Most impact structures have been moderately to severely eroded so that their crater rim morphology is no longer a strong clue to their presence and nature. Worn-down craters are sometimes referred to as astroblemes (literally, "star wounds"). Detection in space images is therefore difficult; breccias with associated shock metamorphic features are then the best indicators. Still, processed imagery can reveal signs of an astrobleme (sometimes drainage will adjust to the underlying structure, with a tendency towards circularity). A relatively young impact crater, the Zhamanshin structure (13 km; 8 miles) in Kazahkstan, is a case in point:

This image was generated from all non-thermal Landsat TM bands regrouped into Principal Components. Shown above are Components 2, 3, and 4 in Red, Green, and Blue.

We close with a look at the most famous impact crater on Earth, Meteor Crater (also called Barringer Crater after the family who owns it), a 50000 year old depression cut into the flat-lying sedimentary layers below the surface of the Colorado Plateau some 73 km (45 miles) east of Flagstaff, Arizona. An aerial oblique view of this 1230 m (4000 ft) wide crater shows its freshness (pieces of the iron meteorite that caused it can still be found in the ejecta); the road allows tourists to visit its overlook and museum.

The flat interior floor, without a central peak, is a characteristic of simple craters; Meteor Crater's outline tends towards a square shape - this departure from circularity is controlled by the dominant set of two orthogonal joints (planar fractures) that run through the layers; and the ejecta deposits outside the rim still retain a hummocky (mound-like) topography. A ground photo from its rim 185 m (600 ft) above the floor gives a sense of its grandiose size; note the displaced (fault-bounded) blocks under the rim in both aerial and ground photos.

Field study of Meteor Crater in the late 1950s by Eugene Shoemaker and its shocked rocks shortly thereafter by Edward Chao led to the first modern concepts of impact crater mechanics. The SiO₂ morph Coesite was first discovered in impact structures at this crater.

A specially processed image made by the airborne Thematic Mapper Simulator (TMS) shows that the ejecta blanket or apron (in reds and yellows) around Meteor Crater is asymmetrically distributed with maximum extension to the northeast. There is a notable tendency for the ejecta deposits to appear elongated to the northeast; this may be mainly an effect of wind-blown re-working rather than impact angle. The ejecta contain fragments of the iron meteorite which caused Meteor Crater., along with iron melt spherules. The red and blue lines are power lines and roadways.

A thermal multiband color image made (courtesy: Dr. J. Garvin) from the airborne TIMS (Thermal Infrared Multispectral Scanner) sensor divulges the expression of this ejecta, with reds and some yellow corresponding largely to Moenkopi Siltstone and Coconino Sandstone (whose spectral properties in the ejecta are influenced by their particulate nature and, possibly, by shock effects) and the blue-greens to the overlying Kaibab Limestone.

What are your chances of being killed from an impact event? Very small, but not zero. A small cometary body exploded (estimated between 10 and 100 megatons) over the Tunguska region in Siberia in 1908 and an iron meteorite made a 30 m [100 ft] crater in Siberia in 1947. Meteor Crater formed not long before North America was settled. Impacting bodies that form 20 km wide craters strike Earth at a frequency of only once every few million years (the Zhamanshin structure in southern Russia 13.4 km [7.5 mile] diameter is 750000 years old and an 8 km crater in Bolivia may be much younger). A Chicxulub-sized collision, capable of destroying much of life 65 m.y. ago through a "nuclear winter" type calamity and thus likely to be fatal to humans, is expected about once every 100 m.y. The now famous multiple impacts of the Shoemaker-Levy comet into Jupiter in 1993 proves convincingly that planets are targets of big hits that have occurred in the past, and will again, during the brief historical span (a few thousand years) when Man has recorded such dramatic events. And there are many thousands of larger asteroids and comets still out there, many not yet found and some destined to pass us nearby. (A paper given in May 1997 by Dr. Louis Frank of the University of Iowa reports on observations made by NASA's Polar satellite that comets in the range of 40 metric tons or less strike the Earth's atmosphere hundreds of times each day; these water-rich bodies may be responsible for significant original deposition and subsequent additions of water in the Earth's oceans.) At present there is no sure defense against these extraterrestrial invaders that would certainly wreak catastrophic havoc on Earth. Pleasant dreams!

This Section, along with the last on Geomorphology, in which several of many scientific uses of space imagery have been demonstrated as adding valuable new information, are good prologues to another of the major applications of remote sensing from spacecraft: the exploration of the planets to be reviewed in the next Section, again with the role of landforms analysis in characterizing surfaces being an integral part of interpretation procedures.

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