Plains Units Research Articles

Volcanism on Mercury: Evidence from the first MESSENGER flyby for extrusive and explosive activity and the volcanic origin of plains

The first MESSENGER flyby of Mercury obtained images of 21% of the surface not seen by Mariner 10, including the center and western half of the Caloris basin and regions near the terminator that show details of the nature of smooth and intercrater plains. These new data have helped to address and resolve a series of longstanding questions on the existence and nature of volcanism on Mercury and the distribution of volcanic materials. Data from the Mercury Dual Imaging System (MDIS) on the MESSENGER spacecraft have shown the following: (1) Numerous volcanic vents, in the form of irregularly shaped rimless depressions, are concentrated around the interior edge of the Caloris basin. (2) These vents appear to be sources for effusive volcanism that in one case built a shield in excess of 100 km in diameter and in some cases formed bright haloes around the vents that are interpreted to represent pyroclastic eruptions. (3) Lobate margins of plains units, seen previously in Mariner 10 data, are documented in MESSENGER images with more clarity and are often distinctive in morphology and color properties, supporting the interpretation that these features are the edges of lava flow units. (4) The interior of the Caloris basin is filled with plains units spectrally distinctive from the rim deposits, and comparison with the lunar Imbrium basin and superposed impact crater stratigraphy provide evidence that these units are volcanic in origin; detailed differences in the mineralogy of lava flow units, so prominent in Imbrium, are not seen in the Caloris interior. (5) Some of the smooth plains surrounding the exterior of the Caloris basin show distinct differences in color and morphological properties, supporting a volcanic origin. (6) Some smooth and intercrater plains units distant from the Caloris basin show evidence of flooding and embayment relations unrelated to Caloris ejecta emplacement; local and regional geological and color relationships support a volcanic origin for these plains. (7) Large impact craters show a sequence of embayment of interior floor and exterior ejecta deposits that supports a volcanic origin for the embayment and filling processes. (8) Crater embayment and flooding relationships in selected areas suggest volcanic plains thicknesses of many hundreds of meters and local thicknesses inside impact craters of up to several kilometers. (9) Impact crater size–frequency distributions for Caloris exterior deposits, including the facies of the Caloris Group and relatively high- and low-albedo smooth plains, show that they are younger than plains interior to Caloris and thus must be dominantly the product of post-Caloris volcanism. These new data provide evidence that supports and confirms earlier hypotheses from Mariner 10 data that volcanism was important in shaping the surface of Mercury. The emerging picture of the volcanic style of Mercury is similar to that of the Moon, the other small, one-plate planetary body: there are no major shield volcanoes (e.g., comparable to Tharsis Montes on Mars), shallow magma reservoirs are rare, and there is little evidence for surface deformation or long-lived volcanic sources related to sites of upwelling mantle. The close association of volcanic plains and surface deformation features suggests that future observations and analyses can help document the relation between the volcanic flux and the evolving state and magnitude of stress in the lithosphere of Mercury.

Stratigraphical evidence of late Amazonian periglaciation and glaciation in the Astapus Colles region of Mars

