Iapetus | Astronomy, Geology & History

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Hubbard</div> <div class="editor-description font-12 font-serif mt-5 clamp-description text-black">Professor of Planetary Sciences, University of Arizona, Tucson. Author of <i>Planetary Interiors.</i></div> </a> <div data-popper-arrow></div> </div> <span class="btn btn-link editor-link p-0 qa-byline-link gtm-byline font-12 byline-contributor text-decoration-underline"> William B. 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They write new content and verify and edit content received from contributors.</div> </a> <div data-popper-arrow></div> </div> <span class="btn btn-link editor-link p-0 qa-byline-link font-12 "> The Editors of Encyclopaedia Britannica</span></div> <div class="last-updated font-12 font-serif"> <a class="byline-edit-history" href="https://www.britannica.com/topic/Iapetus-astronomy/additional-info#history" rel="nofollow">Article History</a> </div></div> </div> <button class="d-flex d-lg-none btn btn-outline-blue border rounded-sm shadow-sm mobile-toc-button gtm-mobile-toc-inline-button d-none d-sm-block js-sections-inline-button module-spacing btn d-lg-none"> <em class="material-icons mr-5 ml-n10 my-n5 md-icon" data-icon="toc"></em> Table of Contents </button> <div class="d-flex d-sm-none flex-row"> <button class="d-flex d-lg-none btn btn-outline-blue border rounded-sm shadow-sm mobile-toc-button gtm-mobile-toc-inline-button js-sections-inline-button module-spacing"> <em class="material-icons mr-5 ml-n10 my-n5 md-icon" data-icon="toc"></em> Table of Contents </button> <button class="ai-ask-button btn border-2 ai-ask-button btn border-2 module-spacing btn-sm js-inline-ai-ask-button btn-outline-red-400 border-red-400 p-10 ml-5"> Ask the Chatbot a Question </button> </div> <span class="marker before-article"></span><section data-level="1" id="ref1"><span class="marker PREMOD1 mod-inline"></span><p class="topic-paragraph"><strong><span id="ref277297"></span>Iapetus</strong>, outermost of <a href="https://www.britannica.com/place/Saturn-planet" class="md-crosslink " data-show-preview="true">Saturn</a>’s major regular <a href="https://www.britannica.com/science/moon-natural-satellite" class="md-crosslink " data-show-preview="true">moons</a>, extraordinary because of its great contrast in surface brightness. It was discovered by the Italian-born French astronomer <span id="ref277298"></span><a href="https://www.britannica.com/biography/Gian-Domenico-Cassini" class="md-crosslink " data-show-preview="true">Gian Domenico Cassini</a> in 1671 and named for one of the <a href="https://www.britannica.com/topic/Titan-Greek-mythology" class="md-crosslink " data-show-preview="true">Titan</a>s of <a href="https://www.britannica.com/topic/Greek-mythology" class="md-crosslink autoxref " data-show-preview="true">Greek mythology</a>.</p><span class="marker MOD1 mod-inline"></span><span class="marker PREMOD2 mod-inline"></span><p class="topic-paragraph">Iapetus has a radius of 718 km (446 miles) and orbits Saturn once every 79.3 <a href="https://www.britannica.com/place/Earth" class="md-crosslink autoxref " data-show-preview="true">Earth</a> days at a distance of 3,561,300 km (2,212,900 miles). Its bulk <a href="https://www.britannica.com/science/density" class="md-crosslink autoxref " data-show-preview="true">density</a> of 1.0 grams per cubic cm implies that it must be made mostly of ices. The closer moons of Saturn orbit within roughly one degree of Saturn’s equatorial plane, but, at Iapetus’s orbit and beyond, the gravitational influence of Saturn’s equatorial bulge becomes less important, permitting larger orbital <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="inclinations" href="https://www.britannica.com/dictionary/inclinations" data-type="EB">inclinations</a>. It has been suggested that Iapetus’s 15° average inclination is a relic of the tilt of the long-vanished gaseous disk from which Saturn’s major regular moons formed.