穀神星：修订间差异

编辑链接
本页使用了标题或全文手工转换
维基百科，自由的百科全书
交互式浏览历史
←上一版本下一版本→
删除的内容 添加的内容
可视化wikitext
Escarbot（留言 | 贡献）
170,308次编辑
小 機器人 修改: ml:സീറീസ്
无编辑摘要
第31行： 第31行：
| albedo=0.113
| albedo=0.113
| temperature=約 230 K}}
| temperature=約 230 K}}
'''穀神星'''（Ceres），由[[意大利]]人[[朱塞普·皮亞齊|皮亞齊]]發現，並於[[1801年]][[1月1日]]公佈，是[[小行星帶]]中最早發現與最大最重的天體，曾是「四大小行星」之首，現被歸類為[[矮行星]]。


'''穀神星'''（Ceres）或'''小行星1'''是[[太陽系]]中最小的、也是唯一一顆位於[[小行星帶]]的[[矮行星]]。由[[意大利]]人[[朱塞普•皮亞齊|皮亞齊]]發現，並於[[1801年]][[1月1日]]公佈。<ref name="hoskin"/>
==命名==

該天體最初命名為Ceres Ferdinandea，其名字取自[[罗马神話]]中的穀物女神[[刻瑞斯]]及[[西西里]]王國的斐迪南國王，但後者的名字不為其他國家接受，因此沒再使用。另外，該天體在[[德國]]曾被稱為「[[赫拉]]」，但為時不長。
穀神星的直徑約有950公里，是[[小行星帶]]之中最大最重的物體，並佔了小行星帶的三分之一質量。<ref name=Michalak2000/>近期的發現表明它是近球體的，並不像[[重力]]更小的物體有著不規則形狀。<ref name=Li2006/>穀神星表面可能由冰和各種含[[氫]]礦物如[[粘土]]。<ref name=Rivkin2006/>Ceres appears to be differentiated into a [[Rock (geology)|rock]]y [[core (geology)|core]] and ice [[mantle (geology)|mantle]].<ref name=Thomas2005/>它可能會擁有一個液態水[[海洋]]，使它成爲一個尋找[[地外生物]]的重要目標。<ref name=Moomaw/>穀神星也有可能存在著一層包含水氣的稀薄大氣層。<ref name=Ahearn1992/>

Ceres' [[apparent magnitude]] ranges from 6.7 to 9.3, hence at its brightest is still too dim to be seen with the naked eye.<ref name="Pasachoff1983"/> On [[September 27]], [[2007]], [[NASA]] launched the [[Dawn Mission]] space probe to explore [[4 Vesta|Vesta]] (2011-2012) and Ceres (2015).<ref name=Russel2006/>


==發現==
==發現==
在[[火星]]和[[木星]]之間可能有一顆未知行星的理論最先由[[Johann Elert Bode]]於1768年提出。<ref name="hoskin"/>他主要是根據[[提丟斯-波得定則]]做出論點，提丟斯-波得定則現在已不被使用，原先由[[戴維•提丟斯]]於1766年提出。<ref name=Hogg1948>{{cite journal|last=Hogg|first=Helen Sawyer|title=The Titius-Bode Law and the Discovery of Ceres|journal=Journal of the Royal Astronomical Society of Canada|volume=242|pages=241-246|year=1948| url=http://adsabs.harvard.edu/abs/1948JRASC..42..241S|accessdate=2007-12-09}}</ref><ref name="hoskin"/>根據定則，這顆行星的軸心應該接近2.8 [[天文單位|AU]]。<ref name=Hogg1948/>[[威廉•赫歇爾]]在1781年發現了[[天王星]]<ref name="hoskin"/>使人們更加相信這個定則。而在1800年，24位有經驗的天文學家一同合作，並對未知行星作出有條理的搜尋工作。<ref name="hoskin"/><ref name=Hogg1948/>這個團隊由[[Franz Xaver, Baron von Zach|Franz Xaver von Zach]]領導。雖然他們並沒有發現穀神星，但是他們後來發現了另外幾個大[[小行星]]。<ref name=Hogg1948/>
在發現小行星之前，[[朱塞普·皮亞齊|皮亞齊]]原是找尋Francis Wollaston的恆星列表中所記載的Mayer 87星，但他在表中所述的位置找不到該星。之後他找到一顆會移動的星，最初他認為這是顆[[彗星]]。
[[Image:Piazzi Cerere.jpg|thumb|left|200px|Piazzi's Book ''"Della scoperta del nuovo pianeta Cerere Ferdinandea"'' outlining the discovery of Ceres]]

穀神星終於在[[1801年]][[1月1日]]被[[朱塞普•皮亞齊]]發現，當時他正在搜尋一顆由[[Francis Wollaston (1737-1815)|Francis Wollaston]]列爲Mayer 87的恆星，因爲這顆恆星並不在[[Tobias Mayer|Mayer's]]的恆星列表中所述的位置上。<ref name="hoskin">{{cite web|last=Hoskin|first=Michael|date=June 26, 1992|url=http://www.astropa.unipa.it/HISTORY/hoskin.html|title=Bodes' Law and the Discovery of Ceres|publisher=Observatorio Astronomico di Palermo "Giuseppe S. Vaiana"|accessdate=2007-07-05 }}</ref>他並沒有發現恆星，而是一顆移動的星體，他當初還以爲是一顆[[彗星]]。<ref name=Forbes1971>{{cite journal|last=Forbes|first=Eric G.|title=Gauss and the Discovery of Ceres|journal=Journal for the History of Astronomy|volume=2|pages=195-199|year=1971| url=http://adsabs.harvard.edu/abs/1971JHA.....2..195F|accessdate= 2007-12-08}}</ref>皮亞齊總共觀測了穀神星24次， 直到[[2月11日]]，他患了病而暫停了他的觀測。他把發現於[[1801年]][[1月24日]]在他寫給和他一起合作的天文學家們（其中包括[[米蘭]]的[[Barnaba Oriani]]）的信裏面發佈。他描述穀神星為一顆彗星但是“由於它的行動如此緩慢和規則，我曾經數次覺得它應該是一些比彗星要好的東西”（Since its movement is so slow and rather uniform, it has occurred to me several times that it might be something better than a comet）。<ref name="hoskin" />在4月，皮亞齊在[[巴黎]]把所有觀測記錄寄給了Oriani、[[Johann Elert Bode|Bode]]和[[Jérôme Lalande|Lalande]]。這些資料發佈於1801年9月的''Monatliche Correspondenz''中。<ref name=Forbes1971/>

不久後，穀神星的位置明顯地發生了變化，主要由於地球的軌道運動。然後，它又運行得太接近太陽光輝，因此一直到年尾其他的天文學家都不能確認皮亞齊的觀測結果。不過經過這麽長的時間，人們已經很難預測它的準確位置。爲了找回穀神星，當時只有24歲的[[高斯]]研究出了一種有效率的方法預計它的軌道。<ref name=Forbes1971/>在短短的幾個星期以内，他準確地預測了它的路徑，並把結果寄給了[[Franz Xaver, Baron von Zach]]，''Monatliche Correspondenz''的編輯。在[[1801年]][[12月31日]]，von Zach和[[奧伯斯]]在預測位置的附近成功找回了穀神星。<ref name=Forbes1971/>

