Category Archives: GIS and Remote Sensing

Remote sensing for fossils

With the growing diversity of data from those parts of the electromagnetic spectrum that pass freely though Earth’s atmosphere, mainly acquired from orbit, an increasing number of attributes of the surface can be mapped remotely. The initial impetus to launch remote sensing satellites in the 1960’s and early 70’s had two strands: to monitor weather conditions and assess vegetation cover with the early metsats, such as TIROS-1, and the first Landsat platform that exploited green plants’ propensity for absorbing visible and largely reflecting near-infrared (NIR) radiation. With the incorporation in the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) instruments of wavelength regions in which minerals show spectral diversity, in the reflected short-wave infrared (SWIR) and emitted thermal infrared (TIR), remote sensing became a viable and useful tool for geologists. It figures strongly in lithological mapping and also in the detection of minerals related to various kinds of alteration associated with metal mineralisation and the migration of hydrocarbon-related fluids. The more wavebands with narrower coverage of radiation wavelengths, the more likely are the subtle differences in mineral spectra able to be detected and mapped. Yet, apart from one experimental system (Hyperion aboard NASA’s EO-1 orbital platform) our home planet is not as well served by such hyperspectral systems as is Mars, blessed by two which have fuelled the on-going search for past habitable zones on the Red Planet.

The May 2014 issue of Scientific American includes an article on remote sensing that follows what to many might seem an odd direction: how to increase the chance of finding rich fossil deposits (Anemone, R.L. & Emerson, C.W. 2014. Fossil GPS. Scientific American, v. 310(5), p. 34-39). Apart from targeting a particular stratigraphic unit on a geological map, palaeontological collection has generally been a hit-or-miss affair depending on persistence and a keen eye, with quite a lot of luck. Once a productive locality turns up, such as the Cambrian Burgess shale, the dinosaur-rich Cretaceous sandstone of the Red Deer River badlands of southern Alberta in Canada and the hominin sites of Ethiopia’s Afar Depression, palaeontologists often look no further until its potential is exhausted. Robert Anemone and Charles Emerson felt, as may palaeobiologists do, that one fossil ‘hotspot’ is simply not enough, yet balked at the physical effort, time and frustration needed to find more by trekking through their area of interest, the vast Tertiary sedimentary basins of Wyoming, USA. They decided to try an easier tack: using the few known fossil localities as digital ‘training areas’ for a software interrogation of Landsat Enhanced Thematic Mapper data in the hope that fossiliferous spots might be subtly different in their optical properties from those that were barren.

Satellite image of the Wyoming Basin, Wyoming,...

Satellite image of the Wyoming Basin, USA. credit: Wikipedia)

The teeth and bones of early Eocene mammals that had drawn them to Wyoming turn up in sandstone beds of the basins. They are pretty distinctive elements of landscape, forming ridges of outcrop because of their relative resistance to erosion, yet for that very reason present a huge selection of possibilities. Being simple mineralogically they also presented a seemingly daunting uniformity. Anemone and Emerson decided on a purely statistical approach using the six visible, NIR and SWIR bands sensed by Landsat ETM, rather than a spectrally oriented strategy using more sophisticated ASTER data with 14 spectral bands. Their chosen algorithm was that based on an artificial neural network that the fossil rich sandstones would train to recognise patterns present in ETM data recorded over them. This purely empirical approach seems to have worked. Of 31 sites suggested by the algorithm 25 yielded abundant vertebrate fossils. Applied to another of Wyoming’s Tertiary basins it also ‘found’ the three most productive known mammal sites there. So, what is it about the fossil-rich sandstones that sets them apart from those that are more likely to be barren? The authors do not offer an explanation. Perhaps it has something to do with reducing conditions that would help preserve organic material better than would sandstones deposited in an oxidising environment. Iron minerals and thereby colour might be a key factor, oxidised sandstones are generally stained red to orange by Fe-3 oxides and hydroxides, whereas reduced sandstone facies may be grey because of iron in the form of sulfides

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An early magma ocean on Mars?

