Wlodek Kofman

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.

The landing(s) of Philae and inferences about comet surface mechanical properties

The Philae lander, part of the Rosetta mission to investigate comet 67P/Churyumov-Gerasimenko, was delivered to the cometary surface in November 2014. Here we report the precise circumstances of the multiple landings of Philae, including the bouncing trajectory and rebound parameters, based on engineering data in conjunction with operational instrument data. These data also provide information on the mechanical properties (strength and layering) of the comet surface. The first touchdown site, Agilkia, appears to have a granular soft surface (with a compressive strength of 1 kilopascal) at least ~20 cm thick, possibly on top of a more rigid layer. The final landing site, Abydos, has a hard surface.

Properties of the 67P/Churyumov-Gerasimenko interior revealed by CONSERT radar

The Philae lander provides a unique opportunity to investigate the internal structure of a comet nucleus, providing information about its formation and evolution in the early solar system. We present Comet Nucleus Sounding Experiment by Radiowave Transmission (CONSERT) measurements of the interior of Comet 67P/Churyumov-Gerasimenko. From the propagation time and form of the signals, the upper part of the “head” of 67P is fairly homogeneous on a spatial scale of tens of meters. CONSERT also reduced the size of the uncertainty of Philae’s final landing site down to approximately 21 by 34 square meters. The average permittivity is about 1.27, suggesting that this region has a volumetric dust/ice ratio of 0.4 to 2.6 and a porosity of 75 to 85%. The dust component may be comparable to that of carbonaceous chondrites.

Dielectric map of the Martian northern hemisphere and the nature of plain filling materials

[1] A number of observations suggest that an extended ocean once covered a significant part of the Martian northern hemisphere. By probing the physical properties of the subsurface to unprecedented depth, the MARSIS/Mars Express provides new geophysical evidences for the former existence of a Late Hesperian ocean. The Vastitas Borealis formation, located inside a putative shoreline of the ancient ocean, has a low dielectric constant compared with that of typical volcanic materials. We show that the measured value is only consistent with low-density sedimentary deposits, massive deposits of ground-ice, or a combination of the two. In contrast, radar observations indicate a distribution of shallow ground ice in equilibrium with the atmosphere in the south polar region. We conclude that the northern plains are filled with remnants of a late Hesperian ocean, fed by water and sediments from the outflow channels about 3 Gy ago.

Comet nucleus sounding experiment by radiowave transmission

Abstract We describe the radio science experiment proposed for the Rosetta cometary mission. The experiment consists in the transmission of electromagnetic waves between the landers and the orbiter through the comet to study its internal structure. In the paper, the electromagnetic model of the comet is presented and used to evaluate the potentiality of the experiment. Various modellings of the radio wave propagation are discussed. Finally, a description of the experiment and the instrument is made.

Aspect angle dependence of HF enhanced incoherent backscatter

Abstract An HF ionospheric interaction experiment was performed in November and December of 1997 using the EISCAT HF transmitter and 931 and 224 MHz incoherent scatter radars, all co-located near Tromso, Norway. During this experiment the pointing of the UHF radar was varied in a predetermined and repeating cycle between elevation angles of 90 and 77.2 degrees south, that is, between vertical and geomagnetic field aligned. The HF transmitter duty cycle was intentionally kept to the relatively low value of 2% (200 ms every 10 s) in order to minimize the effects of ionospheric irregularities. Here we report on variations in the intensity of the enhanced incoherent scatter ion and plasma lines observed during the experiment. Bottomside and topside F region enhanced lines were seen with both radars, and while intensity enhancements observed with the UHF radar were clearly correlated with pointing angles between the Spitze angle and field aligned, no correlation between the intensity of the lines observed with the scanning UHF radar and the vertically pointing VHF radar was observed. Consistent with HF propagation theory, the field aligned backscatter observed by the UHF radar originated several kilometers below the HF reflection height.

