Author: zooks777

Former Senior Lecturer at British Open University, research in remote sensing of arid lands, groundwater exploration, Precambrian tectonics and geochemistry

Two sides to reducing carbon emissions

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Scientists in developed countries are more or less unanimous that climate is warming because of rising CO2 levels from the burning of fossil fuels.  That spurs calls for less reliance on fossil fuels and more use of renewable energy resources, including biomass.  The situation for the other two-thirds of humanity is much different.  The majority depends on biomass fuels (wood products, agricultural waste or animal dung).  Unprotected burning of biofuels releases such levels of carcinogens that 1.6 million people including 400 thousand in sub-Saharan Africa, mainly women and infants, meet an early death each year.  By 2030 this may rise to over 9 million, if current fuel use continues.  Biofuels also devastate woodland cover, and burning animal dung reduces natural fertiliser used on fields: two contributors to the inexorable decline in conditions of life in the “Two-Thirds World”.

Energy researchers at Harvard and the University of California have examined the options for household fuels in the light of these “counter-environmentalism” facts (Bailis, R. et al. 2005.  Mortality and greenhouse impacts of biomass and petroleum energy futures in Africa.  Science, v. 308, p. 98-103).  A safer alternative to wood and dung burning is the use of charcoal, yet that would increase CO2 emissions by around 50%, as well as increasing loss of woodland.  The higher energy content of non-coal fossil fuels would actually decrease the “greenhouse” burden, while improving health dramatically.  They estimate that a shift to petroleum-based household fuels would delay between 1.3 to 3.7 million deaths per annum, by 2030

How the core controls Earth’s magnetic field

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While most geoscientists are well aware that past changes in the geomagnetic field are useful as a means of timing sea-floor spreading and stratigraphic correlation, and that records of the direction of palaeomagnetism are keys to ancient plate movements.  Most, however, understand only vaguely why Earth has a magnetic field that flips polarity from time to time: there is some kind of self-sustaining dynamo due to motion in the liquid-metal outer core.  That aspect of geomagnetism involves tough theory and maths.  So for Scientific American to present an up-to-date review of how that dynamo might work is both surprising and welcome (Glatzmaier, G.A. & Olson, P. 2005.  Probing the geodynamo.  Scientific American April 2005, p. 33-39).  The review covers what is currently known about convective motion in the outer core, both laminar and turbulent, and how the simpler laminar convection has been used in computer modelling that simulates how the geodynamo works.  It is complex even at that level of simplification, because thermal convection is affected by the Coriolis effect: much like that in the atmosphere.  Even though the idea of a dynamo inducing magnetic flux is a basic principle of physics, one based on fluid circulation is in constant motion and change.  Surface monitoring of shifts in the magnetic field help chart that aspect.  The issue of reversal is, literally, the knottiest problem for geomagnetists, and they have to resort to the old idea of lines of flux and the effect of contortions by motion at the core-mantle boundary to grapple with how polarity flips might occur.  Computer simulations show the development of what can only be described as chaos in the geomagnetic field at the core-mantle boundary, and much smoothed, but nonetheless odd variability at the surface, as the poles prepare to reverse.  For a period of around 6 000 years the field wobbles like a massive jelly as it lurches across the planet, sometimes splitting into several “blobs” of different polarity.  Eventually it settles down into its new configuration.  To some extent this strange behaviour is matched by what little is known in detail about the progress of reversals from the geological record (see Magnetic polarity reversals in May 2004 issue of EPN).

Changing the world

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Because humanity and its activities have transformed the vegetated face of our home planet, caused its climate to warm and pushed an increasing number of other species over the edge of extinction, some circles have coined the name “Anthropocene” for the last half of the Holocene Epoch.  Human induced change almost certainly began as soon as settled agriculture arose to dominate most societies (see Did the earliest agriculture kick-start global warming?, in EPN of April 2005).  In terms of atmospheric emissions and mobilizing metals we now push natural rates close: facts that emerge from annual reviews of mining and energy use.  But are we truly significant geological agents as well as influences on the atmosphere and biosphere?  Two articles in April 2005 suggest that we are.

