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NASA NTRS2006

First Directly Retrieved Global Distribution of Tropospheric Column Ozone from GOME: Comparison with the GEOS-CHEM Model

We present the first directly retrieved global distribution of tropospheric column ozone from Global Ozone Monitoring Experiment (GOME) ultraviolet measurements during December 1996 to November 1997. The retrievals clearly show signals due to convection, biomass burning, stratospheric influence, pollution, and transport. They are capable of capturing the spatiotemporal evolution of tropospheric column ozone in response to regional or short time-scale events such as the 1997-1998 El Nino event and a 10-20 DU change within a few days. The global distribution of tropospheric column ozone displays the well-known wave-1 pattern in the tropics, nearly zonal bands of enhanced tropospheric column ozone of 36-48 DU at 20degS-30degS during the austral spring and at 25degN-45degN during the boreal spring and summer, low tropospheric column ozone of <30 DU uniformly distributed south of 35 S during all seasons, and relatively high tropospheric column ozone of >33 DU at some northern high-latitudes during the spring. Simulation from a chemical transport model corroborates most of the above structures, with small biases of <+/-5 DU and consistent seasonal cycles in most regions, especially in the southern hemisphere. However, significant positive biases of 5-20 DU occur in some northern tropical and subtropical regions such as the Middle East during summer. Comparison of GOME with monthly-averaged Measurement of Ozone and Water Vapor by Airbus in-service Aircraft (MOZAIC) tropospheric column ozone for these regions usually shows good consistency within 1 a standard deviations and retrieval uncertainties. Some biases can be accounted for by inadequate sensitivity to lower tropospheric ozone, the different spatiotemporal sampling and the spatiotemporal variations in tropospheric column ozone.

Liu, Xiong↗

NASA NTRS2009

Dewetting and Segregation of Zn-Doped InSb in Microgravity Experiments

In directional solidification, dewetting is characterized by the lack of contact between the crystal and the crucible walls, due to the existence of a liquid meniscus at the level of the solid-liquid interface. This creates a gap of a few tens of micrometers between the crystal and the crucible. One of the immediate consequences of this phenomenon is the dramatic improvement of the quality of the crystal. This improvement is partly due to the modification of the solid-liquid interface curvature and partly to the absence of sticking and spurious nucleation at the crystal-crucible interface. Dewetting has been, commonly observed during the growth of semiconductors in crucibles under microgravity conditions where it appears to be very stable: the gap between the crystal and the crucible remains constant along several centimetres of growth. The physical models of the phenomenon are well established and they predict that dewetting should not occur in microgravity, if sufficient static pressure is imposed on the melt, pushing it towards the crucible. We present the results of InSb(Zn) solidification experiments conducted at the International Space Station (ISS) where, in spite of a spring exerting a pressure on the liquid, partial dewetting did occur. This surprising result is discussed in terms of force exerted .by the spring on the liquid and of possibility that the spring did not work properly. Furthermore, it appears that the segregation of the Zn was not affected by the occurrence of the dewetting. The data suggest that there was no significant interference of convection with segregation of Zn in InSb.

