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).


Superposed glaciers on Mars: what, where, when, and why?

Post contributed by Adam J. Hepburn, Department of Geography and Earth Sciences, Aberystwyth University, UK.

Mars hosts abundant glacier-like landforms throughout its mid-latitudes, the presence of which necessitates major shifts in climate relative to present conditions. These ice-rich viscous flow features (VFFs) are typically found in coalescing, size-hierarchical systems whereby lower-order glacier-like forms (GLFs; ~5 km long) flow from alcoves and merge with higher-order lineated valley fill (LVF; 100s of km long). Several larger VFFs have been dated previously, indicating Mars underwent glaciation in the past several hundred million years, during the late Amazonian epoch.  However, several authors have noted examples of GLFs flowing onto, rather than into, LVFs (Image 1), and hypothesised that these may correspond to a more recent phase of glacial activity. We used crater dating to ascertain that—in addition to the earlier phase of widespread regional glaciation—these superposed GLFs (SGLFs) were formed following at least two major cycles of more recent alpine glaciation, the latter of which ended ~2 million years ago.

Image 1: Superposed glacier-like form (SGLF) flowing onto the underlying viscous flow feature (underlying VFF), in the Protonilus Mensae region of Mars. (A–B) North-up orientated HiRISE image (ESP_018857_2225) image of an SGLF (light blue) emerging from an alcove and flowing onto lineated valley fill (dark blue). Approximate location of image centre is 42.23◦ N, 50.53◦ E. Reproduced from Hepburn et al, 2020.


Impact Crater Degradation on Mercury

Post by Mallory Kinczyk, PhD candidate, Department of Marine, Earth, and Atmospheric Sciences, North Carolina State University

The formation of impact craters may be the most ubiquitous exogenic surface process in the Solar System. These craters take on many shapes and sizes and can hint at underlying rock types, tell us about the nature of the impactor, and can shed light on the body’s geological history. Even on bodies without atmospheres, erosive forces are at play, changing the crater shape through time via processes such as seismic shaking and disruption from debris thrown outward by subsequent, nearby impacts. Because Mercury is the only terrestrial planet without an atmosphere, it maintains a unique snapshot of the inner Solar System’s impactor population (Image 1) and, in turn, can shed light onto Earth’s own geological history.

converted PNM file

Image 1: View of Mercury from the MESSENGER spacecraft, which orbited Mercury between 2011 and 2015 (Image PIA17280). A variety of impact crater sizes and shapes are evident from very fresh craters to subdued to almost completely obliterated crater forms. Bach crater (arrow) hosts a well-defined central peak ring, but its subdued form indicates that it has been disrupted by subsequent craters. Image Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.


Degradation of Titan’s impact craters

Post by Joshua E. Hedgepeth, PhD student, Centre for Planetary Science and Exploration, University of Western Ontario, Canada.

Discovered by Huygens in 1656 (Huygens, 1656), the surface of Titan was obscured by its atmosphere for centuries (Campbell, 2003; Smith et al., 1996; Muhleman et al., 1995, 1990; Kuiper, 1944). In 2004, we finally obtained high resolution images of the surface with Cassini RADAR. Cassini was equipped with a Ku-band (2.17 cm λ) radar instrument with 5 beams for collecting data (Elachi et al., 2004; Stofan et al., 2012). The long wavelength band was able to penetrate Titan’s thick haze to perform radiometry, scatterometry, altimetry and synthetic aperture radar (SAR) imaging of the surface. The SAR mode captured the highest resolution images of the surface of Titan, as high as 175 meters per pixel (Elachi et al., 2004; Lopes et al., 2010). While this may not be as high resolution as the images we have of other moons, it is high enough to obtain some spectacular images. In this blog we have a detailed look at the craters on the surface of Titan.


Image 1: Selk crater (D=84 km) located at 199.1, 6.9 latitude and longitude. The dark crater floor is representative of the smooth material, and around it is the bright rough ejecta material.