Recent modeling of the meteorological conditions during and following times of high obliquity suggests that an icy mantle could have been emplaced in western Utopia Planitia by atmospheric deposition during the late Amazonian period [Costard, F.M., Forget, F., Madeleine, J.B., Soare, R.J., Kargel, J.S., 2008. Lunar Planet. Sci. 39. Abstract 1274; Madeleine, B., Forget, F., Head, J.W., Levrard, B., Montmessin, F., 2007. Lunar Planet. Sci. 38. Abstract 1778]. Astapus Colles (ABa) is a late Amazonian geological unit — located in this hypothesized area of accumulation — that comprises an icy mantle tens of meters thick [Tanaka, K.L., Skinner, J.A., Hare, T.M., 2005. US Geol. Surv. Sci. Invest., Map 2888]. For the most part, this unit drapes the early Amazonian Vastitas Borealis interior unit (ABvi); to a lesser degree it overlies the early Amazonian Vastitas Borealis marginal unit (ABvm) and the early to late Hesperian UP plains unit HBu2 [Tanaka, K.L., Skinner, J.A., Hare, T.M., 2005. US Geol. Surv. Sci. Invest., Map 2888]. Landscapes possibly modified by late-Amazonian periglacial processes [Costard, F.M., Kargel, J.S., 1995. Icarus 114, 93–112; McBride, S.A., Allen, C.C., Bell, M.S., 2005. Lunar Planet. Sci. 36. Abstract 1090; Morgenstern, A., Hauber, E., Reiss, D., van Gasselt, S., Grosse, G., Schirrmeister, L., 2007. J. Geophys. Res. 112, doi:10.1029/2006JE002869. E06010; Seibert, N.M., Kargel, J.S., 2001. Geophys. Res. Lett. 28, 899–902; Soare, R.J., Kargel, J.S., Osinski, G.R., Costard, F., 2007. Icarus 191, 95–112; Soare, R.J., Osinski, G.R., Roehm, C.L., 2008. Earth Planet. Sci. Lett. 272, 382–393] and glacial processes [Milliken, R.E., Mustard, J.F., Goldsby, D.L., 2003. J. Geophys. Res. 108 (E6), doi:10.1029/2002JE002005. 5057; Mustard, J.F., Cooper, C.D., Rifkin, M.K., 2001. Nature 412, 411–414; Tanaka, K.L., Skinner, J.A., Hare, T.M., 2005. US Geol. Surv. Sci. Invest., Map 2888] have been reported within the region. Researchers have assumed that the periglacial and glacial landscapes occur within the same geological unit, the ABa [i.e., Morgenstern, A., Hauber, E., Reiss, D., van Gasselt, S., Grosse, G., Schirrmeister, L., 2007. J. Geophys. Res. 112; doi:10.1029/2006JE002869. E06010; Tanaka, K.L., Skinner, J.A., Hare, T.M., 2005. US Geol. Surv. Sci. Invest., Map 2888]. In this study we use HiRISE (High Resolution Image Science Experiment, Mars Reconnaissance Orbiter) imagery to identify the stratigraphical separation of the two landscapes and show that periglacial landscape modification has occurred in the geological units that underlie the ABa, not in the ABa itself. Moreover, we suggest that the periglacial landscape extends well beyond the perimeter of the ABa and could be the product of “wet” cold-climate processes. These processes involve freeze–thaw cycles and intermittently stable liquid-water at or near the surface. By contrast, we propose that the ABa is a very recent late-Amazonian geological unit formed principally by “dry” cold-climate processes. These processes comprise accumulation (by atmospheric deposition) and ablation (by sublimation).

Phyllosilicate and sulfate‐hematite deposits within Miyamoto crater in southern Sinus Meridiani, Mars

Orbital topographic, image, and spectral data show that sulfate‐ and hematite‐bearing plains deposits similar to those explored by the MER rover Opportunity unconformably overlie the northeastern portion of the 160 km in diameter Miyamoto crater. Crater floor materials exhumed to the west of the contact exhibit CRISM and OMEGA NIR spectral signatures consistent with the presence of Fe/Mg‐rich smectite phyllosilicates. Based on superposition relationships, the phyllosilicate‐bearing deposits formed either in‐situ or were deposited on the floor of Miyamoto crater prior to the formation of the sulfate‐rich plains unit. These findings support the hypothesis that neutral pH aqueous conditions transitioned to a ground‐water driven acid sulfate system in the Sinus Meridiani region. The presence of both phyllosilicate and sulfate‐ and hematite‐bearing deposits within Miyamoto crater make it an attractive site for exploration by future rover missions.

Soil sedimentology at Gusev Crater from Columbia Memorial Station to Winter Haven

A total of 3140 individual particles were examined in 31 soils along Spirit's traverse. Their size, shape, and texture were quantified and classified. They represent a unique record of 3 years of sedimentologic exploration from landing to sol 1085 covering the Plains Unit to Winter Haven where Spirit spent the Martian winter of 2006. Samples in the Plains Unit and Columbia Hills appear as reflecting contrasting textural domains. One is heterogeneous, with a continuum of angular‐to‐round particles of fine sand to pebble sizes that are generally dust covered and locally cemented in place. The second shows the effect of a dominant and ongoing dynamic aeolian process that redistributes a uniform population of medium‐size sand. The texture of particles observed in the samples at Gusev Crater results from volcanic, aeolian, impact, and water‐related processes.