Ostrogorsky, A. G.↗

NASA NTRS2009

Pressure-Energized Seal Rings to Better Withstand Flows

Pressure-energized seal rings intended to withstand flows better than do conventional pressure-energized seal rings have been conceived. The concept applies, more specifically, to seal rings used on some valve stems, pistons, and the like. A conventional pressure-energized seal ring generally has a U-shaped cross section and consists of an elastomer or other suitable polymer with an embedded metal energizing spring (see Figure 1). The working fluid from the high-pressure side that one seeks to seal is allowed into the U-shaped cavity, so that the pressure pushes the sides of the seal ring tighter against the gland and body sealing surfaces, thereby increasing the degree of sealing. Unfortunately, when the seal ring is exposed to flow of the working fluid, under some conditions, the flow grabs the lip of the U-shaped cross section and ejects or deforms the seal ring so that, thereafter, a proper seal is not obtained. Figure 2 depicts one of several alternative seal rings according to the present concept. One element of the concept is to modify the U-shaped cross section from that of the corresponding conventional seal ring to eliminate the exposed lip and prevent entry of the working fluid into the U-shaped cavity. Unlike in the conventional seal, pressurized fluid would not push the seal ring directly against the both gland and body sealing surfaces. Instead, the pressure would directly push the seal ring against a gland sealing surface only. In so doing, the pressure would squash the seal ring into a smaller volume bounded by the gland and body sealing surfaces, and would thereby indirectly press the seal ring more tightly against the body sealing surface. To enhance the desired squashing deformation, a spring having an approximately parallelogram cross section would be embedded in the modified U-shaped cavity. As the pressure pushed two corners of the approximate parallelogram closer together along the axis of the seal ring, the other two corners of the approximate parallelogram would be pushed farther apart along a radius of the ring, thereby causing the polymeric ring material to push radially harder against the body sealing surface. From the radially innermost corner of the approximate parallelogram, the spring material would extend radially, then axially into recesses in the seal gland. These extensions would help to restrain the seal ring against ejection. A seat retainer would hold the sealing ring in the gland and form a mechanical compression seal to prevent or at least reduce leakage of pressurized fluid into the cavity behind the seal. However, because there would likely be a little leakage, the cavity behind the seal should be vented to the low pressure side to prevent buildup of pressure in the cavity over time; otherwise, the built-up pressure could cause ejection of the seal ring when the pressure on the high-pressure side was reduced. Polymeric seal-ring materials may not be able to withstand working conditions in applications that involve abrasive and/or hot working fluids. For such applications, all-metal seal rings may be preferred. The bottom part of Figure 2 shows one example of an alternative gland configuration with an all-metal seal ring.

Farner, Bruce↗

NASA NTRS2007

Improved Quick-Release Pin Mechanism

An improved quick-release pin mechanism supplants a prior such mechanism in which the pin bears a shear load to hold two objects together. The prior mechanism, of a ball-locking design, can fail when vibrations cause balls to fall out. The load-bearing pin is an outer tube with a handle at one end (hereafter denoted the near end). Within the outer tube is a spring-loaded inner tube that includes a handle at its near end and a pivoting tab at its far end. The pin is inserted through holes in the objects to be retained and the inner tube is pushed against an offset pivot inside the outer tube to make the tab rotate outward so that it protrudes past the outer diameter of the outer tube, and the spring load maintains this configuration so that the pin cannot be withdrawn through the holes. Pushing the handles together against the spring load moves the locking tab out far enough that the tab becomes free to rotate inward. Then releasing the inner-tube handle causes the tab to be pulled into a resting position inside the outer tube. The pin can then be pulled out through the holes.

Wright, Jay M.↗

NASA NTRS2010

Novel Thermotolerant Siderophilic Filamentous Cyanobacterium that Produces Intracellular Iron-Rich Phases

Cyanobacteria are the main producers of organic compounds in iron-depositing hot springs despite photosynthetically generated-oxygen and the abundance of reduced iron (Fe2+) that likely leads to enormous oxidative stress within cyanobacterial cells. Therefore, the study of cyanobacterial diversity, phylogeny, and biogeochemical activity in iron-depositing hot springs will not only provide insights into the contribution of CB to iron redox cycling in these environments, but it could also provide insights into CB evolution. This study characterizes the phylogeny, morphology, and physiology of isolate JSC-1, a novel filamentous CB isolated from an iron-depositing hot spring. While isolate JSC-1 is morphologically similar to the CB genus Leptolyngbya, 16S rDNA sequence data indicated that it shares 95 percent sequence similarity to the type strain L. boryanum. Strain JSC-1 fixes N2 and exhibited an unusually high ratio between photosystem (PS) I and PS II and was capable of complementary chromatic adaptation. Further, it synthesized only chlorophyll a and a unique set of carotenoids. Strain JSC-1 not only required high levels of Fe for growth (greater than or equal to 40 microM), but it also accumulated large amounts of extracellular ferrihydrite and generated intracellular ferric phosphates. Strain JSC-1 was found to secrete 2-oxoglutaric acid and possesses one ortholog and one paralog of bacterioferritin. Surprisingly, the latter has 70.13 % identity with a bacterioferritin in marine-proteobacterium HTCC 2080 and has joint node with bacterioferritins found in enterobacteria. Collectively, these observations provide insights into the physiological strategies that might have allowed CB to develop and proliferate in Fe-rich environments. Based on its genotypic and phenotypic characterization of strain, JSC-1 represents a new operational taxonomical unit (OTU) JSC-1.

