Starship Asterisk*

John Bridges wrote:

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Faint Traces of Dark Flows (ESP_064180_1320) (HiClip)

This image shows some faint traces of dark flows along the headwall of an impact crater. These are relics of seasonal recurring slope lineae (RSL) that formed on an equator-facing slope.

They are not expected to be active yet, so we’ll have to wait until later in the Martian spring for any changes. However, we like to monitor these sites as they progress through the seasons, and fully formed RSL have been identified at this site before.

That’s because RSL recur each Mars year at the same places, like this crater wall. RSL activity often happens at predicted temperatures approaching minus 20 degrees Celsius (or minus 4 degrees Fahrenheit). An intermittent flow of brines is possible but dry flow of granules is an alternative explanation to explain RSL formation. Because of this uncertainty, the science community is debating whether these regions should be regarded as “special regions” where rovers or others landers are restricted.

Candy Hansen wrote:

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Exquisite Layering (ESP_064288_1525) (HiClip)

Much of Mars is covered by sand and dust but in some places stacks of sedimentary layers are visible. In this image, exquisite layering is revealed emerging from the sand in southern Holden Crater. Sequences like these offer a window into Mars’ complicated geologic history.

Holden Crater was once a candidate landing area for the Mars Science Laboratory, and is still an intriguing choice today.

Ingrid Daubar wrote:

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Along the Straight Edge (ESP_064445_1475) (HiClip)

Most craters are round, because a high-velocity impact acts like a powerful explosion that expands in all directions. This crater is an exception because the northern rim is almost perfectly straight.

One possibility is that there was a zone of joints or faults in the crust that existed before the impact. When the impact happened, the crater formed along the straight line of these faults. Something similar happened to Meteor Crater in Arizona. Our image doesn’t show any faults, but they could be beneath the surface.

Perhaps some sort of uneven collapse changed the shape of the crater. There are piles of material on the crater’s floor, especially in the northwest and northeast corners. If those piles fell down from the rim, why did it happen there and not in other places? This crater is near the size where larger craters start to show wall slumping and terraces, so this type of collapse could be occurring unevenly.

Our image reveals the crisp detail of the crater rim, with individual boulders around the outside and on the inner walls. That indicates that this crater probably isn't very old, so it hasn’t been heavily modified. So somehow this odd shape probably happened when it first formed, although we still don’t know exactly why.

Candy Hansen wrote:

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Spring Fans at Macclesfield (ESP_064469_0945) (HiClip)

Every Martian spring, fans of dust are blown out from under the seasonal layer of carbon dioxide ice that forms a polar cap over the winter.

Gas blowing out from under the ice carries with it a load of dust that is deposited on the surface in a direction determined by the wind at the time of the eruption. Like windsocks, these fans in a polar area we’ve dubbed Macclesfield, record the direction that the wind was blowing.

A citizen science task at Planet Four enlists the public to outline the fans. Their measurements go into a data base that will ultimately help us to understand weather on Mars.

HiRISE Science Team wrote:

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Two Young Craters (ESP_063947_2310) (HiClip)

This image caught two different targets at once! In the top (northern) part there is a geologically-young crater about 300 meters in diameter, with rocky ejecta. The crater looks very fresh and steep and is not buried or filled in with the smooth deposits that cover the region. Craters like this tell us what is in the shallow subsurface and are very valuable for understanding the geology.

In the bottom (southern) part is a smaller crater, only about 15 meters across. This one is even younger, having formed between 2008 and 2010, when it was detected by MRO’s Context Camera. The smaller crater exposed subsurface ice, and HiRISE has been re-imaging it to see how it changes as the ice slowly sublimates away. Compare this image to ESP_017926_2310 to see what has happened in the last decade!

HiRISE Science Team wrote:

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An Active Gully in Matara Crater (ESP_063969_1300) (HiClip)

Gullies in the sand dunes of Matara Crater are very active. One large gully in particular has had major changes in every Martian winter since HiRISE began monitoring, triggered by the seasonal dry ice frost that accumulates each year.

This time there was an especially large change, depositing a huge mass of sand. The sand divided into many small toes near its end, or perhaps many individual flows descended near the same spot. Additionally, a long sinuous ridge of sand was deposited. This could be a “levee” that formed along one side of a flow, but there is not much sand past the end of the ridge, so it might also be the main body of a flow. How many changes can you see in the cutout?

Colin Dundas wrote:

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Keeping a Watchful Eye (ESP_054687_2055) (HiClip)

Existing images of this impact crater show a couple of dark lineations on the equator-facing wall that resemble small recurring slope lineae (RSL). However, unlike typical RSL, these lines persist for several Mars years with only minor changes.