Quarries, mines and other excavations are obvious signs of human erosive power, but our farming activities produce insidious results by inducing soil erosion.  Although its effects are well known from such areas as the Ethiopian Highlands and the 1930’s “Dust Bowl” of the US mid-west, a global measure of the rates involved requires a careful compilation of  quantitative data.  Bruce Wilkinson of the University of Michigan has made the first attempt (Wilkinson, B.H. 2005.  Humans as geological agents: A deep-time perspective.  Geology, v. 33, p. 161-164).  Throughout the Phanerozoic, the volume of sedimentary rocks suggests that enough erosion has taken place to have stripped a uniform blanket 3 km deep from the continental surface.  That gives an average erosion rate for the last half-billion years of Earth history of the order of tens of metres per million years.  Assembling information about current rates of human-induced stripping, roughly divided 30:70 between excavation and soil erosion, Wilkinson arrives at a staggering figure for anthropogenic denudation: hundreds of metres per million years.  Our activities in the outer part of the rock cycle are an order of magnitude greater than purely natural rates of weathering, erosion and transportation.  He suggests that humanity began to outpace sedimentology sometime around the time of the Norman Conquest.

This awesome picture might seem to indicate that rates of sediment deposition on continental margins are also tremendously elevated by our actions.  That aspect has been studied by geoscientists from the US and Holland (Syvitski, J.P.M. 2005.  Impact of humans on the flux of terrestrial sediment to the global coastal ocean.  Science, v. 308, p. 376-380).  The opposite is now happening.  Syvitski et al.’s analysis of historical sediment loads in the catchments and lower reaches of the worlds major rivers shows that while overall sediment transport has increased by 2.3 billion t per year, since human effects became noticeable in the sedimentary record, the amount delivered to the sea has fallen.  Some 1.4 billion t no longer add to marine sedimentation each year.  Instead, that mass ends up behind dams of one kind or another.  In the last 50 years, more than 100 billion t, containing 1 to 3 billion t of carbon is in silted up reservoirs, or redistributed to farmland by irrigation diversions.  One of the outcomes is that natural coastal protection by spits and sand bars is growing less effective.  Another is that less nutrients are getting to the near-shore marine biosphere, with possible effects on fish stocks, coral reefs and other habitats.

Caring among the Erects

Dmanisi in Georgia provided one great surprise in human evolution by yielding abundant remains of 1.7 Ma old Homo erectus where they might be least expected: north of the Caucasus mountains that would have formed a tremendous barrier to any migration from further south.  The archaeological sites have provided another surprise in the form of a well-preserved skull of a completely toothless individual.  It is clear from the regrowth of bone into the sockets that this “masticatorily impaired” individual survived for years after losing all their teeth (Lordkipanidze, D. et al. 2005.  The earliest toothless hominin skull.  Nature, v. 434, p. 717-718).  It is impossible to believe that the individual could have survived on a tough meat and vegetable diet without special preparation of soft victuals.  Although the person’s survival cannot prove that other Erects helped out, that is a distinct possibility.  Losing teeth through dental disease or trauma would have been immensely painful and debilitating, yet the individual did survive.  We have to move forward to around 40 thousand years ago for compelling evidence that Neanderthal society cared for disadvantaged people, when several near-complete skeletons show evidence of long-term, crippling damage.

New twist for end-Permian extinctions

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There is a Gaelic proverb, which loosely translated goes: “There are more ways of killing a cat than drowning it in butter”.  That seems apt for mass extinctions, particularly the most severe, at the end of the Palaeozoic.  A new hypothesis points the finger towards breathing problems, but not those likely from massive, ground-hugging emissions of sulphur dioxide from the Siberian flood basalts that coincide with the P-Tr extinction: “everyone knows” that they resulted in the universal coughing reflex in all surviving land vertebrates…..  Raymond Huey and Peter Ward of the University of Washington reckon a major contributing factor for terrestrial extinctions was a fall in atmospheric oxygen (Huey, R.B. & Ward, P.D. 2005.  Hypoxia, global warming and terrestrial Late Permian extinctions.  Science, v. 308, p. 398-401).

