Module 13: Planets as Habitats Activity 1:Signs of Life
Summary: • In this Activity, we will investigate: • (a) Astrobiology • (b) Searching for water on Mars • (c) Follow the Water! • (d) A Land of Lakes? • (e) Habitable Zones • (f) Searching for Signs of Ancient Life on Mars, and • (g) Life elsewhere in the Solar System?
(a) Astrobiology • Astronomers, especially planetary astronomers, tend to need a fairly ‘all-round’ knowledge, made up of bits and pieces of several sciences apart from their own - in particular physics, chemistry, geology and meteorology. Until recently the one scientific area an astronomer could safely be ignorant of was the biological sciences, but not any more. An astronomer who is well informed on current Solar System research now needs to know about astrobiology (sometimes called exobiology or bioastronomy), the study of the possibility of life outside Earth, and that involves at least a smattering of palaeontology, genetics, ecology, botany and zoology.
Astrobiologists have a particular interest in the study of when, where and how life started on Earth. The recent discovery of primitive organisms around volcanic vents deep on the ocean floor, and bacterial contamination of samples taken from deep insidethe Earth’s crust, have called into question whether life on Earth started in the oceans, as has been conventionallyassumed.
Traditionally it has been thought that lightning strikes and volcanic eruptions into the primeval atmosphere and seas of the Earth would trigger the formation of complex carbon-based molecules in the ancient seas which could be the precursors to life. Experiments have been conducted where electricity (“lightning”) has been discharged into mixtures attempting to replicate the primeval terrestrial atmosphere and the “primeval soup” of Earth’s ancient seas, and the right sort of complex molecules have indeed been formed. While these experiments are hardly conclusive, they dohighlight a popular premise of astrobiology – that given the raw materials and the right conditions, life will take hold anywhere it can.
That is a fairly big assumption, based on a sample of one: one location where we know life has taken hold (Earth). • If life – even fossilised remains of ancient single celled life – were found in another location in the Solar System, it would make the “life will take hold wherever it can” assumption a lot more respectable. The search for life outside Earth depends on us being able to recognise life if and when we find it.
The forms of life with which we are familiar are based on: • carbon molecules – which are capable of forming long and complex chains which can store genetic information • and water – which has many properties vital for ‘life as we know it’; its properties as a solvent, its ability to absorb a large amount of energy for a small temperature change (“heat capacity”), its ability to stay liquid over a wide temperature range, and its use in evaporative cooling.
We can theorise about lifeforms based on, for example, silicon, not carbon, and other liquids such as ammonia or methyl alcohol, but these alternatives are not as robust or versatile as carbon and water - at least to produce life as we would recognise it. So astrobiologists look for conditions which either support or have supported liquid water, and contain or may have contained the organic, carbon-based compounds associated with life.
(b) Searching for Water on Mars • The low surface pressure on Mars (only 1% of that on Earth) implies that there is no liquid water present, as any liquid water on the surface of Mars today would vaporise rapidly due to the low atmospheric pressures. Only in the very deepest canyons where atmospheric pressure is at a maximum could there possibly be liquid water.
Last century, Giovanni Schiaparelli, an Italian astronomer,reported that he could see what appeared to him to be a number of dark lines criss-crossing the Martian surface, and called them canali, or “water channels”. Translated into English, this was interpreted as “canals”, and it became fashionable to assume that, as Mars appeared to be much like Earth, it was inhabited by intelligent life which had built a sophisticated canal system to bring water from the Martian polar ice caps to irrigate the rest of the planet. (Seasonal variations in the colour of Mars can look greenin contrast to the prevailing red, and were misinterpreted asvegetation.)
By the end of the 1800s, Percival Lowell, a wealthy American, had reported observing 160 Martian canals. In popular stories, Martians were likely to leave theirdesert-like planet and invade Earth, culminating in theWar of the Worlds hoax, where the freeways of New York were clogged by motorists attempting to escape a Martian invasion - panic brought about by a too realistic radio play. Modern telescopes show no sign of canali or canals.Imagine then, the surprise for planetary scientists whenthe Viking and subsequent missions sent back images of features that looked like dried-up riverbeds!
The scale of these“runoff channels” - up to 1500 km long with widths up to 100 km - is many times too small to be seen with modern Earth-based telescopes (much less the telescopes of Schiaparelli and Lowell). They are very old: the amount of cratering in the channels suggests that they are 3 to 3.8 billion years old. No runoff channels were observed by Viking in the younger terrain in the north.
The following Mars Orbital Surveyor images are of Nirgal Vallis, one of a number of possible runoff channels. The debate about these valleys centres on whether they were formed by water flowing across the surface, or by collapse and erosion associated with groundwater (artesian) processes. low resolutionview oblique view
At the resolution of these early images from the Mars Orbital Surveyor, it is not possible to tell whether this is a pattern of debris resulting from groundwater collapse, or a pattern of drainage channels.
