物理特性

由於其低密度、高速自轉和流體的可變性，土星的外形呈現為一個橢球體，也就是極軸相對扁平而赤道相對突出，它的赤道直徑和兩極直徑之比相差大約10%（前者120,536公里，後者108,728公里）。其它氣體行星雖然也是橢球體，但突出程度都較小。雖然土星核心的密度遠高於水，但由於存在較厚的大氣層，土星仍是太陽系中唯一密度低於水的行星，它的比重是0.69 公克/公分³。土星的質量是地球的95倍，相較之下木星質量是地球的318倍，但木星的直徑大約僅為土星的1.21倍。木星和土星一起在太陽系持有總行星質量的92%。

內部構造

土星被稱為氣態行星，但它並不完全是氣態的。行星主要包括氫氣，在密度為0.01 g/cm3以上時氫氣變成了非理想液體。此密度被達到在包含99.9％土星質量的半徑。從行星內部直到的核心的溫度，壓力和密度全都是穩步上升，使在行星的更深層導致氫氣轉變成金屬。

雖然只有少量的直接資料，但標準的行星模型表明，土星的內部結構仍被認為與木星相似，即有一個被氫和氦包圍著的小核心。岩石核心的構成與地球相似但密度更高。在核心之上，有更厚的液體金屬氫層，然後是數層的液態氫和氦層，在最外層是厚達1,000 公里的大氣層，也存在著各種型態冰的蹤跡。估計核心區域的質量大約是地球質量的9–22倍。

土星有非常熱的內部，核心的溫度高達11 700 °C，並且輻射至太空中的能量是它接受來自太陽的能量的2.5倍。大部分能量是由緩慢的重力壓縮（克赫歷程）產生，但這還不能充分解釋土星的熱能製造過程。額外的熱能可能由另一種機制產生：在土星內部深處，液態氦的液滴如雨般穿過較輕的氫，在此過程中不斷地通過摩擦而產生熱。

大氣層

土星外圍的大氣層包括96.3%的氫和3.25%的氦，可以偵測到的氣體還有氨、乙炔、乙烷、磷化氫和甲烷。上層的雲由氨的冰晶組成，較低層的雲則由硫化氫銨（NH4SH）或水組成。相對於太陽所含有的豐富的氦，土星大氣層中氦的豐盈度明顯低得多。

對于比氦重的元素的含量，目前所知不甚精確；但如果假設與太陽系形成時的原始豐盈度是相當的，則可估算出這些元素的總質量是地球質量的19–31倍，而且大部分都存在於土星的核心區域。

雲層

土星的上層大氣與木星相似（在相同定義的前提下），同樣都有著一些條紋；但土星的條紋比較暗淡，並且赤道附近的條紋也比較寬。從底部延展至大約10公里高處，是由水冰構成的層次，溫度大約是-23 °C。在這之後是硫化氫氨冰的層次，延伸出另外的50公里，溫度大約在-93 °C，在這之上是80公里的氨冰雲，溫度大約是-153 °C。接近頂部，在雲層之上200 公里至270 是可以看見的雲層頂端，由數層氫和氦構成的大氣層。土星的風速是太陽系中第二高的，僅次于海王星，航海家計劃的數據顯示土星的東風最高可達500 m/s（1,800公里/時）。直到航海家探測器飛越土星，比較纖細的條紋才被觀測到。然而從那之後，地基望遠鏡也被改善到在通常情況下都能夠觀察到土星的這些細紋。

土星的大氣層通常都很平靜，偶爾會出現一些持續較長時間的長圓形特徵，以及其他在木星上常常出現的特徵。1990年，哈伯太空望遠鏡在土星的赤道附近觀察到一朵極大的白雲，是在航海家與土星遭遇時未曾看見的，在1994年又觀察到另一朵較小的白雲風暴。1990年的白雲是大白斑的一個例子，這是在每一個土星年（大約30個地球年），當土星北半球夏至的時候所發生的獨特但短期的現象。之前的大白斑分別出現在1876、1903、1933和1960年，並且以1933年的最為著名。如果這個週期能夠持續，下一場大風暴將在大約2020年發生。

來自卡西尼號太空船的最新圖像顯示，土星的北半球呈現與天王星相似的明亮藍色（見下圖）。這種藍色非常可能是由瑞利散射造成的，但因為當時土星環遮蔽住了北半球，因此從地球上無法看見這種藍色。

天文學家通過分析紅外線影像發現土星有一個「溫暖」的極地漩渦，這種特徵在太陽系內是獨一無二的。天文學家認為這個點是土星上溫度最高的點，土星上其他各處的溫度是-185 °C，而該漩渦處的溫度則高達-122 °C。

在航海家1號的影像中最先被注意到的是一個長期出現在78°N附近，圍繞著北極的六邊形漩渦。不同於北極，哈勃太空望遠鏡所拍攝到的南極區影像有明顯的「噴射氣流」，但沒有強烈的極區漩渦，也沒有「六邊形的駐波」。但是，NASA報告卡西尼號在2006年11月觀測到一個位於南極像颶風的風暴，有著清晰的眼壁。這是很值得注意的觀測報告，因為在過去除了地球之外，沒有在任何的行星上觀測到眼壁雲（包括伽利略號太空船在木星的大紅斑上都未能發現眼壁雲）。

在北極的六邊形中每一邊的直線長度大約是13 800 公里，整個結構以10h 39 m 24s自轉，與行星的無線電波輻射週期一樣，這也被認為是土星內部的自轉週期。這個六邊形結構像大氣層中可見的其他雲彩一樣，在經度上沒有移動。

這個現象的規律性的起源仍在猜測之中，多數的天文學家認為是在大氣層中某種形式的駐波，但是六邊形也許是一種新型態的極光。在實驗室的流體轉動桶內已經模擬出了多邊型結構。

