Remote sensing
Micro-gravity data chart shrinking ice caps
November 2009
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…
March 2009
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
July 2008
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
July 2008
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
May 2008
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.
Mapping iron minerals – on Mars
November 2007
For geological remote sensers, Mars is the ideal planet; it has virtually no atmosphere. That has four definite advantages: no ‘proper’ geologists are likely to go there for at least several generations, and those that do will not stay long; solar radiation of all wavelengths can illuminate the Martian surface; unlike the Moon, there seems to be a wide diversity of Martian rocks, though not so many as here; there is no vegetation to obscure bare rocks and soils. The Earth’s atmosphere, especially its content of water vapour, oxygen and ozone, carbon dioxide and a few other gases, absorbs many wavelength regions of the radiation spectrum thereby ruling-out several opportunities to explore the mineral diversity of terrestrial rocks from their spectra. There are, however, sufficient atmospheric ‘windows’ to do some useful mineral mapping, especially in the 1.5-2.5 mm region for minerals containing Al-OH, Mg-OH and C-O bonds (e.g. clays, phyllosilicates and carbonates) and in the visible and very-near infrared (0.4-1.5 mm) for various kinds of iron-bearing minerals.
The remote sensing ‘works’ has been put into Mars Orbit; for the reflected and thermally emitted regions. The latter, divided into several narrow wavebands, has enabled assessment of a number of rock forming silicates on Mars and is also available for Earth (given vegetation-poor surfaces) from the US-Japan ASTER instrument. ESA’s Mars Express carries the Observatoire pour la Minéralogie, l’Eau, les Glaces, at l’Activité (OMEGA), whose 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. It has been used to systematically map the occurrence of iron oxides and sulfates in Martian surface (Bibring, J-P. and 11 others 2007. Coupled ferric oxides and sulfates on the Martian surface. Science, v. 317, p. 1206-1210). This interests Mars-focused geologists because of the evidence for hematite nodules and iron sulfates found by the Mars Exploration Rover, Opportunity, a likely sign of hydrous alteration of iron silicates by acid waters.
On Earth these minerals, along with iron hydroxides, are the main colouring agents of sedimentary rocks, soils and the weathered surfaces of igneous and metamorphic rocks. Being able to map them here, using orbital remote sensing would be extremely useful in many geoscience fields. The odd thing is that suitable wavelength bands have never been carried over the Earth by operational remote sensing systems, except in the crudest form of the 80 m resolution Multispectral Scanner data aboard the first five Landsat platforms. In November 2000 NASA launched the experimental Earth Observing-1 satellite (EO-1) carrying the only orbiting hyperspectral imaging system (Hyperion) similar but inferior to OMEGA, and a simpler 9-band system that did deploy three wavebands capable of discriminating iron minerals. This Advanced Land Imager (ALI) was intended to provide information to guide the choice of bands to deploy on the follow-on for the workhorse Landsat Thematic Mapper. (The latest TM has broken down). EO-1 is still up and running, but scientists have to pay handsomely for data gathering, and few parts of the Earth have been covered, unlike with the ASTER system. After due consideration, NASA and the US Geological Survey decided not to deploy bands specifically sensitive to iron minerals in the next TM-like instrument, which will be the same as its predecessors, to all intents and purposes.
How serious the USGS is about systematic geological mapping of the Earth has yet to be divined… In the 21st century the geology of more than half the land surface has still not been mapped at scales better than 1:250 000, mostly in Africa and other economically disadvantaged regions. Just how privileged HG Wells’s Martians might be can be judged from the 21 September 2007 issue of Science, which devotes 14 pages to high-resolution mapping of Mars by the Mars Reconnaissance Orbiter.
An unfortunate mistake
September 2007
April 2007 saw the announcement of an initiative to drill a ‘Thousand Wells for Darfur’ by the University of Boston, USA and the government of Sudan. This was based on Boston remote senser Farouk el Baz having claimed that his team had ‘discovered’ signs of a large former lake in northwestern Sudan. The researchers also claimed that the lake may have drained its waters into underlying sandstone deposits to create a huge reservoir of groundwater. This attractive idea stemmed from interpretation of satellite image data, including that produced by radar illumination, which gives some subsurface information if surface materials are very dry (down to a few metres at most). The media in Africa and the rest of the world over-egged the somewhat breathless scientific briefing, to the extent that some readers began to believe that a lake had somehow migrated underground and may yet have fish in it, as well as presenting a possibly vast freshwater resource.
