Deep Radar Sounding of Martian Polar Deposits: Radiative Transfer Models

Introduction: Investigation of the internal structure of martian polar caps, being a challenging task during past decades [1], is now going to be solved with the ground penetrating radar (GPR) instruments, both orbital and landed. In the present report the propagation of ultra wide band (UWB) chirp pulse in martian polar ice caps is investigated. The specific nature of the problem is that rather inhomogeneous layered structure of the caps lead to formation of diffuse structure of the radar signal field. Application of coherent radar signal processing techniques fails to resolve individual internal scatterers within the bulk of the polar caps, so that some incoherent interpretation technique should be utilized. In the present work the results of application of radiative transfer theory to the problem of radar sounding of the martian polar deposits. Electrical model of the caps: we essentially exploit the electrical model of the caps, previously developed in earlier papers. It has been repeatedly argued that the primary constituent of the northern polar cap is water ice. The following observations provide basis for this statement: high water vapor concentrations over the cap during summer [2], low density [3], a thermal inertia [4] and albedo [5] corresponding to dirty water ice. The surface of the southern polar cap does not look like water ice, but there also are evidences that water ice is its major volatile constituent, too [6,7]. There are also numerous observational evidences of layered structure of Martian polar caps [1]. Thickness of individual layers has been estimated to be 14 – 45 m [8] and 80-120 m [9] for northern and southern polar caps, respectively. Recent studies of layered structure of northern polar cap [10] have revealed its more complicated structure, largely consistent with earlier rough estimates. Precipitational and sedimentational models predict that layers of dirty ice are separated from each other by a dusty cover about 1 m thick [3]. Such cover is formed from the layer material due to ice ablation, which occurs periodically according to variations of Martian orbit parameters. A model of Martian polar caps as a stack of layers of two types, the so-called “icy” and “dusty” layers, has been previously suggested [11]. It has been shown there that only electromagnetic waves at frequencies below 1 MHz can propagate through such a layered medium without significant distortion. Above 1 MHz, there are frequency bands where the medium is opaque. On the other hand, at frequencies below 1 MHz the northern cap is relatively transparent. However, most currently developed and previously proposed radar instruments operate well above 1 MHz.