Category: GIS and Remote Sensing

A friend recently moved to the Napa Valley in California, almost certainly motivated by the vast area given over to the grape and the quality of Napa wines. Shortly after the flit, in the middle of some minor refurbishment, he had quite a shock; a Magnitude 6.0 earthquake at 3:20 a.m. local time on 24 August, the worst in northern California for 25 years. My friend lives only 15 km from the epicentre in South Napa, but his house was undamaged. His confidence in the move remains unshaken, however, as there was no effect on this year’s grape harvest.

The event was monitored by the European Space Agency’s Sentinel-1A high-resolution radar satellite that entered orbit in April 2014. Sentinel revisits any area on the ground every 12 days, has all-weather/day-night imaging capacity, a 250 km-wide image swath with 10 m spatial resolution and is designed to analyse ground movements using interferometry between radar data before and after events. Interferometric radar imaging or InSAR relies on changes in the phase of radar waves between two dates of ‘illumination’ of the ground – radar images normally use only the amplitude of a radar wave, ignoring its phase – and potentially can measure shifts in ground elevation of the order of centimetres.

The image records ground movement in small steps of elevation that are assigned colours, the sequence blue-green-yellow-red-magenta spans a ground shift of about 3 cm. If several of these ‘fringes’ are closely spaced over relatively small areas this is due to significant motions locally. Broad areas with little change in colour have barely moved in the period between the dates of the two images.

The epicentre of the South Napa earthquake clearly shows up at the NE corner of the image, like half a ‘bull’s eye’. A closer look at the enlarged image (click on the image) shows two such features sharply bounded to the west by a line: that coincides with the West Napa Fault.

My friend lives to the west of the faults where the broad areas of colour signify much smaller motions than in the main affected area. He woke and left the building thinking this was a foreshock of a much more destructive event, and had an anxious few days. The United States Geological Survey  estimated that during the main ‘quake 15,000 people experienced severe shaking, 106,000 people felt very strong shaking, 176,000 felt strong shaking, and 738,000 felt moderate shaking. But there was only one fatality and 13 hospitalised casualties.

Related articles

• Sentinel-1 poised to monitor motion

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.

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.

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.

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.

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.

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.

A recent paper provides a clear guide and a new means of addressing one of geoscience’s great puzzles (Andrew Deller, M.E. 2006. Facies discrimination in laterites using Landsat Thematic Mapper, ASTER and ALI data — examples from Eritrea and Arabia. International Journal of Remote Sensing, v. 27, p. 2389–2409). During the early Cenozoic, and perhaps before that, huge areas of the exposed continental surface were subject to a hot, humid climate. Intense chemical weathering broke down every conceivable rock type to a few stable minerals. The resulting residual soils were preserved over vast areas of Africa, South America, India and Australia to form laterites, which M.E. Andrews Deller at the Open University UK points out are distinctly zoned [avoids repeat of layered] mineralogically and stunningly layered in colour. No one can fail to see laterites where they are exposed, if they know what to look for, but few geologists have set out to understand them properly. Andrews Deller documents in detail where these unique rocks occur, highlighting the importance of laterites as a resource; the frightening hazards that they pose to people throughout laterite-mantled Africa, and their relevance to the history of erosion and intraplate deformation.

The central theme of Andrews Deller’s paper is the essential first step of mapping laterites and discriminating their facies. This rests on their mineralogical simplicity, and the unique and distinct spectral properties of those constituent minerals. The author matches these to the spectral coverage of freely available remote sensing data — Landsat TM, ASTER and ALI — each of which offers nuances to be exploited in uniquely discriminating the different laterite horizons. Rather than setting out to ‘unveil’ sophisticated new methods of computer analysis (to which few people in laterite-encrusted areas would have access), Andrews Deller explores the simplest, most revealing approaches to a previously overlooked challenge: laterite facies have never been discriminated and mapped before using remote sensing. The results in this well-illustrated paper are stunning, and any geologist (and probably many lay people) can understand what the figures show and the importance of mapping laterites, thanks to careful discussion. The result is a paper that combines interest, novelty and usefulness.

