Smooth Plains on Europa

Post contributed by Dr. Elodie Lesage, Laboratory of Geosciences Paris-Saclay, University of Paris-Saclay.

The young (60-100 Myrs) and active surface of the Jupiter’s icy moon Europa includes various geological features. In some locations, smooth plains have been observed on Europa, defined as features with no visible texture and an albedo lower than the surrounding terrains. The smooth plains overprint the preexisting terrains, and are confined within basins bounded by topographically high features.

Image 1: Four smooth plains on Europa. The arrows show the sunlight direction. Galileo images: (a) Image 5452r, resolution: 27 m/px; (b) Image 0713r, resolution: 57 m/px; (c) Image 0739r, resolution: 57 m/px; (d) Image 9352r, resolution: 60 m/px.

The best-resolved images of Europa’s surface were acquired by the Galileo spacecraft between 1996 and 2001. Image 1 shows four smooth plains visible in Galileo images, which share the following common characteristics: i) thin features occupying topographic lows, ii) a smooth appearance with little or no visible texture, and iii) kilometer-scale width with a quasi-circular, lobate shape. The morphology of these features suggests that they result from the flow of low-viscosity fluid, such as liquid cryomagma (i.e., briny water coming from Europa’s interior).

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Martian Spiders Recreated in the Laboratory

Post contributed by Dr. Lauren Mc Keown, School of Physical Sciences, The Open University, UK.

Spiders are unusual branched landforms found among the high southern latitudes of Mars (Image 1). They have no Earth analogues and are often accompanied by fans and spots that appear in spring. They are proposed to form when sunlight penetrates the Martian south polar seasonal CO2 ice layer, causing ice at its base to change from ice to gas, and eventually crack. Escaping gas then scours the terrain beneath, carving spider-like patterns and depositing material on top of the ice via a plume (Kieffer et al., 2003). However, although this suggested process is well-accepted, it has never been directly observed on Mars. In order to investigate whether spider patterns could form by CO2 sublimation under Martian atmospheric pressure, experiments were performed (Image 2) at the Open University Mars Simulation Chamber, which simulates Martian atmospheric conditions.

Image 1: Examples of spiders on Mars (HiRISE image ESP_014282_0930). Left shows the ‘classic’ spider morphology which consists of a central depression and radial tortuous dendritic troughs emanating from its centre. Right is a context image. Image Credit: NASA/JPL/University of Arizona.

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Abundant Recurring Slope Lineae (RSL) Following the Great Martian Dust Storm of 2018

Post contributed by Dr. Alfred S. McEwen, Lunar and Planetary Laboratory, University of Arizona, USA.

Recurring slope lineae (RSL) are dark linear markings on steep slopes of Mars that regrow annually and likely originate from the flow of either liquid water or dry granular material. Following the great dust storm (or planet-encircling dust event) of Mars Year 34 (in 2018), the High Resolution Imaging Science Experiment (HiRISE; McEwen et al., 2007) on Mars Reconnaissance Orbiter (MRO)  has seen many more candidate RSL than in typical Mars years (Image 1). These RSL sites show evidence for recent dust deposition and dust devil activity, so dust lifting processes may initiate and sustain RSL activity on steep slopes.

Image 1:  RSL and dust devil tracks on a hill in the southern middle latitudes (41.1ºS, 187.4ºE).  Inset shows some of the RSL at higher resolution.  Hundreds of dark dust devil tracks are seen on the full image as the more diffuse lines that cut across topography.  HiRISE image ESP_058122_1385, acquired after the 2018 dust storm.  Credit: NASA/JPL/University of Arizona

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Antipodal Terrains on Pluto

Post contributed by C. Adeene Denton, Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, USA.

Antipodal terrains are unusual regions of hilly, lineated, or otherwise disrupted terrain that are on the direct opposite side of planetary bodies to large impact basins. These mysterious terrains have been observed at the antipodes to the Caloris basin on Mercury and the Imbrium basin on the Moon, where their formation is considered to be indicative both of the impact’s size and the specificities of the planetary body’s interior structure. Recent revisiting of data from the New Horizons spacecraft revealed an unusual region of disrupted and lineated terrain on Pluto’s far side that is roughly antipodal to the massive Sputnik Planitia basin, the feature sometimes referred to as “Pluto’s Heart” (Image 1). If the lineated terrain is indeed connected to the large impact believed to have formed Sputnik Planitia, then the two geologic features offer a new and unusual way to probe Pluto’s interior: seismology through giant impact.

