Sunday, February 28, 2010

Mars Express to make closest ever approach to Phobos

On 3 March 2010 Mars Express will make its closest ever approach to Phobos, the larger of the two Martian moons. During a series of flybys, spanning six weeks, all seven instruments onboard Mars Express will be utilised to study Phobos. The close approach provides a first opportunity to perform a unique gravity experiment that may reveal the distribution of mass within this intriguing moon.

ESA's Mars Express spacecraft orbits the Red Planet in a highly elliptical, polar orbit that brings it close to Phobos every five months. It is the only spacecraft, currently in orbit around Mars, whose orbit reaches far enough away from the planet to provide a close-up view of Phobos. Over the course of twelve flybys, taking place between 16 February and 26 March 2010, Mars Express will pass within 1400 km of the surface of Phobos. The Mars orbiter will make its closest ever approach to Phobos- just 50 km above the surface - on 3 March 2010.

The suite of seven experiments onboard Mars Express are primarily used to study the atmosphere, surface and subsurface of the Red Planet. These science instruments can also be used to investigate Phobos. During this current series of flybys all Mars Express instruments will be used to study Phobos, taking advantage of not only the close approach to the moon but also, for the gravity experiment during the closest flyby, the proximity of Mars to the Earth.

Phobos – a moon of unknown origin
Phobos, the larger of the two Martian moons, remains one of the few objects in the Solar System whose location cannot be easily explained. By studying Phobos with the Mars Express instruments scientists are hoping to contribute to the understanding of the moon's nature and origin. Phobos (and Deimos) could be captured asteroids – early measurements of the composition of both moons were compatible with this idea – or they could have formed from material that was ejected following a large collision with Mars. Additional theories are that the moons could be survival planetisimals, or formed from the break-up of a moon that was created early in the formation of the Solar System. Knowing how the mass is distributed within Phobos is an important step in understanding the interior of the moon and this in turn will provide crucial insight into the moon’s origin.

Studying Phobos close-up – unique science
At a distance of just 50 km above the surface of Phobos Mars Express will make the most precise measurements to date of the moon's gravity field using the X-band (8.4 GigaHertz) channel of the Mars Radio Science (MaRS) instrument. This instrument relies on the observation of the phase, amplitude, polarisation and propagation times of radio signals transmitted from the spacecraft and received at ground station antennas on Earth. The radio signals are affected by the medium through which the signals propagate, by the gravitational influence of Mars on the spacecraft and finally by the performance of the various systems involved both on the spacecraft and on ground. In addition, during this series of close flybys, the gravitational attraction of Phobos will slightly disturb the trajectory of the spacecraft. The difference between predicted trajectory (without Phobos) and the actually observed trajectory will lead to the determination of the forces acting on the spacecraft and from them the gravity field of the moon. To make these measurements, the spacecraft operates in two-way link mode with an X-band uplink and downlink.

This current series of flybys happen to occur when the orbits of Earth and Mars bring them close together which means that Mars Express will be ideally positioned to maximize the signal-to-noise ratio of the two-way X-band radio-link. NASA’s Deep Space Network (DSN) 70 metre radio station at Robledo, Spain, will track the radio signal from Mars Express and will pick up the subtle changes in the signal due to the doppler effect as the gravity of Phobos affects the spacecraft's velocity. In addition, the ESA Cebreros station will also be listening to the signal.

Mapping the mass distribution of Phobos
Analysis of Mars Express data will provide key coefficients of the gravity field. The most important coefficient, the mass of Phobos, has been determined from previous flybys at higher altitudes, but it does not provide any information about how the mass is distributed. Calculation of the density of Phobos, using the mass and volume, gives a value too low to be consistent with a solid, non-porous body, which has led to speculation about the composition of the moon and about how its mass is distributed.

Measurement of the gravity field coefficients from a lower altitude, as will be achieved during this series of flybys, will provide increased accuracy of the mass and allow the subsequent, smaller coefficients, such as the J2 coefficient, to be determined for the first time. To determine the mass distribution of Phobos these coefficients are required along with the libration, a measure of how Phobos rotates – this has already been determined from Mars Express HRSC images. Knowledge of these various parameters allows the three principle moments of inertia to be derived - these in turn describe the mass distribution of Phobos. Models of Phobos' interior are being developed and will be tested against the findings of the current and future close flybys.

