P41A-01
Resurfacing Revisited: What Mars, Europa, and Io may have to Tell us About Cratering, Volcanism, and Tidal Heating in the Earth-Moon System.
Recent findings involving planetary surfaces are important to our understanding of our own Earth-Moon system. The discovery recently of slope wash and alluvium deposition possibly caused by flowing water, within the past 5 years, indicates a more active Mars and thus solar system in general [1]. Mars Orbiter Camera (MOC) and High Resolution Imaging Science Experiment (HiRISE) data is revealing craters as small as 10 m [2,3]. This new cratering evidence suggests young surface ages on Mars and likely other solid bodies in the solar system. On Europa, for example, evidence points to an overabundance of secondary cratering [4]. The importance of secondary cratering as noted for other bodies can be evaluated in terms of its impact on lunar mare cratering. Voyager I visited the Jupiter system and returned images of volcanic resurfacing of Jupiter's moon, Io, which is very similar to Earth's moon in size and density. Tidal heating prior to the discoveries on Io was not well understood. It was thought that the heat of volcanic resurfacing in the solar system, including in the Earth-Moon system, was driven primarily by accretion and isotopic heating. The discoveries of the Voyager missions introduced the concept of tidal heating as it had not been thought of before [5]. The model of tidal heating for Io can also be used to discuss tidal heating as an explanation for the heating history of the Moon and the formation of the lunar mare. Evidence for such a tidal heating event in the Earth-Moon system should be extant in the geologic record, and indeed the continental flood basalts (CFB) and the formation of the lunar mare provide the most ready examples. The observation that the lunar mare and Earth's CFBs are similar is not new [6]; however, a mechanism to explain the formation of both has been lacking. A tidal heating episode in the Earth-Moon system's past can be used to explain both the formation of the mare and the CFBs. All of these findings point toward much younger surfaces and a more dynamic solar system than has been previously imagined. Yet the estimated age of the mare surface has not been revised significantly since the preliminary examination of Apollo 11 lunar samples in 1969 [7]. Since then, the lunar mare have been used to calibrate planetary surfaces throughout the solar system. Interplanetary surfaces once dated with a crater flux produced from the isotopically dated lunar mare and thought to be billions of years old are now found to be quite young. This cratering paradox can be addressed with a new model in which the lunar mare are shown to be secondary surfaces formed during a tidal heating episode similar to that currently seen on Io. This secondary (tidal) heating, can be correlated with the geologic history of Earth. Hence, an examination of the new information available on cratering, volcanism, and tidal heating from diverse places such as Mars, Io, and Europa can yield clues about the geomorphology in the Earth-Moon system, and other unusual aspects of this system. [1] Malin, M. et al. (2006) Science, 314, 1573. [2] Aharonson, O. (2007) Lunar and Planetary Science XXXVIII, Abstract. [3] McEwen A. et al. (2007) Lunar and Planetary Science XXXVIII, Abstract. [4] McEwen, A. and Bierhaus, E. (2006) Annu. Rev. Earth. Planet. Sci. [5] Consolmagno, G. (1981) Proc. Lunar Planet. Sci. 12B. [6] Alt, D. et al. (1988) J. Geology. [7] Lunar Sample Preliminary Examination Team (1969) Science 165, 1211-17.