磁層

土星有一個簡單的具有對稱形狀的內在磁場——一個磁偶極子。磁場在赤道的強度為0.2 高斯（20 µT），大約是木星磁場的20分之一，比地球的磁場強大，為地球的20倍；由于強度遠比木星的微弱，因此土星的磁層僅延伸至土衛六軌道之外。磁層產生的原因很有可能與木星相似——由金屬氫層（被稱為「金屬氫發電機」）中的電流引起。與其他的行星一樣，土星磁層會受到來自太陽的太陽風內的帶電微粒影響而產生偏轉。衛星土衛六的軌道位于土星磁層的外圍，並且土衛六的大氣層外層中的帶電粒子提供了等離子體。

軌道和自轉

土星和太陽的平均距離超過了1 400 000 000 公里（9天文單位），軌道上運行的平均速度是9.69 公里/秒，所以土星上的一年（即土星繞太陽公轉一周）相當于10 759個地球日（或是29.4571地球年）。土星的橢圓軌道相對於地球軌道平面的傾角為2.48°，因為離心率為0.056，因此土星與太陽在近日點和遠日點（行星在軌道路徑上與太陽最近和最遠的兩個點）之間的距離變化大約為155 000 000 公里。

土星可見的特徵（如六邊型風暴）的自轉速率根據所在緯度的不同而有所不同，各個的區域的自轉周期如下：「系統I」的週期是10 h 14 min 00 s（844.3°/d），包含的是赤道區域，從南赤道帶的北緣延伸至北赤道帶的南緣；其他的緯度都屬於週期為10 h 39 min 24 s（810.76°/d）的「系統II」；基于航海家飛越土星時發現的無線電波，「系統III」的週期為10 h 39 min 22.4 s（810.8°/d）；因為與系統II非常接近，它可以很大程度上替代系統II。

然而，精確的內部周期仍然未能確定。卡西尼太空船在2004年接近土星時，發現無線電的週期又有可察覺的增加，達到10 h 45 m 45 s（± 36 s）。造成變化的原因仍不清楚，但這種變化被認為是由于無線電的來源在土星內部不同的緯度上運動而改變了自轉週期，而不是出自土星本身自轉週期上的變化。

而後，在2007年，無線電發射被發現沒有跟隨著行星一起旋轉，而可能是由等離子體圓盤的對流造成的，它也與除了行星的自轉之外的其他因素有關。有報道指出，這種測量到的自轉週期的變化也許是由土星衛星土衛二上的噴泉活動造成的。由這種活動而散佈進入土星軌道的水蒸氣被電離，從而影響了土星的磁場，使得磁場的旋轉速度相對於土星的自轉被稍稍降低。目前還沒有方法可以直接測定土星核心的自轉速率。

在2007年9月的報告中，根據各種測量結果（包括卡西尼、航海家和先鋒號的報告）綜合而得的對土星自轉的最後估計值是10小時32分35秒。

根據卡西尼號探測器收集的數據，2019年估計10小時33分38秒。

土星環

土星最為人知的莫過於它的行星環系統了，土星環被認為是太陽系內所觀察到的令人印象最深刻的景觀。

歷史

土星因為它美麗的行星環而出名，它也是最早被發現具有光環的行星。1610年7月，伽利略用他自製的望遠鏡觀察到了土星環，但因為望遠鏡成像不好，他並沒有意識到這是一個環。他在寫給托斯卡納大公的信上說到：「土星不是單一的個體，它由三個部份組成，這些部分幾乎都互相接觸著，並且彼此間沒有相對的運動，它們的連線是與黃道平行的，並且中央部份（土星本體）大約是兩側（環的邊緣）的三倍大」。他也把土星描述成是有「耳朵」的。在1612年，土星環以側面朝向地球，因此看起來似乎是消失不見了，伽利略因此而感到困惑不解，「是土星吞掉了它的孩子？」（指的是希臘神話中，農神為了防止他們的子孫造反奪權，會吃掉自己的孩子）。然後，在1613年他又再次看見了環，這使伽利略更加困惑。

在1655年，克里斯蒂安·惠更斯觀測到完整的土星環，他使用了一個比在伽利略時代能得到強大得多的望遠鏡。惠更斯觀測土星並寫道：「它（土星）被一個薄且平坦的環環繞著，環與土星沒有接觸，並且相對黃道傾斜。」

在1675年，喬瓦尼·卡西尼確定土星環由許多較小的環組成，中間並且有縫存在著，其中最明顯的環縫在不久之後被命名為卡西尼縫。卡西尼縫存在於A環和B環之間，寬度有4800 公里。

在1859年，詹姆斯·克拉克·馬克士威提出土星環不可能是固體的，否則將會因為不穩定而碎裂。他認為環是由為數眾多的小顆粒組成的，每個顆粒都獨立地環繞著土星運行。透過光譜學的研究，立克天文台的詹姆斯·基勒在1895年證實了馬克士威的理論。

物理特性

使用簡單的現代望遠鏡或是品質精良的雙筒望遠鏡就可以看見土星環。它在赤道上從距離土星6 630 公里延伸至120 700 公里處，但平均的厚度大約只有20米，主要的成分93%是水冰和少量參雜在其中的複雜有機懸浮物托林，其餘7%是無定型的碳，它們的大小從塵土的斑點到一輛小汽車的大小都有。關於土星環的起源有兩種主要的理論。一種理論是在19世紀提出的起源於洛希極限，認為環原本是土星的一顆衛星，因為軌道的衰減而落入洛希極限的範圍內，因本身不夠緊密而被潮汐力扯碎（參見洛希極限），這種理論又演變出衛星被小行星或彗星撞擊而瓦解的學說。第二種理論認為它並非來自衛星，而是從形成土星的原星雲中直接形成的。

