特徵

金星是太陽系的四顆類地行星之一，因為它的大小、質量、體積與到太陽的距離，均與地球相似，所以經常被稱為地球的姊妹或攣生兄弟。它的直徑是12,092公里（只比地球少 650公里），質量是地球的81.5%。但金星表面的狀況與地球完全不同，其大氣層的質量96.5%是二氧化碳，其餘的3.5%絕大部分是氮氣。

地理

直到行星科學家在20世紀揭示了它的某些秘密之前，金星表面一直是人們猜測的話題。它最後的影像來自麥哲倫號在1990-1991年間的探測，顯示其表面有大量且廣泛的火山活動，大氣層中的硫顯示最近可能還有過噴發。

金星表面的80%被光滑的火山平原覆蓋著，70%的平原有著皺褶脊和10%是平滑或有著碎裂的平原。兩個高原構成其餘30%的表面地區，一個在行星的北半球，另一個正好在赤道的南邊。北方大陸的大小和澳洲差不多，依據巴比倫的愛神，伊師塔（Ishtar）命名為伊師塔地。金星上最高的山峰在伊斯塔地，稱為馬克士威山，它的標高是金星平均表面之上11公里。金星上最高的火山則是瑪阿特山，它高出行星半徑8公里，較周邊平原高出接近5公里。在南半球的大陸是這兩個高原中較大的一個，依據希臘的愛神命名，稱為阿佛洛狄忒陸，大小與非洲大陸相當。這個地區的部分份被斷裂的網狀結構和斷層覆蓋著。

由於缺乏熔岩流的伴隨，隨處可見的破火山口仍然是個謎。這顆行星只有少數的撞擊坑，顯示這顆行星表面相對的年輕，大約只有3-6億年的歷史。除了撞擊坑、山脈、山谷等在岩石行星常見的地形，金星表面有一些獨特的特徵。平頂的火山地形稱為Farra，看起來像薄煎餅，大小的範圍從20至50公里，高度從100至1000公尺；輻射狀、星形的地形系統，稱為novae；有著類似蜘蛛網的輻射狀和同心斷裂外觀的，稱為蛛網膜地形（arachnoid）；coronae是有著同心圓環的凹地；這些都是火山地形。

金星表面的地形幾乎全都以歷史上和神話中的女性命名。少數的例外的是以詹姆斯·克拉克·馬克士威的名字命名馬克士威山，和阿爾法區、貝塔區和奧瓦達區這三個高原地區。前述三個地區是在國際天文學聯合會的行星命名監督機構，通過現行的命名制度之前命名的。

金星上天然的地形以相對於其本初子午線的經度來表示。原本選擇的子午線是通過阿爾法區南部，在雷達下呈現亮點的橢圓形Eve的中心。在金星任務完成後，重新定義的本初子午線為通過阿喇阿德涅火山口中央峰的經線。

表面地質

大部分的金星表面似乎都是火山活動形成的，金星的火山數量是地球的好幾倍，它擁有167座直徑超過100公里的大型火山。地球上，只有夏威夷大島的複雜火山的大小可以和金星比較。這不是因為金星的火山比地球活躍，而是因為它的地殼比地球古老。地球的海洋地殼在板塊的邊界不斷的俯衝而下，使得平均年齡小於一億年，而金星表面的年齡估計在3至6億年間。

幾條線索指出金星上的火山仍在活動中。前蘇聯的金星計劃，金星11號和金星12號探測器偵測到絡繹不絕的閃電，金星12號降落之後不久，就記錄到強大的雷聲。歐洲太空總署的金星特快車記錄到高層大氣中豐富的閃電。 雖然地球上的雷暴伴隨著降雨，但是金星表面不會下雨（儘管在大氣層的上層會落下硫酸雨，但在25公里的高處就會因高溫蒸發）。產生閃電的一種可能是來自火山灰的噴發。另一種證據來自大氣層中的二氧化硫濃度，在1978年至1986年間的測量，其濃度下降了10倍。這意味著，早些時有大型的火山爆發在進行。

金星上有近千個撞擊坑均勻的分布在其表面。在其它天體上的撞擊坑，例如地球和月球，撞擊坑展現出一系列衰退的狀況。在月球，衰退是由於後續的撞擊；在地球，是因為風和雨水的侵蝕。在金星，85%的撞擊坑保持著原始的狀態。撞擊坑的數量，以及其保存在完好的狀態下，顯示這顆行星大約在3億年前經歷了一次全球性的事件，隨後火山活動即開始衰減。地球的地殼是不斷的運動，而金星被認為無法維持這一過程。沒有板塊構造從地函散熱，金星反而經歷一個使地函溫度升高的迴圈，直到它們達到臨界的水準，削弱了地殼。然後，大約在一億年的期間，發生大規模的地殼俯衝，使地殼完全重生。第一個火山活動持續的直接證據，出現在格尼奇峽谷的盾狀火山馬特山的帶狀裂口，發現了3個紅外線的閃光。這些閃光的溫度範圍為527-827℃，相信是氣體或熔岩從火山口釋出的噴發現象。

金星凹面的坑穴大小從3公里至280公里。由於濃稠的大氣影響到進入的天體，所以沒有小於3公里的坑穴。受到大氣層的減速，動能低於某一臨界值的天體，將無法碰撞出撞擊坑。進入的天體直徑若小於50公尺，將在墜落到表面之前就在大氣層中燒毀 。

內部結構

由于沒有地震或轉動慣量的資料，因此只有少許的直接資料可用於了解金星內部的結構和地質化學。與地球相似的大小和密度，顯示它和地球有著相似的共同內部構造：核、地函和地殼。像地球一樣，金星的核心至有一部分是液體，因為這兩顆行星冷卻的速率是相同的。體積略小的金星顯示出內部深處的壓力亦比地球的略小一些。這兩顆行星之間主要的區別在於金星缺乏板塊存在的證據，可能是因為它的外殼太堅硬，隱沒帶缺乏水而使它沒有黏度。這樣的結果使行星的熱難以散逸，阻止了它的冷卻，並提供其內部缺乏生成磁場機制的可能解釋。相反的，金星可能以週期性的重鋪地殼來散逸它內部的熱。

大氣層和氣候

金星有著密度極高的大氣層，其中主要包括二氧化碳和極少量的氮。金星大氣層的質量是地球大氣層的93倍，而其表面上的壓力是地球表面壓力的92倍左右，相當於在地球上深達1公里處的海洋下的壓力。在表面的密度是65公斤/米3，是水的6.5%。富含CO2的大氣層，與薄薄的一層二氧化硫，創造出太陽系最強大的溫室效應，使表面的溫度至少達到，遠遠高於實現滅菌所需要的溫度。這使得金星表面的溫度比水星更高，而水星表面的最低溫是，最高溫也只有。然而，金星的距離比水星遠離太陽將近2倍，所能接受的太陽輻照度只是水星的 25%。金星的表面經常被描述如同地獄般的場所。

研究表明數十億年前的金星大氣層很像現在的地球大氣層，並且表面上可能有許多的液態水，但是經過六億年至數十億年後，受到失控的溫室效應影響，造成原來的水都被蒸發掉，並使得在大氣層中的溫室氣體超過臨界的水準。雖然，在這個事件發生之後，星球的表面條件已不再適合任何像地球生物的生命存在，但在金星雲層的中層和低層是可能有生命存在的。

熱慣量和經由較低層大氣風傳導的熱，意味著儘管這顆行星自轉得很慢，但表面的溫度變化無論是白天或黑夜都不顯著。在表面的風是緩慢的，每小時只移動數公里，但由於表面的大氣密度高，它們施加巨大的壓力對抗障礙物和輸送表面的塵埃和小石塊。即使熱、壓力和缺乏氧氣都不是問題，這依然會使人很難單獨在表面行走移動。

在濃厚的CO2（二氧化碳）大氣層之上的是包含二氧化硫和硫酸水滴的濃厚雲層。這些雲反射和散射90%照射在其上的陽光回到太空中，並阻止了以可見光對金星表面的觀測。永久覆蓋的雲層意味著金星儘管比地球還靠近太陽，但表面不如地球明亮。在雲層頂端的風速高達，每4至5天就可以繞行金星一圈。金星的風速是自轉速度的60倍，地球上的最高風速只是地球自轉速度的10-20% 。