In fact the lake-bed sediments had been discovered by German geoscientists in 1985 and mapped in the 1990s. More to the point, the sandstone aquifer is the Nubian Sandstone that extends through Libya, Chad, northern Sudan and southern Egypt, but which thins out at its southern margin where the lake once existed. It transmits its groundwater northwards and feeds a number of oases. Since the lake evaporated as climate changed in the eastern Sahara, any water that entered the aquifer would probably have been highly saline. Finally, it is far from the real area of need where hundreds of thousands of people have been driven from their homes in western Darfur by proxies for the Khartoum regime.
See: Butler, D. 2007. Darfur lake is a ‘mirage’. Nature, v. 448, p. 394-395.
Ice age mass deficit over Canada
The gravitational potential field over the Earth’s surface changes according to variations in mass beneath it. Among the best-known causes is the depression of low-density continental lithosphere into the asthenosphere as a result of loading by ice sheets during glacial maxima. Once glacial ice cover has melted away the surface once covered by it rises to restore the isostatic balance. There is also post-glacial subsidence beyond the former ice-margins, where displaced asthenosphere had produced broad surface bulging when ice sheets were at their thickest. However, both are very sluggish processes that continue more than 7 ka after the bulk of ice had melted from the Northern Hemisphere: Northern Britain still rises whereas those parts south of the ice front at the last glacial maximum subside to form a drowned coastline.
Mapping the height of raised beaches around the Baltic gave a clue to where Pleistocene ice sheets were at their thickest. The largest ice sheet was that which spread out from the Canadian Shield, and there are few surface clues to where it was thickest and most elevated. One of the objectives of the Gravity Recovery and Climate Experiment (GRACE) has been to map the gravity anomalies remaining from the last glacial maximum. GRACE comprises two satellites launched in 2002, gravitational potential being measured from very accurate GPS and radar measurements of the distance between the two platforms.
Four years worth of GRACE data shows the free-air gravity anomaly—difference between measured and theoretical gravity—trends over Canada (Tamisiea, M.E. et al. 2007. GRACE gravity data constrain ancient ice geometries and continental dynamics over Laurentia. Science, v. 316, p. 881-883). Large mass deficits occur beneath the Canadian Shield west of Hudson’s Bay and to the east below NE Quebec. They are the likely sites of ice-sheet ‘summits’ where most snow accumulated to become ice, and from which ice flowed outwards. Interestingly, a multi-domed ice sheet was the first to be proposed in the early 20th century, but was replaced a single-dome model, so ideas have come full circle. The longer GRACE gathers data, the better the resolution of gravitational fields, so more detail will be added over time. Maybe it will become possible to model the course of post 18 ka melting, throwing some light on likely directions for loss of meltwater to the boreal oceans (see The Younger Dryas and the Flood in June 2006 EPN).
Is Mars a better place to do geology?
March 2007
The various imaging instruments that peer down on the Martian surface from NASA and ESA orbiters have provided a wealth of information about the planet and its evolution. The sophistication of the instruments far surpasses anything available at low or no cost for the Earth. The multi- and hyperspectral approach that helps identify and map a range of minerals using their distinct spectral features in the reflected and thermally emitted parts of the spectrum was pioneered by tests of concept aimed at terrestrial rocks. Several astonishing discoveries on Mars stem from that most traditional of photogeological tools, the stereoscopic potential of image-pairs taken from slightly different angles. Mars is cloud free with a very thin atmosphere and no sign of life: the ideal place to conduct geological remote sensing, and good luck to those who like looking at alien worlds. One thing that gives them a decided edge over Earthward looking image interpreters is the spatial resolution and broad coverage that has been deployed on Mars and made available at virtually no cost to investigators.