For most of the Carboniferous and Early Permian Earth flipped in and out of glacial conditions that dominated the southern supercontinent of Gondwana.  Tropical latitudes were cloaked in dense vegetation for the first time.  Rapid sedimentation buried vast amounts of carbon in the form now taken by the world’s largest and most extensive coal deposits.  Net carbon burial for 90 to 100 Ma resulted in extraordinary oxygen concentrations in the atmosphere. One line of evidence for that is the huge size of Carboniferous and Early Permian insect fossils, such as those of dragonflies.  Insects do not breathe, but take in oxygen by a diffusive process through spiracles on the underside of their bodies.  The more oxygen the larger they can grow.  Carbon burial also links in with the global cooling that made the Carbonierous and Early Permian susceptible to astronomic forcing of glacial-interglacial cyclicity: CO2 fell.

The present-day oxygen concentration in the air is about 22%, whereas estimates for the Carboniferous Permian peak are around 30%.  Most land animals today, including ourselves, have an altitude limit to permanent life of around 4 to 5 km, though the vast majority live much lower.  In the Early to Middle Permian, the availability of oxygen for respiration corresponding to that at sea level today would have been around 6 km altitude, and at the top of a mountain the height of Everest breathing would be easy.  The limit to altitude range of animals would have been temperature rather than oxygen availability.  So, given sufficient warmth, the area available for animal life would have been very high.  Estimates of the oxygen level at the end of the Permian are as low as about 16%.  Even living at sea level would have demanded an ability to survive at about 2.7 km today, and at 6 km during the oxygen-rich Early and Middle Permian.  Evolution of land animals during the 100 Ma long “global winter” would have adjusted to elevated oxygen availability, which Huey and Ward believe would have led to at least a limited altitude stratification of available ecosystems, governed by temperature.  Their hypothesis is that declining oxygen forced extinctions by reducing the habitable range severely, and increased competition among those taxa able to live in the reduced, low-altitude land area: probably patches of “refugia”.

The decline in oxygen was accompanied by global warming.  Permian and Triassic sedimentary records show a dramatic increase in red terrestrial sediments, coloured by iron oxide.  Iron had been released and oxidised to insoluble iron(III), possibly by increased continental weathering, which would have sequestered oxygen by the formation of iron oxide coatings to sedimentary grains.  Increased oxidation would also have encouraged biodegradation by aerobic bacteria, which may have run-away to help boost atmospheric CO2 levels.  One testable outcome of such events is the rate of extinction during the Late Permian, which should have risen slowly, rather than plummeting at the P-Tr event.  Another is that survivors might show signs of adaptation to low oxygen levels, and indeed some Triassic reptiles do.  All in all, those times were stressful on land.  Yet the extinctions were just as severe in marine ecosystems, where the fossil record is more complete.  Less oxygen and warmer seas would have resulted in similar hypoxia for aquatic animals.

Ejecta from the Sudbury impact

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Sudbury in Ontario, Canada hosts one of the largest nickel and platinum-group metal deposits, and it in turn is associated with the world’s second largest impact structure (260 km diameter), dated at 1850 Ma.  About 650 km to the WNW is another of Canada’s Precambrian treasures, the Gunflint Chert beds that contain the earliest incontrovertible fossil cells.  Those cherts are also roughly the same age as the Sudbury impact structure, so what better place to seek material excavated and ejected by the offending meteorite? No need either to thrash around the bush to collect rocks; the succession has been penetrated by 5 drill cores near Thunder Bay and in northern Minnesota.  Sure enough, all the cores show signs of impact ejecta (Addison, W.D. et al. 2005.  Discovery of distal ejecta from the 1850 Ma Sudbury impact event.  Geology, v. 33, p. 193-196).  The proof takes the form of shocked quartz and feldspar grains and melt spherules, but in a sequence of silicified carbonates above the level of the Gunflint Chert.  Ejecta material is about 0.6 m thick.  Because the carbonates contain no volcanic horizons, establishing the age of the ejecta depends on a thin volcanic ash 5 m above it, which yielded zircon U-Pb ages between 1827 to 1832 Ma.  There are no other known impacts around this time, so Sudbury is the most likely source of the ejecta.  Apart from being the oldest impactite layer known that can be tied to a source, there are a couple of intriguing features.  The ejecta layer occurs almost at the top of the Gunflint Formation famous for its cellular remains, yet the overlying strata contain no sign of fossils.  The authors wonder if this might represent mass extinction, but these slightly younger sediments are clastic rocks in which cell microfossils are unlikely to have been preserved.  However, they do show signs of anoxia, including high organic carbon content and sulfide minerals.  Hopefully carbon isotope data from the section might throw light on how impacts in a world exclusively that of single-celled organisms affected the biota: an interesting comparison with the K-T boundary.  The other puzzle is that the ejecta are in shallow-marine sediments.  Being only a few hundred km from the linked impact structure, some sign of disturbance by tsunamis or water-release by huge seismic shocks might be expected within the sediments.  No signs of such disturbances have been reported.