Geologic and spectral mapping of etched terrain deposits in northern Meridiani Planum

A ∼1 km vertical section of etched terrain and hematite‐bearing plains materials and nearby cratered terrain surfaces was mapped in the northern portion of Meridiani Planum using MOC, THEMIS, MOLA, and OMEGA data. The oldest materials are the cratered plains, which have 0.4 to 2.5 μm spectral reflectance dominated by a mix of low and high calcium pyroxenes. Etched plains materials overlie this unit and are exposed within a 120 km NW‐SE trending valley to the south of the cratered plains. Lower etched plains materials exhibit a kieserite signature on a plateau‐forming horizon and polyhydrated sulfate signatures on the main valley floor. The upper etched plains unit exhibits signatures consistent with hydrated iron oxides and is covered by what is interpreted to be a relatively thin layer of basaltic sand and hematitic concretions (the hematite‐bearing plains unit). The youngest unit consists of ejecta deposits from a cluster of six craters that mantle the eastern portion of the study area. The thick section of etched plains materials (∼900 m) with exposures of hydrated sulfates and hydrated iron oxides implies that measurements made by the Opportunity rover (located ∼390 km to the southwest of the study area) are at the top of an extensive section of layered sedimentary materials formed in and/or altered in an aqueous environment and exposed by aeolian erosion.

Radar properties as clues to relative ages of ridge belts and plains on Venus

An ongoing issue regarding the crustal evolution of Venus is whether or not certain features and processes occurred in the same sequence and at the same time globally. This paper seeks to test one aspect of “directional” models for the geologic history: the inference that all ridge belts are older than regional plains. This test involves traditional geologic analysis, but also involves the use of radar properties in a unique way. We use a simplified model for the ridge shape, and an estimate of the diffuse scattering behavior of the Venus surface, to determine the “true” backscatter coefficient of the slope of the ridge tilted away from the radar, and ratio this value to the echoes from nearby plains units. In general, ridge‐forming terrain in these belts appears to have greater inferred backscatter coefficients, and thus decimeter‐scale roughness, than the regional plains. Plains units closely adjacent to many ridge belts have higher backscatter than plains at some distance, which we infer is generally due to aeolian deposition of debris characterized by centimeter‐scale microdunes or ripples where the belt topography interferes with wind patterns. The rougher apparent surface texture within the belt‐forming terrain is not consistent with the gentle folding that forms the ridges, and so is likely a result of weathering or tectonic processes operating prior to belt deformation. We thus conclude that the material incorporated in the ridge belts we have studied predates the surrounding regional plains.

1:1.5,000,000 Geological Map of the Aglaonice Region on Venus

Please click here to download the map associated with this article. A geological map of the Aglaonice region, Venus, is presented. It is the first published attempt at mapping at the full limit of resolution of the available NASA Magellan mission data. It is primarily based on geological interpretation of a 1:1.5,000,000 scale Synthetic Aperture Radar (SAR) image and altimetry data of the same region. The map covers an area approximately 1.6 x 106 km2—roughly 1% of the venusian surface—and demonstrates the stratigraphic history revealed by mapping at this scale. We do not observe a regional plains unit, but rather five tectonically and texturally distinct volcanic plains material units. We have found that almost two-thirds of the mapped units originate from identifiable source regions, suggesting that the origin of most units on Venus can be identified if mapped at the resolution of Magellan data.