Broun, Igor I.↗

NASA NTRS2010

Design and Development of a Model to Simulate 0-G Treadmill Running Using the European Space Agency's Subject Loading System

Develop a model that simulates a human running in 0 G using the European Space Agency s (ESA) Subject Loading System (SLS). The model provides ground reaction forces (GRF) based on speed and pull-down forces (PDF). DESIGN The theoretical basis for the Running Model was based on a simple spring-mass model. The dynamic properties of the spring-mass model express theoretical vertical GRF (GRFv) and shear GRF in the posterior-anterior direction (GRFsh) during running gait. ADAMs VIEW software was used to build the model, which has a pelvis, thigh segment, shank segment, and a spring foot (see Figure 1).the model s movement simulates the joint kinematics of a human running at Earth gravity with the aim of generating GRF data. DEVELOPMENT & VERIFICATION ESA provided parabolic flight data of subjects running while using the SLS, for further characterization of the model s GRF. Peak GRF data were fit to a linear regression line dependent on PDF and speed. Interpolation and extrapolation of the regression equation provided a theoretical data matrix, which is used to drive the model s motion equations. Verification of the model was conducted by running the model at 4 different speeds, with each speed accounting for 3 different PDF. The model s GRF data fell within a 1-standard-deviation boundary derived from the empirical ESA data. CONCLUSION The Running Model aids in conducting various simulations (potential scenarios include a fatigued runner or a powerful runner generating high loads at a fast cadence) to determine limitations for the T2 vibration isolation system (VIS) aboard the International Space Station. This model can predict how running with the ESA SLS affects the T2 VIS and may be used for other exercise analyses in the future.

Caldwell, E. C.↗

NASA NTRS2010

Shape memory thermal conduction switch

A thermal conduction switch includes a thermally-conductive first member having a first thermal contacting structure for securing the first member as a stationary member to a thermally regulated body or a body requiring thermal regulation. A movable thermally-conductive second member has a second thermal contacting surface. A thermally conductive coupler is interposed between the first member and the second member for thermally coupling the first member to the second member. At least one control spring is coupled between the first member and the second member. The control spring includes a NiTiFe comprising shape memory (SM) material that provides a phase change temperature <273 K, a transformation range <40 K, and a hysteresis of <10 K. A bias spring is between the first member and the second member. At the phase change the switch provides a distance change (displacement) between first and second member by at least 1 mm, such as 2 to 4 mm.

Vaidyanathan, Rajan↗

NASA NTRS2002

Life Beneath Glacial Ice - Earth(!) Mars(?) Europa(?)

We are investigating a set of cold springs that deposit sulfur and carbonate minerals on the surface of a Canadian arctic glacier. The spring waters and mineral deposits contain microorganisms, as well as clear evidence that biological processes mediate subglacial chemistry, mineralogy, and isotope fractionation . The formation of native sulphur and associated deposits are related to bacterially mediated reduction and oxidation of sulphur below the glacier. A non-volcanic, topography driven geothermal system, harboring a microbiological community, operates in an extremely cold environment and discharges through solid ice. Microbial life can thus exist in isolated geothermal refuges despite long-term subfreezing surface conditions. Earth history includes several periods of essentially total glaciation. lee in the near subsurface of Mars may have discharged liquid water in the recent past Cracks in the ice crust of Europa have apparently allowed the release of water to the surface. Chemolithotrophic bacteria, such as those in the Canadian springs, could have survived beneath the ice of "Snowball Earth", and life forms with similar characteristics might exist beneath the ice of Mars or Europa. Discharges of water from such refuges may have brought to the surface living microbes, as well as longlasting chemical, mineralogical, and isotopic indications of subsurface life.

Allen, Carlton C.↗

NASA NTRS2011

Mount Protects Thin-Walled Glass or Ceramic Tubes from Large Thermal and Vibration Loads

