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Emptying into the Arctic Ocean, the 2,800-mile-long Lena River in Siberia forms one of the world’s largest deltas. (USGS)

Earth’s Mirror

Landsat shows us the home planet, warts and all.

For the past decade, the USGS and Department of Interior have used Landsat data to show forest service workers where fire damage threatens adjacent developments. If emergency response teams take action quickly, post-fire flooding, mudslides, and erosion can all be prevented.

Light reflected from severely burned soil differs from that reflected by areas where there are still plants and ground cover. Remote-sensing experts have learned that Landsat’s two mid-infrared bands are more sensitive to fire-caused changes in soil than other bands. With Landsat imaging, the forest service is able to generate maps showing four levels of damage, from unburned to severely burned—areas where all the organics like pine needles and duff have burned away and ash predominates. In those areas, fire response teams spread ryegrass seeds, place straw mulch on hillsides, or take some other action to prevent runoff and erosion.

Before 2001, says USGS geographer Randy McKinley, fire perimeters were sketched by teams on the ground or overflying the area in a helicopter; both methods were time-consuming and expensive. “Landsat supplanted the traditional ways that Burned Area Emergency Response teams get information,” says McKinley. With Landsat images, firefighters generate preliminary maps that are usually adjusted with data gathered in the field. But even the preliminary maps provide accurate locations for the perimeters of fires and also show within the perimeters unburned islands that don’t need treatment.

The Landsat archive also provides an atlas of old fires that were never mapped; scientists and environmental groups can study them to determine, for example, whether a fire has made the land more susceptible to invasive plant species.

Context

Some geologic structure is simply more comprehensible viewed from space than it is when you’re standing on top of it. John Spray, the director of the Planetary and Space Science Centre at Canada’s University of New Brunswick, is using the perspective offered by Landsat in a 10-year study of the Manicouagan impact crater in Quebec. “We’re a little bit like an ant walking on an elephant or a rhino,” he says. Though small by solar system standards—many impact craters on the moon and Mars dwarf it—Manicouagan is one of Earth’s largest; it hasn’t been deformed by erosion or squeezed by tectonics. “That huge crater was formed in seconds,” says Spray. “If you took the nuclear arsenal of the Soviet Union and the United States during the cold war and let them off in one place in one moment, it would not create as much energy as the energy that formed Manicouagan.

“What we think is totally weird is that the rocks at Manicouagan came up eight to 10 kilometers [five to six miles]—in a minute or two,” says Spray. The central uplift where those rocks now appear could have formed as the ground beneath the impact rebounded, he says , like a trampoline reacting to a jumper. He believes it was helped along by the almost simultaneous collapse inward of rocks and material around the sides of the excavated bowl. “We don’t really understand how rocks move that fast,” he says.

At the other end of the geologic time scale, the Brandberg Massif in central Namibia rose from the surrounding plain over a period of hundreds of thousands, if not millions, of years during Earth’s Cretaceous period, around 100 million years ago. Geologists call it a granite intrusion. It too is the result of almost unimaginable forces. The ancient surface bulged up because molten rock under pressure pushed through overlying layers into a zone of lower pressure. What drove that action was the movement of tectonic plates.

“Friction between two plates builds up a lot of heat that can’t be dissipated easily and actually gets hot enough to melt some of the overplate,” says USGS scientist Charles Trautwein, “and the melted rock comes up between structural cracks formed by the [movement of one plate beneath the other].” This particular intrusion covers 250 square miles and reaches almost 8,500 feet above the Namib Desert.

One of the things geologists look for in Landsat images are geometric shapes: circles and lines, which can indicate faults, or breaks in the crust. The ridges that ring the Brandberg Massif are an example of circular faults. Faults more often show up as straight lines; in either case, the scale may elude geologists working in the field. “It could continue on for maybe 10 or 20 kilometers,” says John Spray, “but when we’re down in the woods in that valley, we don’t know how big it is.” From space, scientists can see its full length and determine where to target ground studies.

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