金星表面實際上是等溫，不僅是白晝和黑夜之間，包括赤道和南北兩極，都保持一個恆定的溫度。這顆行星自轉軸的傾斜很小－少於3°，相較於地球的23°－也減少了季節的溫度變化。可以察覺到的溫度變化只發在海拔高度的改變，因此金星的最高點，馬克士威山是溫度最低的地點，溫度大約是和大約的大氣壓力。在1995年，麥哲倫號在金星最高峰的頂部拍攝到和地面上的雪相似的高反光物質。儘管在溫度較高的地區，這種過程可以說是類似下雪的現象。較容易揮發的物質在表面上聚集，以氣體的形態上升到較高處，因為高海拔處的氣溫下降而冷凝，於是在那兒如同下雪般跌落回較低的表面。還不知道這種物質的成分，但是投機者的猜測已經從元素的碲到鉛硫化物（方鉛礦）都有。

金星的雲層也像地球上的雲一樣，可以產生閃電。從前蘇聯的金星探測器首度檢測出疑似閃電的色譜開始，金星是否有閃電的爭議就一直存在。在2006–2007年，金星特快車明確發現了閃電的証據哨聲波，它們間歇性出現証明金星存在氣象活動。閃電的比率至少有地球的一半。在2007年，金星特快車還探測到南極存在著巨大的雙大氣渦旋。

在2011年，金星特快車又在金星的大氣層高處發現存在著臭氧層。

在2013年1月29日，歐洲太空總署的科學家報告在金星這顆行星的電離層有著類似於彗星離子尾條件的離子尾流。

磁場和核心

在1967年，金星4號發現金星有磁場，但是比地球的微弱。這個磁場是由電離層和太陽風相互作用誘導，而不是像地球這樣，由行星內部的發電機產生。金星微弱的磁場對大氣層提供的保護不足以抵抗宇宙射線的輻射，因而可以忽略其功能；而這種輻射可能導致雲層的放電。

金星的大小類似地球，在核心應該有類似的發電機機制，因此缺乏內在的磁場令人驚訝。一架發電機需要三樣東西：導電的液體、旋轉和對流。在地球，因為液體層的底部比頂端熱許多，對流出現在核心外層的液體。在金星，整顆星球的表面重新鋪設的事件，導致通過地殼的熱通量減少，並可能使得板塊活動因而結束。這會導致地函的溫度增加，從而減少核心向外的熱通量，來自核心的熱被用於加熱地殼。

對於金星缺乏磁場，目前主要幾種說法如下：

• 理論一：其核心被認為是導電的，雖然它的旋轉很慢，但模擬的結果認為它還是足夠成為「發電機」。這意味著金星的核心只是因為缺少對流，所以不能成為「發電機」。

• 理論二：金星沒有固體的內核，或它的核心已經冷卻，整個核心的液體部分有著幾乎相同的溫度。

• 理論三：核心已經完全固化。核心的狀態與目前尚未知的硫濃度有著密切的關連性。

• 理論四：與理論一相反，2006年金星特快車探勘金星後，認為轉速過慢不足以產生磁場，可能遭遇過類似「大碰撞」的撞擊所導致。

環繞金星的微弱磁圈意味著是太陽風和金星大氣層直接交互作用的結果。此處，氫和氧的離子是中性的分子被紫外線輻射解離所創造的。然後，太陽風提供這些離子足夠逃離金星引力場的速度和能量。這種侵蝕的過程使大氣層內的低質量的氫、氦和氧離子不斷流失，而質量較大的分子，像二氧化碳則更有可能被保留。太陽風對大氣的侵蝕，可能導致金星在形成後的前十億年間就丟失大部分的水分。侵蝕使高質量氘與低質量氫的比率增加，在高層的大氣比低層的高出150倍。

軌道和自轉

金星以平均距離的軌道繞著太陽公轉，完成一圈的時間大約是224.65地球日。雖然所有行星的軌道都是橢圓形，但是金星的軌道最接近圓形，離心率小於0.01。金星它位於地球和太陽的連線之間時，稱為下合（內合）。這時它比任何其他行星更最靠近地球，距離大約是4,100萬公里。它與地球的會合週期平均是584天。歸功於地球的軌道離心率衰減，這個最接近的距離將會以超過10,000年的週期改變。從1至5383年，有526次的距離會小於4,000萬公里；接下來的60,158年都會超過。

從地球的北極方向觀察，太陽系所有的行星都是以逆時針方向在軌道上運行。大多數行星的自轉方向也是逆時針的（稱為順行自轉），但是金星的自轉方向不僅是順時針的（稱為逆行自轉），金星還需243地球日自轉，是所有行星中轉得最慢的。因為它的自轉是如此緩慢，所以它極度的接近球形。金星的恆星日比金星的地球日一年長（243相對於224.7地球日）。金星赤道的線速度為，而地球的則接近。自從麥哲倫號太空船抵達金星之後，它的自轉週期已經延長。因為是逆行的自轉，一個太陽日的長度明顯的短於恆星日，僅為116.75地球日（使得金星的太陽日短於水星太陽日的176個地球日）。一個金星年的長度是金星日（太陽日）的1.92倍。金星上的觀測者會看見太陽從西邊升起，然後從東邊落下；但實際上，由於不透明的雲層，在金星表面是看不見太陽的。

金星可能從太陽星雲中不同轉動週期和轉軸傾角的區域誕生，由於混沌的自旋和其它行星對其濃厚大氣的攝動和潮汐效應，經過數十億年的影響才達到現在的狀況。金星的自轉週期可能代表其潮汐受到太陽引力的鎖定，由太陽熱在濃稠的金星大氣層中創造出金星大氣潮，使旋轉逐漸趨於緩慢。平均584天接近地球一次的會合週期，幾乎正好是金星5個太陽日的長度，但是與地球的自旋軌道共振已經不被採信。

金星沒有天然的衛星，目前僅有小行星2002 VE68維持著準衛星軌道的關係。此外，它還曾有過其它的準衛星：兩顆暫時共軌的小行星，和。在17世紀， 喬瓦尼·卡西尼報告有一顆衛星環繞著金星，還將之命名為尼斯，並在其後的200年還有斷斷續續的觀測報告，但大多數被確認只是鄰近的背景恆星。加州理工學院的Alex Alemi's和大衛·史提芬遜在2006年研究早期太陽系的模型顯示，在數十億年前的巨大撞擊事件中，至少曾為金星創造一顆衛星；大約1,000萬年後，另一個撞擊事件反轉金星的自轉方向，造成金星的衛星逐漸螺旋向內，直到與金星撞擊而合併，若是稍後的撞擊創造出衛星，也會被以相同的方式吸收掉。另一種缺乏衛星的解釋是太陽強大的潮汐力，會使環繞內側類地行星的大型衛星軌道不穩定。

觀測

大部份時候金星比太陽以外的恆星明亮。當它是最靠近太陽的眉型月時，它的最大視星等亮度可以達到-4.9等，當它在太陽的背後最黯淡時，視星等依然有-3等。當高度足夠時，這顆行星的亮度足以在晴朗的夜空下照射出陰影，而且當太陽在接近地平線的低空時，也很容易看見它。由於它是一顆內側行星，所以它與太陽的距角（離日度）永遠小於47度。

金星在繞行太陽的軌道上每584天超越地球一次。當它超越地球時，它會從日落後可見的昏星（長庚星）變成日出之前可見的晨星（啟明星）。雖然水星也是內側的行星，但它的最大離日度只有28°，所以通常很難在晨昏濛影中見到，而金星在它最亮時很難不被看見。它的離日度越大，表示在日落後或日出前的黑暗中可以看見的時間越長。當它是天空中最明亮的光點時，通常會被誤報為不明飛行物（UFO）。美國總統吉米·卡特在1969年宣稱看見不明飛行物，事後分析被認為極可能就是金星。許多人曾誤以為金星是更奇特的東西。

透過望遠鏡觀察在軌道上的金星，它會顯示像月球的相位變化。當它在太陽的另一側時，這顆行星呈現小而圓滿的圖像。當它在最大的離日度時，會呈現半圓形的相位，並顯示較大的視直徑，而當它在靠近地球與太陽的這一側，也就是靠近地球且在夜空中最明亮時，會呈現細長的眉月形。當金星最大並且要呈現新月的相位時，在望遠鏡中可以看見光線被金星大氣層折射後在它周圍形成的光暈。金星的相位變化，曾經被伽利略作為証明哥白尼日心說的有力証據。

凌日

金星的軌道相對於地球的軌道略有傾斜，因此當金星行經地球和太陽之間時，通常不會橫越過太陽的表面。只有當下合時剛好也穿越地球的軌道平面時才會發生金星凌日的現象。目前發生金星凌日的循環週期是，會相隔大約或各出現一對間隔八年的凌日 —這是英國天文學家傑雷米亞·霍羅克斯在1639年首先發現的模式。