Haulani crater on the dwarf planet Ceres

Posted by Dr. Katrin Krohn, German Aerospace Center, Institute of Planetary Research

Haulani is one of the most prominent features on Ceres. The impact crater has bright interior and extensive ejecta with farranging crater rays of about 160 km to 490 km. Haulani shows an overall smooth bright crater floor with flow features and some cracks in the floor’s northwestern part, parallel to the impact crater rim. This crater exhibits a hummocky elongated mountainous ridge in the central part of the crater with flows running downslope the ridge crest ponding toward mass-wasting deposits of the rim. Pits occur on the crater floor and in parts of Haulani’s ejecta. Since Ceres shows evidence of a volatile-rich crust, the pits are likely due to rapid post-impact outgassing of hydrated salts or ground ice.


Image 1: Color mosaic of Haulani, showing the diverse morphology of the crater.


Phobos Grooves from Rolling Boulders

Post by Kenneth R. Ramsley, Department of Earth, Environmental and Planetary Sciences, Brown University.

All but one region of Phobos, the largest moon of Mars, is covered by hundreds of valley-like features, usually described as grooves. Most grooves are ~80 to ~200 meters wide and are found in groups of generally parallel members, or families [see Image 1]. Impact craters typically produce slow-moving boulders, and on Phobos there would be little gravity to halt their motions. Did boulders rolling across the surface of Phobos produce the grooves? To answer this question, using a computer model to calculate the fate of rolling boulders, we compare their motions to the geomorphology of the grooves.

Image 1 - Viking Image

Image 1: Dominated by Stickney Crater, a feature nearly half the radius of the moon itself, Phobos is the larger of the two moons of Mars (average diameter, 22 kilometers). Mostly covered in valley-like features, planetary scientists have struggled for more than 40 years to explain the grooves of Phobos (Viking Project, JPL, NASA; Image mosaic by Edwin V. Bell II NSSDC/Raytheon ITSS).


Is the Xanadu region on Titan an impact basin?

Post by Dr. Mirjam Langhans, GFZ German Research Centre for Geosciences, Helmholtz Centre Potsdam, Germany.

The surface of Titan, Saturn’s largest moon, is subject of great geologic interest, particularly since the arrival of Cassini/Huygens mission in the Saturnian System. Titan’s largest distinct and highly reflective surface feature, named Xanadu, is located close to the equator. The image depicts Xanadu in full extension with a rich diversity of geologic landforms, such as fluvial valleys, mountain ridges and impact craters. Despite the high volume of image data in this region, the geologic history behind Xanadu remains enigmatic to this day.

Geomorphologic map of Xanadu. Data: Cassini SAR data, source: (http://pds-imaging.jpl.nasa.gov/portal/cassini_mission.html). background: Cassini-ISS, source: (http://pds-imaging.jpl.nasa.gov/portal/cassini_mission.html). Inner and outer boundary of the Xanadu Circular Feature (XCF) are highlighted at Western Xanadu (black lines, according to Brown et al. (2011)). Green dots: impact craters listed in Wood et al. (2010) and Neish & Lorenz (2012), red dots: potential impact craters. Fluvial channels are delineated in blue. Dark green: lineations seen in mountain ranges, from Radebaugh et al. (2011). Light green: lineations in mountain ranges (Langhans et al. 2013).

Geomorphologic map of Xanadu. Data: Cassini SAR data, source: (http://pds-imaging.jpl.nasa.gov/portal/cassini_mission.html). background: Cassini-ISS, source: (http://pds-imaging.jpl.nasa.gov/portal/cassini_mission.html). Inner and outer boundary of the Xanadu Circular Feature (XCF) are highlighted at Western Xanadu (black lines, according to Brown et al. (2011)). Green dots: impact craters listed in Wood et al. (2010) and Neish & Lorenz (2012), red dots: potential impact craters. Fluvial channels are delineated in blue. Dark green: lineations seen in mountain ranges, from Radebaugh et al. (2011). Light green: lineations in mountain ranges (Langhans et al. 2013).


Geomorphic activity on asteroid Vesta

Post by Prof. Dr. Ralf Jaumann and Dr. Mary C. Bourke,

German Aerospace Center, Berlin, Germany. 