The design allows for the low-stress mounting of fragile objects, like thin walled glass, by using particular ways of compensating, isolating, or releasing the coefficient of thermal expansion (CTE) differences between the mounted object and the mount itself. This mount profile is lower than true full kinematic mounting. Also, this approach enables accurate positioning of the component for electrical and optical interfaces. It avoids the higher and unpredictable stress issues that often result from potting the object. The mount has been built and tested to space-flight specifications, and has been used for fiber-optic, optical, and electrical interfaces for a spaceflight mission. This mount design is often metal and is slightly larger than the object to be mounted. The objects are optical or optical/electrical, and optical and/or electrical interfaces are required from the top and bottom. This requires the mount to be open at both ends, and for the object s position to be controlled. Thin inside inserts at the top and bottom contact the housing at defined lips, or edges, and hold the fragile object in the mount. The inserts can be customized to mimic the outer surface of the object, which further reduces stress. The inserts have the opposite CTE of the housing material, partially compensating for the CTE difference that causes thermal stress. A spring washer is inserted at one end to compensate for more CTE difference and to hold the object against the location edge of the mount for any optical position requirements. The spring also ensures that any fiber-optic or optic interface, which often requires some pressure to ensure a good interface, does not overstress the fragile object. The insert thickness, material, and spring washer size can be traded against each other to optimize the mount and stresses for various thermal and vibration load ranges and other mounting requirements. The alternate design uses two separate, unique features to reduce stress and hold the object. A release agent is applied to the inside surface of the mount just before the binding potting material is injected in the mount. This prevents the potting material from bonding to the mount, and thus prevents stress from being applied, at very low temperatures, to the fragile object being mounted. The potting material mixing and curing is temperature- and humidity-controlled. The mount has radial grooves cut in it that the potting material fills, thus controlling the vertical position of the mounted object. The design can easily be used for long and thin objects, short and wide objects, and any shape in between. The design s advantages are amplified for long and thin fragile objects. The general testing range was 45 to +45 C, but multiple mounts were successfully tested down to 60 and up to 50 C and the design can be adjusted for larger ranges.

Amato, Michael↗

NASA NTRS2004

Self-Inflatable/Self-Rigidizable Reflectarray Antenna

A report describes recent progress in a continuing effort to develop large reflectarray antennas to be deployed in space. Major underlying concepts were reported in two prior NASA Tech Briefs articles: Inflatable Reflectarray Antennas (NPO- 20433), Vol. 23, No. 10 (October 1999), page 50 and Tape-Spring Reinforcements for Inflatable Structural Tubes (NPO-20615), Vol. 24, No. 7 (July 2000), page 58. To recapitulate: An antenna as proposed would include a reflectarray membrane stretched flat on a frame of multiple tubular booms that would be deployed by inflation. The instant report discusses design concepts and relevant basic mechanical principles. Among the concepts are alternative configurations of booms for holding the reflectarray membrane and its radio-frequency feed horn and the use of catenaries and constant-force springs to stretch the reflectarray membrane on the frame at the required tension. Some emphasis is placed on the need to keep the deployed frame rigid without depending on maintenance of inflation in the presence of impinging micrometeors that could cause leaks: for this purpose, the booms could be made as spring-tape-reinforced aluminum laminate tubes like those described in the second-mentioned prior article.

Fang, Houfei↗

NASA NTRS2004

Improved Electromagnetic Brake

A proposed design for an electromagnetic brake would increase the reliability while reducing the number of parts and the weight, relative to a prior commercially available electromagnetic brake. The reductions of weight and the number of parts could also lead to a reduction of cost. A description of the commercial brake is prerequisite to a description of the proposed electromagnetic brake. The commercial brake (see upper part of figure) includes (1) a permanent magnet and an electromagnet coil on a stator and (2) a rotor that includes a steel contact plate mounted, with tension spring loading, on an aluminum hub. The stator is mounted securely on a stationary object, which would ordinarily be the housing of a gear drive or a motor. The rotor is mounted on the shaft of the gear drive or motor. The commercial brake nominally operates in a fail-safe (in the sense of normally braking) mode: In the absence of current in the electromagnet coil, the permanent magnet pulls the contact plate, against the spring tension, into contact with the stator. To release the brake, one excites the electromagnet with a current of the magnitude and polarity chosen to cancel the magnetic flux of the permanent magnet, thereby enabling the spring tension to pull the contact plate out of contact with the stator. The fail-safe operation of the commercial brake depends on careful mounting of the rotor in relation to the stator. The rotor/stator gap must be set with a tolerance between 10 and 15 mils (between about 0.25 and about 0.38 mm). If the gap or the contact pad is thicker than the maximum allowable value, then the permanent magnetic field will not be strong enough to pull the steel plate across the gap. (For this reason, any contact pad between the contact plate and the stator must also be correspondingly thin.) If the gap exceeds the maximum allowable value because of shaft end play, it becomes impossible to set the brake by turning off the electromagnet current. Although it may still be possible to set the brake by applying an electromagnet current to aid the permanent magnetic field instead of canceling it, this action can mask an out-of-tolerance condition in the brake and it does not restore the fail-safe function of setting the brake when current is lost.