最近的一對是2004年6月8日和2012年6月5-6日。在許多地點都以適當的儀器進行現場觀測和線上直播觀賞這兩次的凌日。

前一次的一對凌日發生在1874年12月和1882年12月；下一次的一對是在2117年12月和2125年12月。在歷史上，凌日的觀測是很重要的，因為這可以讓天文學家確定天文單位的大小，霍羅克斯在1639年即藉此測量太陽系的大小。1768年，庫克船長前往大溪地，於1769年在當地觀測金星凌日之後，還航行到澳大利亞東岸。

合月 / 掩星

金星合月是一種較為常見天文現象，一般為金星的公轉軌道與月球距離較近。若此時金星的公轉軌道與月球呈現一條直線，則會發生月掩金星現象。

灰光

當這顆行星的相位是月牙形時，在黑暗側出現的微弱光照，稱為灰光，長久以來一直是觀測上的謎團。第一個聲稱看見灰光的觀測報告出現在1643年，但從來沒有可證實的可靠照明存在。觀測人員猜測這可能是金星大氣層中的電氣活動，但也可能是觀察明亮的月牙形區域後生理上產生的虛幻。

研究

早期的研究

眾所周知，金星在古文明被視為啟明星（晨星）和長庚星（昏星），反映出在早期假設這是兩顆不同的天體，所以各自有自己的名稱。在西元前1581年的金星碑都表明巴比倫人知道這兩個天體其實是相同的，在這塊板上稱之為明亮的天空女王，可以支持這一觀點和細緻的觀察。直到西元前六世紀的畢達哥拉斯，希臘人都認為這是兩顆不同的天體，凌晨的被稱為磷光體，日落後的才稱為金星（Hesperus）。

羅馬人稱凌晨方位的金星為曉星（Lucifer），字面上的意思是光明使者，晚上的是金星（Vesper），兩者都是承襲希臘名字字面上的翻譯。

首次觀測金星凌日是在1032年，觀測者是波斯天文學家阿維森納，它因此認為金星比地球更靠近太陽，並且認為金星，至少有些時候，是低於太陽。在12世紀，安達盧西亞的天文學家伊本·巴哲觀察到兩顆行星像黑點一樣的從太陽表面經過。後來，13世紀馬拉蓋的天文學家庫特布丁·設拉子認定是金星和水星的凌日。在1639年12月4日（以當時的儒略曆是1639年11月24日），傑雷米亞·霍羅克斯和它的朋友威廉·克萊布崔，在他們自己各自的住宅都觀測了金星凌日。

在17世紀，義大利物理學家伽利略首次觀察到這顆行星時，發現它和月球一樣有著相位變化，從眉形到凸月然後滿月，之後再反過來變化。當金星的距角最大時，它呈現半圓形；離太陽最近時（距角最小）顯示新月或滿月的圓形。只有金星環繞太陽運轉才有這種可能，這是首度觀測到與托勒密的地心模型，地球居於同心圓的太陽系中心矛盾的現象。

在1761年，俄羅斯的學者羅蒙諾索夫發現金星的大氣層。德國天文學家約翰·希羅尼穆斯·施羅特在1790年也觀測到金星的大氣層。施羅特發現這顆行星呈現彎彎的月牙形時，月牙的尖頂延長超過180度。他正確的推論這是因為陽光在稠密的大氣層中散射。後來，美國天文學家觀測到在內合時，在黑暗的一側有完整的光環圍繞著，進一步提供存在大氣層的證據 義大利出生的天文學家卡西尼和羅特，努力觀察金星表面複雜的大氣層在金星表面標示出的標記，不正確的估計金星的自轉週期為。

地形地貌

在金星表面的大平原上有兩個主要的大陸狀高地。北邊的高地叫伊師塔地，擁有金星最高的「馬克士威山脈（大約比喜馬拉雅山高出兩千米）」，它是根據詹姆斯·克拉克·馬克士威命名的。馬克士威山脈包圍了拉克西米高原。伊師塔地大約有澳大利亞那麼大。南半球有更大的阿佛洛狄忒陸，面積與南美洲相當。這些高地之間有許多廣闊的低地，包括有阿塔蘭塔平原低地、圭尼維爾平原低地以及拉維尼亞平原低地。除了馬克士威山脈外，所有的金星地貌均以現實中的或者神話中的女性命名。由于金星濃厚的大氣讓流星等天體在到達金星表面之前減速，所以金星上的隕石坑都不超過3.2千米。

大約90%的金星表面是由不久之前才固化的玄武岩熔岩形成，當然也有極少量的隕石坑。這表明金星近來正在經歷表面的重新構築。金星的內部可能與地球是相似的：半徑約3000千米的地核和由熔岩構成的地幔組成了金星的絕大部分。來自麥哲倫號的最近的數據表明金星的地殼比起原來所認為的更厚也更堅固。可以據此推測金星沒有像地球那樣的可移動的板塊構造，但是卻有大量的有規律的火山噴發遍布金星表面。金星上最古老的特徵僅有8億年曆史，大多數地區都相當年輕（但也有數億年的時間）。最近的發現表明，金星的火山在隔離的地質熱點依舊活躍。

金星本身的磁場與太陽系的其它行星相比是非常弱的。這可能是因為金星的自轉不夠快，其地核的液態鐵因對流產生的磁場較弱造成的。這樣一來，太陽風就可以毫無緩衝地撞擊金星上層大氣。最早的時候，人們認為金星和地球的水在量上相當，然而，太陽風的攻擊已經讓金星上層大氣的水蒸氣分解為氫和氧。氫原子因為質量小逃逸到了太空。金星上氘（氫的一種同位素，質量較大，逃逸得較慢）的比例似乎支持這種理論。而氧元素則與地殼中的物質化合，因而在大氣中沒有氧氣。金星表面十分乾旱，所以金星上的岩石要比地球上的更堅硬，從而形成了更陡峭的山脈、懸崖峭壁和其它地貌。

另外，根據探測器的探測，發現金星的岩漿裡含有水。

人類探索

在太空探測器探測金星以前，有的天文學家認為金星的化學和物理狀況和地球類似，在金星上發現生命的可能性比火星還大。1950年代後期，天文學家用射電望遠鏡第一次觀測了金星的表面。第一個機器人太空探索的金星任務，並且是首次探索任何星球，開始于1961年2月12日發射的金星1號探測器。從1961年起，蘇聯和美國向金星發射了30多個探測器，從近距離觀測，到著陸探測。

日本宇宙航空研究開發機構（JAXA）在2010年5月發射的金星探測器「破曉號」，原定在2010年12月7日進入金星軌道，但「破曉號」開始進行引擎反向噴射、準備減緩速度進入金星軌道時，通訊設備卻發生故障，與地面指揮中心短暫失去聯絡，以至於引擎停擺，與金星擦身而過。「破曉號」必須等到2016年後才能再度接近金星軌道，運作小組表示，屆時「破曉號」若仍完好無損，將再次挑戰。在飛行器繞太陽五年之後，工程師們通過點火其推進器20分鐘而進入一個替代的橢圓形金星軌道，JAXA于2015年12月9日下午六點宣布破曉號于2015年12月7日成功進入金星軌道。

金星殖民

相關條目

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• 金星凌日

• 行星系

• 太陽系流體靜力平衡天體列表

註解

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延伸閱讀

Physical characteristics

Venus is one of the four terrestrial planets in the Solar System, meaning that it is a rocky body like Earth. It is similar to Earth in size and mass and is often described as Earth's "sister" or "twin". Venus is very close to spherical due to its slow rotation. It has a diameter of —only less than Earth's—and its mass is 81.5% of Earth's, making it the third-smallest planet in the Solar System. Conditions on the surface of Venus differ radically from those on Earth because its dense atmosphere is 96.5% carbon dioxide, causing an intense greenhouse effect, with most of the remaining 3.5% being nitrogen. The surface pressure is , and the average surface temperature is , above the critical points of both major constituents and making the surface atmosphere a supercritical fluid of mainly supercritical carbon dioxide and some supercritical nitrogen.

Geography

The Venusian surface was a subject of speculation until some of its secrets were revealed by probes in the 20th century. Venera landers in 1975 and 1982 returned images of a surface covered in sediment and relatively angular rocks. The surface was mapped in detail by Magellan in 1990–91. There is evidence of extensive volcanism, and variations in the atmospheric sulphur dioxide may indicate that there are active volcanoes.

About 80% of the Venusian surface is covered by smooth, volcanic plains, consisting of 70% plains with wrinkle ridges and 10% smooth or lobate plains. Two highland "continents" make up the rest of its surface area, one lying in the planet's northern hemisphere and the other just south of the equator. The northern continent is called Ishtar Terra after Ishtar, the Babylonian goddess of love, and is about the size of Australia. The Maxwell Montes mountain range lies on Ishtar Terra. Its peak is the highest point on Venus, above the Venusian average surface elevation. The southern continent is called Aphrodite Terra, after the Greek mythological goddess of love, and is the larger of the two highland regions at roughly the size of South America. A network of fractures and faults covers much of this area.

