Category Archives: Planetary geology

Life on Mars; what are we searching for?

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I believe alien life is quite common in the universe, although intelligent life is less so. Some say it has yet to appear on planet Earth. Stephen Hawking

I can only imagine H.G. Wells bitter disappointment if he were to learn that Martians were little more than primitive microbes.  All that hype and scare-mongering for nothing. Because that, it seems, is all we are ever likely to find on Mars. They may be intelligent microbes, but microbes nonetheless.

Present conditions on Mars are not conducive to thriving populations of anything living – at least in any life form we are familiar with. Incident UV and other solar radiation, low atmospheric pressure, an atmosphere almost devoid of oxygen, and the presence in soils of oxidizing molecular compounds such as perchlorates and hydrogen peroxide (think bleached hair), all contribute to rather inclement living conditions. It is possible that some life forms have survived these ravages, in sheltered enclaves or buried beneath the scorched earth, but it is more likely that, if life did exist on Mars, we will find the evidence written into ancient sedimentary rocks, or perhaps as chemical signatures.  It is these attributes that current exploration programs, both landed rover expeditions and orbiting satellites, tend to focus on.

Earth-bound life forms, particularly microbes, can thrive in a multitude of environments. We are constantly reminded of this in our own, relatively benign conditions (the right amount of oxygen, clement temperatures and climates), but we also know that microbes and other life forms have evolved in conditions that are extreme; highly acidic geothermal pools, ocean depths devoid of sunlight, the throats of volcanoes, or buried beneath tonnes of ice. The common denominator for all these microbial kitchens is water, water that is used directly, or indirectly via water-soluble compounds such as sulphates, chlorides and carbonates.

Surface water on Mars is scarce; some is locked up as ice at the Martian poles, and perhaps buried in dirt (frozen groundwater – a bit like our Arctic permafrost). However, Mar’s deeper past (measured, like Earth, in billions of years), was quite different.  There is very good evidence (found in sedimentary rocks and minerals), for liquid water that once flowed in rivers, carved ravines and moulded landscapes, rivers that shaped sediment into large deltas, and seas with distant shores.

Scientists can take the watery, Earth-like analogies further in their search for Martian biology, by focussing their attention on muddy rocks. Earthly mud rocks, or mudstones, commonly have evidence for biological activity, either as actual body fossils, or as organic compounds that have survived the vagaries of burial and chemical change; oil and gas deposits represent a kind of end-point of these processes, the culmination of a transformation of organic matter from living creatures or plants.  Certain kinds of molecules, or biomarkers in these deposits, allow us to determine the original kinds of organisms from which the hydrocarbons were derived. We can use the same technology to examine Martian mudstones, to search for biomarker molecules that might point to former organisms.

Specific biomarkers for earth organisms include DNA-RNA, various proteins, amino acids, and steroids, although these molecules tend not to survive intact during geological burial. Compounds like lipids, on the other hand, can be preserved relatively intact – they are found in various hydrocarbons and organic-rich mudstones. A very important distinction needs to be made here, between organic molecules that can be attributed to life forms, and those that form by abiotic processes (not involving life forms). Abiotic organic molecules are no strangers to those who study organic matter on extra-terrestrial flotsam; they have been found on meteorites, identified in the spectra of interstellar gas clouds, and were detected on the European Space Agency’s Rosetta recent mission to comet 67P/Churyumov-Gerasimenko

Mars, as a habitat in its distant, more watery past, may have had some similarity with conditions on earth 3-4 billion years ago, where the earliest life forms (microbes) are hypothesized to have developed in localized prebiotic factories, perhaps associated with hydrothermal and volcanic vents; not unlike Darwin’s “warm little ponds”. Hydrothermal vents, like the black smokers we observe today in deep oceans, could have provided heat and nutrients in the form of chemical compounds. Organisms that obtain their metabolic energy from chemicals like sulphates and iron oxides, instead of sunlight, are called chemotrophs. Mars habitats, it has been suggested, also were localized, but did not have the kind of longevity that earth systems possess. It is also hypothesized that Martian organisms were probably chemotrophic, and, because there was little or no oxygen, anaerobic.

The utility of Earth-Mars analogues, such as organic biomarkers, or comparisons with the life habitats on earth, are useful not because they offer any kind of confirmation of life on Mars, but because they help to focus the search for the kind of evidence that can be interpreted with confidence. This includes a focus on analytical methods, but perhaps more importantly it helps to provide direction on ‘where’ to look, based on knowledge of the geology and Martian landscapes. For example, if the exploration program is to concentrate its search for organic signatures in mudstones, then ancient Martian lakes are good places to start looking. The current mission (Curiosity), and the upcoming ExoMars (European Space Agency, 2020 launch), and NASA’s Mars 2020 missions, will provide initial glimpses into some of these habitats, and perhaps some answers to an age-old question.