P41A-02
Magnetic Signatures of Impact Fractured Rocks from Sierra Madera, Texas, USA - Implications to Magnetic Anomalies on Mars
Mars Express Orbiter (sounding radar data) revealed that craters of ancient origin had been covered by thick sediments in northern hemisphere. Mars MOLA topography mission identified many crater on Mars surface. Thus despite the Mars dichotomy, both northern and southern hemisphere have been covered by impacts to similar density. Mars currently has no global magnetic field of internal origin. In southern hemisphere, magnetic field intensities due to anomalies of remanent origin are much lower over the gigantic impact craters (e.g. Hellas, Prometheus, and Argyre). Low magnetic field may not relate to the absence of internal dynamo but due to impacts. For example, the aerial survey over a two billion year old, largest crater on Earth, Vredefort in South Africa observed much lower magnetic intensity over the crater, despite of the strongly magnetized simgle domain (SD) magnetite in shocked granites. Randomized magnetic vector orientations caused by impact may be the origin of the lower magnetic field observed on both Vredefort and Mars. We conducted magnetic analysis for a suite of Sierra Madera Impact deformed rock sites with complete shatter cone structures and multiple striated joint set (MSJS), and the initial results were intriguing. NRM vector orientations, REM ratios, and AF demagnetization curves showed contrasted magnetic signatures between the sites as well as within the samples. The NRM signatures in small scale shatter cones and larger scale shatter cones indicated shock demagnetization (SDM). The peculiar signatures of the site with MSJS may be both SDM and shock magnetization (SRM). We characterized the complexity and distinct magnetic signatures of impact fractured rocks. The results suggest that the size of the shatter cones and structures may reflect the magnetic signatures of both intensity and directions. Also, the dimensional scale of shatter cones is indicative parameters for randomization of the magnetic vector orientations. Such variations may influence on overall magnetic intensity observed from a distance, which relates to magnetic anomalies on Mars and Moons.
P41A-03
Fine-Scale Topographic Analysis of Rock Size Distributions Derived from High-Resolution Ground-Based LiDAR
Quantitative factors such as RMS height, correlation lengths and surface slope derived from fine-scale topographic datasets hold the potential for characterizing surface morphology in relation to its underlying geologic processes. In an attempt to better understand the relationships between topographic roughness characteristics and geologic processes responsible for creating a distinct surface morphology, we utilize ground-based terrestrial LiDAR and coincidental orthorectified imagery to quantify the variability in RMS heights and correlation lengths. The purpose of this study is to understand directly how various topographic data collection techniques such as LiDAR and manual field-based measurements compare to one another and which techniques are most appropriate for characterizing topography at various scales. Topographic data from several platforms were acquired over desert surfaces in the Mojave Desert near Palm Springs, California and southwestern Arizona. The desert surfaces imaged in the Mojave contained average rock sizes ranging from decimeters to a maximum size near one meter and revealed wide variations in RMS heights and correlation lengths, in keeping with the highly variable surface. Alternately, the Arizona site exhibits less topographic variability and consistent statistics. The data are useful for characterizing the roughness of surfaces for a variety of disciplines, such as penetration of remote sensing signals, upwelling of radiation and characterizing the genetic origin of surfaces. Furthermore, these data become essential to airborne and ground-based imaging sensors and understanding how topographic irregularities affect data fidelity.
P41A-04
Ground-based LiDAR Measurements of Actively Inflating Pahoehoe Flows, Kilauea Volcano, Hawaii: Implications for Emplacement of Basaltic Units on Mars
Remote sensing datasets enable planetary volcanologists to extract information regarding eruption processes. Long-lived effusive eruptions at sites such as Kilauea Volcano (HI) provide opportunities to collect rich observational data sets, including detailed measurements of topography and extrusion rates, that allow comparisons between lava flow surface morphologies and emplacement conditions for use in interpreting similar morphological features associated with planetary lava flows. On Mars, the emplacement of basaltic lava flows is a volumetrically and spatially important process, creating both large-scale and small-scale surface morphologies. On Earth, low effusion rate eruptions on relatively horizontal slopes tend to create inflated lava flows that display hummocky topography. To better understand the processes involved in creating observed surface characteristics, we repeatedly measured the surface topography of an actively flowing and inflating basaltic unit within the Pu'u O'o flow field over a 5-day period. We used a ground-based laser-scanner (LiDAR) system that provided vertical and horizontal accuracies of 4 mm. Comparing DEMs from repeated laser scans yielded the magnitudes and styles of constructional processes, allowing us to quantify the relationship between pre- and post-emplacement surface topography. Our study site (roughly 200 m x 200 m) experienced about 5 m of vertical inflation over a 3 day period and created a new hummocky surface containing several tumuli. The temporal and spatial patterns of inflation were complex and showed no obvious relationship with underlying topography. High-precision morphometric measurements acquired using ground-based LiDAR affords us the opportunity to capture the essential boundary conditions necessary for evaluating and comparing high-resolution planetary data sets, such as those acquired by the MOC, HRSC, and HiRISE instruments.