There is recent evidence of lava flow on Venus (2024), such as flows on Sif Mons, a shield volcano, and on Niobe Planitia, a flat plain. There are visible calderas. The planet has few impact craters, demonstrating that the surface is relatively young, at 300–600million years old. Venus has some unique surface features in addition to the impact craters, mountains, and valleys commonly found on rocky planets. Among these are flat-topped volcanic features called "farra", which look somewhat like pancakes and range in size from across, and from high; radial, star-like fracture systems called "novae"; features with both radial and concentric fractures resembling spider webs, known as "arachnoids"; and "coronae", circular rings of fractures sometimes surrounded by a depression. These features are volcanic in origin.

Most Venusian surface features are named after historical and mythological women. Exceptions are Maxwell Montes, named after James Clerk Maxwell, and highland regions Alpha Regio, Beta Regio, and Ovda Regio. The last three features were named before the current system was adopted by the International Astronomical Union, the body which oversees planetary nomenclature.

The longitude of physical features on Venus is expressed relative to its prime meridian. The original prime meridian passed through the radar-bright spot at the centre of the oval feature Eve, located south of Alpha Regio. After the Venera missions were completed, the prime meridian was redefined to pass through the central peak in the crater Ariadne on Sedna Planitia.

The stratigraphically oldest tessera terrains have consistently lower thermal emissivity than the surrounding basaltic plains measured by Venus Express and Magellan, indicating a different, possibly a more felsic, mineral assemblage. The mechanism to generate a large amount of felsic crust usually requires the presence of a water ocean and plate tectonics, implying that habitable condition existed on early Venus, with large bodies of water at some point. However, the nature of tessera terrains is far from certain.

Studies reported in 2023 suggested for the first time that Venus may have had plate tectonics during ancient times and, as a result, may have had a more habitable environment, possibly one capable of sustaining life. Venus has gained interest as a case for research into the development of Earth-like planets and their habitability.

Volcanism

Much of the Venusian surface appears to have been shaped by volcanic activity. Venus has several times as many volcanoes as Earth, and it has 167 large volcanoes that are over across. The only volcanic complex of this size on Earth is the Big Island of Hawaii. More than 85,000 volcanoes on Venus have been identified and mapped. This is not because Venus is more volcanically active than Earth, but because its crust is older and is not subject to the erosion processes active on Earth. Earth's oceanic crust is continually recycled by subduction at the boundaries of tectonic plates, and has an average age of about 100 million years, whereas the Venusian surface is estimated to be 300–600million years old.

Several lines of evidence point to ongoing volcanic activity on Venus. Sulfur dioxide concentrations in the upper atmosphere dropped by a factor of 10 between 1978 and 1986, jumped in 2006, and again declined 10-fold. This may mean that levels were boosted several times by large volcanic eruptions. It has been suggested that Venusian lightning (discussed below) could originate from volcanic activity (i.e. volcanic lightning). In January 2020, astronomers reported evidence suggesting that Venus is currently volcanically active, specifically the detection of olivine, a volcanic product that would weather quickly on the planet's surface.

This massive volcanic activity is fuelled by a hot interior, which models say could be explained by energetic collisions when the planet was young, as well as radioactive decay as in the case of the earth. Impacts would have had significantly higher velocity than on Earth, both because Venus moves faster due to its closer proximity to the Sun and because high-eccentricity objects colliding with the planet would have high speeds.

In 2008 and 2009, the first direct evidence for ongoing volcanism was observed by Venus Express, in the form of four transient localized infrared hot spots within the rift zone Ganis Chasma, near the shield volcano Maat Mons. Three of the spots were observed in more than one successive orbit. These spots are thought to represent lava freshly released by volcanic eruptions. The actual temperatures are not known, because the size of the hot spots could not be measured, but are likely to have been in the range, relative to a normal temperature of . In 2023, scientists reexamined topographical images of the Maat Mons region taken by the Magellan orbiter. Using computer simulations, they determined that the topography had changed during an 8-month interval, and concluded that active volcanism was the cause.

Craters

There are almost a thousand impact craters on Venus, evenly distributed across its surface. On other cratered bodies, such as Earth and the Moon, craters show a range of states of degradation. On the Moon, degradation is caused by subsequent impacts, whereas on Earth it is caused by wind and rain erosion. On Venus, about 85% of the craters are in pristine condition. The number of craters, together with their well-preserved condition, indicates the planet underwent a global resurfacing event 300–600million years ago, followed by a decay in volcanism. Whereas Earth's crust is in continuous motion, Venus is thought to be unable to sustain such a process. Without plate tectonics to dissipate heat from its mantle, Venus instead undergoes a cyclical process in which mantle temperatures rise until they reach a critical level that weakens the crust. Then, over a period of about 100million years, subduction occurs on an enormous scale, completely recycling the crust.

Venusian craters range from in diameter. No craters are smaller than 3km, because of the effects of the dense atmosphere on incoming objects. Objects with less than a certain kinetic energy are slowed so much by the atmosphere that they do not create an impact crater. Incoming projectiles less than in diameter will fragment and burn up in the atmosphere before reaching the ground.

Internal structure

Without data from reflection seismology or knowledge of its moment of inertia, little direct information has been available about the internal structure and geochemistry of Venus. The similarity in size and density between Venus and Earth suggests that they share a similar internal structure: a core, mantle, and crust. Like that of Earth, the Venusian core is most likely at least partially liquid because the two planets have been cooling at about the same rate, although a completely solid core cannot be ruled out. The slightly smaller size of Venus means pressures are 24% lower in its deep interior than Earth's. The predicted values for the moment of inertia based on planetary models suggest a core radius of 2,900–3,450 km. There is now an estimate of 3,500 km from the moment of inertia based on the rate of axial precession, measured between 2006 and 2020.

The principal difference between the two planets is the lack of evidence for plate tectonics on Venus, possibly because its crust is too strong to subduct without water to make it less viscous. This results in reduced heat loss from the planet, preventing it from cooling and providing a likely explanation for its lack of an internally generated magnetic field. Instead, Venus may lose its internal heat in periodic major resurfacing events.

Magnetic field and core

In 1967, Venera 4 found Venus's magnetic field to be much weaker than that of Earth. This magnetic field is induced by an interaction between the ionosphere and the solar wind, rather than by an internal dynamo as in the Earth's core. Venus's small induced magnetosphere provides negligible protection to the atmosphere against solar and cosmic radiation.

The lack of an intrinsic magnetic field on Venus was surprising, given that it is similar to Earth in size and was expected to contain a dynamo at its core. A dynamo requires three things: a conducting liquid, rotation, and convection. The core is thought to be electrically conductive and, although its rotation is often thought to be too slow, simulations show it is adequate to produce a dynamo. This implies that the dynamo is missing because of a lack of convection in Venus's core. On Earth, convection occurs in the liquid outer layer of the core because the bottom of the liquid layer is much higher in temperature than the top. On Venus, a global resurfacing event may have shut down plate tectonics and led to a reduced heat flux through the crust. This insulating effect would cause the mantle temperature to increase, thereby reducing the heat flux out of the core. As a result, no internal geodynamo is available to drive a magnetic field. Instead, the heat from the core is reheating the crust.

One possibility is that Venus has no solid inner core, or that its core is not cooling, so that the entire liquid part of the core is at approximately the same temperature. Another possibility is that its core has already been completely solidified. The state of the core is highly dependent on the concentration of sulphur, which is unknown at present.

Another possibility is that the absence of a large impact on Venus (contra the Earth's "Moon-forming" impact) left the core of Venus stratified from the core's incremental formation, and without the forces to initiate/sustain convection, and thus a "geodynamo".

The weak magnetosphere around Venus means that the solar wind interacts directly with its outer atmosphere. Here, ions of hydrogen and oxygen are being created by the dissociation of water molecules due to ultraviolet radiation. The solar wind then supplies energy that gives some of these ions sufficient speed to escape Venus's gravity field. This erosion process results in a steady loss of low-mass hydrogen, helium, and oxygen ions, whereas higher-mass molecules, such as carbon dioxide, are more likely to be retained. Atmospheric erosion by the solar wind could have led to the loss of most of Venus's water during the first billion years after it formed. However, the planet may have retained a dynamo for its first 2–3 billion years, so the water loss may have occurred more recently. The erosion has increased the ratio of higher-mass deuterium to lower-mass hydrogen in the atmosphere 100 times compared to the rest of the solar system.