The Mars Pathfinder landed in what had appeared to be an old stream bed. Images sent back by Pathfinder supportthis, with evidence of terracing, and the rocks tilting in the same direction, suggesting a massive water flow in the past.
The images below show a crater in Kasei Vallis that was imaged by the Mars Orbital Surveyor in June 1998, showing a 6 km diameter crater that was once buried by an island of about 3 km of Martian “bedrock”. Kasei Vallis is actually a system of giant channels thought to have been carved by catastrophic floods that occurred more than a billion years ago. top edge of cliff island bottom edge of cliff crater
The crater may have been formed as long as 3.5 billion years ago by meteor impact. Sometime later it was buried by the material that comprises Lunae Planum (the large plains unit of which the island appears to be part). The island is at least partly made of hard rock, however the processes which buried the crater were gentle enough not to destroy it. The crater is like a giant fossil, which has apparently been exposed by the laterfloods through the Kasei region. island cliff moat-like feature partially encircling the crater, probably formed when floodwater encountered the crater wall
Another piece of evidence comes from meteorites found on Earth, which isotopic analysis suggests have come originally from Mars. Meteorites of this type have been discovered to contain water-soaked clay bound up inside them, which suggests that they were exposed to liquid water while still on Mars.
(c) Follow the Water! • The previous discussion summarises the information known and inferred about water on Mars, up to June 2000. • However in that month, NASA and Malin Space Science systems, who are chief investigators with the Mars Orbital Camera (MOC) on the Global Surveyor, held a press conference to release high-resolution images taken with the MOC since March 1999. • The images they released show apparently recent runoff features which suggest that liquid water has existed (and may still exist) at shallow depths below the surface of Mars in geologically recent time.
The planetary geologists associated with the MOC first suspected that liquid groundwater may seep out onto the surface in certain location on Mars when they analysed a low-resolution picture taken during the Orbit Insertion Phase of the mission in December 1997. • The image showed dark, v-shaped scars on the western wall of a 50 kilometer-diameter impact crater in southern Noachis Terra. The MOC scientists interpreted the image to be similar to that of seepage landforms on Earth that form where springs emerge on a slope and water runs downhill. • The following images show the increasing detail seen once the MOC started to make high resolution images, and also shows why these relatively small-scale features were not visible in Viking images. The gullies are too small to have been detected by the Mariner and Viking spacecraft.
Viking mission image of area Higher resolution MOC detail of alcove Original low resolution MOC image Highest resolution MOC detail of alcove
The geologists who made the discovery theorise that there is or has recently been (geologically speaking, this means in the last few million years) a layer of water buried less than 500 m below the Martian surface - an aquifer, somewhat like the Great Artesian Basin in Australia - and that it normally evaporates where it is in contact with air. • However on the colder sides of canyons and craters, the evaporation cools the water down till it forms a surface layer of ice. The pressure behind the ice causes it to dislodge occasionally, resulting in sudden outflows of water down the canyon walls, causing the patterns seen in the MOC images shown here. • The features that have been observed can be explained by groundwater seepage and runoff. They are mostly seen on canyon and crater walls facing away from the equator.
Some of the MOC photographic evidence suggests that some outflows might be very recent indeed, because: • they contain no cratering • they flow over wind patterns in the Martian soil • they flow over polygonal patterns believed to be due to seasonal freezing & melting of ‘permafrost’ ice, and • they contain some regions where the unusually (for Mars) strong contrast in surface colour tends to suggest that dust has not had time to settle:
The existence of liquid water at Mars temperatures and pressures is not easy to explain. However suggestions (backed up by trace analysis of some Martian meteorites) that the water may be very brackish (salty) would help to explain why it might be able to stay liquid at low temperatures. • If liquid water does exist on Mars, or has existed in geologically recent times, it will reopen the debate on whether Mars ever has supported primitive forms of life, or indeed does now. • For more information, see the Internet websitehttp://mars.jpl.nasa.gov/mgs/msss/camera/images/june2000/index.html • If the MOC scientists’ interpretation of these images is correct, Mars may contain subterranean liquid water today.