在環中最大的空隙是卡西尼縫和恩克環縫，土星的恩克環縫是在1837年5月28日由恩克於柏林發現的，從地球上就可以看見。兩艘航海家號太空船都發現環實際上是由數以萬計稀薄的小環和空隙構成的複雜結構體。這些結構的產生被認為有好幾種不同的成因，許多是由于土星衛星引力的拉扯造成的。其中一些明顯的是由土星的微型衛星如土衛十八經過時形成，而其他更多的成因還有待發現；並且有一些小環似乎是由牧羊犬衛星在維護的，像是土衛十六和土衛十七。其他的縫隙可能是與質量較大的衛星軌道週期產生共振造成的，土衛一維繫著卡西尼縫的存在，還有更多的環狀結構因為受到其他衛星週期性的擾動而產生螺旋狀的波浪。

來自卡西尼號太空船的資料顯示土星環有自己的大氣層，與行星本身無關而獨立存在。大氣中有氧分子（O2），這是來自太陽的紫外線作用與環中的冰而產生的。水分子之間的鏈結受到紫外線的刺激產生化學作用釋放出並拋出了氣體，尤其是O2。根據這一模型，大氣層中也存在氫氣（H2）。這種O2和H2組成的大氣層是如此稀薄的，以至于如果均勻分散在環的各處，它的厚度只有一個原子。環中也有稀薄的OH（氧化氫）氣體，如同O2一樣，這些氣體也是水分子的崩解導致的，但這一分解是由高能量離子轟擊土衛二拋射出來的水分子所造成的。這些大氣層儘管是非常的稀薄，依然還是可以被在地球上空的哈伯太空望遠鏡檢測出來。

土星在它的亮度上呈現複雜的樣式。光度的變化大多可以歸咎於環的變化，並且在每個軌道週期有兩個循環的變化。由於行星軌道的離心率，使得疊加在北半球衝的時候比在南半球衝時更為明亮。

在1980年，航海家1號飛越土星時顯示F-環是由三條細環像編辮子一樣的糾結在一起，而呈現出複雜的結構；現在知道是在外面的二個環有突起的瘤，造成交織和糾結成團的假象，比較暗的第三個環則在它們的內側。

光環的黑暗一面

土星光環是連貫成一整個環的，但人為上或會把其分為朝著太陽與背著太陽的一面。而環的背向太陽的一小部分，因為被土星遮擋，只能由太空船如美國國家航空航天局卡西尼—惠更斯號拍攝並傳送回來；比較一下來自2004年3月卡西尼號的與來自先驅者11號的圖象：

環上的輪輻

在1980年以前，對土星環的結構和行為完全都以萬有引力的作用來解釋。航海家太空船在B環上發現被稱為「輪輻」的輻射線狀特徵，這些無法用同樣的方法來解釋，因為它們的存在和繞著環的轉動，是與軌道力學不一致的。這些輪輻在背景散射光下呈現黑暗，而在前景散射光下顯得明亮。它們被假設是懸浮在圓環平面上的微塵，受到電磁的交互作用而聯繫在一起，因此它們的轉動是與土星的磁氣層同步。但是，造成輪輻的確實機制仍然不清楚。

在25年之後，輪輻再度被卡西尼號觀測到。它們看起來有季節性的變化，在土星的仲冬或盛夏時消失不見，當土星接近分點時又再度出現。在2004年初，當卡西尼太空船抵達土星時這些輪輻都未出現。基于目前對于輻條的成因的模型，一些科學家推測這些輪輻要到2007年後才會出現。然而，通過對卡西尼拍攝的環影像的持續尋找，發現輪輻在2005年9月5日重新出現。

衛星

土星有為數眾多的衛星。精確的數量尚不能確定，所有在環上的大冰塊理論上來說都是衛星，而且要區分出是環上的大顆粒還是小衛星是很困難的。已經確認的衛星有82顆，其中53顆已經有了正式的名稱。此外，有證據表明，土星環中有數十至數百個直徑為40-500米的小衛星，不被認為是真正的衛星，現在已經確認的衛星有89顆。

許多衛星都非常的小：34顆的直徑小於10公里，另外13顆的直徑小於50公里，祇有7顆有足夠的質量能夠以自身的重力達到流體靜力平衡，它們與地球的衛星——月球的比較表見下方。

土衛六，土星最大的衛星，是太陽系中第二大的衞星，也是唯一有濃厚大氣層的衛星，而土星絕大多數的衛星都不大。除了太陽、太陽系的八大行星和木星的衛星木衛三之外，土衛六是太陽系內最重的天體。土衛六的質量佔了環繞土星天體（包括土星環和其他質量在土衛六的百分之一到百萬分之一的小天體）的總質量的90%。

土星第二大的衛星土衛五可能有自己的環系統。

傳統上，土星的衛星的英文名稱都以希臘神話中的巨人來命名，這種慣例源自威廉·赫歇爾，土衛一（「Mimas」）和土衛二（「Enceladus」）的發現者，他在自1847年出版的《在好望角的天文觀測成果》中提出了這種命名法，理由是Mimas和Enceladus是克洛諾斯（希臘神話中的Saturn）的兄弟姐妹。

:每一顆衛星發現的時間請參考衛星發現時間列表。

土星的探索

古代觀測

在史前時代就已經知道土星的存在，在古代，它是除了地球之外已知的五顆行星中最遠的一顆，並且有與其特性相符的各式各樣的神話。在古羅馬神話中它是農神，從這顆行星所採用的名字，它是農業和收穫的神祇。羅馬人認為他與希臘神克洛諾斯，希臘人認為最外層的行星是神聖的克洛諾斯，而羅馬人也承襲這個傳統。

在印度占星學，有9個占星用的天體，像是著名的納瓦格拉哈歷（Navagraha，梵文：नवग्रह），土星是其中之一，稱為「Sani」或「Shani」，法官在眾行星之中，由大家共同評判各自的行為是好或是壞。古代的中國和日本文化依據中國的五行之說選定這顆行星是土星，是在傳統上用於自然分類的元素之一。在古希伯來語，土星稱為「Shabbathai」，它的天使是卡西爾（Cassiel），意思是智慧之神或有益於身心的；是Agiel（精靈），它更為黑暗的一面就是惡魔（lzaz）。在奧圖曼土耳其使用的烏爾都語和馬來語，它的名稱是「Zuhal」，是從阿拉伯文زحل轉化過來的。

