P24A-01 INVITED
Seasonal Changes in the Martian North Polar Region from CRISM, MARCI and CTX
Significant mobility and spatial variation in high albedo patches are noted in the summer northern ice cap. These patches are shown to be fine-grained water frost and their motions may provide information on winds or local accumulation and ablation occurring both on seasonal and inter-annual timeframes in the permanent ice deposits. The Mars Reconnaissance Orbiter instruments provide complementary views of the processes involved. MARCI covers the entire northern residual ice and surrounding Polar Layered Deposits (PLD) multiple times per day at spatial resolutions of a few kilometers. CTX has been targeted on one of the largest bright deposits with repeated coverage in the first northern summer. The Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) observes the polar deposits in the wavelength range where multiple, diagnostic features can separate H2O and CO2 ices and allow the determination of effective grain size of ice and level of dust contamination. Several full-resolution and approximately 80 multi-spectral observations were targeted during the first northern summer on a known sustained bright anomaly. Previous work by the OMEGA team has demonstrated the evolution of water frost grain size over the summer season and suggests that the late northern seasonal frost is dominated by water rather than CO2 ice. This late water ice lag in the seasonal cap may also drive the appearance and mobility of residual fine grained ice patches. Spectral properties in exposed layers are more complicated, and even dark lanes within the reentrant troughs still show features associated with water ice. However, within the larger PLD areas that appear as transparent ice to MARSIS and SHARAD have no infrared spectral absorption features of ice. The modern view shows that both seasonal and permanent ice deposits are highly active with complex temporal patterns and understanding these dynamic phenomena are critical to understanding martian climate history. We will report the latest synthesis of these observations.
P24A-02
Persistent summertime water ice deposits in the northern plains of Mars: Observations from MRO CRISM
Analysis of MRO CRISM multispectral mapping data obtained during the late northern summer season on Mars (Ls = 130-180 degrees) reveals small water ice deposits distributed throughout the northern plains at latitudes quite distant from the residual polar cap. These outliers range in size from a few hundreds of meters to several kilometers, and are generally associated with the northward facing slopes of crater rims or other elevated landforms. In a few instances the ice deposits are located on the leeward (southeast-facing) sides of larger craters, and may indicate the presence of wintertime CO2 frost formation from orographic lifting. The brighter frost sublimes more slowly than the surrounding CO2 ice, ultimately forming a late spring cold trap and inducing an accumulation of water ice. During the first 4 months of MRO primary science phase (PSP), CRISM was able to cover ~75% of the northern plains surface (at 75 degrees latitude) with 73 channel visible/near infrared (0.41 to 3.92 μm) multispectral data as part of its systematic global mapping campaign. These long, 10 km-wide strips of data are mosaicked together and resampled to 256 pix/deg (~231 m/pix) spatial resolution. CRISM's wavelength range is particularly well-suited to distinguishing between different types and grain sizes of CO2 and water ice, as well as iron- bearing minerals, sulfates, and phyllosilicates. False color composites of the multispectral data allow consistent mapping of the lowest latitudes of water ice occurrences and comparison to other global datasets. The average minimum latitude that water ice is observed during this time period is 75.5 degrees, with excursions of up to 10 degrees. Albedo appears to be a secondary control after local slope, and large scale topographic trends do not influence the locations of residual ice patches. Further comparison to Mars Odyssey GRS inferred subsurface water ice distribution may provide important clues regarding subsurface-surface-atmospheric volatile interactions and the global water budget on Mars.
P24A-03
HiRISE Observations of Martian Mid-Latitude Fractured Mounds
The High Resolution Imaging Science Experiment (HiRISE) camera has now returned thousands of images of the Martian surface with pixel scale as small as 26 cm/pixel. These have revealed fractured mounds up to several hundred meters in diameter, bearing some morphological resemblance to terrestrial pingos (ice-cored hills formed by freezing groundwater). Pingos on Mars would be valuable indicators of ground ice and have been suggested at a number of sites, but in several cases reexamination has supported different origins. Some differences do exist between the fractured mounds and terrestrial pingos. In several instances, the mounds have roughly trapezoidal topographic profiles with flat, fractured summits. Other morphologies are also seen; we report on the range of morphologies observed so far by HiRISE and similarities and differences with pingos on Earth. The fractured mounds observed to date generally appear in the mid-latitudes, at a range of longitudes. Mars Orbiter Camera (MOC) images of flat-topped mounds in Utopia Planitia (including some previously proposed pingos) show a similar latitudinal dependence, generally occurring between 35-45° N. This supports a ground- ice related origin, particularly since the latitude range is close to the peak-abundance latitude of some other features likely related to water or ice, such as gullies. It is still uncertain whether the formation mechanism of the fractured mounds is the same as terrestrial pingos in detail. We discuss the distribution, properties and settings of fractured mounds observed planet-wide by HiRISE.