For terrestrial geologists, unless they have vast funds for aerial campaigns over big areas, resolution is limited to the order of 15-30 m for the reflected region and 90 m for multispectral thermal data. The joint US-Japanese ASTER mission produces data for both spectral regions in 14 wavebands, with the bonus of 15 m stereopairs of very-near infrared images (a one-off). The old workhorse, the Landsat Thematic Mapper conked out over a year ago, and its replacement is uncertain. That was to be little different from its predecessor and with limited geological potential. Hyperspectral reflected data (of the order of 200 bands covering the 0.4 to 2.4 µm range) from orbit is restricted to a few tiny experimental swaths (about 7.5 km wide) from NASA's experimental Hyperion system.
The High Resolution Imaging Science Experiment aboard the Mars Reconnaissance Orbiter has a resolution as fine as 26 cm, two orders of magnitude better than Landsat and equivalent to aerial images from a few thousand metres above the surface. Because of this superb performance, intricate details of Martian features emerge from the HiRISE images (see Okubo, C.H and McEwan, A.S. 2007. Fracture-controlled paleo-fluid flow in Candor Chasma, Mars. Science, v. 315, p. 983-985). Although 65 cm resolution of the Earth is available in images from the commercial Quickbird satellite, they come at enormous cost for areas as large as those being analysed on Mars. As any user of Google Earth knows, you can spot your own car in the driveway on the outdated Quickbird and Ikonos images obtained by Google for increasingly wide areas. But they are natural colour only – not very useful in geological remote sensing. The perspective Google Earth views, which could substitute for the ‘Swiss hammer' (binoculars), are limited to 90 m terrain resolution by the SRTM elevation data being used to generate them. Stereoscopic Quickbird and Ikonos images can provide elevation data with a one-metre resolution, rivalling those freely available for the Martian surface.
The technology is available to bring routine geological remote sensing of the Earth to the standard of what is on offer from Mars, with the difference that it would be of enormous use in addressing all manner of human problems and opportunities (and a great deal of unresolved geology). Such standards are available to mining and petroleum companies (and a favoured few in UN agencies). And they have long been available to the intelligence community, at resolutions down to about 15 cm (and perhaps better if the ‘twinkling' effect of atmospheric turbulence can be digitally removed). For many years the US government refused to deploy high-resolution public-domain imaging systems in orbit, because it would cause ‘diplomatic difficulties' with governments of some countries who might object to being imaged, or might ‘give away' secrets of the devices being used. Once the private sector saw a profit in taking pictures from space, both arguments disappeared overnight (there is a proviso of a US national-interest override). Despite all the technical advances, very few can afford to look in detail at issues that carry little potential profit, such as finding and managing groundwater resources or monitoring geohazards in the detail necessary for risk assessment.
Detecting, mapping and understanding ancient soils
November 2006
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 of the Open University UK points out are distinctly zoned 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.
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ASTER data and earthquakes
October 2006
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.
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Mineral mapping and the history of Mars' rocks and water
May 2006
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.
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State of the art seismic imaging
November 2005
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.
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Mineral maps of Mars
April 2005
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 com mercially 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.
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The triumph of the old on Mars
April 2004
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 20 th and early 21 st 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.
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Remote sensers now employable
January 2004
A research area could be said to have come of age when those who have participated find that they can get a job. Gone are the days when vast experience in field mapping, skills with mass spectrometers and even encyclopaedic knowledge of tiny fossil remains ensured more than a cursory reading of your CV by potential employers. In the 32 years since the first availability of Landsat data there has been a big shift in the employment prospects of young geoscientists. The dominant trend has been into the broad field of environmental geology. A review of demand for people with skills in Earth observation (Gewin, V. 2004. Mapping opportunities. Nature, v. 427, p. 376-377) shows that recent geopolitical and economic shifts have demonstrated their value in helping decision makers to decide. The prospects are patchy, however. The USA, beset by homeland security and with vast areas mapped at only a superficial level, has a thriving Earth observation jobs market, but Europe lags behind, because of better charting of its land. To a large extent dramatic improvements in spatial and spectral resolution of remotely sensed data in the last 5 years have matched technology to a big range of applications, hence the upturn. Many of the jobs are in governmental agencies, and are not directly related to geological skills. That is a shame, because Earth is less well mapped than the Moon and Mars. Yet, skills and ingenuity that you would learn in addressing purely geological challenges through remote sensing can easily be transferred to any other field.