Studying Phobos close-up – continuing investigation
In addition to the new science performed during the gravity experiment, this series of flybys will see Mars Express build on knowledge gained from previous flybys. HRSC data obtained during previous flybys has led to the development of a new topographical atlas of Phobos (see M. Wählisch et al. (2009) for further details and the Phobos atlas website). On-going investigations include: improving the accuracy of the location of Phobos (see J. Oberst et al. (2006), V. Lainey et al. (2007), K. Willner et al. (2008), P. Rosenblatt et al. (2008)) and therefore knowledge of its constantly changing orbit as it spirals slowly towards Mars; measurements of the surface to determine its composition (see B. Gondet et al. (2008), S. Perrier et al. (2004)), study of the origin of grooves [pdf] (see J. Murray et al. (2006)), shape (see K. Willner et al. (2010)) and sub-surface properties; as well as studying how the surface interacts with the solar wind.

Mars Express data will provide an important contribution to understanding the nature and origin of Phobos but this alone will not provide a definitive answer. Further exploration is required, and in 2011 the Russian Phobos-Grunt (Phobos-Soil) mission is scheduled to launch to retrieve a sample from Phobos to return for study on Earth. Images taken by the Mars Express HRSC instrument during this series of flybys will be used to support the final selection of the Phobos-Grunt landing site.

Digital terrain model of Phobos derived from HRSC data (top), M. Wählisch et al. (2009)
Orbits of Phobos and Mars Express (bottom), ESA

For a very nice animation of Phobos (that, unfortunately, I'm not able to upload for this blog post), see A Complete View of Phobos.

Saturday, February 27, 2010

Central Deposits in Pasteur Crater

This observation shows a portion of the central sedimentary deposits in Pasteur Crater [located in Arabia Terra].

The deposits in this image [are] now being eroded into knobs and ridges. The erosion is probably dominated by wind, as most of the ridges are parallel. This is common in wind-eroded features, with the ridges generally aligned with the prevailing wind.

At high resolution, layering is revealed in many of the knobs and outcrops. The horizontal layers indicate that the material was deposited uniformly over a broad area. Possible origins include volcanic airfall or lacustrine (lake) deposits. After deposition, the rock in this area has been fractured and faulted, forming a diverse array of cracks.

The mottled appearance of much of the image is caused by dark, featureless patches which may be wind-blown dust. These have interacted with lighter-toned ridges and ripples which are probably also formed by aeolian (wind) processes. In places, the dark patches partially cover the ripples, indicating that they have moved more recently, but they must be thin because the ripples frequently stand above surrounding dark material.

The ripples exhibit multiple interacting orientations in some places, producing networks of small ridges which reflect movement in winds from several directions.
Photo Credits: NASA/JPL/University of Arizona

Friday, February 26, 2010

A Field of Secondary Craters

This observation shows a secondary crater field, which form when material ejected from a larger impact event impacts the Martian surface. One impact event, depending on the size of the impactor, can form hundreds of millions of secondary craters at essentially the same time.

Primary craters (those created directly from an impactor from space) can be the same size as secondary craters, which makes dating surfaces based on the number of accumulated craters difficult to near-impossible. Secondary craters are distinguished from primaries based on their morphologies. They are sometimes irregularly shaped, as seen in this image, because they form at relatively low velocities. The velocity of the impactor determines a crater’s size, shape, and depth, with lower energy impacts forming shallow, less-developed craters and higher energy impacts forming deeper, more regular craters.

Secondary craters often occur in clusters, as seen here, as a piece of ejecta breaks up before hitting the surface. Primary craters form at random locations globally. Secondary clusters are more likely to be found in groups because of their formation mechanism.
Note: The craters in this photo are located in the southwestern rim of Utopia Planitia (31.1° North, 89.7° East), in the Casius Quadrangle.

Photo Credit: NASA/JPL/University of Arizona

Thursday, February 25, 2010

Mojave Crater: The Rosetta Stone of Martian Craters?

This Digital Terrain Model (DTM) covers the northwestern portion of the approximately 60 kilometer diameter Mojave Crater, centered at 327.0 degrees East, 7.5 degrees North in Xanthe Terra on Mars.

The perspective views subimage 1 [the black-and-white image above] and subimage 2 shows the entire HiRISE image covering portions of the crater’s northwestern wall-terraces, rim and ejecta blanket (see subimage 3 for context). Subimage 1 is viewed from the southeast and highlights the crater interior. Subimage 2 is viewed from the northwest and highlights the crater rim and ejecta blanket. The vertical exaggeration for these images are set to 3x (meaning that features appear to be 3x taller than they are wide).