Image 1: Comparison of Pluto’s nearside (left) and farside (right) with Sputnik Planitia and its proposed antipodal terrain indicated. The location of Image 3 is also indicated. Images modified from full-scale planetary images taken by the New Horizons spacecraft, via NASA/JHUAPL/SWRI.

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A Canyon in Gale Crater, Mars, and Implications for Exploration by the Curiosity Rover

Post contributed by Divya M. Persaud, University College London, UK/NASA Jet Propulsion Laboratory, USA.

This canyon, Sakarya Vallis, cross-cuts the central mound of Gale Crater, Mars, and was probably formed by fluvial erosion. Gale Crater, the exploration site of the Curiosity rover, has undergone a complex geologic history of aqueous and aeolian processes. The central mound is a topographic high in the center of the crater, on which the ~5.2 km peak Aeolis Mons is situated. This feature sports several canyons (which cut through it), yardangs, and spectacular exposed layers, and its origins are uncertain (likely aeolian and/or fluvio-lacustrine). Image 1 shows the largest canyon on the mound, at 26 km long, up to 3.5 km wide, and up to 400 m deep. The hundreds of meters of exposed layers in this canyon provide a glimpse into the depositional history of the central mound of Gale crater.

Image 1: A 3D perspective view of the interior of the channel, visualized from HiRISE Digital Terrain Model DTEEC_006855_1750_007501_1750_A01 and HiRISE image PSP_007501_1750. This view shows the exposed layers and a possible fracture on the eastern wall (left), while the topography of the bright, inverted feature can be seen on the floor of the canyon. The floor of the canyon is overlaid by ripples. The scalebar is oriented north.

The center of Sakarya Vallis is approximately 27 km from and 700 m higher in elevation than the base of Gediz Vallis, the approximate location of the Curiosity rover (as of February 2021, sol 3038). The layers exposed in Sakarya Vallis therefore represent later depositional events than those encountered by Curiosity to date. As the rover ascends the mound towards Aeolis Mons, this geology could help contextualize rover observations and constrain lateral differences in palaeoenvironment.

The canyon cross-cuts layered hydrated sulfates in the lower mound, while spectra of the upper mound point to a dust composition. The surface of the upper unit (Image 2) is an etched, yardang unit. Yardangs are streamlined features eroded by the wind.

Image 2: A) An overview of the canyon on the central mound from CTX imagery. The slope of the canyon is to the northwest. The surface of the mound is etched by yardangs and in-filled craters. B) A close-up view of the northern, lower half of the canyon, from HiRISE image PSP_007501_1750. The extent of the inverted feature can be seen on the floor.

Within the canyon are possible point bars formed by past rivers (Image 2), mass-wasting features (Image 3), ripples (Image 1), and boulder-scale lag deposits along the floor and clifftops. A bright, topographic feature resembling an inverted channel or meander belt (Image 1) extends along much of the canyon floor and may represent later flow of liquid water, and subsequent topographic inversion by erosion of materials surrounding the channel.

Image 3: Layers exposed in the northern part of the canyon in a possible mass-wasting feature (HiRISE image PSP_007501_1750). These layers are sub-horizontal and dip gently to the northwest. The ripples on the floor have wavelengths of 10-20 m.

Further Reading

Anderson, R. B., et al. 2018. Complex Bedding Geometry in the Upper Portion of Aeolis Mons, Gale Crater, Mars. Icarus 314: 246–64.

Fairen, A. G., et al. 2014. A Cold Hydrological System in Gale Crater, Mars. Planetary and Space Science, 93–94: 101–18.

Fraeman, A. et al. 2016. The Stratigraphy and Evolution of Lower Mount Sharp from Spectral, Morphological, and Thermophysical Orbital Data Sets. Journal of Geophysical Research: Planets 121 (9): 1713–36.

Grotzinger, J. P., et al. 2014. A Habitable Fluvio-Lacustrine Environment at Yellowknife Bay, Gale Crater, Mars. Science 343, 6169.

Hughes, M. N., et al., Mass Movements and Debris Deposits in the Grand Canyon and Gediz Vallis, Gale Crater, Mars, 51st Lunar and Planetary Science Conference, 2020.