P41A-05
The Influence of Slope Variations on the Levees of Large Channelized Lava Flows in the Tharsis Region of Mars
Lava flows in the Tharsis region of Mars extend for hundreds of kilometers over very shallow slopes (<0.1 to 1 degree) and exhibit a variety of morphologies, including prominent leveed lava channels. Such flows are typically greater than 35 m thick, have channels 1 to several km in width, and levees on the order of 1 to 10 km wide; these are extremely large by terrestrial standards, and represent huge magma mass fluxes. The marginal levees can form in a variety of ways, but initial formation conditions are commonly overprinted by channel breakouts or overspills due to unsteady flow processes. Levees and channels consequently have shapes and volumes that are dependent on variations in underlying slope, flow rheology, and effusion rate. The purpose of our study is to examine the effects that small changes in topography have on the morphology of channelized lava flows. To address this issue, we have mapped several large flows in the plains north of Pavonis Mons. Using Mars Orbiter Laser Altimeter (MOLA) profiles co-registered with mosaicked Thermal Emission Imaging System (THEMIS) visible and infrared images, we have acquired channel and levee cross-sectional geometries at multiple locations downflow. In order to link variations in flow morphology with variations in underlying slope, we have developed and applied simple models of levee growth using our data as constraints. We will present a comparison of the different models and discuss our inferences for the downflow evolution of flow rheology.
P41A-06
Gulley Incision Into Basaltic Tephra Deposits And Implications For Mars Valley Network Formation
It's clear from geologic features such as the Tharsis volcanoes, analyses of the SNC meteorites, multispectral
data from orbiting spacecraft, and in situ measurements made by landers that the martian surface is composed
primarily of basalt. Spacecraft data also suggest that this basaltic surface has been broken down into friable
materials produced by weathering, impact cratering and explosive volcanism. We have begun a series of
analyses in the Ka'u Desert of Hawaii to better understand the environmental conditions necessary to initiate
surface runoff and channel incision in brecciated basaltic materials similar to the martian surface. We used
Differential GPS surveys to measure the cross-sections and slopes of gullies that have incised into the
Keanakako'i tephra deposit located on the eastern margin of the Ka'u Desert near the rim of Kilauea volcano. We
conducted grain-size analyses of channel sediments and made estimates of flow depths during peak discharge
based on field evidence. Wherever possible, we also analyzed the stratigraphy of fluvial sedimentary deposits
emplaced over basaltic lava flows of known ages and determined the number of flood events associated with
these materials. Our results suggest that many of the larger gullies have peak discharges of approximately 100
m3/s, and that flood events occur once every 3 years on average. Climate data for the region indicate that runoff
within the gullies occurs during severe thunderstorms. Although rare, these storms are intense and long-lived,
typically producing rain over an 18-48 hour period at a rate of upwards to 1-2 inches per hour. Field observations
suggest that equally intense, but short-lived storms (e.g., a few hours) produce no appreciable runoff. Essentially
the high infiltration capacity and porosity of the Keanakako'i tephra effectively inhibits runoff except in the most
extreme environmental conditions. This implies that the storms on Mars that created the valley networks were
comparable to some of the worse storms on Earth, or, possibly, the release of water from snowmelt was
somehow catastrophic.