Martin, Toby B.↗

NASA NTRS2011

A Conjugate Study of Mean Winds and Planetary Waves Employing Enhanced Meteor Radars at Rio Grande, Argentina (53.8degS) and Juliusruh, Germany (54.6degN)

Two meteor radars with enhanced power and sensitivity and located at closely conjugate latitudes (54.6degN and 53.8degS) are employed for inter-hemispheric comparisons of mean winds and planetary wave structures. Our study uses data from June 2008 through May 2010 during which both radars provided nearly continuous wind measurements from approx.80 to 100 km. Monthly mean winds at 53.8degS exhibit a somewhat stronger westward mean zonal jet in spring and early summer at lower altitudes and no westward monthly mean winds at higher altitudes. In contrast, westward mean winds of approx.5-10 m/s at 54.6degN extend to above 96 km during late winter and early spring each year. Equatorward monthly mean winds extend approximately from spring to fall equinox at both latitudes, with amplitudes of approx.5-10 m/s and more rapid decreases in amplitude at 54.6degN at higher altitudes. Meridional mean winds are more variable at both latitudes during fall and winter, with both poleward and equatorward monthly means indicating longer-period variability. Planetary waves seen in the 2-day mean data are episodic and variable at both sites, exhibit dominant periodicities of approx.8-10 and 16-20 days and are more confined to late fall and winter at 54.6degN. At both latitudes, planetary waves in the two period bands coincide closely in time and exhibit similar horizontal velocity covariances that are positive (negative) at 54.6degN (53.8degS) during peak planetary wave responses.

Fritts, D. C.↗

NASA NTRS2003

Energy-Absorbing, Lightweight Wheels

Improved energy-absorbing wheels are under development for use on special-purpose vehicles that must traverse rough terrain under conditions (e.g., extreme cold) in which rubber pneumatic tires would fail. The designs of these wheels differ from those of prior non-pneumatic energy-absorbing wheels in ways that result in lighter weights and more effective reduction of stresses generated by ground/wheel contact forces. These wheels could be made of metals and/or composite materials to withstand the expected extreme operating conditions. As shown in the figure, a wheel according to this concept would include an isogrid tire connected to a hub via spring rods. The isogrid tire would be a stiff, lightweight structure typically made of aluminum. The isogrid aspect of the structure would both impart stiffness and act as a traction surface. The hub would be a thin-walled body of revolution having a simple or compound conical or other shape chosen for structural efficiency. The spring rods would absorb energy and partially isolate the hub and the supported vehicle from impact loads. The general spring-rod configuration shown in the figure was chosen because it would distribute contact and impact loads nearly evenly around the periphery of the hub, thereby helping to protect the hub against damage that would otherwise be caused by large loads concentrated onto small portions of the hub.

Waydo, Peter↗

NASA NTRS2012

Date of Snowmelt at High Latitudes as Determined from Visible Satellite Data and Relationship with the Arctic Oscillation

Spring snow cover across Arctic lands has, on average, retreated approximately five days earlier since the late 1980s compared to the previous twenty years. However, it appears that since about 1990, the date the snowline first retreats north during the spring has remained nearly unchanged--in the last twenty years, the date of snow disappearance has not been occurring noticeably earlier. Snowmelt changes observed in the 1980s was step-like in nature, unlike a more continuous downward trend seen in Arctic sea ice extent. At latitude 70 deg N, several latitudinal segments (of 10 degrees) show significant (negative) trends. However, only two latitudinal segments at 60 deg N show significant trends, one positive and one negative. These variations appear to be related to variations in the Arctic Oscillation (AO). Additional observations and modeling investigations are needed to better explain past and present spring melt characteristics and peculiarities.

Foster, James↗

NASA NTRS2012

Shape memory-based actuators and release mechanisms therefrom

SM-based actuators (110) and release mechanisms (100) therefrom and systems (500) including one or more release mechanisms (100). The actuators (110) comprise a SM member (118) and a deformable member (140) mechanically coupled to the SM member (118) which deforms upon a shape change of the SM member triggered by a phase transition of the SM member. A retaining element (160) is mechanically coupled to the deformable member (140), wherein the retaining element (160) moves upon the shape change. Release mechanism (100) include an actuator, a rotatable mechanism (120) including at least one restraining feature (178) for restraining rotational movement of the retaining element (160) before the shape change, and at least one spring (315) that provides at least one locked spring-loaded position when the retaining element is in the restraining feature and at least one released position that is reached when the retaining element is in a position beyond the restraining feature (178). The rotatable mechanism (120) includes at least one load-bearing protrusion (310). A hitch (400) is for mechanically coupling to the load, wherein the hitch is supported on the load bearing protrusion (310) when the rotatable mechanism is in the locked spring-loaded position.