使用口徑1.5公分的望遠鏡就能看見土星環，但直到1610年伽利略用望遠鏡看了才知道它的存在。他雖然起初認為是在土星兩側的衛星，直到克里斯蒂安·惠更斯使用倍數更高的望遠鏡才看清楚並認為是環。惠更斯也發現了土星的衛星土衛六。不久之後，卡西尼發現了另外4顆衛星：土衛八、土衛五、土衛三和土衛四。在1675年，卡西尼也發現了著名的卡西尼縫。

之後一段時間都沒有進一步的有意義發現，直到1789年威廉·赫歇爾才再發現兩顆衛星：土衛一和土衛二。形狀不規則的土衛七和土衛六有著共振，是在1848年被英國發現的。

在1899年，威廉·亨利·皮克林發現土衛九，一顆極度不規則衛星，它沒有如同更大衛星般的同步轉動。菲比是第一顆被發現的這種衛星，它以周期超過一年的逆行軌道繞著土星公轉。在20世紀初期，對土衛六的研究在1944年確認他有濃厚的大氣層－ 這是在太陽系的衛星中很獨特的特徵。

先鋒11號飛越

1979年的9月，先鋒11號成為拜訪土星的第一個人造天體，它從距離行星雲層頂端20 000 公里處飛越，獲得了低分辨率的行星和一些衛星的影像，但影像的解析力上不足以分辨表面的特徵。這艘太空船也觀察了環，發現了環縫中有稀薄的物亍，換句話說，環縫不是空無一物的。先鋒11號也測量了土衛六的溫度。

航海家的飛越

在1980年11月，航海家1號太空船拜訪了土星系統，送回了第一批行星、環和衛星的高分辨率影像，這是第一次人們可以看清土星表面的變化和圍繞著它的各式各樣的衛星。航海家1號執行了近掠土衛六的任務，使人們對這顆衛星大氣層的認識增進了許多。但同時，它也證實了可見光是難以穿透土衛六大氣層的，因此還是未能觀察到土衛六表面的詳情。這次的近掠也改變了太空船的航向，使它的飛行軌道偏離了太陽系的平面。

差不多在一年之後的1981年8月，航海家2號繼續對土星系統進行研究，拍攝了更多土星衛星的近距離照片，並且也發現了土星環和大氣發生變化的證據。不幸的是，在飛越期間，太空船的轉動平台故障了兩三天，使得一些計畫中的影像無法拍攝。完成對土星的觀測之後，太空船利用土星的重力拋射朝向天王星飛去。

這艘太空船發現並確認了一些新的衛星在接近環或環的內部環繞著土星，也發現了一些新的小環縫：馬克士威縫（在C環內的縫）和Keeler環縫（在A環內一個寬42 公里的環縫）。

卡西尼太空船

在2004年7月1日，卡西尼-惠更斯號太空船完成SOI（土星軌道切入）的操縱進入了在土星附近環繞的軌道。在SOI之前，它已經廣泛的研究過這個系統。在2004年6月，它首度近距離的飛越土衛九，並送回了高解析度的影像和數據資料。

卡西尼號飛越土星最大的衛星，土衛六，並且用雷達影像獲得了大湖、海岸線以及許多海島和山的影像。在2004年12月25日釋放登陸艇惠更斯號之前，兩度飛越土衛六。惠更斯號在2005年1月14日登陸土衛六的表面，在大氣層中下降的途中和著陸以後送回了大量的數據。在2005年當中，卡西尼號多次飛越土衛六和其它的冰衛星。卡西尼號最後一次飛越土衛六是在2008年3月23日。

從2005年初，科學家追蹤由卡西尼號發現的土星上的閃電。這些閃電釋放出的能量比地球上的閃電強了1,000倍。此外，科學家也相信這場風暴是曾經見過的最強烈的一種。

在2006年3月10日，NASA宣佈經由卡西尼號的影像發現，在土衛二上的間歇泉噴發出的物質中含有液態水的證據，影像也顯示在冰冷的噴泉中有高聳的羽狀物散發出的液體顯示出有水的顆粒。依據加州理工學院安德魯英格索爾博士的解釋："太陽系其他的衛星有被數公里厚的冰凍外殼覆蓋著的液態水海洋，這與此處在地表之下數米，不超過10米的口袋中有液態水，不知會有什麼不同。"

在2006年9月20日，卡西尼號的影像揭露了一個之前未曾發現過的行星環，在較明亮的主要土星環帶之外和G與E環之內。明顯的，這個環的來源是土星的兩顆衛星像隕石一樣碰撞的結果。

在2006年7月，卡西尼號首度證明在土衛六的北極附近有碳氫化合物的湖，並在2007年1月獲得證實。在2007年3月，另外的影像發現在土衛六的北極附近有碳氫化合物的"海洋"，最大的一個幾乎有裏海那麼大。

在2006年10月，太空船在土星的南極偵測到一個直徑5,000公里並有眼牆的颶風。 2013年9月卡西尼號在土星北極地區另發現了一個巨大的颶風，風眼直徑2,000km，為地球上平均直徑的20倍。其風速可達150 m/s。

在2006年當中，太空船發現並證實了四顆新的衛星。它最初的任務在2008年完成第74圈的環繞之後即將結束。然而，美國國家航空暨太空總署在2008年4月15日已經宣布此一任務將再延長兩年。

2017年，卡西尼號完成最後一次任務後進入土星大氣，結束了對其長達十餘年的觀測分析。

最佳的觀測時機

土星是肉眼可見的五顆行星中距離最遠的一顆，其他四顆是水星、金星、火星和木星（天王星和灶神星在黑暗的環境下也能用肉眼看見），並且直到1781年發現天王星之前，是早期的天文學家所知道的最後一顆行星。以肉眼在夜晚看見的土星是一顆明亮的，發出淡黃色光芒的光點，光度通常在+1至0等之間，以29½年的週期在黃道上以黃道帶的眾星作為背景，繞行天球一週。多數人藉助於光學儀器（大的雙筒鏡或望遠鏡）的協助，以20倍以上的倍數，就能清楚的看見土星環。

