Subsurface properties of Lucus Planum, Mars, as seen by MARSIS Roberto Orosei1, Angelo Pio Rossi2, Federico Cantini3, Graziella Caprarelli4,5, Lynn Carter6, Irene Papiano7 1INAF (IT), 2JacobsUni (DE), 3EPFL (CH),4UniSA (AU), 5IRSPS (IT), 6NASA (US), 7Liceo Righi BO (IT) Why Lucus Planum • Located in key area: Dichotomy, Tharsis, Elysium, Medusa Fossae Formation (MFF), 2 (3) landing sites nearby • MFF partially radar-transparent • Lucus Planum variably radar transparent (challenging with SHARAD) • Nature of subsurface material? 2 Geology Gale Gusev Htu AHtu Hesperian Amazonian-Hesperian Source: Tanaka et al. (2014) 3 transitional unit transitional unit Geology Htu AHtu Source: Tanaka et al. (2014) 4 Lucus Planum Htu AHtu Source: Tanaka et al. (2014) 5 Data and Methods • MARSIS radargrams and simulations • Thickness estimation • Subsurface radar reflectors • Estimation of Lucus Planum thickness and ROIs • Interpolation of MOLA topography • Estimation of real depth of subsurface reflector • Derivation of dielectric permittivity • Loss tangent 6 Data • MARSIS, SHARAD respectively on board MEX, MRO • Low-frequency synthetic aperture radar sounders • MARSIS ➔ deep penetration, free- space range resolution of approximately 150 m, a footprint size of 10-20 km in the across-track Image courtesy NASA/JPL-Caltech. direction ranges and 5-10 km in the along-track direction. MARSIS • SHARAD ➔ shallow penetration, but f 1 f 2 f 3 f 4 10x spatial resolution 1.8 Mhz 3.0 Mhz 4.0 Mhz 5 Mhz 7 MARSIS coverage 8 Simulations • Simulations of surface clutter from MOLA MEGDR grid (1/128 px/ deg) • Features visible in the radargram and not in the simulations are most likely real subsurface reflectors • Real reflectors mapped MARSIS - Orbit 4011 Frequency band 1 9 Subsurface reflectors • Position, strength of subsurface echoes extracted manually across Lucus Planum ➔ Echo time delay to apparent depth • Strongest subsurface echoes (weak internal attenuation, strong subsurface reflectivity, or both) in deposits located NW of Apollinaris Patera • No detected subsurface echoes in the central section of Lucus Planum • Subsurface reflections common in the E and NW sectors, up to 2 km (assuming dielectric permittivity = 3) 10 MARSIS radagrams MARSIS visualisation tool —> See Poster Cantini et al. #12545, PS9.5 11 MARSIS - Orbit 13522 Frequency band 1 MARSIS radagrams MARSIS - Orbit 12319 Frequency band 1 MARSIS - Orbit 12319 Frequency band 2 12 MARSIS radagrams MARSIS - Orbit 7013 Frequency band 1 MARSIS - Orbit 7013 Frequency band 2 50 km 13 MARSIS radagrams MARSIS - Orbit 7013 Frequency band 1 MARSIS - Orbit 7013 Frequency band 2 50 km 14 Regions of Interest B • 3 sub-regions selected C within the broader Lucus Planum terrain A • Slightly different response and properties • Surface morphology, erosional stage, age is also slightly different 15 R. Orosei et al.: Radar sounding over Lucus Planum, Mars 3 The depth of reflectors can be estimated from the round- trip timeSubsurface delay between surface and subsurface reflectors echo through the following relation: c ⌧ z = (1) 2p" 5 where z is depth, c the speed of light in vacuo, ⌧ the round- ztrip = depth, time delay between surface and subsurface echo, and " the real part of the relative dielectric permittivity of the Lucus cPlanum = light speed material. in The values of the apparent depths shown in vacuum Fig. 2 have been computed neglecting the effect of ", and thus 10 τrefer = TWT to the distance covered by an electromagnetic wave in εfree = real space part of during the the same round-trip time. As such, appar- relativeent depths dielectric overestimate the thickness of Lucus Planum by a permittivityfactor comprised Lucus between p3 and 3, depending on the nature Planum deposits of the material through which the wave propagates (see e.g., Figure 3. MARSIS coverage over a shaded relief map of Lucus 15 Ulaby et al., 1986, Appendix E). Planum. Ground tracks are plotted as black lines. An estimate the dielectric permittivityassuming for the different propagation re- in vacuum gions of Lucus Planum would provide some insight on their nature and a more precise evaluation of their thickness. Fol- 16 lowing the approach first presented in Picardi et al. (2005) 20 and used also in Watters et al. (2007), we estimate the left side of Eq. 1 by interpolating the topography beneath Lucus Planum from that of the surrounding area. Gλ 2 P = P R 2 (2) s t · 8⇡H ·| s| ✓ ◆ Figure 4. Apparent depth of subsurface echoes detected by MAR- SIS, presumably originating at the base of Lucus Planum. The real depth is obtained dividing the apparent depth by the square root of the relative dielectric permittivity of the medium. 