133 research outputs found
Effects of the passage of Comet C/2013 A1 (Siding Spring) observed by the Shallow Radar (SHARAD) on Mars reconnaissance orbiter
The close passage of Comet C/2013 A1 (Siding Spring) to Mars provided a unique opportunity to observe the interaction of cometary materials with the Martian ionosphere and atmosphere using the sounding radar SHARAD (SHAllow RADar) aboard Mars Reconnaissance Orbiter. In two nightside observations, acquired in the 10 h following the closest approach, the SHARAD data reveal a significant increase of the total electron content (TEC). The observed TEC values are typical for daylight hours just after dawn or before sunset but are unprecedented this deep into the night. Results support two predictions indicating that cometary pickup O+ ions, or ions generated from the ablation of cometary dust, are responsible for the creation of an additional ion layer
UWB processing applied to multifrequency radar sounders. The case of MARSIS and comparison with SHARAD
We readapt ultrawideband (UWB) processing to
enhance the range resolution of the Mars Advanced Radar for
Subsurface and Ionosphere Sounding (MARSIS) up to a factor
of 6 (25 m). The technique provides for the estimation of radar
signature over a wider spectrum via the application of wellknown super-resolution (SR) techniques to adjoining subbands.
The measured spectra are first interpolated and then extrapolated
outside the original bands. The revised algorithm includes the
estimation and removal of ionospheric effects impacting the
two signals. Because the processing requires the realignment of
the echoes at different frequencies, we derived the maximum
tolerable retracking error to obtain reliable super-resolved range
profiles. This condition is fulfilled by low-roughness areas compared to MARSIS wavelength, which proves to be suitable for the
application of our processing. Examples of super-resolved experimental products over different geological scenarios show the
detection of shallow dielectric interfaces not visible from original
MARSIS products. Our results are validated by comparison with
the Shallow Radar (SHARAD) data acquired at the crossovers,
demonstrating the potential of the method to provide enhanced
imaging capabilities
Radar sounding using the Cassini altimeter waveform modeling and Monte Carlo approach for data inversion observations of Titan's seas
Recently, the Cassini RADAR has been used as a sounder to probe the depth and constrain the composition of hydrocarbon seas on Saturn's largest moon, Titan. Altimetry waveforms from observations over the seas are generally composed of two main reflections: the first from the surface of the liquid and the second from the seafloor. The time interval between these two peaks is a measure of sea depth, and the attenuation from the propagation through the liquid is a measure of the dielectric properties, which is a sensitive property of liquid composition. Radar measurements are affected by uncertainties that can include saturation effects, possible receiver distortion, and processing artifacts, in addition to thermal noise and speckle. To rigorously treat these problems, we simulate the Ku-band altimetry echo received from Titan's seas using a two-layer model, where the surface is represented by a specular reflection and the seafloor is modeled using a facet-based synthetic surface. The simulation accounts for the thermal noise, speckle, analog-to-digital conversion, and block adaptive quantization and allows for possible receiver saturation. We use a Monte Carlo method to compare simulated and observed waveforms and retrieve the probability distributions of depth, surface/subsurface intensity ratio, and subsurface roughness for the individual double-peaked waveform of Ligeia Mare acquired by the Cassini spacecraft in May 2013. This new analysis provides an update to the Ku-band attenuation and results in a new estimate for its loss tangent and composition. We also demonstrate the ability to retrieve bathymetric information from saturated altimetry echoes acquired over Ontario Lacus in December 2008
Shallow radar (SHARAD) sounding observations of the Medusae Fossae Formation, Mars
The SHARAD (shallow radar) sounding radar on the Mars Reconnaissance Orbiter detects subsurface reflections in the eastern and western parts of the Medusae Fossae Formation (MFF). The radar waves penetrate up to 580 m of the MFF and detect clear subsurface interfaces in two locations: west MFF between 150 and 155◦ E and east MFF between 209 and 213◦ E. Analysis of SHARAD radargrams suggests that the real part of the permittivity is ∼3.0, which falls within the range of permittivity values inferred from MARSIS data for thicker parts of the MFF. The SHARAD data cannot uniquely determine the composition of the MFF material, but the low permittivity implies that the upper few hundred meters of the MFF material has a high porosity. One possibility is that the MFF is comprised of low-density welded or interlocked pyroclastic deposits that are capable of sustaining the steep-sided yardangs and ridges seen in imagery. The SHARAD surface echo power across the MFF is low relative to typical martian plains, and completely disappears in parts of the east MFF that correspond to the radar-dark Stealth region. These areas are extremely rough at centimeter to meter scales, and the lack of echo power is most likely due to a combination of surface roughness and a low near-surface permittivity that reduces the echo strength from any locally flat regions. There is also no radar evidence for internal layering in any of the SHARAD data for the MFF, despite the fact that tens-of-meters scale layering is apparent in infrared and visible wavelength images of nearby areas. These interfaces may not be detected in SHARAD data if their permittivity contrasts are low, or if the layers are discontinuous. The lack of closely spaced internal radar reflectors suggests that the MFF is not an equatorial analog to the current martian polar deposits, which show clear evidence of multiple internal layers in SHARAD dat
