Category Archives: Interpreting ancient environments

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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Sliced thin; an unfolding story of sandstone

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What do you think of this analogy; sandstone, and a pile of garbage? I jest, of course. Garbage is no laughing matter – it has become a defining environmental issue of global import. Sandstones too are not to be sneered at.

Your pile of refuse will consist of the flotsam and jetsam of a lifestyle – things you no longer need or want, things acquired over time, from any number of localities, some of which may be very far away. Garbage anthropology is a thing. Enlightened folk examine piles of old rubbish because they provide data that allows them to decipher lifestyles and cultural norms.

Sandstone is a sedimentary rock made up of a collection of grains; these too accumulated over time and could have been derived from sources close by or very far afield. Grains of sand consist of many different materials; individual grains may be single mineral crystals, crystal aggregates, or bits of old rock. In fact, the granules making up most sandstones were derived by erosion of some older, pre-existing rock – they are, by analogy, the jetsam of older rocks that over geological periods of time, have been exposed to weather and gravity, and moved by rivers, wind, glaciers, and ocean currents. Equally enlightened folk (sedimentologists) study sand grains (there may also be fossils), asking questions like: where did they come from (where and what was the parent rock?), when and how did they arrive at their destination, and have they changed in any appreciable way during their journey? Like the crystals making up igneous rocks, sand grains tell a story. Some of the images that follow help to illustrate how we unravel these tales.

The first two thin sections show a sandstone made up almost entirely of quartz grains. A few grains are single, clear crystals (labelled Q), but most grains consist of many small quartz crystals seemingly welded together into a composite. If we are to determine the origin of these grains, we need to know the kinds of rocks that have an abundance of quartz. We know a fair bit about the mineral composition of different kinds of rocks, and this knowledge allows us to narrow the search for suitable candidates. In our example, we would not choose basalt, or gabbro (both igneous rocks) as suitable source candidates because they generally have little or no quartz. However, there are three rock types that might fit the bill: rhyolite (a volcanic rock), granite (an igneous intrusive rock), and certain metamorphic rocks.

Rhyolites contain modest amounts of quartz as single crystals, but in granites and metamorphic rocks, quartz abounds. Our experience also shows that metamorphic rocks are good candidates for the kind of grains we see in this example – composite quartz grains. Rhyolite does not have composite grains so we can exclude this as a possible source. The mineral feldspar is also common in these rocks (along with a few darker minerals like pyroxene, mica and hornblende), and yet none of these are present in our sample.

If we invoke either metamorphic or granitic rocks as possible sources for our quartz sandstone (metamorphic grains seem to dominate), then we also need to explain the absence of these other minerals. One clue to evaluate this problem lies in the shape of the grains. When minerals are eroded from a parent rock, they are commonly jagged and sharp-edged for two reasons: growing crystals normally have sharp edges and well-defined faces (like the facets of some precious gems), or they are broken during their removal from rock. These angular grains are tumbled, jostled and ground together as they are moved by rivers, and in the process, have most of their sharp edges smoothed. Minerals like feldspar are easily broken and abraded as they are transported; quartz on the other hand is relatively hard, and does not break easily. Thus, the processes involved in moving grains of sand from one place to another, will preferentially remove those minerals that are more easily broken and smoothed.

Therefore, it seems likely that the sand in this example has travelled far from its original (mostly) metamorphic source rock. The softer fragments and minerals were preferentially removed en route, leaving a pure quartz sand.

The second example tells quite a different story. Individual grains in this sandstone are made up of older sedimentary and volcanic rock fragments, that we can identify as siltstone and basalt lava. Volcanic fragments like these are easily identified because the myriad, microscopic feldspar crystals (within the grains) are characteristic of once-flowing lava (a previous post shows the same kind of texture in a basalt flow). We know from experiments and observations (particularly in rivers) that granular fragments like these are relatively soft compared with quartz; they do not survive as long when subjected to the rigours of river flow, breaking down into progressively smaller grains. Thus, we can infer that the grains in this sandstone didn’t travel far from their source (old sedimentary and volcanic rocks), unlike the quartz grains in the previous example.

When geologists identify grains in sandstone (and other kinds of sedimentary rock), they are rewarded with data that allows them to make sensible interpretations about ancient landscapes, the rivers that flowed there, where ancient shorelines were located, and even ancient climates. If we are lucky, we will actually locate the candidates for the original source rocks, but not finding them is equally rewarding because we can now pose such questions as, were the original metamorphic rocks or basalt flows completely removed by erosion, or were these ancient parents moved to some other distant shore by tectonic forces? Microscope identification of sandstone grains has the power to inform or refute many hypotheses of the ancient earth. There are still tales for the telling.

 

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Sliced thin; time and process recorded in igneous rocks

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This is the second in a series on the geological world under a microscope

Geologists, it seems, are never satisfied with just looking at rocks from a distance; there is some innate need to wield their pointy geological hammer. Break that rock; give it a good bash! To the uninitiated, this may seem a bit pugilistic, a kind of primal wonton destruction. But a good geo won’t hit rocks just for the hell-of it; a good Geo will be selective. Most of my field assistants and post-graduate candidates needed to be reminded of this. Find something of interest? Before you do anything else, sketch and photograph it; no one will be interested in looking at photos of rubble.

