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All sourcesNASA NTRSDOE OSTI
NASA NTRS2001

Gear Crack Propagation Path Studies: Guidelines for Ultra-Safe Design

Design guidelines have been established to prevent catastrophic rim fracture failure modes when considering gear tooth bending fatigue. Analysis was performed using the finite element method with principles of linear elastic fracture mechanics. Crack propagation paths were predicted for a variety of gear tooth and rim configurations. The effects of rim and web thicknesses, initial crack locations, and gear tooth geometry factors such as diametral pitch, number of teeth, pitch radius, and tooth pressure angle were considered. Design maps of tooth/rim fracture modes including effects of gear geometry, applied load, crack size, and material properties were developed. The occurrence of rim fractures significantly increased as the backup ratio (rim thickness divided by tooth height) decreased. The occurrence of rim fractures also increased as the initial crack location was moved down the root of the tooth. Increased rim and web compliance increased the occurrence of rim fractures. For gears with constant pitch radii, coarser-pitch teeth increased the occurrence of tooth fractures over rim fractures. Also, 25 deg pressure angle teeth had an increased occurrence of tooth fractures over rim fractures when compared to 20 deg pressure angle teeth. For gears with constant number of teeth or gears with constant diametral pitch, varying size had little or no effect on crack propagation paths.

Lewicki, David G.↗
NASA NTRS1979

Magnetic suspension system for an Annular Momentum Control Device (AMCD)

A technique to control a rim suspended in a magnetic field was developed. A complete system was developed, incorporating a support structure, magnetic actuators, a rim drive mechanism, an emergency fail-safe system, servo control system, and control electronics. Open loop and closed loop response of the system at zero speed and at 500 revolutions per minute (r/min) of the rim was obtained and analyzed. The rim was then dynamically balanced and a rim speed of 725 r/min was achieved. An analog simulation of the hardware was developed and tested with the actual control electronics connected to the analog computer. The system under development is stable at rim speeds below 700 r/min. Test results indicate that the rim under test is not rigid. The rim has a warp and a number of binding modes which prevented achievement of higher speeds. Further development efforts are required to achieve higher rim speeds.

Source record↗
NASA NTRS1990

The resupply interface mechanism RMS compatibility test

Spacecraft on-orbit servicing consists of exchanging components such as payloads, orbital replacement units (ORUs), and consumables. To accomplish the exchange of consumables, the receiving vehicle must mate to the supplier vehicle. Mating can be accomplished by a variety of docking procedures. However, these docking schemes are mission dependent and can vary from shuttle bay berthing to autonomous rendezvous and docking. Satisfying the many docking conditions will require use of an innovative docking device. The device must provide fluid, electrical, pneumatic and data transfer between vehicles. Also, the proper stiffness must be obtained and sustained between the vehicles. A device to accomplish this, the resupply interface mechanism (RIM), was developed. The RIM is a unique device because it grasps the mating vehicle, draws the two vehicles together, simultaneously mates all connectors, and rigidizes the mating devices. The NASA-Johnson Manipulator Development Facility was used to study how compatible the RIM is to on orbit docking and berthing. The facility contains a shuttle cargo bay mockup with a remote manipulator system (RMS). This RMS is used to prepare crew members for shuttle missions involving spacecraft berthing operations. The MDF proved to be an excellant system for testing the RIM/RMS compatibility. The elements examined during the RIM JSC test were: RIM gross and fine alignment; berthing method sequence; visual cuing aids; utility connections; and RIM overall performance. The results showed that the RIM is a good device for spacecraft berthing operations. Mating was accomplished during every test run and all test operators (crew members) felt that the RIM is an effective device. The purpose of the JSC RIM test and its results are discussed.