Atmosphere and climate

Venus has a dense atmosphere composed of 96.5% carbon dioxide, 3.5% nitrogen—both exist as supercritical fluids at the planet's surface with a density 6.5% that of water—and traces of other gases including sulphur dioxide. The mass of its atmosphere is 92 times that of Earth's, whereas the pressure at its surface is about 93 times that at Earth's—a pressure equivalent to that at a depth of nearly under Earth's ocean surfaces. The density at the surface is , 6.5% that of water or 50 times as dense as Earth's atmosphere at at sea level. The -rich atmosphere generates the strongest greenhouse effect in the Solar System, creating surface temperatures of at least . This makes the Venusian surface hotter than Mercury's, which has a minimum surface temperature of and maximum surface temperature of , even though Venus is nearly twice Mercury's distance from the Sun and thus receives only around a quarter of Mercury's solar irradiance, of 2,600 W/m2 (double that of Earth). Because of its runaway greenhouse effect, Venus has been identified by scientists such as Carl Sagan as a warning and research object linked to climate change on Earth. Therefore Venus has been called a greenhouse planet, a planet under a greenhouse inferno.

Venus's atmosphere is rich in primordial noble gases compared to that of Earth. This enrichment indicates an early divergence from Earth in evolution. An unusually large comet impact or accretion of a more massive primary atmosphere from the solar nebula have been proposed to explain the enrichment. However, the atmosphere is poor in radiogenic argon-40, a proxy for mantle degassing, suggesting an early shutdown of major magmatism.

Studies have suggested that billions of years ago, the atmosphere of Venus may have been much more like the one surrounding the early Earth, and there may have been substantial quantities of liquid water on the surface. After a period of 600 million to several billion years, the rising luminosity of the Sun and possibly large volcanic resurfacing caused the evaporation of the original water. A runaway greenhouse effect was created once a critical level of greenhouse gases (including water) was reached in the atmosphere. Although the surface conditions on Venus are no longer hospitable to any terrestrial-like life that might have formed before this event, there is speculation that life may exist in the upper cloud layers of Venus, above the surface, where atmospheric conditions are the most Earth-like in the Solar System, with temperatures ranging between , and the pressure and radiation being about the same as at Earth's surface, but with acidic clouds and the carbon dioxide air. More specifically, between heights of 48 and 59 km temperature and radiation conditions are suitable for life. At lower elevations water would evaporate and at higher elevation UV radiation would be too strong. The putative detection of an absorption line of phosphine in Venus's atmosphere, with no known pathway for abiotic production, led to speculation in September 2020 that there could be extant life currently present in the atmosphere. Later research attributed the spectroscopic signal that was interpreted as phosphine to sulphur dioxide, or found that in fact there was no absorption line.

Thermal inertia and the transfer of heat by winds in the lower atmosphere mean that the surface temperature does not vary significantly between the hemispheres facing and not facing the Sun, despite Venus's slow rotation. Winds at the surface are slow, moving at a few kilometres per hour, but because of the high density of the atmosphere at the surface, they exert a significant amount of force against obstructions, and transport dust and small stones across the surface. This alone would make it difficult for a human to walk through, even without the heat, pressure, and lack of oxygen.

Above the dense layer are thick clouds 45 to 70 km above the surface, consisting mainly of sulphuric acid, which is formed by a reaction catalyzed by UV radiation from sulphur dioxide molecules and then water, resulting in sulphuric acid hydrate. Additionally, the clouds contain approximately 1% ferric chloride. Other possible constituents of the cloud particles are ferric sulfate, aluminium chloride and phosphoric anhydride. Clouds at different levels have different compositions and particle size distributions. These clouds reflect, like thick cloud cover on Earth, about 70% of the sunlight that falls on them back into space, and since they cover the whole planet they prevent visual observation of the surface. The permanent cloud cover means that although Venus is closer than Earth to the Sun, it receives less sunlight on the ground, with only 10% of the received sunlight reaching the surface, resulting in average daytime levels of illumination at the surface of 14,000 lux, comparable to that on Earth "in the daytime with overcast clouds". Strong winds at the cloud tops go around Venus about every four to five Earth days. Winds on Venus move at up to 60 times the speed of its rotation, whereas Earth's fastest winds are only 10–20% rotation speed.

Although Venus looks featureless in visible light, there are bands or streaks in the UV, whose origin has not been pinned down. The absorption of UV may be due to a compound of oxygen and sulfur, OSSO, which has a double bond between the sulfur atoms and exists in "cis" and "trans" forms, or due to polysulfur compounds from to .

The surface of Venus is effectively isothermal; it retains a constant temperature not only between the two hemispheres but between the equator and the poles. Venus's minute axial tilt—less than 3°, compared to 23° on Earth—also minimizes seasonal temperature variation. Altitude is one of the few factors that affect Venusian temperatures. The highest point on Venus, Maxwell Montes, is therefore the coolest point on Venus, with a temperature of about and an atmospheric pressure of about . In 1995, the Magellan spacecraft imaged a highly reflective substance at the tops of the highest mountain peaks, a "Venus snow" that bore a strong resemblance to terrestrial snow. This substance likely formed by a similar process to snow, albeit at a far higher temperature. Too volatile to condense on the surface, it rose in gaseous form to higher elevations, where it is cooler and could precipitate. The identity of this substance is not known with certainty, but speculation has ranged from elemental tellurium to lead sulfide (galena).

Although Venus has no seasons, in 2019 astronomers identified a cyclical variation in sunlight absorption by the atmosphere, possibly caused by opaque, absorbing particles suspended in the upper clouds. The variation causes observed changes in the speed of Venus's zonal winds and appears to rise and fall in time with the Sun's 11-year sunspot cycle.

The existence of lightning in the atmosphere of Venus has been controversial since the first suspected bursts were detected by the Soviet Venera probes. In 2006–07, Venus Express clearly detected whistler mode waves, the signatures of lightning. Their intermittent appearance indicates a pattern associated with weather activity. According to these measurements, the lightning rate is at least half that on Earth, however other instruments have not detected lightning at all. The origin of any lightning remains unclear, but could originate from clouds or Venusian volcanoes.

In 2007, Venus Express discovered that a huge double atmospheric polar vortex exists at the south pole. Venus Express discovered, in 2011, that an ozone layer exists high in the atmosphere of Venus. In 2013 ESA scientists reported that the ionosphere of Venus streams outwards in a manner similar to "the ion tail seen streaming from a comet under similar conditions."

In December 2015, and to a lesser extent in April and May 2016, researchers working on Japan's Akatsuki mission observed bow-shaped objects in the atmosphere of Venus. This was considered direct evidence of the existence of perhaps the largest stationary gravity waves in the solar system.

Orbit and rotation

Venus orbits the Sun at an average distance of about , and completes an orbit every 224.7 days. It completes 13 orbits in 7.998 years, so its position in our sky almost repeats every eight years. Although all planetary orbits are elliptical, Venus's orbit is currently the closest to circular, with an eccentricity of less than 0.01. Simulations of the early solar system orbital dynamics have shown that the eccentricity of the Venus orbit may have been substantially larger in the past, reaching values as high as 0.31 and possibly impacting early climate evolution.

All planets in the Solar System orbit the Sun in an anticlockwise direction as viewed from above Earth's north pole. Most planets rotate on their axes in an anticlockwise direction, but Venus rotates clockwise in retrograde rotation once every 243 Earth days—the slowest rotation of any planet. This Venusian sidereal day lasts therefore longer than a Venusian year (243 versus 224.7 Earth days). Affected by the strong atmospheric current the length of the day also fluctuates by up to 20 minutes. Venus's equator rotates at , whereas Earth's rotates at . Venus's rotation period measured with Magellan spacecraft data over a 500-day period is smaller than the rotation period measured during the 16-year period between the Magellan spacecraft and Venus Express visits, with a difference of about 6.5minutes. Because of the retrograde rotation, the length of a solar day on Venus is significantly shorter than the sidereal day, at 116.75 Earth days.

One Venusian year is about 1.92Venusian solar days. To an observer on the surface of Venus, the Sun would rise in the west and set in the east, although Venus's opaque clouds prevent observing the Sun from the planet's surface.

Venus may have formed from the solar nebula with a different rotation period and obliquity, reaching its current state because of chaotic spin changes caused by planetary perturbations and tidal effects on its dense atmosphere, a change that would have occurred over the course of billions of years. The rotation period of Venus may represent an equilibrium state between tidal locking to the Sun's gravitation, which tends to slow rotation, and an atmospheric tide created by solar heating of the thick Venusian atmosphere. The 584-day average interval between successive close approaches to Earth is almost exactly equal to 5Venusian solar days (5.001444 to be precise), but the hypothesis of a spin-orbit resonance with Earth has been discounted.