</p><span class="marker MOD2 mod-inline"></span><span class="marker PREMOD3 mod-inline"></span><p class="topic-paragraph">Tidal interactions with Saturn have synchronized the rotation of Iapetus with its orbital period. As a result, the <a href="https://www.britannica.com/science/moon-natural-satellite" class="md-crosslink autoxref " data-show-preview="true">moon</a> always keeps the same face to Saturn and always leads with the same face in its orbital motion. Remarkably, the leading hemisphere is extremely dark, reflecting only a few percent of the sunlight falling on it, whereas the trailing hemisphere reflects as much as 60 percent of incident light. The reflectance at the poles is higher still. Iapetus displays the greatest variation in brightness of any object known in the <a href="https://www.britannica.com/science/solar-system" class="md-crosslink " data-show-preview="true">solar system</a>. Cassini himself wrote that, as Iapetus traveled in its orbit, he could observe it on one side of Saturn but not on the other, and he speculated correctly about the reason for this discrepancy.</p><a class="link-module shadow-sm d-block qa-quiz-module" href="/quiz/space-odyssey" data-link-module-iframe-link=""> <img loading="lazy" src="https://cdn.britannica.com/92/95392-131-F6284677/orbits-planets-bodies-solar-system.jpg" alt="The orbits of the planets and other elements of the solar system, including asteroids, Kuiper belt, Oort cloud, comet" class="rounded-sm mr-15" width="70" /> <div class="line-clamp clamp-5"> <div class="module-title bg-green">Britannica Quiz</div> <div class="font-weight-semi-bold mt-5">Space Odyssey</div> </div> </a><span class="marker MOD3 mod-inline"></span><span class="marker PREMOD4 mod-inline"></span><p class="topic-paragraph">Although the U.S. <span id="ref883393"></span><a href="https://www.britannica.com/technology/Voyager-space-probes" class="md-crosslink " data-show-preview="true">Voyager</a> spacecraft flybys revealed impact craters only on Iapetus’s bright trailing side, subsequent higher-resolution <span id="ref1102358"></span><a href="https://www.britannica.com/event/Cassini-Huygens" class="md-crosslink " data-show-preview="true">Cassini</a> spacecraft images show craters on the leading side as well. The surface material on the bright side is very nearly pure <a href="https://www.britannica.com/science/water" class="md-crosslink " data-show-preview="true">water</a> <a href="https://www.britannica.com/science/ice" class="md-crosslink autoxref " data-show-preview="true">ice</a>, possibly mixed with other ices. The material coating the surface of the dark side, which has a reddish hue, appears to be an <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="opaque" href="https://www.merriam-webster.com/dictionary/opaque" data-type="MW">opaque</a> layer of complex organic <a href="https://www.britannica.com/science/molecule" class="md-crosslink " data-show-preview="true">molecules</a> mixed with <a href="https://www.britannica.com/science/iron-chemical-element" class="md-crosslink " data-show-preview="true">iron</a>-bearing minerals that have been altered by water. The reflectivity difference is caused by dark material—composed of particles that originated from a ring of dust thrown into space by impacts on the outer moon <a href="https://www.britannica.com/place/Phoebe-astronomy" class="md-crosslink " data-show-preview="true">Phoebe</a>—collecting on the leading hemisphere of Iapetus and absorbing more sunlight, which heats up this region enough to cause significant <a href="https://www.britannica.com/science/sublimation-phase-change" class="md-crosslink " data-show-preview="true">sublimation</a> of water ice over <a href="https://www.britannica.com/science/geologic-time" class="md-crosslink autoxref " data-show-preview="true">geologic time</a>. The water vapour condenses onto the colder trailing hemisphere and freezes. From the distance of the Voyager images, the change between the dark and bright material appears to be gradual, but Cassini images taken closer to Iapetus show that the two materials are well segregated down to scales of about 20 metres (65 feet). Radar measurements from Cassini and ground-based radio telescopes, coupled with the presence of small craters on the dark side that have punched through to bright material below, suggest that the dark material is thin, perhaps 30 cm (1 foot) to several metres. The absence of any large fresh craters on the dark material—the craters would be prominent from excavated bright material—suggests that the process forming the dark material is ongoing or at least recent.