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Imaging radar and WMD
August 2003
A short article in New Scientist (Morris, H. 2003. Satellites hunt for buried treasure. New Scientist 12 July 2003, p. 12-13) reminded me of the puzzling failure of British and US forces in Iraq to discover any buried caches of weapons of mass destruction, either before the invasion of Iraq or in the aftermath of Saddam Hussein's disappearance. Researchers at the Ben Gurion University of the Negev in Israel have tested the ground-penetrating capabilities of imaging radar that uses microwave pulses with various wavelengths. One of the principles of radar remote sensing is that microwaves can penetrate beneath the Earth's surface, provided the materials contain little liquid water. The longer the wavelength the greater the depth from which information can be sensed. Ground-penetrating radar is a common tool in archaeological investigations and in glaciology (ice is "dry"), but is usually deployed along ground traverses. The Israeli experiments, which duplicated work done by remote sensing researchers at NASA's Jet Propulsion Laboratory, used airborne imaging radar to detect buried metal target, which are highly reflective to microwaves. They used microwaves with moderately long wavelength, and showed that objects half a metre deep were easily detected.
Radar with a wavelength of around 70 cm is called P-band radar, and has the greatest potential for sub-surface mapping, with penetration up to 9 metres. In 1987, NASA's Jet Propulsion Laboratory first flew an airborne radar imaging system (AIRSAR) that uses P-band, partly to exploit its ability to "see through" dense vegetation but also to produce ground-penetrating images in dry regions. AIRSAR has the potential to produce images with a resolution of 3.3 metres, and data produced by it have been available freely to civilian users. It would be no surprise, therefore, if there were imaging radar systems with P-band radar being used for intelligence gathering. The US National Imagery and Mapping Agency (NIMA), in conjunction with JPL and EarthData International, Inc., developed in 2000 the 2-metre resolution Geographic Synthetic Aperture Radar (GeoSAR) mapping system, that also includes a P-band imager. GeoSAR is funded by the US Defense Advanced Research Projects Agency (DARPA). NIMA, formerly the US Defense Mapping Agency, is a Department of Defense Combat Support and National Intelligence Community agency that provides imagery, image intelligence and geospatial information in support of US national security objectives. The French and Italian space agencies are also discussing the development such systems, perhaps to be deployed from orbit by the European Space Agency.
It was NASA/JPL's Shuttle Imaging Radar missions in the 1980s and 90s that revealed dramatic evidence for former tributaries of the Nile River System that are buried beneath the sands of the arid eastern Sahara desert in Egypt and Libya. Although not so dry, the Tigris-Euphrates plain is a desert, and it would be very surprising if P-band radar imaging has not been used in the search for buried WMD. Since radar energy is barely affected by the atmosphere, and the microwaves used in radar imaging are effectively highly focussed laser beams, systems carried on satellites have the same spatial resolution as those carried on aircraft. Had a P-wave system been deployed on a military surveillance aircraft or satellite, then sizeable buried caches would have been difficult to miss. Even if the ground was damp, one of radar's other features is that it responds to variations in the texture of the ground surface. Reworked soil over excavations would be easily spotted by any radar imaging system, either orbiting or on an aircraft. So it was somewhat odd when the US Secretary of State, Colin Powell, did not use any imaging radar evidence in his submission to the UN Security Council on 5 February 2003.
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Landsat to be privatised, once more?