Investigation of Radar Subsurface Sounding through Seasonal Cycles Collected by Mars Shallow Radar (SHARAD) in the South Polar Area
Using an orbital-based ground-penetrating radar - SHARAD proves to be an effective method for imaging the Martian surface and subsurface layering at the south polar layered deposit. This investigation focuses on examining whether seasonal variation of CO2 thickness on the surface caused by accumulation during winter and sublimation during summer could be observed for the first time by analyzing SHARAD data. Travel time and amplitude analysis between the Martian surface reflection and a reference reflection in the subsurface were conducted on multiple orbital tracks corresponding to varying seasons. Results from the travel time analysis along all four cross-lines show that the average change in CO2 frost thickness ranged from 6.80 m to 9.58 m assuming a medium dielectric constant between 2.12 and 2.77. The CO2 thickness reaches its maximum during winter and minimum during summer likely because of the CO2 frost accumulation and retreat, respectively. This observation agrees with other studies. However, the magnitude of change in CO2 thickness estimated in this study is greater than values reported previously. This difference is attributed to the different locations of the Martian polar region examined in the various studies. Amplitude analysis does not show any relationship to seasonal variations on the Martian surface
Science results from sixteen years of MRO SHARAD operations
In operation for >16 years to date, the Mars Reconnaissance Orbiter (MRO) Shallow Radar (SHARAD) sounder
has acquired data at its nominal 300–450 m along-track and 3-km cross-track resolution covering >55% of the
Martian surface, with nearly 100% overlap in coverage at that scale in the polar regions and in a number of
smaller mid-latitude areas. While SHARAD data have opened a new window into understanding the interior
structures and properties of Martian ices, volcanics, and sedimentary deposits up to a few kilometers in depth,
they have also led to new revelations about the deeper interior and the behavior of the planet’s ionosphere. Here
we summarize the data collected by SHARAD over this time period, the methods used in the analysis of that data,
and the resulting scientific findings. The polar data are especially rich, revealing complex structures that
comprise up to several dozen reflecting interfaces that extend to depths of 3 km, which inform the evolution of
Martian climate in the late Amazonian period. SHARAD observations of mid-latitude lobate debris aprons and
other glacier-like landforms detect strong basal reflections and low dielectric loss, confirming that they are icerich
debris-covered glaciers. In other mid-latitude terrains, SHARAD data demonstrate the presence of widespread
ground ices, likely at lower concentrations. SHARAD signals also probe non-icy materials, mapping out
stacked lava flows, probing low-density materials thought to be ash-fall deposits, and occasionally penetrating
sedimentary deposits, all of which reveal the structures and interior properties diagnostic of emplacement
processes. SHARAD signals are impacted by their passage through the Martian ionosphere, revealing variations
in time and space of the total electron content linked with the remanent magnetic field. Advanced techniques
developed over the course of the mission, which include subband and super-resolution processing, coherent and
incoherent summing, and three-dimensional (3D) radar imaging, are enabling new discoveries and extending the
utility of the data. For 3D imaging, a cross-track spacing at the nominal 3-km resolution is more than sufficient to
achieve good results, but finer spacing of 0.5 km or less significantly improves the spatially interpolated radar
images. Recent electromagnetic modeling and a flight test show that SHARAD’s signal-to-noise ratio can be
greatly improved with a large (~120â—¦) roll of the spacecraft to reduce interference with the spacecraft body. Both
MRO and SHARAD are in remarkably fine working order, and the teams look forward to many more years in
which to pursue improvements in coverage density, temporal variability in the ionosphere, and data quality that
promise exciting new discoveries at Mars
The Global Search for Liquid Water on Mars from Orbit: Current and Future Perspectives
