulated damage such that at 2 m depth in the Martian crust, a D. radiodurans population was estimated to suffer an approximately six order of magnitude reduction in viability after 450,000 years. For a deep subsurface biota just a few meters depth or greater on Mars, particularly one that is active and can repair damage in an environment where liquid water is available, radiation would not render the subsurface uninhabitable.

32 Physical limits to life: pH
In a variety of Martian settings, pH ranges are within the boundaries for life. The pH of the Martian near subsurface was measured at the Phoenix lander site. It was found to be slightly alkaline, , and carbonate-buffered. The pH at Yellowknife Bay, Gale Crater was also inferred to be neutral from Curiosity data. Although many environments in the ancient history of Mars may have been neutral to alkaline, the presence of certain sulfate minerals on the surface of Mars suggests locally acidic conditions. Sulfates are found as Hesperian layered sulfates, polar deposits, sediments in craters, within the globally ubiquitous Martian dust (which may contain 5-10% sulfates) and as sulfate veins within rocks.

33 Physical limits to life: salts
Brines can constrain the boundaries of active life by influencing water activity and other parameters such as chaotropicity (the degree of disorder induced in macromolecules). Extremely low water activities and high chaotropicities can be generated by brines such as chlorides (CaCl2) and mixed sulfate brines. On present-day Mars, seasonally Recurrent Slope Lineae (RSL) could be formed from concentrated briny solutions. Some Martian brines are calculated to have water activities below those required for life and would not be habitable environments. Thus, as the hydrological environment of Mars transitioned from the Noachian into the Hesperian and salt saturated solutions became prevalent in groundwater environments, some of these briny solutions could have rendered localised environments uninhabitable.

34 Physical limits to life: salts
Brines can constrain the boundaries of active life by influencing water activity and other parameters such as chaotropicity (the degree of disorder induced in macromolecules). Extremely low water activities (Chapter 7) and high chaotropicities can be generated by brines such as chlorides (CaCl2) and mixed sulfate brines. On present-day Mars, seasonally Recurrent Slope Lineae (RSL) could be formed from concentrated briny solutions. Some Martian brines are calculated to have water activities below those required for life and would not be habitable environments. Thus, as the hydrological environment of Mars transitioned from the Noachian into the Hesperian and salt saturated solutions became prevalent in groundwater environments, some of these briny solutions could have rendered localised environments uninhabitable.

35 Martian Habitability: Example Trajectories
Supported by this previous synthesis of environmental conditions that would have influenced conditions available to support the activity of organisms, it is possible to think about trajectories of the habitability of Mars that are consistent with these data. All trajectories of Martian habitability begin with the formation of Mars. From early planetesimals an uninhabitable planet formed, much like the Earth. As water condensed and the environment cooled, the planet was at a branch point in its long-term trajectory of biological conditions. In one set of trajectories, the planet is defined by its condition as uninhabited (neither an origin of life occurs, nor does life transfer to the planet from the Earth in meteoritic matter). In the second set of trajectories, the planet is defined by the establishment of life, an event that changes the use of habitable conditions and through feedback effects, would itself change the habitability of environments.

36 Martian Habitability: Example Trajectories

37 The Viking programme and the search for life
The NASA Viking missions were motivated by the possibility of life being present on Mars. The mission had two landers that arrived on the surface of Mars in 1976, Viking I and II and two orbiters. The Viking landers carried out other scientific objectives, including the first surface measurements of the composition of the Martian atmosphere and surface rocks. However the principal objective was to establish whether or not there was life on Mars.

38 The Viking programme and the search for life
The Viking landers had four experiments to look for evidence of active microbial metabolism. Let’s look at these experiments. Although they are now rather old, they are a very good example of applying the scientific method on other planets and the importance of controls. Gas Chromatograph — Mass Spectrometer (GCMS) analysis The GCMS was used to analyse the components of Martian soil, and particularly those components that were released as the soil was heated to different temperatures Gas Exchange Experiment The Gas Exchange (GEX) experiment looked for gases given off by an incubated soil sample. It applied a liquid mixture of organic and inorganic nutrients, first with nutrients added, then with water added as a control. After incubating for 12 days, the instrument sampled the atmosphere of the chamber and used a gas chromatograph to measure the concentrations of several gases, including oxygen, carbon dioxide, nitrogen, hydrogen, and methane. The scientists hypothesized that metabolizing organisms would produce one of these gases. The result of the experiment was the production of oxygen. However, the gas was also produced in controls heated to 145°C, which was not consistent with microorganisms, but could be explained by the presence of reactive compounds in the soil.