Jackson, Stewart W.↗
NASA NTRS2008

Degradation of Victoria Crater, Mars

The $\sim$750 m diameter and $\sim$75 m deep Victoria crater in Meridiani Planum, Mars, presents evidence for significant degradation including a low, serrated, raised rim characterized by alternating alcoves and promontories, a surrounding low relief annulus, and a floor partially covered by dunes. The amount and processes of degradation responsible for the modified appearance of Victoria crater were evaluated using images obtained in situ by the Mars Exploration Rover Opportunity in concert with a digital elevation model created using orbital HiRISE images. Opportunity traversed along the north and northwest rim and annulus, but sufficiently characterized features visible in the DEM to enable detailed measurements of rim relief, ejecta thickness, and wall slopes around the entire degraded, primary impact structure. Victoria retains a 5 m raised rim consisting of 1-2 m of uplifted rocks overlain by 3 m of ejecta at the rim crest. The rim is $\sim$120 to 220 m wide and is surrounded by a dark annulus reaching an average of 590 m beyond the raised rim. Comparison between observed morphology and that expected for pristine craters 500 to 750 m across indicate the original, pristine crater was close to 600 m in diameter. Hence, the crater has been erosionally widened by approximately 150 m and infilled by about 50 m of sediments. Eolian processes are responsible for modification at Victoria, but lesser contributions from mass wasting or other processes cannot be ruled out. Erosion by prevailing winds is most significant along the exposed rim and upper walls and accounts for $\sim$50 m widening across a WNW-ESE diameter. The volume of material eroded from the crater walls and rim is $\sim$20% less than the volume of sediments partially filling the crater, indicating eolian infilling from sources outside the crater over time. The annulus formed when $\sim$1 m deflation of the ejecta created a lag of more resistant hematite spherules that trapped darker, regional basaltic sands.

Wilson, Sharon A.↗
NASA NTRS2014

Space Weathering of Olivine in Lunar Soils: A Comparison to Itokawa Regolith Samples

Regolith particles from airless bodies preserve a record of the space weathering processes that occurred during their surface exposure history. These processes have major implications for interpreting remote-sensing data from airless bodies. Solar wind irradiation effects occur in the rims of exposed grains, and impact processes result in the accumulation of vapordeposited elements and other surface-adhering materials. The grains returned from the surface of Itokawa by the Hayabusa mission allow the space weathering "style" of a chondritic, asteroidal "soil" to be compared to the lunar case. Here, we present new studies of space-weathered olivine grains from lunar soils, and compare these results to olivine grains from Itokawa. Samples and Methods: We analyzed microtome thin sections of olivine grains from the 20-45 micron fractions of three lunar soils: 71061, 71501 and 10084 (immature, submature and mature, respectively). Imaging and analytical data were obtained using a JEOL 2500SE 200kV field-emission scanning-transmission electron microscope equipped with a thin-window energy-dispersive x-ray spectrometer. Similar analyses were obtained from three Hayabusa olivine grains. Results and Discussion: We observed lunar grains showing a range of solar flare track densities (from <10(exp 9) to approx.10(exp 12)/sq cm). The lunar olivines all show disordered, highly strained, nanocrystalline rims up to 150-nm thick. The disordered rim thickness is positively correlated with solar flare track density. All of the disordered rims are overlain by a Si-rich amorphous layer, ranging up to 50-nm thick, enriched in elements that are not derived from the host olivine (e.g., Ca, Al, and Ti). The outmost layer represents impact-generated vapor deposits typically observed on other lunar soil grains. The Hayabusa olivine grains show track densities <10(exp 10)/sq cm and display disordered rims 50- to 100-nm thick. The track densities are intermediate to those observed in olivines in immature and submature lunar soils and indicate surface exposures of approx. 10(exp 5) years. The outermost few nanometers of the disordered rims on Hayabusa olivines are more Si-rich and Mg- and Fe-depleted relative to the cores of the grains and likely represent a minor accumulation of impact-generated vapors or sputter deposits. Nanophase Fe metal particles are less abundant in the Hayabusa rims compared to the rims on lunar grains. Conclusions: The Hayabusa and lunar olivine grain rims have widths and microstructures consistent with formation from atomic displacement damage from solar wind ions. The space weathering features in the Hayabusa grains are similar to those observed in olivines from immature to submature lunar soils. A major difference, however, is that the Hayabusa grains appear to lack the hypervelocity impact products (melt spherules, thick vapor deposits, and abundant nanophase Fe metal particles) that are common in lunar soil grains with a similar exposure history.

