Monitoring ground motions with satellite radar

By using artificially generated microwaves to illuminate the Earth’s surface it is possible to create images. The technology and the theory behind this radar imaging are formidable. After about 30 years of development using aircraft-mounted transmission and reception antennas, the first high resolution images from space were produced in the late 1970s. Successive experiments improved and expanded the techniques, and for the last decade radar surveillance has been routine from a number of orbiting platforms. Radar has two advantages over optical remote sensing: being an active system it can be done equally effectively day or night; it also penetrates cloud cover, which is almost completely transparent to microwaves with wavelengths between a centimetre and a metre. The images are very different from those produced by visible or infrared radiation, the energy returns from the surface being controlled by topography and the roughness of the surface. One of many complicating factors is that images can only be produced by oblique illumination.  That, together with deployment of widely separated transmission and reception antennas, opens up the possibility of extracting very-high precision (millimetre) measurements of topographic elevation.

In 1992 radar data from two overpasses of the European ERS-1 satellite over California were processed to capture interference due to changes in the ground elevation during the time between the two orbits: the first interferometric radar or InSAR. It revealed the regional ground motions that resulted from the magnitude 7.3 Landers earthquake at 4:57 am local time on June 28, 1992. For the last decade InSAR has become a routine tool to monitor globally both lateral and vertical ground movements, whether rapid, as in earthquakes, or slow in the case of continental plate motions, subsidence or the inflation of volcanoes prior to eruptions. Juliet Biggs and Tim Wright, respectively of the Universities of Bristol and Leeds, UK, have summarised InSAR’s potential (Biggs, J. & Wright, T.J. 2020. How satellite InSAR has grown from opportunistic science to routine monitoring over the last decade. Nature Communications, v. 11, p. 1-4; DOI: 10.1038/s41467-020-17587-6).

Ground motions associated with the 2016 Kaiköuea earthquake on the South Island of New Zealand. Each colour fringe represents 11.4 cm of displacement in the radar line-of-sight (LOS) direction. Known faults are shown as thick black lines (Credit: Hamling et al. 2017. Complex multifault rupture during the 2016 Mw 7.8 Kaikōura earthquake, New Zealand. Science, v. 356, article eaam7194; DOI: 10.1126/science.aam7194)

Since the ERS-1 satellite discovered the ground motions associated with the Landers earthquake, InSAR has covered more than 130 large seismic events. Although the data post-dated the damage, they have demonstrated the particular mechanics of each earthquake, allowing theoretical models to be tested and refined. In the image above it is clear that the motions were not associated with a single fault in New Zealand: the Kaikoura earthquake involved a whole network of them, at least at the surface. Probably, displacement jumped from one to another; a complexity that must be taken into account for future events on such notorious fault systems as those in densely populated parts of California and Turkey.

East to west speed of the Anatolian micro-plate south of the North Anatolian Fault derived from the first five years of the EU’s Sentinel-1 InSAR constellation. Major known faults shown by black lines (Credit: Emre, O. et al. 2018. Active fault database of Turkey. Bulletin of Earthquake Engineering, v. 16, p. 3229-3275; DOI: 10.1007/s10518-016-0041-2)

Since its inception, GPS has proved capable of monitoring tectonic motions over a number of years, but only for widely spaced, individual ground instruments. Using InSAR alongside years’ worth of GPS measurements helps to extend detected motions to much finer resolution, as the image above shows for Asiatic Turkey. An important parameter needed for prediction of earthquakes is the way in which crustal strain builds up in regions with dangerously active fault systems.

InSAR image of the Sierra Negra volcano on Isabela Island in the Galapagos Archipelago, at the time of a magma body intruding its flanks. Each colour fringe represents 2.8 cm of subsidence in the LOS direction (Credit: Anantrasirichai, N. et al. 2019. A deep learning approach to detecting volcano deformation from satellite imagery using synthetic datasets. Remote Sensing of Environment, v. 230, article 111179; DOI: 10.1016/j.rse.2019.04.032)

Volcanism obviously involves the movement of large masses of magma beneath the surface before eruptions. GPS and micro-gravity measurements show that charging of a magma chamber causes volcanoes to inflate so InSAR provides a welcome means of detecting the associated uplift, even if it only a few centimetres, as show by the example above from the Galapagos Islands. A volcano’s flanks may bulge, which could presage a lateral eruption or a pyroclastic flow such as that at Mount St Helens in 1980. Truly vast eruptions are associated with calderas whose ring faults may cause collapse in advance.