土星是外行星，在合日（視覺上接近太陽）前後兩個月以外，其他時間也適合觀測。而跟外行星的性質一樣，當衝日時是觀測土星最好時候，因為土星衝日時，土星最亮（約0等）之餘，視直徑（角直徑）也最大，而且衝日前後，整夜可見。

在它出現在天空中可以觀賞的大部分時間，都是值得鼓勵大家觀賞的目標。在接近衝（行星的位置在離日度180°之處，也就是在天空中與太陽相對的方向上）的前後時段是觀賞土星和土星環的最佳時段。土星在2002年12月17日衝的時候，因為土星環以最有利的角度朝向地球，因此有最大的亮度。

Physical characteristics

Saturn is a gas giant composed predominantly of hydrogen and helium. It lacks a definite surface, though it may have a solid core. Saturn's rotation causes it to have the shape of an oblate spheroid; that is, it is flattened at the poles and bulges at its equator. Its equatorial and polar radii differ by almost 10%: 60,268 km versus 54,364 km. Jupiter, Uranus, and Neptune, the other giant planets in the Solar System, are also oblate but to a lesser extent. The combination of the bulge and rotation rate means that the effective surface gravity along the equator, , is 74% of what it is at the poles and is lower than the surface gravity of Earth. However, the equatorial escape velocity of nearly is much higher than that of Earth.

Saturn is the only planet of the Solar System that is less dense than water—about 30% less. Although Saturn's core is considerably denser than water, the average specific density of the planet is due to the atmosphere. Jupiter has 318 times Earth's mass, and Saturn is 95 times Earth's mass. Together, Jupiter and Saturn hold 92% of the total planetary mass in the Solar System.

Internal structure

Despite consisting mostly of hydrogen and helium, most of Saturn's mass is not in the gas phase, because hydrogen becomes a non-ideal liquid when the density is above , which is reached at a radius containing 99.9% of Saturn's mass. The temperature, pressure, and density inside Saturn all rise steadily toward the core, which causes hydrogen to be a metal in the deeper layers.

Standard planetary models suggest that the interior of Saturn is similar to that of Jupiter, having a small rocky core surrounded by hydrogen and helium, with trace amounts of various volatiles. This core is similar in composition to Earth, but is more dense. The examination of Saturn's gravitational moment, in combination with physical models of the interior, has allowed constraints to be placed on the mass of Saturn's core. In 2004, scientists estimated that the core must be 9–22 times the mass of Earth, which corresponds to a diameter of about 25,000 km. This is surrounded by a thicker liquid metallic hydrogen layer, followed by a liquid layer of helium-saturated molecular hydrogen that gradually transitions to a gas with increasing altitude. The outermost layer spans 1,000 km and consists of gas.

Saturn has a hot interior, reaching 11,700 °C at its core, and radiates 2.5 times more energy into space than it receives from the Sun. Jupiter's thermal energy is generated by the Kelvin–Helmholtz mechanism of slow gravitational compression, but such a process alone may not be sufficient to explain heat production for Saturn, because it is less massive. An alternative or additional mechanism may be generation of heat through the "raining out" of droplets of helium deep in Saturn's interior. As the droplets descend through the lower-density hydrogen, the process releases heat by friction and leaves Saturn's outer layers depleted of helium. These descending droplets may have accumulated into a helium shell surrounding the core. Rainfalls of diamonds have been suggested to occur within Saturn, as well as in Jupiter and ice giants Uranus and Neptune.

Atmosphere

The outer atmosphere of Saturn contains 96.3% molecular hydrogen and 3.25% helium by volume. The proportion of helium is significantly deficient compared to the abundance of this element in the Sun. The quantity of elements heavier than helium (metallicity) is not known precisely, but the proportions are assumed to match the primordial abundances from the formation of the Solar System. The total mass of these heavier elements is estimated to be 19–31 times the mass of the Earth, with a significant fraction located in Saturn's core region.

Trace amounts of ammonia, acetylene, ethane, propane, phosphine, and methane have been detected in Saturn's atmosphere. The upper clouds are composed of ammonia crystals, while the lower level clouds appear to consist of either ammonium hydrosulfide or water. Ultraviolet radiation from the Sun causes methane photolysis in the upper atmosphere, leading to a series of hydrocarbon chemical reactions with the resulting products being carried downward by eddies and diffusion. This photochemical cycle is modulated by Saturn's annual seasonal cycle.

Cloud layers

Saturn's atmosphere exhibits a banded pattern similar to Jupiter's, but Saturn's bands are much fainter and are much wider near the equator. The nomenclature used to describe these bands is the same as on Jupiter. Saturn's finer cloud patterns were not observed until the flybys of the Voyager spacecraft during the 1980s. Since then, Earth-based telescopy has improved to the point where regular observations can be made.

The composition of the clouds varies with depth and increasing pressure. In the upper cloud layers, with the temperature in the range 100–160 K and pressures extending between 0.5–2 bar, the clouds consist of ammonia ice. Water ice clouds begin at a level where the pressure is about 2.5 bar and extend down to 9.5 bar, where temperatures range from 185 to 270 K. Intermixed in this layer is a band of ammonium hydrosulfide ice, lying in the pressure range 3–6 bar with temperatures of 190–235 K. Finally, the lower layers, where pressures are between 10 and 20 bar and temperatures are 270–330 K, contains a region of water droplets with ammonia in aqueous solution.