Due to its significance in astrobiology, assessing the amount and state of liquid water present on Mars today has become one of the drivers of its exploration. Subglacial water was identified by the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) aboard the European Space Agency spacecraft Mars Express through the analysis of echoes, coming from a depth of about 1.5 km, which were stronger than surface echoes. The cause of this anomalous characteristic is the high relative permittivity of water-bearing materials, resulting in a high reflection coefficient. A determining factor in the occurrence of such strong echoes is the low attenuation of the MARSIS radar pulse in cold water ice, the main constituent of the Martian polar caps. The present analysis clarifies that the conditions causing exceptionally strong subsurface echoes occur solely in the Martian polar caps, and that the detection of subsurface water under a predominantly rocky surface layer using radar sounding will require thorough electromagnetic modeling, complicated by the lack of knowledge of many subsurface physical parameters. Higher-frequency radar sounders such as SHARAD cannot penetrate deep enough to detect basal echoes over the thickest part of the polar caps. Alternative methods such as rover-borne Ground Penetrating Radar and time-domain electromagnetic sounding are not capable of providing global coverage. MARSIS observations over the Martian polar caps have been limited by the need to downlink data before on-board processing, but their number will increase in coming years. The Chinese mission to Mars that is to be launched in 2020, Tianwen-1, will carry a subsurface sounding radar operating at frequencies that are close to those of MARSIS, and the expected signal-to-noise ratio of subsurface detection will likely be sufficient for identifying anomalously bright subsurface reflectors. The search for subsurface water through radar sounding is thus far from being concluded
Global permittivity mapping of the Martian surface from SHARAD
SHARAD is a subsurface sounding radar aboard NASA's Mars Reconnaissance Orbiter, capable of detecting dielectric discontinuities in the subsurface caused by compositional and/or structural changes. Echoes coming from the surface contain information on geometric properties at metre scale and on the permittivity of the upper layers of the Martian crust. A model has been developed to estimate the effect of surface roughness on echo power, depending on statistical parameters such as RMS height and topothesy. Such model is based on the assumption that topography can be characterized as a self-affine fractal, and its use allows the estimation of the dielectric properties of the first few metres of the Martian soil. A permittivity map of the surface of Mars is obtained, covering several large regions across the planet surface. The most significant correspondence with geology is observed at the dichotomy boundary, with high dielectric constant on the highlands side (7 to over 10) and lower on the lowlands side (3 to 7). Other geological correlations are discussed
Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) after nine years of operation: a summary
Mars Express, the first European interplanetary mission, carries the Mars Advanced Radar for Subsurface
and Ionosphere Sounding (MARSIS) to search for ice and water in the Martian subsurface. Developed by
an Italian–US team, MARSIS transmits low-frequency, wide-band radio pulses penetrating below the
surface and reflected by dielectric discontinuities linked to structural or compositional changes. MARSIS
is also a topside ionosphere sounder,transmitting a burst of short, narrow-band pulses at different
frequencies that are reflected by plasma with varying densities at different altitudes.The radar operates
since July 2005, after the successful deployment of its 40 m antenna, acquiring data at altitudes lower
than 1200 km. Subsurface sounding (SS)data are processed on board by stacking together a batch of
echoes acquired at the same frequency. Onground, SS data are further processed by correlating the
received echo with the transmitted waveform and compensating de-focusing caused by the dispersive
ionosphere. Ground processing of active ionospheric sounding (AIS)data consists in the reconstruction
of the electron density profile as a function of altitude. MARSIS observed the internal structure of Planum
Boreum outlining the Basal Unit, an icy deposit lying beneath the North Polar Layered Deposits thought
to have formed in an epoch in which climate was markedly different from the current one.The total
volume of ice in polar layered deposits could be estimated, and parts of the Southern residual ice cap
were revealed to consist of 10 m of CO2 ice. Radar properties of the Vastitas Borealis Formation point
to the presence of large quantities of ice buried beneath the surface. Observations of the ionosphere
revealed the complex interplay between plasma, crustal magnetic field and solar wind, contributing to
space weather studies at Mars. The presence of three-dimensional plasma structures in the ionosphere
was revealed for the first time. MARSIS could successfully operate at Phobos, becoming the first
instrument of its kind to observe an asteroid-like body. The main goal pursued by MARSIS, the search for
liquid water beneath the surface, remains elusive. However, because of the many factors affecting
detection and of the difficulties in identifying water in radar echoes, a definitive conclusion on its
presence cannot yet be drawn
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