Radar Sounding Research Articles

Long-wave planetary radar: Radar sounding of the soil of Phobos in the Phobos-Grunt project

The specific features of the radar sounding of the soil of Phobos during the performance of the Russian Phobos-Grunt project have been considered. The parameters of the long-wave subsurface sounding radar designed in order to study the electrophysical characteristics of the soil of Phobos, reveal the depth structure, estimate the rock density, and perform radar mapping of the satellite relief and surface roughness have been justified. The scheme of the experimental sounding of the soil of Phobos is presented.

The Lunar Radar Sounder (LRS) Onboard the KAGUYA (SELENE) Spacecraft

The Lunar Radar Sounder (LRS) onboard the KAGUYA (SELENE) spacecraft has successfully performed radar sounder observations of the lunar subsurface structures and passive observations of natural radio and plasma waves from the lunar orbit. After the transfer of the spacecraft into the final lunar orbit and antenna deployment, the operation of LRS started on October 29, 2007. Through the operation until June 10, 2009, 2363 hours worth of radar sounder data and 8961 hours worth of natural radio and plasma wave data have been obtained. It was revealed through radar sounder observations that there are distinct reflectors at a depth of several hundred meters in the nearside maria, which are inferred to be buried regolith layers covered by a basalt layer with a thickness of several hundred meters. Radar sounder data were obtained not only in the nearside maria but also in other regions such as the farside highland region and polar region. LRS also performed passive observations of natural plasma waves associated with interaction processes between the solar wind plasma and the moon, and the natural waves from the Earth, the sun, and Jupiter. Natural radio waves such as auroral kilometric radiation (AKR) with interference patterns caused by the lunar surface reflections, and Jovian hectometric (HOM) emissions were detected. Intense electrostatic plasma waves around 20 kHz were almost always observed at local electron plasma frequency in the solar wind, and the electron density profile, including the lunar wake boundary, was derived along the spacecraft trajectory. Broadband noises below several kHz were frequently observed in the dayside and wake boundary of the moon and it was found that a portion of them consist of bipolar pulses. The datasets obtained by LRS will make contributions for studies on the lunar geology and physical processes of natural radio and plasma wave generation and propagation.

Depth of the Martian cryosphere: Revised estimates and implications for the existence and detection of subpermafrost groundwater

The Martian cryosphere is defined as that region of the crust where the temperature remains continuously below the freezing point of water. Previous estimates of its present thickness have ranged from ∼2.3–4.7 km at the equator to ∼6.5–12.5 km at the poles. Here we revisit these calculations, review some of the assumptions on which they were based, and investigate the effects of several parameters, not previously considered, on the cryosphere's thermal evolution and extent. These include astronomically driven climate change, the temperature‐dependent thermal properties of an ice‐rich crust, the potential presence of gas hydrate and perchlorate‐saturated groundwater, and consideration of recent lower estimates of present‐day global heat flow (which suggest a mean value roughly half that previously thought, effectively doubling the potential thickness of frozen ground). The implications of these findings for the continued survival of subpermafrost groundwater and its potential detection by the MARSIS radar sounder onboard Mars Express are then discussed. Although our estimates of the maximum potential thickness of the cryosphere have significantly increased, consideration of the likely range and spatial variability of crustal heat flow and thermal properties, in combination with the potential presence of potent freezing point depressing salts, may result in substantial local variations in cryosphere thickness. The locations that appear best suited for the detection of groundwater are those that combine low latitude (minimizing the thickness of frozen ground) and low elevation (minimizing the depth to a water table in hydrostatic equilibrium). Preliminary results from a MARSIS investigation of one such area are discussed.