Venus has no natural satellites. It has several trojan asteroids: the quasi-satellite and two other temporary trojans, and . In the 17th century, Giovanni Cassini reported a moon orbiting Venus, which was named Neith and numerous sightings were reported over the following , but most were determined to be stars in the vicinity. Alex Alemi's and David Stevenson's 2006 study of models of the early Solar System at the California Institute of Technology shows Venus likely had at least one moon created by a huge impact event billions of years ago. About 10millionyears later, according to the study, another impact reversed the planet's spin direction and the resulting tidal deceleration caused the Venusian moon gradually to spiral inward until it collided with Venus. If later impacts created moons, these were removed in the same way. An alternative explanation for the lack of satellites is the effect of strong solar tides, which can destabilize large satellites orbiting the inner terrestrial planets.

The orbital space of Venus has a dust ring-cloud, with a suspected origin either from Venus–trailing asteroids, interplanetary dust migrating in waves, or the remains of the Solar System's original circumstellar disc that formed the planetary system.

Orbit in respect to Earth

Earth and Venus have a near orbital resonance of 13:8 (Earth orbits eight times for every 13 orbits of Venus).

Therefore, they approach each other and reach inferior conjunction in synodic periods of 584 days, on average. The path that Venus makes in relation to Earth viewed geocentrically draws a pentagram over five synodic periods, shifting every period by 144°. This pentagram of Venus is sometimes referred to as the petals of Venus due to the path's visual similarity to a flower.

When Venus lies between Earth and the Sun in inferior conjunction, it makes the closest approach to Earth of any planet at an average distance of .

Because of the decreasing eccentricity of Earth's orbit, the minimum distances will become greater over tens of thousands of years. From the year1 to 5383, there are 526 approaches less than ; then, there are none for about 60,158 years.

While Venus approaches Earth the closest, Mercury is more often the closest to Earth of all planets and to any other planet. Venus has the lowest gravitational potential difference to Earth than any other planet, needing the lowest delta-v to transfer between them.

Tidally Venus exerts the third strongest tidal force on Earth, after the Moon and the Sun, though significantly less.

Observability

To the naked eye, Venus appears as a white point of light brighter than any other planet or star (apart from the Sun). The planet's mean apparent magnitude is −4.14 with a standard deviation of 0.31. The brightest magnitude occurs during the crescent phase about one month before or after an inferior conjunction. Venus fades to about magnitude −3 when it is backlit by the Sun, although the exact value depends on the phase angle. The planet is bright enough to be seen in broad daylight, but is more easily visible when the Sun is low on the horizon or setting. As an inferior planet, it always lies within about 47° of the Sun.

Venus "overtakes" Earth every 584 days as it orbits the Sun. As it does so, it changes from the "Evening Star", visible after sunset, to the "Morning Star", visible before sunrise. Although Mercury, the other inferior planet, reaches a maximum elongation of only 28° and is often difficult to discern in twilight, Venus is hard to miss when it is at its brightest. Its greater maximum elongation means it is visible in dark skies long after sunset. As the brightest point-like object in the sky, Venus is a commonly misreported "unidentified flying object".

Because Venus comes close to the earth at inferior conjunction and has an orbit inclined to the plane of the earth's orbit, it can appear more than 8° north or south of the ecliptic, more than any other planet or the moon. Every eight years around March it appears this far north of the ecliptic, in Pisces (such as in mid-March 2025), and every eight years it appears this far south of the ecliptic in August or September in Virgo (as in late August 2023). Venus can thus be north of the sun and appear as a morning star and an evening star on the same day, in the northern hemisphere. The timing of these north or south excursions gets slowly earlier in the year, and over 30 cycles (240 years) the cycle is gradually replaced by another cycle offset by three years, so the situation returns close to the original situation after 243 orbits of Earth, 395 of Venus.

Lunar occultations of Venus, in which the moon blocks the view of Venus for observers in certain parts of the earth, occur on average about twice a year, sometimes several times in a year (though rarely).

Phases

As it orbits the Sun, Venus displays phases like those of the Moon in a telescopic view. The planet appears as a small and "full" disc when it is on the opposite side of the Sun (at superior conjunction). Venus shows a larger disc and "quarter phase" at its maximum elongations from the Sun, and appears at its brightest in the night sky. The planet presents a much larger thin "crescent" in telescopic views as it passes along the near side between Earth and the Sun. Venus displays its largest size and "new phase" when it is between Earth and the Sun (at inferior conjunction). Its atmosphere is visible through telescopes by the halo of sunlight refracted around it. The phases are clearly visible in a 4" telescope. Although naked eye visibility of Venus's phases is disputed, records exist of observations of its crescent.

Daylight apparitions

When Venus is sufficiently bright with enough angular distance from the sun, it is easily observed in a clear daytime sky with the naked eye, though most people do not know to look for it. Astronomer Edmund Halley calculated its maximum naked eye brightness in 1716, when many Londoners were alarmed by its appearance in the daytime. French emperor Napoleon Bonaparte once witnessed a daytime apparition of the planet while at a reception in Luxembourg. Another historical daytime observation of the planet took place during the inauguration of the American president Abraham Lincoln in Washington, D.C., on 4March 1865.

Transits

A transit of Venus is the appearance of Venus in front of the Sun, during inferior conjunction. Since the orbit of Venus is slightly inclined relative to Earth's orbit, most inferior conjunctions with Earth, which occur every synodic period of 1.6 years, do not produce a transit of Venus. Consequently, Venus transits only occur when an inferior conjunction takes place during some days of June or December, when the orbits of Venus and Earth cross a straight line with the Sun. This results in Venus transiting above Earth in a sequence currently of , , and , forming cycles of .

Historically, transits of Venus were important, because they allowed astronomers to determine the size of the astronomical unit, and hence the size of the Solar System as shown by Jeremiah Horrocks in 1639 with the first known observation of a Venus transit (after history's first observed planetary transit in 1631, of Mercury).

Only seven Venus transits have been observed so far, since their occurrences were calculated in the 1621 by Johannes Kepler. Captain Cook sailed to Tahiti in 1768 to record the third observed transit of Venus, which subsequently resulted in the exploration of the east coast of Australia.

The latest pair was June 8, 2004 and June 5–6, 2012. The transit could be watched live from many online outlets or observed locally with the right equipment and conditions. The preceding pair of transits occurred in December 1874 and December 1882.

The next transit will occur in December 2117 and December 2125.

Ashen light

A long-standing mystery of Venus observations is the so-called ashen light—an apparent weak illumination of its dark side, seen when the planet is in the crescent phase. The first claimed observation of ashen light was made in 1643, but the existence of the illumination has never been reliably confirmed. Observers have speculated it may result from electrical activity in the Venusian atmosphere, but it could be illusory, resulting from the physiological effect of observing a bright, crescent-shaped object. The ashen light has often been sighted when Venus is in the evening sky, when the evening terminator of the planet is towards Earth.

Observation and exploration history

Early observation

Venus is in Earth's sky bright enough to be visible without aid, making it one of the classical planets that human cultures have known and identified throughout history, particularly for being the third brightest object in Earth's sky after the Sun and the Moon. Because the movements of Venus appear to be discontinuous (it disappears due to its proximity to the sun, for many days at a time, and then reappears on the other horizon), some cultures did not recognize Venus as a single entity; instead, they assumed it to be two separate stars on each horizon: the morning and evening star. Nonetheless, a cylinder seal from the Jemdet Nasr period and the Venus tablet of Ammisaduqa from the First Babylonian dynasty indicate that the ancient Sumerians already knew that the morning and evening stars were the same celestial object.

In the Old Babylonian period, the planet Venus was known as Ninsi'anna, and later as Dilbat. The name "Ninsi'anna" translates to "divine lady, illumination of heaven", which refers to Venus as the brightest visible "star". Earlier spellings of the name were written with the cuneiform sign si4 (= SU, meaning "to be red"), and the original meaning may have been "divine lady of the redness of heaven", in reference to the colour of the morning and evening sky.

The Chinese historically referred to the morning Venus as "the Great White" ( 太白) or "the Opener (Starter) of Brightness" ( 啟明), and the evening Venus as "the Excellent West One" ( 長庚).

The ancient Greeks initially believed Venus to be two separate stars: Phosphorus, the morning star, and Hesperus, the evening star. Pliny the Elder credited the realization that they were a single object to Pythagoras in the sixth century BC, while Diogenes Laërtius argued that Parmenides (early fifth century) was probably responsible for this discovery. Though they recognized Venus as a single object, the ancient Romans continued to designate the morning aspect of Venus as Lucifer, literally "Light-Bringer", and the evening aspect as Vesper, both of which are literal translations of their traditional Greek names.

