P21B-0141
Late Crustal Growth on Mars: Evidence From Lithospheric Extension
Rift valleys on Mars have attracted considerable attention in the last couple of years and recently been observed with the High Resolution Stereo Camera (HRSC) on board the Mars Express mission. The images provided by the HRSC cover very large areas (up to 10$^5$ km$^2$) with high spatial resolution and are an ideal data base for age determinations based on crater statistics. It was thus possible to constrain the time of rifting at the Coracis Fossae in the Thaumasia region to 3.5-3.9 Gyr, the spatial resolution of the images being ~20 m per pixel. Continental extension on Earth proceeds in either narrow rifting, wide rifting or core complex mode, depending on the ambient initial conditions. The type of lithospheric extension at the Coracis Fossae is narrow rifting, the region subject to intense normal faulting narrower than 150 km. We have applied the pure shear model of lithospheric extension to determine the range of initial conditions such as thermal gradient and crustal thickness that allow for narrow rifts to form. Using the constraints posed on the thermal gradient by the flexure of the rift flank, we have determined the crustal thickness at the Coracis Fossae at 3.5-3.9 Gyr. We find that for the given thermal gradient of 27-33 K km$^{-1}$ the crustal thickness cannot have exceeded 40 km. This is considerably lower than present day estimates of the crustal thickness in that region and interpreted as an indication for continued crustal production at that time.
P21B-0142
Geomorphic and Spectral Mapping of Meridiani Planum Northern Etched Terrains
Mars Orbiter Camera (MOC), Mars Orbiter Laser Altimeter (MOLA), Thermal Emission Imaging System (THEMIS), and Mars Express (OMEGA) data were compiled and coregistered for analysis of a several hundred meter vertical section of etched terrain materials in the northern portion of Meridiani Planum. Seven units were mapped in the study area ($-2.25°E to $0.75°E, $0.25°N to $4°N) based on geomorphology and distinct spectral signatures in OMEGA hyperspectral data (C-spectrometer, 0.9263 to 2.6949 μ m) acquired during orbit 485. The base of the stratigraphic column is occupied by cratered plains materials that exhibit low albedo and spectral signatures dominated by pyroxene. This unit is disconformably overlain to the south by etched terrain materials characterized by widely distributed knobby and mesa-like surfaces. Etched terrain materials show absorption bands at 1.92, 2.10 and 2.40 μ m, consistent with the presence of kieserite (Arvidson et al, 2005, Science). The third unit is topographically and stratigraphically higher than the second unit, also characterized by bumpy and mesa-like terrain, and shows spectral evidence for kieserite. The fourth unit is bright and has a strong 1.92 μ m band, indicative of the presence of water hydration. This unit is extensively exposed just to the north of the hematite-bearing plains defined by Christensen et al, 2001, JGR. The fifth unit is the hematite-bearing plains, extensively exposed in the southern portion of the study area. Finally, the uppermost unit consists of partially dissected mantled terrain, well exposed in the eastern-most section of the study area. This unit has spectral reflectance properties consistent with bright dust and is interpreted as a wind-blown deposit superimposed on etched terrain materials. The geomorphology, spectral properties and stratigraphy indicate that extensive aeolian erosion has exposed a well-defined sequence of sedimentary units deposited over the Noachian cratered terrain.
P21B-0143
Alluvial Fan in Icaria Planum, Mars
The Mars Express HRSC data were used to study fluvial history of southern Claritas on Mars (1, 2). Volatiles, transported downslope to the basin, breached through a saddle valley and formed a channel towards Icaria Planum in the west. Along the channel, sapping provided additional water. The channel broke into a 30-km impact crater and formed a temporary lake. The crater rim has terraces and the floor has smooth deposits. A delta was formed in a standing water. After breaching the crater rim through a neck which is higher than the crater floor, water deposited onto Icaria Planum an alluvial fan. This fan was studied using the HRSC colour data by mapping deposit units of the Icaria lowlands in front of the channel. The flood deposits were made visible by the multichannel HRSC data classification. The alluvial structures reflect topography and regional slopes as well as the amount of available water. The hi-resolution HRSC image provides an additional view into the alluvial structures, erosion and sedimentation in the channel formation. These remote sensing approaches facilitate the mapping of characteristic phases in the fluvial development of the area studied. References. (1) Raitala et al., 2005. LPS XXXVI, \#1307. (2) Korteniemi, J., Raitala, J., Aittola, M., Kostama V.-P., Hauber E., Kronberg P., Neukum G. and the HRSC Co-I Team, 2005. Fluvial channel resulted in alluvial fan formation in Icaria Planum, Mars. Submitted to 42nd Vernadsky-Brown Microsymposium, Moscow 9-12. 10. 2005.