(d) A Land of Lakes? In December 2000, the same scientists studying MOC data released images showing what appears to be layers of sedimentary rock, similar to patterns seen in places on Earth where lakes once existed. To quote Dr. Michael Malin, chief investigator for the MOC*, “We see distinct, thick layers of rock within craters and other depressions for which a number of lines of evidence indicate that they may have formed in lakes or shallow seas. We have never before had this type of irrefutable evidence that sedimentary rocks are widespread on Mars. These images tell us that early Mars was very dynamic and may have been a lot more like Earth than many of us had been thinking.” * The full NASA press release is on the Internet atftp://ftp.hq.nasa.gov/pub/pao/pressrel/2000/00-190.txt
Layered Outcrops of Far West Candor Chasma: 1.5 km Low resolution MOC composite image 2.9 km “Colourised” MOC high resolution image, vertical heights exaggerated by 50%
Alternating Light- and Dark-toned Layers in Holden Crater: Viking image “Colourised” MOC high resolution image
The evidence for liquid water on Mars is not completelyconclusive, but by the end of 2000 it was arguably the simplest explanation for the evidence. Some researchers, however, argue that the features could be caused by other phenomena such as deposits laid down by dust storms, and pyrochlastic outflows caused by release of hypothesized reservoirs of frozen carbon dioxide far beneath the surface of the planet. One version of the latter theory is called the “White Mars” theory, in contrast to the arguments for liquid water having existed or still being present on or in Mars. For more information about White Mars, see: http://www.earthsci.unimelb.edu.au/mars/Enter.html http://www.spacedaily.com/news/mars-water-science-00k1.htmlMake your own density flow:http://www.beloit.edu/coterie/SEPM/public_html/Water_Works/density_currents.html
If liquid water has flown on the Martian surface, where could it have come from? • At the poles the temperature is typically at 160°K, andapart from the water ice in the polar caps there is probably ice frozen under the surface, like permafrost on Earth. • The Martian permafrost under the Martian surface, if it exists, may have occasionally melted in the past due to meteorite impacts or volcanic activity, leading to ground collapse and flash flooding. • The presence of permafrost as a potential source of both water and oxygen, on a planet similar in many ways to our own with soil which may support plant growth in greenhouses, would make Mars an attractive target for colonising and terraforming sometime in the future.
According to some planetary scientists, Gusev crater may have been an ancient lakebed. • Repeated flash flooding is one thing, but lakes are another, and some astronomers believe that they have also identified features on Mars to be shorelines of oceans. To sustain oceans or even lakes for any period of time, ancient Mars must have had a much thicker atmosphere than it has today. • By counting craters, the proponents of Martian oceans conclude that oceans existed up to 2.5 to 3.5 billion years ago, and propose that they managed to remain liquid due to a greenhouse effect caused by gases released by volcanoes.
Others believe that they have identified scars on the old southern Martian landscape due to glaciers. Glaciers need snow to form them, and the formation of snow in turn requires evaporation from an ocean. Ancient Martian seas, let alone glaciers, are highlycontroversial. The more conventional explanation isflash flooding 1–3 billion years ago when the atmosphere was denser and warmer. This assumesthat the climate of Mars has varied in the past, andmay still do so.
The polar caps contain water ice, carbon dioxide ice andaccumulated layers of dust - which are left behind in layers when the water ice caps recede each Martian spring. Variation in theobserved layeringsuggests periodicchanges in climate(perhaps due tochanges in Mars’rotational inclination or orbit around the Sun).
Water Ice on Mars?? In June 2002, NASA announced the findings of an experiment on-board the Mars Odyssey satellite which has been using a neutron spectrometer to search for ice reservoirs just below the surface of Mars. High-energy gamma-rays originating from hydrogen molecules less than one metre below the Martian surface were detected by the spectrometre; scientists currently believe that the hydrogen is locked up in the form of ice crystals. The Mars Odyssey spectrometer is based upon the same design as the Lunar Prospector which discovered ice in the polar regions of the Moon in 1998. Soil enriched with hydrogen is deep blue in colour. Smaller amounts of hydrogen are shown light blue, green, yellow and red. The deep blue areas in the polar regions are believed to contain up to 50 percent water ice in the upper one metre of the soil.
On 18 January 2004, OMEGA, an imaging spectrometer, observed the southern polar cap in three bands – optical, carbon dioxide and water. These observations confirm the presence of carbon dioxide and water ice. H2O ice CO2 ice visible Yes - Ice on Mars! One of the main aims of ESA’s Mars Express mission is to discover water in any of its chemical states.
(e) Habitable Zones • Could Mars have once had a thick enough atmosphere and warm enough temperatures to have supported liquid water and life on or near its surface? • If we look for regions in our Solar System which would support “life as we would recognise it” – i.e. carbon based, requiring liquid water and the temperatures to sustain it – then we can make estimates of the range of distances from the Sun where the development of life would be possible.
The inner edge of the habitable zone for our Sun is the maximum distance from the Sun at which a planet would undergo a runaway greenhouse effect, like Venus. The outer edge of the habitable zone for our Sun is the distance at which a planet’s carbon dioxide in its atmosphere would condense to the ground as dry ice, causing the atmospheric temperature to drop below freezing.