The presence of cavities beneath the surface, formed by natural solution of limestones, deliberately as in extraction of brines from salt deposits or after subsurface mining, present subsidence hazards. There have been several series of alarming TV programmes about sinkhole formation that demonstrate sudden collapse. Yet every case will have been preceded by years of gradual sagging. InSAR allows risky areas to be identified well in advance of major problems. Indeed estate agents (realtors) as well as planners, civil engineers and insurers form a ready market for such survey.

Active volcanic processes on Venus

Earth’s nearest neighbour, apart from the Moon, is the planet Venus. As regards size and estimated density it could be Earth’s twin. It is a rocky planet, probably with a crust and mantle made of magnesium- and iron-rich silicates, and its bulk density suggests a substantial metallic core. There the resemblance ends. The whole planet is shrouded in highly reflective cloud (possibly of CO2 ‘snow’) at the top of an atmosphere almost a hundred times more massive than ours. It consists of 96% CO2 with 3% nitrogen, the rest being mainly sulfuric acid: the ultimate greenhouse world, and a very corrosive one. Only the four Soviet Venera missions have landed on Venus to provide close-up images of its surface. They functioned only for a couple of hours, after having measured a surface temperature around 500°C – high enough to melt lead. One Venera instrument, an X-ray fluorescence spectrometer – did crudely analyse some surface rock, showing it to be of basaltic composition. The atmosphere is not completely opaque, being transparent to microwave radiation. So both its surface textures and elevation variation have been imaged several times using orbital radar. Unlike the Earth, whose dual-peaked distribution of elevation – high continents and low ocean floors thanks to plate tectonics – Venus has just one and is significantly flatter. No tectonics operate there. There are far fewer impact craters on Venus than on Mars and the Moon, and most are small. This suggests that the present surface of Venus is far younger than are theirs; no more than 500 Ma compared to 3 to 4 billion years.

Volcanic ‘pancake’ domes on the surface of Venus, about 65 km wide and 1 km high, imaged by orbital radar carried by NASA’s Magellan Mission.

Somehow, Venus has been ‘repaved’, most likely by vast volcanic outpourings akin to the Earth’s flood basalt events, but on a global scale. Radar reveals some 1600 circular features that are undoubtedly volcanic in origin and younger than most of the craters. They resemble huge pancakes and are thought to be shield volcanoes similar to those seen on the Ethiopian Plateau but up to 100 times larges. Despite the high surface temperature and a caustic atmosphere, chemical weathering on Venus is likely to be much slower than on Earth because of the dryness of its atmosphere. Also, unlike the hydration reactions that produce terrestrial weathering, on Venus oxidizing processes probably produce iron oxides, sulfides, some anhydrous sulfates and secondary silicates. These would change the reflective properties of originally fresh igneous rocks, a little like the desert varnish that pervades rocky surfaces in arid areas on Earth. A group of US scientists have devised experiments to reproduce the likely conditions at the surface of Venus to see how long it takes for one mineral in basalt to become ‘tarnished’ in this way (Filberto, J. et al. 2020. Present-day volcanism on Venus as evidenced from weathering rates of olivine. Science Advances, v. 6, article eaax7445; DOI: 10.1126/sciadv.aax7445). One might wonder why, seeing as the planet’s atmosphere hides the surface in the visible and short-wavelength infrared part of the spectrum, which underpins most geological remote sensing of other planetary bodies, such as Mars. In fact, that is not strictly true. Carbon dioxide lets radiation pass through in three narrow spectral ‘windows’ (centred on 1.01, 1.10, and 1.18 μm) in which fresh olivine emits more radiation when it is heated than does weathered olivine. So detecting and measuring radiation detected in these ‘windows’ should discriminate between fresh olivine and that which has been weathered Venus-style. Indeed it may help determine the degree of weathering and thus the duration of lava flow’s exposure.

