P43A-1381
Volcano Flank Terraces on Mars
Flank terraces are bulge-like structures that occur on the slopes of at least nine large shield volcanoes on Mars, and three on Earth. Terraces have a convex-upward, convex-outward morphology, with an imbricate "fish scale" stacking pattern in plan. They occur at all elevations, are scale-invariant structures, and have similar proportions to thrust faults on Earth. Suggested mechanisms of formation include elastic self-loading, lithospheric flexure, magma chamber tumescence, flank relaxation, and shallow gravitational slumping. Terrace geometries predicted by most of these mechanisms do not agree with our observations, however. Only lithospheric flexure can fully account for terrace geometry on Mars and Earth, and so is the most likely candidate mechanism for flank terrace formation. To verify this hypothesis, we conducted scaled analogue modelling experiments, and investigated the structures formed during flexure. Cones of a sand-gypsum mix were placed upon a deep layer of silicone gel, to simulate volcanic loads upon viscoelastic Martian crust. Key parameters were varied across our experimental program. In all cases convex topographic structures developed on the cones' flanks, arranged in an imbricate, overlapping plan-view pattern. These structures closely resemble flank terraces observed on Mars, and our results provide for a basic kinematic model of terrace formation. Analogue volcanoes experienced a decrease in upper surface area whilst volume was conserved; the contractional surface strain was accommodated by outward verging, circumferentially striking thrusts. The morphology of experimental structures suggests an orientation of the principal stress axes of σ1 = radial, σ2 = concentric, and σ3 = vertical. Elsewhere (J. B. Murray et al., this volume) we detail the relationship between flank terraces and other structures such as pit craters and gräben, using Ascraeus Mons as a case study. We suggest that terraces may influence the distribution and location of these other structures, and thus play a fundamental role in the tectonic development of large shield volcanoes on Mars.
P43A-1382
Tectonic Structures on Ascraeus Mons
Ascraeus Mons is the third largest volcano on Mars, with a volume of 1.1 × 1015 m3 and a vertical relief of 14,900 m. Ascraeus has a convex-upward morphology, a summit plateau, and a nested caldera complex. The volcano is host to a range of structures including pit crater chains, sinuous rilles, concentric gräben, and flank terraces. Pit craters develop when subsurface cavities collapse, whilst sinuous rilles may be channels produced or enlarged by water flow. Elsewhere (P. K. Byrne et al., this volume) we describe flank terraces on Mars, and show that they form in response to flexure of the lithosphere due to volcanic loading. Flexural strain is accommodated within the volcano by a set of tectonic structures that manifest surficially as terraces, which could in turn affect where pit craters and rilles form. Thus, a spatial correlation may exist between flank terraces and the additional surface features of Ascraeus Mons. We investigated this potential correlation by examining the location and distribution of pit craters and rilles on Ascraeus. We constructed a GIS using imagery from the High Resolution Stereo Camera (HRSC) and Context Camera (CTX) datasets. Visible structures were then mapped and classified, and compared to a terrace map derived from the Mars Orbiter Laser Altimeter (MOLA) dataset. Pit craters occur either as discrete near-circular features, or as larger "crater chain" structures. They are generally absent from the volcano's summit, but increase in number towards the base. Many crater chains are spatially coincident with terrace bounding faults on the E flank, where terraces are best preserved. Sinuous rilles are long, rimless channels that issue from many pit craters; concentric rilles can abruptly change orientation and run downslope. Both circumferential crater chains and rilles coalesce into radial gräben on the NE and SW flanks of Ascraeus, forming large, V-shaped vermiform embayments. The distribution and location of pit craters and rilles on Ascraeus Mons thus appear to be influenced by a combination of edifice, flexural, and topographic stresses.
P43A-1383
Amazonian Dike Swarms In Utopia Basin, Mars
Abstract
Hundreds of narrow, linear ridges interpreted as dike segments and dike swarms are found in the transition
zone between Elysium Rise and Utopia basin. The dikes are both modifying and constraining Early
Amazonian flows suggesting intense dike emplacement in the transition zone between Utopia Basin and
Elysium Rise in the Early Amazonian.
Morphology of linear ridges
Single ridges
CTX images reveal that single ridges generally have a sharply defined crest, are up to 30 km long, 200 m-
400 m wide and, according to single MOLA tracks, have a height varying between 5 to 30m.
One of the observed single ridges is emplaced en echelon and another single ridge system penetrates a
lobate flow unit and continues as a ridge on the other side. One singular sharp-crested ridge is also
associated with a rough textured mound with a central ridge, which has been interpreted to be a
möberg ridge.