==命名==
皮亞齊最初建議命名為''Ceres Ferdinandea''，來源於[[羅馬神話]]中的穀物女神[[刻瑞斯]]及[[西西里]]王國的斐迪南國王。<ref name="hoskin"/><ref name=Forbes1971/>但“Ferdinandea”不為其他國家接受，因此沒再使用。另外，該天體在[[德國]]曾被稱為「[[赫拉]]」，但為時不長。<ref>{{cite book|author=Foderà Serio, G.; Manara, A.; Sicoli, P.|editor=W.F. Bottke Jr., A. Cellino, P. Paolicchi, and R.P. Binzel|year=2002 |chapter=Giuseppe Piazzi and the Discovery of Ceres|title=Asteroids III|publisher=University of Arizona Press|pages=17-24|location=Tucson, Arizona| url=http://www.lpi.usra.edu/books/AsteroidsIII/pdf/3027.pdf|format=PDF }}</ref>在希臘，它被稱爲Δήμητρα（[[Demeter]]），是Ceres同等意思的希臘文。在英文裏，Demeter是另一顆小行星（[[小行星1108]]）的名字。由於這個名字並不常用，there is no consensus as to the proper adjectival form of the name, although the [[Nonce word|nonce forms]] ''Cerian'' and ''Cerean''<ref name=Moomaw>{{cite web|author=Bruce Moomaw|title=Ceres As An Abode Of Life| date= [[July 2]], [[2007]]|url=http://www.spaceblogger.com/reports/Ceres_As_An_Abode_Of_Life_999.html|accessdate=2007-11-06|publisher=spaceblooger.com|date=2007-07-02}}</ref> have been used in fiction。在語法上，''Cererean''會是正確的，來源於它的[[拉丁文]][[genitive]]''Cereris''。<ref>{{cite book | last = Simpson | first = D.P. | title = Cassell's Latin Dictionary | publisher = Cassell Ltd. | date = 1979 | edition = 5 | location = London | pages = 883 | id = ISBN 0-304-52257-0|accessdate=2007-12-08}}</ref>穀神星的[[天文學符號|符號]]是一個[[鐮刀]]，（[[Image:Ceres symbol.svg|20px|Sickle variant symbol of Ceres]]），相似於[[金星]]的符號（[[Image:Venus symbol.svg|22px|金星的天文符號]]）which is the female gender symbol and Venus' hand mirror。<ref name=Forbes1971/><ref>{{cite journal|last=Gould|first=B.A.|title=On the symbolic notation of the asteroids|journal=Astronomical Journal|year=1852|volume=2|issue=34|pages=80| url=http://adsabs.harvard.edu/abs/1852AJ......2...80G| accessdate=2007-07-05}}</ref>[[元素]][[鈰]]的拉丁名稱Cerium是以穀神星命名的。<ref>{{cite web|author=Staff|url=http://www.webelements.com/webelements/elements/text/Ce/hist.html|title =Cerium: historical information|publisher=Adaptive Optics| accessdate = 2007-04-27}}</ref>元素[[鈀]]本來也以穀神星命名，可是它的發現者在鈰被命名後將之改以[[智神星]]命名。<ref>{{cite web| url=http://alchemy.chem.uwm.edu/amalgamator/features/feat2003/features.html#yag|date=[[October 30]], [[2003]]|title=Amalgamator Features 2003: 200 Years Ago|accessdate=2006-08-21}}</ref>

==地位==
[[Image:Ceres Earth Moon Comparison.png|200px|thumb|left|Ceres, the Moon and the Earth.]]
穀神星的歸類已經被變更過很多次，並且都是一些天文學家們的意見不和造成的。[[Johann Elert Bode]]相信穀神星就是那顆在[[火星]]和[[木星]]之間、距離太陽4.19億公里（2.8 [[天文單位|AU]]）的“失蹤的[[行星]]”<ref name="hoskin"/>穀神星被賦予一個行星的符號，在更多小行星被發現之前，穀神星（還有[[智神星]]、[[婚神星]]和[[灶神星]]）有將近半個世紀都以行星之名列在天文學書表之中。<ref name="hoskin"/><ref name=Forbes1971/>

然而當其他的天體陸續在同一範圍内被發現，人們慢慢了解到穀神星只是許許多多相似類型天體的第一位而已。<ref name="hoskin"/>Sir [[William Herschel]] coined in 1802 the term ''asteroid'' ("star-like") for such bodies, <ref name=Hilton>{{cite web| url=http://aa.usno.navy.mil/faq/docs/minorplanets.php|first=Dr.J.L.|last=Hilton|authorlink=James L. Hilton|title=When Did the Asteroids Become Minor Planets?|year=[[September 17]] [[2001]]|accessdate=2006-08-16}}</ref> writing "they resemble small stars so much as hardly to be distinguished from them, even by very good telescopes"。<ref>{{cite web| url=http://links.jstor.org/sici?sici=0261-0523%281802%2992%3C213%3AOOTTLD%3E2.0.CO%3B2-R|first=William|last=Herschel| title=''Observations on the two lately discovered celestial Bodies.''|year=[[May 6]] [[1802]]}}</ref>作爲第一顆被發現的小行星，它以現代[[小行星編號]]系統被列爲小行星1穀神星<ref name=Hilton/>

在2006年關於[[冥王星]]到底是不是“行星”的辯論引發了穀神星是否應被歸進行星類別的問題。<ref>{{cite web|last=Battersby|first=Stephen|date=[[August 16]], [[2006]]|url=http://space.newscientist.com/article/dn9762|title =Planet debate: Proposed new definitions|publisher=New Scientist|accessdate = 2007-04-27}}</ref><ref>{{cite news|first=Steve| last=Connor|title=Solar system to welcome three new planets|publisher=NZ Herald|date=[[August 16]], [[2006]]
|url=http://www.nzherald.co.nz/section/story.cfm?c_id=5&ObjectID=10396493|accessdate=2007-04-27}}</ref>在[[國際天文學聯盟]][[行星定義|定義行星]]之前，一個行星會被定義為“一個天體符合：(a) 有足夠的質量，能以自身的重力克服剛體力，因此能呈現流體靜力平衡的形狀（接近圓球體）；(b)在軌道上圍繞一顆[[恆星]]公轉，而且不是一顆恆星或[[衛星]]。“<ref>{{cite web|author=Owen Gingerich ''et al''.|date =[[August 16]], [[2006]]|url=http://www.iau.org/iau0601.424.0.html|title =The IAU draft definition of "Planet" and "Plutons"|publisher =IAU|accessdate = 2007-04-27}}</ref>若然依照這種定義，穀神星將會是從太陽按次序的第五顆行星。<ref>{{cite web|author=Staff Writers|date =[[August 16]], [[2006]]| url=http://www.spacedaily.com/reports/The_IAU_Draft_Definition_Of_Planets_And_Plutons_999.html|title= The IAU Draft Definition Of Planets And Plutons|publisher= SpaceDaily|accessdate=2007-04-27}}</ref>但是，這並沒有被承認，並且同時一個新定義於[[2006年]][[8月24日]]生效：一顆行星符合"(a)和(b)，而且必須將鄰近軌道上的天體清除。”根據這樣的定義，穀神星就不是一顆行星（因爲它在[[小行星帶]]上的軌道佈滿了千千萬萬個小行星），並且現在歸為“[[矮行星]]”（同時還有冥王星和[[鬩神星]]）。它是否還是小行星並未説明。<ref>{{cite web |url=http://www.iau.org/Q_A2.415.0.html|title=Question and answers 2|publisher=IAU|accessdate=2008-01-31}} — Q: What is Ceres? "Ceres is (or now we can say it was)" - but note it then talks about "other asteroids" crossing Ceres' path.</ref>天文學裏存在著[[小行星帶彗星]]等的雙重身份天體，而且屬於一顆矮行星也不代表不能擁有雙重身份。<ref>{{cite web |url=http://cfa-www.harvard.edu/mpec/K06/K06R19.html |title=MPEC 2006-R19 : EDITORIAL NOTICE |author=T. B. Spahr |publisher=Minor Planet Center |date=[[2006-09-07]] |quote=the numbering of "dwarf planets" does not preclude their having dual designations in possible separate catalogues of such bodies. |accessdate=2008-01-31}}</ref>

==物理特性==
[[Image:Moon and Asteroids 1 to 10.svg|thumb|left|Size comparison: the first 10 minor planets profiled against Earth's [[Moon]]. Ceres is far left.]]
[[Image:Ceres Rotation.jpg|thumb|200px|[[Hubble Space Telescope]] images of Ceres, taken in 2003/4 with a resolution of about 30&nbsp;km. The nature of the bright spot is uncertain.]]