P21B-0144
Evidence for Phyllosilicates in the Early Mars Crust at Nili Fossae and Mawrth Valles Using OMEGA and HRSC Data
Hydrated minerals such as Fe-rich smectite clays, have been detected by the spectral imager OMEGA onboard Mars Express (Bibring et al., 2005). These types of phyllosilicates can result from the progressive weathering of volcanic rocks under conditions different from those currently occurring on Mars. Outcrops containing these minerals are located mainly in Noachian-aged regions suggesting that the early Mars conditions before 3.6 Ga were different, with warmer climate permitting weathering by liquid water. Here we examine in detail two regions, Nili Fossae and Mawrth Valles with coordinated OMEGA spectral data and HRSC high resolution images. In Nili Fossae, signatures of smectites are found in fresh outcrops of crust and crater ejecta. Lobate ejecta from a 60 km diameter crater displays abundant smectites that are interpreted to have been excavated during impact from buried reservoirs of smectites. Alluvial fans and sinuous channels are observed in the region but few mineralogic signatures are uniquely correlated to these landforms. Thus while episodes of water flow likely formed these features, the presence of this water did not cause weathering or alteration. In Mawrth valles, Fe-rich, and locally Al-rich smectite clays, are correlated with regions of light toned albedo that were previously interpreted as outcrops of the Noachian crust. The presence of smectites suggests that these rocks are part of the older Martian crust, usually buried under subsequent deposits, but exhumed in Mawrth valles area by erosion. Sediments of the outflow channel are not enriched in hydrated minerals suggesting, like in the Nili area, that the floods were not accompanied by conditions (i.e. sufficient time, or climate) to permit alteration and formation of phyllosilicates.
P21B-0145
The Nature of the Mars Pathfinder "Black Rock" Lithology: Comparisons with SNC Meteorites and OMEGA spectral images of Chryse Planitia
The initial interpretation of the Mars Pathfinder landing site was that it was covered with a single rock type, the "gray rock" spectral class that had variable levels and types of dust coatings. Re-analysis of the Imager for Mars Pathfinder (IMP) SuperPan indicated several occurrences in the near field of pebbles and cobbles of the distinctly different "black rock" spectral class. Where the gray rock spectra were characterized by a weak-to-Nonexistent long wavelength absorption, the black rock spectra had a strong absorption centered at, or longwards of 970 nm. In this paper, black rock spectra from the IMP SuperPan are compared with spectra of SNC meteorites and with OMEGA derived spectra from regions surrounding the MPF landing site in, and near Ares Vallis and in Chryse Planitia. A number of new occurrences of the black rock spectral class have recently been observed in supervised classifications of the SuperPan using a self organizing map artificial neural network approach. Spectra from these identifications, as well as previously recognized black rock occurrences were compared with SNC meteorite spectra. Comparisons were performed using spectral feature fits to the long wavelength absorption, overall spectral comparisons using a spectral angle metric, and comparisons on plots of spectral parameters such as 930 nm band depth and 754 to 1003 nm slope. Wheras there was not one single best match, several meteorites were found to have the most similar spectral character based on the metrics that were applied. These spectrally similar meteorites included olivine-phyric shergottites ALH 77005, EET 79001 lithology A, and Dar al Gani 735 (DaG 735). A paired specimen (DaG 476) of the latter meteorite has recently been noted as a good match to the thermal infrared spectrum of a region in Chryse with a unique spectral character. However, OMEGA derived spectra of this region appear more olivine-rich than DaG 735. More OMEGA derived spectral units from Chryse and Ares Vallis will be compared with the black rock spectra and compared in turn against SNC meteorite spectra in order to present likely candidates for the source region and mineralogic nature of the black rock spectral class.