October 2002
Remote sensing, once the domain of researchers seeking hitherto undiscovered potato fields, lost cities and the intricacies of drainage patterns, entered the commercial domain in a big way about a decade ago. As well as giving lugubrious views of factories reputed to be manufacturing weapons of mass destruction, the aftermath of their bombing and that of villages alleged to harbour agents of the "axis of evil", remote sensing helps find physical resources, spots farmers who fraudulently claim subsidies for non-existent crops and is used to site cell-phone transmitter networks. There are now several orbiting systems launched by commercial outfits that offer pin sharp and spectrally revealing information, at a cost. The workhorse of remote sensing since 1972 has been the US Landsat series. Following the addition of the Thematic Mapper in 1984, pressure grew for Landsat’s privatization in 1988. Prices jumped tenfold, to the horror of researchers, and the venture became uneconomic because of insufficient private-sector interest. Landsat 7, which carries an Enhanced Thematic Mapper, made orbit in 1999, and is administered by the US Geological Survey. Landsat-7 ETM data sell at $600 per scene, which is a bargain. Such has been the demand for data that US authorities are once more trying to shed responsibility for data provision to private hands, by asking for bids to develop, launch and market the next Landsat. Prices will once again leap to profitable levels. The joint US-Japan ASTER system aboard the ostensibly research-oriented Terra satellite rivals Landsat ETM in quality, and many scientists have been trying out the data. Again, to their disquiet, pressure reputedly from the Japanese partners has resulted in once free data being assigned a price of $55 per scene.
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Experimental satellite to have extended mission
November 2001
The Earth Observing-1 (EO-1) satellite, launched by NASA in late 2000, carries two remote-sensing instruments that may become operational devices in the future, given a proven track record on EO-1 and, of course, sufficient funding. One, the Advanced Land Imager (ALI) is a test bed for sensors earmarked for the follow-on to the current Landsat-7 Enhanced Thematic Mapper+ (ETM+). As well as the existing ETM+ six bands, ALI covers three others close to existing bands. Whether by design or good fortune, two of these help define the important VNIR broad absorption by ferric iron minerals, neglected in remote sensing since the early days of the Landsat Multispectral Scanner. Like the ETM+, ALI also carries a panchromatic band that spans the visible range, and which is aimed at providing a means of sharpening detail in images. On ALI, however, this band has an improved resolution of 10 metres as opposed to the current 15.
More innovatory is the Hyperion instrument, a hyperspectral device that spans the visible to short-wave infrared range with 242 bands that are 10 nanometre wide. Hyperion is comparable with airborne hyperspectral devices, such as AVIRIS. In the experiment it captures data swathes that 7.7 km wide, made up from 256 pixels with a resolution of 30 m. After initial difficulties with allowing for atmospheric effects on the data, newly calibrated Hyperion data closely mimic mineral spectra.
Early work on EO-1 data in many fields, including geology, has proved sufficiently promising that NASA has given the mission a year-long extension. Although data are restricted to only a few target areas suggested by the investigators, the extension is good news. It is a reassurance about continuity of the Landsat programme, and a tantalising indication that the ill-fated hyperspectral Lewis satellite may be resurrected.
Information from: http://eo1.gsfc.nasa.gov/
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Mapping with geophysical data
May 2001
In the same way that topographic contours can be transformed to models of continuous elevation change using surface fitting, measurements of gravitational and magnetic field potentials, at points on the ground or along aerial survey lines, are sources of imagery. Expressed as contours joining points with the same value, spatial distributed data are notoriously difficult to interpret, however much information they contain. Not only do contours simplify the data by dividing them into arbitrary steps, how we interpret contour maps depends on how we perceive them. Our eyes evolved to extract information distributed as a continuum across our field of view, and our visual cortex developed many tricks to innately interpret clues to shape, perspective and distance, to extend the limits of stereoscopic vision (we see objects in true 3-D only if they are closer than about 400 metres). Our innate abilities "interpret" contours in terms of the spacing between them; the closer they are together the darker we perceive the area of steep gradient. In other words we have to convert an image that is the "negative" of the first derivative to an understanding of the actual shape represented by contours! Unsurprisingly, we have to learn to "read" maps, and that is a great deal more difficult for those showing potential-field intensity than for topographic elevation. Cartographers long ago latched onto our use of shadows as clues to shape, and designed maps with shading as if the Sun was shining from the top of the sheet. They also use different colours as a second clue to what is high and low. Combining the two aids helps transform images of geographic variables—basically bland shifts from high to low—into visually stunning, and therefore more easily interpreted pictures. Surface modelling of elevation and geophysical data, with such graphic tricks, literally throws hidden, and often unsuspected features into sharp relief.