Kite, E. S., et al. 2016. Evolution of Major Sedimentary Mounds on Mars: Buildup via Anticompensational Stacking Modulated by Climate Change. Journal of Geophysical Research: Planets 121: 2282–2324.

Palucis, M. C., et al. 2016. Sequence and Relative Timing of Large Lakes in Gale Crater (Mars) after the Formation of Mount Sharp. Journal of Geophysical Research: Planets 121 (3).

Thomson, B. J., et al. 2011. Constraints on the Origin and Evolution of the Layered Mound in Gale Crater, Mars Using Mars Reconnaissance Orbiter Data. Icarus 214 (2): 413–32.

The Jezero Crater Western Delta, Mars

Post contributed by Axel Noblet, Laboratoire de Planétologie et Géodynamique de Nantes, CNRS/Université de Nantes, France

Jezero Crater on Mars will soon be explored by NASA’s Perseverance rover. This crater has been interpreted as a paleolake. It contains two fan-shaped deposits in the northern and western portions of the crater. These deposits have been interpreted as ancient deltas. The delta located in the western portion of Jezero (Image 1) displays some of Mars’ best-preserved fluvio-deltaic features, and exhibits a variety of structures such as inverted channels and point-bar strata (Image 2). This delta contains a precious record of various depositional environments, and in situ exploration can give us insight of Mars’ fluvio-lacustrine history. The association of well-preserved lacustrine features with orbital detections of carbonates along the inner margin of Jezero points toward high biosignature preservation potential for these deposits. Hence Jezero’s western delta contains a record of the evolution of Mars’ ancient climate and possible habitability. The presence of a long-lived lake system on Mars is astrobiologically significant, and the deposits within the Perseverance landing site could have preserved biosignatures that could be investigated and cached for a future sample return mission. 

Image 1: 3D view of Jezero western delta, looking north from the center of the crater. The data visualized here is a CTX camera orthorectified mosaic draped over a CTX digital terrain model (horizontal resolution: 20m/px). The triangular ‘birdfoot’ shape of the delta is clearly visible, and inverted channels can be seen radiating from the apex of the delta. The inlet valley goes diagonally from the upper left of the image through the delta deposits. The crater floor appears as the smooth terrains on the lower part of the image.

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The Mysterious Morphology of Hekla Cavus, Pluto

Post contributed by Dr. Caitlin Ahrens, NASA Goddard Space Flight Center, USA.

Cryovolcanism involves the transfer of icy or gaseous subsurface materials either to the surface (eruptive) or through the subsurface (non-eruptive) of an icy planetary body. It differs from magmatism and volcanism on Earth, which involves the migration and eruption of molten rock. Cryovolcanism is thought to have operated on several icy bodies in the Solar System, including Enceladus, Triton, Pluto, and possibly Europa. Cryovolcanism results primarily from internal heat-producing processes, and excludes sublimation and condensation processes at the surface. In the case of Pluto, there is evidence for a subsurface fluid layer, the presence of cryovolcanoes, and cryovolcanic subsurface materials (called cryomagma) which can contain ammonia and methane. Due to the presence of a deformable subsurface layer, it is possible for the material to shift, causing uplift followed by a collapse-type event. This is a possible scenario at Hekla Cavus (Image 1), a large, elongated, and irregular depression situated within a much larger north-south (N-S) ridge-trough system outlined by mountain ranges.

Image 1: Image of Hekla Cavus taken from the LORRI instrument onboard the New Horizons.

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Volatile-rich impact ejecta on Mercury

Post contributed by Dr Jack Wright, School of Physical Sciences, The Open University, UK.

The Caloris basin is the largest (~1,500 km across), well-preserved impact structure on Mercury (Image 1a; Fassett et al., 2009). Hummocky plains around Caloris host numerous, steep-looking, conical knobs (Image 1b). The obvious explanation for the hummocky plains is that they formed from material ejected by the Caloris impact ~3.8 billion years ago. It follows that the knobs probably formed from discrete ejecta blocks. What isn’t obvious is why many of these blocks, which hypothetically could have formed with a variety of shapes, exist as steep cones in the present day. If these knobs really did form as Caloris ejecta, then they offer a rare opportunity to study materials ejected from Mercury’s interior with remote sensing techniques.

Image 1: Mercury and the circum-Caloris knobs. (a) Enhanced colour limb view of Mercury from the MESSENGER spacecraft. The Caloris basin’s interior is made of volcanic plains that appear orange in this data product. The arrow indicates the location of (b). (b) Examples of circum-Caloris knobs just outside the Caloris rim. Mosaic of MESSENGER MDIS WAC frames EW0220807059G, EW0220807071G, and EW0220763870G. ~86 m/pixel.

