Drilling Vostok with Radar
Anyone caught speeding on a freeway is familiar with the detection capabilities of radar. But high from orbit, radar images taken of Earth are offering access to some unlikely places of interest to astrobiologists, including volcanoes that seem to breath and pristine Antarctic lakes that may be sinking or melting. Tiny ground movements that occur too gradually to be seen by the human eye are being detected by satellites looking down to Earth from 800 km away.
|Lake Vostok is believed to contain water millions of years old, which may be the home of ancient organisms. This hidden body of freshwater is the size of Lake Ontario and is the largest of 70 bodies of water first detected under the polar ice-sheet in the 1970s. Credit: LDEO Columbia University|
Researchers are using this ability to monitor volcanoes and earthquake zones, aid oil and gas prospecting, observe urban subsidence and measure the slow flow of glaciers.
Data from Synthetic Aperture Radar (SAR) instruments like those flown aboard the ERS spacecraft and Envisat are the basis for a technique called SAR interferometry, or InSAR for short. InSAR involves combining two or more radar images of the same ground location in such a way that very precise measurements – down to a scale of a few millimetres – can be made of any ground motion taking place between image acquisitions.
Very small movements can potentially be detected across wide areas: tectonic plates grinding past one another, the slow ‘breathing’ of active volcanoes, the slight sagging of a city street due to groundwater extraction, even the thermal expansion of a building on a sunny day.
|The juxtaposition of old and new data shows the change. An aerial or satellite picture of Venice (black and white) is overlaid with the current data of the earth reconnaissance satellite (colored). The space data show that part of Venice is sinking millimetre by millimetre into the sea – more in the red and violet areas than in the blue and green. Such details help concentrate structural protection measures in the places where they are most needed. Credits: GAMMA/ESA|
"Collecting multiple images of the same landscape might at first sound boring, until you realise the extraordinary level of precision with which InSAR shows us how that landscape changes," explained Prof. Fabio Rocca of the Milan Politecnico, who has worked in this field for the last two decades.
"The technique really came into its own since ESA launched its first ERS satellite in 1991. The decision was made to archive all ERS data, which was courageous as data storage was so much more expensive then. Now that decision is paying off because all the archive is available for InSAR use."
Radar images record the backscatter of microwave pulses reflected off the Earth’s surface, and so measure relative surface roughness – the brighter a given point shows up, the higher its roughness, and backscatter. Smoother surfaces tend to bounce radar pulses away from the spacecraft’s field of view.
"It is very different to looking at optical wavelengths, and one of the subjects we are discussing … is how to work out more accurately what we are seeing," said Rocca. "We are looking with different eyes – think of it as like the eyes of the Terminator! At optical wavelengths the surface of a building reflects light, but at radar wavelengths we pierce through the walls of the building to the steel skeleton beneath – its sharp corners give it high radar reflectivity".
Mysteries of Lake Vostok
InSAR has been used to look beneath a polar ice sheet four kilometres deep and learn more about conditions prevailing in one of the strangest environments on Earth. Lake Vostok in the East Antarctic is a 280-km-long freshwater lake that has been buried beneath the ice sheet for at least half a million years.
A combination of crushing pressure, geothermal heat and the insulation of the thick ice above it is thought to keep the waters of Lake Vostok liquid. What remains unknown is whether any life exists in this dark, cold, low-energy environment, entirely cut off from the rest of the world. Researchers have decided not to drill into the Lake until they can be certain they will not contaminate its pristine waters with topside bacteria.
|Envisat wide-swath radar image of ice above the sub-glacial Lake Vostok, acquired 13 March 2003. The freshwater lake’s full extent was first mapped using ERS-1 data in the early 90s, because the ice floating over the lake has a smoother surface than the ground-based ice surrounding it. More recently, ERS InSAR results have comfirmed its waters are influenced by tides. Credits: GAMMA/ESA|
Drilling into Lake Vostok, a huge, Lake Ontario-sized subglacial sea, has intrigued scientists since it was detected four kilometers below the ice in 1996. Proposals to drill into Lake Vostok have met with opposition because of the danger of contamination. In addition, many of the nearly 100 under-ice seas discovered to date may be interconnected, so contaminating one could contaminate them all. An international committee is discussing the issue, which may delay drilling for a decade.
Astrobiologists are interested in the kinds of exotic microbes that might live inside solid ice, either as dormant spores or at a low level of activity. Drilling in Antarctic ice, including to within about 100 meters of Lake Vostok, has turned up some bacteria, according to Russian scientists, but all were known before. Bacteria also have been found in ocean ice. Inspired by finding the sub-glacial lake, scientists hope to discover new species in solid ice, analogous to the novel thermophiles found in hot seafloor vents living at temperatures above the sea-level boiling point of water (100 C or 212 F).
"The ice covers of these lakes represent an oasis for life in an environment previously thought to be inhospitable," wrote John Priscu of Montana State University, who participated in a National Science Foundation project to investigate Antarctica lake covers. "These life forms may possess novel ice-active substances such as antifreezes and ice nucleation inhibitors that allow the organisms to survive the freeze-thaw cycles and come back to life when exposed to liquid water," he said. From taking ice cores, the scientists on the ground have found a layered chemical and biological history preserved in the ice, and revived viable microbes that are at least 2,800 years old.
"Importantly, the cold temperatures preserve DNA extremely well making them perfect ‘ice museums’ for the study of ancient DNA," Priscu added. Research on the ancient DNA will provide an evolutionary and functional history of the microorganisms, he said, and he believes the findings might help scientists draw implications for the type of life that may exist in such inhospitable ecosystems.
|Subsurface oceanic probe of Europa, one of Jupiter’s moons (Icepick, or the Europa Ocean Explorer development study) thought to have a liquid layer heated by tidal forces while orbiting eccentrically around Jupiter, much the way friction heats up a paperclip when bent repeatedly.|
Credit: NASA JPL
Although Lake Vostok is off-limits for now there are indirect ways of seeing beneath the ice. Back in 1993 ERS data was employed to help map the Lake’s full extent, establishing the ice directly over it was much flatter than that around it. More recently, German researchers have used ERS interferograms to establish that – despite their distance from the surface – the waters of Lake Vostok are stirred by daily tides.
During the mission conference, Anja Poetzch of the Dresden Technical University presented details on how pair of interferograms acquired during ERS-1 and 2 tandem operations in 1996 demonstrated a maximum vertical displacement of 15 mm above Lake Vostok, corresponding to tidal motion. Results from in-situ GPS observations carried out during the last two Antarctic summers confirm the conclusion.
Limits to Life
With viable organisms being recovered from a depth of two and half miles (~4 km) and at below freezing (14 F) or -10 C temperatures, many have begun to reconsider how inhospitable the rests of the solar system is. Results from the Galileo spacecraft showed that Jupiter’s moon, Europa, almost definitely has a layer of ice between 60-120 miles (100 and 200 km) thick, but at such depths, most density probes have been unable to differentiate a liquid or solid. Other than the Earth, Europa is currently considered the most likely candidate in the solar system for finding any liquid water (as much as 50 km or 30 miles deep). Probing subsurface bacteria is a frontier that spans both biology and physics. The subsurface of a planetary body for instance, would likely still need a way to recycle or refresh its nutrient supply to the surface atmosphere and back. Of the 61 moons in the solar system only four others (Io, Ganymede, Titan and Triton) are known to have atmospheres.