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Prior to satellites with high resolution, much of the searching for impact craters in more desolate parts of the world used aerial photography, if it existed. The Canadian group at the Dominion Observatory found a number of craters that way. With Landsat, SPOT and now much higher resolution IKONOS and other systems, and worldwide coverage, the possibilities for finding new craters have notably improved. To date, at least 10 more have been found with the aid of space imagery. And, special processing such as the enhancements described in Section 1 can bring out new information about a crater imaged by a digital system. This and the last page in this Section contain examples of impact craters as recorded in both aerial and space imagery.

Remote Sensing of Craters

How has remote sensing played a role in the search for or verification of a supposed impact crater? In two ways: 1) recognizing morphological features that are compatible with an impact origin (but this does not rule out certain volcanic craters) and 2) detecting shock-induced changes as spectral changes. The strategy is one of simply using remote sensing to identify landform anomalies consistent with an impact origin and then confirming them by on-site inspection and examination of rocks for shock metamorphic effects. The absence of shock effects leads to ambiguities, making volcanic craters an alternative hypothesis. Impact-minded searchers have discovered at least ten new impact structures through remote sensing.

We can gain a feel for what to look for, and the variations in morphologic expression due to differences in erosional state, at a crater site, by switching again to the Geological Survey of Canada's Web Page on Impact Cratering. Go to the list of individual craters and check these especially interesting sites: North America: Brent/Clearwater East and West/ Deep Bay/ New Quebec; South America: Araguainha; Africa: Aorounga/ Bosumtwi/Rotor Kamm/Vredefort; Europe: Ries; Asia: Bigach/Popigai; and Australia: Henbury/Wolfe Creek. Another collection of air and space imagery focused on terrestrial craters has been compiled by Koeberl and Sharpton as online slide sets

Some craters are obvious in space and aerial images. Their circularity, and possible rim, afford strong clues. Others, mainly those that have been eroded or were imposed on or involved in mountains, are typically irregular in outline and can easily be missed. Here is an example of a crater in high relief terrain in Siberia, the Lonchatcha crater (12 km [12.5 miles]):

Lonchatcha crater; search for it.

Australia has more impact craters than any area of comparable size (Canada is a close second). This is because of its rock types, the large areas of sparse vegetation, lack of vegetation, limited areas of younger orogenies,and the interest of Australian and other geoscientists in searching for these intriguing features. At last count there were 28 confirmed craters on the continent, plotted here (Bedout is not shown):

Locations of verified impact structures in Australia

One of the younger craters is Wolfe Creek, whose rim is scarcely eroded, suggesting a young age for this structure:

The Wolfe Creek impact structure.

Interior Australia is the home of perhaps the most dramatic exposure of the central peak of a complex crater anywhere in the world, seen below in this aerial oblique view of a ringed mountain at the Gosses Bluff Crater.

Aerial view of the ringed central peak of the Gosses Bluff complex impact crater in the interior of Australia.

With that view showing the surface expression of the central ring, you should be able to pick out the crater complex in this Landsat image:

Gosses Bluff impact structure seen in a Landsat image; the McDonnell Ranges define the lowlands in which it is located.

A more detailed view of part of Gosses Bluff appears in this perspective view produced using topographic data and an ASTER image:

Perspective look at Gosses Bluff, made from SWIR, NIR, and Green bands of the ASTER sensor on Terra.

Another perspective rendition made from Landsat imagery is shown here:

Landsat-DEM perspective of the Gosses Bluff structure.

The 6 km (4 mi) wide ring consists of layers of resistant sandstone, tilted at steep angles as the strata were driven upwards on end, during the rebound of the crater floor into the peak. They have since been breached so that the lower interior now exposes softer rocks being eroded. In the Large Format Camera photo below, taken from the Shuttle, this central peak stands out in sharp contrast to the folded rocks of the McDonnell Range to its north.

Large Format Camera photo of the Gosses Bluff, Australia impact structure.

Erosion nearly obliterated the outer sections of the crater, but they are faintly expressed as a dark band in the photo. Field studies show the approximate diameter of the full crater is 22 km (14 mi).

18-12: With this practice behind you, we challenge you to find the crater in this Landsat scene. ANSWER

Natural color Landsat scene of the Goat Paddock crater in Western Australia.

If you succeed in the hunt, you will have pinpointed the 6 km (4 mile) wide Goat Paddock crater in Western Australia.

A recently discovered crater, 30 km in diameter, has been found 110 km west of Wilana in Western Australia. It has been named Shoemaker crater in honor of Eugene Shoemaker, the famed astrogeologist who was tragically killed in central Australia (see bottom of page 19-23). Here is a Principal Components image of this crater:

PCA image of Shoemaker crater.

An intriguing crater in Western Australia is the Spider Crater, shown first in a vertical Landsat view (in the center left). It has a very strange central peak which gives it its name. An aerial photo shows the spiderlike ridges carved from that peak:

Spider Crater, Western Australia.