These techniques have revitalized desktop interpretation of the world, especially using results of geophysical surveys. However, in the same way that detail of a terrain blurs and loses information as resolving power falls, low-resolution data of other kinds obscure buried features, or give ambiguous hints to what they are and where they go. Reducing the spacing of aerial surveys, and the height from which they are acquired, increases the resolving power of the technique. Stunning examples of the state of this particular art appear in recent work by the US Geological Survey (Grauch, V.J.S. 2001. High-resolution aeromagnetic data, a new tool for mapping intrabasinal faults: example from the Albuquerque basin, New Mexico. Geology, v. 29, p. 367-370. See also: http://rmmcweb.cr.usgs.gov/public/mrgb/airborne.html).
Grauch worked on an area in which superficial materials and rapid rounding of topography result in poor surface expression of all but the largest faults. By using aeromagnetic images modelled from survey lines spaced at 100 to 150 metres, he picked out not only hidden faults, but also the magnetic signatures of pipelines, water tanks and buildings.
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Hands-on planetology
January 2001
Fed up with playing Solitaire or Hearts between those moments of productive inspiration? NASA Ames Research Center has set up a cottage industry (unpaid) to help Mars specialists there build a catalogue of impact craters on the Martian surface. As those flyers tucked under your windscreen wipers say, "No experience needed".
Probably the most important scientific breakthrough from studies of the Moon since the 1960s has been the discovery that its pocked surface resulted from impacts by chunks of interplanetary debris. The rate of impact and the size of the colliding bodies, and therefore the energy that they delivered, has varied since the Moon formed. The lunar cratering record, backed up by accurate dates of its products, is a detailed chronology of how impacts influenced Earth's evolution—vital, since signs of impacts rapidly become masked by our planet's vitality.
Mars, on which NASA scientists and many more besides focus their undivided attention, is also cratered as a result of the same kind of process. Counting craters, measuring their diameters (a proxy for the energy involved in their formation) and looking for their age relative to one another and other features of the Martian scene is an excellent means of assessing aspects of the Red Planet's evolution. But Mars is a great deal bigger than the Moon, and the sheer tedium of doing the work has become a burden. Those geologists who compiled the lunar record have moved on, and few relish the task as a profession, hence Ames' appeal for public participation.
The idea is that the basic information on crater occurrence, size and relative age—that's based on relations between overlapping craters and degradation by Mars' "weather"—can easily be gathered by interested, but untrained people. The statistical work can then be done much more quickly. If you fancy being a NASA "Clickworker", then connect to clickworkers.arc.nasa.gov/top
Since inception on November 17, 2000, all clickworkers combined have contributed 340,070 crater-marking and 93,891 crater-classification entries. It seems better by far than simply running the SETI distributed software to analyse radio frequencies for possible signs of intelligence out there. You get to look at some magnificent high resolution images too.
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Petals unfolding on ASTER
December 2000
The 15-channel imaging system aboard the first of NASA’s Earth Observing System constellation of satellites (Terra) began to demonstrate its potential in November. The 9 month delay between Terra’s launch in December 1999 and the appearance of its first scientific data irked many potential users, already chewing carpets because of the 18 month delay in the launch. However, wrangling between ASTER’s designers at ERSDAC, the Japanese space agency, and NASA was resolved by November 17th. If you are interested, the data can be accessed at the new EOS Data Gateway.
Some 70 000 scenes are already "in the can", but slow processing means that only a trickle of calibrated (Level 1b) data adds to the archive daily, so that cover is very patchy at present. Nonetheless, quality of cloud–free scenes is excellent, and the new potential, especially for geological challenges, is dramatic. It is worth noting that the data are in a somewhat difficult format, and can be had either on tape (tar format) or by ftp downloads (file format 125 Mb per scene). The EOS Data Gateway does plan for release eventually on CDs.
