Why do craters have a central peak

Lava later flowed across the low floors of the basins, giving them a darker, smoother appearance than the surrounding, brighter highlands. The dark basins can be seen by the naked eye. Scientists describe other types of craters as well:. How are large craters different than small ones? Small craters often are simple bowl-shaped depressions. The structure of large craters is more complex because they collapse, forming terraces, central peaks, central pits, or multiple rings. Very large impact craters greater than kilometers miles across are called impact basins.

What influences the size and shape of a crater? The size and shape of the crater and the amount of material excavated depends on factors such as the velocity and mass of the impacting body and the geology of the surface. The faster the incoming impactor, the larger the crater. Typically, materials from space hit Earth at about 20 kilometers slightly more than 12 miles per second.

Such a high-speed impact produces a crater that is approximately 20 times larger in diameter than the impacting object. Smaller planets have less gravitational "pull" than large planets; impactors will strike at lower speeds.

The greater the mass of the impactor, the greater the size of crater. Craters most often are circular. More elongate craters can be produced if an impactor strikes the surface at a very low angle — less than 20 degrees. How can craters be used to determine the age of a planet or moon? Scientists record the size and number of impact craters — and how eroded they are — to determine the ages and histories of different planetary surfaces.

Early in the formation of our solar system before 3. As a rule of thumb, older surfaces have been exposed to impacting bodies meteoroids, asteroids, and comets for a longer period of time than younger surfaces. Therefore, older surfaces have more impact craters. Mercury and the Moon are covered with impact craters; their surfaces are very old. Venus has fewer craters; its surface has been covered recently in the last million years!

Much of Earth's surface is recycled through plate tectonic activity and erosion , so Earth also has few craters. Why does the Moon have so many craters while Earth has so few? On Earth, impact craters are harder to recognize because of weathering and erosion of its surface. The Moon lacks water, an atmosphere, and tectonic activity, three forces that erode Earth's surface and erase all but the most recent impacts.

Essentially, the Moon's surface has not been modified since early in its history, so most of its craters are still visible. Barringer Crater Meteor Crater in Arizona, United States, is a simple crater created when a meter-wide foot-wide iron-rich meteroid struck Earth's surface about 50, years ago — a very recent event to a geologist.

The crater is about 1. Its features, such as the ejecta blanket beyond its rim, are well preserved because of the crater's youth; it has not experienced extensive erosion. Fragments of the Canyon Diablo meteorite were found inside the crater. This is why more of the crater wall is visible on the side closest to the direction of radar illumination. The direction of illumination and the angle of incidence lower the apparent circularity of the crater outline in plan view and impart a bilateral symmetry.

Flows of impact melt or lavas of impact-triggered volcanism breached the crater rim and filled the troughs in the upper-right corner of the image. These flows and the floor of the crater are radar-dark because they are smoother than the surrounding terrain, so more of the radar signal is reflected forward and away from the radar. A rougher terrain would be radar bright because more of the signal would be reflected back to the radar to produce a stronger echo.

The surrounding tesserae are rough and have large-scale slopes, which result in extremely bright returns. A chemical transition at high elevations that leads to a high dielectric constant in the rocks is thought to accentuate the high radar reflectivity [Pettengill et al. Mead crater, with a diameter of km, is the largest impact crater on Venus Figure The inner ring is thought to represent the original rim of the crater cavity, while the outer scarp is thought to be the expression of a ring fault that has downdropped the flank terrace [Schaber et al.

The crater floor has a slightly higher backscatter than the surrounding plain. The surrounding plain is covered by fine debris that decreases the return to the radar, and hence it appears darker on the image. The floor of the crater has several large cracks that show as bright lines due to radarfacing slopes. Ejecta from the crater that appear as diffuse patches surrounding the crater rim are brighter than the surrounding plain because they are rougher and have more slopes facing the radar.

At lower incidence angles, the difference in radar brightness between the ejecta and the surrounding terrain would be minimal because diffuse scattering dominates only at large incidence angles. However, at low incidence angles, quasi-specular scattering from radarfacing slopes of the ejecta will cause stronger backscatter than the flatter surrounding plain.

The topography, emissivity, reflectivity, and rms slope data sets from Magellan are very useful in determining the Mead crater is the largest impact crater on Venus, with a diameter of km. The crater has an inner and an outer ring and a small ejecta blanket surrounding the outer ring. The crater floor looks very similar in morphology to the surrounding plain. The dark vertical bands running through the image are artifacts associated with processing the SAR data.

Illumination is from the left at an incidence angle of 45 deg. The altimetry for the crater Mead is shown in Figure a. The m-contour intervals indicate an altitude above a km planetary radius. The drop in elevation from the crater rim to the center of the crater is approximately 1. This is quite shallow for a crater the size of Mead; it may be that Mead has experienced relaxation of its floor, or a large amount of material has flooded the crater floor.

The rms slopes on the crater floor are about twice as high as those on the surrounding plain 2. This indicates that the floor of the crater is rougher than the surrounding plain at the meter scale. Figure b shows the emissivity for Mead and the surrounding terrain, with a contour interval of 0. While the surrounding plain and western edge of Mead have values near 0.

Because reflectivity is generally the complement of emissivity, the highest reflectivity values for Mead occur at the place of lowest emissivities, which is in the northeast of the crater floor. This latter correlation is not seen in the SAR image of Mead, implying that some other scattering effect e.

The km-diameter crater Yablochkina has one peak ring, seen on Figure as an incomplete circle of bright hills surrounded by a darker, smoother floor. The peak ring is similar to the outer rim found on all craters, except that it is not as well developed.