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Overlapping Lobate Deposits in Martian Gullies

Post contributed by Rishitosh K. Sinha, Planetary Sciences Division, Physical Research Laboratory, India.

Gullies are found on steep slopes on the surface of Mars and appear as a linear-to-sinuous channel linking an alcove at the top to a fan at the bottom. The most interesting interpretation of the past two decades has been that the Martian gullies were carved by the flow of liquid water as discovered from the high-resolution images returned by the Mars Orbiter Camera (MOC) onboard “Mars Global Surveyor (MGS)” in 2000 (Malin and Edgett, 2000). Subsequent observations using MOC and the Mars Reconnaissance Orbiter (MRO) High Resolution Imaging Science Experiment (HiRISE) images revealed that Martian gullies are active today and that sublimation of seasonal carbon dioxide frost – not liquid water – could have played an important role in their formation. In our recent work using HiRISE images we reported global distribution of overlapping lobate deposits in gullies (Image 1) showing that a debris-flow like process may be responsible for gully formation (Sinha et al., 2020).

Image 1: Top: 3D view of gullies on the pole-facing wall of ~8 km diameter Los crater (35.08˚ S, 76.22˚ W) on Mars. HiRISE image ESP_020774_1445 (0.25 m/pixel) is draped over 1 m/pix HiRISE elevation model. The depth of crater floor from the crater rim is ~1 km in elevation and the image spans ~4 km from left to right. The box shows the location of bottom panel. Bottom: Image shows the gully fan surface within Los crater with overlapping lobate deposits, including convex-up and tongue shaped terminal lobes with lateral levees. HiRISE image credits: NASA/JPL/University of Arizona.

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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).

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Lunar lava layers and their Hawaiian analogs

Post contributed by Dr. M. Elise Rumpf, Astrogeology Science Center, US Geological Survey.

Images of the lunar surface reveal layered deposits presumed to be sequences of basaltic lava flows. These sequences have been imaged since the Apollo astronauts acquired both orbital and surface photographs in the 1960s and 1970s. Apollo 15 astronauts visited Hadley Rille, a 130 km long, 200 m deep sinuous feature that was formed by flowing lava, similar to lava channels or tubes on Earth. Photographs taken by the astronauts (such as Image 1) show that the rille cut into the underlying substrate, revealing sequences of layered material. The layers are believed to be basaltic lava flows, based on outcrop morphologies and nearby samples. The thicknesses of ancient lava flows provide insight into the emplacement, dynamics, and history of volcanism on the Moon.

Image 1: Apollo 15 surface image of the interior wall of Hadley Rille (https://www.hq.nasa.gov/alsj/a15/AS15-89-12106HR.jpg). Inset highlights layered deposits presumed to be basaltic lava flows with possible intercalated regolith deposits. Outcrop is approximately 8 meters thick.

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Chaotic Terrain on Pluto, Europa, and Mars

Post contributed by Helle L. Skjetne, PhD candidate, Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, USA.

Chaos terrain is formed by disruption of preexisting surfaces into irregularly shaped blocks with a “chaotic” appearance (Image 1). This typically occurs through fracturing (that can be induced by a variety of mechanisms), and the subsequent evolution of these blocks can follow several paths (Image 2). These distinctive areas of broken terrains are most notably found on Jupiter’s moon Europa, Mars, and Pluto. Although chaos terrains on these bodies share some common characteristics, there are also distinct morphological differences between them (Image 1). The geologic evolution required to shape this enigmatic terrain type has not yet been fully constrained, although several chaos formation models have been proposed. We studied chaotic terrain blocks on Pluto, Europa and Mars to infer information about crustal lithology and surface layer thickness (Skjetne et al. 2020).

Image 1: Examples of chaotic terrain “blocks” (referring to each mountain-like topographic feature). Chaos on Pluto in a) Tenzing Montes and b) Al-Idrisi Montes, respectively (New Horizons image at ~315 m px–1), c) Hydraotes Chaos on Mars (Mars Odyssey THEMIS daytime infrared global mosaic at 100 m px–1), and d) Conamara Chaos on Europa (Galileo 210–220 m px–1 East and West RegMaps).

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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.

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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.
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