Simultaneous Observation of Lunar Radar Sounder and Laser Altimeter of Kaguya for Lunar Regolith Layer Thickness Estimate

Simultaneous observations of Lunar Radar Sounder (LRS) and Laser ALTimeter (LALT) of Kaguya, a Japanese lunar exploration project, were carried out for the purpose of mapping regolith layer thickness of the Moon. Nadir surface echo of a high-frequency (5 MHz) pulse of LRS interferes the shallow (<; 10 m) subsurface echo from the bottom of the regolith layer, which subsequently makes the apparent surface be detected at a range deviated from the actual surface range, while the actual surface range is optically detected by LALT. Regolith layer thickness information is retrieved from this range difference after an inversion process. So far, four major maria on the near side of the Moon (Maria Tranquillitatis, Serenitatis, Imbrium, and Oceanus Procellarum) have been investigated, and the mean regolith layer thicknesses of the four maria were found to be about the same, ranging from 6.3 to 6.9 m. However, spatial distribution of areal regolith thickness appears different in eastern maria from western maria, which implies a difference of the growth history of the regolith layer.

Simulation of radar sounder echo from lunar surface and subsurface structure

Space-borne high frequency (HF) radar sounder is an effective tool for investigation of lunar subsurface structure in lunar exploration. The primary strategy of radar sounder technology for subsurface structure detection is utilization of the nadir echoes time delay and intensity difference from the lunar surface and subsurface. It is important to fully understand electromagnetic wave propagation, scattering, and attenuation through the lunar media in order to retrieve information of lunar layering structure from weak nadir echoes of the subsurface, which is simultaneously interfered by strong off-nadir surface clutters. Based on the Kirchhoff approximation (KA) of rough surface scattering and the ray tracing of geometric optics, a numerical simulation of radar echoes from lunar layering structures is developed. According to the lunar surface feature, the topography of mare and highland surfaces is numerically generated, and the triangulated network is employed to make digital elevations of the whole lunar surface. Scattering from the lunar surface and subsurface is numerically calculated using KA approach. Radar echoes and its range images are numerically simulated, and their dependence on the parameters of lunar layering interfaces is discussed. The approach of this paper can also be utilized to investigate subsurface structures in Mars and other planetary exploration.

Individual lava flow thicknesses in Oceanus Procellarum and Mare Serenitatis determined from Clementine multispectral data

We use multispectral reflectance data from the lunar Clementine mission to investigate the impact ejecta deposits of simple craters in two separate lunar mare basalt regions, one in Oceanus Procellarum and one in Mare Serenitatis. Over 100 impact craters are studied, and for a number of these we observe differences between the TiO 2 (and FeO) contents of their ejecta deposits and the lava flow units in which they are located. We demonstrate that, in the majority of cases, these differences cannot plausibly be attributed to uncorrected maturity effects. These observations, coupled with morphometric crater relationships that provide maximum crater excavation depths, allow the investigation of sub-surface lava flow stratigraphy. We provide estimated average thicknesses for a number of lava flow units in the two study regions, ranging from ∼80 m to ∼600 m. In the case of the Serenitatis study area, our results are consistent with the presence of sub-surface horizons inferred from recent radar sounding measurements from the JAXA Kaguya spacecraft. The average lava flow thicknesses we obtain are used to make estimates of the average flux of volcanic material in these regions. These are in broad agreement with previous studies, suggesting that the variation in mare basalt types we observe with depth is similar to the lateral variations identified at the surface.

Onset and migration of spiral troughs on Mars revealed by orbital radar

The landscape of the north polar layered deposits of Mars (NPLD) is dominated by a pinwheel array of enigmatic spiral troughs. The troughs have intrigued planetary scientists since the Mariner 9 spacecraft returned the first close-up image in 1972, but conclusive evidence of their origin has remained elusive. Debate continues regarding all aspects of the troughs, including the possibility that they have migrated, their age in relation to the current NPLD surface, and whether they are fundamentally erosional or constructional features. The troughs are probably related to climatic processes, yet the nature of this relationship has remained a mystery. Previous data characterizing only the exposed NLPD surface were insufficient to test these hypotheses. Here we show that the central spiral troughs initiated after deposition of three-quarters of the NPLD, quickly reached a stable morphology and migrated approximately 65 kilometres poleward and 600 metres in altitude over the past two million years or so. Our radar stratigraphy rules out hypotheses of erosional incision post-dating deposition, and instead largely validates an early hypothesis for constructional trough migration with wind transport and atmospheric deposition as dominant processes. These results provide hard constraints for palaeo-climate models and a new context for evaluating imagery, spectral data, and now radar sounding data, the better to understand the link between orbital parameters and climate, the role of climate in shaping the polar ice of Mars, and eventually, the age of the polar deposits themselves.

