Roberto Seu

Radar Soundings of the Subsurface of Mars

The martian subsurface has been probed to kilometer depths by the Mars Advanced Radar for Subsurface and Ionospheric Sounding instrument aboard the Mars Express orbiter. Signals penetrate the polar layered deposits, probably imaging the base of the deposits. Data from the northern lowlands of Chryse Planitia have revealed a shallowly buried quasi-circular structure about 250 kilometers in diameter that is interpreted to be an impact basin. In addition, a planar reflector associated with the basin structure may indicate the presence of a low-loss deposit that is more than 1 kilometer thick.

Radar Sounding Evidence for Buried Glaciers in the Southern Mid-Latitudes of Mars

Lobate features abutting massifs and escarpments in the middle latitudes of Mars have been recognized in images for decades, but their true nature has been controversial, with hypotheses of origin such as ice-lubricated debris flows or glaciers covered by a layer of surface debris. These models imply an ice content ranging from minor and interstitial to massive and relatively pure. Soundings of these deposits in the eastern Hellas region by the Shallow Radar on the Mars Reconnaissance Orbiter reveal radar properties entirely consistent with massive water ice, supporting the debris-covered glacier hypothesis. The results imply that these glaciers formed in a previous climate conducive to glaciation at middle latitudes. Such features may collectively represent the most extensive nonpolar ice yet recognized on Mars.

Massive CO2 Ice Deposits Sequestered in the South Polar Layered Deposits of Mars

Radar measurements reveal a substantial buried deposit of carbon dioxide in the south pole of Mars. Shallow Radar soundings from the Mars Reconnaissance Orbiter reveal a buried deposit of carbon dioxide (CO2) ice within the south polar layered deposits of Mars with a volume of 9500 to 12,500 cubic kilometers, about 30 times that previously estimated for the south pole residual cap. The deposit occurs within a stratigraphic unit that is uniquely marked by collapse features and other evidence of interior CO2 volatile release. If released into the atmosphere at times of high obliquity, the CO2 reservoir would increase the atmospheric mass by up to 80%, leading to more frequent and intense dust storms and to more regions where liquid water could persist without boiling.

The bathymetry of a Titan sea

construct the depth profile--the bathymetry--of Titans large sea Ligeia Mare from Cassini RADAR data collected during the 23 May 2013 (T91) nadir-looking altimetry flyby. We find the greatest depth to be about 160 m and a seabed slope that is gentler toward the northern shore, consistent with previously imaged shoreline morphologies. Low radio signal attenuation through the sea demonstrates that the liquid, for which we determine a loss tangent of 3 ± 1*10-5, is remarkably transparent, requiring a nearly pure methane-ethane composition, and further that microwave absorbing hydrocarbons, nitriles, and suspended particles be limited to less than the order of 0.1% of the liquid volume. Presence of nitrogen in the ethane-methane sea, expected based on its solubility and dominance in the atmosphere, is consistent with the low attenuation, but that of substantial dissolved polar species or suspended scatterers is not.

Dielectric properties of lava flows west of Ascraeus Mons, Mars

[1] The SHARAD instrument on the Mars Reconnaissance Orbiter detects subsurface interfaces beneath lava flow fields northwest of Ascraeus Mons. The interfaces occur in two locations; a northern flow that originates south of Alba Patera, and a southern flow that originates at the rift zone between Ascraeus and Pavonis Montes. The northern flow has permittivity values, estimated from the time delay of echoes from the basal interface, between 6.2 and 17.3, with an average of 12.2. The southern flow has permittivity values of 7.0 to 14.0, with an average of 9.8. The average permittivity values for the northern and southern flows imply densities of 3.7 and 3.4 g cm ―3 , respectively. Loss tangent values for both flows range from 0.01 to 0.03. The measured bulk permittivity and loss tangent values are consistent with those of terrestrial and lunar basalts, and represent the first measurement of these properties for dense rock on Mars.

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.

RIME: Radar for Icy Moon Exploration

This paper presents the Radar for Icy Moons Exploration (RIME) instrument, which has been selected as payload for the JUpiter Icy moons Explorer (JUICE) mission. JUICE is the first Large-class mission chosen as part of the ESAs Cosmic Vision 2015-2025 programme, and is aimed to study Jupiter and to investigate the potentially habitable zones in the Galilean icy satellites. RIME is a radar sounder optimized for the penetration of Ganymede, Europa and Callisto up to a depth of 9 km in order to allow the study of the subsurface geology and geophysics of the icy moons and detect possible subsurface water. In this paper we present the main science goals of RIME, the main technical challenges for its development and for its operations, as well as the expected scientific returns.

The SHAllow RADar (SHARAD) Onboard the NASA MRO Mission

This paper describes the mission concepts, design, and achievements of the Italian Space Agency (ASI)-provided Mars SHAllow RADar (SHARAD) sounder high-frequency (HF) sounding radar, used onboard the National Aeronautics and Space Administration (NASA) Mars Reconnaissance Orbiter (MRO) Spacecraft. Its goals are the detection of liquid or solid water below the surface, and the mapping of subsurface geologic structures. Following a brief overview of the MRO mission and of its main science objectives, the paper introduces the basic principles of operation of the radar sounder, and addresses the major design issues faced by such a system. The greatest challenges faced in the design are the control of the interference from off-nadir echoes and the need for a high signal fidelity over a very large fractional bandwidth. The core of the paper is devoted to describing how the above problems have been tackled in the design of the SHARAD instrument, and the main characteristics of its architecture. The two key features of the instrument system design are 1) generation of the transmitted signal directly at the transmitted frequency; and 2) sampling performed directly at the radio frequency (by means of a subsampling technique). The careful design of these features, intended to keep the analog signal path very simple, minimizes distortions and stability problems. An overview of the calibration approach of both the system impulse response and the antenna gain at nadir versus solar array position, an assessment of the in-flight performance of the instrument, and a short summary of the achieved science results are also provided.

The Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS): concept and performance

Describes the key features and expected performance of a new radar sounder instrument currently under development by a team of Italian and US researchers and industrial partners, selected to fly with the ESA Mars Express orbiter scheduled for launch to Mars late in 2003. Very low transmitted frequency (1-5 MHz), large instantaneous bandwidth and coherent on-board processing techniques will make it possible to acquire a large amount of science-relevant data about the Mars interior, surface and atmosphere ensuring global coverage at all latitudes while respecting the Mars Express mission constraints.

An incoherent simulator for the SHARAD experiment

SHARAD (shallow radar) is a sounder and altimeter with synthetic aperture radar (SAR) capability provided by the Italian Space Agency (ASI) as a Facility Instrument to NASApsilas 2005 Mars Reconnaissance Orbiter. Primary objective of this nadir-looking sounder is to map Martian surface and subsurface down to several hundred meters depth with vertical resolution of 15 m and horizontal resolution of a few hundred meters (300 m-1 Km). In this paper we are going to present an incoherent simulator for the surface echoes received from SHARAD that utilizes only geometrical projections and the operating principle of the synthetic aperture radar. Making use of a convenient model of the Martian surface, our simulator is an important instrument for discriminating subsurface echoes from clutter artifacts.