Remote sensing and geologic studies of the Balmer‐Kapteyn region of the Moon

The Balmer‐Kapteyn (B‐K) region is located just east of Mare Fecunditatis on the east limb of the Moon. It is centered on the Balmer‐Kapteyn basin, a pre‐Nectarian impact structure that exhibits two rings, approximately 225 km and 450 km in diameter. Clementine multispectral images and Lunar Prospector (LP) gamma‐ray spectrometer (GRS) data were used to investigate the composition, age, and origin of geologic units in the region. A major expanse of cryptomare was mapped within the B‐K basin. Spectral and chemical data obtained for dark‐haloed craters (DHCs) established that these impact craters excavated mare basalt from beneath higher‐albedo, highland‐rich surface units. The buried basalts exposed by DHCs in the region are dominated by low‐titanium mare basalts. The fresh DHC FeO values (15.0–15.7 wt.%) that best represent those of buried mare basalts are well within the range of values exhibited by high‐alumina mare basalts. While most cryptomare deposits occur beneath surfaces that range in age from Imbrian to Nectarian, it is possible that some mare flows were emplaced during pre‐Nectarian time. Most cryptomare deposits in the B‐K region were formed by the contamination of mare surfaces by highland‐rich distal ejecta from surrounding impact craters. These Balmer‐type cryptomare deposits are usually associated with light plains units. Major LP‐GRS FeO enhancements are associated with cryptomaria in the Balmer‐Kapteyn, Lomonosov‐Fleming, Schiller‐Schickard, and Mendel‐Rydberg regions.

MIOCENE MAMMALIAN FAUNAS, OGALLALA GROUP, PAWNEE BUTTES AREA, WELD COUNTY, COLORADO

Abstract The Pawnee Buttes of northeastern Colorado lie within spectacular outcrops of Oligocene (White River Group) and Miocene (Ogallala Group) strata that form part of the Chalk Bluffs, an 150 mile long escarpment near the state line. These deposits were some of the earliest High Plains units to be explored by vertebrate paleontologists (Marsh, 1870; Cope, 1873). Consequently their fossil remains yielded the first information on North American faunas of mid- and late Tertiary time. Many parties subsequently exploited these deposits; their work, and his own investigations, were monographed by Galbreath in 1953. The present paper reviews the literature, a substantial part of the collections, as well as later stratigraphic work to present a revised litho- and biostratigraphy focused on the Pawnee Buttes area of eastern Weld County, Colorado. The oldest post-White River Group unit is the Martin Canyon Formation, the type locality of which is in adjacent Logan County. It contains an early Hemingfordian faun...

Land Capability in Gobeh Sub Watershed Wonogiry Regency Central Java Province

The research held on Sub DAS of Gobeh, Wonogiri Regency, have the aims to kno about class and sub-class of land capability in the research area. The other aim is to know about some area developed to the farming land, and to evaluate about land apability currently. The method used in this research is field-survey method, including about observation, measurenment, reording, and laboratory analysis. The sampling method applied in this research is stratified sampling, with land unit stratum. Measurenment and recording in the field inluding about degree of slope, erosion level, soil-deepening, drainage, stone or gravel and flood hazard. The laboratory analysis including about land erodibility, texture, and permeability. The research result identify that, researh fields have three lanf form unit, they are the fluvio volanic foot plain of old Lawu vulcan area flat relief slightly dissected, the fluvio volcanic foot plain of old Lawu mountain area rolling relief medium dissected, and flat foot plain unit can be separated to thirty two land unit. Land capability of the research field identify between level II to level VII. The wide of each land capability are, level II 3 – 8 Ha (2,4%), level III 38,18 Ha (23,7%), level IV 10 Ha (6,33%), level V 107,62 Ha (66,8%), level VII 1,2 Ha (0,75%).

