P14B-01 INVITED
Radar techniques to study subsurfaces and interiors of the solar system objects.
The radar techniques are widely used in the planetary exploration to map the surfaces. The observations from Earth or from spacecrafts were developed during the last decades. However, the idea to use this technique to study the subsurface started to develop during the last 10-15 years. The ability of the radio waves to penetrate the ice, permafrost and arid surface was at the origins of the development of the Ground Penetrating Radars (GPR) with a large number of the scientific work and industrial applications on Earth. The measurements from the surface can not replace the global mapping from orbiting platforms. In this presentation, on the example of MARSIS radar on the Mars Express mission measurements we evaluate the general capabilities of radar sounders for planetary exploration. The CONSERT is the experiment on board of the ROSETTA mission that will provide information about the deep interior of the comet (Kofman et al, 1998, 2007). The CONSERT instrument is an original concept of spaceborne transmission radar based on the propagation throughout the nucleus while the classical radars are based on the reflection. In this experiment, an electromagnetic signal is transmitted between the lander, located on the comet surface, and the orbiter. The transmitted signal will be measured as a function of time and as a function of the relative position of the orbiter and the lander for a number of orbits. Any signal that has propagated through the medium contains information concerning this medium. With a sufficient number of orbits one will be able to obtain many cuts of the interior of the comet and therefore to build up a tomographic image of the interior. On the CONSERT experiment example we discuss the main advantages and difficulties of the techniques using radiowaves to study the interior of asteroids and comets. The capacity of radar technique to do the tomography of the interior of the asteroids and comets is emphasized.
P14B-02 INVITED
Results From the SHARAD Sounding Radar Experiment at Mars
The SHARAD radar on the Mars Reconnaissance Orbiter has carried out successful subsurface soundings in a number of locales since operations began in late 2006. NORTH POLAR LAYERED DEPOSITS (NPLD): These consist of a thick (up to 2 km+), layered ice-rich unit (Apl) overlying a basal unit (BU), which differs from APl, principally in its lower albedo but additionally because it is platy and irregular compared to Apl. The circumpolar erg, most prevalent at Olympia Undae, is likely material shed from the BU. SHARAD shows that Apl is divided into a finely-layered upper facies of variable thickness, containing dozens of apparent reflectors that likely under-resolve a more finely layered ice-dust sequence, and a lower facies, typically a kilometer or more thick, which contains a limited number of distinct reflectors. SHARAD shows that BU is up to approximately a kilometer thick under the main lobe of the NPLD and is largely absent beneath the minor lobe (centered on the prime meridian), as was suggested earlier by mapping exposures in chasmata on the periphery of the NPLD. SHARAD does not see a sharp reflector at the base of the BU, likely due to volume scattering within the BU, but its sister radar MARSIS, operating at lower frequencies, does see such an interface and its extension beneath Olympia Undae. SOUTH POLAR LAYERED DEPOSITS (SPLD): SHARAD typically does not penetrate to the base of the SPLD, but maps fine-scale layering within this unit. Dipping layers are truncated at the surface and at depth there is good evidence for angular unconformities in the layers, which are likely tied to surface erosion followed by new episodes of ice and dust deposition. AMAZONIS PLANITIA: This roughly circular lowland north of the hemispheric dichotomy boundary is one of the flattest places on Mars, with a smooth cover of sediment. Younger flow-like features with ridged and platy surface structures occur in southwestern Amazonis. These flows are widely held to be volcanic, but some workers suggest that they are instead related to past and/or present ground ice. SHARAD shows clear evidence in places for a subsurface reflector at ~40-90 m depth that is well correlated with areas of high Earth-based radar backscatter at 12.6-cm wavelength, likely indicative of lava flows. Loss tangent estimates are in the range of lunar basalts and seem to preclude a major component of ice. ELYSIUM PLANITA: SHARAD has detected a subsurface reflector as deep as 200 m over a ~500,000 km2 area. The evidence from SHARAD is consistent with volcanic material, not ice-rich material, for the intervening layer.