The division of the lunar surface into two petrological domains – ancient anorthositic highlands and younger basaltic maria – spurred the idea, as long ago as the early 1970s, that the early Moon had a deep ocean of magma at the surface, whose cooling caused fractional crystallization. Low density plagioclase feldspar, dominated by high-calcium anorthite and bytownite, floated to the surface to form the lunar anorthosites leaving a more mafic mantle from which the mare basalts formed by partial melting. The key evidence in support of this hypothesis lies in the rare-earth elements of the two terrains. Because plagioclase feldspar has a much stronger affinity to incorporate the element europium (Eu) than the other REEs, the lunar anorthosites are enriched in Eu compared with its related elements. If the highland anorthosites did form by fractional crystallisation the remaining magma that formed the lunar mantle would be depleted in Eu yet enriched in the remaining REE. Although there are no samples of the Moon’s mantle there are plenty of the mare basalts that formed when it partially melted, probably as a result of huge impacts around 3.8 billion years ago. They should have inherited dominant features of mantle geochemistry, and indeed they do show characteristic depletion of Eu.

Lunar Highlands, near Descartes Crater. Collec...

Lunar Highland anorthosite, collected by the crew of Apollo 16. (credit: Wikipedia)

The giant-impact hypothesis for the Earth-Moon system presupposes that such a cataclysm would have left much of the outer Earth in much the same molten condition and destined to fractionate in the same manner. There are geochemical hints from terrestrial rocks that do support such an idea. An important target for exploration of Mars has been to check if a magma ocean also existed early in its history. Of the various missions in recent years only two have the capacity to shed useful light on the issue: the US Mars Reconnaissance Orbiter and Mars Odyssey. Both orbiters carry more sophisticated remote sensing instruments than any circling the Earth. The first has the hyperspectral Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) that senses visible to short-wave infrared (VNIR) radiation, the other deploys  the Thermal Emission Imaging System (THEMIS) that captures different parts of the longer wavelength thermal infrared (TIR) spectrum emitted by surface materials. Both allow spectra of surface materials to be reconstructed and compared with the features of known minerals from the Earth and Moon.

Feldspars are highly reflective for the most part of  the VNIR range but show a shallow, broad absorption feature centred on a wavelength of 1.26 micrometres. Such spectra have been detected using CRISM from parts of the Martian surface in the highlands of its southern hemisphere (Carter, J. & Poulet, F. 2013. Ancient plutonic processes on Mars inferred from the detection of possible anorthositic terrains. Nature Geoscience, v. 6, p. 1008-1012). The authors, from Chile and France, acknowledge that the plagioclase-rich rocks occur only in small patches, unlike the vast tracts on the Moon, and also that on Earth anorthosites are known to have formed by a variety of processes from far smaller magma systems than a veritable ocean of molten rock. Feldspars also show spectral features in the TIR, though not so distinctive, both plagioclase and alkali feldspars being very similar. Moreover, THEMIS deploys sensor for only 10 thermal wavebands, compared with 544 on CRISM.  A team of US remote sensers (Wray, J.J. and 8 others 2013. Prolonged magmatic activity on Mars inferred from the detection of felsic rocks. Nature Geoscience, v. 6, p. 1013-1017) used both CRISM and THEMIS data. While noting resemblances to lunar anorthosites, they adopt a more cautious approach to the spectra and prefer the broad, ‘sack’ term ‘felsic rocks’. It seemed possible from their work that feldspar-rich magmas may have formed by partial melting of common andesitic crust noted from the Martian surface: high spatial resolution images of the occurrences bear some resemblance to outcrops of granitic rocks in arid environments on Earth. That is, there may be highly evolved rocks akin to terrestrial continental crust.