Venus VNIR
Colour-coded image of night-time thermal emissivity over Venus’s southern hemisphere as sensed by VIRTIS on Venus Express (Credit: M. Gilmore 2017, Space Sci. Rev. DOI 10.1007/s11214-017-0370-8; Fig. 3)

The European Space Agency’s Venus Express Mission in 2006 carried a remote sensing instrument (VIRTIS) mainly aimed at the structure of Venus’s clouds and their circulation. But it also covered the three CO2 ‘windows’, so it could detect and image the surface too. The image above shows significant areas of the surface of Venus that strongly emit short-wave infrared at night (yellow to dark red) and may be slightly weathered to fresh. Most of the surface in green to dark blue is probably heavily weathered. So the data may provide a crude map of the age of the surface. However, Filberto et al’s experiments show that olivine weathers extremely quickly under the surface conditions of Venus. In a matter of months signs of the fresh mineral disappeared. So the red areas on the image may well be lavas that have been erupted in the last few years before VIRTIS was collecting data, and perhaps active eruptions. Previous suggestions have been that some lava flows on large volcanoes are younger than 2.5 Ma and possible even younger than 0.25 Ma. Earth’s ‘evil twin’ now seems to be vastly more active, as befits a planet in which mantle-melting temperatures (~1200°C) are far closer to the surface as a result of the blanketing effect of its super-dense atmosphere.

Ediacaran glaciated surface in China

It is easy to think that firm evidence for past glaciations lies in sedimentary strata that contain an unusually wide range of grain size, a jumble of different rock types – including some from far-off outcrops – and a dominance of angular fragments: similar to the boulder clay or till on which modern glaciers sit. In fact such evidence, in the absence of other signs, could have formed by a variety of other means. To main a semblance of hesitancy, rocks of that kind are now generally referred to as diamictites in the absence of other evidence that ice masses were involved in their deposition. Among the best is the discovery that diamictites rest on a surface that has been scored by the passage of rock-armoured ice – a striated pavement and, best of all, that the diamictites contain fragments that bear flat surfaces that are also scratched. The Carboniferous to Permian glaciation of the southern continents and India that helped Alfred Wegener to reconstruct the Pangaea supercontinent was proved by the abundant presence of striated pavements. Indeed, it was the striations themselves that helped clinch his revolutionising concept. On the reconstruction they formed a clear radiating pattern away from what was later to be shown by palaeomagnetic data to be the South Pole of those times.

striae
29 Ma old striated pavement beneath the Dwyka Tillite in South Africa (credit: M.J Hambrey)

The multiple glacial epochs of the Precambrian that extended to the Equator during Snowball Earth conditions were identified from diamictites that are globally, roughly coeval, along with other evidence for frigid climates. Some of them contain dropstones that puncture the bedding as a result of having fallen through water, which reinforces a glacial origin. However, Archaean and Neoproterozoic striated pavements are almost vanishingly rare. Most of those that have been found are on a scale of only a few square metres. Diamictites have been reported from the latest Neoproterozoic Ediacaran Period, but are thin and not found in all sequences of that age. They are thought to indicate sudden climate changes linked to the hesitant rise of animal life in the run-up to the Cambrian Explosion. One occurrence, for which palaeomagnetic date suggest tropical latitude, is near Pingdingshan in central China above a local unconformity that is exposed on a series of small plateaus (Le Heron, D.P. and 9 others 2019. Bird’s-eye view of an Ediacaran subglacial landscape. Geology, v. 47, p. 705-709; DOI: 10.1130/G46285.1). To get a synoptic view the authors deployed a camera-carrying drone. The images show an irregular surface rather than one that is flat. It is littered with striations and other sub-glacial structures, such as faceting and fluting, together with other features that indicate plastic deformation of the underling sandstone. The structures suggest basal ice abrasion in the presence of subglacial melt water, beneath a southward flowing ice sheet

Chang’E-4 and the Moon’s mantle

The spacecraft Chang’E-4 landed on the far side of the Moon in January; something of a triumph for the Peoples’ Republic of China as it was a first. It was more than a power gesture at a time of strained relations between the PRC and the US, for it carried a rover (Yutu2) that deploys a panoramic camera, ground penetrating radar, means of assessing interaction of the solar wind with the lunar surface, and a Visible and Near-infrared Imaging Spectrometer (VNIS). The lander module itself bristles with instrumentation, but Yutu2 (meaning Jade Rabbit) has relayed the first scientific breakthrough.

ChangE
Variation in topography (blue – low to red – high) over the Moon’s South Pole, showing the Aitken Basin and the Chang’E-4 landing site. (Credit: NASA/Goddard)

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