We are continuing to monitor this site to understand how they differ from “standard” recurring slope lineae.

John Bridges wrote:

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Disrupted Sediments in Acidalia Planitia (ESP_064090_2250) (HiClip)

This color HiRISE view shows a pitted, blocky surface, but also more unusually, it has contorted, irregular features.

Although there are impact craters in this area, some of the features (like in the lower center of the cutout) are too irregular to be relic impact craters or river channels. One possibility is that sedimentary layers have been warped from below to create these patterns. The freezing and thawing of subsurface ice is a mechanism that could have caused this.

Acidalia Planitia is part of the northern plains of Mars, at a latitude of 44 degrees north.

James Wray wrote:

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Colorful Bedrock in a Northern Plains Crater (ESP_063376_2315) (HiClip)

This image covers the inside of an impact crater on the northern plains of Mars. It was intended to provide a baseline image of sand dunes on the crater floor, which could be monitored for potential motion in future pictures.

Much more than sand is visible. The dark, undulating dunes sit atop a colorful surface of exposed bedrock. Based on the crater's diameter of roughly 25 kilometers, these rocks may have been previously buried over a mile beneath the surface. The varying colors likely reflect diverse mineral compositions. (The CRISM instrument, also on MRO, has detected different minerals in the neighboring larger Micoud Crater, whose rim lies about 50 kilometers east-southeast of this image.)

Excavated by impact, the colorful rocks here remain visible in part thanks to the winds that shape the overlying sand dunes, which help to sweep the crater's center clear of surface dust.

Alfred McEwen wrote:

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Pitted Material from Tooting Crater (ESP_062934_2040) (HiClip)

Tooting Crater is one of the youngest craters on Mars that is larger than 20-kilometers in diameter. Relatively low areas inside and outside the crater are covered by a distinctive pitted and ponded material. The pits are not impact craters, as they lack ejecta and are very closely spaced.

There is one small impact crater near the lower right corner of our picture, which is much more circular than the pits and has a raised rim and ejecta. One interpretation is that this pitted and ponded material was hot impact ejecta from Tooting, and loss of volatiles from this material or underlying materials created the pits as it cooled.

M. Ramy El-Maarry wrote:

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Where the Wind Blows (ESP_063901_1710) (HiClip)

Sand dunes commonly form when particles that are being moved around by the wind find a natural barrier to accumulate and build a hill-like formation. Scientists study dunes because their shape and size can give us valuable information about the wind directions and speeds in current and past climates.

For instance, barchan dunes are crescent-shaped, and they form when the wind blows mainly from one direction (perpendicular to the crescent long edge). On the other hand, “star” dunes have three or more “arms,” and form in environments that that are affected by multiple wind directions. Our image shows an area on Mars with both star and barchan dunes next to each other. This implies that wind directions have changed with time, or that the surrounding landscape is creating complex wind patterns.

Scientists can study HiRISE images collected over time of the same dunes to observe whether they are moving or not, and if so, how fast. By observing multiple dune systems over many seasons, we can get a better picture of wind regimes on Mars and possibly how they have evolved with time.

M. Ramy El-Maarry wrote:

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Monitoring Active Gullies (ESP_063775_1295) (HiClip)

Gullies are common on steep slopes of many impact craters on Mars. When gullies were first observed, there was a lot of excitement surrounding them because similar features form on Earth through the action of liquid water.

However, liquid water is currently unstable on the surface of Mars. Long-term observations have prompted many scientists to question a liquid water origin for the gullies, and alternative ideas have been suggested. These include flows of salt-rich (briny) water, as the salt would allow water to be liquid under lower temperatures than those for pure water. Also “dry” processes, which do not require the action of liquid water at all.

Monitoring of gullies by HiRISE could help scientists better understand the conditions where the gullies are active, and in doing so, help understand how they form.

Ross A. Beyer wrote:

Looking for Slope Streaks (ESP_063204_1800) (HiClip)

Dust and sand slide down slopes on Mars in little avalanches. Dark slope streaks are thought to be the result of the relatively bright colored dust avalanching down slopes, revealing the darker, coarser sand underneath.

This image is the latest in a sequence of images of this crater that started in 2013. The goal is to watch the dusty slopes, and try to understand more about the processes that drive these little avalanches.

An animation shows this sequence of 14 images taken over seven Earth years (about 3 and a half Mars years), and shows where new streaks have occurred on the slopes of this crater. The shape of the crater’s rim appears to “wobble” because the spacecraft looks at the crater from slightly different directions. This could be corrected by creating a 3D terrain model and properly projecting each image onto it.