The interesting spectral observations on Mars can only be validated by actual rock samples. While rovers still operating on the Martian surface are well able to produce geochemical data that would petrologically characterise most rocks that they encounter, none of them is in a terrain suitable for resolving this particular issue. Yet, coincidentally, a meteorite found in West Africa shows hallmarks of having been blasted from the surface of Mars and sheds useful light on various hypotheses about the Martian crust http://earth-pages.co.uk/2013/11/21/a-glimpse-of-early-martian-crust/. It is a breccia that may represent the soil or regolith that accumulated from early impacts that shattered and melted surface materials, and it is extremely old: zircons yielded an age of 4428 Ma. The clasts set in a fine matrix consist of a variety of igneous rocks, none of which are anorthosites. Some are coarse grained, plutonic rocks containing both alkali feldspars and plagioclase, which match terrestrial monzonites; broadly speaking members of the granite family. Having formed from the ejecta of large impacts, such regolith materials represent the breadth of compositions across the planet and extending deep into its crust. This one suggests that anorthosites may have been rare on early Mars.

The Grand Greenland Canyon

One of the properties of radar is that it can pass through hundreds of metres of ice to be scattered by the bedrock beneath and return to the surface with sufficient remaining power to allow measurement of ice depth from the time between transmission of a pulse and that when the scattered energy returns to the antenna. Liquid water simply absorbs the radar energy preventing any return from the subsurface. As far as rocks and soils are concerned, any water in them and the structure of minerals from which they are composed limit penetration and energy return to at most only a few metres. While radar images that result from scattering by the Earth’s solid surface are highly informative about landforms and variations in the surface’s small-scale texture, outside of seismic reflection profiling, only ice-penetrating radar (IPR) approaches the ‘holy grail’ of mapping what lies beneath the surface in 3-D. Unlike seismic surveys it can be achieved from aircraft and is far cheaper to conduct.

English: Topographic map of Greenland bedrock,...

Greenland’s topography without the ice sheet. (Photo credit: Wikipedia)

It was IPR that revealed the scattering of large lakes at the base of the Antarctic ice cap, but a survey of Greenland has revealed something even more astonishing: major drainage systems. These include a vast canyon that meanders beneath the thickest part of the ice towards the island’s north coast (Bamber, J.L. et al. 2013. Palaeofluvial mega-canyon beneath the central Greenland ice sheet. Science, v. 341, p. 997-999). At 750 km long and a maximum depth of 800 m it is comparable with active canyon systems along the Colorado and Nile rivers in the western US and Ethiopia respectively. A less-well publicised feature is ancient leaf-shaped system of buried valleys further south that emerges in a great embayment on West Greenland’s coast near Uummannaq, which may be the catchment of another former river system. In fact much of the data that revealed what appears to be pre-glacial topography dates back to the 1970s, though most was acquired since 2000. The coverage by flight lines varies a great deal, and as more flights are conducted, yet more detail will emerge.

The British, Canadian and Italian discoverers consider that glacial meltwater sinking to the base of the ice cap continues to follow the canyon, perhaps lubricating ice movement. The flatter topography beneath the Antarctic ice cap is not so easy to drain, which probably accounts for the many sub-glacial lakes there whereas none of any significance have been detected in Greenland. The earliest time when Greenland became ice-bound was about 5 Ma ago, so that is the minimum age for the river erosion that carved the canyon

On-line global geological maps

Note: Earth-Pages will be closing as of early July, but will continue in another form at Earth-logs

Global geological map (credit: Commission for the Geological Map of the World

Global geological map (credit: Commission for the Geological Map of the World

Getting hold of geological maps on-line has been a hit or miss affair until recently, and those made available for free are at a variety of scales (generally less than 1:10 million) and vary in reliability and information content.  Scanned versions of paper sheets rendered with JPEG compression can leave a lot to be desired. If you are able to pay, then the situation improves as there are on-line vendors of printed geological  maps. But, all told, browsing the world’s geological features is a slow and generally frustrating task. The best bet might seem to be the Commission for the Geological Map of the World (http://www.ccgm.org/) . They do, as you might expect, sell global maps, but at 1:50 million detail is sparse, although there is an alternative 3-sheet set (Old World, Americas and Polar regions) at 1:25 million, and it is possible to purchase digital versions and a variety of geophysical sheets. Maps at 1:5 million are available for Europe, Africa (6 sheets), the Middle East and South America plus various tectonic maps. However, to explore full planetary-scale geology at the modestly informative scale of 1:5 million demands visiting a lot of on-line vendors, as there is no one-stop shop for geologists