Radar Signal Propagation and Detection Through Ice

In this paper we describe the existing and planned radar measurements of the planetary bodies. The dielectric properties of water ice and other potential surface and subsurface materials are discussed, as well as their dependency on temperature and structure. We then evaluate the performance of subsurface sounding radars using these parameters. Finally we describe some laboratory technique to help interpret the radar data, presenting some results obtained using dielectric spectroscopy methods.

New Concepts and Tools for Geological Mapping of Mars: Geological Mapping of Mars: A Workshop on New Concepts and Tools; Tuscany, Italy, 12–14 October 2009

Geological mapping is a key tool for understanding the evolution of any planetary surface. The availability of ever growing data sets (e.g., multispectral and hyperspectral imaging and subsurface radar sounding) requires increasing effort in analyzing, integrating, and exploiting them for mapping purposes.To discuss these issues, about 80 planetary geoscientists gathered in Italy at a workshop co‐organized by the Italian Space Agency (ASI), the International Research School of Planetary Sciences (IRSPS), and the U.S. Geological Survey (USGS). The workshop focused on both data and concepts and covered a range of scientific and technical topics.

Detectability of subsurface interfaces in lunar maria by the LRS/SELENE sounding radar: Influence of mineralogical composition

The Lunar Radar Sounder (LRS) of the SELENE mission has detected horizontal subsurface features at depths of a few hundreds of meters within all major lunar maria. We have mapped these features at global scale and found a heterogeneous geographical distribution, which correlates negatively with the maps of TiO2 and FeO obtained from UV‐VIS measurements by the Clementine probe. High concentration of ilmenite (FeTiO3) in the basaltic mare lava can explain this correlation, this mineral being a strong absorber for low frequency electromagnetic waves. Thus, absence of detection of subsurface interfaces by LRS on a large portion of lunar maria does not necessarily imply that these interfaces are actually absent.

Magnetic anomalies near Apollinaris Patera and the Medusae Fossae Formation in Lucus Planum, Mars

The nature of strong martian crustal field sources is investigated by mapping and modeling of Mars Global Surveyor magnetometer data near Apollinaris Patera, a previously proposed volcanic source, supplemented by large-scale correlative studies. Regional mapping yields evidence for positive correlations of orbital anomalies with both Apollinaris Patera and Lucus Planum, a nearby probable extrusive pyroclastic flow deposit that is mapped as part of the Medusae Fossae Formation. Iterative forward modeling of the Apollinaris Patera magnetic anomaly assuming a source model consisting of one or more uniformly magnetized near-surface disks indicates that the source is centered approximately on the construct with a scale size several times larger and comparable to that of the Apollinaris Patera free-air gravity anomaly. A significantly lower rms deviation is obtained using a two-disk model that favors a concentration of magnetization near the construct itself. Estimates for the dipole moment per unit area of the Lucus Planum source together with maximum thicknesses of ∼3 km based on topographic and radar sounding data lead to an estimated minimum magnetization intensity of ∼50 A/m within the pyroclastic deposits. Intensities of this magnitude are similar to those obtained experimentally for Fe-rich Mars analog basalts that cooled in an oxidizing (high fO 2) environment in the presence of a strong (⩾10 μT) surface field. Further evidence for the need for an oxidizing environment is provided by a broad spatial correlation of the locations of phyllosilicate exposures identified to date using Mars Express OMEGA data with areas containing strong crustal magnetic fields and valley networks in the Noachian-aged southern highlands. This indicates that the presence of liquid water, which is a major crustal oxidant, was an important factor in the formation of strong magnetic sources. The evidence discussed here for magnetic sources associated with relatively young volcanic units suggests that a martian dynamo existed during the late Noachian/early Hesperian, after the last major basin-forming impacts and the formation of the northern lowlands.

