Venus Craters

Spring 2003

	Possible Remnants of a Meteoroid in Lakshmi Region: This full resolution mosaicked image covers an area of approximately 100 kilometers by 120 kilometers (62 by 74 miles) and is located in the Lakshmi region of Venus at 47 degrees north latitude and 334 degrees east longitude. Due to the dense Venusian atmosphere, primary impact craters of less than a 3 kilometer (2 mile) diameter are nonexistent. The dark circular region and associated central bright feature in this image are thought to be the remnants of a meteoroid smaller than the size necessary to create an impact crater entering the atmosphere at a low velocity (approximately 350 meters/second.) The central bright feature appears to be a cluster of small secondary impacts, ejecta and debris from the original meteor that broke up in the atmosphere. Even though most of the meteorite did not hit the surface, the atmospheric shock wave could be great enough to modify the surrounding region. One explanation for this radar dark circular formation, called dark margins, could be that the shock wve was energetic enough to pulverize the surface (smooth surfaces generally appear radar dark.) Another explanation is that the surface could be blanketed by a fine material that was formed by the original meteor's breakup through the atmosphere. More than half of the impact craters on Venus have associated dark margins, and most of these are prominently located left of center of the crater. This is another effect which could be caused by the dense atmosphere of Venus.
	Complex Crater "Dickinson": This Magellan image is centered at 74.6 degrees north latitude and 177.3 east longitude, in the northeastern Atlanta Region of Venus. The image is approximately 185 kilometers (115 miles) wide at the base and shows Dickinson, an impact crater 69 kilometers (43 miles) in diameter. The crater is complex, characterized by a partial central ring and a floor flooded by radar-dark and radar-bright materials. Hummocky, rough-textured ejecta extend all around the crater, except to the west. The lack of ejecta to the west may indicate that the impactor that produced the crater was an oblique impact from the west. Extensive radar-bright flows that emanate from the crater's eastern walls may represent large volumes of impact melt, or they may be the result of volcanic material released from the subsurface during the cratering event.
	Impact Crater Isabella: Crater Isabella, with a diameter of 175 kilometers (108 miles), seen in this Magellan radar image, is the second largest impact crater on Venus. The feature is named in honor of the 15th Century queen of Spain, Isabella of Castile. Located at 30 degrees south latitude, 204 degrees east longitude, the crater has two extensive flow-like structures extending to the south and to the southeast. The end of the southern flow partially surrounds a pre-existing 40 kilometer (25 mile) circular volcanic shield. The southeastern flow shows a complex pattern of channels and flow lobes, and is overlain at its southeastern tip by deposits from a later 20 kilometer (12 mile) diameter impact crater, Cohn (for Carola Cohn, Australian artist, 1892-1964). The extensive flows, unique to Venusian impact craters, are a continuing subject of study for a number of planetary scientists. It is thought that the flows may consist of 'impact melt,' rock melted by the intense heat released in the impact explosion. An alternate hypothesis invokes 'debris flows,' which may consist of clouds of hot gases and both melted and solid rock fragments that race across the landscape during the impact event. That type of emplacement process is similar to that which occurs in violent eruptions on Earth, such as the 1991 Mount Pinatubo eruption in the Philippines.
	Multi-Floor Irregular Crater: This Magellan image shows an irregular crater of approximately 14-kilometer (8.7-mile) mean diameter. The crater is actually a cluster of four separate craters that are in rim contact. The noncircular rims and multiple, hummocky floors are probably the result of the breakup and dispersion of an incoming meteoroid during passage through the dense Venusian atmosphere. After breaking up, the meteoroid fragments impacted nearly simultaneously, creating the crater cluster. The area shown is 40 kilometers (25 miles) in width and 76 kilometers (47 miles) in length, it is centered at -21.4 degrees latitude, 335.2 degrees longitude in the northern Lavinia Region of Venus.
	Impact Crater Jeanne: This Magellan full-resolution image shows Jeanne crater, a 19.5 kilometer (12 mile) diameter impact crater. Jeanne crater is located at 40.0 degress north latitude and 331.4 degrees longitude. The distinctive triangular shape of the ejecta indicates that the impacting body probably hit obliquely, traveling from southwest to northest. The crater is surrounded by dark material of two types. The dark area on the southwest side of the crater is covered by smooth (radar-dark) lava flows which have a strongly digitate contact with surrounding brighter flows. The very dark area on the northeast side of the crter is probably covered by smooth material such as fine-grained sediment. This dark halo is asymmetric, mimicking the asymmetric shape of the ejecta blanket. The dark halo may have been caused by an atmospheric shock or pressure wave produced by the incoming body. Jeanne crater also displays several outflow lobes on the northwest side. These flow-like features may have formed by fine-grained ejecta transported by a hot, turbulent flow created by the arrival of the impacting object. Alternatively, they may have formed by flow of impact melt.