穀神星是迄今[[小行星帶]]中最大的天體。<ref name=Rivkin2006/>[[古柏帶]]中包含著更大的天體，包括[[冥王星]]、[[創神星]]和[[厄耳枯斯]]，而在[[黃道離散天體]]中的[[鬩神星]]是所有小行星中最大的。<ref name="Stansberry 2007">{{cite journal|last=Stansberry|first=J.| coauthors=Grundy, W.; Brown, M.; et.al.|title=Physical Properties of Kuiper Belt and Centaur Objects: Constraints from Spitzer Space Telescope|url=http://arxiv.org/abs/astro-ph/0702538v1|date=[[5 November]] [[2007]]|accessdate=2007-12-08}}</ref>

穀神星的質量的測量可以由觀測它對其它小行星的影響得知。<ref name=Michalak2000/>不同觀測者的不同結果會有少許的差異。<ref name=Kovacevic2007>{{cite journal|last= Kovacevic|first=A.|coauthors=Kuzmanoski, M.|title=A New Determination of the Mass of (1) Ceres|journal=Earth, Moon, and Planets|volume=100|pages=117-123|year=2007|doi=10.1007/s11038-006-9124-4|url=http://adsabs.harvard.edu/abs/2007EM&P..100..117K|accessdate=2007-12-08}}</ref> The average of the three most recent values as of 2008 is 9.4{{e|20}}&nbsp;kg.<ref name="Carry2007"/>With this mass Ceres comprises about a third of the estimated total 3.0&nbsp;±&nbsp;0.2 {{e|21}}&nbsp;kg mass of the asteroids in the solar system,<ref name="Pitjeva2005" /> together totalling about 4% of the mass of the [[Moon]]. Ceres' size and mass are sufficient to give it a nearly spherical shape.<ref name="Thomas2005"/> That is, it is close to [[hydrostatic equilibrium]]. In contrast, other large asteroids such as [[2 Pallas]],<ref name=Carry2007>{{cite web|last=Carry|first=B.|coauthors=Kaasalainen, M.; Dumas, C.; et.al.|title=Asteroid 2 Pallas Physical Properties from Near-Infrared High-Angular Resolution Imagery|journal=ISO|accessdate=2007-11-05|year=2007|publisher= ESO Planetary Group: Journal Club|url= http://www.sc.eso.org/santiago/science/PlanetaryGroup/journal_club/slides/ESO.JournalClub-2007.08.14-BenoitCARRY.pdf| format=pdf}}</ref> [[3 Juno]],<ref name=Kaasalainen2002>{{cite journal|last=Kaasalainen|first=M.|coauthors=Torppa, J.; Piironen, J.|title=Models of Twenty Asteroids from Photometric Data|journal=Icarus|volume=159|pages=369-395|year=2002|doi=10.1006/icar.2002.6907| url=http://www.rni.helsinki.fi/~mjk/IcarPIII.pdf|format=pdf}}</ref> and [[4 Vesta]]<ref name=Thomas1997>{{cite journal|last=Thomas|first=Peter C.|coauthors=Binzel, Richard P.; Gaffey, Michael J.; et.al.|title=Impact Excavation on Asteroid 4 Vesta: Hubble Space Telescope Results|journal=Science|volume=277|pages=1492-1495|year=1997| doi=10.1126/science.277.5331.1492|url=http://www.sciencemag.org/cgi/content/abstract/277/5331/1492|accessdate=2007-12-08}}</ref> are known to be quite irregular.

The surface composition of Ceres is broadly similar to that of [[C-type asteroid]]s.<ref name=Rivkin2006/> However, some differences do exist. The ubiquitous features in the [[IR]] [[spectrum|spectra]] of Ceres are that of hydrated materials, which indicates the presence of significant amounts of water in the interior of this body. Other possible surface constituents include [[iron]]-rich [[clay]]s ([[cronstedtite]]) and [[carbonate]]s ([[dolomite]] and [[siderite]]), which are common minerals in [[carbonaceous chondrite]] meteorites.<ref name=Rivkin2006/> The spectral features of carbonates and clay are usually absent in the spectra of other C-type asteroids.<ref name=Rivkin2006>{{cite journal|last=Rivkin|first=A.S.|coauthors=Volquardsen, E.L.; Clark, B.E.|title=The surface composition of Ceres:Discovery of carbonates and iron-rich clays|journal=Icarus|volume=185|pages=563-567|year=2006|doi=10.1016/j.icarus.2006.08.022| url=http://irtfweb.ifa.hawaii.edu/~elv/icarus185.563.pdf|format=pdf|accessdate=2007-12-08}}</ref> Sometimes Ceres is classified as [[G-type asteroid]].<ref name=Parker2002/>

The surface of Ceres is relatively warm. The maximum temperature with the [[Sun]] overhead was estimated from measurements to be 235&nbsp;[[kelvin|K]] (about &minus;38 °[[Celsius|C]]) on [[May 5]] [[1991]].<ref name="Saint-Pe1993"/> Taking into account also the [[heliocentric]] distance at the time, this gives an estimated maximum of about 239&nbsp;K at [[perihelion]]. There are some indications that Ceres may have a tenuous [[Celestial body atmosphere|atmosphere]] and water [[frost]] on the surface.<ref name=Ahearn1992>{{cite journal|last=A’Hearn|first=Michael F.|coauthors=Feldman, Paul D.|title=Water vaporization on Ceres|journal=Icarus|volume=98|pages=54-60|year=1992| doi=10.1016/0019-1035(92)90206-M|url= http://adsabs.harvard.edu/abs/1992Icar...98...54A|accessdate=2007-12-08}}</ref> [[Ultraviolet]] observations by [[IUE]] spacecraft detected statistically significant [[hydroxide]] water vapour near the Cererean north pole.<ref name=Ahearn1992/>

[[Image:Ceres Cutaway.jpg|left|thumb|250px|Diagram showing internal structure of Ceres]]

Peter Thomas of [[Cornell University]] has proposed that Ceres has a differentiated interior;<ref name="Thomas2005">{{cite journal|first=P.C|last=Thomas|coauthors=Parker J.Wm.; McFadden, L.A.; et.al.|title=Differentiation of the asteroid Ceres as revealed by its shape|year=2005|journal=Nature|volume=437|pages=224-226|doi=10.1038/nature03938| url=http://adsabs.harvard.edu/abs/2005Natur.437..224T|accessdate=2007-12-09}}</ref> its [[oblateness]] appears too small for an undifferentiated body, which indicates that it consists of a rocky core overlain with an icy [[Mantle (geology)|mantle]].<ref name=Thomas2005/> This mantle of thickness from 120 to 60&nbsp;km could contain 200 million cubic kilometres of water (16&ndash;26% of Ceres by mass; 30&ndash;60% by volume), which is more than the amount of [[fresh water]] on the Earth.<ref>{{cite news|url=http://space.com/scienceastronomy/050907_ceres_planet.html| title=Largest Asteroid Might Contain More Fresh Water than Earth|first=Bjorn|last=Carey| publisher=SPACE.com|date=[[7 September]], [[2005]]|accessdate=2006-08-16}}</ref>

There has been some ambiguity regarding surface features on Ceres. High resolution [[ultraviolet]] [[Hubble Space Telescope]] images taken in 1995 showed a dark spot on its surface which was nicknamed "Piazzi" in honour of the discoverer of Ceres.<ref name="Parker2002"/> This was thought to be a crater. Later [[near-infrared]] images with a higher resolution taken over a whole rotation with the [[Keck telescope]] using [[adaptive optics]] showed no sign of "Piazzi".<ref name=Keck/> However, two dark features were seen to move with the dwarf planet's rotation, one with a bright central region. These are presumably craters. More recent visible light [[Hubble Space Telescope]] images of a full rotation taken in 2003 and 2004 showed eleven recognizable surface features, the nature of which is currently unknown.<ref name=Li2006>{{cite journal|last=Li|first=Jian-Yang|coauthors=McFadden, Lucy A.; Parker, Joel Wm.|title=Photometric analysis of 1 Ceres and surface mapping from HST observations|journal=Icarus|volume=182|pages=143-160|year=2006|doi=10.1016/j.icarus.2005.12.012| url=http://www.astro.umd.edu/~jyli/publications/2006.Icar.182.143.pdf|format=pdf|accessdate=2007-12-08}}</ref><ref name="Hubbl12003-4">{{cite news| url=http://hubblesite.org/newscenter/newsdesk/archive/releases/2005/27/|title=Largest Asteroid May Be 'Mini Planet' with Water Ice|date=[[September 7]] [[2005]]|accessdate=2006-08-16|publisher=HubbleSite}}</ref> One of these features corresponds to the "Piazzi" feature observed earlier.<ref name=Li2006/> The dark albedo features seen with Keck are, however, not immediately recognizable in these images.<ref name=Keck/>

