Venus's craters

Impact craters are the most numerous and most easily recognized surface features in the solar system. Because of their pristine nature, Venusian impact craters provide a unique opportunity for astrogeologists to study the effects of atmospheric variabilities and gravity in the formation of planetary surfaces.

Overview

One of the earliest and most crucial phases in the early development of the solar system was the Great Bombardment. During this phase, planetary surfaces were under intense bombardment by cosmic debris. For the Earth and Moon, the Great Bombardment began about 4.6 billion years ago and declined about 3. 8 billion years ago. Because the Moon has no atmospheric processes, and it was long thought to have no tectonic activity, scientists deduce that the impact craters there are mostly scars left from the time of intense bombardment. Because the Earth, by contrast, is continuously subjected to weather processes and is tectonically active, craters of such ancient ages are not visible on Earth’s surface and have been identified only on the cratons of continents, which undergo relatively little resurfacing from weathering, tectonism, volcanism, or other processes. The majority of impact craters identified on Earth, therefore, are geologically young and do not represent impacts from the Great Bombardment. Venus, Earth’s closest planetary neighbor, presumably was also subjected to this intense cosmic bombardment, but it appears that Venus totally lacks the ancient, heavily cratered surface that occurs on Mercury, Mars, the Moon, and the rocky moons of other planets.

The detailed morphology of the surface of Venus is known through research done on remote-sensing data obtained from the launching of successful spacecraft mounted by the National Aeronautics and Space Administration (NASA)—the Mariner (1962-1975), Pioneer (1978), Galileo (1990), Magellan (1990-1994), Cassini-Huygens (1998-1999), and MESSENGER (2004) uncrewed spacecraft—as well as the Soviet Union’s Vega (1985) and Venera missions (1967-1984) and the European Space Agency's (ESA)Venus Express (2005). Other data have been accumulated through the study of high-resolution images from Earth-based radar. The Magellan spacecraft, inserted into Venus orbit in 1990, yielded radar images with a resolution of a few hundred meters, covering nearly 98 percent of the planet. In addition, the Soviet Venera mapped nearly 25 percent of Venus with additional radar imaging. These data have resulted in the mapping of nearly 100 percent of Venus and suggest that Venus lacks the ancient, heavily cratered surface occurring on other terrestrial planets and rocky satellites. The Venus Express provided valuable data on the atmosphere of Venus and evidence suggesting that past oceans may have existed on the Venusian surface. The JapanAerospace Exploration Agency (JAXA) launched the Venus mission Planet-C in 2010. Throughout the 2010s and into the 2020s, flybys of Venus by unmanned spacecraft continued with the BepiColombo, the Solar Orbiter, and the Parker Solar Probe, joint efforts between NASA and the European Space Agency.

Astrogeologists use the density of impact craters to determine the age of planetary surfaces. The older the surface, the more impact craters it will have accumulated over time. On Venus, this dating technique is problematic because there are relatively few impact craters. Based on the density of Venus’s impact craters, larger than thirty kilometers in diameter, estimates of cratering rates scaled for other terrestrial planets and rocky satellites, and the known population of asteroids crossing Venus’s orbit, the planet’s average surface age is estimated at between 450 million and 250 million years old, the younger age being more likely. This suggests the surface terrain of Venus may be less than 5 percent of the age of the solar system. However, these ages are average estimates, and based on superposition, some Venusian impact craters, volcanic structures, and tectonic terrains are thought to be as young as fifty million years old. Average age estimates aside, for the last 700 million years, Venus has been subjected to significant surface volcanism, which probably is the reason that so few impact craters are visible on the planet’s surface: older impact craters have been covered over with lava flows or destroyed during episodes of catastrophic volcanic eruptions.

Venus has the densest atmosphere of any terrestrial planet. The Venusian surface pressure is equivalent to 94 bars—more than ninety times the pressure humans feel from Earth’s atmosphere (ninety bars is approximately the weight of water at one kilometer below the surface of Earth’s oceans). In addition, Venus’s atmosphere is composed of 96 percent carbon dioxide and trace amounts of nitrogen, water vapor, argon, carbon monoxide, and other gases. These clouds seal in the Sun’s heat, creating a perpetual greenhouse effect that boosts surface temperatures on Venus to around 753 kelvins. Clouds within the Venusian atmosphere are composed mainly of sulfuric acid and small amounts of hydrochloric and hydrofluoric acid. The presence of such a dense atmosphere effectively filters the numbers of potential impactors by severely decreasing their kinetic energy during transit and preventing all but the largest incoming objects from impacting the Venusian surface. Craters smaller than 1.5 kilometers appear not to exist on Venus. Many craters of this size are distinctly non-circular and form groups or clusters of craters. This phenomenon is attributed to the impactor’s becoming fragmented as it passes through the dense Venusian atmosphere and hits the surface like a shotgun blast rather than like an artillery shell.

The variety of morphologies seen in Venus’s impact craters tends to depend on their size. As the diameter of Venusian craters increases, changes in crater morphology take place and appear to correlate directly with Venus’s surface gravity and dense atmosphere. Much of the morphology of Venusian impact craters is unique. Craters larger than eleven kilometers in diameter exhibit morphological characteristics similar to comparable complex craters on other planets: a circular shape, surrounding ejecta blankets, well-defined rims, terraced walls, central ring structures or central peak complexes, and, in the largest craters, multiple-ring basins. However, smaller Venusian craters tend to display a wide variation in shape and structural complexity—the opposite of the cratering patterns seen on other terrestrial planets and rocky satellites.