Mutiple Ridges
Five occurrences of multiple ridge systems were observed within the study area. These usually have a
wedge-like shape, are 15-45 km long and 1-7 km wide being broadest in the middle of the transect.
In the westernmost part of the study area HiRISE images reveal that some of the multiple ridge systems have
a distinct, symmetric fracture indicating that the ridge material is competent. Moreover short stubby flows
originate from some of the ridges.
Origin of the linear ridges
The ridges are most likely to be either dikes or möberg ridges because they are very uniform, linear,
crosscutting different units and sometimes being emplaced en echelon . The observed fractures along the
crest of the ridges are not observed in hyaloclastite ridges on Earth, which probably indicates that the
material is not loose hyaloclastite. This support the conclusion that the observed ridges either are normal
dikes or that they are dikes emplaced subglacially as part of an effusive eruption.
Geologic relationships and preservation
Some ridges clearly crosscut flows that are mapped as Early Amazonian while others have constrained Early
Amazonian outflow activity suggesting intense dike emplacement in the Early Amazonian.
Some of the observed ridges are also crosscutting a rough textured knobby unit, which displays small
elongated ridges, which are interpreted to be yardangs. Moreover different stages of inverted craters are
observed within this unit. This indicates that erosion of geologic units has taken place in the area making it
plausible that normal dikes have been exposed.
Conclusions and implications
The observed linear ridges are interpreted to be single dikes and dike swarms,- either emplaced as normal
dikes or as subglacially in association with an effusive eruption.
The finding of yardangs and different stages of inverted craters indicate that erosion of units has taken place
in the area making it feasible that normal dikes have been exposed. However, evidence for möberg
ridges has been reported within the area making subglacial intrusions also plausible.
class="ab'>
P43A-1384
CRISM-OMEGA Observations of Compositionally Distinct Crater Ejecta in the Syrtis Major Region of Mars.
The Syrtis Major volcanic region on Mars exhibits a number of craters with compositionally distinct ejecta (CDE), showing high concentrations of high-calcium pyroxene (HCP) compared to the general background. The examination of these craters provides insight into the true composition of Syrtis Major and possible temporal changes in spectral signatures of exposed materials that resulted in only the young craters displaying CDE. OMEGA observations have suggested that the Hesperian aged Syrtis Major region has a mafic composition with a pyroxene distribution slightly enriched in HCP (40/60 LCP/HCP) in contrast to the surrounding Noachian aged crust enriched in low-calcium pyroxene (LCP) (60/40 LCP/HCP). This indicates a fundamental difference between the petrology of early crustal formation and Hesperian volcanics. Observations from the CRISM instrument on the MRO spacecraft using MGM data processing indicates that just below a thin surface covering of material slightly enriched in HCP is a volcanic edifice highly enriched in HCP (10/90 LCP/HCP). We are examining these features with TES observations to expand the analysis into the thermal infrared region. This underlying material is only exposed as CDE from young impacts dated to have occurred since the Hesperian-Amazonian boundary (~2Ga). This indicates that the Hesperian volcanism in this region underwent a much greater compositional change, as seen in the pyroxene mineralogy, from the Noachian to the Hesperian than previously thought. Also, crater ejecta exposed since the Hesperian-Amazonian boundary appear less enriched in HCP in near infrared observations than older crater ejecta. While the exact nature of this difference is still unknown, three possibilities have been considered; layering, mixing, and coatings. A compositionally distinct layer could exist throughout Syrtis Major that the craters are tapping, although this layer would need to have been emplaced throughout much of the feature, near surface, late in the volcano's life and without noticeable effect to the surface expression. Given these considerations, the layered model seems unlikely. It is also possible that mixing occurred with surrounding LCP rich material, although the uniform values across Syrtis Major and a sharp mineralogical boundary around its edge make regional scale mixing unlikely. Finally, coatings may have formed near the end of the Hesperian, muting the HCP signature of older deposits. Similar CDE features have been detected in other regions, including Hesperia Planium, Meridiani Planium, and to the south of Valles Marineris, indicating a global effect. Martian global events that could have coincided with the crater type transition include the formation of outflow channels, possibly the last major water release event, and the cessation of Hesperian volcanism, leading to a decrease in volcanic gases in the atmosphere. Laboratory studies have shown that basaltic rock with an initial HCP signal and thin hydrated coatings can experience absorption flattening and addition of 1.9μm water absorption that could be modeled as additional LCP content. Coatings result from either leaching out non Si cations, or the dissolution of the basalt followed by the precipitation of a silica rich coating.