Small-scale extract from the OneGeology portal with 1:2 million maps for Ethiopia, Kenya, Tanzania and Uganda, and at 1:10 million covering surrounding areas (credit:OneGeology portal)

Small-scale extract from the OneGeology portal with 1:2 million maps for Ethiopia, Kenya, Tanzania and Uganda, and at 1:10 million covering surrounding areas (credit:OneGeology portal)

Such frustration is set to change, because in the last few years there have been moves to compile digital geology in a manner akin to Google Earth, now available at the OneGeology portal (http://portal.onegeology.org/). As soon as you enter the portal, the reason why the Commission for the Geological Map of the World is so irritating immediately becomes clear: the CGMW world map is what shows at the global scale and it doesn’t show much. Progressive zooming-in removes the 1:50 million map, to be replaced by a compilation of regional maps at scales ranging from 1:2 million to 1:12.5 million scales that does cover the entire Earth’s continental surface. A mouth-watering prospect until you start to look for legends! In fact, the associated tool box provides a means of pointing to individual stratigraphic units on the maps to get information (metadata), but whether and how it works depends on the source of the maps and the scale of viewing. For instance, the 1:10 million map of Africa gives no information, while the 1:5 million map of Europe gives quite a lot.

With a zoom to better than 1:10 million display, lots more detail appears in the form of country maps, but coverage is not comprehensive. In East Africa country maps are available for Ethiopia, Kenya, Rwanda and Tanzania – ranking with the current offerings from the USA. Moving to Europe, the range of scales improves on a country-by-country basis, generally 1:1 million to 1:250 thousand, but the UK truly grabs attention by providing digital geology at up to 1:50 thousand scale. The British Geological Survey has systematically rendered all its bedrock map data digitally to this scale, and is to be congratulated at making the ‘Full Monty’ available on the OneGeology portal. Full BGS metadata shows for all the visible stratigraphic and lithological units, together with faults and superficial deposits.

British Geological Survey bedrock mapping in Cumbria at 1:50 thousand scale. (credit: OneGeology portal)

British Geological Survey bedrock mapping in Cumbria at 1:50 thousand scale. (credit: OneGeology portal)

It soon becomes clear that OneGeology is a work in progress, but what a work it will be! If I have a criticism it is that geology is not linked to topography and cartographic features. The ever-present base data is the NASA Blue Marble mosaic of natural colour MODIS imagery. Unfortunately, outside of areas bare of vegetation this does not have any useful lithological connection, and is presented at such a large pixel size that only the coarsest topography shows up. At scales better than 1:2 million it is an irritating patchwork of square pixels. Far better would be shaded relief based on the ubiquitous ASTER GDEM data at up to 30 m resolution, especially as it is possible to vary the opacity of the geological maps to show the link with surface morphology. Maybe that is on its way and possibly oblique perspective 3-D viewing: one has to bear in mind that Google Earth wasn’t built in a day and geoscientific data are not yet standardised – a hugely costly endeavour, as that would involve not only digitising all maps but lengthy negotiations.