During the early Cenozoic, and perhaps before that, huge areas of the exposed continental surface were subject to hot humid climatic conditions. That broke down every conceivable rock type to a few simple minerals that were both stable and insoluble. Such intense weathering possibly affected 30% of the land area during those ‘hothouse’ times. Where the surface was flat, the resulting residual soils were preserved to form laterites, strongly layered mineralogically. Since one of the common components is bright-red hematite, and its brown hydrous equivalent goethite, and another is brilliant white kaolinite, laterites are also stunningly layered in colour from white iron-poor clays at their base through an middle mottled yellow, orange, pink and white zone, to brick-red iron-rich ferricrete at the top of the sequence. No-one can fail to see laterites where they are exposed, but few geologists have set out to understand them. A recent paper provides a clear guide to begin that work on a grand scale, and also to chart where their unique properties and socio-economic pros and cons can be developed or avoided respectively (Andrew Deller, M.E. 2006. Facies discrimination in laterites using Landsat Thematic Mapper, ASTER and ALI data—examples from Eritrea and Arabia. International Journal of Remote Sensing, v. 27, p. 2389–2409).

The key to the long and complex chemical and mineralogical evolution of laterites lies in the different layers or facies in these palaeosols. Because they are thin and once present over vast areas of Africa, South America, India and Australia, their presence or absence today is a guide to the history of erosion and intraplate deformation after they formed. Each facies has very different chemical and physical properties, some advantageous, and some decidedly a threat of some kind, recognised and well documented by M.E. Andrews Deller of the British Open University. For instance, the clay zone is a lubricant that can encourage landslides of great thicknesses of overlying rock, yet is a potential resource — it is China Clay. Hard and porous ferricrete, containing both iron minerals and clays, makes it a cheap source of bricks and even road aggregate. But hematite can pose a frightening risk. Its open structure soaks up dissolved ions, including infamously those of arsenic, which lateritisation can set in motion from the rocks on which it develops. Hematite dissolves under reducing conditions, and should those develop on old laterites arsenic might be liberated to groundwater. Another associated compound that laterites can release is magnesium sulfate (Epsom Salts), an natural emetic but also a potential remedy for eclampsia that threatens mothers and their babies throughout laterite-mantled Africa.

Andrews Deller’s paper is a mine of laterite-related information, yet its central theme is the essential first step of mapping them and discriminating their facies. Her starting point is their mineralogical simplicity, and the unique and distinct spectral properties of those constituent minerals. She matches these to the spectral coverage of freely available remote sensing data — Landsat TM, ASTER and ALI — each of which offers nuances to be exploited in uniquely discriminating the zones. Rather than setting out to ‘unveil’ sophisticated new methods of computer analysis (to which few in laterite-encrusted areas would have access), she chose the simplest useful approaches to a previously overlooked challenge: laterite facies have never been discriminated and mapped before. The results in this well-illustrated paper are stunning, and any geologist, and quite likely many lay people can understand what they show, thanks to careful discussion. The result is a paper that combines interest, novelty and usefulness. The last is the best aspect: geologists can learn from the paper how confidently to make highly informative maps cheaply and quickly.

ASTER data and earthquakes

NASA’s Jet Propulsion Laboratory in Pasadena, California is a huge engine of across-the-board innovation. In my field, remotely sensed geology, everyone pounces eagerly on publications by its scientists because they are bound to push techniques and applications forwards, often in surprising contexts, such as archaeology from space. One such nugget is about to be published (probably this month) in the premier geoscience journal EPSL (Avouac, P. et al. 2006. The 2005, Mw 7.6 Kashmir earthquake: Sub-pixel correlation of ASTER images and seismic waveforms analysis. Earth and Planetary Science Letters, in press doi:10.106/j.epsl.2006.06.025) and amply justifies my impatient preview here. It offers great potential for monitoring the effects of natural hazards that involve mass motion using free (for bona fide researchers and, hopefully, humanitarian organizations) satellite image data.