	Sag Caldera Sachs Patera: This image of Sachs Patera on Venus is centered at 49 degrees north, 334 degrees east. Defined as a sag-caldera, Sachs is an elliptical depression 130 meters (81 feet) in depth, spanning 40 kilometers (25 miles) in width along its longest axis. The morphology implies that a chamber of molten material drained and collapsed, forming a depression surrounded by concentric scarps spaced 2-to-5 kilometers (1.2-to-3 miles) apart. The arc-shaped set of scarps, extending out to the north from the prominent ellipse, is evidence for a separate episode of withdrawal; the small lobe-shaped extension to the southwest may represent an additional event. Solidified lava flows 10-to-25 kilometers (6-to-16 miles) long, give the caldera its flower-like appearance. The flows are a lighter tone of gray in the radar data because the lava is blockier in texture and consequently returns more radar waves. Much of the lava, which was evacuated from the chamber, probably traveled to other locations underground, while some of it may have surfaced further south. This is unlike calderas on Earth, where a rim of lava builds up in the immediate vicinity of the caldera.
	Impact Crater in E. Navka Region: This Magellan image, which is 50 kilometers (31 miles) in width and 80 kilometers (50 miles) in length, is centered at 11.9 degrees latitude, 350 degrees longitude in the eastern Navka Region of Venus. The crater, which is approximately 8 kilometers (5 miles) in diameter, displays a butterfly symmetry pattern. The ejecta pattern most likely results from a oblique impact, where the impactor came from the south and ejected material to the north.
	Transitional Crater: During orbits 423 through 424 on 22 September 1990, Magellan imaged this impact crater that is located at latitude 10.7 degrees north and longitude 340.7 degrees east. This crater is shown as a representative of Venusian craters that are of the proper diameter (about 15 kilometers) to be 'transitional' in their morphology between 'complex' abe 'irregular' craters. Complex craters account for about 96 percent of all craters on Venus with diameters larger than about 15 kilometers; they are thought to have been formed by the impact of a large, more or less intact, mass of asteroidal material that has not been excessively affected during its passage through the dense Venusian atmosphere. Complex craters are characterized by circular rims, terraced inner wall slopes, well-developed ejecta deposits, and flat floors with a central peak or peak ring. Irregular craters make up about 60 percent of the craters with diameters less than about 15 kilometers. Irregular craters are thought to form as the result of the impact of asteroidal projectiles that have been aerodynamically crushed and fragmented during their passage through the atmosphere. Irregular craters are characterized by irregular and/or discontinuous rims and hummocky or multiple floors. The 'transitional' crater shown here has a somewhat circular rim like larger complex craters, but has the hummocky floor and asymmetric ejecta characteristic of smaller irregular craters.
	Large Impact Crater in Eistla Region: This Magellan image shows an impact crater in the central Eistla Region of the equatorial highlands of Venus. It is centered at 15 degrees north latitude and 5 degrees east longitude. The image is 76.8 kilometers (48 miles) wide. The crater is slightly irregular in planform and approximately 6 kilometers (4 miles) in diameter. The walls appear terraced. Five or six lobes of radar-bright ejecta radiate up to 13.2 kilometers (8 miles) from the crater rim. These lobes are up to 3.5 kilometers (2 miles) in width and form a 'starfish' pattern against the underlying radar-dark plains. The asymmetric pattern of the ejecta suggests the angle of impact was oblique. The alignment of two of the ejecta lobes along fractures in the underlying plains is apparently coincidental.
	Mosaic of Large Impact Craters:One of the most useful Magellan standard data products is the full resolution mosaic, the F-MIDR (Full-Resolution Mosaicked Image Data Record). These products are mosaics of about 500 kilometer (300 mile) segments of 30 or more individual image strips. This image is an F-MIDR made from orbits 376 to 407, obtained between September 15 and September 19, 1990, part of the first orbits in which the Magellan flight team operated the radar system in the mapping mode. The mosaic is centered at 27 degrees south latitude, 339 degrees longitude, in the Lavinia region of Venus. Three large impact craters with diameters ranging from 37 kilometers (23 miles) to 50 kilometers (30 miles) can be seen located in a region of fractured plains. The craters show many features typical of meteorite impact craters, including rough, radar-bright ejecta, terraced inner walls and central peaks. Numerous domes of probable volcanic origin can be seen in the southeastern corner of the mosaic. The domes range in diameter from 1-12 kilometers (0.6-7 miles), and some have central pits typical of volcanic shields or cones. Resolution of the Magellan data is about 120 meters (400 feet).

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Last updated 06/05/2003