P24A-04 INVITED
HiRISE Images of the Sublimation of the Southern Seasonal Polar Cap of Mars
The High Resolution Imaging Science Experiment (HiRISE) on the Mars Reconnaissance Orbiter (MRO) has returned images with unprecedented resolution of Mars southern seasonal CO2 polar cap. Several high latitude sites were selected for systematic monitoring throughout the spring as the seasonal cap sublimed away. The capability of MRO to turn off-nadir enabled acquisition of stereo pairs to study the topography. HiRISE color capability distinguishes processes involving dust and frost. Color images show evidence of localized migration of frost as dark spots sublimate. Unique landforms are found in the cryptic terrain[1] region of Mars polar cap. These unusual landforms have narrow channels emanating radially, dubbed spiders[2]. Fans of dust blown by the prevailing wind are hypothesized to come from gas jets of CO2 subliming beneath translucent seasonal ice [3]. HiRISE images show a wide variety of morphologies of narrow channels. In some regions deep narrow channels converge radially, while in others the high channel density is more akin to lace. A smooth evolution of one form to another has been observed. Channels converge dendritically, often uphill, consistent with formation by flowing gas, not liquid. More dust fans are observed in regions of spiders than in lace, suggesting that the sublimating gas under the seasonal ice builds up more pressure and can entrain more dust in spidery areas. These differing terrain types are found within a single 6 x 10 km image, which has presumably homogeneous weather, thus a uniform layer of ice and exposure to atmospheric dust. HiRiSE images show that the dust fans tend to emerge from low spots, where the subsurface is accessed, then are blown up and out onto the surface of the seasonal ice. The fans evolve from a thin diffuse covering to thick blankets filling in the narrow channels. We hypothesize that dust collects in the channels, and that these relatively more permeable dust-filled channels form pathways for the next seasons gas to escape from below, a positive feedback which would tend to erode the same channels a little more every season. In another region of the cryptic terrain the surface has a texture reminiscent of lizard skin. Fans are seen to form and grow in a time period of less than 5 days, evidence for a very active process modifying the martian landscape. We also see the formation and disappearance of bright streaks within a short time interval. There are also sand dunes at high latitudes that get covered with seasonal frost. Dry ice is affecting the landscape and may be responsible for the formation of gullies on dunes. [1]Kieffer, H., (2000) LPI #1057. [2]Piqueux, S., S. Byrne, and M. Richardson, (2003) JGR 108(E8):3-1. [3]Kieffer, H., (2006) accepted for publication in JGR. This work was partially supported by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.
P24A-05
Bright Fans in Mars Cryptic Region Caused by Adiabatic Cooling of CO2 Gas Jets.
Over the last decade, observations of the retreat of the southern seasonal cap of Mars have revealed the presence of exotic processes within an area now informally referred to as the cryptic region. The appearance of dark spots, fans, blotches, and halos have been a "hot" topic of scientific discussion since they were first observed by the Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) [Malin et al., 1998]. Further observations by the Mars Odyssey (ODY) Thermal Emission Imaging System (THEMIS) showed that the dark features remained cold throughout the early-to-mid spring, suggesting that these features were either CO2 ice or were in thermal contact with CO2 ice [Kieffer et al., 2006]. In this paper, we present observations in the near-infrared at spatial resolutions that have previously been unavailable. We present further evidence that many of these features in the cryptic region are the result of cold jets, as first described by Kieffer [2000, 2007]. The adiabatic cooling of gas spewing downwind from the jets produces CO2 frost, thus forming the bright fans. The bright fans appear to be devoid of H2O ice, thus further supporting the hypothesis that they are formed from the downwind settling of CO2 frost. In some areas, the bright fans are adjacent to dark fans and appear to start from common vertices, while in other areas, bright fan-like deposits occur without the strong presence of dark fans. References: Kieffer, H.H. (2000) Annual Punctuated CO2 Slab-Ice and Jets on Mars, International Conference on Mars Polar Science and Exploration, p. 93. Kieffer, H.H. et al. (2006) Nature, 442,793-796. Kieffer, H.H. (2007) JGR, in press. Malin, M.C., M.H. Carr, G.E. Danielson, M.E. Davies, W.K. Hartmann, A.P. Ingersoll, P.B. James, H. Masursky, A.S. McEwen, L.A. Soderblom, P. Thomas, J. Veverka, M.A. Caplinger, M.A. Ravine, and T.A. Soulanille (1998) Early views of the Martian surface from the Mars orbiter camera of Mars global surveyor, Science, 279, 1681-1685.