P21B-0146
Quantifying valley network parameters on Mars using DEM from HRSC/MEX data
Valley networks are ubiquitous in Martian, old, densely cratered terrains. They show a well developed branching-like pattern and are thus interpreted as the result of fluvial activity during the Noachian period (>3.6 Gy) despite other hypothesis such as subsurface flows. On the plateaus surrounding Valles Marineris, some well preserved dendritic valley networks have been identified, suggesting the same process of formation (fluvial activity). They show the same organisation as the Noachian valley networks but valleys appears much shallower than the former. Based on the cross-cutting relationships and crater counts, they are dated of the Late Hesperian time. The high spatial resolution (few 10 m/pixel) of HRSC images acquired with stereo-mode allows us to perform DEMs (Digital Elevation Model) on these areas to study quantitatively the two different kinds of valley networks. HRSC DEM have a spatial resolution of ~50 m and a height accuracy of 100 m. In HRSC DEMs, Noachian valley networks located in Aeolis region [157°E-30.5°S] show a great number of tributaries (few 100s) arranged in 5-6 Strahler's orders, with a drainage density close to 0.3 km-1. Lengths of major tributaries are few 100s km to 1000 km. The major tributaries are often 100-200 m deep. In contrast, the Hesperian valley networks that incised the plateau of Echus Chasma have much smaller number of tributaries (few 10s) arranged in 5 Strahler's orders with a drainage density close to 1. Lengths of major tributaries are few 10s km and the major tributaries are shallow less than 100 m deep. In the two studied examples, the valley networks show the same spatial organisation, e.g. dendritic pattern, which suggests that they formed by the same erosive process involving fluvial activity. The difference in depth and length suggests that the fluvial activity was more significant and efficient during the Noachian time eroding important volume of material and/or the fluvial activity was spending over a long period of time. As a consequence, the climate in the Noachian was highly warm during geological long duration to incise these valleys, whereas the Hesperian valleys have formed during shorter episodes of time enabling surface runoff.
P21B-0147
Comparing the Late Stage Effusive Histories of Olympus, Pavonis, and Arsia Montes, Mars, Inferred From Lava flow Abundance Mapping of Mars Express HRSC Data
The abundances and temporal relationships between lava flow types provide insight into the most recent styles of effusive activity of a volcano, as they tend to form during different eruptive conditions. HRSC and THEMIS lava flow abundance mapping shows a high ratio of lava channels to tubes for Olympus Mons, with superposition relations indicating that channels are typically younger. This suggests that a change in eruptive style occurred from tube-forming (long-lived, steady effusions of lower viscosity lavas) to channel-forming (shorter-lived, unsteady effusions of higher viscosity lavas) eruptions. The southern flanks of Pavonis and Arsia Mons are typified by a lack of lava tubes. Highly subdued channels dominate their flanks, along with a mottled surface likely to be nearly completely buried channels. These results suggest that Pavonis and Arsia have likely completed the evolutionary trend from tube- to channel-forming eruptions as seen at Olympus, or never underwent conditions favorable for tube formation. Following main edifice formation, Pavonis and Arsia Mons formed similar northeast-trending rifts that acted as source regions for large lava aprons to the northeast and southwest. The aprons, which are lacking at Olympus Mons, are composed mostly of channel-fed flows. Arsia's southern apron appears to be the source for some long lava flows that extend into southern and western Tharsis. No detectable vents are present along the apron, and were likely buried by younger flows from the rift. Although Pavonis and Arsia Mons display similar flank morphologies, effusive styles on the aprons differ significantly. The rift style of eruption at Pavonis' south flank appears to have transitioned towards plains-style volcanism, typical of the Snake River Plains, Idaho. This area of the Pavonis apron is composed of >31 low shields and fissure-fed flow fields, covering the southeastern portion of the rift apron with lava tubes and sheets. Both aprons represent a change from major shield building to Tharsis plains formation. These results support previous Mariner and Viking era classifications of Tharsis shield building, but also suggest that main construct formation involves a change from tube- to channel-forming eruptions, while the advanced stages achieved by the Tharsis Montes are more complex than previously suspected.
P21B-0148
Martian Gravity Field at Short Wavelength: Results From Mars Express Radio Science Experiment MaRS
During the first two years of the Mars Express mission, the radio science experiment MaRS has returned high quality gravity data above selected targets such as the Tharsis volcanoes. The resulting acceleration residuals are used as an independent test of the quality of the existing global gravity solutions at short wavelength. A good agreement between MEX gravity data and MGS-derived gravity solutions is demonstrated in the Tharsis area down to a wavelength of 300 km. Furthermore, the combined analysis of adjacent gravity profiles allows the investigation of the density of the crust, the value of which is still controversial.