These last observations also determined that Ceres' north pole points in the direction of [[right ascension]] 19&nbsp;h 24&nbsp;min (291°), [[declination]] +59°, in the [[constellation]] [[Draco (constellation)|Draco]]. This means that Ceres' [[axial tilt]] is very small&mdash;about 3°.<ref name="Thomas2005"/><ref name=Li2006/>

==軌道==
[[Image:Ceres Orbit.svg|thumb|left|300px|Orbit of Ceres]]
Ceres follows an orbit between Mars and Jupiter, within the main [[asteroid belt]], with a period of 4.6 Earth years. The orbit is moderately inclined (''i'' = 10.6° compared to 7° for Mercury and 17° for Pluto) and moderately [[Orbital eccentricity|eccentric]] (''e'' = 0.08 compared to 0.09 for Mars).<ref name=Bowell/>

The diagram illustrates the orbits of Ceres (blue) and several planets (white/grey). The segments of orbits below the ecliptic are plotted in darker colours, and the orange plus sign is the Sun's location. The top left diagram is a polar view that shows the location of Ceres in the gap between Mars and Jupiter. The top right is a close-up demonstrating the locations of the [[pericenter|perihelia]] (q) and [[apocenter|aphelia]] (Q) of Ceres and Mars. The perihelion of Mars is on the opposite side of the Sun from those of Ceres and several of the large main belt asteroids, including [[2 Pallas]] and [[10 Hygiea]]. The bottom diagram is a perspective view showing the inclination of the orbit of Ceres compared to the orbits of Mars and Jupiter.

In the past, Ceres had been considered to be the largest member of an [[asteroid family]].<ref name=Cellino>A. Cellino ''et al''. "Spectroscopic Properties of Asteroid Families", in ''Asteroids III'', p. 633-643, University of Arizona Press (2002). (Table on page 636, in particular).</ref> These groupings of asteroids share similar orbital elements, which may indicate a common origin through an asteroid collision some time in the past. Ceres, however, was found to have spectral properties different from other members of the family, and so this grouping is now called the [[Gefion family]], named after the lowest-numbered family member, [[1272 Gefion]].<ref name=Cellino/> Ceres appears to be merely an interloper in its own family, coincidentally having similar orbital elements but not a common origin.<ref name=Kelley>{{cite journal|author=Kelley, M. S.; Gaffey, M. J.|title=A Genetic Study of the Ceres (Williams #67) Asteroid Family|journal=Bulletin of the American Astronomical Society|year=1996|volume=28|pages=1097| url=http://adsabs.harvard.edu/abs/1996BAAS...28R1097K|accessdate=2007-04-27 }}</ref>
Self rotation period of Ceres is 9 hours and 4 minutes.<ref name=NSSDC>{{cite journal|author=Dr. David R. Williams|title=Asteroid Fact Sheet|year=2004|url=http://nssdc.gsfc.nasa.gov/planetary/factsheet/asteroidfact.html}}</ref>

==Origin and evolution==
The observations imply that Ceres is a remaining [[protoplanet]] – planetary embryo, which formed 4.57 billion years ago in the [[asteroid belt]].<ref name=Petit2001>{{cite journal|last=Petit|first=Jean-Marc.|coauthors=Morbidelli, Alessandro|title=The Primordial Excitation and Clearing of the Asteroid Belt|journal=Icarus|volume=153|pages=338-347|year=2001| doi=10.1006/icar.2001.6702|url=http://www.gps.caltech.edu/classes/ge133/reading/asteroids.pdf|format=pdf}}</ref> While the majority of embryos (including all lunar- to Mars-sized bodies) were ejected from the [[Solar System]] by [[Jupiter (planet)|Jupiter]] or merged with other embryos to form [[terrestrial planet]]s,<ref name=Petit2001/> Ceres survived relatively intact.<ref name=Li2006/> Two other possible remaining [[protoplanet]]s are [[2 Pallas|Pallas]] and [[4 Vesta|Vesta]],<ref name=Russel2006/> but they do not have relaxed shapes, in the case of Vesta perhaps only because it suffered a catastrophic impact after solidifying.<ref name=Thomas1997/>

Further [[Evolution (disambiguation)|evolution]] of Ceres was relatively simple. Heated by the energy of [[accretion (astrophysics)|accretion]] and, possibly, by decay of the short-lived [[radionuclide]]s like Al<sup>26</sup>, Ceres differentiated into a rocky [[core (geology)|core]] and icy [[mantle (geology)|mantle]] soon after its formation.<ref name=Li2006/><ref name=Castillo-Rogez2007>{{cite journal|last=Castillo-Rogez|first=J.C.|coauthors=McCord, T.B.; and A.G. Davis|title=Ceres: evolution and present state|journal=Lunar and Planetary Science|volume=XXXVIII|pages=2006-2007|year=2007|url= http://www.lpi.usra.edu/meetings/lpsc2007/pdf/2006.pdf|format=pdf }}</ref> This event caused resurfacing by the water [[volcanism]] and [[tectonics]] erasing many geological features. However due to the fast depletion of heat sources Ceres cooled down quickly.<ref name=Castillo-Rogez2007/> The ice on the surface gradually [[sublimation (chemistry)|sublimated]] leaving behind various hydrated minerals like [[clay]]s and [[Carbonate minerals|carbonate]]s. Now Ceres is a geologically dead body, whose surface is being sculptured only by [[impact crater|impact]]s.<ref name=Li2006/>

The existence of significant amounts of water ice in Ceres<ref name=Thomas2005/> raises a possibility that it has or had a layer of liquid water in the interior.<ref name=Castillo-Rogez2007/> This hypothetical layer is often called an [[ocean]].<ref name=Rivkin2006/> The water layer is (or was) probably located between the rocky core and ice mantle like in [[Europa (moon)|Europa]].<ref name=Castillo-Rogez2007/> The existence of the ocean is more likely if [[ammonia]] or other [[antifreeze]] is dissolved in water. The possible existence of liquid water inside Ceres makes it a target in the search for [[extraterrestrial life]].<ref name=Moomaw/>

==觀測==
When Ceres has an opposition near the perihelion, it can reach a visual magnitude of +6.7.<ref name="Pasachoff1983">{{cite book|author=Donald H. Menzel and Jay M. Pasachoff|year=1983|title=A Field Guide to the Stars and Planets|edition=2nd edition|publisher=Houghton Mifflin|pages=391|location=Boston, MA|id=ISBN 0395348358}}</ref> This is generally regarded as being just barely too dim to be seen with the [[naked eye]], but under exceptional viewing conditions a very sharp-sighted person may be able to see this [[dwarf planet]]. The only asteroids that can reach so bright a magnitude are [[4 Vesta#Visibility|4 Vesta]], and, during rare oppositions near perihelion, [[2 Pallas#Characteristics|2 Pallas]] and [[7 Iris]].<ref>Martinez, Patrick, ''The Observer's Guide to Astronomy'', page 298. Published 1994 by Cambridge University Press</ref> At a [[astronomical conjunction|conjunction]] Ceres has a magnitude of around +9.3, which corresponds to the faintest objects visible with 10×50 [[binoculars]]. It can thus be seen with binoculars whenever it is above the horizon of a fully dark sky.

Some notable observation milestones for Ceres include:
*An [[occultation]] of a [[star]] by Ceres observed in [[Mexico]], [[Florida]] and across the [[Caribbean]] on [[November 13]] [[1984]].<ref name=Millis1987>{{cite journal|last=Millis|first=L.R.|coauthors=Wasserman, L.H.; Franz, O.Z.; et.al.|title=The size, shape, density, and albedo of Ceres from its occultation of BD+8 deg 471|journal=Icarus|volume=72|pages=507-518|year=1987|doi=10.1016/0019-1035(87)90048-0|url= http://adsabs.harvard.edu/abs/1987Icar...72..507M|accessdate=2007-12-08}}</ref>
* [[Ultraviolet]] [[Hubble Space Telescope]] images with 50&nbsp;km resolution taken in 1995.<ref>{{cite web| url=http://www.swri.org/press/ceres.htm| title=Observations reveal curiosities on the surface of asteroid Ceres| accessdate = 2006-08-16}}</ref><ref name="Parker2002">{{cite journal|last=Parker|first=J.W.|coauthors=Stern, Alan S.; Thomas Peter C.;et.al.|title=Analysis of the first disk-resolved images of Ceres from ultraviolet observations with the Hubble Space Telescope|year=2002|journal=The Astrophysiscal Journal|volume=123|pages=549-557| url=http://adsabs.harvard.edu/abs/2002AJ....123..549P|doi=10.1086/338093|accessdate=2007-12-08}}</ref>
* [[Infrared]] images with 30&nbsp;km resolution taken with the [[Keck telescope]] in 2002 using [[adaptive optics]].<ref name=Keck>{{cite web|author=Staff|date=[[October 11]], [[2006]]|url=http://www.adaptiveoptics.org/News_1006_2.html|title=Keck Adaptive Optics Images the Dwarf Planet Ceres|publisher=Adaptive Optics|accessdate=2007-04-27}}</ref>
* Visible light images with 30&nbsp;km resolution (the best to date) taken using [[Hubble Space Telescope|Hubble]] in 2003 and 2004.<ref name="Hubbl12003-4"/><ref name=Li2006/>