Keller, L. P.↗
NASA NTRS2013

Transmission Electron Microscopy of Itokawa Regolith Grains

Introduction: In a remarkable engineering achievement, the JAXA space agency successfully recovered the Hayabusa space-craft in June 2010, following a non-optimal encounter and sur-face sampling mission to asteroid 25143 Itokawa. These are the first direct samples ever obtained and returned from the surface of an asteroid. The Hayabusa samples thus present a special op-portunity to directly investigate the evolution of asteroidal sur-faces, from the development of the regolith to the study of the effects of space weathering. Here we report on our preliminary TEM measurements on two Itokawa samples. Methods: We were allocated particles RA-QD02-0125 and RA-QD02-0211. Both particles were embedded in low viscosity epoxy and thin sections were prepared using ultramicrotomy. High resolution images and electron diffraction data were ob-tained using a JEOL 2500SE 200 kV field-emission scanning-transmission electron microscope. Quantitative maps and anal-yses were obtained using a Thermo thin-window energy-dispersive x-ray (EDX) spectrometer. Results: Both particles are olivine-rich (Fo70) with μm-sized inclusions of FeS and have microstructurally complex rims. Par-ticle RA-QD02-0125 is rounded and has numerous sub-μm grains attached to its surface including FeS, albite, olivine, and rare melt droplets. Solar flare tracks have not been observed, but the particle is surrounded by a continuous ~50 nm thick, stuctur-ally disordered rim that is compositionally similar to the core of the grain. One of the surface adhering grains is pyrrhotite show-ing a S-depleted rim (~8-10 nm thick) with nanophase Fe metal grains (<5 nm) decorating the outermost surface. The pyrrhotite displays a complex superstructure in its core that is absent in the S-depleted rim. Particle RA-QD02-0211 contains solar flare particle tracks (~2x109 cm-2) and shows a structurally disordered rim ~100 nm thick. The track density corresponds to a surface exposure of ~103-104 years based on the track production rate of [1]. The dis-ordered rim is nanocrystalline with minor amorphous material between crystalline domains. Quantitative element maps show the outermost ~10 nm of the disordered rim is Si-rich. Discussion and Conclusions: Both particles record the ef-fects of space weathering processes on Itokawa. Noguchi et al. [2] proposed that the disordered rims they observed on Itokawa particles largely result from solar wind radiation damage and we arrive at a similar conclusion for the two particles we analyzed. The microstructure of the S-depleted layer on the pyrrhotite grain in RA-QD02-0125 is similar to that observed in troilite irradiated with ~1018 4 kV He+ [3, 4]. Prolonged irradiation has also been shown to disorder pyrrhotite such that the superstructure reflec-tions are lost [5].

Keller, Lindsay P.↗
NASA NTRS1994

Dynamics Explorer 1, retarding ion mass spectrometer summary spectrograms: 81/280 to 81/365 spin-time spectrograms for H(+), He(+), O(+), N(+), O(++), M/Z=2, and molecular ions

The Retarding Ion Mass Spectrometer (RIMS) experiment onboard the Dynamics Explorer 1 (DE 1) satellite was designed to perform energy and mass-per-charge analysis on low-energy ions (less than 50 eV) with mass/charge ratios ranging from 1 to 40 amu/Z. The DE 1 satellite, carrying the RIMS experiment, was launched into an elliptical polar orbit on August 3, 1981. The approximately 7.5 hour orbit has perigee of 675 km altitude and apogee of 24,875 km altitude. This document, as well as those that follow in this series, contains summary RIMS data spectrograms for each orbit for which RIMS data are available. The RIMS instrument began returning science data on day 280 of 1981 and continued to return usable data until the end of the DE mission in March 1991. It should be noted that studies of the RIMS data set should be conducted only with a thorough awareness of the material described in the introduction section presented here, or in collaboration with a scientist familiar with RIMS data analysis.

Source record↗
NASA NTRS1994

Dynamics Explorer 1, retarding ion mass spectrometer summary spectrograms: 82/001 to 82/109 spin-time spectrograms for H(+), He(+), O(+), N(+), O(++), M/Z=2, and molecular ions

The Retarding Ion Mass Spectrometer (RIMS) experiment onboard the Dynamics Explorer 1 (DE 1) satellite was designed to perform energy and mass-per-change analysis on low-energy ions (less than 50 eV) with mass/charge ratios ranging from 1 to 40 amu/Z. The DE 1 satellite, carrying the RIMS experiment, was launched into an elliptical polar orbit on August 3, 1981. The approximate 7.5 hour orbit has perigee of 675 km altitude and apogee of 24,875 km altitude. This document, and those that follow in this series, contains summary RIMS data spectrograms for each orbit for which RIMS data are available. The RIMS instrument began returning science data on day 280 of 1981 and continued to return usable data until the end of the DE mission in March 1991. It should be noted that studies of the RIMS data set should be conducted only with a thorough awareness of the material described in the introduction section presented here, or in collaboration with a scientist familiar with RIMS data analysis.