Alfred McEwen wrote:

Dark Sand at the Margin (ESP_062827_2620) (HiClip)

This image covers the boundary between north polar ice and nearby polar sand dunes. The color data clearly distinguishes between the bright ice, dark sand, and reddish dust.

An animation compares an exact same area to how it appeared in March 2009 at the same Martian time of year. The dark sand appears to be on the move, covering much of this area that was formerly bright ice or dust-covered ice. However, this may also show year-on-year variability of seasonal processes. In other words, this area may have looked similar in 2009 a month or so after the HiRISE image was acquired. The seasonal defrosting patterns vary from year to year, perhaps depending on dust storm activity.

Alfred McEwen wrote:

A Crater Enlarged by the Sublimation of Ice (ESP_062951_2255) (HiClip)

This image covers part of the ejecta from an impact crater (about 6-kilometers in diameter) to the west in Utopia Planitia. The ejecta lobes have morphologies suggesting icy flow.

Several small (about 100 to 200 meters in diameter) craters on top of those lobes have a distinctive formation. One interpretation is that the impact crater exposed nearly pure water ice, which then sublimated away where exposed by the slopes of the crater, expanding the crater’s diameter and producing a scalloped appearance. The small polygons are another indicator of shallow ice.

Alfred McEwen wrote:

A New Impact Marking Fades Away (ESP_062948_2175) (HiClip)

A HiRISE observation in 2010 covered a new impact crater that formed after December 2007 and before August 2010, based on Context Camera images. HiRISE has been re-imaging these sites to see how rapidly the dark ejecta and blast zone markings disappear as dust is deposited or redistributed.

An animation compares the two images and shows that the dark material has faded into the background, while the new 6.3-meter diameter crater persists.

Alfred McEwen wrote:

Bright Dust Devil Tracks over RSL in Eos Chasma (ESP_062917_1640)

This image (see cutout) features large candidate recurring slope lineae (RSL), which some have considered evidence for seeping water. They are “candidate” RSL because we have not seen them change over time in ways that definitely identify them as RSL

Recent research has favored dry models for RSL formation, in which the darkening is due to removal of bright dust. There are often dark dust devil tracks associated with RSL sites, supporting this interpretation. At this site, however, we see bright dust devil tracks where they cross the RSL. The dust devil tracks are the diffuse streaks that cross the topography at various angles, not following the downhill direction. How can dust devil tracks be bright from dust removal or redistribution while RSL are dark from dust removal?

The answer may be that the grain size and roughness of the surface is changing. Small grains and smoother surfaces tend to be brighter than coarser-grained or rougher surfaces. Bright dust devil tracks are seen elsewhere on Mars, and one example on Earth in which the passing dust devil produces a smoother surface. Downhill is to the upper left of this cutout image, and the dark RSL flow directly down the slope gradient.

Alfred McEwen wrote:

Northern Dunes and Snowfall (ESP_062901_2560)

Every year we see new slumps on dune slip faces at high northern latitudes, and old slumps are erased by windblown sand. This image was acquired as part of a joint study with MRO’s Mars Climate Sounder (MCS), to determine if the rate of slumping seen by HiRISE corresponds to winter snowfall tracked by MCS.

Be sure to check out the stereo anaglyph using red-green glasses. The slumps look strange in the anaglyph, because they change every year and the other image for the stereo pair was acquired one Mars year ago.

This is a stereo pair with ESP_054158_2560.

Alfred McEwen wrote:

Olivine-Rich Terrain in Ganges Chasma (ESP_062930_1720)

HiRISE has been striving to image locations that have high-resolution infrared spectral coverage by CRISM showing interesting mineralogy, but lacks HiRISE coverage to better interpret the geologic setting.

This image covers a location in which the mineral olivine was detected, and shows that the surface is ancient fractured bedrock, eroded by wind, and with a few wind-blown deposits.

Matthew Chojnacki wrote:

Dynamic Activity in a Crater North of Antoniadi Crater (ESP_023261_2065)

There are many overlapping and competing geologic processes acting on the surface of Mars today. One area where this is most clearly illustrated is a small crater north of Antoniadi Crater. A digital terrain model was constructed of this site, which allowed all images to be corrected in a way to more easily identify and track changes (e.g., sand dunes, wind streaks).

In this image, we see a number of deposits that collectively form a large star-shaped megadune, just over 500-meters tall. Brighter surfaces of red Martian dust are vacuumed up by dust devils, leaving behind darker tracks that change dramatically over time. Over the Martian year, the surface changes in brightness as dust falls from the atmosphere. The dust is removed during windier portions of the year by sand ripples and dust devils. (Note the greatest change occurs at the end of the animations in Mars year 34 (MY34) and the period following the planet-encircling dust storm that occurred in mid-2018.)