Saturn's usually bland atmosphere occasionally exhibits long-lived ovals and other features common on Jupiter. In 1990, the Hubble Space Telescope imaged an enormous white cloud near Saturn's equator that was not present during the Voyager encounters, and in 1994 another smaller storm was observed. The 1990 storm was an example of a Great White Spot, a unique but short-lived phenomenon that occurs once every Saturnian year, roughly every 30 Earth years, around the time of the northern hemisphere's summer solstice. Previous Great White Spots were observed in 1876, 1903, 1933 and 1960, with the 1933 storm being the most famous. If the periodicity is maintained, another storm will occur in about 2020.

The winds on Saturn are the second fastest among the Solar System's planets, after Neptune's. Voyager data indicate peak easterly winds of . In images from the Cassini spacecraft during 2007, Saturn's northern hemisphere displayed a bright blue hue, similar to Uranus. The color was most likely caused by Rayleigh scattering. Thermography has shown that Saturn's south pole has a warm polar vortex, the only known example of such a phenomenon in the Solar System. Whereas temperatures on Saturn are normally −185 °C, temperatures on the vortex often reach as high as −122 °C, suspected to be the warmest spot on Saturn.

North pole hexagonal cloud pattern

A persisting hexagonal wave pattern around the north polar vortex in the atmosphere at about 78°N was first noted in the Voyager images. The sides of the hexagon are each about long, which is longer than the diameter of the Earth. The entire structure rotates with a period of (the same period as that of the planet's radio emissions) which is assumed to be equal to the period of rotation of Saturn's interior. The hexagonal feature does not shift in longitude like the other clouds in the visible atmosphere. The pattern's origin is a matter of much speculation. Most scientists think it is a standing wave pattern in the atmosphere. Polygonal shapes have been replicated in the laboratory through differential rotation of fluids.

South pole vortex

HST imaging of the south polar region indicates the presence of a jet stream, but no strong polar vortex nor any hexagonal standing wave. NASA reported in November 2006 that Cassini had observed a "hurricane-like" storm locked to the south pole that had a clearly defined eyewall. Eyewall clouds had not previously been seen on any planet other than Earth. For example, images from the Galileo spacecraft did not show an eyewall in the Great Red Spot of Jupiter.

The south pole storm may have been present for billions of years. This vortex is comparable to the size of Earth, and it has winds of 550 km/h.

Other features

Cassini observed a series of cloud features found in northern latitudes, nicknamed the "String of Pearls". These features are cloud clearings that reside in deeper cloud layers.

Magnetosphere

Saturn has an intrinsic magnetic field that has a simple, symmetric shape – a magnetic dipole. Its strength at the equator – 0.2 gauss (20 µT) – is approximately one twentieth of that of the field around Jupiter and slightly weaker than Earth's magnetic field. As a result, Saturn's magnetosphere is much smaller than Jupiter's. When Voyager 2 entered the magnetosphere, the solar wind pressure was high and the magnetosphere extended only 19 Saturn radii, or 1.1 million km (712,000 mi), although it enlarged within several hours, and remained so for about three days. Most probably, the magnetic field is generated similarly to that of Jupiter – by currents in the liquid metallic-hydrogen layer called a metallic-hydrogen dynamo. This magnetosphere is efficient at deflecting the solar wind particles from the Sun. The moon Titan orbits within the outer part of Saturn's magnetosphere and contributes plasma from the ionized particles in Titan's outer atmosphere. Saturn's magnetosphere, like Earth's, produces aurorae.

Orbit and rotation

The average distance between Saturn and the Sun is over 1.4 billion kilometers (9 AU). With an average orbital speed of 9.68 km/s, it takes Saturn 10,759 Earth days (or about  years) to finish one revolution around the Sun. As a consequence, it forms a near 5:2 mean-motion resonance with Jupiter. The elliptical orbit of Saturn is inclined 2.48° relative to the orbital plane of the Earth. The perihelion and aphelion distances are, respectively, 9.195 and 9.957 AU, on average. The visible features on Saturn rotate at different rates depending on latitude and multiple rotation periods have been assigned to various regions (as in Jupiter's case).

Astronomers use three different systems for specifying the rotation rate of Saturn. System I has a period of (844.3°/d) and encompasses the Equatorial Zone, the South Equatorial Belt, and the North Equatorial Belt. The polar regions are considered to have rotation rates similar to System I. All other Saturnian latitudes, excluding the north and south polar regions, are indicated as System II and have been assigned a rotation period of (810.76°/d). System III refers to Saturn's internal rotation rate. Based on radio emissions from the planet detected by Voyager 1 and Voyager 2, System III has a rotation period of (810.8°/d). System III has largely superseded System II.

A precise value for the rotation period of the interior remains elusive. While approaching Saturn in 2004, Cassini found that the radio rotation period of Saturn had increased appreciably, to approximately . An estimate of Saturn's rotation (as an indicated rotation rate for Saturn as a whole) based on a compilation of various measurements from the Cassini, Voyager and Pioneer probes is . Studies of the planet's C Ring yield a rotation period of  .

In March 2007, it was found that the variation of radio emissions from the planet did not match Saturn's rotation rate. This variance may be caused by geyser activity on Saturn's moon Enceladus. The water vapor emitted into Saturn's orbit by this activity becomes charged and creates a drag upon Saturn's magnetic field, slowing its rotation slightly relative to the rotation of the planet.

An apparent oddity for Saturn is that it does not have any known trojan asteroids. These are minor planets that orbit the Sun at the stable Lagrangian points, designated L4 and L5, located at 60° angles to the planet along its orbit. Trojan asteroids have been discovered for Mars, Jupiter, Uranus, and Neptune. Orbital resonance mechanisms, including secular resonance, are believed to be the cause of the missing Saturnian trojans.

Natural satellites

Saturn has 82 known moons, 53 of which have formal names. In addition, there is evidence of dozens to hundreds of moonlets with diameters of 40–500 meters in Saturn's rings, which are not considered to be true moons. Titan, the largest moon, comprises more than 90% of the mass in orbit around Saturn, including the rings. Saturn's second-largest moon, Rhea, may have a tenuous ring system of its own, along with a tenuous atmosphere.