Source record↗
NASA NTRS1995

Dynamics Explorer 1, retarding ion mass spectrometer summary spectrograms-82/110 to 82/229 spin-time spectrograms for H(+), He(+), O(+), N(+), O(++), M/Z = 2, and molecular ions

The retarding ion mass spectrometer (RIMS) experiment onboard the Dynamics Explorer 1 (DE 1) satellite was designed to perform energy and mass-per-charge analysis on low-energy ions (less than 50 eV) with mass/charge ratios ranging from 1 to 40 amu/Z. The DE 1 satellite, carrying the RIMS experiment, was launched into an elliptical polar orbit on August 3, 1981. The approximately 7.5 hour orbit has perigee of 675 km altitude and apogee of 24,875 km altitude. this document and those that following in this series, contains summary RIMS data spectrograms for each orbit for which RIMS data are available. The RIMS instrument began returning science data on day 280 of 1981 and continued to return usable data until the end of the DE mission in March 1991. It should be noted that studies of the RIMS data set should be conducted only with a thorough awareness of the material described in the introduction section presented here, or in collaboration with a scientist familiar with RIMS data analysis.

Source record↗
NASA NTRS1995

Dynamics Explorer 1, retarding ion mass spectrometer summary spectrograms-82/230 to 82/265 spin-time spectrograms for H(+), He(+), O(+), N(+), O(++), M/Z = 2, and molecular ions

The retarding ion mass spectrometer (RIMS) experiment onboard the Dynamics Explorer 1 (DE 1) satellite was designed to perform energy and mass-per-charge analysis on low-energy ions (less than 50 eV) with mass/charge ratios ranging from 1 to 40 amu/Z. the DE 1 satellite, carrying the RIMS experiment, was launched into an elliptical polar orbit on August 3, 1981. The approximately 7.5 hour orbit has perigee of 675 km altitude and apogee of 24,875 km altitude. This document and those that follow in this series, contain summary RIMS data spectrograms for each orbit for which RIMS data are available. The RIMS instrument began returning science data on day 280 of 1981 and continued to return usable data until the end of the DE mission in March 1991. It should be noted that studies of the RIMS data set should be conducted only with a thorough awareness of the material described in the introduction section presented here, or in collaboration with a scientist familiar with RIMS data analysis.

Source record↗
NASA NTRS1996

Detecting Gear Tooth Fatigue Cracks in Advance of Complete Fracture

Results of using vibration-based methods to detect gear tooth fatigue cracks are presented. An experimental test rig was used to fail a number of spur gear specimens through bending fatigue. The gear tooth fatigue crack in each test was initiated through a small notch in the fillet area of a tooth on the gear. The primary purpose of these tests was to verify analytical predictions of fatigue crack propagation direction and rate as a function of gear rim thickness. The vibration signal from a total of three tests was monitored and recorded for gear fault detection research. The damage consisted of complete rim fracture on the two thin rim gears and single tooth fracture on the standard full rim test gear. Vibration-based fault detection methods were applied to the vibration signal both on-line and after the tests were completed. The objectives of this effort were to identify methods capable of detecting the fatigue crack and to determine how far in advance of total failure positive detection was given. Results show that the fault detection methods failed to respond to the fatigue crack prior to complete rim fracture in the thin rim gear tests. In the standard full rim gear test all of the methods responded to the fatigue crack in advance of tooth fracture; however, only three of the methods responded to the fatigue crack in the early stages of crack propagation.

Zakrajsek, James J.↗
NASA NTRS2008

Suzaku Observations of Thermal and Non-Thermal X-Ray Emission from the Middle-Aged Supernova Remnant G156.2+5.7

We present results from X-ray analysis of a Galactic middle-aged supernova remnant (SNR) G156.2+5.7 which is bright and largely extended in X-ray wavelengths, showing a clear circular shape (radius approx.50'). Using the Suzaku satellite, we observed this SNR in three pointings; partially covering the northwestern (NW) rim, the eastern (E) rim, and the central portion of this SNR. In the NW rim and the central portion, we confirm that the X-ray spectra consist of soft and hard-tail emission, while in the E rim we find no significant hard-tail emission. The soft emission is well fitted by either a one-component or two-component non-equilibrium ionization (NEI) model. In the NW and E rims, a one-component (the swept-up interstellar medium) NEI model well represents the soft emission. On the other hand, in the central portion, a two-component (the interstellar medium and the metal-rich ejecta) NEI model fits the soft emission better than the one-component NEI model from a statistical point of view. The relative abundances in the ejecta component suggest that G156.2+5.7 is a remnant from a core-collapse SN explosion whose progenitor mass is less than 15 Solar Mass. The origin of the hard-tail emission detected in the NW rim and the central portion of the SNR is highly likely non-thermal synchrotron emission from relativistic electrons. In the NW rim, the relativistic electrons seems to be accelerated by a forward shock with a slow velocity of APPROX.500 km/sec.

