Surface of Venus - Oxford Research Encyclopedia of Planetary Science
METEORITE BOMBARDMENT AND DATING OF PLANETARY SURFACES. G. Neukum. Importance of the Determination of Age in the study of. During this time, ~80%–85% of the surface of the planet was renovated. Online Publication Date: Mar . away earlier in their planetary history and has not continued at a high level after the waning of the meteorite bombardment. . Map showing landing sites of the Soviet landers of the Venera and Vega missions. METEORITE BOMBARDMENT AND DATING OF PLANETARY SURFACES. G. Neukum. Importance of the Determination of Age in the study of the. Physical .
Pioneer Venus PV was the first orbiter that systematically observed the entire surface of Venus Table 2. These data allowed compilation of a global radar map of the planet, the spatial resolution of which was still low, about 30 km. The low resolution prevented a confident geological interpretation of features detected on the surface. Despite their low resolution, the topographic data have indicated that the hypsogram of Venus is unimodal, which is in a sharp contrast with the bimodal hypsogram of Earth Figure 5.
These differences in the global topography distribution reflect fundamental differences between Venus and Earth and are related to both the crustal structure and the dominant tectonic styles on both planets. Click to view larger Figure 5. Hypsograms of Earth dashed line and Venus solid thick line that show a fraction of the planetary surface within a specific elevation interval.
Where do meteorites come from?
The curves show a drastic difference in the distribution of global topography on both planets. The bimodal hypsogram of Earth reflects the different topographic configurations of oceanic left peak and continental right peak lithosphere. The hypsogram of Venus is strongly unimodal. If plate tectonics were to be operating on Venus, the unimodal hypsogram would be more consistent with the crust that is made of one component of about the same density.
If plate tectonics do not work on Venus, its crust could be single- or poly-component, but the unimodal hypsogram would indicate that the components are overlapping each other, as occurs on Moon. During this mission, about the upper half of the northern hemisphere of Venus was mapped at unprecedented high spatial resolution, 1—2 km Barsukov et al.
The incidence angle of the radar systems the angle between the vertical and radar beam to a great degree relates the strength of the returned signal and the scale of the surface roughness. Images taken by the Magellan radar system SAR, synthetic aperture radar showed no evidence for the structures that characterize plate tectonics on Earth Solomon et al.
The geometry of the Magellan observations enhanced the visibility of morphologically homogenous terrains and allowed their confidential delineation and the determination of their relative ages. The surface geology of Venus. The map is in equal-area Mollweide projection. Main Type of Terrains Composing the Surface of Venus The massive CO2 atmosphere of Venus has caused a strong greenhouse effect, which limits the range of possible geological processes operating on the planet.
Three geological processes are important on the planet: Only about a thousand impact craters have been detected on Venus. Thus, volcanism and tectonics have been the principal geological processes during the observable geologic history of Venus. Volcanic Landforms The dimensions of the occurrences, topographic configuration, and relative importance of features of volcanic and tectonic origin permit the subdivision of the volcanic landforms of Venus into three broad categories: Volcanic Plains This category includes vast plains-like surfaces that are either mildly deformed by tectonic structures or nondeformed Figure 7.
Click to view larger Figure 7. Examples of major volcanic plains on Venus. The brightness of the lower subunit rp1 is moderate and uniform, and the surface of the upper subunit rp2 is brighter. A narrow channel cuts the surface of unit rp1 and appears as a prominent topographic feature arrow.
Material of unit rp2 almost completely fills the channels arrow ; center of the image is at Others occur as a breccia, where fragments of stony and iron material have been cemented together by either heat or chemical reactions. Origin of Meteorites Most meteorites appear to be fragments of larger bodies called parent bodies.
These could have been small planets or large asteroids that were part of the original solar system. There are several possibilities as to where these parent bodies, or their fragments, originated. It consists of a swarm of aboutobjects called asteroids.
Asteroids are small rocky bodies with irregular shapes that have a cratered surface.
About 4, of these asteroids have been officially classified and their orbital paths are known. Once they are so classified they are given a name. The asteroids are either remnants of a planet that formed in the region between Mars and Jupiter but was later broken up by a collision with another planetary body, or are fragments that failed to accrete into a planet. The latter possibility is more likely because the total mass of the asteroids is not even equal to our moon.
Meteorites, Impacts, & Mass Extinction
It does appear that some of the asteroids are large enough to have undergone internal differentiation. Differentiation is a process that forms layering in a planetary body i. If these larger asteroids did in fact undergo differentiation, then this could explain the origin of the different types of meteorites.
Because of the shapes of the asteroids it also appears that some of them have undergone fragmentation resulting from collisions with other asteroids. Such collisions could have caused the larger bodies to be broken up into the smaller objects we observe as meteorites.Planet Formation and the Origin of Life
The Asteroids as Parent Bodies of Meteorites Much evidence suggests that the asteroids could be the parent bodies of meteorites.
The larger ones could have differentiated into a core, mantle, and crust. Fragmentation of these large bodies would then have done two things: First the fragments would explain the various types of meteorites found on Earth - the stones representing the mantle and crust of the original parent body, the irons representing the cores, and the stony irons the boundary between the core and mantle of the parent bodies.