One of the most interesting features in Mojave are the “pitted ponds” that appear to be “backed-up” behind massive wall-terrace blocks of bedrock. Pitted materials are currently recognized in hundreds of fresh and well-preserved Martian craters, and are thought to represent impact melt bodies that were captured behind the wall-terraces as they faulted off the rim and into the crater interior. With the exception of the dense population of pits, these “ponds” are quite similar to impact melt ponds observed on the wall-terraces of lunar craters (see the Kaguya image gallery).

The DTM shows terrain spanning -4803.65 meters to -2744.87 meters (-3 miles to -1.7 miles) with respect to the Martian datum (just over 2 kilometers [1.3 miles] from the lowest to highest points; see color altimetry image [above]). Based on observations and modeling of fresh craters, a crater the size of Mojave should be approximately 2,595 meters (1.62 miles) deep in its most pristine state. The DTM here does not show the lowest point in the crater, but still demonstrates nicely that Mojave has minimal infilling or erosion, and is very close to its pristine state – estimates from other datasets indicate that Mojave is approximately 2,604 meters (1.63 miles) deep. A survey of Mojave’s features indicates that there are very few overprinting craters on them.

A statistical analysis of Mojave’s overprinting craters further indicates the youthfulness of Mojave, as models indicate the crater may be as “young” as approximately 10 million years old – well within the Amazonian Era, and indeed a very young Martian crater for its size.

Mojave no doubt gives us a glimpse of what a very large complex crater should look like on Mars; and perhaps in a sense, it is a “Rosetta Stone” of craters, given that it’s so “fresh” and most others – especially this size – have been affected by erosion, sedimentary infilling and overprinting by other geologic processes. Such craters like Mojave, especially when accounting for size, location and target properties, are one-of-a-kind ... , but give tremendous insight into the impact process (e.g., ejecta, melt-generation and deposition, etc.).

Mojave’s fans and channels are most intriguing, and hint that impacts such as Mojave may have unleashed water/water-ice from the subsurface to flow across the surface and, perhaps, condense as rain or snow for only a brief period of Martian time. This further suggests that early climate on Mars could have been heavily influenced by its most intense bombardment when many Mojave-sized craters (and far larger) were more common, approximately 3.9 billion years ago.
Credit: NASA/JPL/University of Arizona/USGS

Wednesday, February 24, 2010

Changes at the Site of a New Impact Cluster

Nineteen new impact sites were discovered by the Mars Orbital Camera on the Mars Global Surveyor, and HiRISE has re-imaged these sites to learn more about them, including detection of many smaller craters since the objects often break up in the atmosphere and make clusters of craters.

The biggest cluster--shown here--with over 1,000 craters, formed between September 2005 and February 2006, over the dusty region between Ascraeus and Pavonis Mons, giant shield volcanoes. We have imaged some of these new impact sites multiple times to look for changes, which provides information on aeolian (wind-driven) processes.

Many of these impact sites are remarkably unchanged over several years time, but the site shown here has changed dramatically. In the subimage [above right] are cutouts of the impact cluster from PSP_003172_1870 (top), PSP_007431_1870 (middle), and this newest image (bottom). Blowing winds through the pass between shield volcanoes has darkened some regions and brightened others, probably largely by removing and depositing dust.
Photo Credit: NASA/JPL/University of Arizona

Tuesday, February 23, 2010

Evidence of Multiple Episodes of Gully Formation

This observation shows gullies in a crater in Terra Sirenum. The gullies unusually emanate from different elevations along the crater wall. Several of the gullies are extremely developed and incised, while others have very narrow, shallow channels.

Many of the gullies appear to have extensive debris aprons, but that could be deceiving. Based on their surroundings, the topography underlying the debris aprons is likely not flat or gently sloping. This might cause the debris apron material to cover a wider surface area, without being as large of a volume as it might appear visually, than it otherwise would.

The subimage shows a gully with many channels. Several of the channels overlap or are overlapped by debris aprons suggesting that multiple flow episodes occurred here. In particular, there is a large channel that sticks out from underneath the main debris apron with a debris apron of its own. If this channel originated where the alcove currently is, then it is possible that the past flow contained more liquid and that the source of liquid to form the gullies in this region is now available in smaller amounts for an unknown reason.
Photo Credit: NASA/JPL/University of Arizona

Monday, February 22, 2010

Crater Floor in Arabia Terra Region

This observation shows a northwestern portion of the floor of a crater in the Arabia Terra region of Mars.