Katsuda, Satoru↗
NASA NTRS2015

Solar Ion Processing of Itokawa Grains: Constraints on Surface Exposure Times

Analytical TEM observations obtained to date reveal that a significant sub-population of grains returned from the surface of asteroid Itokawa have had their outer 30-100 nm processed by space weathering effects. Although the effects include some surface deposition of condensed impact vapor and isolated impact melt splashes, much of the width of the space weathered outer margins or "rims" on grains is derived from solar wind processing of the original host grain. Similar to what has long been reported for some lunar grains, the ion-processed rims on Itokawa grains exhibit varing degrees and depths of penetration of atomic-displacement ion damage, resulting in complete amorphization for some rims (particularly in plagioclase), or formation of highly defective but still crystalline structures in others (particularly in pyroxene and olivine). Possibly different from lunar grains, however, is the presence of isolated internal cavities or voids in Itokawa grain rims, which may be implantation "bubbles" due to accumulating implanted solar wind H and/or He. For a given mineral exposed at a particular set of long term solar wind conditions, the level of ion damage in a given grain rim, the depth of damage penetration represented by the rim width, and the formation or lack of formation of implantation bubbles can all be linked to the time spent by the grain in an uncovered state on the topmost, space-exposed, regolith surface. For the lunar case, we have previously shown that with reasonable assumptions about solar wind characteristics over time, a model can be developed to estimate this exposure time based on the width of amorphous rims on lunar grains. Here we report on an expansion of the model to cover exposure time information contained in the array of solar ion-induced features in Itokawa grains.

Christoffersen, R.↗
NASA NTRS1974

Omnidirectional wheel

The apparatus consists of a wheel having a hub with radially disposed spokes which are provided with a plurality of circumferential rim segments. These rim segments carry, between the spokes, rim elements which are rigid relative to their outer support surfaces, and defined in their outer contour to form a part of the circle forming the wheel diameter. The rim segments have provided for each of the rim elements an independent drive means selectively operable when the element is in ground contact to rotatably drive the rim element in a direction of movement perpendicularly lateral to the normal plane of rotation and movement of the wheel. This affords the wheel omnidirectional movement.

Blumrich, J. F.↗
NASA NTRS1982

Capture-ejector satellites

A satellite in the form of a large rotating rim is described which can be used to boost spacecraft from low-Earth orbit to higher orbits. The rim rotates in the plane of its orbit such that the lower portion of the rim is travelling at suborbital velocity, while the upper portion is travelling at greater than orbital velocity. Ascending spacecraft or payloads arrive at the lowest portion of the rim at suborbital velocities, attach to the perimeter, and remain until they reach the highest point, where the payloads are released on a trajectory for higher orbits; descending payloads employ the reverse procedure. Electric thrusters placed on the rim maintain rim rotational speed and altitude. From the standpoint of currently known materials, the capture-ejector concept may be useful for relatively small velocity increments.

Macconochie, I. O.↗
NASA NTRS1983

Capture-ejector satellites

A satellite in the form of a large rotating rim which can be used to boost spacecraft from low-Earth orbit to higher orbits is described. The rim rotates in the plane of its orbit such that the lower portion of the rim is traveling at suborbital velocity, while the upper portion is travelling at greater than orbital velocity. Ascending spacecraft or payloads arrive at the lowest portion of the rim at suborbital velocities, where the payloads are released on a trajectory for higher orbits; descending payloads employ the reverse procedure. Electric thrusters placed on the rim maintain rim rotational speed and altitude. From the standpoint of currently known materials, the capture-ejector concept may be useful for relatively small velocity increments.