NEAR-FIELD SCATTERING OF WAVES FROM A STATISTICALLY ROUGH SURFACE: III. TIME CORRELATION FUNCTION AND INTENSITY OF THE FIELD FLUCTUATIONS IN THE CASE OF OBLIQUE MONOSTATIC RADAR SOUNDING

Asymptotic expressions are presented for the integrals of rapidly oscillating functions which determine the correlation function and intensity of the scattered field in the case of monostatic radar sounding of a statistically rough surface. The expressions have been obtained using the method of stationary phase corresponding to the case of isolated stationary points. The transition to the result known from the literature is analyzed.

Response of the midlatitude ionosphere to extreme geomagnetic storms of the 23rd solar cycle

The ionospheric response in the Irkutsk region (52.3° N, 104.3° E) to the extreme geomagnetic storms of solar cycle 23 was studied based on the data of the Irkutsk incoherent scatter radar (ISR) and DPS-4 vertical sounding digital ionosonde. The deviations of parameters from the undisturbed level, i.e., from the monthly medians or the values obtained on a quiet day, were considered as an ionospheric response. Values of the electron concentration maximum (NmF2) and electron temperature (Te) at a height of 350 km were chosen as parameters. The ionospheric response is interpreted in the scope of the concept of a thermospheric storm and penetration of the magnetospheric electric field.

Structural Characteristics of the No.12 Glacier in Laohugou Valley, Qilian Mountain Based on the Ground Penetrating Radar Sounding

The inner-structure of glacier was analyzed from the profile images of ground-penetrating radar at No.12 Glacier in Laohugou Qilian Mountain western China.By calculating the difference of dielectric constants among water,air,bedrock,debris and ice,integrated with changes in radar wave polarity and velocity,the geomorphic character within the glacier was described and caves,caves with water,channels beneath the ice,the interface between glacier and bedrock were identified and positioned,with the comparison of wave patterns introduced in some literatures.The results suggest that there exist debris layers between ice-bedrock interfaces indicating that strong basal erosional processes may even prevail in certain part of a glacier of the continental characteristic,contrary to the early knowledge about this kind of glaciers.

Distribution of the subsurface reflectors of the western nearside maria observed from Kaguya with Lunar Radar Sounder

The Lunar Radar Sounder (LRS) performed global soundings and detected possible subsurface echoes at the nearside maria. This paper examines the relationship between the distribution of the detected subsurface echoes and the surface ages in the western nearside maria. Continuous subsurface reflectors are clearly detected only in a few areas consisting of about 10% of the western nearside maria at apparent depths from hundreds to more than a thousand meters. These reflectors are not generally the basements of the mare but are the interface between different basaltic rock facies. Comparison between the distribution of the detected subsurface boundaries and the surface ages suggests that most of the detected subsurface reflectors were formed more than 3.4 billion years ago. Based on the strong connection between the accumulation rate of regolith and the absolute age, the detected subsurface boundaries appear to be relatively thick regolith layers accumulated during the depositional hiatuses of basaltic lavas.

Differences between general relativity and dynamical theory of gravitation in the resolution of radar sounding phenomenon to the order of C&lt;sup&gt;-5&lt;/sup&gt;

Differences between general relativity and dynamical theory of gravitation in the resolution of radar sounding phenomenon to the order of C&lt;sup&gt;-5&lt;/sup&gt;

Slope characteristics of Europa: Constraints for landers and radar Sounding

Limited Galileo‐derived topographic mapping of four major terrain types on Europa provides slope characteristics useful for designing spacecraft instruments and for terrain comparisons. Topographic data are derived using stereo and photoclinometric analysis of Galileo images at resolutions of 10 to 200 m (with a few sites sampled at ∼500 m resolution). Similar data are also presented for dark and bright terrains on Ganymede as well as preliminary results for polar terrains on Enceladus. RMS slopes for ridged plains, matrix, and crater deposits on Europa are typically between 10 and 15° at 10–100 m length scales, significantly steeper overall than typical landing sites on Mars. Terrains on Ganymede may be even rougher. Smooth dilational bands on Europa, however, have RMS slopes of ∼5°, the shallowest slope statistics of any major terrain type on Europa. Aside from these areas almost no smooth areas larger than a few kilometers across occur on Europa.

