Stewart Mukhopadhyay, Sarah

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Stewart Mukhopadhyay, Sarah

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Stewart Mukhopadhyay

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Sarah

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Stewart Mukhopadhyay, Sarah

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Tidal evolution of the Moon from a high-obliquity, high-angular-momentum Earth
(Springer Science and Business Media LLC, 2016-10-31) Stewart Mukhopadhyay, Sarah; Ćuk, Matija; Hamilton, Douglas P.; Lock, Simon
In the giant impact hypothesis for lunar origin, the Moon accreted from an equatorial circumterrestrial disk; however the current lunar orbital inclination of 5◦ requires a subsequent dynamical process that is still debated. In addition, the giant impact theory has been challenged by the Moon’s unexpectedly Earth-like isotopic composition. Here, we show that tidal dissipation due to lunar obliquity was an important effect during the Moon’s tidal evolution, and the past lunar inclination must have been very large, defying theoretical explanations. We present a new tidal evolution model starting with the Moon in an equatorial orbit around an initially fast-spinning, high-obliquity Earth, which is a probable outcome of giant impacts. Using numerical modeling, we show that the solar perturbations on the Moon’s orbit naturally induce a large lunar inclination and remove angular momentum from the Earth-Moon system. Our tidal evolution model supports recent high-angular momentum 1 giant impact scenarios to explain the Moon’s isotopic composition and provides a new pathway to reach Earth’s climatically favorable low obliquity.
High-Pressure X-ray Diffraction and Raman Spectroscopy of Ice VIII
(American Institute of Physics, 2005) Yoshimura, Yukihiro; Stewart Mukhopadhyay, Sarah; Somayazulu, Maddury; Mao, Ho-kwang; Hemley, Russell J.
In situ high-pressure/low-temperature synchrotron x-ray diffraction and optical Raman spectroscopy were used to examine the structural properties, equation of state, and vibrational dynamics of ice VIII. The x-ray measurements show that the pressure-volume relations remain smooth up to 23 GPa at 80 K. Although there is no evidence for structural changes to at least 14 GPa, the unit-cell axial ratio c/a undergoes changes at 10-14 GPa. Raman measurements carried out at 80 K show that the nu(Tz)A(1g)+nu Tx,yEg lattice modes for the Raman spectra of ice VIII in the lower-frequency regions (50-800 cm(-1)) disappear at around 10 GPa, and then a new peak of similar to 150 cm(-1) appears at 14 GPa. The combined data provide evidence for a transition beginning near 10 GPa. The results are consistent with recent synchrotron far-IR measurements and theoretical calculations. The decompressed phase recovered at ambient pressure transforms to low-density amorphous ice when heated to similar to 125 K.
Martian Subsurface Properties and Crater Formation Processes Inferred From Fresh Impact Crater Geometries
(Meteoritical Society, 2006) Stewart Mukhopadhyay, Sarah; Valiant, Gregory J.
The geometry of simple impact craters reflects the properties of the target materials, and the diverse range of fluidized morphologies observed in Martian ejecta blankets are controlled by the near-surface composition and the climate at the time of impact. Using the Mars Orbiter Laser Altimeter (MOLA) data set, quantitative information about the strength of the upper crust and the dynamics of Martian ejecta blankets may be derived from crater geometry measurements. Here, we present the results from geometrical measurements of fresh craters 3-50 km in rim diameter in selected highland (Lunae and Solis Plana) and lowland (Acidalia, Isidis, and Utopia Planitiae) terrains. We find large, resolved differences between the geometrical properties of the freshest highland and lowland craters. Simple lowland craters are 1.5-2.0 times deeper (>= 5 sigma(o) difference) with >50% larger cavities (>= 2 sigma(o)) compared to highland craters of the same diameter. Rim heights and the volume of material above the preimpact surface are slightly greater in the lowlands over most of the size range studied. The different shapes of simple highland and lowland craters indicate that the upper similar to 6.5 km of the lowland study regions are significantly stronger than the upper crust of the highland plateaus. Lowland craters collapse to final volumes of 45-70% of their transient cavity volumes, while highland craters preserve only 25-50%. The effective yield strength of the upper crust in the lowland regions falls in the range of competent rock, approximately 9-12 MPa, and the highland plateaus may be weaker by a factor of 2 or more, consistent with heavily fractured Noachian layered deposits. The measured volumes of continuous ejecta blankets and uplifted surface materials exceed the predictions from standard crater scaling relationships and Maxwell's Z model of crater excavation by a factor of 3. The excess volume of fluidized ejecta blankets on Mars cannot be explained by concentration of ejecta through nonballistic emplacement processes and/or bulking. The observations require a modification of the scaling laws and are well fit using a scaling factor of similar to 1.4 between the transient crater surface diameter to the final crater rim diameter and excavation flow originating from one projectile diameter depth with Z = 2.7. The refined excavation model provides the first observationally constrained set of initial parameters for study of the formation of fluidized ejecta blankets on Mars.