Macconochie, I. O.↗
NASA NTRS1999

Space Weathering in the Fine Size Fractions of Lunar Soils: Soil Maturity Effects

The effects of space weathering on the optical properties of lunar materials have been well documented. These effects include a reddened continuum slope, lowered albedo, and attenuated absorption features in reflectance spectra of lunar soils as compared to finely comminuted rocks from the same Apollo sites. However, the regolith processes that cause these effects are not well known, nor is the petrographic setting of the products of these processes fully understood. A Lunar Soil Characterization Consortium has been formed with the purpose of systematically integrating chemical and mineralogical data with the optical properties of lunar soils. Understanding space-weathering effects is critical in order to fully integrate the lunar sample collection with remotely-sensed data from recent robotic missions (e.g., Lunar Prospector, Clementine, and Galileo) We have shown that depositional processes (condensation of impact-derived vapors, sputter deposits, accreted impact material, e.g., splash glass, spherules, etc.) are a major factor in the modification of the optical surfaces of lunar regolith materials. In mature soils, it is the size and distribution of the nanophase metal in the soil grains that has the major effect on optical properties. In this report, we compare and contrast the space-weathering effects in an immature and a mature soil with similar elemental compositions. For this study, we analyzed <10 micron sieve fractions of two Apollo 17 soils, 79221 (mature, Is/FeO = 81) and 71061 (immature, Is/FeO = 14). Details of the sieving procedures and allocation scheme are given else where. The results of other detailed chemical, mineralogical, and spectroscopic analyses of these soil samples are reported elsewhere. A representative sample of each soil was embedded in low-viscosity epoxy, and thin sections (about 70nm thick) were obtained through ultra microtomy. The thin sections used for these analyses typically contained cross sections of up to 500 individual grains. The thin sections were studied using a JEOL 2010 transmission electron microscope (TEM) equipped with a thin window energy-dispersive X-ray (EDX) spectrometer. An individual thin section was selected from each soil, and for each grain in the section we determined (1) the elemental composition by EDX; (2) whether the grain was crystalline or glassy using electron diffraction and darkfield imaging; (3) the presence or absence of rims and accreted material; and (4) the distribution of nanophase Fe where present. Most of the categories are self-evident; however, we divide the agglutinate derived material into agglutinitic glass (glass with approximately the same composition as the bulk soil that contains nanophase Fe with or without vesicles) and agglutinate fragments, which are composed of crystalline grains and agglutinitic glass. Lithic fragments are defined as polymineralic grains with no glass. Pyroxene grains have been divided into high- and low-Ca groups. As expected, there are a number of differences in the petrography of the <10-microns fractions of 79221 and 71061 given the great difference in their respective maturities, but we focus here on two major distinctions: agglutinate content and the number of grains with micropatina. Slightly over 50% of the particles in 79221 consist of agglutinitic glass and agglutinate fragments, while the remainder are predominantly crystalline mineral grains. The agglutinic glass particles contain abundant nanophase Fe and vesicles. Angular particles are rare, with most showing smooth, rounded exteriors, Of the mineral grains analyzed thus far, over 90% of the grains have amorphous rims that contain nanophase Fe (these rims are believed to have formed by vapor deposition and irradiation effects). The nanophase Fe in these rims probably accounts for a significant fraction of the increase in Is/FeO measured in these size fractions. In addition to the rims, the majority of particles also show abundant accreted material in the form of glass splashes and spherules that also contain nanophase Fe. In stark contrast, the surfaces of the mineral grains in the 71061 sample are relatively prisitine, as only about 14% of the mineral grains in the sample exhibited amorphous rims. Furthermore, the mineral particles are more angular and show greater surface roughness than in the mature sample. Accreted material on particle surfaces is rare. Agglutinitic material is a major component of the 71061 sample; however, nanophase Fe and vesicles are not as well developed as in the 79221 sample. It is now recognized that nanophase Fe is probably the main agent in modifying the optical properties of lunar soil grains. The most important result of this study is the observation that in the fine size fractions of mature soils, nearly every grain has nanophase Fe within 100 run of the particle surface. (Additional Information contained in original)