In this animation (bottom of the sequence) we can view four of the smaller duneforms and their steepest slopes facing to the south. These “slip faces” are broadly perpendicular to the primary winds, indicating the dunes are slowly migrating south to south-southeast.

At the finest scale, a third animation reveals a single slip face where dark streaks are traces of meter-scale sand avalanches that indicate that the landform is currently active and moving. These types of sand streaks which seemingly form, fade, and then reappear have been described as recurring slope lineae (RSL) and have been observed elsewhere, including steep rocky cliffs without sand dunes. HiRISE is continuing to monitor dynamic locations like this to better understand how landscape evolution on Mars differs from that of Earth.

This is a stereo pair with ESP_023327_2065.

Alfred McEwen wrote:

Sedimentary Rock Layers in Terby Crater (ESP_062860_1525)

Sedimentary layers record a history of Mars’ erosion and deposition by water and wind, and they make great landscapes for future interplanetary parks.

Alfred McEwen wrote:

Dunes and Bedrock (ESP_062863_1980)

These sand dunes in a crater south of Mawrth Vallis are being monitored to measure changes. However, active dunes also clear the dust off of the bedrock between the dunes, which may have diverse colors and compositions, as in the enhanced-color cutout.

Eric Pilles, Will Yingling and Livio L. Tornabene wrote:

Possible Impact Melt Deposits in a Multiple Impact Structure (ESP_062296_1840)

This image shows the eastern portion of a triple impact structure that is approximately 21 by 14 kilometers, visible in this Context Camera image.

In this close-up we can see possible impact melt-bearing deposits flowing from the higher elevation in the eastern portion of the crater into the larger central crater. In the eastern part of this image, where the topography is more level, the impact melt material appears to have ponded to form relatively dark and smooth deposits on the floor of the crater. The materials that were ejected outside of the crater, observed in the northeast section of this image, are much rougher in appearance.

This is a stereo pair with ESP_062586_1840.

Alfred McEwen wrote:

Ice-Exposing Scarps (ESP_062853_2355)

There are steep scarps facing the poles of both hemispheres that expose thick (up to 100 meters or more) sections of nearly pure water ice.

This is a valuable resource for future settlers on Mars, if that happens.

Ross A. Beyer wrote:

Staring into a Pit (ESP_063262_1755)

This observation was meant to examine a pit identified in a Context Camera image to see if HiRISE could resolve any details inside. In this cutout, we see the “normal” view of the HiRISE image on the left, while the right shows what happens when we try to “enhance” the brightness of the pixels in the pit.

Fortunately, HiRISE is sensitive enough to actually see things in this otherwise dark pit. Since HiRISE turned by almost 30 degrees to capture this image, we can see the rough eastern wall of the pit. The floor of the pit appears to be smooth sand and slopes down to the southeast. The hope was to determine if this was an isolated pit, or if it was a skylight into a tunnel, much like skylights in the lava tubes of Hawai’i. We can’t obviously see any tunnels in the visible walls, but they could be in the other walls that aren’t visible.

Leah Sacks, Livio Leonardo Tornabene, Vidhya Ganesh Rangarajan, and Chimira Andres wrote:

A T Party on Mars (ESP_062151_2540)

The dunes of Mars clearly sent out an invitation to a “T Party,” but it looks like none of us were invited. Forming a veritable maze of sand and rock, these unusually shaped dunes are located in the north polar region.

The shape and the form of the dunes serve as weathervanes. In crescent or “barchan” dune forms, the pointed tips of the sand dunes align with the dominant wind direction. The sand grains move with the wind, progressing the dunes forward over time. The T Party dunes are similar to “barchan” dunes, but they deviate from the characteristic crescent shape, thus it is less clear which direction is indicated. They may be suggesting varying wind conditions and perhaps the dunes are in the process of changing directions.

These polar dunes form as piles of basaltic sands that are covered with bright carbon dioxide frost as the Martian winter descends every year. In this early northern summer image, the dunes have thawed, the frost has sublimated to gas, and the underlying dark sand is exposed. Small ripples on the dunes and the underlying polygonized surface and boulders are also visible in this enhanced color cutout.

Sharon Wilson wrote:

The Devil is in the Details (ESP_061787_2140)

The HiRISE camera has done it again: here is yet another stunning image of an active dust devil on Mars.