http:www.nasm.si.edu/ceps/research/earth/craddock_kaudesert.cfm
P41A-07
A Global Look at the Composition of the Martian Surface
The Mars Odyssey spacecraft gamma-ray spectrometer (GRS) is providing us with our first global database of the chemical composition of Mars. GRS has measured the concentrations of K, Th, Fe, Si, Al, Ca, Cl, and H in the upper few tens of cm at an intrinsic instrumental spatial resolution of about 500 km. The surface is compositionally heterogeneous, but summing the data into 5x5 degrees bins shows that each element has close to a normal distribution. K and Th are strongly correlated, Cl and H, and Si and Fe are weakly correlated, and Cl and Si are weakly anti-correlated. Correlations among other elements are statistically less significant. Cluster analysis on the elemental data indicates that in spite of the weak elemental correlations and normal distributions, large areas are similar to each other in composition. The clusters differ in key elemental variables, such as the concentrations of one or two elements (e.g., K, Th, and Fe) or elemental ratio (e.g., Ca/Al). We are searching for areas in which two or three elements all differ from the mean by more than one standard deviation. These areas are much smaller than those identified by cluster analysis, typically containing only several 5x5 grid points. The high Cl (0.2 to 0.8 wt%) and H2O (1.5 to 7 wt%, calculated from H concentrations) indicate the ubiquitous presence of the products of aqueous alteration. An initial (very rough) attempt to correct for such products (up to 10-15 wt% of each GRS measurement) using meteorite and landing-site data suggests that the primary igneous rocks are largely olivine basalts with a range in Fe/Mg and Al concentrations, similar to aqueously unaltered rocks reported at Gusev and Meridiani. The aqueous alteration is widespread but did not significantly affect the K/Th ratio, indicating either highly acid conditions or short wet periods. We will be able to determine the concentrations of S and U over large regions (but not be able to make maps). These elements will shed more light on the conditions of aqueous alteration on Mars.
P41A-08
Zinc on Mars
Unambiguous detections of zinc on Mars, with certain measurements in excess of 1500 ppm, have been established by the Alpha Particle X-ray Spectrometers onboard the Mars Exploration Rovers. Several distinct trends in the behavior of zinc are evident in this data set: (1) Zn is clearly correlated with sulfur, chlorine, and, as measured by the Moessbauer spectrometers, a nanophase ferric oxide in over 50 analyses of martian soils at landing sites separated by 180 degrees of longitude. This relationship suggests that Zn, S, and Cl may be volcanic emissions condensed on soil grains and/or that these components (including the nanophase iron oxide) are associated with minor amounts of weathering in these ferrous iron dominated samples. No distinct sulfate, chloride, or zinc mineral phase is present in these soils. (2) A certain class of rocks (Wishstone/Watchtower) at the Gusev Crater landing site, characterized by elevated P and Ti with Cr at or near the detection limit, shows higher levels of Zn for samples with a higher ratio of ferric to total iron. In this case, Zn is an indicator of alteration in these nearly isochemical samples. (3) In the Wishstone/Watchtower samples and in other altered rocks (Clovis class), Zn is correlated with Cl. In terrestrial Cl-rich fluids, the Zn:Cl ratio is an indicator of the temperature of the fluids which interacted with the deposits. The slope of the Zn-Cl line in these rocks is shallower than the fit through the soil data, consistent with a lower temperature origin of this zinc signature. Finally, (4) the highest levels of Zn detected on Mars thus far approach 2300 ppm and are correlated with Cl. In this sample, hematite dominates the iron mineralogy, and thermal alteration is suggested.
P41A-09
Water in the Equatorial Regions of Mars
The Gamma Subsystem (GS) of the Gamma Ray Spectrometer (GRS) suite of instruments on board the 2001 Mars Odyssey spacecraft has detected the signature of hydrogen on Mars. This signature is the 2223 MeV gamma ray given off by hydrogen when its nucleus is excited by capture of a thermal neutron. The number of such gamma rays emitted is not only a function of the concentration of hydrogen, but also of the number of cosmic rays, the atmospheric composition and thickness, and the composition of the martian regolith, all of which are involved in producing and moderating the thermal neutrons responsible for raising the hydrogen nucleus to an excited state. Fortunately, we are able to take these parameters into account and normalize our results since silicon produces gamma rays by both inelastic scatter and thermal capture, is relatively evenly distributed, and has been measured in situ. We report the results of this detection as the equivalent wt% of water in the martian regolith for the equatorial region (~±45°). Values range from a low of 1.5 (± 0.3) up to 7.5 (± 0.6) wt%. Results assume that any water is uniformly distributed within the top meter or so of the surface, the depth to which gamma ray spectrometry is sensitive. Uncertainties in the data are seen to be relatively small, on the order of ±10% relative, and are dominated by counting statistics. Thus the regions of greatest uncertainty are those areas where the atmospheric thickness is greatest, e.g. Hellas Basin, and are well known. Our results for water concentration are generally lower than those reported by the Neutron Spectrometer (NS) aboard the same spacecraft. It has been suggested that variation in water content with depth, having a wetter layer buried below a dryer surface layer, might account for this discrepancy. However, such a distribution would result in our measured gamma ray flux returning a lower derived value for the concentration, exacerbating the problem. Hydrogen is the most variable of any of the elements that the GS has mapped to date, ranging over a factor of 5. High concentrations of water occur in Arabia Patera and Apollinaris Patera. The distribution of water generally correlates with that of chlorine particularly within Apollinaris Patera and to a lesser extent in Arabia Terra. There are no significant correlations of water with the other elements GS has mapped to date: iron, silicon, potassium and thorium.
