P11B-0537
Radar backscattering by densely packed particulate surfaces: first exact theoretical results
Analyses of radar backscattering by densely packed particulate surfaces based on direct solutions of the Maxwell equations are virtually absent. This makes uncertain the correctness of conclusions derived on the basis of the conventional theories of radiative transfer and coherent backscattering applicable to sparsely distributed scatterers. To address this fundamental problem, we use the numerically exact superposition T-matrix method and perform extensive computations of electromagnetic scattering by a 3D volume filled with randomly distributed wavelength-sized particles. These computations are used to simulate and analyze the effect of randomness of particle positions as well as the onset and evolution of various multiple-scattering effects with increasing number of particles. Our exact results illustrate and substantiate the methodology underlying the microphysical theories of radiative transfer and coherent backscattering. Furthermore, we show that even in densely packed media, electromagnetic waves multiply scattered along strings of widely separated particles still provide a significant contribution to the total scattered signal and thereby make quite pronounced the classical radiative transfer and coherent backscattering effects. The model results are consistent with the values of the linear and circular polarization ratios observed for various ice-covered planetary surfaces.
P11B-0538
Modeling Lunar Radar Scattering From Icy Regoliths
For lunar orbital synthetic aperture radars, such as the Chandrayaan Mini-RF operating at S-Band (13cm) wavelength and the Lunar Reconnaissance Orbiter Mini-RF operating at S-Band and X-Band (3-cm) wavelengths, it is important to understand and model the radar backscattering characteristics of the icy regoliths. If ices in the permanently shadowed areas of the lunar poles backscatters like the ices on Mercury, Mars and the Galilean satellites, then it will have a substantial radar enhancement characterized by a Circular Polarization Ratio (CPR) greater than unity. We examine the possibilities that these distinct signatures may be diminished by factors such as a thin regolith covering and/or the ice occupies small patches within a larger radar pixel. Our first model for scattering from lunar surface assumes a simple mixing model consisting of diffuse and quasi- specular components. The quasi-specular component results from the surface and sub-surface layers that are oriented perpendicular to the radar's line-of-sight. The diffuse component associated with either rocks or ice is assumed to be uniformly bright, where backscatter is proportional to the cosine of the incidence angle. Rocks are assumed to have CPRs of unity while ices are assumed to have CPRs of 2 like those observed on Mercury, Mars and Galilean Satellites. This first model shows that radar signatures for ice and rocks are separable if the depolarized (SC, same sense circular) enhancements are larger than about 4 and are indistinguishable for smaller depolarized (SC) enhancements. Our second model addresses CPR changes for ice filling the pores of the regolith. Here only the quasi-specular backscatter from the surface and the diffuse backscattering from sub-surface rocks will change with increased abundances of ice in the regolith. This model indicates that only small indistinguishable changes in CPRs would occur.
P11B-0539
Effects of Impact Gardening on the Coherence Length of Ice Deposits in Lunar Cold Traps
Volatiles that may be present in permanently shadowed craters on Mercury and the Moon appear to be quite different from one another based on radar observations. Mercury appears to have relatively pure deposits buried beneath 20-30 cm of dry regolith. These deposits are thought to contain water ice, but other constituents are also possible. In contrast, if ice exists on the Moon, the observations indicate that it is buried, patchy, and/or not very pure. Although some radar data may be indicative of ice deposits on the Moon from Clementine, they are inconclusive regarding the contents of the cold traps because similar signals are found in locations where ice is not expected and may be due to blocky regolith. Neutron measurements indicate that if any lunar ice exists in the topmost meter, it is buried beneath ~10 cm dry regolith and has a concentration of around 0.5 - a few wt. %. Nonetheless, the distribution of volatiles within permanently shadowed regions on Mercury and the Moon is clearly quite different. We perform 2-D Monte Carlo modeling of the evolution of ice layers on the Moon over time due to impact gardening to examine the relationship between the coherence length and time. The model traces the water content as a function of depth in the lunar regolith in several columns of regolith at set spacing intervals. An initial column ice profile is assumed, for example reflecting ice layer(s) deposited by comets, for each regolith column. The program then simulates a series of impacts onto the region based on the crater frequency function. Each column is modified appropriately for each impact. We examine the ice profiles of the different regolith columns as a function of time, correlating ice thickness, peak concentration, depth, and total ice content over the lateral spacings of the columns. This provides an appropriate view of how well ice layers remain coherent as a function of time, initial thickness, initial concentration, and lateral distance. This information will aid in the interpretation of radar data of lunar ice deposits. We constrain the initial thickness of an ice layer as a function of age that would be consistent with the radar measurements of the Moon.
