Category: GIS and Remote Sensing

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.

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

The triumph of the old on Mars

Except perhaps for some of the current generation of geologists, who are immersed in their remote sensing training by false colour images of spectrally revealing multispectral image data, a great many professionals who engage in mapping cut their teeth on what is known simply as photogeology.  And it is simple.  Provided images are taken of an area from different angles, with the simplest of instruments most people’s innate stereoscopic vision enables them to see startling illusions in three dimensions.  Stereoscopy has been to geologists of the mid to late 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.

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.

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.

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

Outside of tides, two fundamental processes shift mass in our planet, the convective motion of the mantle and lithosphere, and that of the oceans.  A second-order means of mass transfer is that of water via the atmosphere, from sources of evaporated vapour to sites of precipitation and temporary storage (as soil moisture and in snow and ice).  Any movement of mass should, theoretically, result in changes in the Earth’s gravitational field.  Exploiting that simple notion presents two practical challenges, sufficiently precise measurements of gravity and its continuous monitoring.  Gravimeters used for surveys at the surface are now sensitive enough to give a reading for the mass of a person, provided he or she moves close enough to the instrument (gravity obeys an inverse-square law), but ground-based monitoring is so slow and expensive that continuous monitoring is impossible, except at permanent stations that check micro-gravitational changes near active volcanoes and fault zones.  Variations in the height at which satellites orbit the Earth stem from changes in gravity.  Although the inverse-square law of gravitational attraction smoothes out gravity anomalies at orbital altitudes, such measurements have been used for three decades to assess the shape of the Earth’s surface, were it completely covered with water (the geoid).  However, they are not accurate enough to do much more than that.

A project jointly funded by NASA and the German space agency DLR aims to improve the precision of satellite gravity measurements by more than 100 times that of the best to date (Adams, D.  2002.  Amazing grace.  Nature, v. 416, p. 10-11).  The Gravity Recovery and Climate Experiment (GRACE), launched in March 2002, uses two satellites that follow the same orbit with a spacing of 220 km.  Range finders on each measure their separation distance, and so their ups and downs as gravity varies, with far greater accuracy than any other method.  Every month they will have gathered enough data to assess the global variation of gravity at their orbital height.  That will produce movies of annual and longer term fluctuations, with sufficient detail even to track variations in the Gulf Stream and rises and falls in soil moisture and snow cover, as well as details that relate to deep ocean currents and mantle convection.  Unfortunately, gravity and the drag of Earth’s atmosphere limits GRACE’s lifespan to a mere 5 years.

See: http://www.csr.utexas.edu/grace and http://op.gfz-potsdam.de/grace/index_GRACE.html