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Which satellite is that? What does it measure?

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Space may well be the final frontier (there are one or two on earth that still require some work), but the space around our own planet is decidedly crowded. Folk at NASA’s Goddard Space Center (Maryland) estimate about 2300 satellites now orbit Earth; vehicles in various states of repair, use or disuse, of which a little more than 1400 are operational Continue reading

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Volcanism does not cause glaciations; let’s turn this statement on its head

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It is almost a truism that volcanic eruptions influence climate. Cold winters and even failed crops, particularly in the northern hemisphere, followed on the heels the Tambora, Krakatoa, and Pinatubo eruptions.  But these climate aberrations were relatively short-lived, counted in years; the stratospheric aerosols and fine volcanic ash that reflect solar radiation back into space, eventually succumb to gravity and fall to earth.  Eruptions of this kind do not result in long-lived, or permanent changes; they are temporary blips on an evolving earth and an evolving climate. Continue reading

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A measure of the universe; Renaissance slide-rules and Heavenly spheres

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Measurement is a cornerstone of science, in fact of pretty well everything we do: How far? How fast? How long?  We take most measurement for granted, with little thought to how the process originated.  We demand accuracy and precision, forgetting that these are relatively modern luxuries.  Before the universal clock chimed GMT in 1884, there were more than 200 time zones in the US.  A league in France was shorter than a league in Spain, a discrepancy for which the 16th C French scribe François Rabelais had an imaginative, if rollicking explanation.  In his tale, The Life of Gargantua and Pantegruel (1532-1564), a king required a standard distance to be determined (after all, if he was going to send his armies to battle it would be best if his advisors new how far they had to go).  He sent a trusted Knight, instructing him to ride to Spain, stopping every league to “roger and swive”; hence the discrepancy.  The leagues gradually became longer. The amusing satire of this explanation had its roots in real Medieval measures; the width of a hand, the distance one could walk in an hour. Continue reading

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Galileo’s finger

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“Their final resting place…” a sepulchral phrase, redolent of a fate that awaits us all.  There is no doubt as to its finality, but resting…?  A nice metaphor that may convey a sense of comfort to the living, rather than the deceased.  Wander through any church or cathedral in Europe and Britain, and you will inevitably walk over cold marble slabs, engraved with the details of those who lie beneath, polished by the feet of a myriad worshipers and tourists.  The Basilica di Santa Croce in Florence is, in many respects, like any other magnificent church; it is old, construction beginning in 1295, with alterations and additions during the 14th -15th century overlapping the earlier Gothic forms.  The Basilica is stunning, but differs from many of its contemporaries in that it became THE place in Italy to be buried. Continue reading

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A Watery Mars; Canals, a duped radio audience, and geological excursions

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Deceptive news is the art of pulling wool over the eyes of the populace, a tool (recently resurrected by certain politicians) for persuasion or dissuasion.  Orson Welles got more than he bargained for when, on October 30, 1938, he orchestrated a radio adaptation of H.G. Wells The War of The Worlds, a 1898 sci-fi that pits intelligent Martians against Victorian Britain.  Welles broadcast created a mix of amusement in some commentators, and in others panic and anger; panic in the unwitting, anger in the duped (especially other broadcasters), and amusement in all the above.

Well’s novel, apart from being the product of an agile mind, was influenced by some of the popular astronomical ideas of his time.  Italian astronomer Giovanni Schiaparelli produced, in 1888 a wonderfully detailed map of Mars showing (above image), among features such as seas, islands, and other landmasses, a network of ‘canali’, or channels.  Canali was misinterpreted in English as canals, and along with all its connotations of intelligent life, the idea of Martian canals entered popular belief.

Were the anthropomorphic connotations of the word canali deliberate?  Percival Lowell, a self-made American astronomer certainly thought so.  In 1894 Lowell announced his own findings, that there were indeed canals, 100s of them, many of them straight, intricately networked, and all artificial, which of course meant intelligent beings.  H.G. Wells simply expanded Lowell’s ideas to the point of delightful absurdity.  That O. Welles would later foist his version of events on an unsuspecting public seems quite reasonable.

Since the 1970s we have been projecting our own intelligence and sense of puzzlement on Mars, using satellites and landed vehicles.  There are no artificial canals, but there are canyons, channels and gullies, landforms that bear an uncanny resemblance to terrestrial analogues.  There is now a significant body of evidence to indicate that Mars was once watery.

