Monthly Archives: August 2017

Io; Zeus’s fancy and Jupiter’s moon

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Zeus, the head-honcho of assorted Greek gods, heroes, nymphs, and mortals, was chiefly the God of the Sky, or Heavens. One of his minor portfolios was the upholding of Honour, but, as mythology relates, he didn’t put much energy into that particular task; he was a philanderer, much to the annoyance of his own wife, Hera (I guess his energies were directed elsewhere).  One such misdirection was Io, a mortal woman, who had the misfortune to be turned into a heifer by Zeus, to hide the infidelity from Hera. Io’s memory now survives as planetary body; one of the Galilean moons of Jupiter is named after her (to be named a moon of the Roman God Jupiter, seems like a historical slap in the face to the Greek deity).

Io is the third largest of Jupiter’s 60 (known) moons, fractionally larger than our own moon, and one of the four moons discovered by Galileo in 1610. It is also one of the most unusual planetary bodies in our solar system, and the only one with active volcanism – over 400 eruptive centres have been mapped. Io’s elliptical orbit brings it to about 420,000 km from the gaseous giant. The resulting gravitational forces produce huge tides such that the surface bulges up to 100m.  These tidal forces generate enough heat to melt rock and sulphur.

Io is quite different to the other Jovian moons in that it’s density is more like that of the rocky planets (Earth, Mars, Venus, Mercury) – in fact it is almost the same density as Mars (3.5g/cm3). In comparison, Jupiter’s average density is little more than that of water, at 1.3g/cm3.  The core of Io probably consists of iron or iron sulphide, surrounded by a mix of solid and molten rock, the composition of which may be similar to that on earth. The outer solid carapace is an intriguing mix of sulphur, which in solid form at the surface, and silicate rock. Sulphur is also the main constituent of Io’s thin atmosphere, but in this case in the form of sulphur dioxide.

Spectacular images taken by the space craft Galileo in 1995, show a moon splashed with colour, a kind of post-impressionist experiment; NASA refers to it in more colloquial terms as a giant pizza. Frequent volcanic eruptions of lava and fine (sulphur?) particles mean that Io’s surface is constantly being renewed; a consequence of this is that impact craters are poorly preserved because they too are recycled.

One of the more exciting discoveries from the Galileo images was active eruption plumes, extending more than 300km above the moon surface (the blue colour is probably due to dispersion of light by plume aerosols).  Several plumes were imaged during different orbits by the Galileo explorer.

Active eruption of lava was also discovered. In the two images below, eruptions about a year apart produced lava flows that contributed to the ridged and fractured surface topography. Also visible are steep-sided depressions, thought to be the remnants of once active calderas, formed by collapse of the surface as lavas were erupted. The calderas are analogous to structures seen on earth.  Older lava flows and deposits from the plumes are buried and eventually incorporated, or recycled into the molten subsurface. On earth, the recycling of rock and water into the crust and mantle is driven by plate tectonics; on Io, sulphur and rock recycling is driven by tidal forces and volcanism.

Space exploration over the last couple of decades has produced a myriad images. There seems to be an endless supply of astronomical vistas, all spectacular, even surreal. Despite this surfeit, I still feel a sense of amazement, a thrill, to witness the details of active Martian sand dunes, the jumbled surface of comet 67P/Churyumov-Gerasimenko, or the stark ruggedness of Pluto’s frozen surface. I am not yet exhausted by this surfeit.

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An Italian job; seismic risk-assessment at risk

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Life is a risky business. Not a day goes by when some aspect of our lives comes under the gaze of risk assessment, an analysis of potential adversity, the probability that some event will impact our well-being. No black and white determinism here; we have become probabilistic entities. The seemingly simple act of driving your car, is translated into an actuarial assessment that determines the cost of insurance, a government’s health budget, a funeral director’s business plan, or a vehicle manufacturer’s liability. All are predicated on the probability of some event taking place – one chance in x occurrences. No luck, or absence of luck? Luck is when you win the lottery without buying a ticket.