P41A-10
A Mechanism for Mars Subsurface Warming at Low Obliquity: Its Effects on the State and Distribution of Volatiles
We present a modeling study of a mechanism that has not previously been considered but is likely to have generated significant subsurface warming during the periodic intervals when Mars' obliquity was lower than 25°. Orbital dynamics calculations show that Mars' obliquity, which is currently 25°, oscillates between 10° and 45° - with a dominant periodicity of ~120,000 years and a modulation period of ~1.3 million years - due to long-term perturbations by the other planets [1]. The present Martian atmosphere is 95% CO2 with a mean surface pressure of 6 mbar, but model calculations [2] show that it could drop to as low as 0.3 mbar at low obliquity because the global surface pressure would be controlled by the annual-average temperature of the perennial CO2 ice at the poles [3]. At such low pressures, the thermal conductivity of a porous regolith can be significantly reduced as the mean free path of gas molecules approaches the size of pore spaces [4]. This decreased conductivity leads to increased subsurface temperatures as the geothermal gradient steepens to maintain a constant internal heat flux (estimated to be ~ 30 mW/m2). We have performed model simulations of the resulting time evolution of subsurface temperatures, ground ice, and adsorbed volatiles. This mechanism could explain many of the geomorphological features attributed to liquid water without invoking a thicker atmosphere or increased geothermal heat flow. [1] Laskar, J. et al., Icarus 170, 343-364, 2004. [2] Manning, C. V. et al., Icarus 180, 38-59, 2006. [3] Toon, O. B., et al., Icarus 44, 552-607, 1980. [4] Presley, M. A. and P. R. Christensen, JGR 102, 6535-6549, 1997.
P41A-11
On the Universality of the Elemental Depletion Pattern Between a Star and its Rocky Planets
The elemental abundances of the Sun provided the raw material for the terrestrial planets. The most obvious pattern is the extreme depletion in the terrestrial planets of the most volatile elements: hydrogen (H) and the noble gases (He, Ne, Ar). The abundances of the most refractory elements, (Mg, Si, Fe Al, Ca, Ni, Cr, Ti and Co) are virtually identical in the Sun and terrestrial planets. The distribution of elemental abundances in Mercury, Venus, Earth and Mars is a measure of how different terrestrial objects can be, starting from the same solar material. The observed range then gives us preliminary estimates of the range of elemental depletion in the terrestrial planets orbiting nearby stars. An increasingly large body of observations of circumstellar accretion disks, including infra-red spectroscopy sensitive to mineralogy suggests that the fundamental aspects of the accretion and fractionation processes that led to the depletion pattern in our Solar System are universal. Elemental volatility is universal within the range of temperatures and pressures expected in protoplanetary disks. That is, terrestrial planets around other stars will have refractory elemental abundances that match their host stars. Also, terrestrial planets will be severely depleted in the noble gases compared to their host stars. These simple plausible examples have major implications for the composition of terrestrial planets orbiting other stars - - stars whose relative abundances of refractory elements can vary by factors of at least two compared to the Sun. Based on this and other evidence we assess the universality of the elemental depletion pattern seen in our Solar System.