==探索==
皮亞齊持續觀測至[[2月11日]]，但他的發現卻未受注意，之後該小行星已公轉至太陽背面而無法觀測。及後德國[[數學家]][[高斯]]憑著皮亞齊的三次觀測結果去估計其軌道，並於翌年由Franz Xaver, Baron von Zach和[[奧伯斯]]成功尋回該天體。
[[Image:Dawn Flight Configuration 2.jpg|thumb|right|250px|Artist's conception of Dawn visiting Ceres and Vesta]]


To date, no space probe has visited Ceres. However, [[NASA]] launched the [[Dawn Mission]] on [[27 September]] [[2007]], which will explore the asteroid [[4 Vesta]] in 2011 before arriving at Ceres in 2015.<ref name=Russel2006/>
==歸類==
自1801年發現谷神星後，天文學家根據與「[[提丟斯-波得定則]]」軌道相若的關係判別其為一直失蹤的[[行星]]之一，但其後數年分別發現另外三顆[[小行星]]後，四顆天體曾一併並列為行星之列，但其後再發現5號小行星[[義神星]]後，天文學家相信火星與木星之間散佈著十分零碎而且數量不菲的[[小行星]]，因此英國天文學家[[威廉·赫歇爾]]把這類天體稱為「[[小行星]]」（asteroid），穀神星則為「四大小行星」之首。


The mission profile calls for the Dawn Spacecraft to enter orbit around Ceres at an altitude of 5,900&nbsp;km. After five months of study, the spacecraft will reduce the orbital
而在2006年8月24日，於布拉格舉行之「第26屆國際天文學大會」上，由2500多位天文學家表決，開設並定義[[矮行星]]這一名詞，而把穀神星劃入其中。
distance to 1,300&nbsp;km, then down to 700&nbsp;km
after another five months.<ref>{{cite web|first=Marc|last=Rayman|date=[[13 July]], [[2006]]|url=http://www-ssc.igpp.ucla.edu/dawn/mission.html
|title=Dawn: mission description|publisher = UCLA &mdash; IGPP Space Physics Center|accessdate=2007-04-27}}</ref> The spacecraft instrumentation includes a framing camera, a visual and infrared [[spectrometer]], and a [[gamma-ray]] and [[neutron]] detector. These will be used to examine the dwarf planet's shape and element abundance.<ref name=Russel2006>{{cite journal|last=Russel|first=C.T.|coauthors=Capaccioni, F.; Coradini, A.; et.al.|title=Dawn Discovery mission to Vesta and Ceres: Present status|journal=Advances in Space Research|volume=38|pages=2043-2048|year=2006|doi=10.1016/j.asr.2004.12.041| url=http://adsabs.harvard.edu/abs/2006AdSpR..38.2043R|accessdate=2007-12-08}}</ref>


Radio signals from spacecraft in orbit around [[Mars]] and on its surface have been used to estimate the mass of Ceres from the perturbations induced by it onto the motion of Mars.<ref>{{cite journal|last=Pitjeva|first=E.V.|authorlink=Elena V. Pitjeva|title= High-Precision Ephemerides of Planets — EPM and Determination of Some Astronomical Constants |journal=Solar System Research|year= 2005|volume=39|issue=3|pages=176|url=http://iau-comm4.jpl.nasa.gov/EPM2004.pdf|format=[[PDF]]|doi= 10.1007/s11208-005-0033-2|accessdate=2007-12-09}}</ref>
==特徵==
[[Image:Ceres Rotation.jpg|thumb|由[[哈勃太空望遠鏡]]拍攝的谷神星影像，解像度達每[[像素]]30[[千米]]。當中亮點的性質不詳。[http://www.astronomy.com/asy/default.aspx?c=a&id=3478 動畫][http://www.astronomy.com/asy/default.aspx?c=a&id=3478]</sup>]]
穀神星是迄今[[小行星帶]]中最大的天體，其平均直徑為952公里，但隨著[[凱柏帶]]及其天體的發現，比穀神星大的天體也隨之被找到，包括[[小行星28978]]（伊克西翁）、[[小行星50000]]（誇歐爾）、[[小行星90482]]（厄耳枯斯）以及矮行星[[鬩神星]]等，而新發現的最遠天體[[小行星90377]]（塞德娜）也可能比穀神星大，它可能來自[[奧特雲]]內層。[[冥王星]]有時也會被認為是[[柯伊伯带|-{zh-hans:柯伊伯带;zh-hant:凱柏帶}-]]天體。


==參見==
[[2003年]]底及[[2004年]]初，[[哈勃太空望遠鏡]]首度攝得穀神星的外貌，發現它相當接近球形，而且表面具有不同的反照率，相信擁有複雜的地形，有天文學家甚至推測穀神星的具有冰質的幔及金屬的核心。
* [[Ceres in fiction]]
* [[List of Solar System bodies formerly regarded as planets]]
* [[Colonization of Ceres]]
* [[Planets in astrology#Ceres|Ceres in astrology]]
* [[Asteroids in astrology]]
{{portal|Solar System|Solar system.jpg}}


==參考資料==
[[美國太空總署]]在[[2007年]][[9月]]發射[[黎明號]]（Dawn）探測器前往穀神星，預計於[[2015年]][[2月]]到達。
{{reflist|2}}
<div class="references-small">
===Ephemerides===
* [[James L. Hilton]], [http://aa.usno.navy.mil/publications/reports/asteroid_ephemerides.html ''U.S. Naval Observatory Ephemerides of the Largest Asteroids''] The Astronomical Journal, Vol. 117 pp. 1077 (1999). <!-- not an accurate mass determination, but interesting reading, and online -->
* {{cite web | last = Yeomans | first =Donald K. | url = http://ssd.jpl.nasa.gov/?horizons | title = Horizons system | publisher = NASA JPL | accessdate = 2007-03-20 }} &mdash; Horizons can be used to obtain a current ephemeris
</div>


==附加資料==
==外部鏈接==
* [http://anon.nasa-global.speedera.net/anon.nasa-global/HST_ceres/640.mov Movie of one Ceres rotation (processed Hubble images)]
[[元素]][[鈰]]的拉丁名稱Cerium是以穀神星命名的。
* [http://www.keplersdiscovery.com/Asteroid.html How Gauss determined the orbit of Ceres] from keplersdiscovery.com
* [http://planetary.org/explore/topics/asteroids_and_comets/ceres.html An up-to-date summary of knowledge about Ceres, plus an Earth-Ceres size comparison (the Planetary Society)]
* [http://orbitsimulator.com/gravity/articles/ceres.html A simulation of the orbit of Ceres]
* [http://home.comcast.net/~eliws/ceres/ A website dedicated entirely to 1 Ceres]


{{穀神星}}{{太阳系}}
{{穀神星}}{{太阳系}}

2008年4月6日 (日) 09:34的版本

Ceres
发现
發現者皮亞齊
軌道參數
曆元 2006年3月6日
遠日點2.988 AU
近日點2.544 AU
半長軸2.765 AU
離心率0.080
軌道週期1681.243 日 (4.60 a)
平近點角129.983°
軌道傾角10.587°
升交點黃經80.409°
近日點參數73.231°
物理特徵
大小959.2×932.6 km
質量9.445×1020 千克
平均密度2.05 克/立方厘米
0.51 km/s
自轉週期0.3781日
反照率0.113

穀神星（Ceres）或小行星1是太陽系中最小的、也是唯一一顆位於小行星帶的矮行星。由意大利人皮亞齊發現，並於1801年1月1日公佈。[1]