P14B-03
An Overview of MARSIS and SHARAD Radar Sounding Observations of the Polar Deposits of Mars
A recent development in the exploration of Mars has been the deployment of two orbital subsurface radar sounding instruments, MARSIS (Mars Advanced Radar for Subsurface and Ionospheric Sounding) on Mars Express, and SHARAD (Shallow Radar) on the Mars Reconnaissance Orbiter. Both experiments are fully operational, MARSIS since July 2005, and SHARAD since October 2006. The two radars are complementary, with MARSIS operating at deep-sounding low frequencies (2-5 MHz) with low vertical resolution (100 m), and SHARAD operating at a shallower-sounding higher frequency (15-25 MHz), but with much higher vertical resolution (10 m). The materials on Mars that are most consistently amenable to subsurface sounding are found in the polar regions. These include the several-km-thick, water-ice-rich polar layered deposits (PLD), and in the south, a surrounding ancient sedimentary terrain known as the Dorsa Argentea Formation. MARSIS sounding of the PLD consistently reaches the base of the deposits, to an interface between the ice-rich PLD materials and a presumably less ice-rich lithic substrate. This interface lies at a maximum depth of more than 3 km in both polar regions. The signal power reflected at the basal interface indicates minimal attenuation in the PLD material, consistent with a sediment-poor (less than 0.1) composition. No evidence is seen of liquid zones at the PLD base. Internal banding related to the layered structure of the deposits is observed in both MARSIS and SHARAD data, with the high vertical resolution SHARAD data revealing spectacular detail. Preliminary correlation of the internal radar banding with optical images of exposed layers indicates the bands are related to packets of layers that show a consistent erosional style in outcrop. SHARAD data show clear evidence of erosional unconformities, but evidence of deformation due to flow is sparse at best. In the north PLD, MARSIS data commonly show a diffusely reflecting layer above the basal interface. The SHARAD signal only partially penetrates this lower layer. The map distribution and thickness of this radar layer suggest a correlation with the previously recognized "basal unit" that in optical images appears to have a large component of incorporated sediment.
P14B-04
Marsis Data Inversion Approach: Preliminary Results on Mars South Polar Region
An approach to the inversion of the data available from the MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) instrument on Mars Express is described. The data inversion gives an estimation of the materials composing the different detected interfaces, including the impurity (inclusion) of the first layer, if any, and its percentage, by the evaluation of the values of the permittivity that would generate the observed radio echoes. The data inversion method is based on the analysis of the surface to subsurface power ratio and the relative time delay as measured by MARSIS. A volume scattering and a multilayer analysis has been performed in order to analyze the influence of these scattering process on the obtained results. The data inversion has been performed at several frequencies to estimate the frequency dependent parameters affecting the behaviour of the radar echoes. The main complexity, pertaining to the data inversion, is related to the accuracy needed on the values of the dielectric constant on the surface (ε'm(0)), as well as on the accuracy in the radar data influenced by various causes as, for instance, the ionosphere residual distortion. Taking into account that along the orbits the echo frames exhibit a non stationary behaviour, due to the shape of the surface and subsurface, in order to obtain a proper inversion, the frames have been selected only in regions of MARS that are moderately flat as can be determined, a priori, from MOLA data and by the echoes' behaviour. Few models of the subsurface has been considered and a list of inclusion material considered as end to end members of possible scenarios have been assumed.
P14B-05
Radar Subsurface Sounding Over the Putative Frozen Sea in Cerberus Palus, Mars
We present observations acquired by the orbiting radar sounders MARSIS and SHARAD over Eastern Elysium Planitia, Mars. This area encompasses Cerberus Palus, where the Mars Express High Resolution Stereo Camera acquired images that were interpreted as evidence for a frozen sea close to Mars' equator. MARSIS and SHARAD are synthetic-aperture, orbital sounding radars, carried respectively by ESA's Mars Express and NASA's Mars Reconnaissance Orbiter. They work by transmitting a low-frequency radar pulse that is capable of penetrating below the surface, and is reflected by any dielectric discontinuity present in the subsurface. Whereas MARSIS is optimized for deep penetration, having detected echoes down to a depth of 3.7 km over the South Polar Layered Deposits, SHARAD is capable of a tenfold-finer vertical resolution, namely 15 m or less, depending on the dielectric constant of the material being sounded. MARSIS is capable of transmitting at four different bands between 1.3 MHz and 5.5 MHz, with a 1 MHz bandwidth. SHARAD operates at a central frequency of 20 MHz transmitting a 10 MHz bandwidth. Mapping the location of subsurface echoes detected by either instrument, we find that there are different structures at different depths, with a deeper interface seen by MARSIS in the westernmost part of the area under examination, and shallower reflectors found over parts of Cerberus Palus by SHARAD. Analyzing the properties of the radar signal, we find that SHARAD echoes from the easternmost part of Cerberus Palus are stronger than those detected over other areas. One possible explanation is that the material being sounded through is a more transparent medium, such as dirty ice or very porous rock or regolith. At this stage, however, other interpretations consistent with the data exist and cannot be ruled out. We discuss the issue of the nature of the material in the context of what is known of the geologic history of Elysium.