Snowball Earth gets a boost

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enveloping glaciations during the Neoproterozoic Eon, that notion of “Snowball” conditions has received many severe knocks, charted by numerous items in EPN.  Geochemists and geologists from the Universities of Vienna and Witwatersrand realised that a good test of the hypothesis would be to concentrate on a rather obvious property of an ice-bound planet (Bodiselitsch, B. et al. 2005.  Estimating duration and intensity of Neoproterozoic Snowball glaciations from Ir anomalies.  Science, v. 308. P. 239-242).  Whatever falls on an ice sheet, whether it is cosmic dust from outside the Earth or ash from volcanoes, becomes trapped in the annual layers of ice.  When the ice melts, that accumulated content is transferred to the oceans very quickly.  With weathering in suspended animation during the glacial epoch, transport of many elements would have slowed to very low levels.  So, marine sediments deposited immediately after the diamictites that are allegedly glaciogenic ought to contain anomalously high levels of several elements.  The most important of these would be those which show very different abundance patterns in meteorites form those in terrestrial rocks.

Bodiselitsch et al. hit what seems to be “paydirt” in carbonates above a prominent diamictite in central Africa.  Their samples are impeccable, being from diamond-drill cores produced during evaluation of sediment-hosted mineralization in the famous Neoproterozoic Copper Belt of Zambia and Congo.  The core contains a prominent iridium anomaly at the very base of the carbonates, with a “signature” relative to other anomalous elements that points to a cosmic origin.  Normally such an anomaly would be ascribed to a meteorite impact, but in this case the coincidence would be too good to be true.  Instead, the authors use the magnitude of the anomaly to estimate how long cosmic dust had to accumulate to build up such a high level if it was released by rapid deglaciation.  Deep-ocean sediments from the last 80 Ma are a guide to the long-term accumulation rate of cosmic material.  If that rate is applied to the cap-carbonate anomaly, it gives a total time for accumulation in the hypothesised global ice cover of around 12 Ma.  Presumably this would have been from ice immediately overlying the area being studied.  An ice age that long defies any idea of more “normal”, astronomically forced glaciation, which would be expected to have cyclically formed and receded many times, thereby releasing the dust particles much more gradually.  Any anomalies would be expected in the diamictites themselves, yet there are none.  Although sample spacing is rather patchy through the entire succession, they are most dense around the anomaly itself.  Moreover, another suspected glaciogenic “package” higher in the sequence shows exactly the same iridium “spike”. 

Arguing against such support for the “Snowball Earth” hypothesis will be difficult, but other sequences require similar tests, most importantly those of Namibia, where Hoffman and colleagues developed their ideas, and the much more extensive deposits of Australia.  This diamictite sequence is reckoned to represent both postulated deep-freeze events of the Neoproterozoic, around 710 Ma (Sturtian) and 635 Ma (Marinoan). There is one nagging problem.  Data from one area are likely to record ice-retained cosmic dust only from ice in its immediate vicinity, and therefore do not represent the entire planet.  Much of the controversy is between supporters of a whole-Earth ice cover, and those who favour patchy glaciation (the “Slushball” model).  Unfortunately, Neoproterozoic stratigraphic correlation and radiometric age calibration is not sufficiently good to detect the same intervals elsewhere and look for anomalies there.  In fact, the stratigraphy is generally correlated from place to place by matching the diamictites themselves.  There is plenty of evidence that they may all coincide in time.