Jean-Phillipe Avouac and colleagues at JPL applied a well-tried approach in remote sensing — comparison of images captured on different dates—in trying to assess the extent and magnitude of ground motion involved in the 8 October 2005 Kashmir earthquake that claimed at least 80 thousand lives. But theirs is a before-and-after study with a revolutionary new slant. ASTER data from the joint US-Japanese Terra satellite resolves the ground with a resolution as sharp as 15 m, in several wavebands of EM radiation. In their own right, these bands contain huge amounts of information about vegetation, rocks and soils, and many other environmental attributes. Particularly with vegetation, comparing data from different years or seasons soon shows up changes and clues as to why they occurred. But ASTER has another potential view to offer. Two of its sensors, one pointing vertically downwards, the other obliquely back along its ground track, constitute a stereopair. They can be viewed together to give dramatic 3-D visualizations of terrain. With the appropriate software, the parallax difference between the location of each point on the ground in the two images produces a map of terrain elevation. The novelty and potential in Avouac et al. is to combine ASTER data from two instants in time to find places that have shifted in position in the meantime. So that they match geographically, they used stereo-derived terrain elevation to remove geometric distortions caused by viewing rugged relief with effectively a wide-angle camera. The key to extracting deformation parameters is applying shape-detection software to images from before and after an event, and then finding the magnitude and direction of the differences between landform shapes to chart movement. The 15 m resolution poses a limit, but the sophistication of the algorithms enables shifts of the order of less than a metre to be detected at a coarse resolution of 150 m. But that is quite sufficient to show what happened in Kashmir along the entire length of fault movement in 2005. Applied to commercially available stereo data (up to 0.65 m resolution) the results would be awesome.

The principal mineral and rock mapping tool for Mars is the Observatoire pour la Minéralogie, l’Eau, les Glaces, at l’Activité. 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. For over 18 months it has been steadily building up mineralogical maps of the Martian surface in a series of narrow swathes would round the planet in the manner of wool in a ball (see Mineral maps of Mars in April 2005 issue of EPN for early results). The 90% complete data, combined with dating of surface regions from crater counts and other means of stratigraphic analysis, is beginning to chart the history of the Martian surface in familiar terms of geology and the effects of water (Bibrin, J-P, and a great many others in the OMEGA team 2006. Global mineralogical and aqueous Mars history derived from OMEGA/Mars Express data. Science, v. 312, p. 400-404).

An interesting correlation is emerging. Where Mars’s surface is dominated by large amounts of pyroxene – the stratigraphically older regions of heavily cratered volcanic rocks – there is evidence of hydrated clay minerals (products of non-acid water alteration) and sulfates (formed by acid, hydrous alteration). The younger, brighter regions, which probably formed by surface processes after about 3.5 Ga, are dominated by anhydrous iron(III) oxides that give Mars its overall red colour. Although on Earth this hematite commonly forms by dehydration of iron(III) hydroxide or goethite, there is no sign of relic goethite on Mars. The authors attribute the red-staining hematite to direct oxidation of iron-rich silicates, without the role of water. It seems that in terms of surface processes, water played a role in the very earliest weathering to form clays. For a while conditions became acidic by the oxidative breakdown of igneous sulfides, thereby encouraging the formation of sulfate encrustations and sediments. This ‘wet’ phase may well have involved water vapour emanating from early, huge volcanoes. Once global volcanism became extinguished the supply of water was shut off, and since 3.5 Ga the planet has been hyper-arid. Hydrated minerals above the 5% level are not common on Mars, and if they did in fact encourage some life forms to emerge, the search for them can be finely focused by the OMEGA results.

For many decades the primary tool of petroleum exploration has been reflection seismic surveying. As oil has become harder to find, industry has hugely improved means of processing seismic waves that return to detectors and expanded data gathering as a means of showing subtle structures and sedimentological detail.  From individual seismic sections up to the 1970s, seismic surveys have moved towards multiple lines with ever-decreasing spacing as a means of producing 3-dimensional subsurface maps. Until recently the results of 3-D seismics have been glimpsed only rarely by the academic community, but once their commercial usefulness has been exploited they are increasingly becoming accessible.  Richard Davies of the 3DLab at the University of Cardiff, UK and Henry Posamentier of Anadarko Canada provide an exquisite overview of the possibilities for research in the October 2005 issue of GSA Today (Davies, R.J. & Posamentier, H.W. 2005.  Geologic processes in sedimentary basins inferred from three-dimensional seismic imaging. GSA Today, v. 15(10), p. 4-9).  They show examples of derivatives from 3-D seismics, produced by a variety of image-processing techniques as well as the basic seismic processing, which demonstrate the depth to which these data can be interrogated.  Featured are an example of meandering Pleistocene channels beneath the Gulf of Mexico, structures produced by sediment compaction between the Shetland and Faeroe islands in the North Atlantic, and the shapes taken by basaltic sills as they flowed into place.  The graphics are wonderful, and would certainly tempt an IT-literate researcher.  However, no funding agency could afford to commission such revealing surveys, and the geoscience community will always rely on the activities and generosity of the petroleum industry to enter this awesome world.  Some might think of midnight meetings at lonely crossroads or an armful of long-handled spoons.  Yet the potential results far transcend the kind of information one might extract from exposed geology.