穀神星的直徑約有950公里，是小行星帶之中最大最重的物體，並佔了小行星帶的三分之一質量。[2]近期的發現表明它是近球體的，並不像重力更小的物體有著不規則形狀。[3]穀神星表面可能由冰和各種含氫礦物如粘土。[4]Ceres appears to be differentiated into a rocky core and ice mantle.[5]它可能會擁有一個液態水海洋，使它成爲一個尋找地外生物的重要目標。[6]穀神星也有可能存在著一層包含水氣的稀薄大氣層。[7]

Ceres' apparent magnitude ranges from 6.7 to 9.3, hence at its brightest is still too dim to be seen with the naked eye.[8] On September 27, 2007, NASA launched the Dawn Mission space probe to explore Vesta (2011-2012) and Ceres (2015).[9]

發現

在火星和木星之間可能有一顆未知行星的理論最先由Johann Elert Bode於1768年提出。[1]他主要是根據提丟斯-波得定則做出論點，提丟斯-波得定則現在已不被使用，原先由戴維•提丟斯於1766年提出。[10][1]根據定則，這顆行星的軸心應該接近2.8 AU。[10]威廉•赫歇爾在1781年發現了天王星[1]使人們更加相信這個定則。而在1800年，24位有經驗的天文學家一同合作，並對未知行星作出有條理的搜尋工作。[1][10]這個團隊由Franz Xaver von Zach領導。雖然他們並沒有發現穀神星，但是他們後來發現了另外幾個大小行星。[10]

Piazzi's Book "Della scoperta del nuovo pianeta Cerere Ferdinandea" outlining the discovery of Ceres

穀神星終於在1801年1月1日被朱塞普•皮亞齊發現，當時他正在搜尋一顆由Francis Wollaston列爲Mayer 87的恆星，因爲這顆恆星並不在Mayer's的恆星列表中所述的位置上。[1]他並沒有發現恆星，而是一顆移動的星體，他當初還以爲是一顆彗星。[11]皮亞齊總共觀測了穀神星24次， 直到2月11日，他患了病而暫停了他的觀測。他把發現於1801年1月24日在他寫給和他一起合作的天文學家們（其中包括米蘭的Barnaba Oriani）的信裏面發佈。他描述穀神星為一顆彗星但是“由於它的行動如此緩慢和規則，我曾經數次覺得它應該是一些比彗星要好的東西”（Since its movement is so slow and rather uniform, it has occurred to me several times that it might be something better than a comet）。[1]在4月，皮亞齊在巴黎把所有觀測記錄寄給了Oriani、Bode和Lalande。這些資料發佈於1801年9月的Monatliche Correspondenz中。[11]

不久後，穀神星的位置明顯地發生了變化，主要由於地球的軌道運動。然後，它又運行得太接近太陽光輝，因此一直到年尾其他的天文學家都不能確認皮亞齊的觀測結果。不過經過這麽長的時間，人們已經很難預測它的準確位置。爲了找回穀神星，當時只有24歲的高斯研究出了一種有效率的方法預計它的軌道。[11]在短短的幾個星期以内，他準確地預測了它的路徑，並把結果寄給了Franz Xaver, Baron von Zach，Monatliche Correspondenz的編輯。在1801年12月31日，von Zach和奧伯斯在預測位置的附近成功找回了穀神星。[11]

命名

皮亞齊最初建議命名為Ceres Ferdinandea，來源於羅馬神話中的穀物女神刻瑞斯及西西里王國的斐迪南國王。[1][11]但“Ferdinandea”不為其他國家接受，因此沒再使用。另外，該天體在德國曾被稱為「赫拉」，但為時不長。[12]在希臘，它被稱爲Δήμητρα（Demeter），是Ceres同等意思的希臘文。在英文裏，Demeter是另一顆小行星（小行星1108）的名字。由於這個名字並不常用，there is no consensus as to the proper adjectival form of the name, although the nonce forms Cerian and Cerean[6] have been used in fiction。在語法上，Cererean會是正確的，來源於它的拉丁文genitiveCereris。[13]穀神星的符號是一個鐮刀，（Sickle variant symbol of Ceres），相似於金星的符號（金星的天文符號）which is the female gender symbol and Venus' hand mirror。[11][14]元素鈰的拉丁名稱Cerium是以穀神星命名的。[15]元素鈀本來也以穀神星命名，可是它的發現者在鈰被命名後將之改以智神星命名。[16]

地位

Ceres, the Moon and the Earth.

穀神星的歸類已經被變更過很多次，並且都是一些天文學家們的意見不和造成的。Johann Elert Bode相信穀神星就是那顆在火星和木星之間、距離太陽4.19億公里（2.8 AU）的“失蹤的行星”[1]穀神星被賦予一個行星的符號，在更多小行星被發現之前，穀神星（還有智神星、婚神星和灶神星）有將近半個世紀都以行星之名列在天文學書表之中。[1][11]

然而當其他的天體陸續在同一範圍内被發現，人們慢慢了解到穀神星只是許許多多相似類型天體的第一位而已。[1]Sir William Herschel coined in 1802 the term asteroid ("star-like") for such bodies, [17] writing "they resemble small stars so much as hardly to be distinguished from them, even by very good telescopes"。[18]作爲第一顆被發現的小行星，它以現代小行星編號系統被列爲小行星1穀神星[17]

在2006年關於冥王星到底是不是“行星”的辯論引發了穀神星是否應被歸進行星類別的問題。[19][20]在國際天文學聯盟定義行星之前，一個行星會被定義為“一個天體符合：(a) 有足夠的質量，能以自身的重力克服剛體力，因此能呈現流體靜力平衡的形狀（接近圓球體）；(b)在軌道上圍繞一顆恆星公轉，而且不是一顆恆星或衛星。“[21]若然依照這種定義，穀神星將會是從太陽按次序的第五顆行星。[22]但是，這並沒有被承認，並且同時一個新定義於2006年8月24日生效：一顆行星符合"(a)和(b)，而且必須將鄰近軌道上的天體清除。”根據這樣的定義，穀神星就不是一顆行星（因爲它在小行星帶上的軌道佈滿了千千萬萬個小行星），並且現在歸為“矮行星”（同時還有冥王星和鬩神星）。它是否還是小行星並未説明。[23]天文學裏存在著小行星帶彗星等的雙重身份天體，而且屬於一顆矮行星也不代表不能擁有雙重身份。[24]

物理特性

Size comparison: the first 10 minor planets profiled against Earth's Moon. Ceres is far left.
Hubble Space Telescope images of Ceres, taken in 2003/4 with a resolution of about 30 km. The nature of the bright spot is uncertain.

穀神星是迄今小行星帶中最大的天體。[4]古柏帶中包含著更大的天體，包括冥王星、創神星和厄耳枯斯，而在黃道離散天體中的鬩神星是所有小行星中最大的。[25]

穀神星的質量的測量可以由觀測它對其它小行星的影響得知。[2]不同觀測者的不同結果會有少許的差異。[26] The average of the three most recent values as of 2008 is 9.4×1020 kg.[27]With this mass Ceres comprises about a third of the estimated total 3.0 ± 0.2 ×1021 kg mass of the asteroids in the solar system,[28] together totalling about 4% of the mass of the Moon. Ceres' size and mass are sufficient to give it a nearly spherical shape.[5] That is, it is close to hydrostatic equilibrium. In contrast, other large asteroids such as 2 Pallas,[27] 3 Juno,[29] and 4 Vesta[30] are known to be quite irregular.