Dust devils are rotating columns of dust that form around low-pressure air pockets, and are common on both Earth and Mars. This Martian dust devil formed on the dust-covered, volcanic plains of Amazonis Planitia. The dust devil is bright, and its core is roughly 50 meters across. The dark streak on the ground behind the dust devil is its shadow. The length of the shadow suggests the plume of rotating dust rises about 650 meters into the atmosphere!

There are several HiRISE images of tracks left behind by dust devils, but it is rare to catch one in motion. Check out this amazing image of a nearby dust devil.

Alfred McEwen wrote:

Exposing Colorful Deep Bedrock (ESP_062877_1690)

Large impacts produce uplifted central structures, either peaks, or pits, or an uplifted peak with a central pit. This crater south of Aurorae Chaos has a central pit exposing bedrock units with diverse colors, indicating diverse rock compositions.

This crater includes clay-rich minerals identified by the CRISM instrument on MRO. See this enhanced-color cutout over the eastern half of the central pit.

This is a stereo pair with ESP_044930_1690.

Sharon Wilson wrote:

Pollywog Craters on Mars (ESP_061768_2200)

This crater is approximately 2.3 kilometers across and is located in northern Arabia Terra near where the cratered highlands meets the northern lowlands (called a “dichotomy boundary”). Small craters with an exit channel, such as this one, are nicknamed “pollywog” craters, as they resemble tadpoles.

The channel is consistent with flow *out of* the crater, rather than flow *into* the crater, because 1) the valleys do not cut down to the level of the interior crater floor, and 2) there are no deposits of material on the floor associated with the mouth of the valley.

This small crater was probably once filled with an ice-covered lake that overflowed, forming the exit channel. Young craters with exit channels are intriguing because they record a relatively recent (during the Amazonian epoch) wet environment on Mars.

This is a stereo pair with ESP_062045_2200.

Alfred McEwen wrote:

A Candidate Landing Site in Utopia Planitia (ESP_062898_2060)

This image samples the smooth plains within one of the areas being considered for setting down China’s lander and rover, expected to launch in 2020.

While smooth on large scales, HiRISE reveals small-scale roughness elements, including craters, boulders, and other features. Such hazards may be avoided by using “terminal hazard avoidance,” a technology China has demonstrated on the Moon.

Utopia Planitia may have been extensively resurfaced by mud flows, so it is an interesting place to investigate potential past subsurface habitability.

Chimira Andres, Livio Leonardo Tornabene, Leah Sacks, and Vidhya Ganesh Rangarajan wrote:

A Slice of Polar Layer Cake (ESP_062216_2660)

The Martian ice cap is like a cake with every layer telling a story. In this case, the story is one of climate change on Mars.

In this image is an exposed section of the north polar layered deposits (NPLD). Like a delicious slice of layered tiramisu, the NPLD is made up of water-ice and dust particles stacked one on top of the other. However, instead of icing, layers are topped with seasonal carbon dioxide frost. We can observe lingering frost adhering to one of the layers.

The high-resolution and color capabilities of HiRISE provide details on the variations in the layers. Scientists are also using radar data, which show us that they have continuity in the subsurface. During deposition, these complex layers might encapsulate tiny air pockets from the atmosphere which, if sampled, could be studied to understand linkages to previous climates.

In the end, it’s not always a piece of cake studying NPLD on Mars but, where there is cake, there is hope!

Will Yingling, Eric Pilles, and Livio L. Tornabene wrote:

An Oblong Impact Crater in Terra Cimmeria (ESP_062344_1910)

Here we observe a portion of an impact crater that is elliptical rather than circular. How do we know this is a crater and not a volcanic or tectonic feature?

First, a raised rim around the edge of the depression is characteristic of all impact craters. Secondly, in this image from MRO’s Context Camera, there is a distinct ejecta blanket deposited to the northwest and southeast of the cavity, referred to as “butterfly” ejecta.

Why is the crater so oblong and the ejecta distributed thus? Most craters are generally circular. The ejecta distribution and oblong crater shape are due to a lower impact angle. Most impactors hit the surface around 45 degrees, yet they still form circular craters. Models show at the lowest impact angles (less than 15 degrees) that we get an elliptical shape and ejecta that is not equally distributed around the entire cavity.

The impactor likely originated from the southwest. A lack of ejecta, referred to as a “forbidden zone,” tells us which direction the impactor came from. However, in this case ejecta is lacking in both the southwest and northeast. Fortunately, another indicator that tells us about the impact direction is the smaller more circular cavity that comprises the northeast portion of the crater. This cavity likely formed when a piece of the impactor broke off, referred to as a “decapitated impactor,” and struck the surface downrange.