The US and ESA satellites orbiting Mars have so far deployed remote sensing instruments that detect visible to thermal infrared radiation from the planet’s surface.  Ultimately the energy involved is from the Sun: these are passive instruments.  Engrossing as they are, images from these sensors reveal only details of surface mineralogy and the Martian topography.  So far, virtually nothing is known about what lies buried beneath it, apart from inferences about ground ice.  The ESA Mars Express has one last imaging trick up its sleeve, which uses energy generated on board and beamed obliquely down to the surface.  This is the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS).  Radar remote sensing on Earth generally uses high-frequency microwaves in the wavelength range from 0.01 to 0.1 metres, and the images produced show how much energy is scattered by surfaces of varying roughness, to be received by antennae deployed from an aircraft or satellite.  The longer the wavelength the greater the height of small-scale surface irregularities that cause scattering and therefore a received signal.  Smooth perfectly surfaces reflect all the energy away from the antennae, like a mirror, so no energy returns to be sensed.  How microwaves interact with the Earth’s surface depends on the electrical properties of the materials.  Good electrical conductors, such as metals and liquid water are extremely efficient reflectors, whereas minerals are poor conductors and tend to absorb microwaves to some extent.  If soils are extremely dry, with less than 1% moisture content, as in some deserts, some of the absorbed energy is scattered by materials below the surface and images show subsurface features.  This lies behind the principle of ground penetrating radar, but since many soils are damp, only radar waves generated at the surface give good signals in most areas, to be exploited by civil engineers and archaeologists.  Ice is very different from liquid water, being so poorly conductive that it is almost transparent to microwaves.  Consequently it has proved possible to sound the depth of glaciers and ice sheets using ground penetrating radar deployed from aircraft.  The depth of penetration, and of course that involves energy returning to the surface in order to get a signal, is governed by the radar wavelength.  For instance, unknown former courses of the River Nile’s tributaries have been detected by 0.25 m radar waves beneath the hyperarid eastern Sahara through about 3 metres of dry sand.

MARSIS can transmit microwaves with 4 wavelengths 170 , 100 , 80  and 60 m.  Given rocks and soils free of liquid water, which comprise most of Mars’s surface, or ice, it can penetrate as deep as almost 5 km.  The multi-wavelength arrangement can also potentially discriminate water ice from rock and soil.  A great deal of speculation and some evidence suggest that parts of Mars may be underlain by permafrost, that is melted only under unusual conditions, such as after meteorite impacts.  There are also suggestions that glaciogenic-like landforms may still be underlain by ice, and bizarrely that there are frozen seas (see The triumph of the old on Mars in April 2005 EPN).  MARSIS may well throw Mars investigations into a turmoil, but maybe not.  The delay in sparking it up has been caused by fears that deploying its antennae might damage the whole spacecraft, and the first attempt seems to have got stuck.  It’s other drawback is limited power so that horizontal resolution will be between 5 to 10 km and vertically only 100 m, so results may be so blurred as to be inconclusive.  NASA plans a similar device aboard its Mars Reconnaissance Orbiter (launch date August 2005).  The Shallow Subsurface Radar (SHARAD) will use microwaves with 12 to 20 m wavelengths that give penetration to 1 km, but horizontal and vertical resolutions of 300 and 15 metres.

See: Reichhardt, T. 2005.  Going underground.  Nature, v. 435, p. 266-267.