Spider Crater, Western Australia

Canada has the large number of craters (as indicated on the bottom of page 18-1) largely because much of that country is exposed Precambrian Shield - made up mainly of hard, resistant igneous and metamorphic crystalline rocks. Glaciation has carved into some of these craters but glacial debris and boreal forests can mask smaller structures. Typical of a crater in ridges of crystalline rock is the Gow structure.

The Gow Structure

East of the Nastapoka Arc (previous page) are the two Clearwater Lakes structure, formed simultaneously by the breakup of the incoming meteoroid. West Clearwater is a complex crater, with a circular ridge as a remnant of the central peak; East Clearwater is a simple crater.

West and East Clearwater Lakes

A different kind of complex crater is Manicouagan, a 100 km (62 mi) structure in southern Quebec, Canada, which has a great central peak area of igneous and metamorphic rocks, among which are feldspar-rich rocks. In these rocks, much of the feldspar has transformed by shock into glass, known as maskelynite. Similar to Manson, there is a depression or moat between the peak and rim (now eroded) that formed annular valleys, which filled with water when a hydroelectric power dam blocked the draining rivers. Because of this contrasting surface expression, astronauts journeying back from the Moon could see this crater from well out in space.

Landsat image of the Manicouagan structure in southern Quebec, Canada.

A SIR-B radar image of southern Ontario highlights two juxtaposed but unrelated craters that are very different in age, in size, and in structural state.

 SIR-B radar image of the Sudbury impact structure (elliptical because of deformation by Grenville thrusting) and the nearby Wanapitei crater (lake-filled) formed much later.

The partially circular lake-filled structure on the right (east) is the 8 km (5 mi) wide Wanapitei crater, estimated to have formed 34 million years (m.y.) ago. The far larger Sudbury structure appears as a pronounced elliptical pattern, more strongly expressed by the low hills to the north. This huge impact crater, with its distinctive outline, was created about 1800 m.y. ago. Some scientists argue that it was at least 245 km (152 mi) across when it was circular. The strong northwestward thrusting of the Grenville Province terrane against the Superior Province (containing Sudbury) subsequently deformed it, more than 900 m.y. later, into its present elliptical shape(geologists will recognize this as a prime example of the "strain ellipsoid) model. After Sudbury was initially excavated, magmas from deep in the crust invaded the breccia filling, to emplace against its walls. Some investigators think that the resulting norite rocks are actually melted target rocks. This igneous rock (called an "irruptive") is host to vast deposits of nickel and copper, making this impact structure a 500 billion dollar source of ore minerals since mining began in the last century.

18-13: Which part of the elliptical rim of Sudbury seems to have better topographic expression? ANSWER

A geologic map shows the general geologic units present within and around the main structure (just north of the town of Sudbury):

Map of the units composing the deformed Sudbury structure.

Part of this structure has been displayed in this Landsat image in which classification procedures indicate many of the surface constituents:

A classified Landsat image of surface units around Sudbury.

Radar can sharpen the appearance of an impact structure, as demonstrated with this aerial radar image of the Haughton crater (24 km, 15 mi wide) in Canada. Although about 23 m.y. old, much of the crater's morphology has survived erosion.

Aerial radar image of the Haughton crater in Canada.

18-14: Where does the crater rim of Haughton appear to be? ANSWER

We move on now to craters elsewhere in the world:

Water often fills complex craters sufficiently intact to retain a central depression. A Seasat radar image of the Elgygytgyn Crater in Siberia (its diameter is 18 km [11 mi]) is a good example. The aerial oblique view offers a different perspective.

Seasat radar image of the Elgygytgyn Crater, Siberia.

Color aerial oblique photograph of the Elgygytgyn Crater, Siberia.

18-15: The Elgygytgyn Crater interior, defined by the lake, seems almost squarish rather than round. Speculate on the cause of this departure from the normal inner shape of impact craters. ANSWER

A Radarsat image provides a color view of the newly-discovered Aorounga crater (about 17 km [10.5 miles] in rim diameter found in inclined sandstone units and desert sand in the Sahara Desert of northern Chad.

Radarsat color composite showing the recently discovered Aorounga impact structure in the Chad desert.

Another well known African crater is Bosumtwi, in the jungle of Ghana, Africa. This 10.5 km (6.5 mile) crater may be the source of the Ivory Coast tektite strewnfield.

The Bosumtwi crater.

The second largest impact crater (about 300 km [200 miles] wide) on Earth is the Vredefort Dome (once considered that type of structure until shatter cones revealed its extraterrestrial origin as a depression) in South Africa southeast of Johannesburg. Here it is as seen from space:

The Vredefort impact crater; hills on its northwest side indicate a more distinct rim structure.

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Primary Author: Nicholas M. Short, Sr. email: [email protected]