Leah Sacks, Livio Leonardo Tornabene, Chimira Andres, Vidhya and Ganesh Rangarajan wrote:

An Impact North of Valles Marineris (ESP_011425_1775)

Repeat imaging of the same location on Mars allows us to detect changes, including new impacts. This recent crater is known to have formed between February 2005 and July 2005.

Before and after images enable us to “age” a crater to within a few months or years. HiRISE often confirms the existence of craters identified in pre-existing lower resolution images.

Incoming impactors form new craters and deposit rock, in what is called an ejecta blanket that is outside the crater. The ejecta blanket resembles a splash pattern when seen from above. The dark colors in the image show a portion of the blanket, including far-flung small pieces of rock. The blue likely represents dark basaltic rocks, a volcanic rock commonly found in places like Hawaii, on top of the dust-covered surface.

The radial features of the crater are comprised of ejecta and often termed “rays.” Rays are used to help identify more recent craters and find them in images. Older craters do not have rays as they have been eroded away. As is clear from an example like this, impact craters allow us to study the subsurface portions of planetary bodies.

This is a stereo pair with ESP_012282_1775.

Chimira Andres, Livio Leonardo Tornabene, Leah Sacks, and Vidhya Ganesh Rangarajan wrote:

The Life of a Glacier on Mars (PSP_008809_2215)

The life of glacier-like forms (GLFs) on Mars can be quite a drag, especially when having to push through all the Martian dust and rocks for millions of years. In this image, we see a potential debris-covered glacier spilling out onto relatively flat plains in Protonilus Mensae, spreading into a bulb-like lobe while bulldozing the surface in front of it.

After years of surface modification, different types of landforms develop. The most common glacial landforms on Mars are viscous flow features and curved, raised ridges at the terminus of a GLF called “moraines.”

This close-up image of the GLF surface also shows linear features resembling fractures, as seen on many terrestrial glaciers. Also visible on the sides of the valley walls are a series of parallel lines that could potentially be exposed layers or lines that mark the GLF’s past levels.

Ice is one of the many powerful agents that can modify the surface of a planet. On Mars, water-ice tends to be more common in the mid- to high latitude regions and can serve as paleoclimate indicators or windows into the past climate of Mars.

This is a stereo pair with PSP_009455_2215.

Vidhya Ganesh Rangarajan, Livio Leonardo Tornabene, Leah Sacks, and Chimira Andres wrote:

Central Peak of a Large Crater (ESP_014361_1585)

Crater formation is an intense phenomenon that sends shock waves into the surface that scours and displaces material to form a cavity. Larger craters are observed to possess a central structure formed as a result of bedrock uplifted from the subsurface. High pressures and temperatures experienced during impact cause irreversible changes to target-surface materials that contribute to the formation of rocks called “impactites.” These include impact melts which, as a consequence of melting and re-solidifying, are younger than the target-surface.

Identifying and studying well-preserved bedrock exposures associated with central uplifts may provide insights into subsurface composition and the geologic history of the target prior to impact. Here, we see a 41-kilometer diameter crater in Terra Sabaea that shows massive and well-exposed bedrock in its central uplift that is highly fractured, possibly due to the formative impact event. In addition, HiRISE color imaging facilitates the identification of at-least two different kinds of material that comprise the uplift. Erosional remnants of impact melt coating the exposed bedrock of the uplift is visible in the north-west part of the color strip. Detailed studies of such deposits could inform us about various modification processes that the crater underwent after initial impact.

This is a stereo pair with ESP_021587_1585.

Will Yingling, Eric Pilles and Livio L. Tornabene wrote:

Impact Melt Flows and Ponds (PSP_006993_1875)

This image shows various crater-related features. Specifically, we see a raised mound in the middle, called a central uplift, and terraces (or ledges) on the crater wall. Within these locations are dark-toned deposits consistent with impact melt-bearing substances that behave as flows and ponded materials.

This Context Camera mosaic image shows a large 60-kilometer diameter crater named Mojave to the east of this HiRISE image. The impact melt-bearing material within the smaller crater likely originated from Mojave Crater. Therefore, we know that Mojave formed after this smaller crater.

As a result of the Mojave Crater impact, ejected material was transported to the smaller crater, some of which flowed around and some spilled inside and filled the topographic lows, such as on the terraces or the crater floor. Within the melt, we see evidence of “pitting.” Pitting comes from the release of volatile gases. These pits are commonly associated with and diagnostic of impact melt-bearing material on Mars and other rocky and ice-rich bodies in the Solar System.

This is a stereo pair with PSP_006703_1875.

Eric Pilles, Will Yingling, and Livio L. Tornabene wrote:

Degradation of Craters in Noachis Terra (ESP_062388_1450)

Small impact craters usually have simple bowl shapes, but in some cases surface properties or processes can alter this shape in unusual ways.

