Improved discrimination of volcanic complexes, tectonic features, and regolith properties in Mare Serenitatis from Earth-based radar mapping

Abstract

Radar images at 70 cm wavelength show 4-5 dB variations in backscatter strength within regions of relatively uniform spectral reflectance properties in central and northern Mare Serenitatis, delineating features suggesting lava flow margins, channels, and superposition relationships. These backscatter differences are much less pronounced at 12.6 cm wavelength, consistent with a large component of the 70 cm echo arising from the rough or blocky transition zone between the mare regolith and the intact bedrock. Such deep probing is possible because the ilmenite content, which modulates microwave losses, of central Mare Serenitatis is generally low (2-3% by weight). Modeling of the radar returns from a buried interface shows that an average regolith thickness of 10 m could lead to the observed shifts in 70 cm echo power with a change in TiO 2 content from 2% to 3%. This thickness is consistent with estimates of regolith depth (10-15 m) based on the smallest diameter for which fresh craters have obvious blocky ejecta. The 70 cm backscatter differences provide a view of mare flow-unit boundaries, channels, and lobes unseen by other remote sensing methods. A localized pyroclastic deposit associated with Rima Calippus is identified based on its low radar echo strength. Radar mapping also improves delineation of units for crater age dating and highlights a 250 km long, east-west trending feature in northern Mare Serenitatis that we suggest is a large graben flooded by late-stage mare flows.

Planetary Geology with Imaging Radar: Insights from Earth-based Lunar Studies, 2001–2015

“…Comparison of visible (left) and high-resolution 70 cm radar (right) views of Mare Serenitatis. The long-wavelength, SC polarization radar view detects very subtle changes in the ilmenite content of basalt units, revealing an intricate pattern of flow boundaries not evident in other data sets (Campbell et al 2014). …”

“…The total radar echo, expressed as a dimensionless backscatter coefficient, σ 0 , is (Campbell et al 2014):…”

“…Short wavelengths (X-band or 3-4 cm) penetrate less than a meter, and are scattered by even quite small (sub-cm diameter) rocks. Since these rocks are typically far more numerous in ejecta than rocks of decimeter to meter diameter (Section 4), X-band images often show a strong signature from the abundant small impact crater population on any airless (Campbell et al 2014). The relative importance of surface, volume, and substrate scattering will vary with the regolith thickness, composition, and the illuminating radar wavelength.…”

“…Despite the shorter baud, the increase of about a factor of four in pulse length (from 3 to 13 μs) offsets the lower echo strength of the smaller surface resolution cells. Observations on multiple days are added to obtain a 5-to 9-look final product to reduce speckle effects (Campbell et al 2014). To date, we have obtained coverage of many of the nearside mare regions (Figure 8) and the north pole with this higher spatial resolution.…”

Lunar regolith thickness estimation using dual frequency microwave brightness temperature and influence of vertical variation of FeO+TiO2

“…An empirical relationship was derived for the thickness estimation, which is discussed in detail in Section 3.2.2. Along with the lateral variation, recent studies (Fa and Wieczorek, 2012;Campbell et al, 2014) have shown that the loss tangent depends on the abundance of ilmenite that can be indicated by TiO 2 . However, in our study we utilised the Fa and Jin (2010) model and considered the influence from both FeO and TiO 2 content.…”