P14B-06 INVITED
Radar Sounding at Decameter Wavelengths: Applying the lessons learned at Mars to the investigation of icy bodies
Following the pioneering radar sounding results from the Apollo Lunar Sounder Experiment (ALSE) and recently the two successful Mars Advanced Radar Sounder for Subsurface and Ionospheric Sounding (MARSIS) and Shallow Radar (SHARAD) radar sounders at Mars, it is evident that radar sounders can provide unique information about the interior of icy bodies. The next target for the radar sounder are the icy moons of Jupiter and/or Titan and possibly Earth. In this talk, we will provide a background on MARSIS and SHARAD instruments and provide representative examples from both instruments. Finally, the relevance of lessons learned from these two instruments to future sounding mission to icy bodies will be discussed.
P14B-07 INVITED
Array Processing for Radar Clutter Reduction and Imaging of Ice-Bed Interface
A major challenge in sounding of fast-flowing glaciers in Greenland and Antarctica is surface clutter, which masks weak returns from the ice-bed interface. The surface clutter is also a major problem in sounding and imaging sub-surface interfaces on Mars and other planets. We successfully applied array-processing techniques to reduce clutter and image ice-bed interfaces of polar ice sheets. These techniques and tools have potential applications to planetary observations. We developed a radar with array-processing capability to measure thickness of fast-flowing outlet glaciers and image the ice-bed interface. The radar operates over the frequency range from 140 to 160 MHz with about an 800- Watt peak transmit power with transmit and receive antenna arrays. The radar is designed such that pulse width and duration are programmable. The transmit-antenna array is fed with a beamshaping network to obtain low sidelobes. We designed the receiver such that it can process and digitize signals for each element of an eight- channel array. We collected data over several fast-flowing glaciers using a five-element antenna array, limited by available hardpoints to mount antennas, on a Twin Otter aircraft during the 2006 field season and a four-element array on a NASA P-3 aircraft during the 2007 field season. We used both adaptive and non-adaptive signal-processing algorithms to reduce clutter. We collected data over the Jacobshavn Isbrae and other fast-flowing outlet glaciers, and successfully measured the ice thickness and imaged the ice-bed interface. In this paper, we will provide a brief description of the radar, discuss clutter-reduction algorithms, present sample results, and discuss the application of these techniques to planetary observations.
P14B-08
Geophysical Investigations of Ground Ice in the Arctic: Considerations for Mars
Our understanding of Mars has been advanced with the continuing successes of MARSIS and SHARAD. As we move forward with studying ice deposits on Mars, it is important to consider how to efficiently collect data with available (or future) instrument resources. Although a suite of instruments are currently operating at Mars, little is known about the shallow subsurface, up to depths of several meters, except where outcrops can be extrapolated into the subsurface. When considering deposits of ground ice that are most accessible, it will be those encountered within a few meters of the surface that require the least amount of energy and effort to sample or mine, whether by robots or humans. The only planned mission that will investigate the top meters of Mars is the ESA ExoMars rover which includes the WISDOM ground penetrating radar (GPR). In order to understand how to quickly and efficiently detect potential resources of ground ice, field studies in the Mackenzie Delta, NWT, Canada, were undertaken using a combination of ground penetrating radar and capacitive-coupled resistivity (CCR) measurements. We will present survey data collected along coincident transects with commercial GPR and CCR systems at a variety of locations and ground ice settings throughout the Mackenzie Delta. In addition, we will show data collected with the Strata Mars GPR prototype antenna which produced data comparable to (and sometimes indistinguishable from) the commercial GPR antennas. Part of the motivation for using these two geophysical techniques was to demonstrate the capabilities of combined measurements to provide information about ice content and distribution beyond what could be accomplished using either technique alone. In addition to showing that combined GPR and CCR geophysical surveys have the ability to map massive ground ice, ice-rich sediments, ice wedges, thermokarst, and basic stratigraphic relationships, field measurements also reaffirmed that these geophysical measurements are a fast, relatively easy, and arguably necessary precursor to more destructive, complicated, and expensive drilling or mining operations.