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The study of craters in our Solar System is growing in scope and significance, as images of the planets increase along with our knowledge of the correlation of impact events with changing planetary climates. One of the best sources of these images is the planet Venus, nearly the same size as Earth although very different in atmospheric history and in some aspects of its geologic makeup. Over 900 craters have so far been mapped on Venus by a series of survey missions including Venera, Pioneer, and Magellan. These impact features, comprising several main types, occur throughout the surface of Venus.
Most information on Venusian craters comes from the Magellan spacecraft, which mapped the planet's surface from Sept. 1990 to Oct. 12, 1994 (fig.1), after which radio contact was lost during a controlled descent into Venus' thick, sulfur- and CO2-laden atmosphere. During its four-year survey, Magellan returned high-resolution images of 98% of the surface using synthetic aperature radar, altimetry, thermal emissions, and gravity maps. Similar radar and gravity techniques have been used to map Chicxulub Crater in Yucatán (fig.7) and other craters on Earth including Aorounga (fig.8).
Fig.1: Composite radar image of the globe of Venus gathered by Magellan. The complex, braided terrain at mid latitudes includes Aphrodite Terra (NASA/JPL Magellan).
Major geological features on Earth's sister planet include Maxwell Montes in the northern Ishtar Terra highlands, Venus's highest mountain range which contains the crater Cleopatra (fig.4). Along the eastern equator lies Aphrodite Terra, a complex highland region containing the mountain Atla Regio. In highlands to the west towers the mountain Beta Regio. Scattered dark patches show relatively young craters from impacts 500 to 300 million years ago.
Based on form and origin, Venusian craters fall into several main types (Schaber et al. 1992). Largest are the Multi-Ringed craters, similar to Mare Orientale (fig.2) and other large basins on the Moon, Mars, and Mercury. Typically larger than 100 km in diameter with smooth floors, examples include Mead (fig.3), some 280 km in diameter, and Klenova (140 km diamaeter), both generally comparable in form and scale to the crater at Chicxulub, Yucatán (fig.7).
[Fig.2: The Moon's Mare Orientale, one of the largest impact basins in the solar system. The outer ring is the Cordillera Mountain range, 900 km in diameter (NASA Lunar Orbiter)]
Mead crater (fig.3), the largest impact feature on Venus, is named for the American anthropologist Margaret Mead (1901-1978). The multi-ringed Mead is located north of Aphrodite Terra and east of Eistla Regio (12.50° N, 57.20° E). The flat, radar-bright floor (A) has several cracks showing as brighter lines. The crater has both an inner (C) and an outer (D) ring. The inner ring appears to be the rim of the transient crater. The patchy outer ejecta of Mead (E) are rougher than the crater floor, with more slopes facing the radar, and thus appear brighter than it and the surrounding plain (G) which is covered by fine debris that shows up darker on the image.
[Fig.3: Mead Crater, Venus (NASA/JPL Magellan)]
Double Ring craters are of medium size, including most examples over 40 km diam., with an outer rim and an inner ring. One example is Cleopatra (65.90° N, 7.00° E), a crater as controversial as the 1st century BC Egyptian queen providing its namesake. Images from Venera 15/16 orbiters and Arecibo radar showed it had great depth and extensive deposits to the east, but a lack of visible rim deposits. All this suggested a volcanic caldera rather than a crater (Masursky et al. 1980; Schaber et al. 1986).
[Fig.4: Cleopatra Crater on Venus (NASA/JPL Magellan)]
Magellan's higher resolution images of Cleopatra (fig.4) soon allowed scientists to interpret the structure as a peak-ring impact crater with an inner basin, an outer basin, and rough ejecta deposits (Basilevsky and Ivanov 1990). Although the crater rim (C) appears volcanic, the inner, radar-dark crater floor (A) and the surrounding ejecta blanket (E), which appears darker than the outlying terrain (G), provide compelling evidence of an impact origin. In the upper right of the image are darker flows (F) of lava-like impact melt which breached the crater rim and filled the troughs.