Many of the other moons are small: 34 are less than 10 km in diameter and another 14 between 10 and 50 km in diameter. Traditionally, most of Saturn's moons have been named after Titans of Greek mythology. Titan is the only satellite in the Solar System with a major atmosphere, in which a complex organic chemistry occurs. It is the only satellite with hydrocarbon lakes.

On 6 June 2013, scientists at the IAA-CSIC reported the detection of polycyclic aromatic hydrocarbons in the upper atmosphere of Titan, a possible precursor for life. On 23 June 2014, NASA claimed to have strong evidence that nitrogen in the atmosphere of Titan came from materials in the Oort cloud, associated with comets, and not from the materials that formed Saturn in earlier times.

Saturn's moon Enceladus, which seems similar in chemical makeup to comets, has often been regarded as a potential habitat for microbial life. Evidence of this possibility includes the satellite's salt-rich particles having an "ocean-like" composition that indicates most of Enceladus's expelled ice comes from the evaporation of liquid salt water. A 2015 flyby by Cassini through a plume on Enceladus found most of the ingredients to sustain life forms that live by methanogenesis.

In April 2014, NASA scientists reported the possible beginning of a new moon within the A Ring, which was imaged by Cassini on 15 April 2013.

Planetary rings

Saturn is probably best known for the system of planetary rings that makes it visually unique. The rings extend from outward from Saturn's equator and average approximately in thickness. They are composed predominantly of water ice, with trace amounts of tholin impurities and a peppered coating of approximately 7% amorphous carbon. The particles that make up the rings range in size from specks of dust up to 10 m. While the other gas giants also have ring systems, Saturn's is the largest and most visible.

There are two main hypotheses regarding the origin of the rings. One hypothesis is that the rings are remnants of a destroyed moon of Saturn. The second hypothesis is that the rings are left over from the original nebular material from which Saturn was formed. Some ice in the E ring comes from the moon Enceladus's geysers. The water abundance of the rings varies radially, with the outermost ring A being the most pure in ice water. This abundance variance may be explained by meteor bombardment.

Beyond the main rings, at a distance of 12 million km from the planet is the sparse Phoebe ring. It is tilted at an angle of 27° to the other rings and, like Phoebe, orbits in retrograde fashion.

Some of the moons of Saturn, including Pandora and Prometheus, act as shepherd moons to confine the rings and prevent them from spreading out. Pan and Atlas cause weak, linear density waves in Saturn's rings that have yielded more reliable calculations of their masses.

History of observation and exploration

The observation and exploration of Saturn can be divided into three phases. The first phase is ancient observations (such as with the naked eye), before the invention of modern telescopes. The second phase began in the 17th century, with telescopic observations from Earth, which improved over time. The third phase is visitation by space probes, in orbit or on flyby. In the 21st century, telescopic observations continue from Earth (including Earth-orbiting observatories like the Hubble Space Telescope) and, until its 2017 retirement, from the Cassini orbiter around Saturn.

Ancient observations

Saturn has been known since prehistoric times, and in early recorded history it was a major character in various mythologies. Babylonian astronomers systematically observed and recorded the movements of Saturn. In ancient Greek, the planet was known as Phainon, and in Roman times it was known as the "star of Saturn". In ancient Roman mythology, the planet Phainon was sacred to this agricultural god, from which the planet takes its modern name. The Romans considered the god Saturnus the equivalent of the Greek god Cronus; in modern Greek, the planet retains the name Cronus—Κρόνος|label=none: Kronos.

The Greek scientist Ptolemy based his calculations of Saturn's orbit on observations he made while it was in opposition. In Hindu astrology, there are nine astrological objects, known as Navagrahas. Saturn is known as "Shani" and judges everyone based on the good and bad deeds performed in life. Ancient Chinese and Japanese culture designated the planet Saturn as the "earth star" (土星). This was based on Five Elements which were traditionally used to classify natural elements.

In ancient Hebrew, Saturn is called 'Shabbathai'. Its angel is Cassiel. Its intelligence or beneficial spirit is 'Agȋȇl (אגיאל|ʿAgyal), and its darker spirit (demon) is Zȃzȇl (זאזל|Zazl). Zazel has been described as a great angel, invoked in Solomonic magic, who is "effective in love conjurations". In Ottoman Turkish, Urdu and Malay, the name of Zazel is 'Zuhal', derived from the Arabic language (زحل|Zuhal).

European observations (17th–19th centuries)

Saturn's rings require at least a 15-mm-diameter telescope to resolve and thus were not known to exist until Christiaan Huygens saw them in 1659. Galileo, with his primitive telescope in 1610, incorrectly thought of Saturn's appearing not quite round as two moons on Saturn's sides. It was not until Huygens used greater telescopic magnification that this notion was refuted, and the rings were truly seen for the first time. Huygens also discovered Saturn's moon Titan; Giovanni Domenico Cassini later discovered four other moons: Iapetus, Rhea, Tethys and Dione. In 1675, Cassini discovered the gap now known as the Cassini Division.

No further discoveries of significance were made until 1789 when William Herschel discovered two further moons, Mimas and Enceladus. The irregularly shaped satellite Hyperion, which has a resonance with Titan, was discovered in 1848 by a British team.

In 1899 William Henry Pickering discovered Phoebe, a highly irregular satellite that does not rotate synchronously with Saturn as the larger moons do. Phoebe was the first such satellite found and it takes more than a year to orbit Saturn in a retrograde orbit. During the early 20th century, research on Titan led to the confirmation in 1944 that it had a thick atmosphere – a feature unique among the Solar System's moons.

Modern NASA and ESA probes

Pioneer 11 flyby

Pioneer 11 made the first flyby of Saturn in September 1979, when it passed within 20,000 km of the planet's cloud tops. Images were taken of the planet and a few of its moons, although their resolution was too low to discern surface detail. The spacecraft also studied Saturn's rings, revealing the thin F-ring and the fact that dark gaps in the rings are bright when viewed at high phase angle (towards the Sun), meaning that they contain fine light-scattering material. In addition, Pioneer 11 measured the temperature of Titan.