On earth, sediment is distributed far and wide by flowing water. Very fine sediment from rivers or wind-blown dust is commonly suspended in water; the sediment gradually settles on the sea or lake floor.  Coarser sediment, like sand and gravel tends to be ‘entrained’ close to the sea floor or river bed by fast flowing water.  Sediment that is moved in this way forms a variety of structures such as ripples and larger dune-like structures.

Rivers in particular, generally move sediment to larger repositories, or basins such as seas or lakes; The kinds of landforms that represent these processes are very distinctive.  On Mars, there are several landform-indicators of flowing liquid (most likely water), most of which have direct terrestrial counterparts; deltas, straight and meandering river, point bars, alluvial fans, and gullied crater margins.  One such Martian landform, imaged by NASA’s Mars Global Surveyor, is the Eberswalde Delta which contains many of the ingredients that also make up terrestrial deltas.  In this case, sediment making up the delta was probably derived from outside the Eberswalde Crater and subsequently transported by rivers into the crater:

  • The delta consists of one or two main river channels (left side) that split into many smaller channels,
  • Bifurcating channels form distinct lobes – there are at least 6 of these, where each lobe represents a specific period of delta formation.
  • Switching of delta lobes is common in terrestrial delta. Each lobe represents a period of sediment movement and deposition, in this case into the deep crater basin.  At a certain point in time, the channel will switch direction and begin to build a new lobe.
  • Each new lobe partly overlaps older lobes, such that the younger deposits appear to lie on top of older deposits.
  • The Lena Delta in Russia provides a nice analogue for the overall shape of channels, with some active parts of the delta (especially the centre-right) juxtaposed with less active segments.

The Eberswalde Delta has another remarkable set of structures.  Meander loops (opposite image), seemingly identical to those seen in meandering rivers on Earth, contain patterns of progressive channel movement.  The Martian meander channel loop was eventually cut off, perhaps forming an oxbow lake like its terrestrial counterpart.

Martian landforms like these are mostly found in regions assigned to the Noachian Period, a geological interval that extended from 4.1 to 3.7 billion years.  All the evidence (so far) points to a time when surface water was common as rivers, lakes, and possibly seas; groundwater can be added to this mix.  If this was the case, there must also have been water vapour in the atmosphere.  The surface must have been significantly warmer than the present frigid temperatures; water vapour probably provided some degree of greenhouse protection.  Overland flow of water also produced sediment, much of which ended up in impact craters and broad lowlands.

 

However, some extremely large outflow channels, such as the Kasei Valles formed sporadically during a later, generally drier and colder time known as the Hesperian Period (3.7 to 2.9 billion years).  This massive system of channels and canyons extends about 3000km from its source in the Tharsis volcanic region, and through about 4km of topographic relief. The overall form of the channels, plus more detailed images of flow-like structures within the channels, indicates possible catastrophic outbursts of humongous volumes of water.  One popular hypothesis to account for this involves massive volumes of frozen groundwater being released either during meteorite impact or volcanic activity and heating.

The comparison between Eberswalde Delta and Kasei Valles mega-floods is quite stark; the delta represents relatively continuous river flow over a long period of time, into a crater.  The Kasei Valles outflow formed almost instantly, driven by the forces of impact and directed away from the crater.

Scientific understanding of Martian geology will continue to evolve; some hypotheses will stand the test of experimental and observational rigour; others will become history.  Modern science has developed the technology to actually do the field work, albeit remotely.  Perhaps we shouldn’t be too hasty to consign Schiaparelli’s and Lowell’s ideas to the theatrically amusing; their observations and explanations were not without context. Keep in mind the possibility that another H.G. Wells may point a satirical finger at 21st century science.

NASA, ESA and other organizations have multiple sites to access imagery and general information on all space missions.  SEPM (Society for Sedimentary Research) also has a Special Publication (number 102; 2012) with 12 papers that describe aspects of sediments and sedimentary rocks on Mars.  The Introductory chapter by John Grotzinger and Ralph Milliken provides an excellent technical summary of the Martian sedimentary realm.

 

 

 

 

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Sand dunes but no beach; a Martian breeze

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When James Hutton, in 1785 presented to the Royal Society of Edinburgh his ideas on the uniformity of earth processes (over vast tracts of time), he did so with both feet planted firmly on good Scottish ground.  Hutton’s Principle, for which Archibald Geikie later (1905) coined the phrase “The present is the key to the past” gave to geologists a kind of warrant to interpret the geological past using observations and experiments of processes we see in action today (see an earlier post for a bit more discussion on this philosophy).  One wonders whether either of these gentlemen gave thought to the Principle being used to interpret processes elsewhere in our solar system.

There is of course, no logical reason why we cannot use terrestrial environments and physical-chemical-biological processes to unravel the geology on our solar neighbours.  We may need to extend our thinking beyond purely earth-bound processes, but the Principle remains a starting point for scientific thinking, interpretation, and discovery.  Mars provides the perfect opportunity for this scientific adventure.