About 37% of the craters on Venus are the intermediate-sized Central Peak craters with a radar-bright, jagged central peak or mounds and smooth floor. Among these is the 49 km diameter Danilova (fig.5; 26.4° N, 337.2° E), named for Alexandra Danilova, a premier Russian ballerina born in 1904. Danilova (fig.17) has a crater floor (A) with a central peak (B), surrounded by a crater wall (C). Outside is an ejecta blanket (E) and crater outflow deposits (F) surrounded by a radar-dark plain (G).
[Fig.5: Danilova Crater on Venus (NASA/JPL Magellan)]
Recent images of a central peak crater named Pwyll have been recovered further out in the solar system on Europa, one of Jupiter's planetary satellites now being studied through the Galileo spacecraft. The computer-generated topographic image of Pwyll Crater in fig. 6 clearly shows the central peak (B), crater ring (C), ejecta (E), and outflow deposits (F) of this impact feature of 26 km. diameter.
[Fig.6: Pwyll Crater on Europa (NASA/JPL Galileo PIA 01175)]
Because of the dense Venusian atmosphere, some aspects of crater formation and morphology on Venus are different from those on Earth. Venus has fewer small craters since meteors often vaporize or break up in its thick atmosphere before they reach the surface. Small Irregular craters, often less than 16 km diam., tend to have bowl-shaped floors which are usually radar bright because they are rough and complex. Such small, simple bowl-shaped craters, quite common on the Moon and Mars and even known on Earth (ie, Barringer Crater, 1.2 km diameter), are actually scarce on Venus, where no craters smaller than 3 kilometers (1.9 miles) in diameter have been observed (Phillips et al., 1991). Most small Venusian craters form tight, overlapping clusters called Multiple craters, produced by projectiles that impacted very close to each other; in some cases, the crater rims may overlap. Another example is the Shakespeare Quadrangle of Mercury, which also includes the much larger Strindberg Crater.
On Earth, craters are rapidly degraded and destroyed by surface weathering processes. In contrast, most Venusian craters remain in a relatively pristine condition, due to being formed after Venus was resurfaced about 500 myr. Evidence for such resurfacing includes a lack of flooded craters. This suggests that the surface might have been covered by lava flows about 500 million years ago, burying all existing craters (Schaber et al., 1992). Since then there has been very little geologic activity and weathering to degrade and destroy the craters. They are clearer examples of the same types of impact features that occur on Earth, a planet of nearly the same size and gravity as Venus but unlike it, covered with eons of the remains of biological life and terrestial weathering.
[Fig.7: Gravity map of Chicxulub Crater,Yucatán (Sharpton, Lunar and Planetary Institute)]
The Chicxulub structure (fig.7) is classified as a multi-ring crater, the largest of several major types now known from planetary study. Impact craters comparable to that at Chicxulub include the Strindberg Crater on Mercury , and the the multi-ringed Klenova impact basin on Venus.
Another catastrophic impact event on Earth is thought to have caused mass extinctions ca. 214 million years ago between the Carnian and Norian stages of the Triassic (225-190 myr), when today's continents were combined in the giant land mass known as Pangaea. A series of three craters in France, Quebec, and Manitoba, all of similar date and equidistant from the equator, may have formed within hours of each other in a single collision event. The Rochechouart impact in France, along with the Manicouagan crater in Quebec and the Saint Martin impact structure in Manitoba would have had a combined impact strength of the object which struck Chicxulub. Two other craters, one in North Dakota and the other in the Ukraine, may also be part of the multiple impact.
A much smaller but more easily visible impact is seen from radar imaging in the 17 km diameter Aorounga Crater in Chad, (fig.8) where an asteroid or comet struck the Earth over 200 myr. New images show an apparent ring structure revealed after covering sediments had eroded. Dark streaks throughout the image are deposits of windblown sand filling eroded valleys. A possible second crater is shown by the darker band in the upper right corner of the image.
The emerging picture is that the Earth, like Mars, Venus, Mercury, the Moon, Europa, and other planetary bodies of the solar system, is literally covered with ancient craters. These are detectable through state-of-the-art gravitational, radar, seismic, and other remote sensing techniques joined with ongoing ground and aerial photo survey.
[Fig.8: SIR-C radar image of the Aorounga Crater in Chad (NASA/JPL)].
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