Mapping of the Culann–Tohil region of Io from Galileo imaging data

We have used Galileo spacecraft data to produce a geomorphologic map of the Culann–Tohil region of Io's antijovian hemisphere. This region includes a newly discovered shield volcano, Tsũi Goab Tholus and a neighboring bright flow field, Tsũi Goab Fluctus, the active Culann Patera and the enigmatic Tohil Mons-Radegast Patera–Tohil Patera complex. Analysis of Voyager global color and Galileo Solid-State Imaging (SSI) high-resolution, regional (50–330 m/pixel), and global color (1.4 km/pixel) images, along with available Galileo Near-Infrared Mapping Spectrometer (NIMS) data, suggests that 16 distinct geologic units can be defined and characterized in this region, including 5 types of diffuse deposits. Tsũi Goab Fluctus is the center of a low-temperature hotspot detected by NIMS late during the Galileo mission, and could represent the best case for active effusive sulfur volcanism detected by Galileo. The Culann volcanic center has produced a range of explosive and effusive deposits, including an outer yellowish ring of enhanced sulfur dioxide (SO 2), an inner red ring of SO 2 with short-chain sulfur (S 3–S 4) contaminants, and two irregular green diffuse deposits (one in Tohil Patera) apparently produced by the interaction of dark, silicate lava flows with sulfurous contaminants ballistically-emplaced from Culann's eruption plume(s). Fresh and red-mantled dark lava flows west of the Culann vent can be contrasted with unusual red–brown flows east of the vent. These red–brown flows have a distinct color that is suggestive of a compositional difference, although whether this is due to surface alteration or distinct lava compositions cannot be determined. The main massif of Tohil Mons is covered with ridges and grooves, defining a unit of tectonically disrupted crustal materials. Tohil Mons also contains a younger unit of mottled crustal materials that were displaced by mass wasting processes. Neighboring Radegast Patera contains a NIMS hotspot and a young lava lake of dark silicate flows, whereas the southwest portion of Tohil Patera contains white flow-like units, perhaps consisting of ‘ponds’ of effusively emplaced SO 2. From 0°–15° S the hummocky bright plains unit away from volcanic centers contains scarps, grooves, pits, graben, and channel-like features, some of which have been modified by erosion. Although the most active volcanic centers appear to be found in structural lows (as indicated by mapping of scarps), DEMs derived from stereo images show that, with the exception of Tohil Mons, there is less than 1 km of relief in the Culann–Tohil region. There is no discernable correlation between centers of active volcanism and topography.

Remote sensing studies of the Lomonosov‐Fleming region of the Moon

Lomonosov‐Fleming is a 620 km, pre‐Nectarian impact basin centered at 19°N, 105°E. Clementine multispectral images and a variety of spacecraft photographs were used to investigate the composition and origin of geologic units in the region. Mare basalt deposits in the Lomonosov‐Fleming region are small and exhibit varying degrees of highlands contamination. Four previously unrecognized dark mantle deposits of probable pyroclastic origin were identified and mapped. A major expanse of cryptomare was mapped in and around the Lomonosov‐Fleming basin. Spectral and chemical data obtained for dark‐haloed craters (DHC) established that these impact craters excavated mare basalt from beneath higher‐albedo highlands‐rich surface units. The buried basalts exposed by DHCs in the region exhibit a variety of compositions, which range from very low titanium basalt to intermediate TiO2 mare basalt. Nectarian‐aged units with subjacent cryptomare are much more extensive than Imbrian‐aged deposits with underlying basalt. At least some of the buried mare basalts in the region may have been emplaced during pre‐Nectarian time. While both impact basins and craters played a role in cryptomare formation by transporting highlands material to mare surfaces, the evidence indicates that most of the cryptomare inside the basin are of the distal basin ejecta type. The enhanced FeO and TiO2 abundances exhibited by these cryptomare surfaces, usually light plains units, demonstrate that ballistic erosion and sedimentation played an important role in their formation.

Graben–fissure systems in Guinevere Planitia and Beta Regio (264°–312°E, 24°–60°N), Venus, and implications for regional stratigraphy and mantle plumes

Detailed mapping in a 14,000,000 km 2 area of northwestern Guinevere Planitia and northern Beta Regio bounded by 264°–312°E, 24°–60°N has revealed thousands of long extensional lineaments (graben, fissures and related fractures). These can be grouped into radiating, circumferential and linear systems. Thirty four radiating systems have been identified, of which 16 have radii greater than 300 km and eight have radii greater than 1000 km. Twenty six linear (straight) systems with a length greater than 300 km have been distinguished of which six have a length greater than 1000 km. Linear systems are generally associated with rifts, although some may represent distal portions of radiating systems. In addition, 19 circumferential systems, some associated with coronae, have been identified. The distribution of each system is compared with the host geology in order to place the graben–fissure systems in a regional stratigraphic framework. The majority of systems are: (1) younger than tesserae, ridge belts and densely fractured plains, (2) coeval with, and in many cases, define fracture belts, (3) partially flooded by wrinkle-ridged plains units, and (4) older than smooth and lobate plains units and young rifts. The inventory of radiating graben–fissure systems that we catalogue represents a database of tectono-magmatic centers that complements the centers defined using other criteria, e.g., large volcanoes, coronae, and shield fields. We have attempted to identify those systems that are underlain by dike swarms in order to evaluate their relationship to mantle plumes. At least 11 of the radiating systems extend well beyond any central topographic uplift and are therefore interpreted to be underlain by dike swarms.