P11B-0540
Imaging of the Internal Structure of Comet 67P/Churyumov-Gerasimenko from Radiotomography CONSERT Data by Using Grid Computing Techniques (Rosetta Mission).
We investigate the inverse problem of imaging the internal structure of comet 67P/Churyumov-Gerasimenko from radiotomography CONSERT data (radiowave transmitted back and forth from the Rosetta orbiter to the Philae lander through the comet) by using grid computing techniques. For this purpose we use a massively parallel approach based on the discretization of the interior dielectric permittivity and of the shape of the comet, as well as the interior /exterior electric field, along suitable basis of spectral functions, in the frame of the Helmholtz equation.
P11B-0541
Probing cometary ice: Finite Difference Time Domain Simulation of Radar Wave Propagation Through Comet Nuclei Dielectric Models
The 90 MHz radar-wave experiment, CONSERT (COmet Nucleus Sounding Experiment by Radiowave Transmission), on board the Rosetta mission (ESA, 2004) is expected to probe the nucleus of the comet 67P/Churyumov-Gerasimenko (67P/C-G) to reveal information on its physical properties, chemical composition, and internal structure [Kofman et al., 1998]. This investigation assesses the potential to recognise lithological structure in the comet nucleus with a radar experiment such as CONSERT. Radar simulations at 90 MHz were performed with a Finite Difference Time Domain (FDTD) method. The amplitude and losses of the transmitted and reflected electric field components of an incident radar pulse were evaluated as a function of time. Seven different dielectric models of sections of a hypothetical comet nucleus were used, representative of existing theories of comet nuclei. Values of dielectric constant assigned to these models are based on mixing laws for a porous mixture of ice and meteoritic dust, employing laboratory measured values of relative permittivity for mainly chondritic meteorites. Our results confirm that structural differences such as layers or inclusions are discernable from transmitted and reflected radar signals at 90 MHz, the central frequency of the CONSERT instrument. Such simulations help to constrain the ambiguities that might exist in future radar data associated with the nature of the comet nuclei, whether conglomerate or layered in nature.
P11B-0542
Parametric Dielectric Map of the Martian North Polar Layered Deposits in Support of the MARSIS and SHARAD data analysis
Laboratory dielectric characterizations of Ice-dust mixtures are crucial for the quantitative analysis of radar sounding data as for the case of the MARSIS and SHARAD experiments. Understanding the range of the dielectric properties of the Martian northmen Polar layer deposits as well as their geographical an vertical distribution result in a better topographical mapping of the basement material below the northern polar cap and help constrain the ambiguities on the identification of layering and any potential subglaciar melting. In order to achieve this task, we constructed first order modeled maps of the surface dielectric properties oh the NPLD. We first used the recent Mars Global Surveyor Thermal Emission Spectrometer (TES) thermal inertia observations in order to derive a map of the dust mass fraction in the ice at the top of the permanent cap. Then we used parametric laboratory measurements of the dielectric properties of Martian polar ice analogs with various temperatures, radar frequencies and mass fractions and compositions of dust in order to obtain the parametric dielectric maps. Thermal inertia maps have been derived from recent TES observations of the surface temperatures of Mars taken over three Mars-years from orbit 1583 to 24346. Laboratory dielectric characterization of ice-dust mixtures has been performed using TES dust calibration samples provided by the ARES group at NASA JSC. Our Maps suggest that surface dielectric properties of the northern Polar cap ranges from 2.72 to 3.23 in the 2-20 MHz band for a dust inclusion typical of Martian basalt. Parametric maps of loss tangent, penetration depth for several dust types will be presented at the conference.