Department of Geography, Trinity College Dublin, Ireland.

The NASA Dawn spacecraft was launched in September 2007 to characterize the conditions and processes of the solar system’s earliest epoch by investigating in detail two of the largest protoplanets remaining intact since their formation. Ceres and Vesta reside in the extensive zone between Mars and Jupiter together with many other smaller bodies, called the asteroid belt. Each has followed a very different evolutionary path constrained by the diversity of processes that operated during the first few million years of solar system evolution. The Dawn mission entered orbit around Vesta on 16 July 2011 for a one-year exploration and left orbit on 5 September 2012 heading towards Ceres.

Image A. A composite digital terrain model, and high resolution albedo mosaic and imbedded color channels of three cratres on the surface of Vesta. The image is composed of many individual photographs taken between October and December 2011 by Dawn’s framing camera during the high-altitude mapping orbit, at about 680 kilometers above Vesta’s surface. The image is centered on ~ 13° north latitude and ~ 195° eastern longitude. South is to the top of the image. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.


Fluvial flow triggered by impact events on Mars

Post by Andrea Jones, Lunar and Planetary Institute/NASA Goddard Space Flight Center

 In memory of our dear friend and colleague Dr. Elisabetta (Betty) Pierazzo

Hale crater is a 125×150 km impact crater located near the intersection of Uzboi Vallis and the northern rim of Argyre basin on Mars, at 35.7ºS, 323.6ºE. Hale is an unusual crater on Mars because it is modified by fluvial channels. The channels originate from the outer edges of Hale’s ejecta and extend as far as 460 km from the crater rim (Image 1). They are upto a few kilometers wide, exhibit a braided planform (Image 2), and had sufficient stream power to incise and transport the crater ejecta. Most of the channels are found to the south-southwest of Hale crater, on the northern slope of Argyre basin (Image 3).

Image 1

Image 1: Channels in the southeastern ejecta of Hale crater, Mars in a THEMIS daytime-infrared mosaic. The channels were likely carved with water mobilized by the Hale-forming impact event. White box is location of Image 2. North is up in all images.

Image 2

Image 2: Detailed view of fluvial channel flowing through crater ejecta. CTX image
Location is shown in Image 1.


Impact Craters on Earth and Mars: Monturaqui and Bonneville

Post by Nathalie Cabrol and the High Lakes Science Expedition.

Impact processes are fundamental in the creation of planets, the modification of their landscape, and for Earth, in the evolution of life. However, unlike the other planets of our solar system, Earth has not kept a large record of its impact history. Plate tectonic and erosional processes have erased most of them with time. Small impact craters, in particular, are difficult to preserve but there are still a few left, including the Monturaqui impact crater (23°56’S/68°17’W) located in the Atacama Desert in Chile.

Maturaqui Crater, Chile, Earth

Image 1: Monturaqui impact crater in the Atacama Desert of Chile. Credit: Planetary Spherules Project, Nathalie A. Cabrol, NASA Ames/SETI CSC.


Mars’s Moon Phobos

Post by Dr William Hartmann


The image is of Mars’s 27 x 19 km satellite, Phobos. It hints at many mysteries that await us there. The surface (contrary to the appearance of this well-exposed image) is dark black, probably similar to carbonaceous asteroids, but the exact composition and spectral properties are still uncertain (due partly to scattered reddish light from Mars). Spectra show the surface soil lacks any water, but that soil has been blasted off and recycled through dust belts circling Mars, and then re-accreted onto Phobos. This process likely removes any initial water from the dust, so we can’t be sure whether the surface represents the interior material.

Medusae Fossae Formation on Mars


Landslide deposits on Mars

Posted by Bill Hartmann, Planetary Science Institute, Tucson, Arizona, USA.

(Re-posted from IAG Image of the month, March 2007)

This high-resolution MGS MOC image shows overlapping landslide deposits at the foot of the wall in the Ganges region of the Valles Marineris canyon complex on Mars. (more…)

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