Most geologists are likely to be interested in maps that show rock units with stratigraphic age, but Jens Hartmann and Nils Moosdorf of the University of Hamburg, German have mined regional geological maps to assemble a global, purely lithological database (Hartmann, J. & Moosdorf, N. 2012. The new global lithological map database GLiM: A representation of rock properties at the Earth surface. Geochemistry, Geophysics, Geosystems, v. 13, doi:10.1029/2012GC004370) in cooperation with CGMW. Their Global Lithological Map (GLiM) consists of over 1.25 million digital polygons (ESRI shape or *.shp format), classified lithologically in three levels to give a total of 42 rock-type classes, 16 used in previous global lithological maps and two more lithologically specific sets of 12 and 14 subclasses . Though the database is said to be presentable at up 1:3.75 million scale, the version of GLiM that the reader can download is not in vector format but as a series of cells numerically coded according to class in a georeferenced grid. Since that is 360 rows x 720 columns, i.e. 0.5 degrees of latitude by 0.5 degrees of longitude, that version is useful only for rough statistics, such as the percentage of North America that is covered by evaporates, for instance. Perhaps the most useful aspect of the GLiM paper is the comprehensive referencing of the source maps. GLiM, apparently, is not an on-line resource, but no doubt the authors can provide interested parties with the *.shp files (contact jens.hartmann@zmaw.de or nils.moosdorf@zmaw.de)

More wet minerals on Mars

A remote-sensing geologist who focuses on terrestrial matters would likely grind their teeth on seeing papers that use far better data captured from the Martian or lunar surface than are ever likely to be available from the bulk of Earth’s land surface over the next decade at least. Mine are even closer to the gums after reading about hyperspectral data from Mars with high spatial resolution (~20 m), used to locate rocks altered by water on Mars (Carter, J. et al. 2010. Detection of hydrated silicates in crustal outcrops in the northern plains of Mars. Science, v. 328, p. 11682-1686). And, of course, there is no vegetation and not much of an atmosphere to cryptify spectral features of minerals: if there is enough of a mineral exposed to show up, the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) carried by NASA’s Mars Reconnaissance Orbiter will spot it. If the mineral has unique features in its spectrum, and most of the hydrated silicates do, it can be classified nicely. Less spatially sharp hyperspectral data from the Observatoire pour la Minéralogie, l’Eau, les Glaces et l’Activité (OMEGA) carried by ESA’s Mars Express is equally discriminating for larger patches.

The two instruments have shown up hundreds of small outcrops of minerals in the southern hemisphere that formed by reactions between the dominantly anhydrous minerals of Mars’s dominantly igneous crust and water. They record an early phase when liquid water was available at the surface. The question is, are they merely a thin veneer? As a check, John Carter (did bearing the same name as Edgar Rice Burroughs’ hero in his Mars novels encourage his fascination with the Red Planet?) of the University of Paris and colleagues used OMEGA and CRISM data to look at deep crust exhumed in several of Mars’s northern hemisphere craters. Clay minerals, chlorite and prehnite do show up clearly, and the hydration reactions must therefore have penetrated up to a kilometre into the crust. The same suite of minerals occur in the southern hemisphere, so during this early wet episode water was available far and wide across the Martian surface. Minerals like prehnite and chlorite are most familiar as products of low-grade metamorphism, which presents a puzzle. Maybe they formed as a result of the temperatures and pressure generated by the impacts themselves. But if that were the case they would be expected to pervade all the excavated rock, whereas they occur in distinct patches next to pristine, highly reactive olivine-rich rocks. One absentee mineral is serpentine that would definitely have formed by the reaction of water with olivine during impacts. So it looks like water pervaded the whole Martian crust down to maybe a kilometre, then this ‘weathered’ layer was blanketed much later by a thick volcanic layer which has been removed in some places by impact excavation.

• Tectonics
Underpinnings of Mediterranean tectonics
The region of the Mediterranean Sea, especially in the Aegean area, has among the most complex active tectonics on Earth. Both the African and Eurasian plates are now barely moving. The basic shaping of the region stems from Africa’s protracted collision with Europe since 40 Ma that resulted in the closure of the Mesozoic Tethys seaway and jumbled both its sedimentary fill and the continental lithosphere that lay on either side of the collision zone. But if surface motion has largely stopped, why is the Mediterranean region so tectonically active? It now seems as though it links to flow in the mantle beneath (Facenna, C. & Becker, T.W. 2010. Shaping mobile belts by small-scale convection. Nature, v. 465, p. 602-605). A mix of GPS tracking of surface motions, evaluation of surface uplift and subsidence, and analysis of seismic tomography of the mantle. Vertical motion of the mantle is most pronounced at shallow mantle depth (250 km), suggesting vigorous convection in quite small cells. The relations to tectonics are complex, but they are interlinked. For instance subducting slabs interfere with shallow mantle flow so that compensating upwellings result, and in turn help drive subduction and volcanism, as in Italy. Overall, the lithospheric motion, from GPS tracking, has a distinct vortex-like pattern in the eastern Mediterranean and Middle East, which can be modelled from the underlying mantle flow.