This image shows an approximately 300-meter impact crater that appears to have narrow terraces around the rim. How could these different crater morphologies form? One explanation is that the impact occurred into a surface with layers of differing strengths. However, the clearest example of this type of crater are better-preserved than this one.

Additional clues come from other craters that have a raised mound in the center as opposed to a depression and are sometimes referred to as inverted craters due to their topography. These craters were filled with sediment (or some material stronger than the surrounding material), and subsequent erosion removed the terrain around the filled material, leaving a small mound behind.

Vidhya Ganesh Rangarajan, Livio Leonardo Tornabene, Leah Sacks, Chimira Andres wrote:

Layered Bedrock in the Central Uplift of Betio Crater (ESP_016805_1565)

The road to the top involves challenges along the way and the higher you move, the knowledge acquired throughout your journey becomes more abundant and priceless. Such is the case with complex impact craters whose central structures struggle their way from deep within the subsurface to rise higher and higher. As they peek out through the surface, they give us a snapshot of the intense stresses and temperatures they were subjected to, in their path to see the sky.

However, not all of these crater central structures have sufficient strength to maintain a peak so they collapsing inwards upon loss of all their energy, forming a central pit or depression. Such is the case with the uplift of the 32-kilometer diameter Betio Crater, located south of Valles Marineris. This HiRISE color composite cutout shows a part of the central depression in Betio.

The presence of tilted light-toned layers at the base of the central depression suggests that bedrock may have been initially uplifted but later collapsed to form a pit. The image also shows how the layers rotated, deformed and competed with each other to be a part of the central structure we observe today.

This is a stereo pair with ESP_017583_1565.

HiRISE wrote:

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Dulovo Crater Region Barchan Dunes (ESP_055303_1835)

This image was a HiRISE Picture of the Day on 27 January 2020.

Alfred McEwen wrote:

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Possible Landing Region for a Future Mars Sample Return (ESP_062886_1990)

Although the “Mars 2020” rover (to be renamed) is planned to land inside Jezero Crater, HiRISE continues to image the regions to the west of the crater because the rover may drive into this area in its extended mission.

If so, this western region may be a potential location to set down a future mission that might carry an ascent vehicle and a “fetch” rover. The enhanced color cutout highlights an interesting portion of this image, with fractured bedrock and wind-blown dunes.

Alfred McEwen wrote:

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The Schiaparelli Crash Site (ESP_062731_1780)

The ExoMars Schiaparelli Lander (Entry, Descent, and landing Module, or EDM) crashed on the Martian surface on 19 October 2016. Also on that day the Trace Gas Orbiter successfully entered Mars orbit.

The HiRISE images acquired soon after the crash showed diffuse dark markings surrounding a shallow crater, plus small bright spots. HiRISE re-imaged this location on 25 March 2019, while dust was still settling from the planet-encircling dust storm, so surface features had low contrast.

HiRISE re-imaged this spot again through a much clearer atmosphere on 14 December 2019 (see animation). Much of the diffuse dark material has faded, perhaps from dust fallout, such that the crater is now more distinct. At least two bright spots are still visible.

In 2020 we expect three launches to Mars leading to landing attempts in early 2021: NASA’s unnamed (Mars 2020) rover, that will collect samples for return to Earth; the ESA/Roscosmos ExoMars lander and Rosalind Franklin rover; and an orbiter, lander, and Huoxing-1 (Mars-1) rover from China. HiRISE will be ready to see what happens.

Alfred McEwen wrote:

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Watching the InSight Lander Region (ESP_062884_1845)

HiRISE periodically images the InSight lander region in case anything changes, such as the appearance of new dust devil tracks. InSight has detected many passing atmospheric vortices, but they do not necessarily disturb the surface sufficiently to create a new track visible from orbit.

The cutout (with the the lander in the upper left corner) was given a “hard” stretch, saturating the brightest and darkest regions, to better detect subtle dust devil tracks. There are several southeast-to-southwest trending streaks to the east of the lander that may be from dust devils, but they were also present in a prior HiRISE image acquired about one month earlier, so they did not form in the past month. The bright spot next to the lander is a specular reflection from the smooth hemispherical cover over the seismometer.

Alfred McEwen wrote:

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North Polar Block Party (ESP_062866_2640)

The steep edge of the north polar cap is falling apart. This animation shows where a section of the slope at right has collapsed since 3 Mars years ago and deposited a field of ice blocks.