The surface composition of Ceres is broadly similar to that of C-type asteroids.[4] However, some differences do exist. The ubiquitous features in the IR spectra of Ceres are that of hydrated materials, which indicates the presence of significant amounts of water in the interior of this body. Other possible surface constituents include iron-rich clays (cronstedtite) and carbonates (dolomite and siderite), which are common minerals in carbonaceous chondrite meteorites.[4] The spectral features of carbonates and clay are usually absent in the spectra of other C-type asteroids.[4] Sometimes Ceres is classified as G-type asteroid.[31]

The surface of Ceres is relatively warm. The maximum temperature with the Sun overhead was estimated from measurements to be 235 K (about −38 °C) on May 5 1991.[32] Taking into account also the heliocentric distance at the time, this gives an estimated maximum of about 239 K at perihelion. There are some indications that Ceres may have a tenuous atmosphere and water frost on the surface.[7] Ultraviolet observations by IUE spacecraft detected statistically significant hydroxide water vapour near the Cererean north pole.[7]

Diagram showing internal structure of Ceres

Peter Thomas of Cornell University has proposed that Ceres has a differentiated interior;[5] its oblateness appears too small for an undifferentiated body, which indicates that it consists of a rocky core overlain with an icy mantle.[5] This mantle of thickness from 120 to 60 km could contain 200 million cubic kilometres of water (16–26% of Ceres by mass; 30–60% by volume), which is more than the amount of fresh water on the Earth.[33]

There has been some ambiguity regarding surface features on Ceres. High resolution ultraviolet Hubble Space Telescope images taken in 1995 showed a dark spot on its surface which was nicknamed "Piazzi" in honour of the discoverer of Ceres.[31] This was thought to be a crater. Later near-infrared images with a higher resolution taken over a whole rotation with the Keck telescope using adaptive optics showed no sign of "Piazzi".[34] However, two dark features were seen to move with the dwarf planet's rotation, one with a bright central region. These are presumably craters. More recent visible light Hubble Space Telescope images of a full rotation taken in 2003 and 2004 showed eleven recognizable surface features, the nature of which is currently unknown.[3][35] One of these features corresponds to the "Piazzi" feature observed earlier.[3] The dark albedo features seen with Keck are, however, not immediately recognizable in these images.[34]

These last observations also determined that Ceres' north pole points in the direction of right ascension 19 h 24 min (291°), declination +59°, in the constellation Draco. This means that Ceres' axial tilt is very small—about 3°.[5][3]

軌道

Orbit of Ceres

Ceres follows an orbit between Mars and Jupiter, within the main asteroid belt, with a period of 4.6 Earth years. The orbit is moderately inclined (i = 10.6° compared to 7° for Mercury and 17° for Pluto) and moderately eccentric (e = 0.08 compared to 0.09 for Mars).[36]

The diagram illustrates the orbits of Ceres (blue) and several planets (white/grey). The segments of orbits below the ecliptic are plotted in darker colours, and the orange plus sign is the Sun's location. The top left diagram is a polar view that shows the location of Ceres in the gap between Mars and Jupiter. The top right is a close-up demonstrating the locations of the perihelia (q) and aphelia (Q) of Ceres and Mars. The perihelion of Mars is on the opposite side of the Sun from those of Ceres and several of the large main belt asteroids, including 2 Pallas and 10 Hygiea. The bottom diagram is a perspective view showing the inclination of the orbit of Ceres compared to the orbits of Mars and Jupiter.

In the past, Ceres had been considered to be the largest member of an asteroid family.[37] These groupings of asteroids share similar orbital elements, which may indicate a common origin through an asteroid collision some time in the past. Ceres, however, was found to have spectral properties different from other members of the family, and so this grouping is now called the Gefion family, named after the lowest-numbered family member, 1272 Gefion.[37] Ceres appears to be merely an interloper in its own family, coincidentally having similar orbital elements but not a common origin.[38] Self rotation period of Ceres is 9 hours and 4 minutes.[39]

Origin and evolution

The observations imply that Ceres is a remaining protoplanet – planetary embryo, which formed 4.57 billion years ago in the asteroid belt.[40] While the majority of embryos (including all lunar- to Mars-sized bodies) were ejected from the Solar System by Jupiter or merged with other embryos to form terrestrial planets,[40] Ceres survived relatively intact.[3] Two other possible remaining protoplanets are Pallas and Vesta,[9] but they do not have relaxed shapes, in the case of Vesta perhaps only because it suffered a catastrophic impact after solidifying.[30]

Further evolution of Ceres was relatively simple. Heated by the energy of accretion and, possibly, by decay of the short-lived radionuclides like Al26, Ceres differentiated into a rocky core and icy mantle soon after its formation.[3][41] This event caused resurfacing by the water volcanism and tectonics erasing many geological features. However due to the fast depletion of heat sources Ceres cooled down quickly.[41] The ice on the surface gradually sublimated leaving behind various hydrated minerals like clays and carbonates. Now Ceres is a geologically dead body, whose surface is being sculptured only by impacts.[3]

The existence of significant amounts of water ice in Ceres[5] raises a possibility that it has or had a layer of liquid water in the interior.[41] This hypothetical layer is often called an ocean.[4] The water layer is (or was) probably located between the rocky core and ice mantle like in Europa.[41] The existence of the ocean is more likely if ammonia or other antifreeze is dissolved in water. The possible existence of liquid water inside Ceres makes it a target in the search for extraterrestrial life.[6]

觀測

When Ceres has an opposition near the perihelion, it can reach a visual magnitude of +6.7.[8] This is generally regarded as being just barely too dim to be seen with the naked eye, but under exceptional viewing conditions a very sharp-sighted person may be able to see this dwarf planet. The only asteroids that can reach so bright a magnitude are 4 Vesta, and, during rare oppositions near perihelion, 2 Pallas and 7 Iris.[42] At a conjunction Ceres has a magnitude of around +9.3, which corresponds to the faintest objects visible with 10×50 binoculars. It can thus be seen with binoculars whenever it is above the horizon of a fully dark sky.

Some notable observation milestones for Ceres include:

探索

Artist's conception of Dawn visiting Ceres and Vesta

To date, no space probe has visited Ceres. However, NASA launched the Dawn Mission on 27 September 2007, which will explore the asteroid 4 Vesta in 2011 before arriving at Ceres in 2015.[9]

The mission profile calls for the Dawn Spacecraft to enter orbit around Ceres at an altitude of 5,900 km. After five months of study, the spacecraft will reduce the orbital distance to 1,300 km, then down to 700 km after another five months.[45] The spacecraft instrumentation includes a framing camera, a visual and infrared spectrometer, and a gamma-ray and neutron detector. These will be used to examine the dwarf planet's shape and element abundance.[9]

Radio signals from spacecraft in orbit around Mars and on its surface have been used to estimate the mass of Ceres from the perturbations induced by it onto the motion of Mars.[46]