P24A-06 INVITED
Stratigraphy of the South Polar Layered Deposits in SHARAD Radar and Images
SHARAD (Shallow Radar) is a synthetic-aperture, orbital sounding radar on the Mars Reconnaissance Orbiter (MRO) operating at 20 MHz, with a 10-MHz bandwidth that provides a free-space range resolution of approximately 15 m after pulse compression. SHARAD observes many tens of internal reflections in the Promethei Lingula (90°-140°E) region of the south polar layered deposits (SPLD). Multiple parallel reflections are traceable over distances of several hundred km in individual radargrams. There are several packets of multiple reflections separated by non-reflective regions with depth. By comparing crossing orbits and using stratigraphic correlations between orbits, a three dimensional picture of the internal structure of the SPLD as observed by SHARAD can be assembled. Reflections extend many hundreds of km throughout the Promethei Lingula region, and generally decrease in elevation towards the margins of the deposit. An angular unconformity between ~ 108°E and 123°E is observed in multiple orbits, and may be related to erosional features observed in the nearby canyon of Australe Sulci. Additional unusual reflections are observed in two orbits near 108°E, 84°S and are currently undergoing closer analysis. Comparisons of SHARAD reflections that intersect with the surface and THEMIS and MOC images indicate that an individual reflection may correlate to multiple (3-7) layers at MOC resolutions (~ 6 m/pxl). The layers in this region are eroding in groups, resulting in a stair-stepped topographic profile; the underlying physical properties of the layers causing this erosional behavior may also be the cause of a radar reflection. By fitting a surface to correlated reflections in multiple orbits using an inverse distance weighting technique and extrapolating that surface to the wall of Chasma Australe, it is possible compare the elevations of the reflectors to the sequence of layers exposed on the wall in THEMIS images. Preliminary results indicate that three major reflectors correlate with changes in layering styles within the SPLD. In particular, a sequence of dark, thin layers near the bottom of the layer stratigraphy in this region may correlate to a single reflection. A shift in layering style between a region where individual layers tend to be subtle and hard to distinguish and a region of thicker (~ 40 m), bright, erosion- resistant layers may correlate to another reflection.
P24A-07
Density of Mars' South Polar Layered Deposits
The Martian south polar layered deposits (SPLD) contain the south polar residual ice cap and smooth, low- albedo surroundings that collectively rise about 3 km above surrounding cratered highlands terrain. The residual polar cap component is believed to be composed of water ice with an unknown admixed dust component that is overlain by a thin (1-10 m) predominantly CO2 cover. The CO2 veneer contains "swiss cheese-like" shallow depressions that reveal the underlying water ice at their bases. The more spatially extensive part of the SPLD has a low albedo and dust-like spectral signature, which raises the question whether the dominant component of the SPLD as a whole is volatile (H2O and/or CO2) or dust. In this study we use initial high-resolution gravity observations from X-band (8.4 GHz) Doppler tracking of the Mars Reconnaissance Orbiter (MRO), together with the volume obtained by combining surface topography from the Mars Orbiter Laser Altimeter (MOLA) and basal topography from the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) to calculate the density of the SPLD and constrain its composition. We find the best-fit density by calculating the predicted gravity field from the observed structure of the SPLD and underlying structure. We determine the contributions to the gravity from the topography along the Moho, crustal surface, and SPLD surface, accounting for the finite amplitude of the topography. We iterate the SPLD density to find the best fit between the modeled and observed gravity. Results indicate a best-fit density of 1220 kg m-3, which is consistent with water ice with approximately 15% admixed dust. Our results indicate that despite the dust-like albedo and spectral signature over most of its surface, the SPLD are likely composed of relatively clean water ice. The result refines the Martian surface water inventory. These deposits represent the largest known surface reservoir of water on Mars today, and the largest in the inner solar system outside the Earth.