Tracking ocean circulation during the last glacial period

The use of various ocean-floor sediment proxies for climate change, such as the ups and downs of heavy 18O that chart waxing and waning continental ice cover, has progressively revealed the complexity of shifts during glacial and interglacial periods. Yet more emerged from finer-resolution time-series contained with Greenland and Antarctic ice cores.  The diversity of information that proxy for many different, climate-related processes has in the last decade enabled palaeoclimatologists to begin piecing together possible causative mechanisms, beyond the initial discovery of an astronomical signal in early oxygen-isotope records.  One of enormous significance is the possibility that sudden millennial-scale cooling and warming link to changes in ocean circulation, especially that performed by the Gulf Stream driven by thermohaline processes at high northern latitudes.  Shutting down that poleward transfer of heat, probably because freshwater made high-latitude surface water less dense, has been implicated in sudden cooling or “stadials”, and its restart linked to warming or “or interstadials”.  The last such sudden climate event, the Younger Dryas between about 12 and 11 thousand years ago, is widely believed to have resulted from a collapse of the Gulf Stream.  That has raised fears that current anthropogenic warming might achieve the same thing, thereby plunging Western Europe into a counterintuitive frigid period through loss of its maritime warming.

Ocean circulation has lacked a proxy that might help resolve such worrying scenarios, but it seems that one has arrived, because of improvements in mass spectrometry (Piotrowski, A.M. et al. 2005.  Temporal relationships of carbon cycling and ocean circulation at glacial boundaries.  Science, v. 307, p. 1933-1938).  Different bodies of ocean-surface water have subtly different chemical compositions, due to the varied geochemistry of surrounding landmasses.  Weathering of exposed rocks results in some elements entering solution in river water, and that mixes with surface water in the nearby ocean.  Among the most useful elements are those with an isotope to which radioactive decay of unstable isotopes of another element contributes.  A good example is 87Sr that is formed when 87Rb decays.  Where continents expose  large expanses of very ancient rocks they contribute more 87Sr to seawater than do continents veneered with younger rocks.  Strontium isotopes have been used successfully for charting very-long term changes in the overall erosion of continental crust, in relation to climate shifts, but being related to calcium are taken up quickly by carbonate secreting organisms, such as foraminifera, at many different levels in the ocean as it circulates.  So they are not very useful for short-term studies.  A more useful isotopic system involving an daughter of slow radioactive decay is that of neodymium, because it does not get taken up in this way.  It does however enter the manganese minerals that slowly precipitate on the deep ocean floor.  Moreover, its isotopic composition varies greatly in different ocean-water masses.  Piotrowski et al. used neodymium isotopes from deep ocean cores to see if changes in this circulation proxy coincided with known climate proxies.  For interstadial, warming events there is a match, so a Gulf-stream control over millennial-scale climate shifts is indeed supported.  But for the start and end of the full glacial period control by ocean circulation did not happen.  Instead, changes in the neodymium record lag behind the climate proxies, suggesting climatic control of circulation, which then “kicked in” to boost changes that were well underway.

See also: Kerr, R.A. 2005.  Ocean flow amplified, not triggered, climate change.  Science, v. 307, p. 1854.

Tree-ring heaven

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Growth rings in tree trunks are among the best records of local climate variation that there are: they provide an annual “stratigraphy”.  So intricate are the records that it has proved possible to match ring sequences in ancient but still growing trees to those found in logs of even greater antiquity, thereby building up a “dendrochronology” that extends back into history.  Tree rings help historians link human affairs to a background of changing conditions for life.  Henri Grissino-Mayer of the University of Tennessee has brought together a wealth of dendrochronological information in his Ultimate Tree Ring Pages at web.utk.edu/%7Egrissino/default.html.