The morphological divergence is most directly attributed to the greater atmospheric density of Venus. Large multi-ring basins on Venus display at least two, and sometimes three or more, rings and near-pristine morphology; are surrounded by blocky ejecta distributed in lobes or raylike patterns; and in some cases produce lavalike flows of ejecta traveling several radii from the crater. The ejecta patterns are attributed to the dense Venusian atmosphere’s slowing the travel path and speed of debris exiting the crater during impact. Many of the largest Venusian impact craters appear to have little to no topographic relief. Shallowness of these craters may be linked to Venus’s lower gravity, producing slower impact speeds, and the planet’s high surface and crustal temperatures, producing a large volume of impact-generated melt that remains in a near-molten state, allowing it to flow over long periods of time and eventually fill in the crater.

It is also suggested large Venusian impacts could trigger the subsequent volcanic or tectonic activity that disguises or eventually erases, them within the landscape. One of the more difficult aspects of studying Venusian craters is distinguishing impact craters from circular volcanic calderas. High-resolution radar images help in defining the morphology of impact craters versus volcanic features by distinguishing ejecta deposits from lava flows. Unfortunately, lavas generated from impact-triggered volcanism can complicate discerning these structures because the lava flows may infill the impact craters, making them look like calderas.

One of the most unusual phenomena associated with Venusian impact craters is parabolic halos. These halos surround about 10 percent of the youngest craters and usually expand westward. The halos are attributed to the formation of a pre-impact bow-shock wave created by the impactors producing strong turbulence as they travel through the dense Venusian atmosphere. The turbulence lifts surface dust high into the air, and then prevailing easterly winds resettle the dust after the impact. Because the halos appear unaffected by volcanic, tectonic, or atmospheric processes, the haloed craters may be no more than fifty million years old, making them useful dating horizons.

Knowledge Gained

Identifiable impact craters on Venus are rare—slightly less than one thousand, or approximately one crater per million square kilometers—and large craters and basins are uncommon. Impact craters on Venus are randomly distributed and range in size from 1.5 to 270 kilometers in diameter. Venusian impact craters are unusual in that, almost without exception, they appear to be fresh, characterized by sharp rims and well-preserved ejecta deposits. This morphology suggests that the craters have not been subjected to significant erosional, volcanic, or tectonic activity. Only about 40 percent of Venusian craters appear slightly modified, 5 percent appear embayed by volcanic deposits, and 35 percent appear modified by tectonic activity. Venus’s dense atmosphere filters out small meteors, so there is a lack of small impact craters to chip away at larger craters. This situation favors the preservation of existing large craters. Furthermore, while there is currently no hydrogeologic cycle on Venus, there is evidence to suggest that there may once have been liquid oceans of some kind on the surface.

The pristine appearance of craters on Venus makes it appear the surface is both geologically young and of a relatively uniform age. This observation has significant implications for the geologic history of Venus. While resurfacing processes have most likely removed Venusian craters older than 450 million years, the morphology of existing craters is not what is expected for a steady balance between crater formation and crater loss caused by tectonic, volcanic, or erosional processes. The unique observation is that Venusian craters of all ages look “fresh,” suggesting that most of the present surface characteristics of Venus date from the end of a global resurfacing event that ceased about 450 million years ago.

Because of their lack of weathering, Venusian craters provide a unique opportunity for scientists to study the effects atmospheric variabilities and gravity have in forming planetary surfaces. While the total number of impact craters on Venus is not comparable to those on Mars, Mercury, and the rocky satellites, they do fall into morphological and age classifications similar to those of impact craters on Earth. Crater density and morphology suggest that cratering records of Venus and Earth are similar. Because the cratering data from these two planets are complementary, they provide interpretive guidelines for researching the roles that volcanism, tectonics, and erosional processes play in planetary resurfacing.

Context

The high temperature and dense atmosphere of Venus slow incoming projectiles, destroying the smaller, high-velocity objects. This shielding effect influences the size of Venusian craters. Smaller craters appear to be absent on Venus because only large impactors can penetrate the Venusian atmosphere to reach the surface. It is estimated that as many as 98 percent of the craters between 1.5 and 35 kilometers in diameter that could have formed on Venus did not as a result of its dense atmosphere. Venus’s high temperature and dense atmosphere also impeded the emplacement of ejecta during cratering by limiting flight distance and decelerating fragments, resulting in lobate ejecta blankets that are sharply defined and make up coarse blocks. Because of Venus’s high surface temperature, rocks tend to be softer, less solid, and somewhat viscous. During an impact, these viscous rocks produce large amounts of impact melt, which works to fill the craters and make them topographically low. It is also suggested that large Venusian impacts could trigger regional tectonic or volcanic events by transferring their tremendous heat and shock energies into the planet’s thin crust.

Since the research of Venus first began, approximately 900 impact craters have been discovered on the planet's surface. The largest of these craters was named Crater Mead. It was named after American anthropologistMargaret Mead.

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