39 The Viking programme and the search for life
Labelled Release Experiment In the Labelled Release Experiment (LR), a sample of Martian soil was inoculated with a 1 mL drop of very dilute nutrient solution tagged with radioactive 14C. After incubation for 10 days, the air above the soil was monitored for the evolution of radioactive 14CO2 gas as evidence that microorganisms in the soil had metabolized one or more of the nutrients. The result was the production of radioactive gases. The control samples heated to 160°C did not give the same result, which was consistent with biology. However, subsequent injections into the experimental chambers a week later did not give the same reaction, which would not be expected for biology. Pyrolytic Release Experiment Light, water, and an atmosphere of carbon monoxide (CO) and carbon dioxide (CO2), simulating that on Mars, were introduced into the experimental chamber with a 0.25 cm3 soil sample. The carbon-bearing gases were made with 14C. If there were photosynthetic organisms, they would incorporate some of the carbon as biomass. After 120 hours of incubation using a xenon arc lamp to provide light, the experiment removed the gases, baked the remaining soil at 625 °C and collected the products in a device which counted radioactivity. If any of the 14C had been converted to biomass, it would be vaporized during heating and the radioactivity counter would detect it as evidence for life. Controls using heated soil were also investigated. In both the experimental and heated control samples, gases were released from apparently small amounts of fixed carbon. This was not consistent with microorganisms. Overall, the results from Viking are thought to be explained by active chemical components in the Martian soils, not biology.

40 Martian Meteorites The number of Martian meteorites identified is small. There are just over 130, although this number continues to grow. Their very different composition from Earth rocks and other meteorites marked them out from the early days of meteorite hunting. The meteorites contain pockets of gas trapped in glasses when the rock was melted during ejection from the surface of Mars. These pockets of gas have sampled the atmosphere of Mars, one of the lines of evidence that demonstrates they are from Mars. The gases have elemental and isotopic compositions that are similar to atmospheric analyses on Mars. Furthermore, the mineral composition of the rocks themselves is consistent with a Martian origin. The abundance of gases found trapped in the glasses of the Martian shergottite meteorite EET A79001 compared with the Martian atmosphere (as determined from spacecraft) suggests that samples of Martian atmosphere was trapped in melted glass when the meteorite was ejected from the Martian surface.

41 Martian Meteorites The Martian meteorites are divided into 3 groups, the shergottites, nakhlites and chassignites (or SNC meteorites, the general name given to Martian meteorites). There are also two ‘grouplets’ called the orthopyroxenite and basaltic breccias. Perhaps the most famous Martian meteorite is ALH The rather cumbersome name does have a logic. The ‘ALH’ refers to the Allan Hills region of Antarctica where it was found; the ‘84’ refers to the year it was collected – 1984; the 001 designates that it was the first sample to be analysed when returned for curation. The meteorite falls into the grouplet of the orthopyroxenites (an igneous rock composed of pyroxene). ALH84001 is about 4.1 billion years old, in other words contemporaneous with the evidence for early liquid water on the planet, and it has been the focus of controversy about life on Mars.

42 Martian Meteorites In 1996 a bold claim was made in the journal, Science, by a group led by David McKay ( ) at the NASA Johnson Space Centre, stating that evidence for primitive Martian life had been found in ALH Many lines of evidence were presented to prove the case for Martian life. An examination of the meteorite showed that: 1) it contained tiny carbonate globules which were interpreted as being formed at low temperatures conducive to life, 2) it contained polycyclic aromatic hydrocarbons (PAHs) associated with the carbonate globules, 3) it contained shapes that resembled microfossils of primitive bacteria about 300 nm long it contained magnetite mineral phases of ~40-60 nm in size, consistent with by-products of bacterial activity and particularly magnetotactic bacteria. These organisms are known to produce chains of iron oxide minerals with which they can orient themselves along the Earth’s magnetic fields as they search for the optimum oxygen concentration. These were claimed to be the products of biological activity. The original claim was that each one of these lines of evidence could be separately accounted for by non-biological processes, but together they constitute evidence for life. Today this evidence remains controversial with non-biological origins being claimed for each finding.

43 Martian Meteorites Putative microfossils in ALH The putative microfossil shape is ~400 nm long. These images are highly suggestive and caught the imagination of the media, but morphology alone is not compelling evidence for life.

