Using MARSIS signal attenuation to constrain SPLD basal temperature and composition

Introduction 

The presence of liquid water at the base of the Southern polar cap of Mars has been inferred from exceptionally strong radar echoes detected by MARSIS aboard the MEX spacecraft [1]. The identification of water from strong radar reflections is based on the high value of the dielectric permittivity of water-bearing materials compared to that of dry rocks. This identification has been challenged based on thermal models of the Martian polar cap, which could not produce basal temperatures compatible with the presence of liquid water, and alternative interpretations have been proposed like wet clays and iron-rich basalts [2-4]. However, a quantitative determination of which hypothesis best explains the strong radar echoes detected by MARSIS requires a careful modelling of electromagnetic propagation within the Martian South Polar Layered Deposits (SPLD), the dust-laden ice sheet covering most of the Martian polar regions. In addition to the dielectric properties of the material beneath the Martian ice cap, the main factor determining the strength of basal radar echoes is the attenuation experienced by the radio waves as they propagate within the SPLD. Here we estimate the bulk loss tangent (ratio of imaginary to real part of the complex dielectric permittivity) of the SPLD from differential attenuation of basal echoes detected by MARSIS at different frequencies.  

 

Data and Methods  

The used MARSIS dataset consists of 132 radar observations collected at 3MHz and 4MHz or 4MHz and 5MHz, acquired at Ultimi Scopuli between 2010 and 2019 (Fig.1): 36 at 3 MHz, 132 at 4MHz and 96 at 5MHz. Such observations have been collected on a large region, were both bright and non-bright areas were detected [5]. The basal reflectivity is lower at higher frequencies, with a systematic difference between each frequency pair (3/4 MHz and 4/5 MHz) regardless the acquisition inside or outside the bright area (Fig.2). This behavior can be ascribed to different causes: the attenuation in the SPLD; the scattering generated by the basal interface which, in turns, depends on the interface roughness and the dielectric contrast between the SPLD and the underlying material. From data analysis, it is possible to ascribe the frequency behavior of the MARSIS observations mostly to the signal attenuation in the SPLD.  Under these assumptions, loss tangent is computed from the measurements of the normalized basal echo power observed at different depths and frequencies. 

                                                                                               (P_b/P_s)_dB ≃ R₀−𝜉 𝜈 tan𝛿 𝜏, 

where P_b is the basal echo power, P_s is the surface echo power,  𝜉=2𝜋 10log₁₀(⁡e), and R₀ is a constant which depends on surface and basal Fresnel reflection coefficients, 𝜈 is the frequency and 𝜏 is the two-way travel time. 

Fig.1 Mars Orbiter Laser Altimeter topographic map of the investigated area at Ultimi Scopuli. The white lines represent the MARSIS observations in the region. The gray region indicates the main bright area studied in [1]. Black lines are the observations illustrated in Fig.2. 

 

Fig. 2 The plots refer to observations collected inside (a) and outside the bright areas (b) of Fig. 1, after applying an along track average.  

Results 

In the entire investigated region, the estimated loss tangent value is of the order of 10−3 . This value implies an attenuation of several dB's over the thickness of the SPLD in the area where strong echoes were detected by MARSIS, thus increasing the value of the dielectric permittivity of the basal material required to produce such echoes. The observed frequency behavior of basal echoes requires the presence of a significant amount of dust within the SPLD, similar to what has been deduced from gravity measurements, and puts an upper limit to the basal temperature of the SPLD. Furthermore, the extrapolation of the observed attenuation at higher frequencies explains why SHARAD, the other radar sounder at Mars, is unable to detect the strong basal echoes found by MARSIS. The upper limit on basal temperature retrieved in this analysis, when compared with literature data about the electrical conductivity of geomaterials at low temperature, rules out the possibility that clays or other dry minerals can produce the strong echoes detected by MARSIS. The most likely explanation for such echoes thus remains the presence of perchlorate brines at the base of the SPLD. 

References 

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Sori, M. M., & Bramson, A. M. (2019). GRL, doi.org/10.1029/2018GL080985. 

Smith, I. B. et al., (2021). GRL, doi.org/10.1029/2021GL093618.  

Grima, C., et al., (2022). GRL, doi.org/10.1029/2021GL096518.   

Lauro, S.E., et al.. Nat Astron, doi.org/10.1038/s41550-020-1200-6