In the second century, in his astronomical treatise Almagest, Ptolemy theorized that both Mercury and Venus were located between the Sun and the Earth. The 11th-century Persian astronomer Avicenna claimed to have observed a transit of Venus (although there is some doubt about it), which later astronomers took as confirmation of Ptolemy's theory. In the 12th century, the Andalusian astronomer Ibn Bajjah observed "two planets as black spots on the face of the Sun"; these were thought to be the transits of Venus and Mercury by 13th-century Maragha astronomer Qotb al-Din Shirazi, though this cannot be true as there were no Venus transits in Ibn Bajjah's lifetime.

Venus and early modern astronomy

When the Italian physicist Galileo Galilei first observed the planet with a telescope in the early 17th century, he found it showed phases like the Moon, varying from crescent to gibbous to full and vice versa. When Venus is furthest from the Sun in the sky, it shows a half-lit phase, and when it is closest to the Sun in the sky, it shows as a crescent or full phase. This could be possible only if Venus orbited the Sun, and this was among the first observations to clearly contradict the Ptolemaic geocentric model that the Solar System was concentric and centred on Earth.

The 1631 transit of Venus, while not recorded, was the first one successfully predicted, by Johannes Kepler and his calculations, which he published in 1629. The following 1639 transit of Venus was accurately predicted by Jeremiah Horrocks and observed by him and his friend, William Crabtree, at each of their respective homes, on 4December 1639 (24 November under the Julian calendar in use at that time).

The atmosphere of Venus was discovered in 1761 by Russian polymath Mikhail Lomonosov. Venus's atmosphere was observed in 1790 by German astronomer Johann Schröter. Schröter found when the planet was a thin crescent, the cusps extended through more than 180°. He correctly surmised this was due to scattering of sunlight in a dense atmosphere. Later, American astronomer Chester Smith Lyman observed a complete ring around the dark side of the planet when it was at inferior conjunction, providing further evidence for an atmosphere. The atmosphere complicated efforts to determine a rotation period for the planet, and observers such as Italian-born astronomer Giovanni Cassini and Schröter incorrectly estimated periods of about from the motions of markings on the planet's apparent surface.

Early 20th century advances

Little more was discovered about Venus until the 20th century. Its almost featureless disc gave no hint what its surface might be like, and it was only with the development of spectroscopic and ultraviolet observations that more of its secrets were revealed.

Spectroscopic observations in the 1900s gave the first clues about the Venusian rotation. Vesto Slipher tried to measure the Doppler shift of light from Venus, but found he could not detect any rotation. He surmised the planet must have a much longer rotation period than had previously been thought.

The first ultraviolet observations were carried out in the 1920s, when Frank E. Ross found that ultraviolet photographs revealed considerable detail that was absent in visible and infrared radiation. He suggested this was due to a dense, yellow lower atmosphere with high cirrus clouds above it.

It had been noted that Venus had no discernible oblateness in its disk, suggesting a slow rotation, and some astronomers concluded based on this that it was tidally locked like Mercury was believed to be at the time; but other researchers had detected a significant quantity of heat coming from the planet's nightside, suggesting a quick rotation (a high surface temperature was not suspected at the time), confusing the issue. Later work in the 1950s showed the rotation was retrograde.

Space age

Humanity's first interplanetary spaceflight was achieved in 1961 with the robotic space probe Venera 1 of the Soviet Venera programme flying to Venus, but it lost contact en route.

The first successful interplanetary mission, also to Venus, was Mariner 2 of the United States' Mariner programme, passing on 14 December 1962 at above the surface of Venus and gathering data on the planet's atmosphere.

Additionally radar observations of Venus were first carried out in the 1960s, and provided the first measurements of the rotation period, which were close to the actual value.

Venera 3, launched in 1966, became humanity's first probe and lander to reach and impact another celestial body other than the Moon, but could not return data as it crashed into the surface of Venus. In 1967, Venera 4 was launched and successfully deployed science experiments in the Venusian atmosphere before impacting. Venera 4 showed the surface temperature was hotter than Mariner 2 had calculated, at almost , determined that the atmosphere was 95% carbon dioxide, and discovered that Venus's atmosphere was considerably denser than Venera 4 designers had anticipated.

In an early example of space cooperation the data of Venera 4 was joined with the 1967 Mariner 5 data, analysed by a combined Soviet–American science team in a series of colloquia over the following year.

On 15 December 1970, Venera 7 became the first spacecraft to soft land on another planet and the first to transmit data from there back to Earth.

In 1974, Mariner 10 swung by Venus to bend its path towards Mercury and took ultraviolet photographs of the clouds, revealing the extraordinarily high wind speeds in the Venusian atmosphere. This was the first interplanetary gravity assist ever used, a technique which would be used by later probes.

Radar observations in the 1970s revealed details of the Venusian surface for the first time. Pulses of radio waves were beamed at the planet using the radio telescope at Arecibo Observatory, and the echoes revealed two highly reflective regions, designated the Alpha and Beta regions. The observations revealed a bright region attributed to mountains, which was called Maxwell Montes. These three features are now the only ones on Venus that do not have female names.

In 1975, the Soviet Venera 9 and 10 landers transmitted the first images from the surface of Venus, which were in black and white. NASA obtained additional data with the Pioneer Venus project, consisting of two separate missions: the Pioneer Venus Multiprobe and Pioneer Venus Orbiter, orbiting Venus between 1978 and 1992. In 1982 the first colour images of the surface were obtained with the Soviet Venera 13 and 14 landers. After Venera 15 and 16 operated between 1983 and 1984 in orbit, conducting detailed mapping of 25% of Venus's terrain (from the north pole to 30°N latitude), the Soviet Venera programme came to a close.

In 1985 the Soviet Vega programme with its Vega 1 and Vega 2 missions carried the last entry probes and carried the first ever extraterrestrial aerobots for the first time achieving atmospheric flight outside Earth by employing inflatable balloons.

Between 1990 and 1994, Magellan operated in orbit until deorbiting, mapping the surface of Venus. Furthermore, probes like Galileo (1990), Cassini–Huygens (1998/1999), and MESSENGER (2006/2007) visited Venus with flybys en route to other destinations.

In April 2006, Venus Express, the first dedicated Venus mission by the European Space Agency (ESA), entered orbit around Venus. Venus Express provided unprecedented observation of Venus's atmosphere. ESA concluded the Venus Express mission in December 2014 deorbiting it in January 2015.

In 2010, the first successful interplanetary solar sail spacecraft IKAROS travelled to Venus for a flyby.

Between 2015 and 2024 Japan's Akatsuki probe was active in orbit around Venus and BepiColombo performed flybys in 2020/2021.

Active and future missions

Several probes are under development as well as multiple proposed missions still in their early conceptual stages. NASA approved two missions to the planet, VERITAS and DAVINCI, planned to be launched not earlier then 2031. ESA plans to launch EnVision also in 2031. Indian ISRO is working on Venus Orbiter Mission, aiming to launch it in 2028. Besides these large missions, Rocket Lab is working on the first private mission to Venus, Venus Life Finder. UAE mission to asteroids, MBR Explorer, will perform a flyby of the planet.

Venus has been identified for future research as an important case for understanding:

• the origins of the solar system and Earth, and if systems and planets like ours are common or rare in the universe.

• how planetary bodies evolve from their primordial states to today's diverse objects.

• the development of conditions leading to habitable environments and life.

Search for life

Speculation on the possibility of life on Venus's surface decreased significantly after the early 1960s when it became clear that conditions were extreme compared to those on Earth. Venus's extreme temperatures and atmospheric pressure make water-based life, as currently known, unlikely.

Some scientists have speculated that thermoacidophilic extremophile microorganisms might exist in the cooler, acidic upper layers of the Venusian atmosphere. Such speculations go back to 1967, when Carl Sagan and Harold J. Morowitz suggested in a Nature article that tiny objects detected in Venus's clouds might be organisms similar to Earth's bacteria (which are of approximately the same size):

:While the surface conditions of Venus make the hypothesis of life there implausible, the clouds of Venus are a different story altogether. As was pointed out some years ago, water, carbon dioxide and sunlight—the prerequisites for photosynthesis—are plentiful in the vicinity of the clouds.

In August 2019, astronomers led by Yeon Joo Lee reported that long-term pattern of absorbance and albedo changes in the atmosphere of the planet Venus caused by "unknown absorbers", which may be chemicals or even large colonies of microorganisms high up in the atmosphere of the planet, affect the climate. Their light absorbance is almost identical to that of micro-organisms in Earth's clouds. Similar conclusions have been reached by other studies.

