Iapetus’s Equatorial Ridge

Post contributed by Charlene Detelich, Department of Geological Sciences, University of Alaska Anchorage, USA.

Voyager flybys of the Saturnian system in the early 1980s discovered a ring of mountains on Saturn’s icy, two-toned moon, Iapetus. These mountains are up to 20 km tall, over twice the height of Mt. Everest, yet Iapetus is almost 650 times smaller than Earth. Later flybys from the Cassini spacecraft (Image 1) revealed these mountains to be part of a ridge encircling nearly all of Iapetus’s equator, resulting in the moon’s bizarre walnut shape. Numerous endogenic (internal) processes have been proposed for the origin of the equatorial ridge, including tidal spindown, convection, despinning coupled with global volume change, global contraction, or intrusion. However, a recent study by Detelich et al. (2021) found that evidence from photogeological mapping and crater statistics support an exogenic (external) origin best explained by the accretion of an ancient ring system onto Iapetus’s surface.

Image 1: Cassini image PIA08376 (https://photojournal.jpl.nasa.gov/catalog/PIA08376) taken on September 10th, 2007 shows the enigmatic equatorial ridge of Saturnian moon, Iapetus. The ridge runs vertically down the center of the image before disappearing into the shadows of the non-lit portion of Iapetus. Image enhanced by Charlene Detelich.


Polygonal Impact Craters on Miranda, Charon, and Dione

Post contributed by Dr. Chloe B. Beddingfield, The SETI Institute and NASA Ames Research Center

Some impact craters are classified as polygonal impact craters (PICs), which have at least one straight rim segment, as shown in Image 1. The morphologies of PICs are shaped by pre-existing, sub-vertical structures in the target material, such as normal and strike-slip faults, joint sets, and lithologic boundaries. Because the straight rim segments of PICs only form where pre-existing structures are present, PIC morphologies can be used to analyze fractures that are buried by regolith or too small to be seen in available spacecraft images. On the icy Uranian moon Miranda, PICs are widespread across its southern hemisphere, which was imaged by the Imaging Science System (ISS) onboard the Voyager 2 spacecraft. Some of these PICs reveal previously undetected fractures that suggest Miranda has experienced multiple periods of tectonic activity.

Image 1: Examples of two PICs identified on the Uranian moon Miranda. Black arrows indicate the straight rims of these PICs. The Voyager 2 ISS image mosaic shown here includes the following images, from top to bottom: c2684620 (light blue box), c2684629, c2684617 (dark blue box).


Titan’s labyrinth terrain

Post contributed by Michael J. Malaska, PhD, Scientist, Jet Propulsion Laboratory / California Institute of Technology, USA.

Saturn’s moon Titan is where organic chemistry and surface geomorphology intersect to create an enigmatic landscape with many features in common with Earth, but that are made of completely different materials. Much of Titan’s surface is made up of organic sedimentary materials; recent mapping shows that plains and dunes cover over 80 percent of the globe. The Cassini spacecraft’s Synthetic Aperture Radar (SAR) was able to penetrate Titan’s thick haze and reveal areas of highly dissected plateaux on the surface that are called labyrinth terrain. Image 1 shows an SAR image of an example of this type of terrain, the Sikun Labyrinth. Detailed examination of Titan’s labyrinth terrain can tell us a lot about Titan’s geological history and surface evolution.

Image 1. Top: Image of the Sikun Labyrinth in the south polar terrain of Titan. The blue arrow and number at top left indicates direction of radar illumination and incidence angle for this scene. Bottom: diagram showing how radar illumination interacts with terrain of valleys and plateaux. Image credit: Mike Malaska.

Pit chains on Enceladus

Post contributed by Dr. Emily S. Martin, Research Fellow, Center for Earth and Planetary Studies, National Air & Space Museum, Smithsonian Institution.

Pit chains are linear assemblages of circular to elliptical pits and have been observed across the solar system. Pit chains have been found on Venus, Earth, Mars, Phobos, Eros, Gaspra, Ida, and Vesta. Across the solar system, pit chains may form through a variety of mechanisms including the collapse of lava tubes, karst, venting, extensional fracturing, or dilational faulting. Saturn’s tiny icy moon Enceladus is the first body of the outer solar system on which pit chains have been identified. Enceladus is only 500 km in diameter and is best known for its warm south pole and its watery plume emanating from prominent ridges known as tiger stripes. The source of the plume is likely a global liquid water ocean beneath an icy shell.


Image 1: The morphology of pit chains across the solar system. a. Eros from NEAR. Image no. 135344864. b. Phobos. Image PIA10367. c. Albalonga Catena, Vesta. d. Venus. Right-look Magellan data near 13°S, 112°E. e. Kilauea Volcano, Hawaii centered at 19.3909°N 155.3076°W. Image taken 12/06/2014, acquired from Google Earth on 04/20/2016. f. Ida, modified from image PIA00332. g. Gaspra, modified from Galileo image PIA00332. h. Pit chains in north-eastern Iceland centered near 65.9902°N and 16.5301°W. Image taken on 7/27/2012, acquired from Google Earth 04/20/2016. i. Pit chains on Mars from the Mars Global Surveyor Mars Orbiter Camera, centered near 6.5398°S and 119.9703°W on the flank of Arsia Mons. Image PIA02874.


Unexpected variety among small inner satellites of Saturn

Post by Peter Thomas, Cornell University, Ithaca, New York

 Small satellites (< 150 km mean radius) usually resemble potatoes. Their irregular shapes are formed by a history of impact cratering without the benefit of internally-driven processes of volcanism, tectonics, or atmospheric effects (Castillo-Rogez et al., 2012).  During its 9 years orbiting Saturn, the Cassini spacecraft has shown that the small satellites orbiting close to Saturn have a variety of shapes, most of which deviate from the expected familiar battered potato appearance.  These objects are likely dominated by water ice as determined from mean densities and spectroscopy (Thomas et al., 2010; Buratti et al. 2010).  Satellites within rings have equatorial ridges (Charnoz et al. 2007; Porco et al., 2007).  Others, such as Janus and Epimetheus, the “co-orbitals” are almost lunar-like in appearance, close to the expected potato variety.

Image 1: Best available view of Helene. N1687119756, UV3 filter, phase = 97°, sub-spacecraft point is 2.7°N, 124.8°W.  North is down in this presentation.  Taken June 18, 2011.

Image 1: Best available view of Helene. N1687119756, UV3 filter, phase = 97°, sub-spacecraft point is 2.7°N, 124.8°W. North is down in this presentation. Taken June 18, 2011.


  • Enter your email address to follow this blog and receive notifications of new posts by email.

  • Io

  • Blog Stats

    • 143,891 hits