P43A-1385
Spectral and Morphological Analysis of Daedalia Planum Lava Field
Daedalia Planum is one of the Tharsis volcanic plains and is located southwest of the Arsia Mons. According to MOLA data, the flanks of Arsia have an average slope < 5°, while the surrounding regions, including Daedalia Planum, have slopes < 0.5° and commonly < 0.1°. MOC and THEMIS images show a plain covered by a huge number of lava flows. Older and larger lava flows on the field have a length greater than 1500, even if determining their absolute length is difficult as subsequent lava flows have buried the source vents. MEX/OMEGA data reveal that Daedalia Planum lavas have a spectral shapes comparable to those observed in laboratory for rock slabs of Earth's basalts. Moreover most of the Daedalia flows are associated to wrinkly and ropy surfaces, typical of pahoehoe lavas. The Daedalia Planum flow surfaces show several morphological features that remember the inflation fingerprints. This suggests that also Daedalia Planum could have been interested by inflation. However these features appear dissimilar to inflation forms on Elysium Planitia flows. Different degrees of erosion could explain such dissimilarities. In particular Daedalia Planum flow surfaces appear heavily modelled by wind erosion whereas the Elysium Planitia features seem fresher. The different age between the two areas support this hypothesis. Our crater counting dated the most recent Daedalia Planum flows to about 230 Myr , by contrast the Elysium Planitia lava flows range from 100 to 10 My. In conclusion, the inflation process on Martian flows could be more frequent than previously supposed and, consequently, effusion rates and rheological properties of Martian lavas more variable.
P43A-1386
Constraints on the Evolution of the Upper Crust and Volcanic Deposits of Mars
The spatial distributions, abundances, and grain sizes of mineral phases on Mars have important petrogenetic implications for the Martian crust. One of the major science objectives of OMEGA (Observatoire pour la Minéralogie, l'Eau, les Glaces et l'Activité) is to map the mineralogy of the Martian surface, in order to improve understanding of the composition of the crust and the volcanic deposits of Mars. The first global analysis of reflectance OMEGA spectra showed that low albedo regions could generally be grouped into different classes (Poulet et al., JGR 112, 2007): distinct mafic, rock-forming minerals low-calcium pyroxene (LCP), and high-calcium pyroxene (HCP) are identified in equatorial and southern low albedo regions; the regions exhibiting the strongest LCP signatures are found mainly in the ancient Noachian-aged units; olivine with high iron content and/or large grain size (>100 microns) is only detected in isolated areas, olivine (Mg- rich composition) is detected in extensive regions of the pyroxene-rich zones; extended dark regions in the northern plains exhibit shallow pyroxene absorptions. The objective of this presentation is to: 1) determine the modal mineralogy of a variety of volcanic mafic-rich regions using OMEGA data, 2) compare the modal and chemical composition with those derived from Thermal Emission Spectrometer (TES) analyses (McSween et al., JGR 108, 2003; Rogers and Christensen, JGR 112, 2007), 3) identify possible relationship with the diverse compositions of SNCs, 4) evaluate constraints on the chemistry and petrology of the volcanic deposits through time.
P43A-1387
Io from Ground-based Eclipse Observations: Implications for the Eruptive History of Loki
Loki is the most powerful volcano on Io, the most volcanically active body in the solar system. We have been observing infrared thermal emission from Io's volcanoes using NASA's Infrared Telescope Facility (IRTF) for nearly two decades. Measurements of Loki's 3.5 micron brightness from Jupiter occultation lightcurves have indicated that it often erupts in a regular and predictable manner and have been used to constrain models of Loki's eruption behavior, indicating the behavior at this wavelength is consistent with a periodically overturning lava lake. Knowledge of Loki's brightness at additional wavelengths would help to further constrain these models. Our observations also include eclipse images of Io's volcanic emission at 2.3, 3.5, and 4.7 microns, which can also be used to constrain the eruption models if Loki's flux can be separated from that of other volcanoes. We will analyze all of the already obtained eclipse observations of Io and measure Io's brightness at the three wavelengths. We will also determine the number of active volcanoes on a given night based on the occultation lightcurve. From this information, we will determine how to extract approximate Loki brightnesses from the integrated Io brightnesses. Finally, we will compare the calculated multi-wavelength Loki brightnesses to the model of Loki as an overturning lava lake.