Calculations suggest that the Sun’s habitable zone now stretches from about 0.95 AU (just inside the orbit of the Earth) to about 1.4 AU (just inside the orbit of Mars). The Sun is gradually brightening - the inner edge of the zone was probably located at about 0.8 AU approx 5 billion years ago. As the Sun ages and becomes gradually brighter, the Earth will eventually heat up to the point where it lies outside the habitable zone - but not for another 5 billion years or so!
However there are some inconsistencies in this story. If the Sun is gradually becoming brighter, then it should have been dimmer and put out less energy in the early history of the Earth (and Moon) - but palaeontological and geological studies of the early history of the Earth do not appear to show any effects of a weaker Sun. • This makes it difficult to make definitive statements about whether Mars was positioned within the Sun’s habitable zone early in its history. • This background slide shows stills from a Mars ‘virtual reality’ movie made by the Swinburne Centre for Astrophysics & Supercomputing, using Mars Global Surveyor data to construct (vertical scale-enhanced) Martian topology, then ‘terraforms’ Mars by adding an atmosphere and ocean. • Click here to see the animation.
(f) Searching for Signs of Ancient Life on Mars • The Viking landers carried out experiments to look for signs of life on the Martian soil at their landing sites - without success. The current set of Martian probes are mainly designed to look for evidence of water rather than signs of life on Mars. However the issue of whether life has existed on Marsin the past became a ‘hot topic’ again with the announcement in 1996 that a meteorite, originally fromMars, contained several forms of evidence that life hadonce existed in cracks contained in it, while it was stillon Mars.
Click here to see ananimation Possible Ancient Life on Mars? Found in Antarctica in 1984 Identified as formed on Mars4.5 billion years ago Water penetrated fractures 3.6 – 4 billion years ago 16 million years ago rock ejected from Mars due to a large impact ~2kg meteorite Rock landed in Antarcticaapprox. 13,000 years ago
Microscopic analysis: fractured rock carbonate mineralsfound infractures
concentrationincreases towards interior? In more detail: Polycyclic aromatichydrocarbons (PAHs) - from dead organisms? fractured carbonate minerals possible microscopic fossilstructures similar to “nanobacteria” found in hotsprings on Earth iron sulfides & magnetite- produced by anaerobic bacteria?
So, in summary, a meteorite found in Antarctica - but originating on Mars - is claimed to contain several pieces of evidence which suggest that ancient life once existed in its cracks. The concentration of the deposits increases with depth of the cracks, suggesting that the deposits did not infiltrate the rock while on Antarctica. • The announcement of this discovery provoked a vast amount of press coverage and interest. However other scientific teams have since disputed the conclusions, claiming that the deposits did settle in the rock on Antarctica rather than on Mars, that the deposits were transported into the rock by a hot gas rather than water flow, and that the claimed nanofossils are instead crystals.
On the principle that ‘extraordinary claims require extraordinary evidence’, the broader scientific community is yet to be convinced on this claim of evidence for ancient Martian life. However the ‘liquid water on Mars’ debate (see earlier) has now reopened the whole issue of life on Mars, even in geologically recent times. In December 2000 another dramatic twist in the story occurred - a group of scientists published the conclusions of a four-year long study of the magnetite crystals (see earlier) found in ALH84001. They concluded that the crystals originated on Mars, and that a significant proportion of the magnetite crystals are identical to those found in aqueous bacteria on Earth.
Magnetite crystals act as very efficient compasses which are believed to assist bacteria in locating and maintaining optimal positions for survival in environments containing, for example, dissolved oxygen in water. Today Mars has only localised magnetic fields of any significant size. It was originally believed that Mars had never had a strong magnetic field, but instruments on the Mars Global Surveyor have since observed magnetised strips in the Martian crust which suggest that Mars once possessed a strong magnetic field, perhaps at the same time as the magnetite crystals found in ALH84001 were formed. To find out more, see the NASA-Johnson Space Centre press release on the Internet at http://spaceflightnow.com/news/n0012/14marslife/
This research, as with the original claims about ALH84001, will be subjected to considerable scrutiny by the scientific community. Whatever the outcome, this and the recent Mars Express announcement about water ice on Mars have ensured that future (successful!) missions to the red planet will be followed with considerable interest. • To follow the debates, visit the following Internet sites: • Mars Today • http://humbabe.arc.nasa.gov/MarsToday.html • Signs of Past Life on Mars? • http://www.fas.org/mars/aaas_001.htm • What’s New with Life on Mars • http://www.fas.org/mars/new.htm • Centre for Mars Exploration (NASA) • http://cmex-www.arc.nasa.gov • Mars Global Surveyor • http://mars.jpl.nasa.gov/mgs/index.html