Flow field stratigraphy surrounding Sekmet Mons Volcano, Kawelu Planitia, Venus

Detailed mapping has revealed several stratigraphic components among lobate plains in the vicinity of Sekmet Mons volcano (44.5°N. lat., 240.5° long.), located on the northern lowland plains of Kawelu Planitia on Venus. Volcanic effusion events produced discrete lobate plains throughout the area. Superposition of flow margins between adjacent lobate plains components indicate that the effusive activity generally progressed from southwest to northeast across the Sekmet Mons area, leading to a cumulative total of 1.5 million km2 covered by adjacent lobate plains components. Source areas for the effusion responsible for the lobate plains components occur primarily along fracture zones or from concentrations of low volcanic domes (“shield fields”) instead of from single constructs. Magellan radar backscatter values from a mixture of both radar bright and radar dark flow components within the flow fields are well below values typical of clinkery a′a lava flows on Earth, but they are consistent with values from terrestrial pahoehoe flows. Detailed mapping of Strenia Fluctus (centered on 41.5° N. lat., 251.0° long.), one of the latest lobate plains units in the area, does not show a systematic trend among effusive centers that contributed to the generation of this flow field. Instead, the Strenia Fluctus flow field is a complex mixture of flows that emanated from a shield field along Mist Chasma and that flowed down a very gentle (>0.1°) regional slope to the east. A 4‐km‐diameter cone north of Strenia Fluctus was the source of a flow complex traceable for more than 200 km over a slope of only 0.03°. Individual flows within Strenia Fluctus are composed of intermixed lobes, similar to relationships observed on the distal portion of the 75‐km‐long Carrizozo basalt flow in New Mexico, which also displays abundant inflation features and a predominant pahoehoe surface texture. If the Venusian lobate plains consist of pahoehoe flows comparable to the Carrizozo flow, they were most likely emplaced at modest effusion rates (∼50 to 500 m3/s), and even with inflation the flows are likely ≤15 m in thickness. Under these conditions all of the lobate plains surrounding Sekmet Mons could have been emplaced in ∼14,000 to 1400 Earth years, assuming continuous effusion with only one source vent active at a time. At similar rates all of the lowland plains in the northern hemisphere of Venus could have been resurfaced by 15‐m‐thick flows through continuous single‐vent effusion in ∼1.4 million to ∼140 thousand Earth years.

Geologic History of the Polar Regions of Mars Based on Mars Global Surveyor Data: I. Noachian and Hesperian Periods

During the Noachian Period, the south polar region of Mars underwent intense cratering, construction of three groups of volcanoes, widespread contractional deformation, resurfacing of low areas, and local dissection of valley networks; no evidence for polar deposits, ice sheets, or glaciation is recognized. South polar Hesperian geology is broadly characterized by waning impacts, volcanism, and tectonism. Emplacement of the polar Dorsa Argentea Formation (DAF) occurred during the Hesperian Period. Mars Orbiter Laser Altimeter topographic data and Mars Orbiter Camera images elucidate stratigraphic, morphologic, and topographic relations, permitting the dividing of the DAF into eight members, which surround and underlie about half of the Amazonian south polar layered deposits. The lobate fronts and lack of typical volcanic-flow morphology of the six plains units indicate that they may be made up of debris flows. We think that these flows, tens of meters to 200 m thick, may have originated by the discharge of huge volumes of slurry fluidized by ground water or liquid CO 2, perhaps triggered by local impacts, igneous activity, or basal melting beneath polar deposits. The cavi and rugged members include irregular depressions that penetrate the subsurface; some of the pits have raised rims. The depressions may have formed by collapse due to expulsion of subsurface material in which local explosive activity built up the raised rims. Further, smaller eruptions of volatile-rich material may have resulted in narrow, sinuous channel deposits within aggrading fine-grained unconsolidated material perhaps produced by gaseous discharge of subsurface volatiles; preferential erosion of the latter material could have produced the Dorsa Argentea-type sinuous ridges associated mainly with the DAF. Alternatively, the ridges may be eskers, but the lack of associated glacial and fluvial morphologies casts doubt on this interpretation. The knobby, degraded materials forming Scandia Colles may represent the only Noachian geologic record exposed in the north polar region. Most of the north polar region was buried by water- or debris-ocean sediments during the Hesperian Period, originating from uplands areas and perhaps knobby terrains in the northern plains. The sediments either mantle or were deformed by wrinkle ridges radial and concentric to Utopia basin and concentric to northern Tharsis. Sources of stress probably included sediment loading in the northern plains and regional magmatic and loading activity at Tharsis. Polar layered deposits began piling up during the Early Amazonian or later.