In Situ Raman Spectroscopy of Low-Temperature/High-Pressure Transformations of H2O
(American Institute of Physics, 2007) Stewart Mukhopadhyay, Sarah; Hemley, Russell J.; Mao, Ho-kwang; Yoshimura, Yukihiro
In situ Raman spectra of transformations of H[sub]2[sub]O as functions of pressure and temperature have been measured starting from high-density amorphous ice (HDA). Changes above T[sub]x[sub], the crystallization temperature of HDA, were observed. The spectra provide evidence for an abrupt, first-order-like, structural change that appears to be distinct from those associated with the transformation between low-density amorphous ice (LDA) and HDA. In separate experiments, in situ Raman spectra of ice XII transformed from HDA have been measured at various P-T regions, in order to improve the understanding of the stability limits of ice XII. The spectra of ices VI and XII differ in shape, but the vibrational frequencies are very close in the same P-T regimes. A metastable phase of ice found to form within the stability field of ice VI appears to be distinct from ice XII.
Modeling Impact Cratering in Layered Surfaces
(American Geophysical Union, 2007) Senft, Laurel E.; Stewart Mukhopadhyay, Sarah
Impact craters are potentially powerful tools for probing large-scale structure beneath planetary surfaces. However, the details of how target structure affects the impact cratering process and final crater forms remain poorly understood. Here, we present a study of cratering in layered surfaces using numerical simulations. We implement the rheologic model for geologic materials described by Collins et al. ( 2004) into the shock physics code CTH; this model includes pressure, temperature, and damage effects on strength as well the option to include acoustic fluidization. The model produces reasonable final crater shapes and damaged zones from laboratory to planetary scales. We show the effects of varying material strength parameters and discuss choosing appropriate strength parameters for laboratory and planetary situations. Results for cratering into idealized terrains with layers of differing material strength are presented. The presence of such layers in the target can significantly alter the ejecta curtain structure and the final crater morphology. Finally, we reproduce the morphologic variations that are observed in small lunar craters by modeling a weak regolith overlying competent rock.
Effects of Planet Curvature and Crust on the Shock Pressure Field Around Impact Basins
(American Geophysical Union, 2009) Louzada, Karin; Stewart Mukhopadhyay, Sarah
We investigate the effects of planetary curvature and the crust-mantle boundary on the shock pressure field around impact basins on Mars using acoustic ray path calculations and hydrocode simulations. Planet curvature and, to a lesser extent, increasing sound speed with depth shallow the zone of wave interference, where shock pressures decay rapidly to the surface. The depth to the interference zone boundary diverges from the flat surface solution for projectile-to-Mars radius ratios greater than ~1% (transient craters greater than ~300 km); the difference increases with distance from the impact point and projectile size. In hydrocode simulations (but not the simple ray path model), the presence of the crust-mantle boundary produces nearly vertical pressure contours in the crust. Around Hellas basin, demagnetization occurs at shock pressures between 1.1 (±0.2) and 3.4 (±0.7) GPa, where the range is due to the uncertainty in the transient crater diameter.
Impact Crater Formation in Icy Layered Terrains on Mars
(Meteoritical Society, 2008) Senft, Laurel E.; Stewart Mukhopadhyay, Sarah
We present numerical simulations of crater formation under Martian conditions with a single near-surface icy layer to investigate changes in crater morphology between glacial and interglacial periods. The ice fraction, thickness, and depth to the icy layer are varied to understand the systematic effects oil observable crater features. To accurately model impact cratering into ice, a new equation of state table and strength model parameters for H2O are fitted to laboratory data. The presence of an icy layer significantly modifies the cratering mechanics. Observable features demonstrated by the modeling include variations in crater morphometry (depth and rim height) and icy infill of the crater floor during the late stages of crater formation. In addition, an icy layer modifies the velocities, angles, and volumes of ejecta, leading to deviations of ejecta blanket thickness from the predicted power law. The dramatic changes in crater excavation are a result of both the shock impedance and the strength mismatch between layers of icy and rocky materials. Our simulations suggest that many of the unusual features of Martian craters may be explained by the presence of icy layers, including shallow craters with well-preserved ejecta blankets, icy flow related features, some layered ejecta structures, and crater lakes. Therefore, the cratering record implies that near-surface icy layers are widespread on Mars.