In the subimage, several light-toned layered outcrops are visible, surrounded by dunes of varying sizes. The outcrops exhibit multiple alternating light and dark layers with extensive fracturing and small fault offsets. The outcrops represent the eroded remains of sedimentary rocks that formed from sediments once deposited within the crater. Possible origins for the sediments include windblown debris, volcanic ash falling from the sky, or sediments that accumulated in a lake on the crater floor.

The dark filamentary streaks in the right half of the full image were most likely created by the disruption and/or removal of thin surface coatings of dust by the passage of a dust devil. Streak patterns such as these have been found to change over periods of several months to an Earth year, suggesting that the ones seen here probably formed relatively recently.
Photo Credit: NASA/JPL/University of Arizona

Sunday, February 21, 2010

Sinuous Ridges Near Aeolis Mensae

This observation covers part of a fan-shaped deposit of material in the Aeolis Mensae region of Mars.

The dominant surface texture is a series of parallel linear ridges. In addition, there are several sinuous, flat-topped ridges. The sinuous ridges do not follow the trend of the linear ridges, and various intersecting relations are observed.

The southernmost sinuous features in this image are partially buried by linear ridge material, while in the northern part of the image they appear to stand above it. This could indicate that the linear unit has been more eroded in the north than the south, but may also be due to a more complex geological history in which different sinuous ridges formed at different times. In the northeast part of the image one sinuous ridge appears superposed on another, supporting this hypothesis.

The linear ridges may be yardangs. Yardangs form when material is eroded by wind, producing elongated features aligned with the prevailing wind. Many of the ridges expose layers and appear to have broken into boulders. Layering indicates multiple episodes or pulses of deposition, while the occurrence of boulders shows that the material has been consolidated to some degree.

The sinuous ridges could be former stream channels outcropping in inverted relief, where a formerly low-lying feature is now relatively high-standing. This occurs when the stream channel is more resistant to erosion that the surroundings, either due to cementation by water or to the presence of large rocks which are not easily eroded.

In this case, the sinuous ridges contain few boulders resolvable by HiRISE, generally appearing uniform and smooth. They also contain fractures which in places cut across the entire ridge. Both of these observations are consistent with cementation of former channel floors.
Photo Credit: NASA/JPL/University of Arizona

Saturday, February 20, 2010

Layers in Olympus Mons Basal Scarp

This observation shows a small portion of the scarp (cliff) that surrounds the largest volcano in the solar system, Olympus Mons.

The scarp is of unknown origin. It may have formed from faulting or other tectonic processes resulting from the heavy loading of the Martian crust in this location. The bottom of the image shows the cratered flanks of Olympus Mons.

Olympus Mons is a large shield volcano, like the Hawaiian volcanoes on Earth. Shield volcanoes have very shallow slopes and gentle eruptions. The Hawaiian volcanoes form when a plate of crust moves over a hot spot. The hot spot produces magma that gradually forms the volcanoes. Since Earth has plate tectonics, the crustal plate moves over the hot spot producing a chain of volcanoes.

Mars does not have plate tectonics, which causes the magma to build a volcano in one location making Olympus Mons so large.

Photo Credit: NASA/JPL/University of Arizona

Friday, February 19, 2010

Eroding Layers in an Impact Crater

This image shows a stack of layers on the floor of an impact crater roughly 30 kilometers across, located in the Iapygia Quadrangle (MC-21) of Mars. Many of the layers appear to be extremely thin, and barely resolved.

In broad view, it is clear that the deposit is eroding into a series of ridges, likely due to the wind. Below the ridges, additional dark-toned layered deposits crop out. These exhibit a variety of textures, some of which may be due to transport of material.

The light ridges are often capped by thin dark layers, and similar layers are exposed on the flanks of the ridges. These layers are likely harder than the rest of the material, and so armor the surface against erosion. They are shedding boulders which roll down the slope, as shown in the subimage. Although these cap layers are relatively resistant, the boulders do not seem to accumulate at the base of the slope, so it is likely that they also disintegrate relatively quickly.

The subimage itself is 250 meters wide. The light is from the left. Boulders are visible on the slopes of the ridges along with thin dark layers including the cap layer, but they are absent on the spurs where the resistant cover has been eroded. This demonstrates that the boulders come only from the dark layers, and are not embedded in the rest of the deposit.
Photo Credit: NASA/JPL/University of Arizona

Thursday, February 18, 2010

Slope Streaks in Terra Sabaea

This observation shows the rim of a crater in the region of Terra Sabaea in the northern hemisphere of Mars.

