Studies of lava flows in the Tharsis region of Mars using SHARAD

Abstract

The Tharsis region of Mars is covered in volcanic flows that can stretch for tens to hundreds of kilometers. Radar measurements of the dielectric properties of these flows can provide information regarding their composition and density. SHARAD (shallow radar), a sounding radar on the Mars Reconnaissance Orbiter, detects basal interfaces beneath flows in some areas of Tharsis northwest and west of Ascraeus Mons, with additional detections south of Pavonis Mons. Comparisons with 12.6 cm ground-based radar images suggest that SHARAD detects basal interfaces primarily in dust or regolith-mantled regions. We use SHARAD data to estimate the real relative permittivity of the flows by comparing the measured time delay of returns from the subsurface with altimetry measurements of the flow heights relative to the surrounding plains. In cases where the subsurface interface is visible at different depths, spanning tens of meters, it is also possible to measure the loss tangent (tan δ) of the material. The permittivity values calculated range from 7.6 to 11.6, with an average of 9.6, while the mean loss tangent values range from 7.8 × 10À3 to 2.9 × 10 À2with an average of 1.0 × 10 À2. These permittivity and loss tangent estimates for the flows northwest of Ascraeus Mons, west of Ascraeus Mons, and south of Pavonis Mons are consistent with the lab-measured values for dense, low-titanium basalt.

New Insights Into Subsurface Stratigraphy Northwest of Ascraeus Mons, Mars, Using the SHARAD and MARSIS Radar Sounders

“…Dry, dense terrestrial basalts have a typical range of permittivity values between 7 and 11 whereas pumice, ash, and tuff typically fall between 2.5 and 3.5 (Campbell & Ulrichs, 1969;Carter, Campbell, Holt, et al, 2009;Morgan et al, 2015;Ulaby et al, 1988). We therefore interpret the measured deposits in this region to be basaltic lava flows based on our measurements (see Table 1), in good agreement with previous estimates of permittivity from two lava flows in this region of Tharsis by Carter, Campbell , Holt, et al (2009) and Simon et al (2014). We interpret the reflectors we observe within the margins of the northern, western, and southern flows to be the base of those lava flows in contact with other, smaller flows in the plains emplaced from prior episodes of volcanism.…”

“…(2009) and Simon et al. (2014). We interpret the reflectors we observe within the margins of the northern, western, and southern flows to be the base of those lava flows in contact with other, smaller flows in the plains emplaced from prior episodes of volcanism.…”

“…The black box indicates the region of interest in this study (Figure 2) investigated using the SHAllow RADar (SHARAD) and Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) currently in orbit around Mars. The stars indicate a general region where multiple SHARAD or MARSIS detections of subsurface interfaces have been made by various workers (Carter, Campbell, Holt, et al, 2009;Ganesh et al, 2020;Simon et al, 2014;Shoemaker et al, 2019Shoemaker et al, , 2020. Mars Orbiter Laser Altimeter hillshade map.…”

“…Surface roughness has been shown to control where SHARAD detects subsurface interfaces (Campbell et al, 2013). Simon et al (2014) suggested that the SHARAD detections are concentrated within "radar dark" backscatter regions in ground-based S-band (12.6 cm) Arecibo observations of Mars (Harmon et al, 2012). Indeed, Harmon et al (2012) interpret these regions to be mantling deposits of low-density material.…”

Dielectric Properties of the Medusae Fossae Formation and Implications for Ice Content

“…Figure 17 shows the numerical solution of Equation for two values of ϕ o (40% and 60%) and K (500 m and 1,800 m) when ρ o = 2.8, corresponding to a “solid” rock dielectric constant of ∼8. The latter is a conservative value for basalt, consistent with low‐end estimates for lava flows in the Tharsis Montes (Carter, Campbell, Holt, et al., 2009b; Simon et al., 2014).…”