Yet more Indian Ocean earthquakes? Sadly, yes

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The shores of the Indian Ocean and the people who live near them will take years and maybe decades to recover from the awful events of 26 December 2004.  While relief and reconstruction efforts are underway, so too is the scientific analysis of what happened.  Throwing a malevolent shadow is the uncertainty of whether there may yet be more tsunamis so soon after the first in the region for 150 years.  The Sunda trench where the massive earthquake took place had remained stable for a long time.  Stresses built up, eventually to cause the subduction zone to fail catastrophically.  However stress relief in one place redistributes that which remains along other fault lines, and can create space in which new breaks might occur.  Geophysicists from the University of Ulster have analysed the likely disruption of stress in the eastern Indian Ocean (McCloskey, et al. 2005.  Earthquake risk from co-seismic stress.  Nature, v. 434, p. 291) following the distribution of about 20 m displacement on the Sunda subduction zone over a N-S length of around 500 km.  They feared that such a huge perturbation may activate other large faults.  A changed stress field seems to have been the cause of the Izmit earthquake that devastated central Turkey and also set in motion repeated seismicity along the subduction system off Japan in the past. McCloskey and colleagues foresaw two worrying possibilities for the Sunda subduction system: stress localised just to the south of the Boxing Day event could migrate southwards to trigger release again on the subduction zone; a large strike-slip fault that runs down the centre of Sumatra, itself linked to subduction, may fail soon. fear that the second is the more likely.  Since modern seismology emerged, so few earthquakes have occurred in the area compared with other large subduction settings that prediction is difficult.  The Ulster scientists were correct, very soon after their prediction was published.  On 28 March 2005, a magnitude 8.7 earthquake occurred on the subduction zone about 150 km south-west of that on Boxing Day 2004.  Its motion involved vertical displacement, so it was feared to trigger yet more tsunamis and sirens sounded throughout the previously devastated areas.  The warnings were heeded.  Apart from some panic that cause two deaths in Sri Lanka, people moved quickly to safe ground.  Thankfully, perhaps miraculously considering an energy release not far short of that at the end of 2004, there were no tsunamis of any consequence.  Yet the places on the nearby Indonesian island of Nias were devastated by the shock waves, killing upwards of a thousand people.  This is a grim warning that McCloskey and colleagues’ interpretation of stresses moving southwards along the main ocean floor fault system is happening.  The risk of further devastation soon is by no means over.

Mineral maps of Mars

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Lots of space has been devoted in science journals to results from NASA’s robot rovers on Mars.  Well, haven’t they been exciting?  Iron-oxide “blueberries, a cliff with bedded sediments and some iron-aluminium sulphate in a combined traverse of a kilometre at most: imagine a geologist coming back from a terrestrial field trip costing a year’s GDP of a small poor country and writing a report for the funding agency!  That is a bit cruel, for in planetary exploration the themes are context, context and context, but we did know that Mars is red and orange, which is enough for most of us to feel happy with a lot of iron coloration.  At the same time as the rovers were deployed, the European Space Agency’s Mars Express was going into orbit (so named because it was assembled in something of a hurry).  That bristles with the geoscientist’s other modern tools: those aimed at sensing materials from their electromagnetic spectra.  There is the High-Resolution Stereo Camera that produces images to rival high-altitude aerial photos of the Earth, and with stereoscopic overlap from which accurate models of Mars’ topographic elevation can be calculated, of which more in the next item.  The principal mineral and rock mapping tool is the Observatoire pour la Minéralogie, l’Eau, les Glaces, at l’Activité (OMEGA), that builds on the spectral mapping by NASA’s Thermal Emission Spectrometer deployed by the earlier Mars Global Surveyor and a similar instrument aboard Mars Odyssey.  OMEGA is every remote sensing geologist’s dream machine, because its coverage of the short-wave end of electromagnetic radiation by 350 narrow bands can match spectra reflected from rocks and soils with those measured under laboratory conditions for several hundred important minerals.  Research geologists don’t get much of that quality of data from Earth, mainly because it is commercially successful in mineral exploration, and very expensive (for much of the Earth, such hyperspectral data is not very useful, because vegetation masks most mineral signatuires).  But data are free from Mars Express (or will be when the main investigators have had a reasonable time to satisfy their curiosity) and has a terrestrially useful resolution down to 100m.  They also cover an awful lot of the planet’s surface and should eventually give 100% coverage.. The 11 March 2005 issue of Science devotes 24 pages (p. 1574-1597) to summarising OMEGA results.  Various papers reveal variations in the composition of pyroxenes in the predominantly mafic Martian surface rocks, those minerals, such as the sulphates gypsum and jarosite, which contain water and signs of weathering by water, and an awful lot about water and CO2 ices around the poles.  But this is not the geology in full of course, but driven by the search for potential habitability.  Common rocks are not made of sulphates and ice, but silicates, which can be assessed by multispectral thermal emission data that prove very useful on Earth.  The lack of information about such fundamental divisions of Martian igneous rocks as ultramafic, mafic, intermediate and felsic is a great disappointment, but perhaps the thermal instrument aboard Mars Odyssey will eventually come up with those more mundane goodies.  Oddly, the planetary treasures of Mars are not being revealed by such sophisticated instruments, but by what is still the work horse for a great deal of  geological image interpretation, black and white stereo images.