The subimage is a close-up view of the crater rim revealing dark and light-toned slope streaks. Slope streak formation is among the few known processes currently active on Mars. While their mechanism of formation and triggering is debated, they are most commonly believed to form by downslope movement of extremely dry sand or very fine-grained dust in an almost fluidlike manner (analogous to a terrestrial snow avalanche) exposing darker underlying material.

Other ideas include the triggering of slope streak formation by possible concentrations of near-surface ice or scouring of the surface by running water from aquifers intercepting slope faces, spring discharge (perhaps brines), and/or hydrothermal activity.

Several of the slope streaks in this subimage, particularly the three longest darker streaks, show evidence that downslope movement is being diverted around obstacles such as large boulders. Several streaks also appear to originate at boulders or clumps of rocky material.

In general, the slope streaks do not have large deposits of displaced material at their downslope ends and do not run out onto the crater floor suggesting that they have little reserve kinetic energy. The darkest slope streaks are youngest and can be seen to cross cut and superpose older and lighter-toned streaks. The lighter-toned streaks are believed to be dark streaks that have lightened with time as new dust is deposited on their surface.
Photo Credit:  NASA/JPL/University of Arizona

Wednesday, February 17, 2010

Exhumed Layers Near the Nili Fossae

This subimage shows (near center) densely fractured light-toned rock in the vicinity of the Nili Fossae. The light-toned material is finely layered; these layers are visible in cross-section along a scarp face at the bottom of the image.

At full resolution, the light-toned layered materials resemble those seen in other HiRISE images of Nili Fossae and its surroundings, some of which have been identified on the basis of their infrared spectra (by OMEGA and CRISM) as containing phyllosilicates (clays), which require the presence of water to form. These layers likely formed very early in Martian history, but must have been rapidly buried due to the lack of overprinting impact craters.

Presently, the light-toned materials are being exhumed as the overlying material is eroded away by wind.

Additionally, the light-toned layers are overlain by a darker, densely pitted, rubbly layer. The areal extent of this darker layer, which has no apparent internal layering, is visible in the full image. The dark layer may represent lava flows, possibly extruded from the Nili Fossae fissures or from the Syrtis Major volcano, 1000 kilometers (620 miles) to the southwest.

In the full image, the large valleys cutting into the dark material and its underlying layers may have formed by groundwater seepage and erosion, or by tectonic processes related to the opening of the Nili Fossae fissure system, to which the valleys connect just southeast of this image.

South of the large area capped by dark material is a complex terrain of irregularly shaped pits and mesas, some of which are also capped by dark, pitted rock. The lighter, layered, densely fractured material is well exposed here. The pits are filled with relatively dark-toned, fine-grained material, and lighter wind-blown ripples are also present in some cases. Large boulder-sized fragments of light-toned rock are also visible in some pits, especially near the eroding scarp face highlighted in the sub-image above.

Photo Credit: NASA/JPL/University of Arizona

Tuesday, February 16, 2010

Debris Flow Near Hale Crater

Channels are found all around Hale Crater. The largest channels were there before the formation of Hale, such as Uzboi Vallis.

The impact that created Hale Crater smashed directly into Uzboi Vallis, a very large channel thought to have periodically transported hundreds of thousands of cubic meters of water per second. Another nearby channel is Nirgal Vallis, an approximately 700 kilometer (430 miles)-long channel interpreted to have formed from groundwater sapping. Nirgal Vallis is about 300 kilometers (190 miles) from Hale.

Other, smaller channels also radiate from Hale. The relationship between the channels and the ejecta from Hale Crater strongly suggests the Hale-forming impact event created, or at least heavily modified, the channels.

One of the types of channels thought to have been formed at the same time as Hale Crater is a channel with raised margins. These channels are relatively short (less than 5 kilometers, or 3 miles) and less than 0.5 kilometers (0.3 miles) wide. They are found on the slope break at the very edge of the northern rim of Argyre Basin, which opens to the bottom left of this image. We interpret these channels to be the result of debris flows.

Photo Credit: NASA/JPL/University of Arizona

Sunday, February 7, 2010

Frost-Covered Dunes in the North Polar Region

This image shows dunes on the northern plains of Mars, and appears similar to images taken when the surface was covered by frost.

However, CRISM spectra taken at the same time do not show evidence for either water or carbon dioxide frost here. Possibly, and consistent with the CRISM spectra, this area is covered by dust, obscuring the dark material that is typically present in dunes of this type.