Vaidyanathan, Rajan↗

NASA NTRS2011

From Space Weather Toward Space Climate Time Scales: Substorm Analysis from 1993 to 2008

Magnetic activity in the Northern Hemisphere auroral region was examined during solar cycles 22 and 23 (1993- 2008). Substorms were identified from ground-based magnetic field measurements by an automated search engine. On average, 550 substorms were observed per year, which gives in total about 9000 substorms. The interannual, seasonal and solar cycle-to-cycle variations of the substorm number (R(sub ss)), substorm duration (T(sub ss)), and peak amplitude (A(sub ss)) were examined. The declining phases of both solar cycles 22 and 23 were more active than the other solar cycle phases due to the enhanced solar wind speed. The spring substorms during the declining solar cycle phase (absolute value of A(sub ss,decl)) - 500 nT) were 25% larger than the spring substorms during the ascending solar cycle years ((absolute value of A(sub ss,asc) = 400 nT). The following seasonal variation was found: the most intense substorms occurred during spring and fall, the largest substorm frequency in the Northern Hemisphere winter, and the longest-duration substorms in summer. Furthermore, we found a winter-summer asymmetry in the substorm number and duration. which is speculated to be due to the variations in the ionospheric conductivity. The solar cycle-Io-cycle variation was found in the yearly substorm number and peak amplitude. The decline from the peak substorm activity in 1994 and 2003 to the following minima took 3 years during solar cycle 22, while it took 6 years during solar cycle 23.

Tanskanen, E. I.↗

NASA NTRS2012

Use of MODIS Snow-Cover Maps for Detecting Snowmelt Trends in North America

Research has shown that the snow season in the Northern Hemisphere has been getting shorter in recent decades, consistent with documented global temperature increases. Specifically, the snow is melting earlier in the spring allowing for a longer growing season and associated land-cover changes. Here we focus on North America. Using the Moderate-Resolution Imaging Radiometer (MODIS) cloud-gap-filled standard snow-cover data product we can detect a trend toward earlier spring snowmelt in the approx 12 years since the MODIS launch. However, not all areas in North America show earlier spring snowmelt over the study period. We show examples of springtime snowmelt over North America, beginning in March 2000 and extending through the winter of 2012 for all of North America, and for various specific areas such as the Wind River Range in Wyoming and in the Catskill Mountains in New York. We also compare our approx 12-year trends with trends derived from the Rutgers Global Snow Lab snow cover climate-data record.

Hall, Dorothy K.↗

NASA NTRS2012

Seasonally Active Slipface Avalanches in the North Polar Sand Sea of Mars: Evidence for a Wind-Related Origin

Meter-scale MRO/HiRISE camera images of dune slipfaces in the north polar sand sea of Mars reveal the presence of deep alcoves above depositional fans. These features are apparently active under current climatic conditions, because they form between observations taken in subsequent Mars years. Recently, other workers have hypothesized that the alcoves form due to destabilization and mass-wasting during sublimation of CO2 frost in the spring. While there is evidence for springtime modification of these features, our analysis of early springtime images reveals that over 80% of the new alcoves are visible underneath the CO2 frost. Thus, we present an alternative hypothesis that formation of new alcoves and fans occurs prior to CO2 deposition. We propose that fans and alcoves form primarily by aeolian processes in the mid- to late summer, through a sequence of aeolian deposition on the slipface, over-steepening, failure, and dry granular flow. An aeolian origin is supported by the orientations of the alcoves, which are consistent with recent wind directions. Furthermore, morphologically similar but much smaller alcoves form on terrestrial dune slipfaces, and the size differences between the terrestrial and Martian features may reflect cohesion in the near-subsurface of the Martian features. The size and preservation of the largest alcoves on the Martian slipfaces also support the presence of an indurated surface layer; thus, new alcoves might be sites of early spring CO2 sublimation and secondary mass-wasting because they act as a window to looser, less indurated materials that warm up more quickly in the spring.

Horgan, Briony H. N.↗