Both the peak ring and the outer rim are bright because they are rough and have radar-facing slopes. These slopes must therefore be oriented perpendicular to the radar illumination, which is from the left in this Cycle 1 image. The crater wall facing the radar the right side of the crater appears compressed compared to the wall on the opposite side-the same phenomenon that occurred on the image of Cleopatra, discussed earlier.

A good estimate of the slope and depth of a crater can be calculated using the diagram shown in Figure It should be noted that these equations apply only to craters with no layover i.

In the case of Yablochkina, the values of X and Y measured from the radar image are 4. Application of the formulas in Figure results in a depth H of 1. Altimetry for the crater shows a depth of 0. While the depth of the crater varies by only 0. Hence, the depths measured by the altimeter for the smaller craters will generally be too shallow.

The crater shown in Figure has two other interesting features: a radar-dark halo and radar-bright outflow deposits. The crater and its ejecta are surrounded by a dark halo. These areas of low backscatter cross sections partially or wholly surround approximately half of the impact craters on Venus [Phillips et al. It is likely that the dark margins seen on the Magellan images, which were taken at higher incidence angles, represent smooth areas with little surface roughness at the scale of the radar wavelength.

Atmospheric shock waves produced as the meteoroid passed through the thick atmosphere may have removed wavelength-size structures from the existing terrain and pulverized the surface materials to produce these dark margins.

Alternatively, fine debris produced by the destruction of the target material or the meteoroid as it passed through the atmosphere and exploded at the surface may have been deposited before the crater formed.

In addition to dark halos, many Venusian craters have bright halos, also thought to have formed from atmospheric shock waves. Also surrounding the Yablochkina crater in many locations, but particularly to the northeast, are deposits or flows that are often brighter than the crater ejecta. These flows originate predominantly downrange from the point of the impact. Decreasing the impact angle measured from the surface appears to increase the runout flow mass [Schultz, ].

The great distances that these deposits travel and the fact that they follow the topography suggest that they consist of low viscosity material [Schaber et al, ; Asimow and Wood, ; Schultz, ]. The disruption of Yablochkina's ejecta to the east by the laminar-style runout flows and the radar brightening of the surface to the northeast led Schultz [] to suggest an impact direction from the west at an angle of at least 45 deg from the vertical.

The high backscatter of the outflow deposits suggests either a very rough surface or wavelength-size facets facing the radar, or both. The similar emissivity and reflectivity values of these deposits Radiophysical properties of Mead crater superimposed on the SAR image: a altimetry contours at m intervals-the reference altitude is km planetary radius, and the drop in elevation from the crater rim to the cen [ 83 ] ter of the crater is approximately 1.

Yablochkina, a km-diameter crater: a Magellan image- illumination is from the left at a deg incidence angle; b geologic sketch map. Profile outline and equations showing calculations for crater depth and slope. These equations are applicable only for craters where no radar layover has occurred. Smaller impactors may be broken up as they enter the Venusian atmosphere [Basilevsky et al.

Except for the smallest members of some crater clusters, no craters smaller than 3 km in diameter have been observed [Phillips et al. Figure shows an irregular crater of approximately km mean diameter. The crater is actually a cluster of four separate craters in rim contact. The noncircular rims and multiple, hummocky floors are probably the result of the breakup and dispersion of a meteoroid during its passage through the dense Venusian atmosphere; subsequently, the meteoroid fragments impacted simultaneously to create the cluster.

Meteoroids that would form craters smaller than the observed cutoff diameter of 3 km either are not able to penetrate the atmospheric column or they decelerate to velocities insufficient to form impact craters [Phillips et al. However, the shock or pressure wave created as such a meteoroid travels through the atmosphere may still have energy sufficient to deform the surface.

Figure shows three dark splotches on the plains of Venus. The impact crater in the splotch at the right indicates that the meteoroid was not completely destroyed and reached the surface to produce a crater.

The other two splotches, at near center and the extreme left, have no associated impact crater, indicating that only a shock wave disturbed the surface. Evidence that these splotches represent a deposit of material is the change in brightness of the underlying lava flows from the center of the splotches outward. The dark margin to the left has associated wind streaks, suggesting that the splotch is composed of material fine enough to be moved by the wind. This indicates that impact craters have not been significantly altered by surficial processes.

In only a few cases have craters been modified by lava flows or tectonism. Figure shows the crater Somerville, a km-diameter crater in Beta Regio, which has been cut by many fractures and faults.

The crater was split in half during the formation of a rift that is up to 20 km wide and apparently quite deep. A north-south profile through the center of this crater is visible as a result of the downdropping. Most of the central peak is visible as a bright spot in the middle of the crater. A radar-bright ejecta blanket is also visible through the fractures. A small portion of the Cluster of four craters in rim contact.

A small projectile broke up in the atmosphere to form four smaller impactors that struck nearly simultaneously to form this crater cluster. Illumination is from the left at an incidence angle of 38 deg. While the majority of large impact craters on Venus have floors that are radar-dark probably due to flooding of the crater floor by lavas from below after the crater was produced , craters that have been modified by volcanism not associated with the impact process are rare on Venus.

One such rarity is Alcott, a km-diameter crater extensively flooded by lava Figure A remnant of rough, radar-bright radial ejecta is preserved outside the crater's southeast rim.

A type of complex crater. The central peak is the simplest interior feature of complex craters. Many central peak craters have scalloped rims, terraced inner walls, and hummocky floors, on both rocky and icy bodies. These are inferred to represent failure by slumping and mass wasting of materials onto the floor Greeley et al.

The central peak itself can be a simple peak at or near the center of the crater floor, or can be composed of multiple uplift segments.

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Authors Authors and affiliations Veronica J. Reference work entry First Online: 20 November How to cite. Definition Complex crater with a single central uplift, a tight cluster of peaks, or a tightly spaced ring-like arrangement of peaks e.