I am constantly amazed with the images that various satellites and extraterrestrial vehicles provide of our celestial neighbours.  The surface of Mars is no exception.  There is clear evidence for ancient processes involving water; landforms and landscapes, strata, and minerals, all pointing to a once-watery planet – ancient rivers, lakes, and seas, beset by Noachian floods.  But all this aqueous excitement was in the distant past; more than 3.7 billion years past.  The Martian surface is now cold and bone dry (although the solid carbon dioxide ice does melt from time to time).  Active surface processes on Mars, i.e. those operating now, mainly involve loose sediment (sand, dust), and wind.  On earth, our experience tells us that if there is enough wind and enough sand, then sand dunes will form. Martian sand dunes look pretty much like our earth varieties.

The formation of terrestrial ripples and dunes requires certain (well understood) sediment attributes and physical processes.  We define sand grains as particles of rock or mineral that range in size from about 0.1mm to 2mm diameter.  Most dunes and ripples on earth form in sands that are in the 0.1mm to 0.3mm range.

  • The sand needs to be dry.
  • Sand grains must be loose, and not stick to one another (in technical jargon they are cohesionless).
  • Wind speed must be greater than the threshold for moving the grains. Stronger winds can move larger grains.

Satellite imagery of the Martian surface has revealed that sand dunes and ripples are common. They have formed, and continue to form in crater floors, broad flat plains, and ancient river valleys; basically anywhere there is sufficient loose sand available.

The variety of dune sizes and shapes is similar to that seen on earth.  Dune shape, like the Barchan dunes shown in the above images, provides clues to prevailing wind directions.  Other dune forms suggest greater variability in wind direction and strength, like the smaller, ripple-like structures (below) imaged by Curiosity ; here, changes in wind direction and speed are indicated by the very small ripples that move in different directions across the larger structures.  These patterns of sand movement are identical to those we commonly see on earth.

The larger dune imaged by Curiosity (below) is moving directly towards the camera.  Sand blown over the top of the dune (the crest) tumbles or slides down the steep front, or lee face, a process that continues as the dune moves forwards.  The inset, taken with a special camera that acts like a small magnifying glass (or hand lens), shows that the sand grains are indeed loose and cohesionless.

All the above images show dunes and ripples that are currently active.  It is also possible to identify fossil dunes in much older strata, using structures within the rocks.  In the examples below, the Martian fossil dune structures are very similar to those we see in some familiar rocks at Zion National Park (Utah).  The kind of layering visible in both Martian and earth examples is called crossbedding (see here for an explanation of how crossbedding forms).

Despite the obvious similarities between Martian and Terrestrial dunes, one in-depth analysis has led a couple of researchers (Gary Kocurek and Ryan Ewing, see below) to suggest that there are also important differences.  The Martian atmosphere is much less dense than that on Earth; there is no oxygen or water vapour.  Mars is consistently much colder; permafrost is probably widespread but in this case the ice is solid carbon dioxide.  Even if the CO2 melts, it will sublime (to gas) rather than form a liquid.  In some cases, permafrost may even stabilise, or solidify dunes.

One important difference between Earth and Mars is in the availability of loose sand.  On earth, much sand is produced by weathering of rock, by all the elements of weather plus chemical reactions that commonly involve oxygen.  Sand is available along most ocean, lake and river shores.  On Mars, physical weathering is very slow; sand is no longer moved by water, as it is on Earth.  Chemical weathering on Mars is also extremely slow.  Thus, the only sand available will likely be from local permafrost melt, the very slow breakdown of rocks, and the occasional meteorite impact.  Some sand may be left over from the period on Mars when water was abundant, the Noachian Period 4.1 to 3.7 billion years ago, but that seems like an awfully long time for sand grains to hang around.

Fossil dunes on Mars, like those on earth, provide clues to the kinds of environments that once existed. Some of these very ancient dunes may even have been associated with Martian lakes and seas.  Modern, active dunes on Mars have little or no association with water.  But they do provide information on sediment availability, wind direction, and wind strength.  The latter may be important when deciding where to locate future sites for human habitation.

NASA, ESA and other organizations have multiple sites to access imagery and general information on all space missions.  SEPM (Society for Sedimentary Research) also has a Special Publication (number 102; 2012) with 12 papers that describe aspects of sediments and sedimentary rocks on Mars.  The paper by Kocurek and Irwin, cited above, is one of these papers.

Kocurek and Ewing, 2012.  Source-to-sink; an Earth/Mars comparison of boundary conditions for eolian dune systems, p. 151-168.

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