Sounding the subsurface of Athabasca Valles using MARSIS radar data: Exploring the volcanic and fluvial hypotheses for the origin of the rafted plate terrain

To test the volcanic and fluvial hypotheses for the origin of the rafted plate terrain observed in the vicinity of Athabasca Valles (5°N, 150°E, Central Elysium Planitia), we investigated the subsurface radar echo from the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) 5‐MHz band data over this area. The backscattered signal losses were compared to finite difference time domain (FDTD) simulations of those arising from three hypothetical geoelectrical subsurface models, which differed in their assumed composition (percentage basalt versus ice) and assumed mode of origin (fluvial discharge/“frozen sea,” mudflow, and low‐viscosity lavas). The dielectric values used in these models are derived from laboratory measurements of Mars analog materials under Mars‐like conditions. FDTD simulations suggest that if the near‐surface environment is ice‐rich, it will result in an average loss rate of 0.053 dB/m for massive ice (having less than 1% of suspended particulates) and 0.065 dB/m for a mudflow (consisting of a 50/50 mixture of ice and basaltic dust). Whereas the losses associated with a lava flow model increase to 0.19 dB/m. In comparison, the actual signal losses experienced by MARSIS within this region were on the order of 0.18 dB/m within the first 160 m beneath the surface. This suggests that propagation characteristics of Athabasca's near‐subsurface are more consistent with a volcanic rather than a fluvial or mudflow origin of the rafted plate terrain, although limitations on radar sounding depth in this region cannot rule out the possibility of more deeply buried massive ice deposits.

Marine ice in Larsen Ice Shelf

It is argued that Larsen Ice Shelf contains marine ice formed by oceanic freezing and other mechanisms. Missing basal returns in airborne radar soundings and observations of a smooth and healed surface coincide downstream of regions where an ocean model predicts freezing. Visible imagery suggests that marine ice currently stabilizes Larsen C Ice Shelf and implicates failure of marine flow bands in the 2002 Larsen B Ice Shelf collapse. Ocean modeling indicates that any regime change towards the incursion of warmer Modified Weddell Deep Water into the Larsen C cavity could curtail basal freezing and its stabilizing influence.

A plasma flow velocity boundary at Mars from the disappearance of electron plasma oscillations

The Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) on the Mars Express (MEX) spacecraft is capable of measuring ionospheric electron density by the use of two main methods: remote radar sounding and from the excitation of local plasma oscillations. The frequency of the locally excited electron plasma oscillations is used to measure the local electron density. However, plasma oscillations are not observed when the plasma flow velocity is higher than about 160 km/s, which occurs mainly in the solar wind and magnetosheath. As a consequence, in many passes, there is a sudden disappearance of the plasma oscillations as the spacecraft enters into the magnetosheath. This fact allows us to identify a flow velocity boundary on the dayside, between the ionosphere of Mars and the shocked solar wind. This paper summarizes the results of the measurement of 552 orbits mostly over a period from August 4, 2005 to August 17, 2007. The boundary points found using MARSIS have been verified by measurements from the Analyzer of Space Plasma and Energetic Atoms (ASPERA-3) Electron Spectrometer (ELS) instrument on Mars Express. The average position of the flow velocity boundary is compared to flow velocity simulations computed using hybrid model and other boundaries. The boundary altitude is slightly lower than the magnetic pile-up boundary determined using Phobos 2 and Mars Global Surveyor (MGS) crossings, but it is in good agreement with the induced magnetospheric boundary determined by ASPERA-3. Investigation of the effect of the crustal magnetic field revealed that the flow velocity boundary is raised at the locations with strong crustal magnetic fields.