P24A-08 INVITED
New Insights into Gemina Lingula of the Northern Polar Layered Deposits, Mars from SHARAD Observations
The Shallow Radar (SHARAD) instrument on Mars Reconnaissance Orbiter expands our ability to study internal features of the Polar Layered Deposits (PLDs) by complementing optical and spectral imagery in addition to radar sounding by MARSIS on Mars Express. The theoretical vertical resolution is approximately 8 m in water ice. Horizontal resolution is 0.3 1 km along-track and 3 6 km across track. We present results of SHARAD over the northern PLD (NPLD) with an emphasis on layer character in Gemina Lingula, the NPLD lobe that extends along the southern edge of Chasma Boreale. Initial observations show two distinct units, an upper unit approximately 600 m thick exhibiting many strong radar reflectors with extensive lateral continuity, and a lower unit that is typically over 1 km thick with fewer and weaker reflectors that also appear to be continuous over large distances. Angular unconformities are evident between the units. These radar-defined units likely correspond to a smooth upper PLD unit and a lower, polygonally cracked PLD unit as observed in HiRISE imagery. Both would therefore make up the "finely layered unit" of Byrne and Murray (2002; JGR). A diffuse radar reflector underlies these units and is presumably the contact with the Vastitas Borealis Formation. If this interpretation proves correct, there is no significant "platy unit" of Byrne and Murray (2002) or "basal unit" of Fishbaugh and Head (2005; Icarus) existing under Gemina Lingula. After importing over 50 SHARAD data passes with over 300 crossovers into a seismic data interpretation package, we have interpreted radar reflectors as discrete horizons in order to map interface continuity and determine radar-detected layer thicknesses across the extent of Gemina Lingula. These results provide a 3-dimensional, first-order look at past accumulation patterns (and possibly the effects of flow) assuming that these radar interfaces are isochrones. Some evidence for brittle deformation is seen, particularly near Chasma Boreale. The geometrical relationships of radar horizons with Chasma Boreale and the spiral troughs are also examined in order to constrain the relative timing of their formation with respect to the deposits themselves.
P24A-09
A Study of the Subsurface Structure of a Region of the Martian South Polar Layered Deposits by Means of Sharad High Resolution Radar Data
SHARAD (SHAllow RADar) is a Ground Penetrating Radar provided by the Italian Space Agency, which is participating as a facility instrument to NASA's Mars Reconnaissance Orbiter mission. Goal of this nadir-looking, low-frequency SAR is to investigate surface and subsurface of Mars and subsequently provide data, including high resolution views of the stratigraphy within the layered deposits, that supply unique information concerning dielectric interfaces. The latter can thus be analyzed and interpreted in terms of occurrence and distribution of expected materials, including water, ice, rock and regolith. Among the most interesting results obtained so far, the numerous and continuous subsurface reflectors detected below the Promethei Lingula area, extending for approximately 1 km in depth, can give a relevant contribution in understanding the geophysical features of this part of the South Polar Layered Deposits, in particular the still unknown phenomenon that is at the base of the layers formation. In this work we will analyze whether the reflectors detected are a result of a real physical interaction between electromagnetic wave and subsurface structures or are due to phenomena such as multiple reflections. At the same time we are able to exclude possible off-nadir clutter by means of a surface simulator developed at the SHARAD operational center. We then give an exhaustive description of the reflectors, especially their structure shape as well as the horizontal and vertical extension. In addition to that we put together SHARAD radar data in 3D images to better understand shape and slope of the subsurface reflectors. Moreover we make a comparison with terrestrial areas, that show similar features.
P24A-10
Residual South Polar Cap of Mars: MY28 MARCI Observations
MGS observations revealed that the Residual South Polar Cap (RSPC) consists of several identifiable units that show evidence of deposition and erosion with characteristic times on the order of ~ 100 years (Thomas et al., 2005). These authors suggested that significant changes observed in the RSPC between Mariner 9 and Viking represent deposition of a new CO2 layer following erosion of a previous layer(s) by unidentified processes. One possibility is that the a layer erodes gradually due to growth of the pits observed by MGS (Thomas et al., 2000; Malin et al., 2001). However, the changes in the small-scale structure over the four MGS years were not accompanied by changes in the albedo (James et al., in press) or surface area (Piqueux and Christensen, 2007) of the residual cap. Thus the current expansion of the pits may not result in any net sublimation of the cap as would be required to remove a layer. A possible mechanism for removal of a layer is a change in absorbed insolation caused by interannual variability in the dust content of the atmosphere near perihelion. In particular a major perihelic dust storm, such as the event observed by Mariner 9, would accelerate cap sublimation; such a storm, or a succession of such storms, could lead to the erosion of a CO2 layer(s) (Bonev et al., in press). MRO observed a major dust event in MY28 that obscured the south polar cap in much the same way that the Mariner 9 storm obscured the cap in MY9. At the time of this meeting Mars Reconnaissance Orbiter (MRO) will have observed the RSPC for the entire summer following the storm; these observations will be compared to the four complete summers recessions observed by MGS in order to see if there are changes in the RSPC and test the dust hypothesis. This work was funded through JPL Contract 1275776. References: Bonev, B.P. et al. Planetary and Space Science (in press). James, P.B. et al. Icarus (in press). Malin, M. C. et al., Science 294, 2146-2148 (2001). Piqueux, S. and Christensen, P.R. Seventh International Mars Conference, Abstract 3068 (2007). Thomas, P.C. et al. Nature 404, 161-164 (2000). Thomas, P.C. et al.. Icarus 174, 535-559 (2005).