Iapetus is the outermost regular moon of Saturn. It has a diameter of almost 1500 kilometers. One of my responsibilities for Cassini was the imaging observation planning of Iapetus as well as the observation planning of the sole close flyby during the mission (“049IA“) at the end of orbit 49 on 10 Sep 2007.

Many unusual properties are found on this object. Two of them are: (1) The global albedo dichotomy divides the surface globally into a very dark (almost as black as coal) and a quite bright part (as bright as snow which is not completely fresh anymore); (2) a giant ridge spans more than half of the surface exactly at the equator.

The albedo dichotomy was already recognized in 1677 by Jean-Dominique Cassini, the discoverer of Iapetus. However, the physical reason remained mysterious over 330  years, until the Cassini spacecraft sent images and surface temperature data of Iapetus back to Earth. The giant ridge was discovered in Cassini images from 25 Dec 2004. Hints for unusual mountains were already seen in data obtained by the Voyager-2 spacecraft in 1981. This web site compiles a short history of Iapetus research through related publications and web posts.

Iapetus ist der äußerste der sogenannten regulären Saturnmonde. Sein Durchmesser beträgt fast 1500 Kilometer. Während der Cassini-Mission war ich für die Kamera-Beobachtungsplanung dieses Mondes zuständig sowie für die Planung des einzigen nahen Vorbeiflugs an diesem Mond (“049IA“) am Ende von Orbit 49 am 10 Sep 2007.

Iapetus zeigt sehr viele ungewöhnliche Eigenschaften. Zwei davon sind: (1) Die globale Helligkeitsdichotomie, welche die Oberfläche global in eine sehr dunkle (fast so schwarz wie Kohle) und eine recht helle Seite (so hell wie nicht mehr ganz frischer Schnee) unterteilt; (2) ein gigantischer Bergrücken, der mehr als die Hälfte des Iapetus-Umfangs exakt am Äquator umspannt.

Die Helligkeitsdichotomie war bereits von Jean-Dominique Cassini, dem Entdecker von Iapetus, im Jahr 1677 beschrieben worden. Die physikalische Ursache hingegen blieb über 330 Jahre mysteriös und konnte erst mit Bildern und Temperaturdaten der Cassini-Sonde enträtselt werden. Der Bergrücken wurde in Bildern vom 25 Dez 2004 entdeckt. Hinweise auf ungewöhnlich hohe Berge waren schon zuvor in Aufnahmen gefunden worden, die der Raumsonde Voyager-2 im Jahr 1981 gelangen. Diese Webseite bietet eine kurze Geschichte der Iapetus-Forschung.

Iapetus in Google Maps — Link

2010 — Iapetus Global Brightness Dichotomy Enigma Solved

Science Magazine (22 Jan 2010):
Denk, T., Neukum, G., Roatsch, Th., Porco, C.C., Burns, J.A., Galuba, G.G., Schmedemann, N., Helfenstein, P., Thomas, P.C., Wagner, R.J., West, R.A. (2010): Iapetus: Unique Surface Properties and a Global Color Dichotomy from Cassini Imaging. Science 327, 435-439. doi:10.1126/science.1177088.
Spencer, J.R., Denk, T. (2010): Formation of Iapetus’s Extreme Albedo Dichotomy by Exogenically-Triggered Thermal Migration of Water Ice. Science 327, 432-435. doi:10.1126/science.1177132.

Sterne und Weltraum (April 2010):
Denk, T. (2010): Das Iapetus-Rätsel ist gelöst. Sterne und Weltraum 4/2010, S.40-49.

Press release / Pressemitteilung Freie Universität Berlin (Dec 2009):
10Dec2009 10Dez2009.

2007 — Targeted Cassini Flyby of Iapetus

Information on my Cassini planning site about the Iapetus flyby 049IA on 10 Sep 2007.

Denk, T. (2007): Cassini besucht Iapetus. Sterne und Weltraum 9/2007, S.18-20.

 Denk, T. (2008): Cassini at Iapetus: A Bumpy but Successful Flyby. The Planetary Report, Vol. XXVIII, no. 1, pp. 10-16, January/February 2008.

 Pressemitteilungen Freie Universität Berlin: 13Sep200709Okt2007.

Planetary.org blog: 15Aug200717Aug200731Aug200705Sep200706Sep200709Sep200710Sep2007a10Sep2007b11Sep2007a11Sep2007b12Sep2007a12Sep2007b12Sep2007c13Sep200720Sep200724Sep200722Oct2007.

Planetary.org radio: Broadcast of 17Sep2007 (28:50 min).

2004/05 — Discovery of the Equatorial Ridge and Cassini’s ‘B/C’ Flyby

Content will be added soon 🙂 [note from 28 Sep 2020]

1980/81 — Distant Flybys of the Voyager Spacecraft

Content will be added soon 🙂 [note from 28 Sep 2020]

1879 — Photometric Lightcurve of Iapetus

While the fact of Iapetus’ strong brightness variation was known since 200 years, it was now quantitatively measured for the first time with a photometer at Harvard Observatory. Iapetus was measured relative to Saturn and relative to a star of similar brightness (labelled “star b”; located at RA = 23h 50m 33s; Dec = -3°38.7′ (1880.0)).

The plot to the right is a visualization of Table LI in the report (Pickering et al. 1879), showing Iapetus’ relative magnitude over rotational phase (binned to 30° steps). Zero degrees corresponds to the Saturn-facing side of Iapetus (superior conjunction), 90° to the dark leading side (eastern elongation), 180° to the anti-Saturn side (inferior conjunction), and 270° to the bright trailing side (western elongation).

Pickering, E.C., Searle, A., Upton, W. (1879): Photometric Observations. Part II, Chapter IX: Satellites of Saturn. Annals of Harvard College Observatory 11, 247-270. (pdf: here)

1673/77 — Discovery of Iapetus and its Global Brightness Dichotomy

Articles from Jean-Dominique Cassini in French and English

Cassini, J.D. (1673): Découverte de deux nouvelles planètes autour de Saturne. Fol. chez Sébastien Mabre-Cramoisy, Imprimeur du Roy, Paris, pp. 5-14 + 5 figures. (Fichier pdf: ici)

Cassini, J.D. (1673): A Discovery of Two New Planets about Saturn, Made in the Royal Parisian Observatory by Signor Cassini, Fellow of Both the Royal Societys, of England and France; English’t Out of French. Philosophical Transactions Vol. 8, No. 92, 5178-5185 (25 Mar 1673). doi:10.1098/rstl.1673.0003.

Cassini, J.D. (1677): Histoire de la Découverte de deux Planetes autour de Saturne, faite à l’Observatoire Royal par M. Cassini. Journal des Sçavans, No. VI, 70-72 (15. Mars 1677).

Cassini, J.D. (1677): Some New Observations Made by Sig. Cassini and Deliver’d in the Journal Des Scavans, Concerning the Two Planets about Saturn, Formerly Discover’d by the Same, as Appears in N. 92. of these Tracts. Philosophical Transactions Vol. 12, No. 133, 831-833 (25 Mar 1677). doi:10.1098/rstl.1677.0004.

Black and white is not unique to Iapetus… / Schwarz-weiße Flecken sind nicht auf Iapetus beschränkt…

© Tilmann Denk (2020)