Second, the collisions that caused the fragmentation could send the fragments into Earth-crossing orbits.
- Earth's gold and platinum arrived in a meteor shower lasting 200MILLION years
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Some of the asteroids have orbits that bring them close to Earth. These are called Amor objects. Some have orbital paths that cross the orbital path of the Earth. These are called Earth-crossing asteroids or Apollo objects. About NEOs with diameters between 1 and 8 km are known, but this is only a fraction of the total number. Many NEOs will eventually collide with the Earth. These objects have unstable orbits because they are under the gravitational influence of both the Earth and Mars.
The source of these objects is likely the asteroid belt. These orbits are not circular like those of the planets and are not necessarily within the same plane as the planets. Most comets have elliptical orbits which send them to the far outer reaches of the solar system and back toward a closer approach to the sun. As a comet approaches the sun, solar radiation generates gases from evaporation of the comet's surface.
These gases are pushed away from the comet and glow in the sun light, thus giving the comet its tail. While the outer surface of comets appear to composed of icy material like water and carbon dioxide solids, they likely contain a more rocky nucleus. Because of their eccentric orbits, many comets eventually cross the orbit of the Earth. Many meteor showers may be caused by the Earth crossing an orbit of a fragmented comet.
The collision of a cometary fragment is thought to have occurred in the Tunguska region of Siberia in The blast was about the size of a 15 megaton nuclear bomb.
It knocked down trees in an area about square miles, but did not leave a crater. Although still controversial, the general consensus among scientists is that a cometary fragment about 20 to 60 meters in diameter exploded in the Earth's atmosphere just above the Earth's surface. A similar event if it happened over a large city, would be devastating. Other Sources While the asteroid belt seems like the most likely source of meteorites, some meteorites appear to have come from other places.
Some meteorites have chemical compositions similar to samples brought back from the moon. Others are thought to have originated on Mars. These types of meteorites could have been ejected from the Moon or Mars by collisions with other asteroids, or from Mars by volcanic eruptions. Impact Events When a large object impacts the surface of the Earth, the rock at the site of the impact is deformed and some of it is ejected into the atmosphere to eventually fall back to the surface.
This results in a bowl shaped depression with a raised rim, called an Impact Crater. The size of the impact crater depends on such factors as the size and velocity of the impacting object and the angle at which it strikes the surface of the Earth. Meteorite Flux and Size Meteorite flux is the total mass of extraterrestrial objects that strike the Earth.
Much of this material is dust-sized objects called micrometeorites. The frequency at which meteorites of different sizes strike the Earth depends on the size of the objects, as shown in the graph below.
Note the similarity between this graph and the flood recurrence interval graphs we looked at in our discussion of flooding. Tons of micrometeorites strike the Earth each day.
Because of their small size, they do not usually burn up when entering the Earth's atmosphere, but instead settle slowly to the surface. Meteorites with diameters of about 1 mm strike the Earth about once every 30 seconds. Upon entering the Earth's atmosphere the friction of passage through the atmosphere generates enough heat to melt or vaporize the objects, resulting in so called shooting stars.
Meteorites of larger sizes strike the Earth less frequently. If they have a size greater than about 2 or 3 cm, they only partially melt or vaporize on passage through the atmosphere, and thus strike the surface of the Earth.
Objects with sizes greater than 1 km are considered to produce effects that would be catastrophic, because an impact of such an object would produce global effects. Larger objects would not be slowed down much by the friction associated with passage through the atmosphere, and thus would impact the Earth with high velocity.
Such a meteorite struck at Meteor Crater, Arizona the Barringer Crater about 49, years ago leaving a crater m in diameter and m deep.
The amount of energy released by an impact depends on the size of the impacting body and its velocity. For comparison, the amount of energy needed to create a nuclear winter on the Earth as a result of nuclear war is about 8, megatons, and the energy equivalent of the world's nuclear arsenal is about 60, megatons.
Cratered Surfaces Looking at the surface of the Moon, one is impressed by the fact that most of the surface features of the moon are shaped by impact craters.
The Earth is subject to more than twice the amount of impacting events than the moon because of its larger size and higher gravitational attraction. Yet, the Earth does not show a cratered surface like the moon. The reason for this is that the surface of the Earth is continually changing due to processes like erosion, weathering, tectonism, sedimentation, and volcanism.
Thus, the only craters that are evident on the Earth are either very young, very large, or occurred on stable continental areas that have not been subject to intense surface modification processes. Currently, approximately terrestrial impact structures have been identified, with the discovery rate of new structures in the range of per year. The Mechanics of Impact Cratering When a large extraterrestrial object enters the Earth's atmosphere the initial impact with the atmosphere will compress the atmosphere, sending a shock wave through the air.
Frictional heating will cause the object to heat and glow. Melting and even vaporization of the outer parts of the object will begin, but if the object is large enough, solid material will remain when it impacts the surface of the Earth.
Impacts of large meteorites have never been observed by humans. Much of our knowledge about what happens next must come from scaled experiments. As the solid object plows into the Earth, it will compress the rocks to form a depression and cause a jet of fragmented rock and dust to be expelled into the atmosphere.