Geochemical prospecting on Mars

Since its atmosphere is so thin, there are things you can achieve from orbit around Mars that would be unthinkable for the Earth. One is imagery free of atmospheric shimmer or scattering, another is analysing gamma rays emitted by Martian rocks using a gamma-ray spectrometer (GRS), as carried by Mars Odyssey. Two processes produce the gamma rays: the decay of long-lived naturally-occurring radiogenic isotopes of potassium, uranium and thorium with their daughter isotopes, and by the interactions of high-energy cosmic-ray particles with other elements in surface materials. Again, with little atmosphere the Martian surface is heavily bombarded by cosmic rays. Using far larger gamma-ray detecting crystals carried on low-flying aircraft it is possible to remotely sense K, U and Th concentrations at the Earth’s surface. To get data on other terrestrial elements from far off would involve unsociable irradiation of the surface by artificial means.

Results from the Mars Odyssey GRS are somewhat blurred as the analysed radiation comes from 0.5º x 0.5º sampling ‘bins’ and is then filtered to a level of 5º x 5º (~ 25 x 25 km) (Taylor G.J. et al. 2010. Mapping Mars geochemically. Geology, v. 38, p. 183-186). So, the approach cannot match geological maps made by interpretation of high resolution images of reflected or thermal radiation. However, as well as K, U and Th estimates, the data cover Fe, Si, Ca, Cl and H2O: sufficient to crudely distinguish mafic and felsic igneous rocks and to detect any regional hydrothermal or groundwater alteration. The authors claim that the GRS separates  much of the Equatorial region of Mars into six kinds of geochemical province, all of roughly basaltic composition. With an estimated SiO2 range from 46.7 to 49.8% that doesn’t promise much by way of fractionation on the scale of terrestrial magmagenesis; i.e. there are no significant intermediate or felsic igneous rocks. A CaO range of 7.5 to 11.4 does indicate varying plagioclase feldspar content, but no anorthosites, unlike the Moon. The greatest variation is in K and Th content, but that does not match the much larger ranges in terrestrial basalts. The geochemical provinces do not match even a simplified photogeological map of the planet, and it seems quite likely that such variation as there is could have resulted from slight weathering and movement of dust and sand. Will a single returned sample of Mars basalt be all that is needed to characterise the Red Planet? More to the point, how does the estimated chemistry match that of purported Martian meteorites, or for that matter the analyses performed on the surface by the Martian rovers Spirit and Opportunity and by the earlier Mars Pathfinder? There is no comment…but Mars Pathfinder surface analyses revealed andesitic rocks at its landing site with up to 55% SiO2.

Micro-gravity data chart shrinking ice caps

The NASA and German Aerospace Centre Gravity Recovery and Climate Experiment (GRACE) launched in 2002 aims to measure variations over time in the Earth’s gravity field by gauging tiny changes in distance between two satellites using radar. Briefly, mass in the Earth tugs first on the leading satellite and then on the one trailing it, so if mass distribution stays constant so does the separation between the craft. If mass below a point on the Earth’s surface does change, GRACE detects this from a change in separation between the two craft. Between April 2002 and February 2009, monthly measurements over Greenland and Antarctica reveal losses in the amount of ice, and the rate at which the ice caps are shrinking is accelerating (Velicogna, I. 2009. Increasing rates of ice mass loss from the Greenland and Antarctic ice sheets revealed by GRACE. Geophysical Research Letters, v. 36, L19503 doi:10.1029/2009GL040222). Isabella Velicogna of NASA/JPL shows that the Greenland ice cap (total mass~3 x 1015 t) lost 1.37 x 1011 t a-1 in 2002–3, rising to 2.86 x 1011 t a-1 in 2007–9 , the loss is accelerating at 3.0 ± 1.1 x 1010 t a-2). The ten times more massive Antarctic ice cap lost 1.04 x 1011 t a-1 in 2002–6 rising to 2.46 x 1011 t a-1 in 2006–9, giving an acceleration of 2.6 ± 1.4 x 1010 t a-2. Proportionate to size the Greenland ice cap is dwindling faster than Antarctica, but at these rates it still has 10 thousand years before it disappears.

