Smaller craters punch through the ejecta to reveal a darker substrate. Fresh young impacts on the Moon often display magnificent ejecta blankets so called because they "blanket" the surrounding terrain. Ejecta is unevenly distributed, which gives rise to its interfingered appearance. Since space weathering tends to lower the albedo of material on an airless planet, the relative brightness of this ejecta blanket speaks to the young age of the parent crater.
In this case, the parent crater is just to the south of the opening image, and can be seen in the context image. But what is providing the small circular patches of dark material? It is now understood that most material is ejected at an angle of about 45 degrees. Data from multiple explosion cratering experiments black labels and four impact craters white labels indicate that the mass of the largest ejected block mb scales with the total mass of ejected material Me. The inset illustrates that the mass of the largest ejected block also scales with crater diameter.
Blocks of rock ejected from the 1. Ballistic Ejection of Boulders at Meteor Crater. Approximately million tons of rock were excavated to produce Barringer Meteorite Crater popularly known as Meteor Crater , which is the best preserved impact crater in the world. The impact processes that carved the crater blanketed the surrounding landscape in a matter of seconds with high-velocity debris. To illustrate that process, a team of graduate students measured the locations of several large boulders around the crater and calculated the ballistic trajectories of that material from where it was launched inside the crater to where it landed beyond the crater rim.
One of the largest blocks visible near the crater rim is called Monument Rock or House Rock. Blocks of rock that landed farther from the crater rim had longer flight times and hit the ground with higher speeds. The time-of-flights and re-impact speeds of several boulders are illustrated here in an oblique aerial view of the 1. The data indicate the Meteor Crater impact occurred quickly and deposited debris in a violent, ground-shaking crash.
In larger impact events, like the one that produced the 24 km diameter Ries Crater in Germany, debris hits the ground with such fast speeds that it erodes the underlying surface and creates a mixture of ejected and local rock. That process is called ballistic sedimentation.
Cumulative-size frequency distributions for particles in a variety of volcanic and sedimentary deposits modified after Greeley and Iversen, The data for the Heimaey scoria and Ukinrek maars are courtesy of S.
The data for the sands and loess are from Krumbein and Sloss Cumulative-size frequency distributions for particles in deposits at Meteor Crater and in an impact experiment. The Meteor Crater data were measured in three types of deposits: i a mixed breccia containing shocked Coconino sandstone, Kaibab dolomite, Moenkopi siltstone, and meteoritic material on the interior crater wall, which is interpreted to be a fallback breccia; ii a monomict breccia of Coconino sandstone ejected from the crater and deposited in the ejecta blanket; and iii a polymict breccia of Coconino sandstone, Kaibab dolomite, and Moenkopi siltstone that occurs in the ejecta blanket.
Those data are compared with those of an impacted basalt that was experimentally disrupted by Don Gault and others Cumulative-size frequency distributions for particles in a variety of volcanic and sedimentary deposits modified after Greeley and Iversen, , integrated with data for impact crater deposits.
Finally, the floor of the crater rebounds, forming the central peak. Three kinds of deposits can be found around a crater.
One kind is the lines of ejecta thrown out along ballistic paths that are lines of secondary craters. On the Moon, these form the bright rays that extend for 10 to 30 crater diameters. A second type occurs when a continuous blanket of ejecta extends outward one to two crater diameters: such a covering formed by movement of the ejected material along the surface is the so-called "base surge.
Molten ejecta can flow down the interior [ ] walls of an old crater in its path and form a puddle on the crater floor or flow down the outside sloping rim of the crater. The third type of deposit is the lines of ejecta radiating from the central peak. These materials are the last to be ejected from the crater. Craters fall into three groups.
First, there are primary craters, which are generally randomly placed. Occasionally cosmic debris has struck the Earth in long lines, forming features such as the Campo de Cielo line of meteorite craters in South America; such an alinement of primary craters could occur on the Moon.
Second, there are radiating and looped patterns of smaller craters surrounding the larger primary impact crater. These secondary craters in turn may have fans of ejecta thrown out from them in the direction away from the primary crater.
Third, there are craters of internal volcanic origin. These have a different form, are often alined along fractures, and have ejecta blankets of different style and form from those of impact craters. Impacted ejecta has many blocks and forms very hummocky deposits; volcanic ejecta is fine "rained and usually smooth. The far side, even more than the near, presents a tortured record of the bombardment suffered by the Moon throughout its history.
This scene exemplifies the relentless attack of impacting objects from space and from the lunar surface that has characterized most of lunar history. The craters in this far-side area come in various shapes, sizes, and degrees of degradation attesting to a variety of formative processes, energies of formation, and ages. Each individual circular crater was probably produced by the impact of a body from interplanetary space- the larger the crater, the higher the energy; that is, the larger the body, or the greater its velocity upon impact.
The first and largest such impact erased all earlier features and produced the crater that fills most of the scene, Gagarin, km in diameter rim crest outlined.
A series of smaller craters followed, starting with crater A 46 km , itself heavily cratered, and ending with the sharp funnel-shaped craters and crater C 14 km. The young age of crater C is demonstrated by the sharpness of its rim crest and its halo of extremely fine, fresh ejecta and secondary craters. During the rain of objects from space, clots of lunar material ejected from impact craters outside this area landed here to form irregular secondary craters. Examples D are the elongate partly filled crater near the upper right corner and the elongate but deeper craters in the upper left.
Conceivably, however, some irregular craters were formed by volcanism, a process that at one time was widely believed to be the cause of most irregular craters on the Moon.
At some time late in the history of the region, an even more distant impact hurled a loose cluster of debris to form the group of sharp, circular high-energy craters in the left center of the picture. In this section of the book other primary and secondary craters will be illustrated as will some possible volcanic craters and some craters whose properties are too obscured to reveal their origin-like crater B and its twin Iying inside the older crater A in this picture.
Slightly less than 3 km in diameter, it is located on the west flank of the large crater Gagarin on the lunar far side. The youthfulness of this small crater is illustrated by its sharply defined rim crest; by its bright continuous blanket of ejecta extending outward for 1 to 2 crater diameters; and, most particularly, by its exceptionally welldeveloped radial pattern of bright rays.
The rays consist of narrow, diffuse streaks and shorter bright spots. They are formed when ejected material from the impact site is redeposited as projectiles in the form of discrete blocks and clusters of disaggregated debris. The brightening is probably caused by the shock effects on the rock that was excavated from the impact site and deposited onto the older, darker surface materials. The western rim of Gagarin is marked by a dashed line.
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