P11B-0543
SHARAD radar stratigraphy of the Martian North Pole
SHARAD (SHAllow RADar) is a nadir looking synthetic aperture subsurface sounding radar and altimeter provided by the Italian Space Agency (ASI) to NASA's Mars Reconnaissance Orbiter (MRO). The primary objective of SHARAD is the investigation of the subsurface at shallow depth to detect geological signatures for water reservoirs. The Mars polar regions and their ice caps are among the highest priority targets for SHARAD. In its first several months of data acquisition, SHARAD made many successful observations of the Martian north pole, detecting the diverse stratification of the polar layer deposits (NPLD) down to hundreds of meters. Some spacecraft-rolled observations resulted in increased performance, allowing identification of even more complex stratigraphy in the NPLD and variable depth of the basal unit. One major north polar campaign of SHARAD observations targeted the Gemina Lingula region of Mars. In addition to establishing the context for these observations, this paper presents a detailed reconstruction of the subsurface layers as a function of location and geometrical setting based on several crossing groundtracks above Gemina Lingula. This allows a three dimensional view of the sequences revealed by the radar to be compared with surface information. Moreover, these orbit crossovers permit the systematic removal of clutter and noise, yielding more precise measurements of the subsurface layer depths.
P11B-0544
Internal Structure of the North Polar Layered Deposits on Mars From SHARAD Observations
The Shallow Radar (SHARAD) instrument onboard the Mars Reconnaissance Orbiter is revealing detailed layering patterns within the North Polar Layered Deposits (NPLD) in Planum Boreum. Since the beginning of its primary science mission in November of 2006, SHARAD has acquired over 400 subsurface sounding observations (radargrams) that cross the NPLD. Each radargram consists of a two-dimensional profile beneath the instrument's ground track and shows a series of returns corresponding to dielectric contrasts in the subsurface to depths of 2 to 3 km. We have imported a subset of these data together with associated location information into an interactive subsurface data interpretation software package, thereby allowing us to delineate and map horizons and faults within the NPLD in three dimensions. Our initial results show: 1) several discrete units of sub-parallel reflections separated by regional unconformities and situated above a diffusely reflective zone (DRZ) that extends throughout the main lobe of the NPLD and appears to correspond to a previously identified Basal Unit (Byrne and Murray 2002, JGR 107 E6, 5044); 2) apparent large-scale faulting or imbrication of the DRZ in portions of the main lobe to the east of Chasma Boreale; 3) the absence of the distinct basal reflections beneath the main lobe and Olympia Planum that have been identified in MARSIS results (Picardi et al. 2005, Science 310, #5756, pp. 1925-1928; Phillips et al. 2007, LPSC XXXVIII, Abstract 1925); and 4) shallow subsurface layering in portions of Olympia Planum that are proximal to the main lobe. These findings have significant implications for the history of accumulation and erosion of the NPLD, which may provide a record of the global climate history for much of the Amazonian epoch. Efforts to correlate the internal units unveiled by SHARAD with those mapped on the basis of images and other surface data are in progress. Acknowledgments: Thanks to the Italian Space Agency (ASI) and NASA/JPL. http://nathaniel.putzig.com/research/agu2007
P11B-0545
MARSIS and SHARAD radar reflections within Promethei Lingula, South Polar Layered Deposits, Mars
Recently, two sounding radar instruments have begun collecting information about the subsurface of the south polar layered deposits (SPLD) of Mars, which are made up of layers of primarily water ice mixed with small amounts of dust. Both MARSIS, the radar onboard Mars Express (operating at 1.8-5 MHz) and SHARAD, the radar onboard Mars Reconnaissance Orbiter (operating at 20 MHz), detect subsurface reflections at multiple depths within the SPLD in several locations, in particular the Promethei Lingula (90˚-140˚E) region of the SPLD. MARSIS observes up to three strong internal reflections, plus a strong basal reflection at approximately 1.2 – 1.5 km depth (assuming pure ice, consistent with the strength of the basal reflection). Reflections where orbits cross are at the same elevations in each radargram (within 100 m, or the resolution of the data) and the reflections in the remaining orbits are at similar elevations; we therefore conclude that the same reflectors are being observed in each orbit. SHARAD detects many tens of reflections with several packets of multiple reflections separated by non-reflective regions with depth; the lowest reflection is observed intermittently at approximately 1 km depth. Comparisons of neighboring and crossing orbits indicate that MARSIS reflections correlate to the boundaries of packets of reflections in the SHARAD data; whatever change in composition of the SPLD that causes the SHARAD reflections to occur in packets may also be the source of the MARSIS reflections. The deepest observable reflection in each SHARAD radargram is at a shallower depth than the deepest reflection in many MARSIS radargrams. The lack of any deeper reflectors in SHARAD may be due to attenuation of the radar signal; however, the contact between the SPLD and the underlying cratered plains as observed in images and topography data at the margins occurs at a similar elevation to the SHARAD basal reflection rather than the MARSIS basal reflection. Thus, SHARAD may be penetrating to the interface between the base of the SPLD and the underlying plains, and MARSIS may be detecting an ice-rich deposit underlying the SPLD in this region. Such a deposit may be related to the nearby volatile-rich Dorsa Argentea Formation.