The orientation of the dunes indicates that they were formed by winds blowing generally from upper right to lower left. Ripples on the dunes show that the wind patterns that formed them are more complex, with the dune shapes affecting the wind direction.

It is not known whether these dunes are currently active (being moved by wind today) or have been in this location for a very long time, but if they are indeed covered by dust they cannot have been recently active.

Between the dunes, the underlying surface of the northern plains can be seen. In places, it has been fractured into polygonal blocks, suggesting that water ice is or was present below the surface. Meter-size blocks are also seen in places in this image and elsewhere on the northern plains. The origin of these blocks is not known, but they may be remnants of erosion of material that once covered this region.

Photo Credit: NASA/JPL/University of Arizona

Saturday, February 6, 2010

Layered Deposits in Terby Crater

Terby Crater is a large (approximately 165 kilometer), Noachian-aged crater located on the northern rim of the Hellas impact basin.

Terby hosts a very impressive sequence of predominantly light-toned layered deposits, up to 2.5 kilometers thick that are banked along its northern rim and extend toward the center of the crater.

The full image shows this stack of layered rocks as they are exposed westward facing scarp. The layered sequence consists of many beds that are repetitive, relatively horizontal and laterally continuous on a kilometer scale. Many beds are strongly jointed and fractured and exhibit evidence of small-scale wind scour.

The light-toned layers are typically at least partially covered with dark mantling material that obscures the layers as well as debris and numerous, meter-scale boulders that have cascaded down slope. The processes responsible for formation of these layers remain a mystery, but could include deposition in water, by the wind, or even volcanic activity.

This HiRISE image is a proposed landing site for the Mars Science Laboratory (MSL) in Terby Crater.

Photo Credit: NASA/JPL/University of Arizona

Friday, February 5, 2010

Mojave Crater Floor and Central Uplift

This observation shows a portion of the central uplift structure in Mojave Crater.

Central uplifts are a typical feature of large impact craters on the Earth, the Moon and Mars; craters larger than 6 or 7 kilometers in diameter on Mars typically form this mountain-like peak in the central portion of the crater interior.

This peak consists of rocks originating from several kilometers beneath the pre-impact surface. Mojave has a very prominent central uplift as it has a diameter of 60 kilometers (37 miles). In this image, boulders as large as 15 meters (50 feet) across have been eroded from the massive uplifted rock and have rolled downslope. Fine-grained debris has also collected in the topographic lows, and has been shaped by the wind into dunes and ripples.

Notably absent from this image are the striking drainage channels and alluvial fans that are abundant on the wall-terraces and ejecta of Mojave Crater (see PSP_001415_1875). These features were likely formed by surface runoff of liquid water, which may have been released from the subsurface during the impact event that formed Mojave.

Previously, it had been suggested that a brief, torrential downpour over Mojave Crater delivered the water. However, Mars Orbiter Camera's (MOC) images of Mojave's central uplift have previously shown no evidence for surface runoff, and the higher resolution of this HiRISE image confirms that this part of the crater appears untouched by liquid water.

So the question remains: by what means was the water, in the form of runoff, supplied to Mojave? This question, in addition to several others regarding this phenomenon, are currently being investigated by the HiRISE team and their collaborators.

The full HiRISE image shows that the crater floor south of the central uplift is densely pitted and fractured. These pits, many of which are partially filled with dark sand, lack raised rims and a circular form. This suggests that they are not impact craters. In fact, very few definite impact craters are seen on the floor and walls of Mojave, implying that it is incredibly young and relatively well preserved for a crater of its size.

Photo Credit: NASA/JPL/University of Arizona

Thursday, February 4, 2010

Central Uplifted Region of Crater in Phlegra Dorsa

This image covers part of the central uplifted region of an unnamed crater in Phlegra Dorsa.

This complex crater is approximately 30 kilometers (about 18 miles) in diameter and is centered at 23 N latitude and 176 E longitude. The transition from a simple bowl-shaped crater to a complex crater exhibiting central peaks or pits, flat floors and terraced walls takes place in craters that are larger than about 15 kilometers (about 9 miles) in diameter on Mars. Because the central uplifts of complex craters expose rocks and materials that originated deep below the surface, researchers can use these regions as possible "windows" to view the rocks beneath the surface.

A northeast-southwest linear valley or trough transects this region dividing the uplift in two. This valley, or lineation, may have resulted from processes occurring during the uplift event or subsequent to crater formation. When seen at HiRISE resolution the center of this valley seems to bisect what may be a small (less than a kilometer wide) central pit.

Photo Credit: NASA/JPL/University of Arizona