A characterization of a comet nucleus interior: inversion of simulated radio frequency data

Abstract Radio transmission through the nucleus of the comet Wirtanen, as developed in the CONSERT experiment on the ROSETTA mission, is designed to provide information on the electrical properties of the cometary interior. In order to determine what information can most readily be obtained, we first introduce an analytical model of the nucleus permittivity. We then use the WKB approximation to describe the propagation of waves through the nucleus model assuming a radio frequency of 90 MHz, the frequency chosen for the experiment. With one end of the propagation path on the surface of the comet, and the other in a vehicle orbiting the nucleus, it is shown that the signal at the orbiting end of the path must be back-propagated to the surface of the nucleus of the comet in order to obtain a correct interpretation of the data. We show how the mean permittivity along each propagation path can be obtained and how this can be used to obtain information about the large scale structure of the nucleus. Finally we discuss how the permittivity distribution found can be used to constrain the nature of the material in the interior of the nucleus.

HF radio wave attenuation due to a meteoric layer in the atmosphere of Mars

In the atmosphere of Mars, the effect of an ionospheric layer of meteoric origin on an HF radio wave propagation is investigated. We consider the putative magnesium ion layer which results from the ablation of sporadic meteors from a work recently published. To account for day-night variability, two electron profiles are considered in an altitude range around 80 km. First we argue that there may be some observational evidence of this magnesium layer in early Mars observations that were not noticed before. Then we study the effect of this ionospheric layer on the attenuation of HF waves. For a propagation path through the ionosphere as envisaged for HF subsurface sounding on future Mars missions, the one-way attenuation may range from up to 360 dBs at 1.8 MHz and up to 18 dBs at 9 MHz.

Subsurface Radar Sounding of the Jovian Moon Ganymede

This paper provides an overview of the Europa Jupiter System Mission (EJSM) and of its scientific objectives, focusing the attention on the subsurface radar (SSR) instrument included in the model payload of the Jupiter Ganymede Orbiter (JGO). The SSR instrument is a radar sounder system at low frequency (HF/VHF band) designed to penetrate the surface of Ganymede icy moon of Jupiter for performing a subsurface analysis with a relatively high range resolution. This active instrument is aimed at acquiring information on the Ganymede (and partially on the Callisto during flybys) shallow subsurface. The paper addresses the main issues related to SSR, presenting its scientific goals, describing the concept and the design procedure of the instrument, and illustrating the signal processing techniques. Despite the fact that SSR can be defined on the basis of the heritage of the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) and SHAllow RADar (SHARAD) instruments currently operating at Mars, the EJSM mission poses additional scientific and technical challenges for its design: 1) the presence of a relevant Jupiter radio emission (which is very critical because it has a significant power spectral density in proximity of the expected SSR central frequency); 2) the properties of the subsurface targets, which are different from those of the Mars subsurface; 3) the different orbit conditions; and 4) the limited available resources (in terms of mass, power, and downlink data rate). These challenges are analyzed and discussed in relation to the design of the instrument in terms of: 1) choice of the central frequency and the bandwidth; 2) signal-to-noise ratio (SNR); 3) signal-to-clutter ratio (SCR); and 4) definition of the synthetic aperture processing. Finally, the procedure defined for SSR performance assessment is described and illustrated with some numerical examples.

A two dimensional simulation of the CONSERT experiment (radio tomography of comet Wirtanen)

Abstract We simulate two-dimensional (2-D) maps of the internal permittivity of comet Wirtanen from CONSERT (Comet Nucleus Sounding Experiment by Radiowave Transmission) phase measurements. We study the influence of a priori covariance matrices on the mapping process through Tikhonov or linear regression inverse formulas. We show that this a priori information permits a smooth imaging of the internal permittivity, sometimes affected by ghosts (false anomalies). Accounting for indirect propagation paths through the comet material (rays totally reflected by the comet surface) can alleviate this problem.