Predicting natural phenomena, like volcanic eruptions, tornadoes, or earthquakes, is the stuff of science. The problem with these kinds of events is that they can have global impacts. How does one prepare for, or mitigate, meteorite impacts or Boxing Day tsunamis? We know there are risks, but quantifying those risks remains, in many cases, elusive. We know a great deal about earthquakes for example, how and where they occur (in terms of the mechanics of the earth’s crust), the most likely impacts, and patterns of aftershocks, but pin-pointing the time and place of the actual events, other than in rather vague, general terms, still eludes us. Seismologists frequently speak of earthquake recurrence intervals for certain faults, based on historical, archeological, and geological records, but there is a world of difference between knowing a fault will probably be active sometime in the next 300 years, and foretelling an earthquake three weeks hence. Seismologists tend to be quite vague, as well they should be, when asked to comment on the likelihood of an event; there are simply no established tools that permit such predictions.  A group of Italian scientists learned this lesson the hard way.

In the early hours of April 6, 2009, a M6.3 earthquake, its epicentre near the medieval city of L’Aquila in the central Italian Apennines, killed 309 people and caused severe damage, rendering more than 65,000 people homeless. The main shock was preceded over several months, by 100s of smaller events (a seismic swarm), mostly less than M3. Understandably, the L’Aquila townsfolk were unsettled by the earthquake swarm. L’Aquila and neighbouring towns are no strangers to earthquakes, but this event had a different outcome in the context of scientific knowledge and its role in risk assessment and management.

Six days prior to the M6.3 event, a group of six seismologists and seismic engineers, plus one government official, met with local L’Aquila officials, presumably to ascertain the level of risk. The government official (head of the Civil Protection Department), announced (as it turned out, unbeknownst to the 6 scientists) that there was no real cause for alarm, that the series of small earthquakes were actually quite helpful because they released some of the pent-up seismic energy, and that the seismic swarm did not elevate the probability of a larger event. It is not clear why the official presented such definitive conclusions, that from a scientific perspective were unjustified – did he not understand the science of seismology or the concept of probability, or was it the response of a bureaucrat trying to please everyone? Six days later, all his pronouncements were shown to be incorrect.

Shock and dismay of a very different kind, rippled through the global scientific community in 2011, after it was announced that the seven experts were to stand trial for manslaughter, based on their inadequate assessment of the risks. While recognising that precise earthquake prediction is not scientifically possible (at the present time), the court did determine that the public had been misled by the Government official’s announcement, and that this had contributed to the death toll. Their sentence – 6 years in prison, pay compensation of 7.8 million Euro to the 29 families who had collectively brought the charges, and a ban from public service.

In 2015, an Appeal Court exonerated the 6 scientists, but not the government official. The court argued that the official had taken it upon himself to make statements that the scientists did not agree with, and that he made the statements before the meeting with the scientists had taken place – the scientists clearly could not be blamed for statements they had no knowledge of, and likely would have disagreed with.

The science community’s collective dismay centred around several issues:

  • the science of earthquake prediction is inexact. Earthquake swarms are not uncommon; some appear to precede major shocks, but many do not. However, the L’Aquila debacle has lent some focus to the study of seismic swarms, in the hope that something predictive may be teased from them,
  • it was an attack on science,
  • that the court did not understand the nature of earthquake science, the nature of probability and risk assessment-management. For example, many seismologists argue that the existence of a seismic swarm does increase the probability of a major event, although the actual probability may still be quite small. In other words, the swarms are possible signals of impending calamity,
  • and, if the convictions had stood, no scientist in future would willingly make public comment about ‘acts of God’. This last point probably extends to disciplines other than scientific, where risks are evaluated.

I expect the six scientists felt a sense of relief, that their judgements about potential risks to L’Aquila were not unreasonable, at least within the context of scientific knowledge at the time. But I also suspect no small degree of frustration, that they were unable to be more definitive in their predictions. At present, earthquake prediction doesn’t extend past statements of recurrence intervals. But I’m optimistic that the science of seismology will eventually find the tools to be more precise.

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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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