</p><div class="one-good-fact-module"> </div><span class="marker MOD4 mod-inline"></span><span class="marker PREMOD5 mod-inline"></span><p class="topic-paragraph">The Cassini spacecraft imaged a remarkable narrow ridge encircling much of Iapetus’s equator. The ridge is about 20 km (13 miles) high and 20 km wide, and some areas are punctuated by a system of mountains about 10 km (6 miles) high. The heavily cratered surface of the ridge implies that it was formed very early in the history of Iapetus. Models suggest that it was formed by motions of a thin, active ice lithosphere when deeper layers of the moon were warm. On the other hand, the moon’s observed impact basins and other <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="topography" href="https://www.merriam-webster.com/dictionary/topography" data-type="MW">topography</a> generally require a thicker lithosphere. Possibly most of the features were formed when temperatures within the moon were changing rapidly during its first few million years of existence.</p><span class="marker MOD5 mod-inline"></span></section> <span class="md-signature"><a href="/contributor/William-B-Hubbard/3949">William B. Hubbard</a></span> <span class="md-signature"><a href="/contributor/Bonnie-Buratti/8059292">Bonnie Buratti</a></span> <span class="marker end-of-content"></span><span class="marker after-article"></span></div> <div id="chatbot-root"></div> </div> </div> </div> <div class="ai-dialog-placeholder"></div> </div> </div> <aside class="col-md-da-320"></aside> </div> </div> </div> </div> </article></div> </div></div> </div> </main> <div id="md-footer"></div> <noscript><iframe src="//www.googletagmanager.com/ns.html?id=GTM-5W6NC8" height="0" width="0" style="display:none;visibility:hidden"></iframe></noscript> <script type="text/javascript" id="_informizely_script_tag"> var IzWidget = IzWidget || {}; (function (d) { var scriptElement = d.createElement('script'); scriptElement.type = 'text/javascript'; scriptElement.async = true; scriptElement.src = "https://insitez.blob.core.windows.net/site/f780f33e-a610-4ac2-af81-3eb184037547.js"; var node = d.getElementById('_informizely_script_tag'); node.parentNode.insertBefore(scriptElement, node); } )(document); </script>  <script> window.ap3c = window.ap3c || {}; var ap3c = window.ap3c; ap3c.cmd = ap3c.cmd || []; ap3c.cmd.push(function() { ap3c.init('ZO4siT4cLwnykPnzZWJtd3Byb2Q', 'https://engage.email.britannica.com/'); ap3c.track({v: 0}); }); ap3c.activity = function(act) { ap3c.act = (ap3c.act || []); ap3c.act.push(act); }; var s, t; s = document.createElement('script'); s.type = 'text/javascript'; s.src = "https://engage.email.britannica.com/app.js"; t = document.getElementsByTagName('script')[0]; t.parentNode.insertBefore(s, t); </script> <script class="marketing-page-info" type="application/json"> {"pageType":"Topic","templateName":"DESKTOP","pageNumber":1,"pagesTotal":1,"pageId":280464,"pageLength":652,"initialLoad":true,"lastPageOfScroll":false} </script> <script class="marketing-content-info" type="application/json"> [] </script> <script src="https://cdn.britannica.com/mendel-resources/3-130/js/libs/jquery-3.5.0.min.js?v=3.130.14"></script> <script type="text/javascript" data-type="Init Mendel Code Splitting"> (function() { $.ajax({ dataType: 'script', cache: true, url: 'https://cdn.britannica.com/mendel-resources/3-130/dist/topic-page.js?v=3.130.14' }); })(); </script> <script class="analytics-metadata" type="application/json"> {"leg":"C","adLeg":"C","userType":"ANONYMOUS","pageType":"Topic","pageSubtype":null,"articleTemplateType":"MEDIUM","gisted":false,"pageNumber":1,"hasSummarizeButton":false,"hasAskButton":false} </script> <script type="text/javascript"> EBStat={accountId:-1,hostnameOverride:'webstats.eb.com',domain:'www.britannica.com', json:''}; </script> <script type="text/javascript"> ( function() { $.ajax( { dataType: 'script', cache: true, url: '//www.britannica.com/webstats/mendelstats.js?v=1' } ) .done( function() { try {writeStat(null,EBStat);} catch(err){} } ); })(); </script> <div id="bc-fixed-dialogue"></div> </body> </html>

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