2 2 Gλ 2 Pss = Pt 1 Rs · 8⇡(H + z) · − | | · both, are found within the deposits located NW of Apolli- ✓ ◆ 2 ⇣ ⌘ naris Patera, while no subsurface echoes could be detected in 25 Rss exp( 4⇡f tan⌧) (3) | | · − the central section of Lucus Planum, in spite of several high- SNR observations. Subsurface reflections are common in the Eastern and Northwestern sectors, in some cases to depths of 35 more than 2000 m assuming a dielectric permittivity of about 3 (Watters et al., 2007; Carter et al., 2009). Because subsurface echoes were clustered in specific ar- The positions and strengths of subsurface echoes were ex- eas, Lucus Planum has been subdivided in three Regions of tracted manually from radargrams and mapped across Lu- Interest (ROI), as shown in the map below (Fig. 5) 40 cus Planum, converting echo time delay to apparent depth To estimate the dielectric permittivity, the topography be- (Fig. 4). The strongest subsurface echoes, resulting from neath Lucus Planum has been extrapolated from that of the 30 weak internal attenuation, strong subsurface reflectivity, or surrounding terrains. If this extrapolation is sufficiently ac- Interpolation 1 2 Interpolation of basal topography with MOLA MEGDR 1 2 interpolated base vs. MOLA surface 17 Apparent vs interpolated B A C 18 equation April 15, 2016 equation Dielectric permittivityR. Orosei et al.: Radar sounding over Lucus Planum, Mars 3 1 Introduction The depth of reflectors can be estimated from the round- • In a medium characterized by permittivity ε, the speed of propagation of an EM wave is: trip time delay between surface and subsurface echo through v April=thec/ followingp✏ 15, 2016 relation: (1) • In such medium, the relationship between echo time delay � and depth z is thus: 2 Gλc ⌧ 2 Ps = Pt z = Rs (2) (1) · 8⇡H2p!" ·| | 1 Introduction B • The ratio between the extrapolated thickness and the 5 where z is depth, c the speed of light in vacuo, ⌧ the round- A apparent thickness provides an estimate of 2 trip timep delay✏ between surface and subsurfaceC echo,(1) and " Gλ 2 2 • Pss = Pt the real part of1 the relativeRs dielectric permittivity of the Lucus The relative dielectric permittivity· 8⇡(H estimated+ z) for! ROIs· A − | | · and C is comprised between 5 and Planum6. For ROI material.B, the⇣ The values⌘ of the apparent depths shown in estimated permittivity is above2 10, which2.7 ⌧isf characteristictan δ of Rss 10−Fig. 2v have= c/ beenp✏ computed neglecting the effect(3) of ",(2) and thus dense rocks (lava flows?| bedrock?)| · . 10 refer to the distance covered by an electromagnetic wave in free space during2 the same round-trip19 time. As such, appar- Gλ 2 Ps =entPt depths overestimateRs the thickness of Lucus Planum(3) by a factor· comprised8⇡H ! between·| | p3 and 3, depending on the nature of the material through which the wave propagates (see e.g., Figure 3. MARSIS coverage over a shaded relief map of Lucus 15 Ulaby et al., 1986,2 Appendix E). Planum. Ground tracks are plotted as black lines. AnG estimateλ the dielectric permittivity2 2 for the different re- Pss = Pt 1 Rs · gions8⇡(H of+ Lucusz)! Planum· − would| | provide· some insight on their ⇣ ⌘ nature2 and2.7⌧f atan moreδ precise evaluation of their thickness. Fol- Rss 10− (4) | |lowing· the approach first presented in Picardi et al. (2005) 20 and used also in Watters et al. (2007), we estimate the left side of Eq. 1 by interpolating the topography beneath Lucus Planum from that of the surrounding area. 1 Gλ 2 P = P R 2 (2) s t · 8⇡H ·| s| ✓ ◆ Figure 4. Apparent depth of subsurface echoes detected by MAR- 1 SIS, presumably originating at the base of Lucus Planum. The real depth is obtained dividing the apparent depth by the square root of the relative dielectric permittivity of the medium. 2 2 Gλ 2 Pss = Pt 1 Rs · 8⇡(H + z) · − | | · both, are found within the deposits located NW of Apolli- ✓ ◆ 2 ⇣ ⌘ naris Patera, while no subsurface echoes could be detected in 25 Rss exp( 4⇡f tan⌧) (3) | | · − the central section of Lucus Planum, in spite of several high- SNR observations. Subsurface reflections are common in the Eastern and Northwestern sectors, in some cases to depths of 35 more than 2000 m assuming a dielectric permittivity of about 3 (Watters et al., 2007; Carter et al., 2009). Because subsurface echoes were clustered in specific ar- The positions and strengths of subsurface echoes were ex- eas, Lucus Planum has been subdivided in three Regions of tracted manually from radargrams and mapped across Lu- Interest (ROI), as shown in the map below (Fig. 5) 40 cus Planum, converting echo time delay to apparent depth To estimate the dielectric permittivity, the topography be- (Fig. 4). The strongest subsurface echoes, resulting from neath Lucus Planum has been extrapolated from that of the 30 weak internal attenuation, strong subsurface reflectivity, or surrounding terrains. If this extrapolation is sufficiently ac- R. Orosei et al.: Radar sounding over Lucus Planum, Mars 3 The depth of reflectors can be estimated from the round- trip time delay between surface and subsurface echo through the following relation: R.