P11B-0546
Incoherent Simulator for Mars Surface Applied to the Analysis of Sharad Radar Data
SHARAD (SHAllow RADar) is a Synthetic Aperture Radar (SAR) and Altimeter provided by ASI as a Facility Instrument to NASA's 2005 Mars Reconnaissance Orbiter. Primary objective of this nadir-looking GPR instrument is to map Martian surface and subsurface up to 2 Km depth with vertical resolution of 15 m and horizontal resolution of a few hundred meters (300 m-1 Km). Within this frame, one of the SHARAD goals is to identify Mars dielectric interfaces and to interpret them in terms of the occurrence and distribution of expected materials, including water and ice. The transmitted signal is a chirp with duration of 85 microsec and bandwidth of 10 MHz around a carrier frequency of 20 MHz. The received one is the result of the combination of surface and subsurface echoes and off-nadir surface reflection (clutter), reaching the radar after nadir surface echoes thus appearing as subsurface reflections (artifacts). In order to discern surface and subsurface echoes from clutter, we developed an incoherent simulator for the surface echoes. Input data are: DEM MOLA, SHARAD ancillary data and surface scattering models (Optical geometry, Hagfors). We modeled the Mars area to be simulated with plane square facets whose dimensions are those of the MOLA data set sampling. We applied incoherent backscattering models for rough surfaces to each facet, which is characterized by a point scatterer located in its center and a normal vector. Incoherent sum of the echoes from all point scatterers located in the illuminated area generates as a result a single surface echo. The latter is calculated for each spacecraft orbital position. Output of our processor are SHARAD simulated radar data that we can visualize in order to allow to distinguish artifacts from subsurface reflectors.
P11B-0547
Radar Stratigraphy of Ice on Earth and Mars: What are we Missing? An Evaluation of Multiple Radars and Processing Techniques.
Radar sounding has been used for decades on Earth to map sub-ice topography, yet we are only beginning to fully make use of the information contained within the radar-detected, ice-internal layering. This internal layering serves as a guide to estimate accumulation rates and flow reorganization, to detect geothermal anomalies and to extrapolate ice core results over large regions. Radar layering in snow and ice on Earth is generally caused by variations in acidity due to deposits from volcanic eruptions, changes in ice crystal fabric, or variations in density (near the surface). Radar studies in Antarctica have been undertaken by a variety of means, most commonly airborne systems operating at 60 or 150 MHz, typically with 10 – 15 MHz bandwidths, but also sled-mounted systems generally operating in the 1 – 10 MHz range. The stratigraphy of icy deposits on Mars is also thought to hold important information about past climatic variations there and radar sounding has started to reveal new stratigraphic information to complement optical and spectral studies. Two orbital radar sounders are currently operating at Mars. MARSIS on Mars Express operates at 2 – 5 MHz, while SHARAD on Mars Reconnaissance Orbiter operates in the 15 – 25 MHz band. This is a tremendous advance over our ability to probe the subsurface of Mars just a few years ago; however, we don't know how much information we may be missing due to limited over-ice data at these frequencies on Earth. We therefore examine the impact of different wavelengths, bandwidths, and pulse types on the reconstruction of ice stratigraphy on both Earth and Mars by comparing data obtained from different radar systems over the same locations. Simulated results are also compared, as are the effects of data reduction schemes such as unfocused and focused synthetic aperture radar (SAR) processing.