Wow! Columnar joints found in Martian lava flow…

From time to time I wear my spleen on my sleeve over issues of scientific priority. Orbiting Mars are imaging devices whose data, if they were of the Earth’s surface, would cost geoscientists the proverbial arm and a leg.  ‘Astrogeologists’ get those from Mars for nothing. The latest result explains why I get annoyed; and I hope many others do as well (Milazzo, M.P. and a great many others 2009. Discovery of columnar jointing on Mars. Geology, v. 37, p. 171-174). The High Resolution Imaging Science Experiment (HiRISE) aboard the Mars Reconnaissance Orbiter, can resolve pixels 30 cm across (about the same as the best, classified military data of Earth from spy satellites). It has stereoscopic capacity capable of producing not only stunningly informative 3-D visualisations but also topographic elevation data sufficiently precise that they could be used ‘at home’ for large-scale civil engineering, for instance routing water pipelines. The US Department of Defence vetoes access by scientists to near-global SRTM DEMs with even a 30 m resolution, the degraded 90 m version being freely available. Sub-metre DEMs can be produced from aircraft for the Earth’s surface, but at very high cost.

The paper reports one of the most common features exhibited by thick lava flows and other tabular bodies of igneous rock that cooled slowly. Visit the Giant’s Causeway in Antrim to see columnar joints, and put your child on one for scale. In fact there are thousands of such sights on Earth, and any planet that has a volcanic history will have columnar joints. Similar quality data is awaited from the Moon, and you can bet your intimate garments that some bright spark will report much the same. Meanwhile, there are over a billion people drinking hazardous water when geologists armed with data this good – and the inclination – could find safe supplies in the rocks beneath them.

Entire Landsat archive now accessible by all, free of cost

May 2008 saw probably the most significant announcement for geologists of this century (The Landsat Science Team 2008. Free access to Landsat imagery. Science, v.  320, p. 1011; and see landsat.usgs.gov/images/squares/USGS_Landsat_Imagery_Release.pdf). Given a broadband internet connection, it will soon be possible to download Landsat data (MSS, TM and ETM+) covering any area on Earth free of charge from the US Geological Survey, provided it occurs among the >2 million scenes archived by their EROS Data Center. This act of open-handed generosity by the USGS marks a key step in revolutionising the activities of geologists of the Third World, especially those in Africa; the least well-mapped continent. Landsat data and those from the Japanese-US ASTER instrument aboard the Terra satellite offer huge potential for mapping rocks and soils, especially in dry lands, at scales of up to 1:50 000. Africans need to know about their physical resources, especially water, instead of well-heeled mining, petroleum and consulting companies from rich countries, who have more or less monopolised (and sometimes eked out) knowledge of the continent’s riches. Now they can begin to find out for themselves.

Satnavs useful to hydrogeologists as well as white-van drivers

Microwave radiation emitted by radar remote sensing systems does not merely produce useful images of the Earth when all else fails because of cloud cover. They interact with the surface in such a way that their characteristics change, specifically when the moisture content of surface materials such as soil varies. This phenomenon has spurred development of satellite-borne estimation of soil moisture. But since the launch of constellations of satellites aimed at precise navigation, such as the well-known US Global Positioning System (GPS) and Europe’s Galileo system, everywhere on the Earth is continually bathed in weak microwaves. Researchers at the University of Colorado, Boulder have done a test of the concept using a single GPS receiver recording continuously at one site in Tashkent, Uzbekistan (Larson, K.M. et al. 2008. Using GPS multipath to measure soil moisture fluctuations: initial results. GPS Solutions, v. 12, p. 173-177).