Mapping of crustal magnetic anomalies on the lunar near side by the Lunar Prospector electron reflectometer

Lunar Prospector (LP) electron reflectometer measurements show that surface fields are generally weak in the large mare basalt filled impact basins on the near side but are stronger over highland terranes, especially those lying antipodal to young large impact basins. Between the Imbrium and Nectaris basins, many anomalies correlate with the Cayley and Descartes Formations. Statistical analyses show that the most strongly magnetic nearside terranes are Cayley‐type light plains, terra materials, and pre‐Imbrian craters. Light plains and terrae include basin impact ejecta as a major component, suggesting that magnetization effects from basin‐forming impacts were involved in their formation. The magnetization of pre‐Imbrian craters, however, may be evidence of early thermal remanence. Relatively strong, small‐scale magnetic anomalies are present over the Reiner Gamma feature on western Oceanus Procellarum and over the Rima Sirsalis rille on the southwestern border of Procellarum. Both Apollo subsatellite and LP data show that the latter anomaly is nearly aligned with the rille, though LP magnetometer and reflectometer data show that the anomaly peak is actually centered over a light plains unit. This anomaly and the Reiner Gamma anomaly are approximately radially aligned with the center of Imbrium, suggesting an association with ejecta from this basin.

Initial mapping and interpretation of lunar crustal magnetic anomalies using Lunar Prospector magnetometer data

Maps of relatively strong crustal magnetic field anomalies detected at low altitudes with the magnetometer instrument on Lunar Prospector are presented. On the lunar nearside, relatively strong anomalies are mapped over the Reiner Gamma Formation on western Oceanus Procellarum and over the Rima Sirsalis rille on the southwestern border of Oceanus Procellarum. The main Rima Sirsalis anomaly does not correlate well with the rille itself but is centered over an Imbrian‐aged smooth plains unit interpreted as primary or secondary basin ejecta. The stronger Reiner Gamma anomalies correlate with the locations of both the main Reiner Gamma albedo marking and its northeastward extension. Both the Rima Sirsalis and the Reiner Gamma anomalies are extended in directions approximately radial to the center of the Imbrium basin. This alignment suggests that Imbrium basin ejecta materials (lying in many cases beneath the visible mare surface) are the sources of the nearside anomalies. If so, then the albedo markings associated with the stronger Reiner Gamma anomalies may be consistent with a model involving magnetic shielding of freshly exposed mare materials from the solar wind ion bombardment. Two regions of extensive magnetic anomalies are mapped in regions centered on the Ingenii basin on the south central farside and near the crater Gerasimovic on the southeastern farside. These regions are approximately antipodal to the Imbrium and Crisium basins, respectively. The Imbrium antipode anomaly group is the most areally extensive on the Moon, while the largest anomaly in the Crisium antipode group is the strongest detected by the Lunar Prospector magnetometer. A consideration of the expected antipodal effects of basin‐forming impacts as well as a combination of sample data and orbital measurements on the nearside leads to the conclusion that the most probable sources of magnetic anomalies in these two regions are ejecta materials from the respective impacts. In both regions the strongest individual anomalies correlate with swirl‐like albedo markings of the Reiner Gamma class visible on available orbital photography.