P21B-0149
A new Solution of Mars' Time Variable Gravity Field From a Joint Inversion of MEX and MGS Radio-tracking Data
The MEX orbit is quite different from the MGS orbit (ellipticity of 0.6 against 0.01), and thus offers a unique opportunity to improve solutions of the time-variable gravity field of Mars (mainly) obtained from MGS orbiter tracking data. By using almost 2 years of MaRS radio tracking of MEX, an accurate MEX orbit is estimated and solutions for time variable zonal coefficients (J2 to J5) are presented. Then the MEX tracking data are also stacked with the MGS data to obtain new solutions of J2 to J5 variations. This new solution is compared with estimates from Global Circulation Model and from data of the HEND experiment onboard Mars Odyssey.
P21B-0150
MARSIS Radar Echo Ionospheric Correction and The Estimation of Mars Ionosphere's Total Electron Content
On June 19th, 2005, Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) recorded its first radar echoes from the surface of Mars. MARSIS is capable of radiating between 0.1 and 5.5 MHz. The subsurface mode of MARSIS consists of four one-MHz bands with center frequencies at 1.8, 3.0, 4.0, and 5.5 MHz. The primary objectives of the subsurface sounding mode are to detect subsurface structures and search for water reservoirs. The amplitude and phase of MARSIS echoes are distorted by the Mars ionosphere by amounts depending on the number of free electrons and their distribution profile in the ionosphere. The compensation for the ionospheric distortion is one of major steps in MARSIS subsurface mode data processing. In this paper, we present the first results of the de-dispersion processing which yields very detailed information about the total electron content (TEC) between the radar and the Mars surface and to some degree how it is distributed in the ionosphere. For a given orbit, the TEC can be measured at high spatial resolution of approximately 5-10 km. We will also discuss issues related to variability of the TEC in Mars ionosphere and how it impacts MARSIS subsurface echo interpretation. The research described in this paper was carried out by the Jet Propulsion Laboratory, California Institute of Technology under a contract with the National Aeronautical and Space Administration.
P21B-0151
Atmospheric soundings by SPICAM occultation observations: aerosol and ozone vertical profiles
The SPICAM instrument is a highly versatile, dual spectrometer probing both the UV and the NIR spectral region and is currently flying around Mars onboard Mars Express. Since the beginning of MEx operations, SPICAM has collected about thousand atmospheric profiles while observing in a solar or a stellar occultation mode. UV spectra bear the signatures of several species; i.e carbon dioxide, ozone and aerosols, while infrared spectra potentially bring information on atmospheric condensates and on water vapor. This presentation will focus on the measured aerosol, ozone and water vapor profiles. For the aerosol, we will emphasize the numerous observations made in the polar night and will also discuss some high altitude clouds discovered in the southern hemisphere. Ozone and water vapor profiles will be presented along with some General Circulation Model comparisons. This work has been supported by CNES.
P21B-0152
Insights into Highland Patera Volcanism using Mars Express HRSC Data
We have used images obtained by the High Resolution Stereo Camera (HRSC) on the ESA Mars Express orbiter to assess geologic activity at two of Mars' highland volcanoes: Hadriaca Patera and Tyrrhena Patera. HRSC images cover wide swaths at consistent lighting conditions and resolutions, making them ideal resources for assessing surfaces ages using crater statistics. Additionally, multi-channel HRSC images are processed to produce Digital Terrain Models (DTMs) that are of greater spatial resolution than MOLA-derived DTMs, which are useful to assess regional and local topographic variations. Crater size-frequency analyses and cratering model age estimates show both Hadriaca and Tyrrhena Paterae have complex surfaces shaped by volcanic, fluvial, and eolian processes. These ancient shields were formed early in martian history, 3.7-4.0 Ga. At Hadriaca Patera, the earliest detectable caldera activity occurred at 3.5 Ga, followed by explosive volcanic and fluvial activity on the flanks at 3.3-3.4 Ga. Later caldera activity occurred at 2.2-2.5 Ga and again at 1.1-1.6 Ga. At Tyrrhena Patera, explosive volcanic activity and emplacement of pyroclastic deposits occurred 3.5-3.6 Ga, with later fluvial erosional activity at 1.9-2.0 Ga and again at 1.2-1.5 Ga. Slopes on Tyrrhena Patera are generally shallower (0.09-0.4 degrees) than those on Hadriaca Patera (up to 0.7 degrees). Hadriaca's north flank trends uphill, suggesting that Hadriaca Patera settled due to removal of material during formation of Dao Vallis. Further study is underway to use HRSC topographic data and computer modeling to better understand pyroclastic volcanism at these two volcanoes.