Alfred McEwen wrote:

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Mounds Cut by a Fissure (ESP_062943_2230)

This image shows four relatively bright mounds along a linear, curving feature that appears to be a rift zone, where the shallow surface materials have pulled apart. The mounds also appear to be deformed.

A possible geologic interpretation is that as the rift began to open, subsurface material (perhaps mud) erupted to create the mounds, which were then deformed as the rift continued to spread. This region (Chryse Planitia) is a low-elevation basin in which large outflow channels deposited water and sediments billions of years ago.

This is a stereo pair with ESP_062877_2230.

Alfred McEwen wrote:

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A Giant Gully in Kaiser Crater Dunes (ESP_062928_1325)

HiRISE has been monitoring this dune field since 2008, and it changes every year from gully erosion in the winter and blowing sand in the summer.

This cutout shows an especially large gully. The bright white materials are seasonal frost, persisting on shaded slopes.

Alfred McEwen wrote:

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Dunes in Briault Crater (ESP_062912_1700)

Active sand dunes are common on Mars, and have a variety of surface textures. The “braided” texture visible here may be typical of dunes that are transitioning into sand sheets.

Also see this image for a Context Camera view of this area.

This is a stereo pair with ESP_060855_1700.

Alfred McEwen wrote:

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Frosted Gullies (ESP_062894_1435)

Gullies on Mars form during the winter, fluidized by carbon dioxide frost, so we monitor these sites for activity throughout the year.

This mid-winter scene is almost completely frosted over the pole-facing slope within the shadow.

Alfred McEwen wrote:

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Sandstone in West Candor Chasma (ESP_062839_1740)

Candor Chasma in central Valles Marineris is filled with light-toned layered deposits thought to be sandstones, perhaps formed in an ancient wet and potentially habitable environment.

The CRISM instrument on MRO has acquired thousands high-resolution spectral images across Mars, often with simultaneous coverage by HiRISE, but sometimes, for a variety of reasons, without HiRISE coverage. We are now trying to complete coordinated coverage over such locations, to enable geologic interpretations based on both the compositional information of CRISM and the high-resolution imaging of HiRISE.

HiRISE Science Team wrote:

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A First Look at a Gullied Slope (ESP_062714_2350)

HiRISE has been operating since 2006, and lately many of our observations of gullies are repeat images designed to study changes. However, we are also collecting data over gullies never before seen at this resolution, to study their morphology and allow us to look for changes in the future.

This is the first HiRISE look at a cluster of gullies that appear modified or degraded—the gully fans have ripples and ridges that have formed since the last major gully activity, suggesting that they don’t change very often, but we won’t know for sure unless we look!

Candy Hansen wrote:

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Streamers of Frost (ESP_062556_1415)

When we acquired this image, it was northern summer and southern winter on Mars, but signs of spring are already starting to appear at latitudes not far from the equator. This image of Penticton Crater, taken at latitude 38 degrees south, shows streamers of seasonal carbon dioxide ice (dry ice) only remaining in places in the terrain that are still partially in the shade.

The turquoise-colored frost (enhanced color) is protected from the sun in shadowed dips in the ground while the sunlit surface nearby is already frost-free.

Candy Hansen wrote:

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Dunes Frozen in Time (ESP_062562_1670)

Sand dunes are found in many places on Mars. At most of these places the dunes are slowly moving, blown by the wind, just like on Earth. However, in this location in south Melas Chasma they appear to have turned to stone.

The large dunes are slowly being eroded and disappearing, replaced by smaller structures of scalloped sand.

HiRISE Science Team wrote:

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Barchan and Linear Dunes (ESP_062731_2645)

This image shows two types of sand dunes on Mars. The small dots are called barchan dunes, and from their shape we can tell that they are upwind. The downwind dunes are long and linear.

These two types of dunes each show the wind direction in different ways: the barchans have a steep slope and crescent-shaped “horns” that point downwind, while the linear dunes are stretched out along the primary wind direction. Linear dunes, however, typically indicate a wind regime with at least two different prevailing winds, which stretch out the sand along their average direction.

In several places in this image, you can find barchan dunes turning into linear dunes as they are stretched out, but they both seem into indicate the same wind direction.

Alfred McEwen wrote:

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North Polar Changes over 6 Mars Years (ESP_062793_2655)

MRO has been observing Mars for 6 Mars Years (MY), each of which lasts for 687 Earth days. Shown here is an impact crater on the north polar ice cap, which contains an icy deposit on the crater floor.

These inter-crater ice deposits shrink and expand or change shape or surface texture from year to year, In this animation, we can see the appearance of this crater fill in MY 29 (2/2008), 30 (8/2010), 31 (7/2012), 33 (2/2016), 34 (1/2018), and 35 (12/2019).