Geology of the Reull Vallis Region, Mars

The geology and stratigraphy of the Reull Vallis region (27.5–47.5°S, 245–270°W) of the southern highlands of Mars are documented through geomorphic analyses and geologic mapping at 1:2,000,000 scale. Crater size–frequency distributions are used to constrain relative ages of geologic units and determine the timing and duration of the various geologic events. The origin and evolution of the Reull Vallis outflow system and the extent to which Reull Vallis and other fluvial features modified the surrounding highlands are also evaluated. Rugged highland terrains were formed by repeated impact events and were extensively degraded and embayed by a series of plains. Highland terrains incised with well developed valley networks record some of the earliest fluvial activity in the region. Reull Vallis originates in Hesperia Planum, and dissects units of various types and ages. Truncated and bisected wrinkle ridges in Hesperia Planum indicate that the formation of Reull Vallis postdates ridge formation. The morphology of Reull Vallis suggests it had a long and complex history, including subsurface and surface movement of fluids followed by extensive modification by mass wasting. Volcanic and sedimentary plains units in the region record evidence of erosion by surface flow possibly related to development of Reull Vallis. Features produced by mass wasting, such as debris aprons and crater fill material, and aeolian processes appear to form the youngest deposits exposed in the region.

Biodiversity and terrestrial ecology of a mid-Cretaceous, high-latitude floodplain, Alexander Island, Antarctica

The biodiversity and terrestrial ecology of the Late Albian Triton Point Formation (Fossil Bluff Group), Alexander Island, Antarctica is analysed to improve our understanding of polar biomes during the mid-Cretaceous thermal optimum. This formation was deposited on a high-latitude (75°S) floodplain and consists of two facies associations, a lower braided alluvial plain unit and an upper coastal meander-belt unit. Analysis of fossil plants in well exposed palaeosols reveals the existence of spatially complex plant communities. Braidplains supported patchy, low-density (91 trees/ha) stands of podocarp and taxodioid conifers on floodbasin substrates, and conifer–cycadophyte–fern–angiosperm thickets in riparian settings. Coastal meander-belts supported medium density (568 trees/ha) podocarp–araucarian conifer forests on mature floodbasin soils, and fern–angiosperm–ginkgo thickets in riparian settings. Growth-ring analysis indicates plants experienced stressful growing conditions on the braidplain characterized by high-frequency flood events, but more favourable growing conditions on the coastal plain. Additional vegetation disturbances were caused by arthropod–fungal attack, frost and wildfire. In terms of structure, composition, ecology and productivity these predominantly evergreen, broad-leafed conifer forests bear similarities to the extant temperate rainforests of New Zealand.

Results of the Imager for Mars Pathfinder windsock experiment

The Imager for Mars Pathfinder (IMP) windsock experiment measured wind speeds at three heights within 1.2 m of the Martian surface during Pathfinder landed operations. These wind data allowed direct measurement of near‐surface wind profiles on Mars for the first time, including determination of aerodynamic roughness length and wind friction speeds. Winds were light during periods of windsock imaging, but data from the strongest breezes indicate aerodynamic roughness length of 3 cm at the landing site, with wind friction speeds reaching 1 m/s. Maximum wind friction speeds were about half of the threshold‐of‐motion friction speeds predicted for loose, fine‐grained materials on smooth Martian terrain and about one third of the threshold‐of‐motion friction speeds predicted for the same size particles over terrain with aerodynamic roughness of 3 cm. Consistent with this, and suggesting that low wind speeds prevailed when the windsock array was not imaged and/or no particles were available for aeolian transport, no wind‐related changes to the surface during mission operations have been recognized. The aerodynamic roughness length reported here implies that proposed deflation of fine particles around the landing site, or activation of duneforms seen by IMP and Sojourner, would require wind speeds >28 m/s at the Pathfinder top windsock height (or >31 m/s at the equivalent Viking wind sensor height of 1.6 m) and wind speeds >45 m/s above 10 m. These wind speeds would cause rock abrasion if a supply of durable particles were available for saltation. Previous analyses indicate that the Pathfinder landing site probably is rockier and rougher than many other plains units on Mars, so aerodynamic roughness length elsewhere probably is less than the 3‐cm value reported for the Pathfinder site.