Shock Properties of H2O Ice
(American Geophysical Union, 2005) Stewart Mukhopadhyay, Sarah; Ahrens, Thomas J.
To understand the mechanics and thermodynamics of impacts on, and collisions between, icy planetary bodies, we measured the dynamic strength and shock states in H2O ice. Here, we expand upon previous analyses and present a complete description of the phases, temperature, entropy, and sound velocity along the ice shock Hugoniot. Derived from shock wave measurements centered at initial temperatures (T-0) of 100 K and 263 K, the Hugoniot is composed of five regions: (1) elastic shocks in ice Ih, (2) ice Ih deformation shocks, and shock transformation to (3) ice VI, (4) ice VII, and (5) liquid water. In each region, data obtained at different initial temperatures are described by a single U-S-D Delta u(p) shock equation of state. The dynamic strength of ice Ih is strongly dependent on initial temperature, and the Hugoniot Elastic Limit varies from 0.05 to 0.62 GPa, as a function of temperature and peak shock stress. We present new bulk sound velocity measurements and release profiles from shock pressures between 0.4 and 1.2 GPa. We report revised values for the shock pressures required to induce incipient melting (0.6 +/- 0.05, 1.6 +/- 0.3 GPa) and complete melting (2.5 +/- 0.1, 4.1 +/- 0.3 GPa) upon isentropic release from the shock state (for T-0 = 263, 100 K). On account of the > 40% density increase upon transformation from ice Ih to ices VI and VII, the critical shock pressures required for melting are factors of 2 to 10 lower than earlier predicted. Consequently, hypervelocity impact cratering on planetary surfaces and mutual collisions between porous cometesimals will result in abundant shock-induced melting throughout the solar system.
Excess Ejecta Craters Record Episodic Ice-Rich Layers at Middle Latitudes on Mars
(American Geophysical Union, 2008) Black, Benjamin A.; Stewart Mukhopadhyay, Sarah
We infer Amazonian climate change events from the presence of fresh craters with excess volumes of ejecta. Using the Mars Orbiter Laser Altimeter data, the geometrical properties of 572 fresh impact craters with rim diameters between 2.5 and 102 km were compiled in lowland and highland plains. The data reveal a class of fresh craters with anomalously high ejecta volumes preferentially located in Utopia Planitia. These fresh, "excess ejecta'' craters have volumes of material above the preimpact surface larger than the crater cavity volumes by factors of 2.5 to 5.8. The excess volume corresponds to an excess thickness of ejecta of about 20 to 100 m averaged over the continuous ejecta blanket. The excess material cannot be accommodated by ejecta bulking alone and requires an external process to increase the apparent volume of the ejecta and/or the uplifted surface. On the basis of the geologic setting, ejecta morphology, and calculations of increased ice stability with burial, we conclude that the most likely origin of the excess ejecta volume is the presence of an ice-rich layer tens of meters thick at the time of impact. The icy layer or a lag deposit is partially preserved beneath the ejecta blanket today. In this scenario, the icy layer has since been removed from areas unprotected by ejecta blankets, creating an apparent preimpact surface lower than the original elevation. The statistical occurrence of excess ejecta craters is consistent with climate model predictions of recent glacial periods on Mars.
Effect of Shock on the Magnetic Properties of Pyrrhotite, the Martian Crust, and Meteorites
(American Geophysical Union, 2007) Louzada, Karin L.; Stewart Mukhopadhyay, Sarah; Weiss, Benjamin P.
We performed planar shock recovery experiments on natural pyrrhotite at pressures up to 6.9 GPa. We find that high-field isothermal remanent magnetization in pyrrhotite is demagnetized up to 90% by shock due to preferential removal of low coercivity components of magnetization. Contrary to static experiments, we do not observe complete demagnetization. Post shock permanent changes in magnetic properties include increasing saturation isothermal remanent magnetization, bulk coercivity and low-temperature memory, and changes in squareness of hysteresis. These changes are consistent with an increase in the volume fraction of single domain grains. The lack of magnetic anomalies over large Martian impact basins is not expected to be solely due to shock demagnetization of the crust. We find that pyrrhotite-bearing rocks and meteorites can retain records of Martian magnetic fields even if shocked to pressures approaching 7 GPa. However, some paleointensity techniques may underestimate this field.