Voyager flybys

In November 1980, the Voyager 1 probe visited the Saturn system. It sent back the first high-resolution images of the planet, its rings and satellites. Surface features of various moons were seen for the first time. Voyager 1 performed a close flyby of Titan, increasing knowledge of the atmosphere of the moon. It proved that Titan's atmosphere is impenetrable in visible wavelengths; therefore no surface details were seen. The flyby changed the spacecraft's trajectory out from the plane of the Solar System.

Almost a year later, in August 1981, Voyager 2 continued the study of the Saturn system. More close-up images of Saturn's moons were acquired, as well as evidence of changes in the atmosphere and the rings. Unfortunately, during the flyby, the probe's turnable camera platform stuck for a couple of days and some planned imaging was lost. Saturn's gravity was used to direct the spacecraft's trajectory towards Uranus.

The probes discovered and confirmed several new satellites orbiting near or within the planet's rings, as well as the small Maxwell Gap (a gap within the C Ring) and Keeler gap (a 42 km-wide gap in the A Ring).

Cassini–Huygens spacecraft

The Cassini–Huygens space probe entered orbit around Saturn on 1 July 2004. In June 2004, it conducted a close flyby of Phoebe, sending back high-resolution images and data. Cassini flyby of Saturn's largest moon, Titan, captured radar images of large lakes and their coastlines with numerous islands and mountains. The orbiter completed two Titan flybys before releasing the Huygens probe on 25 December 2004. Huygens descended onto the surface of Titan on 14 January 2005.

Starting in early 2005, scientists used Cassini to track lightning on Saturn. The power of the lightning is approximately 1,000 times that of lightning on Earth.

In 2006, NASA reported that Cassini had found evidence of liquid water reservoirs no more than tens of meters below the surface that erupt in geysers on Saturn's moon Enceladus. These jets of icy particles are emitted into orbit around Saturn from vents in the moon's south polar region. Over 100 geysers have been identified on Enceladus. In May 2011, NASA scientists reported that Enceladus "is emerging as the most habitable spot beyond Earth in the Solar System for life as we know it".

Cassini photographs have revealed a previously undiscovered planetary ring, outside the brighter main rings of Saturn and inside the G and E rings. The source of this ring is hypothesized to be the crashing of a meteoroid off Janus and Epimetheus. In July 2006, images were returned of hydrocarbon lakes near Titan's north pole, the presence of which were confirmed in January 2007. In March 2007, hydrocarbon seas were found near the North pole, the largest of which is almost the size of the Caspian Sea. In October 2006, the probe detected an 8,000 km diameter cyclone-like storm with an eyewall at Saturn's south pole.

From 2004 to 2 November 2009, the probe discovered and confirmed eight new satellites. In April 2013 Cassini sent back images of a hurricane at the planet's north pole 20 times larger than those found on Earth, with winds faster than . On 15 September 2017, the Cassini-Huygens spacecraft performed the "Grand Finale" of its mission: a number of passes through gaps between Saturn and Saturn's inner rings. The atmospheric entry of Cassini ended the mission.

Possible future missions

The continued exploration of Saturn is still considered to be a viable option for NASA as part of their ongoing New Frontiers program of missions. NASA previously requested for plans to be put forward for a mission to Saturn that included the Saturn Atmospheric Entry Probe, and possible investigations into the habitability and possible discovery of life on Saturn's moons Titan and Enceladus by Dragonfly.

Observation

Saturn is the most distant of the five planets easily visible to the naked eye from Earth, the other four being Mercury, Venus, Mars and Jupiter. (Uranus, and occasionally 4 Vesta, are visible to the naked eye in dark skies.) Saturn appears to the naked eye in the night sky as a bright, yellowish point of light. The mean apparent magnitude of Saturn is 0.46 with a standard deviation of 0.34. Most of the magnitude variation is due to the inclination of the ring system relative to the Sun and Earth. The brightest magnitude, −0.55, occurs near in time to when the plane of the rings is inclined most highly, and the faintest magnitude, 1.17, occurs around the time when they are least inclined. It takes approximately 29.5 years for the planet to complete an entire circuit of the ecliptic against the background constellations of the zodiac. Most people will require an optical aid (very large binoculars or a small telescope) that magnifies at least 30 times to achieve an image of Saturn's rings in which clear resolution is present. When Earth passes through the ring plane, which occurs twice every Saturnian year (roughly every 15 Earth years), the rings briefly disappear from view because they are so thin. Such a "disappearance" will next occur in 2025, but Saturn will be too close to the Sun for observations.

Saturn and its rings are best seen when the planet is at, or near, opposition, the configuration of a planet when it is at an elongation of 180°, and thus appears opposite the Sun in the sky. A Saturnian opposition occurs every year—approximately every 378 days—and results in the planet appearing at its brightest. Both the Earth and Saturn orbit the Sun on eccentric orbits, which means their distances from the Sun vary over time, and therefore so do their distances from each other, hence varying the brightness of Saturn from one opposition to the next. Saturn also appears brighter when the rings are angled such that they are more visible. For example, during the opposition of 17 December 2002, Saturn appeared at its brightest due to a favorable orientation of its rings relative to the Earth, even though Saturn was closer to the Earth and Sun in late 2003.

From time to time, Saturn is occulted by the Moon (that is, the Moon covers up Saturn in the sky). As with all the planets in the Solar System, occultations of Saturn occur in "seasons". Saturnian occultations will take place monthly for about a 12-month period, followed by about a five-year period in which no such activity is registered. The Moon's orbit is inclined by several degrees relative to Saturn's, so occultations will only occur when Saturn is near one of the points in the sky where the two planes intersect (both the length of Saturn's year and the 18.6-Earth year nodal precession period of the Moon's orbit influence the periodicity).