參見

參考資料

  1. ^ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 Hoskin, Michael. Bodes' Law and the Discovery of Ceres. Observatorio Astronomico di Palermo "Giuseppe S. Vaiana". June 26, 1992 [2007-07-05]. 
  2. ^ 2.0 2.1 引证错误：没有为名为Michalak2000的参考文献提供内容
  3. ^ 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 Li, Jian-Yang; McFadden, Lucy A.; Parker, Joel Wm. Photometric analysis of 1 Ceres and surface mapping from HST observations (pdf). Icarus. 2006, 182: 143–160 [2007-12-08]. doi:10.1016/j.icarus.2005.12.012. 
  4. ^ 4.0 4.1 4.2 4.3 4.4 4.5 Rivkin, A.S.; Volquardsen, E.L.; Clark, B.E. The surface composition of Ceres:Discovery of carbonates and iron-rich clays (pdf). Icarus. 2006, 185: 563–567 [2007-12-08]. doi:10.1016/j.icarus.2006.08.022. 
  5. ^ 5.0 5.1 5.2 5.3 5.4 5.5 Thomas, P.C; Parker J.Wm.; McFadden, L.A.; et.al. Differentiation of the asteroid Ceres as revealed by its shape. Nature. 2005, 437: 224–226 [2007-12-09]. doi:10.1038/nature03938. 
  6. ^ 6.0 6.1 6.2 Bruce Moomaw. Ceres As An Abode Of Life. spaceblooger.com. 2007-07-02 [2007-11-06]. 
  7. ^ 7.0 7.1 7.2 A’Hearn, Michael F.; Feldman, Paul D. Water vaporization on Ceres. Icarus. 1992, 98: 54–60 [2007-12-08]. doi:10.1016/0019-1035(92)90206-M. 
  8. ^ 8.0 8.1 Donald H. Menzel and Jay M. Pasachoff. A Field Guide to the Stars and Planets 2nd edition. Boston, MA: Houghton Mifflin. 1983: 391. ISBN 0395348358. 
  9. ^ 9.0 9.1 9.2 9.3 Russel, C.T.; Capaccioni, F.; Coradini, A.; et.al. Dawn Discovery mission to Vesta and Ceres: Present status. Advances in Space Research. 2006, 38: 2043–2048 [2007-12-08]. doi:10.1016/j.asr.2004.12.041. 
  10. ^ 10.0 10.1 10.2 10.3 Hogg, Helen Sawyer. The Titius-Bode Law and the Discovery of Ceres. Journal of the Royal Astronomical Society of Canada. 1948, 242: 241–246 [2007-12-09]. 
  11. ^ 11.0 11.1 11.2 11.3 11.4 11.5 11.6 Forbes, Eric G. Gauss and the Discovery of Ceres. Journal for the History of Astronomy. 1971, 2: 195–199 [2007-12-08]. 
  12. ^ Foderà Serio, G.; Manara, A.; Sicoli, P. Giuseppe Piazzi and the Discovery of Ceres. W.F. Bottke Jr., A. Cellino, P. Paolicchi, and R.P. Binzel (编). Asteroids III (PDF). Tucson, Arizona: University of Arizona Press. 2002: 17–24. 
  13. ^ Simpson, D.P. Cassell's Latin Dictionary 5. London: Cassell Ltd. 1979: 883. ISBN 0-304-52257-0. 
  14. ^ Gould, B.A. On the symbolic notation of the asteroids. Astronomical Journal. 1852, 2 (34): 80 [2007-07-05]. 
  15. ^ Staff. Cerium: historical information. Adaptive Optics. [2007-04-27]. 
  16. ^ Amalgamator Features 2003: 200 Years Ago. October 30, 2003 [2006-08-21]. 
  17. ^ 17.0 17.1 Hilton, Dr.J.L. When Did the Asteroids Become Minor Planets?. September 17 2001 [2006-08-16]. 
  18. ^ Herschel, William. Observations on the two lately discovered celestial Bodies.. May 6 1802. 
  19. ^ Battersby, Stephen. Planet debate: Proposed new definitions. New Scientist. August 16, 2006 [2007-04-27]. 
  20. ^ Connor, Steve. Solar system to welcome three new planets. NZ Herald. August 16, 2006 [2007-04-27]. 
  21. ^ Owen Gingerich; et al. The IAU draft definition of "Planet" and "Plutons". IAU. August 16, 2006 [2007-04-27]. 
  22. ^ Staff Writers. The IAU Draft Definition Of Planets And Plutons. SpaceDaily. August 16, 2006 [2007-04-27]. 
  23. ^ Question and answers 2. IAU. [2008-01-31].  — Q: What is Ceres? "Ceres is (or now we can say it was)" - but note it then talks about "other asteroids" crossing Ceres' path.
  24. ^ T. B. Spahr. MPEC 2006-R19 : EDITORIAL NOTICE. Minor Planet Center. 2006-09-07 [2008-01-31]. the numbering of "dwarf planets" does not preclude their having dual designations in possible separate catalogues of such bodies. 
  25. ^ Stansberry, J.; Grundy, W.; Brown, M.; et.al. Physical Properties of Kuiper Belt and Centaur Objects: Constraints from Spitzer Space Telescope. 5 November 2007 [2007-12-08]. 
  26. ^ Kovacevic, A.; Kuzmanoski, M. A New Determination of the Mass of (1) Ceres. Earth, Moon, and Planets. 2007, 100: 117–123 [2007-12-08]. doi:10.1007/s11038-006-9124-4. 
  27. ^ 27.0 27.1 Carry, B.; Kaasalainen, M.; Dumas, C.; et.al. Asteroid 2 Pallas Physical Properties from Near-Infrared High-Angular Resolution Imagery (pdf). ISO. ESO Planetary Group: Journal Club. 2007 [2007-11-05].  参数|journal=与模板{{cite web}}不匹配（建议改用{{cite journal}}或|website=） (帮助)
  28. ^ 引证错误：没有为名为Pitjeva2005的参考文献提供内容
  29. ^ Kaasalainen, M.; Torppa, J.; Piironen, J. Models of Twenty Asteroids from Photometric Data (pdf). Icarus. 2002, 159: 369–395. doi:10.1006/icar.2002.6907. 
  30. ^ 30.0 30.1 Thomas, Peter C.; Binzel, Richard P.; Gaffey, Michael J.; et.al. Impact Excavation on Asteroid 4 Vesta: Hubble Space Telescope Results. Science. 1997, 277: 1492–1495 [2007-12-08]. doi:10.1126/science.277.5331.1492. 
  31. ^ 31.0 31.1 31.2 Parker, J.W.; Stern, Alan S.; Thomas Peter C.;et.al. Analysis of the first disk-resolved images of Ceres from ultraviolet observations with the Hubble Space Telescope. The Astrophysiscal Journal. 2002, 123: 549–557 [2007-12-08]. doi:10.1086/338093. 
  32. ^ 引证错误：没有为名为Saint-Pe1993的参考文献提供内容
  33. ^ Carey, Bjorn. Largest Asteroid Might Contain More Fresh Water than Earth. SPACE.com. 7 September, 2005 [2006-08-16]. 
  34. ^ 34.0 34.1 34.2 Staff. Keck Adaptive Optics Images the Dwarf Planet Ceres. Adaptive Optics. October 11, 2006 [2007-04-27]. 
  35. ^ 35.0 35.1 Largest Asteroid May Be 'Mini Planet' with Water Ice. HubbleSite. September 7 2005 [2006-08-16]. 
  36. ^ 引证错误：没有为名为Bowell的参考文献提供内容
  37. ^ 37.0 37.1 A. Cellino et al. "Spectroscopic Properties of Asteroid Families", in Asteroids III, p. 633-643, University of Arizona Press (2002). (Table on page 636, in particular).
  38. ^ Kelley, M. S.; Gaffey, M. J. A Genetic Study of the Ceres (Williams #67) Asteroid Family. Bulletin of the American Astronomical Society. 1996, 28: 1097 [2007-04-27]. 
  39. ^ Dr. David R. Williams. Asteroid Fact Sheet. 2004. 
  40. ^ 40.0 40.1 Petit, Jean-Marc.; Morbidelli, Alessandro. The Primordial Excitation and Clearing of the Asteroid Belt (pdf). Icarus. 2001, 153: 338–347. doi:10.1006/icar.2001.6702. 
  41. ^ 41.0 41.1 41.2 41.3 Castillo-Rogez, J.C.; McCord, T.B.; and A.G. Davis. Ceres: evolution and present state (pdf). Lunar and Planetary Science. 2007,. XXXVIII: 2006–2007. 
  42. ^ Martinez, Patrick, The Observer's Guide to Astronomy, page 298. Published 1994 by Cambridge University Press
  43. ^ Millis, L.R.; Wasserman, L.H.; Franz, O.Z.; et.al. The size, shape, density, and albedo of Ceres from its occultation of BD+8 deg 471. Icarus. 1987, 72: 507–518 [2007-12-08]. doi:10.1016/0019-1035(87)90048-0. 
  44. ^ Observations reveal curiosities on the surface of asteroid Ceres. [2006-08-16]. 
  45. ^ Rayman, Marc. Dawn: mission description. UCLA — IGPP Space Physics Center. 13 July, 2006 [2007-04-27]. 
  46. ^ Pitjeva, E.V. High-Precision Ephemerides of Planets — EPM and Determination of Some Astronomical Constants (PDF). Solar System Research. 2005, 39 (3): 176 [2007-12-09]. doi:10.1007/s11208-005-0033-2. 

Ephemerides

外部鏈接

穀神星（矮行星）
分類
哈柏太空望遠鏡拍攝優化後的穀神星
發現
地質
探測
其它
太阳水星金星月球地球火星火卫一和火卫二穀神星小行星主带木星木星的卫星木星环土星土星的卫星土星环天王星天王星的卫星天王星环海王星海王星的卫星海王星环冥王星冥王星的卫星妊神星妊神星的卫星鸟神星柯伊伯带阋神星阋卫一離散盤山形云奥尔特云
太阳
•
太陽圈
行星
矮行星
太阳系
小天体
小行星
（微型行星）
小行星族
另见：小行星列表。
海王星
外天体
古柏带
冥族小天體
傳統古柏帶天體
離散盤
彗星
前一小行星：
- 		小行星列表 後一小行星：
(2)智神星

Template:Link FA Template:Link FA Template:Link FA

取自“https://zh.wikipedia.org/w/index.php?title=穀神星&oldid=6628068”