P43A-1388
Lava Fountains on Io: Implications for the Interior and Future Observations
Lava fountains provide some of the most spectacular volcanic activity on Io, the innermost large moon of Jupiter. However, properly interpreting observations of this style of eruption is challenging. In the past, infrared emissions from fountains were interpreted using models derived for lava flows. Such modeling has been highly successful for lava lakes and lava flows. However, these models are appropriate for infinite half- spaces of hot lava, not the small droplets that are expected. A simple fountain model, derived from the cooling of small spheres, shows that the earlier temperature estimates were probably >200 °C too high. The 1997 eruption at Pillan Patera was carefully re-examined, since this is the only observation that appeared to require ultramafic eruption temperatures. The new estimates of observational uncertainties, coupled with the new thermal model, allow lava temperatures consistent with basalts or basaltic komatiites. These eruption temperatures suggest that the upper mantle source region is at ~1300 °C. This in turn implies that a small part of the uppermost mantle reaches ~25% partial melting, and interconnected melt extends down to ~600 km depth. The lava droplet model also places difficult constraints on obtaining useful color temperatures from future high-resolution imaging of Ionian lava fountains. A critical goal of such observations would be to detect the highest possible temperatures near the vent to provide better constraints on lava composition. However, if the droplets are similar in size to lunar pyroclastics (i.e., ~100 microns), then the color temperature would drop 100 °C in less than 0.1 seconds. This means that color temperatures should ideally be derived from images acquired simultaneously through different filters. If not simultaneous, the separate colors should be acquired much less than 0.1 s apart. Another procedure that can assist is to obtain images that rapidly alternate between the different colors. These constraints need to be considered by future imaging systems sent to the Jovian system. Eruption temperatures remain the best hope for estimating lava composition since the lava surface is expected to be highly glassy, lacking distinct spectral features.
P43A-1389
Using Lava Tube Skylight Thermal Emission Spectra to Determine Lava Composition on Io: Quantitative Constraints for Observations by Future Missions to the Jovian System.
Deriving the composition of Io's dominant lavas (mafic or ultramafic?) is a major objective of the next missions to the jovian system. The best opportunities for making this determination are from observations of thermal emission from skylights, holes in the roof of a lava tube through which incandescent lava radiates, and Io thermal outbursts, where lava fountaining is taking place [1]. Allowing for lava cooling across the skylight, the expected thermal emission spectra from skylights of different sizes have been calculated for laminar and turbulent tube flow and for mafic and ultramafic composition lavas. The difference between the resulting mafic and ultramafic lava spectra has been quantified, as has the instrument sensitivity needed to acquire the necessary data to determine lava eruption temperature, both from Europa orbit and during an Io flyby. A skylight is an excellent target to observe lava that has cooled very little since eruption (<0.1 K per km from source vent [2]). Using skylights has a number of advantages over outbursts. Lava fountains have a complex physical and thermal structure, and many model inputs can only be roughly estimated. Outburst events are also relatively rare. Finally, fluctuations in fountain activity mean that multi-spectral observations ideally have to be contemporaneous [3] to yield usable results. Skylights provide an unvarying thermal signal on timescales of 1 minute or longer, and expose a restricted range of temperatures close to lava eruption temperature. Skylights are therefore easily discernible against a cool background, and are detectable from great distances at night or with Io in eclipse with imagers covering the range 0.4 to 5.0 μm. To distinguish between ultramafic and mafic lavas, multispectral (or hyperspectral) observations with precise exposure timing and knowledge of filter response are needed in the range 0.4 to 0.8 μm, with (minimally) an additional model-constraining measurement at ~4-5 μm. As with many lava tube systems on Earth, skylights should be common on Io (for example, at Prometheus, Culann and Amirani). The possible superheating of lava prior to eruption complicates the analysis [4], but is likely to be significant only for deep- seated, often explosive, eruptions. Effusive activity at the aforementioned three locations is likely fed from shallow reservoirs [5], minimising superheating effects. This work was carried out at the Jet Propulsion Laboratory-California Institute of Technology, under contract to NASA. AGD is supported by a grant from the NASA OPR Program. References: [1] Davies, A. G., 1996, Icarus, 124, 45-61. [2] Keszthelyi, L., et al., 2006, JGS, 163, 253-264. [3] Davies, A. G., 2007, Volcanism on Io, Cambridge University Press. [4] Keszthelyi, L., et al., 2007, Icarus, 192, 491-502. [5] Davies, A. G., et al., 2006, Icarus, 184, 460-477.