Looking ‘inside’ rocks serves a unique purpose; it allows you to travel back in time, to picture the ancient world, ancient events, outcomes of processes that involve the benign and the brutal, terrifyingly beautiful. Rocks contain memories of all these. And that is why we sometimes break them apart. The optical, or polarizing microscope allows us to unlock these rock memories in a uniquely visual way.  Continue reading

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Sliced thin; kaleidoscopes with a geological purpose

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This is the first post in a series on the geological world under a microscope

As a kid visiting my Scottish grandparents, I would make a bee-line for two delights in their house (after the hugs); the kitchen (following my nose) to the inevitable trays of homemade donuts and shortbread, and the living room credenza wherein was kept an old kaleidoscope. It was a triangular prism (most modern forms are tubes), filled with glitter, two mirrors at one end, and a peep-hole at the other. This simple toy introduced me to the world of symmetrical, kaleidoscopic, never-repeated patterns.  Years later, as a geology student, I was introduced to optical mineralogy, the science and art of identifying minerals under a polarizing microscope – flashbacks to my childhood. Continue reading

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The brutality of Surtsey’s laboratory

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I recently came across a local newspaper article describing a new volcanic island, rising from its own ashes above the sea floor, off the coast of Tonga.  The subtlety of memory returned me to 1963, and an announcement over our morning radio, of the birth… of a volcanic island off the coast of Iceland. Images, arriving a couple of days later (this was 1963 after all), gave witness to a natural brutality I had not seen before; the sea in boiling turmoil, torn by erupting columns of rock and steam. Beautiful, in an awe-filled way.

It has been fifty years since the cessation of volcanic activity. Surtsey has become home to plants and birds, a laboratory for the adaptable, the dispersible, and the colonial.  The only sounds that resonate now are noisy gulls and pounding North Atlantic waves. Continue reading

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A burp and a hiccup; the volcanic contribution of carbon dioxide to the atmosphere

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Of the two certainties in life, volcanoes offer the most excitement (death and taxes are basically the same thing).  They are magnificent while asleep; a primeval ruggedness that stirs the imagination. We paint them, we eulogise them. And when they awaken, we run for cover. Whether in a state of dormancy or high agitation, they leave an impression on our inner and outer landscapes.

All active volcanoes emit gas; pre-, during and post-eruption. On average, 96% of volcanic gases are water vapour, the remaining components being CO2, SO2 (most common), plus a little helium, nitrogen, carbon monoxide, hydrogen sulphide, and a few halides. Volcano-derived carbon dioxide is frequently cited as a culprit for increasing atmospheric CO2 concentrations in Continue reading

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The life of a Tuscan wall

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Montefioralle, Chianti country, Tuscany, Italy, and from where I’m sitting (happily sampling a Chianti Classico) I see rolling, wooded hills, next season’s vintage, olive groves a scattering of farm dwellings, and rock walls. Quintessential Tuscany. Except for a few ratty road cuts, there is little native rock exposed in this part of Tuscany from which a keen geologist might ascertain something of ancient pre-Tuscan history (farther south this changes).  But in fact, there are rocks aplenty. Most walls (houses, defensive, retaining, decorative) are made of limestone and sandstone, some quarried and deliberately shaped as in churches and castello (that date back to the 10th -11th century), and others that made use of whatever was handy at the time. Most of these materials were collected locally; stones that littered the hillsides, and stones brought to the surface during ploughing.  Even today, ploughs bring stones to the surface; the local clay-loam soils are incredibly stony (an important part of Chianti terroir).

So, despite the paucity of hard-rock exposure, one can make a reasonable guess at the geology beneath the hills and vineyards, based on the stone composition in local buildings. About 50-60% of the stones are cream-coloured marls; marl is an old name (medieval Latin) given to very fine grained, usually muddy limestone that breaks along curved, sharp-edged surfaces (referred to as conchoidal).  The Tuscan marls are very hard – ideal for building stone.  Variations on this theme include sandy limestones, some of which contain intricate contorted layering, and small crossbeds that indicate flowing water many millions of years ago.

Grey sandstone is also common; in fact it is found as paving stone throughout most of Tuscany.  All kinds of structures are visible in these stones, especially cut stones in larger buildings and roads; fossil ripples that indicate flowing water, trails and burrows of critters that moved across or below the ancient seafloor in search of food or finding a place to live. Some of the sandstones are not as hard as the marls, and in places show quite advanced damage where bits of rock fritter away with the vagaries of weather.

An assortment of red bricks, some rumoured to be of Roman or Etruscan derivation, has been used in most walls. It looks like odd-shaped bricks are filling equally odd-shaped gaps, but they have also been used to replace stone arches over doors, or fill holes in walls left by marauding armies (of which there were many) or neglect.

The rocks were originally deposited as sediment in an ancient and vast ocean called Tethys, that separated two supercontinents – Gondwana, and Laurasia (most of Europe and Asia).  The Tethys was closed when the Africa plate (part of Gondwana) drifted north and crunched into Laurasia, beginning about 65 million years ago. The resulting uplift produced the Apennine Ranges that now course the length of Italy.

Montefioralle is a picturesque hill-top village, typical of many in Tuscany.  Its medieval origins are still visible, but frequent battles between neighbouring villages, as well as larger fracas between Florence and Siena, put a few dents in the outer wall and houses. The hill top is crowned by a small church and tower; the last refuge in the event of siege.  There is clear evidence of repairs made over the last 800 plus years, including, I suspect, some from a more recent European conflict.

Stones in these Tuscan walls weave their tales in different threads. The limestones and sandstones have a geological story that spans 10s of millions of years, the disappearance of an ocean and the collision of continents.  Each stone and brick can also relate centuries of local history; each was carefully placed by someone, a stone-mason or perhaps a Renaissance DIY. Nameless, we can admire their handy-work, wonder what they talked about with their fellow workers, what they ate, who they loved.  There are centuries of these former lives everywhere in Tuscany. Chianti Classico loosens all their tongues.

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