Keller, L. P.↗
NASA NTRS1998

Gale Crater: An Amazonian Impact Crater Lake at the Plateau/Plain Boundary

Gale is a 140-km diameter impact crater located at the plateau/plain boundary in the Aeolis Northeast subquadrangle of Mars (5S/223W). The crater is bordered in the northward direction by the Elysium Basin, and in eastward direction by Hesperian channels and the Aeolis Mensae 2. The crater displays a rim with two distinct erosion stages: (a) though eroded, the south rim of Gale has an apparent crest line visible from the north to the southwest (b) the west and northwest rims are characterized by a strong erosion that, in some places, partially destroyed the rampart, leaving remnant pits embayed in smooth-like deposits. The same type of deposits is observed north, outside Gale, it also borders the Aeolis Mensae, covers the bottom of the plateau scarp, and the crater floor. The central part of Gale shows a 6400 km2 subround and asymmetrical deposit: (a) the south part is composed of smooth material, (b) the north part shows spectacular terraces, streamlines, and channels. The transition between the two parts of the deposit is characterized by a scarp ranging from 200 to 2000 in high. The highest point of the scarp is at the center of the crater, and probably corresponds to a central peak. Gale crater does not show a major channel directly inflowing. However, several large fluvi systems are bordering the crater, and could be at the origin of the flooding of the crater, or have contributed to. One fluvial system is entering the crater by the southwest rim but cannot be accounted alone for the volume of sediment deposited in the crater. This channel erodes the crater floor deposit, and ends in a irregular-shaped and dark albedo feature. Gale crater shows the morphology of a crater filled during sedimentation episodes, and then eroded Part of the lower sediment deposition contained in Gale might be ancient and not only aqueous in origin. According to the regional geologic history, the sedimentary deposit could be a mixture of aeolian and pyroclastic material, and aqueous sedimentary material that can originate both from drainage of the regional subsurface aquifer, and/or from surface flood. The central deposit shows three main levels: (a) the current crater floor (north of Gale), (b) an ancient level about 200 rn higher (south of Gale), and (c) the massive terraced deposits. A crater statistics on the 15,400 kM2 area of the crater floor and deposit [3,41 gave: 259+/-112.4 craters, most of them partly embayed in the sedimentary deposit, and all inferior to 5-km diameter. For superimposed crater population only, the result is 194+/-112. The deduced relative ages ranges from Early to Middle Amazonian. The population of craters are comparable for the three levels, implying that the last sedimentation/erosion episode on Gale was recent and affected the whole crater. The streamlined morphology of the border of the deposit, the layering, the channels, and the terraces are compatible with a significant fluvio-lacustrine history of the site. Multiple levels may suggest different episodes, but the common statistical age of the three levels shows that the last episode involved the whole crater. The origin of the lake water in Gale may have varied in time. Three major contributions have been proposed: (a) the drainage of the regional underground aquifer by Gale crater over an area of 110-km radius around the crater which would have provided approximately 1,600 cubic km of water, (b), surface drainage entering Gale by the south and north rims. In the south, a 250-km long system originates in the cratered uplands in a Noachian crater material plain (Nc), and crosses Hesperian and Amazonian crater material plains (AHc) northward [1]. Several fluvial systems originate in the Aeolis Mensae, east of Gale. They may had two functions in time: to recharge, the underground aquifer in the region of Gale, and to supply surface water in the crater by overspilling the northern rim, and (c) surface floods that originated from the rising of the water level in the Elysium Basin. According to the Amazonian age of Gale's floor, and the erosion direction in the crater, a flood from Elysium Basin is the most likely event to explain the material observed in Gale, and the formation of the last lake. This last flood may have been important enough to flood the central deposit up to about 1400 m above the crater floor, leaving two islands (non stream lined features) at the center of the deposit. Terrace spacing suggests a regular drop of the lake level in time. Fractures in terraces perpendicular to the shoreline can be interpreted either as: (a) the result of the drainage systems during the waning of the lake, or (b) traces of the pressure of an ice-covered sheet associated with subglacial drainage. The presence of a lake of such volume during the Amazonian period is one more evidence that water was still active on Mars relatively recently. Gale crater offers the rare opportunity to unveil a key-period of the martian history. The Amazonian might proved not as cold and dry as previously thought. The presence of large lakes and basins (Elysium Basin is large as the Mediterranean Sea), reinforces the model of an extensive water activity during the Amazonian that has still to be understood in the context of an assumed cooling and drying planet. The sediments and rocks that were left of this period in Gale keep the record of the climatic conditions of the Amazonian and the clues that are missing to understand the climatic evolution of Mars. In addition, Gale crater presents the advantage to be located at the plateau/plain boundary, which has never been studied and contains information about the two main martian geological units. As a conclusion, we propose a table that summarizes the worthiness of a mission in Gale crater, and the expected science return relative to the objectives to be met by the Surveyor Program. Additional information contained in the original.

Cabrol, N. A.↗