P43A-1390
Lithospheric Structure and Patera Formation on Io: Implications for Future Observations
On Jupiter's moon Io, the interaction between volcanism and tectonism is strongly modulated by the structure of the lithosphere. Previous work has shown that Io's prodigious volcanism leads to the rapid burial of its surface and, consequently, the global subsidence of the materials that compose its lithosphere. This, in turn, generates large horizontal compressive stresses, which drive the thick-skinned thrust faulting that uplifts Io's mountains. Because horizontal compression is an ongoing process that proceeds ever more rapidly with increasing depth, we assume in our model that the lithosphere is pervasively fractured and its stress state is governed by frictional sliding. The result is that pore space within the crust disappears rapidly with depth, and SO2 (which is the dominant volatile species and which melts at a relatively low pressure) is confined to the near-surface region. Thus, Io's lithosphere is compositionally stratified, with a low-density SO2-dominated layer up to a few kilometers thick overlying a denser mafic or ultramafic silicate layer a few tens of kilometers thick. Rising magma should achieve neutral buoyancy near the interface between these two layers, and if that interface is abrupt, the magma may spread laterally there for purely mechanical reasons as well. The heat from these shallow sill-like intrusions is expected to mobilize the overlying volatiles, eventually unroofing the sill or lava lake. The overburden can be removed both by the melting and lateral flow of the volatiles and by their sublimation. (SO2 atop warm silicate lavas is predicted to sublimate at a rate of ~100 m/yr.) Hence, ionian paterae may be more analogous to the depressions formed in ice during terrestrial subglacial eruptions than to true volcanic calderas. These models for Io's lithospheric structure and its patera formation should be tested by future spacecraft missions. Three observations that would be particularly useful are (a) height and slope measurements of patera walls, which will provide a lower limit on the strength of the near-surface materials, (b) repeat stereo imaging of active paterae for ~1 year with a vertical precision of at least 100 m/pixel, and (c) high-resolution imaging of steep (i.e., exposed rock) scarps on mountain flanks, which will provide direct cross-sectional views of deeper parts of the lithosphere.
P43A-1391
The rheology of water-methanol slurries: Implications for cryovolcanism on Titan
Cassini SAR imagery has revealed the presence of landforms on the surface of Titan that may be cryovolcanic flows and domes [1,2]. In order to relate the observed surface features to the geological processes and chemistries that produced them, it is necessary to construct rheological flow models at cryogenic temperatures. We report preliminary cryogenic rheological measurements on a binary 40 wt% methanol-water composition, used as a path finding analog for characterizing the rheological properties of candidate cryo-magmas and eruptant materials [3]. Work by Kargel et al. [4] used a cryogenic rotational viscometer and a viscous drop experiment to determine the viscosity of ammonia-water slurries, a likely composition of Titan cryomagma. This work revealed that the materials in question have viscosities that were controlled by the pure liquid viscosity and the solid fraction, the latter also resulting in shear-rate dependence. Our cryogenic rheological measurements were conducted between 90-300 K using a home- built LN2 cooled cryogenic rotational viscometer system, with data acquisition and control achieved using the National Instruments LabView program. We report the results of a series of measurements performed as a function of temperature and rotational strain rate. The methanol-water mixture exhibited a variety of rheological response behaviors under these experimental conditions; i.e., development of yield stress-like behaviors, shear-rate dependence, and thixotropic behavior, even at relatively low crystal fractions, which to our knowledge have not been previously observed or reported. At fixed shear rate our data are fit well by the Andrade equation, with the activation energy modified by the solid volume fraction. At fixed temperature, depending on shearing history, a Cross model describes our data well over a wide shear rate range. A Bingham plastic model appears to be a good constitutive model for the data measured at high shear rates when the shear was global, but at low shear stresses the approximation becomes inaccurate because the Bingham yield stress is only an approximation to what is actually a high viscosity creep behavior. This yield-stress-like creep behavior implies that initialization of levees in cryolava flows is more likely than would be inferred from previous cryo-rheological studies and may provide a partial explanation for features observed by the Cassini spacecraft on Titan, which are interpreted as steep-sided volcanic constructs [2]. This analysis will be critical in the development of future experiments designed to measure all the parameters controlling cryomagma rheologies for input into flow models. [1] Elachi et al. (2005) Science 308, 970-974. [2] Lopes et al. (2007) Icarus 186, 395-412. [3] Zhong et al. (in review) Icarus. [4] Kargel et al. (1991) Icarus 89, 93-11.