The triumph of the old on Mars

Except perhaps for some of the current generation of geologists, who are immersed in their remote sensing training by false colour images of spectrally revealing multispectral image data, a great many professionals who engage in mapping cut their teeth on what is known simply as photogeology.  And it is simple.  Provided images are taken of an area from different angles, with the simplest of instruments most people’s innate stereoscopic vision enables them to see startling illusions in three dimensions.  Stereoscopy has been to geologists of the mid to late 20th and early 21st centuries what the binoculars were to those earlier scientist who discovered the great nappes of the Alps and thrust belts of the Rockies.  A stereoscope of some kind is the latter-day analogue of that “Swiss Hammer”.  Two stereo images reveal a great deal more than twice the information of one flat image, no matter how detailed.  Using complex software, which converts the parallax differences that enable us to see 3-D to the differences in topographic elevation that cause relative shifts in the position of features on overlapping images creates accurate models of the elevation itself.  That enables quantitative measure of many features related to topography, and allows the images to be viewed in perspective, as if they were indeed captured by binoculars from a high view point.  Results from the Mars Express High-Resolution Stereo Camera (HRSC) have proved able to revolutionise our understanding of the Martian surface.  The 17 March 2005 issue of Nature reports three important new results that stem from HRSC data.  For several years the possibility of glaciers having carved some features on Mars have been suspected from lower resolution elevation data.  Now it is certain from exquisite perspective views of debris aprons that record the flow of smashed rock from large mountains, almost certainly because the debris was once extremely dirty glacial ice (Head, J.W. et al. 2005.  Tropical to mid-latitude snow and ice accumulation, flow and glaciation on Mars.  Nature, v.  434, p. 346-351).  The flows are reminiscent of rock-rich glaciers in the hyper-arid Dry Valleys of Antarctica.  These authors present evidence that suggests that the flows are as young as 130 Ma, and may yet contain water ice.  A second paper also reveals the influence of near-surface ice on Mars (Hauber, E. et al. 2005.  Discovery of a flank caldera and very young glacial activity at Hhecates Tholus, Mars.  Nature, v. 434, p. 356-361).  In its case it seems to have been mobilised by an explosive volcanic eruption, possibly as young as 20 Ma, to produce debris flows and also very well preserved drainage channels at a much smaller scale than those known from Mars’ earliest history.  The drainages might have resulted from subsurface ice melting by high heat flow and emergence of the “groundwater” to carve the meandering channels.  There is an important caution: any dating on Mars depends on assuming a timescale based on counting impact craters and noting their relations to each other and different kinds of surface.  The third paper observes something very different (Murray, J.B. et al. 2005.  Evidence from the Mars Express High Resolution Stereo Camera for a frozen sea close to Mars’ equator.  Nature, v. 434, p. 352-356).  HRSC images reveal an area about the same size as the North Sea that is not only completely flat, but shows features very like those associated with pack ice in the Arctic and around Antarctica.  They are plates whose edges can be fitted together, and in some cases islands have resulted in pressure ridges very like those seen where terrestrial pack ice meets land.  There are even examples of impact craters that have been flooded.  Murray and colleagues attribute all this to a large volume of subsurface water released by very recent volcanism along fissures close to the Martian equator.  Basalt floods had been identified in the region before, but not evidence for a possible sea-sized, frozen lake.  Similar, but not so revealing features elsewhere on Mars have been interpreted as lava rafts that once floated on flood basalts.  Naturally, Mars scientists are very excited about the possibility of a large ice sheet at the equatorial surface, which may be as much as 45 metres deep.  Unfortunately, the observations are from an area not yet covered by spectral data that would resolve whether the surface is ice-rich or more mundane lavas.