Multipath signals are received when an electromagnetic signal arrives at an antenna, not along a direct path from its source, but indirectly due to reflection of the signal by an object or surface near the antenna. Multipath contaminates all GPS measurements, leading to small positional errors, because the receiver locks onto a signal that mixes the direct and reflected signal. It is difficult to isolate the effects of multipath in GPS carrier phase signals. However, the signal-to-noise ratio (SNR) data computed by a GPS receiver are also affected by multipath and provide an easier route to quantifying multipath effects.  In fact the authors found that the amplitude of the SNR varies over time and correlates well with variations in local soil moisture following rainy and dry episodes. Although a first test of concept, the results are sufficiently encouraging that specialist GPS receivers may be developed that allow both precise positioning and accurate measurements of soil moisture – what may become a must for hydrogeologists, especially in arid and semi-arid terrains.

Desert varnish

Just as vultures are annoyed by glass eyes, so geologists who use remote sensing detest vegetation cover. But the spectral blanket thrown over geology by grass and other plants is not the only irritation and one occurs where least expected. Arid terrain usually pays the best dividends in remote geological mapping, because the spectral properties of rocks and their constituent minerals emerge in reflected and emitted radiation  and bear close relationships to those determined in laboratories. Images captured from orbit that use carefully chosen wavebands are often stunningly informative in deserts. The bugbear is desert varnish, an often shiny black coating that completely masks what lies beneath, be it basalt, granite, sandstone or carbonate, even in the field. Generally it is no more than a millimetre thick, and often far thinner. Close examination often shows a minutely botryoidal texture and parallel laminae in cross section, very like a tiny stromatolite. Basically, desert varnish is such a biofilm deposit, and the responsible organisms are cyanobacteria, as in stromatolites, but exceptionally sturdy ones. However, the bulk of the material is inorganic, and it is spectrally featureless, hence the problem in remote sensing.

Widespread as it is in arid environments, desert varnish has not been deemed an appropriate subject of study, so any information is welcome (Garvie, L.A.J. et al. 2008. Nanometer-scale complexity, growth and diagenesis in desert varnish. Geology, v. 36, p. 215-218). Hailing from Arizona University, the authors are well placed. Their approach is no so much directed at organic aspects, which is a shame, but at the geochemistry of this annoying gunk. As previously known, they show the dominance of manganese phases, but mixed in with very fine-grained quartz, clays and iron oxy-hydroxides. The varnish seems to contain a wind-blown component, but the manganese and probably the iron is derived in some other way, having grain sizes less than 100 nanometres. Iron and manganese minerals dominate the fine laminae, and at very high electron microscope resolutions their grains show yet finer structure at 1 nm scale. The authors ascribe the cyclical structures and mineralogy to repeated wetting and drying, with leaching and oxidation of Fe and Mn. Both iron and manganese are multi-valent, Mn more so than Fe. For both to be leached, i.e. drawn into solution as Fe2+ and Mn2+ ions, requires strongly reducing conditions, and then oxidation to precipitate Fe3+ and Mn4+ or Mn7+ minerals. At this minute scale, whatever the source of the Fe and Mn, a biological influence seems crucial.

Renewed interest in desert varnish seems to be connected with Mars – the study was partly financed by NASA. Yet, none of the Martian remote sensing studies report annoyance with huge tracts blacked out by manganese minerals. Such surface alteration that has been analysed by the Mars Rovers proved to be iron-enriched with little significant manganese enrichment. If desert varnish is biogenically mediated, then its occurrence on Mars would be cause for excitement bordering on hysteria. The cyanobacteria in terrestrial varnishes are tough, and may date back into Precambrian times as the first colonisers of dry land. As yet, there have been no attempts to examine their genetic affinities.