P11B-0548
The Italian Radio Echo Sounding System : Improvement Solutions.
Radio Echo Sounding (RES) system are widely used RADAR systems in glaciology, to obtain information on the level of the bedrock, the ice thickness and its inhomogeneties, i.e. the internal layering of glaciers and subglacial lake exploration. Since, 1997 the Istituto Nazionale di Geofisica e Vulcanologia (INGV) in Italy was involved in the development of the airborne radio echo sounding system (Glacio RADAR) which is continuously upgraded. This radar was used in several Italian Antarctic Expeditions (1997, 1999, 2001 and 2003). The Glacio RADAR mounted on an aircraft flies at an altitude around 300m above the ice surface during the survey. The first model operates at 60 MHz with a pulse width variable between 0.3μs and 1μs. Two wired folded dipole antennas were mounted beneath the aircraft wings; one for transmission and the other for receiving. The listening time is 64μs which implies a maximum penetration depth (range) in the ice of about 5.3 km. The horizontal sampling rate is 10 traces/s at a mean aircraft speed of about 70 m/s. This would produce roughly 143 traces per kilometre (horizontal resolution of 1 trace every 7 m). The Navigation and geographical information is based on a GPS receiver on board giving longitude, latitude, altitude and time for the acquired radar trace. In this presentation two enhanced RES systems developed by INGV are described. These systems, with a new carrier signal (phased coded), operates at 150 MHz and 300 MHZ with a new antenna system (8 folded dipoles) and with the implementation of powerful signal processing techniques to increase signal to noise ratio. Amplitude information is used to recognise the electromagnetic characteristics of the reflector. This new RADAR will improve horizontal and vertical resolutions in the ice with a capability to discriminate the internal layers (less than 1m) obtaining the measure of the ice accumulation rate through the knowledge of the depth of the known isochronal horizons.
P11B-0549
Planetary Radars Operating Centre PROC
Planetary exploration by means of radar systems, mainly using Ground Penetrating Radars (GPR) plays an important role in Italy. Numerous scientific international space programs are currently carried out jointly with ESA and NASA by Italian Space Agency, the scientific community and the industry. Three important experiments under Italian leadership ( designed and manufactured by the Italian industry), provided by ASI either as contribution to ESA programs either within a NASA/ASI joint venture framework, are now operating: MARSIS on-board Mars Express, SHARAD on-board Mars Reconnaissance Orbiter and CASSINI Radar on-board Cassini spacecraft. In order to support all the scientific communities, institutional customers and experiment teams operation three Italian dedicated operational centers have been realized, namely SHOC, (Sharad Operating Centre), MOC (Marsis Operating Center) and CASSINI PAD ( Processing Altimetry Data). Each center is dedicated to a single instrument management and control, data processing and distribution. Although they had been conceived to operate autonomously and independently one from each other, synergies and overlaps have been envisaged leading to the suggestion of a unified center, the Planetary Radar Processing Center (PROC). PROC is conceived in order to include the three operational centers, namely SHOC, MOC and CASSINI PAD, either from logistics point of view and from HW/SW capabilities point of view. The Planetary Radar Processing Center shall be conceived as the Italian support facility to the scientific community for on-going and future Italian planetary exploration programs. Therefore, scalability, easy use and management shall be the design drivers. The paper describes how PROC is designed and developed, to allow SHOC, MOC and CASSINI PAD to operate as before, and to offer improved functionalities to increase capabilities, mainly in terms of data exchange, comparison, interpretation and exploitation. Furthermore, in the frame of an operative experimental platform, where a specific payload ( to be developed by the Italian Industry) a GPR will be accommodated on-board the Italian Space Agency stratospheric balloon and the data analysed by PROC; as a minimum two flight campaigns over polar regions are foreseen. The system shall be capable of acquiring radar data upon scientists requests in order to help them refine their models, experiment new algorithms, improve data interpretation capabilities. The paper also describes how the system will be integrated in the PROC, sharing the operational resources and aiding scientists to increase their knowledge in the field of surface radar sounding. A specific PROC Web facility is foreseen to allow data gathering, request submission, data exchange and dissemination.