TVBNOW 含有熱門話題，最新最快電視，軟體，遊戲，電影，動漫及日常生活及興趣交流等資訊。; `( o( d( [ y& ~6 z% _# CAsteroid impact craters are among the most interesting geological structures on any planet. Many other
planets and moons in our solar system, including our own moon, are pock-marked with loads of craters. 公仔箱論壇! C# C2 m# X& Z
But because Earth has a protective atmosphere and is geologically active — with plate tectonics and volcanic + t. V# {+ Z$ {+ V; h) ~7 Q
eruptions, mostly relatively young oceanic crust, and harsh weathering from wind and water — impact
structures don’t last long and can be tough to come by.
But on a few old pieces of continent, especially in arid deserts, the marks of asteroids have been preserved.
One well-known example is our own Barringer crater, also known as Meteor Crater, in Arizona. The images here
show some of the biggest, oldest and most interesting impact craters on the planet.
Aorounga crater, pictured above and below, is one of the best preserved impact craters on Earth, thanks
in part to its location in the Sahara Desert in Chad. The 10 mile-wide crater is probably around 350 million
years old. The stripes are alternating rock ridges and sand layers, known as yardangs, caused by persistent
unidirectional wind. The image above was taken by astronauts in the International Space Station in July. tvb now,tvbnow,bttvb6 H/ ?: H) F* d* R: Q- b
In the radar image below, taken from the space shuttle in 1994, reveals that Aorounga may be one of two or three craters.
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' R# l' ^4 I: g# a& J; XThe Shoemaker crater in Western Australia, formerly known as the Teague crater, was renamed in honor of the planetary
geologist Eugene Shoemaker for whom the Comet Shoemaker-Levy 9 is also named. The age of the crater unclear, but it
could be 1.7 billion years old, which makes it the oldest known impact in Australia. The brightly colored splotches are 5.39.217.768 k. R7 @0 r/ F: @5 s/ W
seasonal salt-water lakes. This image was taken by the Landsat 7 satellite.tvb now,tvbnow,bttvb; Y. f3 e* {2 S7 V0 r
( L2 d6 a% b. B& j4 ]The Manicouagan Crater in northern Canada is one of the largest impact craters known on Earth. The impact occurred
around 210 million years ago at the end of the Triassic period and may have caused a mass extinction that killed around
60 percent of all species. Though the crater has mostly eroded, Lake Manicouagan outlines what is left of the 43-mile wide 6 T+ w! Z) t8 s* e# w
impact structure. The asteroid that created the crater is thought to have been about three miles wide. Today the lake is a
reservoir and popular salmon fishing location.
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This crazy looking structure in Western Australia is called the Spider crater, for obvious reasons. Geologists determined it was TVBNOW 含有熱門話題，最新最快電視，軟體，遊戲，電影，動漫及日常生活及興趣交流等資訊。/ H- X, K8 h, n8 E% v' c) B3 y% p8 V- Z
an impact crater when they found shatter cones, telltale cone-shaped, grooved rocks found only around impacts. The crazy
looking legs surrounding the impact are mostly due to erosion of different rock layers. The harder sandstone ridges withstood $ l* D8 S6 n, `+ B+ r" W9 D; H
the weathering of wind and water better than the softer intervening layers. The crater is somewhere between 600 million 公仔箱論壇/ U$ s6 \& j! B I3 z$ K* k7 w7 B
and 900 million years old and the raised area at its center is around 1,600 feet wide.TVBNOW 含有熱門話題，最新最快電視，軟體，遊戲，電影，動漫及日常生活及興趣交流等資訊。7 Z& F5 I8 G; |( f+ D6 o
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The image above is a false-color image taken by the Advanced Spaceborne Thermal Emission and Reflection Radiometer on tvb now,tvbnow,bttvb( i) j5 Z. x6 ^( P2 s$ p: N
NASA’s Terra. The true-color image below was captured by Taiwan’s Formost-2 satellite
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The Gosses Bluff crater in Australia’s Northern Territory sits between two mountain ranges. The raised circular feature in the 公仔箱論壇2 n9 J+ I, @ c: D6 }& B
center of the structure is around 2.8 miles across, but the original crater rim was probably at the edge of the surrounding
grayish area. The crater was formed caused by an asteroid impact around 140 million years ago.' P8 u8 K7 p) f% \* o
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Clearwater Lakes in Quebec, Canada were formed by a pair of asteroid impact craters. The impacts probably occurred
simultaneously around 290 million years ago. The larger crater measures 22 miles across.
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The Roter Kamm crater in Namibia is hard to see in visible light, but shows up more clearly with imaging radar. The crater’s
1.5 mile-wide rim stands out as a bright ring in the lower left of the image, which was taken by a radar instrument on boardTVBNOW 含有熱門話題，最新最快電視，軟體，遊戲，電影，動漫及日常生活及興趣交流等資訊。0 b( F5 }1 b+ N; T9 ?' f
the space shuttle. Irregular surfaces show up brightly and smooth areas are dark. The white splotch at the bottom of the
photo is a rocky hill, the darkest areas are wind-blown sand dunes, the blue around the crater’s rim may be material that was
ejected during the impact, the red is limestone outcrop and the green areas are mostly vegetation. The impact occurred TVBNOW 含有熱門話題，最新最快電視，軟體，遊戲，電影，動漫及日常生活及興趣交流等資訊。* ?4 a: Y' v7 n9 H W5 r+ W
around 5 million years ago.
The Lonar crater in Maharashta, India is around 6,000 feet wide and 500 feet deep and contains a saltwater lake. 公仔箱論壇3 A, i3 g% ~# }! {1 w7 S
Scientists determined the structure was caused by an asteroid through clues such as the presence of maskelynite,
a glass that is only formed by extremely high-velocity impacts. The impact occurred around 50,000 years ago. $ p6 h# j+ A' Z. y6 k, x( p$ f
This image was captured by the Advanced Spaceborne Thermal Emission and Reflection Radiometer on NASA’s tvb now,tvbnow,bttvb# O3 A5 e. g5 }9 y) J; Y
Terra satellite. It is a simulated true-color image4 D& m0 ~* |3 x7 Q( \( W8 w9 c
The Vredefort Dome in South Africa is possibly the oldest and largest clearly visible asteroid impact on Earth. tvb now,tvbnow,bttvb1 E' i2 r2 x; d* N0 Y
The 155-mile crater is approximately 60 miles southwest of Johannesburg and was formed around 2 billion years ago.
tvb now,tvbnow,bttvb7 N7 f1 U/ G# l2 G' Z. B4 F0 V: T' n) v, ^Artist’s rendering of a space object or phenomenon, so we couldn’t resist this beauty. This is one man’s idea of what
a 300-mile wide asteroid would look impacting Earth. This event probably would have killed everything on the planet. 公仔箱論壇" q$ ]5 N) @# S. }
Fortunately, NASA says nothing this big is headed our way anytime soon.
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