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Asteroid impact craters are among the most interesting geological structures on any planet. Many other
% R* g* G8 }3 U }" qTVBNOW 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。planets and moons in our solar system, including our own moon, are pock-marked with loads of craters. tvb now,tvbnow,bttvb* c* J& J( G E |7 L+ r
But because Earth has a protective atmosphere and is geologically active — with plate tectonics and volcanic
& Z4 h# K$ c: f4 c# }" j* y% M公仔箱論壇eruptions, mostly relatively young oceanic crust, and harsh weathering from wind and water — impact 公仔箱論壇. ~8 [; G& I3 T G
structures don’t last long and can be tough to come by.公仔箱論壇 X$ ^$ J1 y% V- R
! ]# S) u. v/ B* QBut on a few old pieces of continent, especially in arid deserts, the marks of asteroids have been preserved. 公仔箱論壇; }5 t* n$ d: m. @
One well-known example is our own Barringer crater, also known as Meteor Crater, in Arizona. The images here
' B/ X% h. q7 N* L5.39.217.76show some of the biggest, oldest and most interesting impact craters on the planet.
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q( R, |7 M' a7 x wtvb now,tvbnow,bttvbAorounga crater, pictured above and below, is one of the best preserved impact craters on Earth, thanks 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。* I8 U3 t5 w( M- O* E G0 ^9 ^
in part to its location in the Sahara Desert in Chad. The 10 mile-wide crater is probably around 350 million
8 j& N: x& P, X1 MTVBNOW 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。years old. The stripes are alternating rock ridges and sand layers, known as yardangs, caused by persistent
0 K+ b8 k8 N5 ltvb now,tvbnow,bttvbunidirectional wind. The image above was taken by astronauts in the International Space Station in July. tvb now,tvbnow,bttvb% k& X4 j/ `4 I4 t& m, f
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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The Shoemaker crater in Western Australia, formerly known as the Teague crater, was renamed in honor of the planetary 7 Y, M, W; }9 s: ^1 K% `
geologist Eugene Shoemaker for whom the Comet Shoemaker-Levy 9 is also named. The age of the crater unclear, but it
2 S. h. ^9 u4 h8 z" O! k, \公仔箱論壇could be 1.7 billion years old, which makes it the oldest known impact in Australia. The brightly colored splotches are ) @4 Q4 c8 y! N% w+ G
seasonal salt-water lakes. This image was taken by the Landsat 7 satellite.
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The Manicouagan Crater in northern Canada is one of the largest impact craters known on Earth. The impact occurred 1 _- D2 x2 D! o; l0 t4 Z
around 210 million years ago at the end of the Triassic period and may have caused a mass extinction that killed around
3 e& X- E; Y, d" ~; P" C% j- R4 P! YTVBNOW 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。60 percent of all species. Though the crater has mostly eroded, Lake Manicouagan outlines what is left of the 43-mile wide
8 t: U0 g7 G: o, bimpact structure. The asteroid that created the crater is thought to have been about three miles wide. Today the lake is a
2 W/ j' k- J1 d) @4 u Z- ^tvb now,tvbnow,bttvbreservoir and popular salmon fishing location.tvb now,tvbnow,bttvb* a ~& p- ]+ D" P3 i
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This crazy looking structure in Western Australia is called the Spider crater, for obvious reasons. Geologists determined it was - p5 I x5 `# c; O( t: \/ d) ]
an impact crater when they found shatter cones, telltale cone-shaped, grooved rocks found only around impacts. The crazy
; u4 }3 a$ z8 ~2 D: _5.39.217.76looking legs surrounding the impact are mostly due to erosion of different rock layers. The harder sandstone ridges withstood 5.39.217.768 o) S% A9 c8 c* x. p' a; o( A
the weathering of wind and water better than the softer intervening layers. The crater is somewhere between 600 million
1 Q7 O: \5 i1 L8 Q+ x+ I' \and 900 million years old and the raised area at its center is around 1,600 feet wide.公仔箱論壇7 z, N( p) y! [ b0 l3 O
' A/ K0 C7 g" h$ s, }The image above is a false-color image taken by the Advanced Spaceborne Thermal Emission and Reflection Radiometer on 4 d, C9 S' t- R5 ?; P8 a" h. D/ v$ T
NASA’s Terra. The true-color image below was captured by Taiwan’s Formost-2 satellite
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& s; X9 f; t% w) uTVBNOW 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。The Gosses Bluff crater in Australia’s Northern Territory sits between two mountain ranges. The raised circular feature in the
6 w1 M3 m9 _+ @, ]+ H; \7 ~, Q/ }center of the structure is around 2.8 miles across, but the original crater rim was probably at the edge of the surrounding
/ {8 t7 ^+ P9 I- Y, Kgrayish area. The crater was formed caused by an asteroid impact around 140 million years ago.
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Clearwater Lakes in Quebec, Canada were formed by a pair of asteroid impact craters. The impacts probably occurred
7 s P5 C& J3 `: i5 _simultaneously around 290 million years ago. The larger crater measures 22 miles across.公仔箱論壇- C# d$ v9 d, e1 r M D4 i; B
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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
- W: f: Y5 W# J+ Z. d1.5 mile-wide rim stands out as a bright ring in the lower left of the image, which was taken by a radar instrument on board
Z; a6 B% {; ]: u; v1 ~, v公仔箱論壇 the space shuttle. Irregular surfaces show up brightly and smooth areas are dark. The white splotch at the bottom of the 8 q# ?- L% P, `/ C4 @
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
2 X/ k2 [$ A1 o @公仔箱論壇ejected during the impact, the red is limestone outcrop and the green areas are mostly vegetation. The impact occurred tvb now,tvbnow,bttvb4 U, u8 I* R* Z3 ~4 C) B
around 5 million years ago.
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The Lonar crater in Maharashta, India is around 6,000 feet wide and 500 feet deep and contains a saltwater lake. 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。5 a n! o d8 f+ U/ \5 {
Scientists determined the structure was caused by an asteroid through clues such as the presence of maskelynite,
5 ]2 E2 O$ ^8 B) R* t7 S! Wa glass that is only formed by extremely high-velocity impacts. The impact occurred around 50,000 years ago.
7 h1 y7 i% q2 c7 N6 E" f5.39.217.76This image was captured by the Advanced Spaceborne Thermal Emission and Reflection Radiometer on NASA’s 3 V P% W* D8 G- \2 I- V7 F& P
Terra satellite. It is a simulated true-color imagetvb now,tvbnow,bttvb' e/ M5 }6 j* }) a3 i
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$ A* U7 v& h, L) YThe Vredefort Dome in South Africa is possibly the oldest and largest clearly visible asteroid impact on Earth. tvb now,tvbnow,bttvb, W1 K o* C" Q+ ?, t
The 155-mile crater is approximately 60 miles southwest of Johannesburg and was formed around 2 billion years ago.
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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。* f: D) o5 v: X3 P
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) [: Q# ?4 w" u9 p) QTVBNOW 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。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 B% k! W: u2 J0 b- J& Y9 za 300-mile wide asteroid would look impacting Earth. This event probably would have killed everything on the planet.
' l x( I& S) }+ g B+ M7 X1 ^2 \tvb now,tvbnow,bttvbFortunately, NASA says nothing this big is headed our way anytime soon.
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發表於 2009-12-22 08:55 AM
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