P21B-0153
Evidence for Enhanced Hydration and Water Ice Stability at Tropical Latitudes on Mars
New spectroscopic observations performed by the OMEGA instrument on board Mars Express show evidence for a small region of enhanced hydration on the northern scarp of Olympus Mons. Day and night measurements of surface temperatures in this region indicate that the soil has an high thermal inertia and exhibits the lowest daytime temperatures in the area. Thermodynamic modeling suggests that water ice in this region is stable during most of the Martian year due to the saturation of the atmosphere.
P21B-0154
Strengths and Limitations of Spectral Parameters for Mapping Surface Composition on Mars
OMEGA, the visible to near-infrared (352 bands from 0.35 - 5.1 microns) imaging spectrometer on Mars Express, has achieved near-global coverage with spatial resolution varying between 300 m and 4.8 km depending on the pericenter altitude of the spacecraft's highly elliptical orbit. Data from OMEGA have dramatically increased our understanding of the types and distributions of mafic minerals, sulfates, and phyllosilicates on the martian surface resulting in a new perception of the surface composition of Mars. Employing the technique designed for CRISM multispectral operations, we use these data to calculate and globally map 46 spectral parameters formulated to highlight mineralogic and atmospheric properties. The parameters vary in utility. The most robust parameters highlight mafic mineralogy, sulfates, hydrated silicates, and surface ices, the least robust are those tailored for narrow absorptions or those that span atmospheric absorptions, which show variations with time indicating sensitivity to atmospheric corrections. However, even the most robust parameters are occasionally susceptible to false positives; therefore regions of mineralogic interest identified with this approach must always be evaluated with the full spectral resolution observations for verification. Nevertheless locations of strong mineralogic signatures previously identified by the OMEGA team are well defined in these maps and demonstrate the validity of this approach as a rapid assessment tool. Thus, this simple technique will be a useful and reasonable tool to aid in Mars exploration in the following manners: i) to identify scientifically interesting areas to target with both present and future orbiting spacecraft, ii) to support landing site selection for Mars missions already slated for launch such as Phoenix and MSL, and iii) to identify scientifically interesting areas as potential landing sites for future surface missions. Here we focus on the most robust parameters highlighting mafic mineralogy, sulfates, hydrated silicates, and surface ices for a subset of relevant and interesting regions. These parameter maps are an important resource for identifying mineralogically-diverse areas and, following CRISM, will be available at 200m/pixel global coverage.
P21B-0155
SPICAM on MARS Express: An Overview of UV and Near IR Results
SPICAM, a light-weight (4.8 kg) UV-IR dual spectrometer on board Mars Express orbiter, is dedicated primarily to the study of the atmosphere and ionosphere of Mars, but is providing important results on the surface albedo of Mars. A UV imaging spectrometer (118 - 320 nm, resolution 1.5 nm, intensified CCD) operates in nadir viewing, limb viewing and atmospheric vertical profiling by stellar and solar occultation. An IR spectrometer (1.0-1.7 microns, resolution 0.5-1.2 nm, or = 1300, mass 0.8 kg) is dedicated primarily to nadir measurements of H2O abundances, but detects also water ice and CO2 ice both on perennial and seasonal caps. It is based on AOTF technology; and it is the first time that such a spectrometer is flying in deep space. Night glow emissions from NO recombination and aurora over crustal magnetic anomalies have been discovered. Several hundreds of atmospheric vertical profiles by UV star occultation were obtained. The main absorbers are CO2 (below 200 nm) and aerosols/dust (above 200 nm). The atmospheric pressure and temperature are retrieved from 150 km down to 25-40 km, for the construction of an empirical model of the atmosphere. In 60 % of cases there is a detached layer of dust, or cloud. Ozone is also measured and compared to model predictions. The UV absorption of ozone is well identified in routine reflectance nadir viewing. The ozone vertical quantity is recovered along the orbital track, and compared to the water vapour distribution measured by SPICAM in the near Infra-red around 1.38 m. SPICAM has thus obtained for the first time from an orbiter simultaneous measurements of ozone and H2O, which are found to be somewhat anti-correlated as expected on the ground of chemistry consideration: catalytic destruction of ozone from OH and HO2 radicals. We observed also the O2 Airglow at 1.27 μm in nadir and limb viewing produced by photo-dissociated ozone, and all UV limb dayglow emissions that were observed by Mariner spacecraft.