In September 2020, a team of astronomers led by Jane Greaves from Cardiff University announced the likely detection of phosphine, a gas not known to be produced by any known chemical processes on the Venusian surface or atmosphere, in the upper levels of the planet's clouds. One proposed source for this phosphine is living organisms. The phosphine was detected at heights of at least above the surface, and primarily at mid-latitudes with none detected at the poles. The discovery prompted NASA administrator Jim Bridenstine to publicly call for a new focus on the study of Venus, describing the phosphine find as "the most significant development yet in building the case for life off Earth".

Subsequent analysis of the data-processing used to identify phosphine in the atmosphere of Venus has raised concerns that the detection-line may be an artefact. The use of a 12th-order polynomial fit may have amplified noise and generated a false reading (see Runge's phenomenon). Observations of the atmosphere of Venus at other parts of the electromagnetic spectrum in which a phosphine absorption line would be expected did not detect phosphine. By late October 2020, re-analysis of data with a proper subtraction of background did not show a statistically significant detection of phosphine.

Members of the team around Greaves, are working as part of a project by the MIT to send with the rocket company Rocket Lab the first private interplanetary space craft, to look for organics by entering the atmosphere of Venus with a probe named Venus Life Finder.

Planetary protection

The Committee on Space Research is a scientific organization established by the International Council for Science. Among their responsibilities is the development of recommendations for avoiding interplanetary contamination. For this purpose, space missions are categorized into five groups. Due to the harsh surface environment of Venus, Venus has been under the planetary protection category two. This indicates that there is only a remote chance that spacecraft-borne contamination could compromise investigations.

Human presence

Venus is the place of the first interplanetary human presence, mediated through robotic missions, with the first successful landings on another planet and extraterrestrial body other than the Moon. The most recent orbital visit was by the Japanese Akatsuki (2015-2024). Other probes routinely use Venus for gravity assist manoeuvres capturing some data about Venus on the way.

The only nation that has sent lander probes to the surface of Venus has been the Soviet Union. The American Pioneer Venus Multiprobe has brought the only non-Soviet probes to enter the atmosphere, and one of its atmospheric entry probes was able to briefly send signals after impacting the surface.

Crewed flight

Studies of routes for crewed missions to Mars have since the 1960s proposed opposition missions instead of direct conjunction missions with Venus gravity assist flybys, demonstrating that they should be quicker and safer missions to Mars, with better return or abort flight windows, and less or the same amount of radiation exposure from the flight as direct Mars flights.

Early in the space age the Soviet Union and the United States proposed the TMK-MAVR and Manned Venus flyby crewed flyby missions to Venus, though they were never realized.

Habitation

While the surface conditions of Venus are inhospitable, the atmospheric pressure, temperature, and solar and cosmic radiation 50 km above the surface are similar to those at Earth's surface ("clement conditions"). With this in mind, Soviet engineer Sergey Zhitomirskiy (Сергей Житомирский, 1929–2004) in 1971 and NASA aerospace engineer Geoffrey A. Landis in 2003 suggested the use of aerostats for crewed exploration and possibly for permanent "floating cities" in the Venusian atmosphere, an alternative to the popular idea of living on planetary surfaces such as Mars. Among the many engineering challenges for any human presence in the atmosphere of Venus are the corrosive amounts of sulfuric acid in the atmosphere.

NASA's High Altitude Venus Operational Concept is a mission concept that proposed a crewed aerostat design.

In culture

Venus is a primary feature of the night sky, and so has been of remarkable importance in mythology, astrology and fiction throughout history and in different cultures.

Several hymns praise Inanna in her role as the goddess of the planet Venus. Theology professor Jeffrey Cooley has argued that, in many myths, Inanna's movements may correspond with the movements of the planet Venus in the sky. The discontinuous movements of Venus relate to both mythology as well as Inanna's dual nature. In Inanna's Descent to the Underworld, unlike any other deity, Inanna is able to descend into the netherworld and return to the heavens. The planet Venus appears to make a similar descent, setting in the West and then rising again in the East. An introductory hymn describes Inanna leaving the heavens and heading for Kur, what could be presumed to be, the mountains, replicating the rising and setting of Inanna to the West. In Inanna and Shukaletuda and Inanna's Descent into the Underworld appear to parallel the motion of the planet Venus. In Inanna and Shukaletuda, Shukaletuda is described as scanning the heavens in search of Inanna, possibly searching the eastern and western horizons. In the same myth, while searching for her attacker, Inanna herself makes several movements that correspond with the movements of Venus in the sky.

The Ancient Egyptians and ancient Greeks possibly knew by the second millennium BC or at the latest by the Late Period, under mesopotamian influence that the morning star and an evening star were one and the same. The Egyptians knew the morning star as Tioumoutiri and the evening star as Ouaiti. They depicted Venus at first as a phoenix or heron (see Bennu), calling it "the crosser" or "star with crosses", associating it with Osiris, and later depicting it two-headed with human or falco heads, and associated it with Horus, son of Isis (which during the even later Hellenistic period was together with Hathor identified with Aphrodite). The Greeks used the names Phōsphoros (ΦωσΦόρος), meaning "light-bringer" (whence the element phosphorus; alternately Ēōsphoros (ἨωσΦόρος), meaning "dawn-bringer"), for the morning star, and Hesperos (Ἕσπερος), meaning "Western one", for the evening star, both children of dawn Eos and therefore grandchildren of Aphrodite. Though by the Roman era they were recognized as one celestial object, known as "the star of Venus", the traditional two Greek names continued to be used, though usually translated to Latin as Lūcifer and Vesper.

Classical poets such as Homer, Sappho, Ovid and Virgil spoke of the star and its light. Poets such as William Blake, Robert Frost, Letitia Elizabeth Landon, Alfred Lord Tennyson and William Wordsworth wrote odes to it. The composer Holst included it as the second movement of his The Planets suite.

In India, Shukra Graha ("the planet Shukra") is named after the powerful saint Shukra. Shukra which is used in Indian Vedic astrology means "clear, pure" or "brightness, clearness" in Sanskrit. One of the nine Navagraha, it is held to affect wealth, pleasure and reproduction; it was the son of Bhrgu, preceptor of the Daityas, and guru of the Asuras.

The English name of Venus was originally the ancient Roman name for it. Romans named Venus after their goddess of love, who in turn was based on the ancient Greek goddess of love Aphrodite, who was herself based on the similar Sumerian religion goddess Inanna (which is Ishtar in Akkadian religion), all of whom were associated with the planet. The weekday of the planet and these goddesses is Friday, named after the Germanic goddess Frigg, who has been associated with the Roman goddess Venus.

Venus is known as Kejora in Indonesian and Malaysian Malay.

In Chinese the planet is called Jīn-xīng (金星), the golden planet of the metal element. Modern Chinese, Japanese, Korean and Vietnamese cultures refer to the planet literally as the "metal star" (金星), based on the Five elements.

The Maya considered Venus to be the most important celestial body after the Sun and Moon. They called it Chac ek, or Noh Ek', "the Great Star". The cycles of Venus were important to their calendar and were described in some of their books such as Maya Codex of Mexico and Dresden Codex. The Estrella Solitaria ("Lone Star") Flag of Chile depicts Venus.

Modern culture

The impenetrable Venusian cloud cover gave science fiction writers free rein to speculate on conditions at its surface; all the more so when early observations showed that not only was it similar in size to Earth, it possessed a substantial atmosphere. Closer to the Sun than Earth, the planet was often depicted as warmer, but still habitable by humans. The genre reached its peak between the 1930s and 1950s, at a time when science had revealed some aspects of Venus, but not yet the harsh reality of its surface conditions. Findings from the first missions to Venus showed reality to be quite different and brought this particular genre to an end. As scientific knowledge of Venus advanced, science fiction authors tried to keep pace, particularly by conjecturing human attempts to terraform Venus.

Symbols

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The symbol of a circle with a small cross beneath is the so-called Venus symbol, gaining its name for being used as the astronomical symbol for Venus. The symbol is of ancient Greek origin, and represents more generally femininity, adopted by biology as gender symbol for female, like the Mars symbol for male and sometimes the Mercury symbol for hermaphrodite. This gendered association of Venus and Mars has been used to pair them heteronormatively, describing women and men stereotypically as being so different that they can be understood as coming from different planets, an understanding popularized in 1992 by the book titled Men Are from Mars, Women Are from Venus.

The Venus symbol was also used in Western alchemy representing the element copper (like the symbol of Mercury is also the symbol of the element mercury), and since polished copper has been used for mirrors from antiquity the symbol for Venus has sometimes been called Venus mirror, representing the mirror of the goddess, although this origin has been discredited as an unlikely origin.

Besides the Venus symbol, many other symbols have been associated with Venus, other common ones are the crescent or particularly the star, as with the Star of Ishtar.