44 Mars analogue environments
A Mars analogue environment is an environment on Earth that hosts conditions comparable to either past or present environments on Mars and in some way allows us to understand the geophysics, geochemistry or the potential habitability of the planet. Examples of Mars analogue environments include: 1) Acidic environments, which provide insights into the geochemistry and habitability of Hesperian acidic terrains. An example is the acidic Rio Tinto River, Spain. 2) Permafrost, which is used to understand possible processes in Martian permafrost. Examples include the Canadian High Arctic (e.g. Axel Heiberg Island, Devon Island) and Greenland. 3) Dry regions used to understand the habitability of extreme desiccated environments relevant particularly to present-day Mars. Examples are the Dry Valleys of Antarctica and the Atacama Desert, Chile. 4) Volcanic environments used to study ice-volcano interactions and the habitability of igneous rocks and volcanic environments. An example is the many terrains of Iceland (Figure 21). 5) Caves, used to investigate near-surface processes and the habitability of enclosed subsurface regions. An example would be lava tubes and caves on the island of Lanzarote, Spain. 6) Underground laboratories used to study deep subsurface processes that could be of relevance to Mars

45 Mars analogue environments
Iceland is an environment with volcanic rock-ice interactions which provides an analogue location for understanding the past geochemistry and habitability of Mars. This image shows lava flows in the south-east of the country.

46 Panspermia – transfer of life between planets?
Are planets biogeographical islands? A schematic showing the dispersal filters that life must survive to be transferred from one planet to another. The thickness of the atmosphere is exaggerated.

47 Panspermia – transfer of life between planets?
Ejection from a planet. Shock experiments using gas gun and plate-flyer apparatus carried out by numerous researchers show clearly that survival depends both on the nature of the cell and its local microenvironment. Spores of Bacillus subtilis show high resistance to shock, with survival of shock pressures greater than 50 GPa. The experiments show that even for desiccated vegetative cells, shock pressures at the lower end required for escape velocity from Mars (and Mars-like planets) can be survived. A light gas gun used to simulate asteroid impact.

48 Panspermia – transfer of life between planets?
Interplanetary transfer phase. Organisms must also survive the journey through space. During the interplanetary transit phase organisms near the surface of the rocks are potentially subject to high UV radiation. A diversity of published work shows that layers of different mineral types can significantly reduce UV radiation. Lichens, cyanobacteria, fungi and a range of bacteria have all been exposed on the European Space Agency’s EXPOSE facility outside the International Space Station and can survive the desiccation and extreme vacuum of space for years, albeit generally much shorter than transit times between planets (on the order of thousands to millions of years). The EXPOSE facility. The apparatus is attached to the outside of the International Space Station.

49 Panspermia – transfer of life between planets?
Arriving at the destination planet. Both the launch of a rock and its landing on the destination planetary body require that it pass through the dispersal filter of transit through the origin and destination planetary atmospheres. In the context of the STONE experiment, an experimental campaign designed to investigate the survival of artificial meteorites during atmospheric entry, the effects of atmospheric entry on photosynthetic organisms was also studied. The organisms on the surface and near-surface of the rocks did not survive. However, meteorites can remain cool on their insides and so in principle they could survive atmospheric transit without the interior becoming heated enough to kill organisms. The Foton spacecraft returns to Earth in the steppes of Kazakhstan. Embedded within the heat shield (the small white circles on the left of the spacecraft) are artificial meteorites to study how rocks respond to atmospheric entry and whether organisms could survive.

50 Panspermia – transfer of life between planets?
In summary, at the current time there is strong evidence that some microbes can survive the conditions required for ejection from a planetary surface and that the interior of rocks remains cool enough for organisms deep within rock to potentially survive atmospheric entry. Although some rocks may be transferred between planets in short time periods, a major dispersal filter to organisms is the usually long journey through interplanetary space. The question of whether planets are biogeographical islands remains a fascinating question in astrobiology. Although there is no evidence that it has occurred in our Solar System, it nevertheless drives us to ask questions about the limits of microbial survival and draws us into extrapolating ecological questions beyond the home world. It will eventually be needed to explain why Mars is inhabited or lifeless.

51 What have we learned? Mars had much more liquid water in its past and may have liquid water today. Mars seems to have many of the elements and nutrients required for life to exist, although the global distribution of them, particularly nitrogen is not well known. Life could potentially be transferred between planets. Mars lacks plate tectonics and so the regeneration of energy sources and nutrients in the crust may be a problem for life. The first serious experiments sent there were the Viking biology experiments. The Viking experiments can be explained by non-biological processes. The evidence for ancient Martian life in Martian meteorites remains highly controversial.

52 What have we learned? The idea that life can be transferred between planets is an old one. It addresses the question of whether planets are biological islands. Microbes can apparently survive the three phases of transfer: launch, the journey in space and atmospheric entry. We don’t know what the maximum time for survival of microbes in space is. Panspermia has not yet be shown to have occurred, but whether it can occur remains an important question in island biogeography and astrobiology. Panspermia has implications for the origin of life.

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