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Geology from Orbit
Using satellite images for geological mapping and exploration, or for monitoring short-lived phenomena, such as volcanic eruptions, is now standard Earth sciences technology. But it involves substantial costs for data and for the software needed for analysis, or so it was. Access to the most recent images from the US Landsat-7 and French SPOT systems is now on-line using sophisticated browsing sites on the Web. Both enable guest users, as well as those who have signed up for slightly more sophisticated services, to browse and download reduced-resolution JPEG versions of archived images, and to order data, if needs be. For Landsat-7, go to landsat7.usgs.gov though this means going through several pages.
You may also jump straight to the Earth Observation System (EOS) Data Gateway. This currently opens a data search and order form. Choose a search keyword first using the Data Set button, selecting Landsat-7 Level 1 data. You can choose several options for the geographic search area, and simply enter a date range (e.g. 2000-01-01 and 2000-05-25 for this year's archives. Then Start Search. Sometimes your search will take quite a while, dues to pressure on the server's bandwidth. The good news is, you can disconnect and go back later to the relevant page using Internet Explorer or Netscape History listing. For SPOT, access is via the DALI server at www.spotimage.fr/home/proser/whatdali/daligst/daligst.htm or the Sirius server at sirius.spotimage.fr/anglais/Welcome.htm—the Sirius service is a little more complicated than DALI, but is set to become SPOT-Image's standard browser.
Image quality in both cases is excellent, with the Landsat-7 browse images having a roughly 250 m resolution, and SPOT data showing at about 120 m (4 to 8 times better than similarly available data from meteorological satellites). Use the right mouse button with cursor over the image and select Save Image As: assigning your own name instead of the default given by the server, e.g. geology1.jpg. You can then make some cosmetic changes to contrast and colour balance using MS PhotoEditor or Adobe PhotoShop.
Remember that SPOT data of whatever kind are covered by SPOT-Image copyright, but the USGS who distribute Landsat-7 data make no such claim. Clearing copyright for publication or acknowledging sources is an important responsibility for uses in research or publications.
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SRTM and ASTER
There are several other web sites to watch. In February 2000 NASA, the US National Image and Mapping Agency (NIMA), and the Italian and German Space Agencies deployed the Shuttle Radar Topographic Mission (SRTM) aboard a Space Shuttle flight. The SRTM uses radar reflection received by two antennae separated by a long arm deployed from the Shuttle to estimate topographic elevation of the Earths surface, with a method known as radar interferometry. The resulting data are in the form of a digital elevation model (DEM) with elevation values for cells 30 metres square. A DEM is therefore a 3-D model of topography, and shows landforms in stunning detail, together with geological features that control them. Because radar relies on energy transmitted from the spacecraft and radar waves can penetrate cloud, the SRTM produces data whatever the time or the weather. The mission successfully captured the entire continental surface between 60°N and 60°S, and will revolutionize both geomorphology and geology. From November 2001 the US Geological Survey and the German Space Agency (DLR) will release DEMs publicly and at low cost, but 'tasters' are available from the following web sites:
NASA—www.jpl.nasa.gov/srtm
DLR—www.dlr.de/srtm
For the next decade or so, the main Earth-oriented thrust by NASA is the Earth Observing System (EOS), which will be a constellation of satellites that orbit from pole to pole to give coverage of the entire surface. On 18 December 2000 NASA launched the first of these, named Terra. This satellite carries several payloads that produce images of various kinds, the most geologically important of which is the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), designed and built by the Japanese space agency (ERSDAC), but operated jointly with NASA. ASTER captures images for several ranges of wavelength in the visible, reflected and emitted infrared, which are designed to highlight the spectral properties of common minerals. So, ASTER is a geological remote-sensing system par excellence. The system is working properly, but will begin to produce scientific data sometime in late 2000. Like the SRTM, the plan for ASTER is eventually to produce full coverage of the continental surface during its lifetime. For the moment, you can keep a watching brief by visiting NASA—terra.nasa.gov and ERSDAC—astweb.ersdac.or.jp
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