Earthshots: Satellite Images of Environmental Change

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» Petermann Glacier, Greenland

Petermann Glacier made headline news in 2010 and again in 2012 when large pieces broke off the end of the glacier and floated out to sea. Located on the northwestern coast of Greenland, Petermann Glacier covers 1,295 square kilometers. Its floating ice tongue is 15–20 kilometers wide and 70 kilometers long—the longest floating glacier in the Northern Hemisphere.

A glacier is made up of fallen snow that has been compressed into a large thickened ice mass over many thousands of years. Tidewater glaciers flow like very slow rivers to the ocean, and at the boundary between the glacier and the sea, ice breaks, or calves, from the end, creating icebergs.

This calving is normal, but it’s worth watching Petermann and other Greenland glaciers closely. Petermann is one of the major marine-terminating glaciers of Greenland. Ice loss from the Greenland Ice Sheet has increased recently. An article in Nature concluded that climate change may cause Petermann and other Greenland glaciers to contribute to sea level rise. Landsat helps glaciologists keep a close eye on this remote but significant glacier.

Map of the featured area.

Scientists observed rifts in Petermann Glacier throughout the first decade of the 2000s. The rift that caused the 2010 break was first spotted in satellite imagery in 2001. See below for the location of this rift in a 2001 Landsat image.

The massive calving event in 2010 removed 28 kilometers of the ice shelf. The result was an ice island four times the size of Manhattan, about 270 square kilometers. It was the largest iceberg to form in the Arctic since 1962.

The last image displayed in this section shows a size comparison of Manhattan Island (yellow outline) to the iceberg.

(Black stripes run through the images because of the Scan Line Corrector failure on Landsat 7 in May 2003.)



July 21, 2001, Landsat 7 (path/row 43/1) — rift that began the calving events, Petermann Glacier, Greenland     July 21, 2001, Landsat 7 (path/row 43/1) — area showing rift that began the calving events, Petermann Glacier, Greenland

Just two years later, another large iceberg broke off Petermann Glacier. This one was estimated at 130 square kilometers—about half the size of the 2010 iceberg. But this calving broke off the glacier tongue farther upstream and moved the front end of the glacier farther inland than has been observed since 1876, the first reported measurements of the glacier.

The floating ends of glaciers like Petermann are known as ice shelves. They act as doorstops. When these ice shelves suddenly splinter and weaken, the glaciers that feed them speed up. The result is more ice flowing into the ocean, which could raise global sea levels.

The development of these icebergs is a natural process; however, when there are two major breaks in two years, scientists must take notice. Even large breaks do not amount to a collapse of the floating extension; nevertheless, they are important events.

(Black stripes run through the images because of the Scan Line Corrector failure on Landsat 7 in May 2003.)

Landsat 8 has a 16-day repeat cycle. That means it images the same spot on the ground every 16 days. However, there is some overlap at the sides of the images, and that overlap increases at higher latitudes.

Here in northern Greenland, at about 81 degrees north latitude, the repeat cycle of Landsat 8 is in reality much more frequent than every 16 days during spring, summer, and fall. Displayed in this section are several clear images available from Landsat 8 of Petermann Glacier from May 23 to September 19, 2014. As you can see by the acquisition dates, Landsat visits this spot more frequently than every 16 days. This was especially true in August and September, 2014, when there were more non-cloudy days.

Seasonal changes are noticeable in this image series. For example, the high cliffs on the west side of Petermann Glacier cast longer shadows in September than on June 24, close to the summer solstice.

The patterns in the ice also change throughout the year. The amount of ice in the water diminishes through September, and by the next May will be packed in again. On the ground, the amount of snow cover also decreases, but the ice fields remain. The snow cover does begin increasing again in the late September images indicating winter is returning. The later images are also darker as the sun approaches a lower angle in the sky as it gets closer to the autumnal equinox.

Other lucky places that receive a slightly better than 16-day repeat cycle from Landsat 8 include Green Bay, Wisconsin; New York City; London, England; and Valparaiso, Chile. It's not just high latitude places that lie in these overlapping scenes. Jacksonville, Florida, at about 30.3 degrees north latitude, is imaged approximately once a week.

Compare the two Landsat images from June 24, 2014, and September 19, 2014. At this high latitude, the sun angle becomes very low later in the year.

The June 24 image is close to the summer solstice. The September 19 image is close to the autumnal equinox, when the sun angle is much lower in northern Greenland. This lower sun angle is very noticeable in the longer shadows. The shadows also provide hints about the height and shape of the mountain peaks along the glacier.

Landsat does not acquire images this far north during winter. In the Northern Hemisphere’s winter, it becomes too dark for satellite imaging.

A new rift formed on Petermann Glacier in 2017. An older crack to the right of the new rift also extends toward the glacier’s center. By 2020, this new rift had met up with the older crack. If an iceberg breaks off, it would be Petermann’s third massive iceberg calving since 2010. It also could place the new calving face much farther upstream than the 2010 break.

Close-up images from Sentinel-2 take advantage of its 10-meter spatial resolution to show the new rift in greater detail. Landsat 8 and Sentinel-2 complement each other by imaging glacial movement and possible calving events.

The prominent vertical line could be from deformation of the ice as it flows over the bedrock farther upstream. The irregular topographic surface of the underlying bedrock could have caused the ice to develop this longitudinal crevasse as it moved over bedrock, resulting in a line being drawn the length of the glacier as it flows. Besides the new rift, other bumps and lines extend from this longitudinal line, which are stress fractures from the glacial movement.

Also note that the rifts move downstream between 2017 and 2020. Along with becoming longer, the rifts are also moving downstream as the glacier flows.

Landsat 8 normally images all Earth landmasses every 16 days. However, at high latitudes, there is considerable overlap because Landsat 8’s orbital tracks converge at the Poles. As a result, this increases the temporal frequency of Landsat 8 coverage over northern Greenland.

Building on this imaging overlap, Landsat 8 takes advantage of long hours of daylight in the Arctic to acquire “nighttime” sunlit images, increasing temporal coverage even more. The two Landsat 8 images were acquired a little over 3 hours apart, one on its descending orbit and one ascending. Having multiple images increases the chances of acquiring more cloud-free images and helps scientists monitor iceberg calving events.


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