彗星的pristine relics
The long-held perspective that comets are pristine remnants from the formation of the solar system has evolved from the prevailing views of 30 years ago, finds planetary scientist Dr.S. Alan Stern in a paper published in the journal Nature.Its fair to say that a sea change has taken place, says Stern, director of the Space Studies Department in the SwRI Space Science and Engineering Division. We used to consider comets as wholly unchanged relics that had been stored ever since the era of solar system formation in a distant, cold, timeless deep freeze called the Oort cloud. We now appreciate that a variety of processes slowly modify comets during their storage there, he says. As a result,its become clear that the Oort cloud and its cousin the Kuiper Belt are not such perfect deep freezes.
The first evolutionary process to be recognized as affecting comets during their long storage was radiation damage, followed by the discovery that sandblasting from dust grains in the interstellar medium plays an important role. Next, researchers theorized that comets in the Oort cloud are heated to scientifically significant temperatures by passing stars and supernovae,says Stern. More recently, researchers are finding that comets in the Kuiper Belt are heavily damaged by collisions.It also now seems inevitable that most comets from the Kuiper Belt, though constructed of ancient material, cannot themselves be ancient -- instead they must be recently created chips off larger Kuiper Belt Objects, formed as a result of violent impacts, says Stern. This is truly a paradigm shift. Many of the short-period comets we see arent even ancient!
The classical view that comets do not evolve while they are stored far from the sun in the Oort cloud and Kuiper Belt began to change as far back as the 1970s, but the pace of discoveries about the way comets evolve picked up considerably in the 1980s and 1990s.As a result of these findings, astronomers now better appreciate that comets, though still the most pristine bodies known, have been modified in several important ways since their birth,says Stern.The realization that comets evolve during their long storage in the Oort cloud and Kuiper Belt provides insight and context to more confidently evaluate the results of astronomical and space mission observations of comets. So, too, it suggests that cometary sample return 16 missions now on the drawing board for NASA should employ relatively deep subsurface sampling if truly pristine, ancient material is to be collected.
七年级英语 Unit 7 How much are these pants教案
Where’s the post office
Unit 11 What do you think of game shows
Unit 1 Have we got any oranges 教案
What time do you go to school
Where’s your pen pal from
Starter Moudle 2
新目标初一英语下册Unit 7 What does he look like 导学案
七年级英语上册Unit 1-2复习提纲
Could you help me please
新目标初一英语下册Unit 2 Where is the post office 导学案
Unit 3 Language in use学案
七年级英语上册unit5-6复习教案
Unit 8 When is your birthday
Unit 10 Can you play the guitar教学设计
牛津7B Unit 1导学案1
Unit 1 The Dragon Boat Festival教学案
七年级英语上册全册教案(外研版)
七年级英语What time do you go to school教案
Unit 1 Hello! Nice to meet you !
I’m watching TV教案
七年级英语What does he look like教案
仁爱版Unit 1 What class are you in Section 学案
七年级英语How was your weekend教案
Unit 12 Don’t eat in class
七年级英语上册Unit 1词组和句子
七年级英语上册Unit 4复习提纲
七年级英语下册Unit 6学案
Unit 12 A birthday party 教案
Could you please tell me where the restrooms are教案
| 不限 |
| 英语教案 |
| 英语课件 |
| 英语试题 |
| 不限 |
| 不限 |
| 上册 |
| 下册 |
| 不限 |