NEW CONSTRAINTS ON MID-LATITUDE GLACIER DEBRIS LAYER COMPOSITION FROM SHARADAND HIRISE E. I. Petersen1, J. W. Holt1, J. S. Levy1, T. A. Goudge2, and E. A. McKinnon3 1Institute for Geophysics,University of Texas at Austin, Austin TX (eric petersen@utexas.edu),2Jackson School of Geosciences, University ofTexas at Austin, 3Bureau of Economic Geology, University of Texas at Austin

Introduction: Features thought to be debris-coveredglaciers (DCG) are abundant in the northern and south-ern mid-latitudes of Mars [1]. Many DCG exhibit basalreflectors in MRO Shallow Radar (SHARAD) soundingdata, consistent with a composition of nearly pure waterice under a debris cover no thicker than 10 m [2,3].

Not all DCG exhibit basal reflectors. In the di-chotomy boundary region of Deuteronilus Mensae someDCG exhibit weak, fading reflectors (Site C, Fig. 1) orno reflectors at all (Site D, Fig. 1).

Varied radar properties may be the result of differingbulk compositions, differing physical properties of thesurface debris layer, or differing properties of the basalinterface. In this work, which focuses on 5 DCG com-plexes in Deuteronilus Mensae (2 each with SHARADdetections and nondetections), we show that surface mor-phologies attributable to glacial and periglacial processesmay play a strong role, while there is little evidence forthe role of varied internal characteristics.

Methods: SHARAD Reflection Strength: We ex-tracted SHARAD reflection amplitudes for observedsubsurface reflections. These amplitudes are normal-ized to the surface return and fit to a linear regressionas a function of the thickness of the glacial deposit toparametrize the attenuation of the radar signal throughthe DCG interior, as well as compare surface to subsur-face reflection strength. If there are significant differ-ences in DCG internal composition, this will be reflectedin our empirical treatment of attenuation.

HiRISE Geomorphology: We examined surface geo-morphology at the tens to hundred meter scale using 3-4HiRISE images over each site. Surfaces were charac-terized into 3 broad categories and 8 subcategories basedupon general appearance, surface roughness, and hypoth-esized formation mechanism. These include brain ter-rain, polygons, and other.

Brain terrain exhibits high roughness at the tens ofmeters scale. This scale is relevant to SHARAD as it pro-duces incoherent summation losses. Brain terrain is hy-pothesized to form via infilling of extensional or thermalcracking in an icy substrate with overlying sediments,followed by ice table lowering [5].

Thermal contraction-crack polygons [6] are verysmooth at the scale relevant to SHARAD and may serveto mask or smooth out underlying and neighboring mor-phologies.

HiRISE DTM Surface Roughness: HiRISE stereo ob-servations are currently available for Sites A, C, and D.We produced DTMs for these using the Ames Stereo

Figure 1: MOLA-derived map of Deuteronilus Mensaewith confirmed detections and non-detections of LDAbasal reflectors mapped. Selected LDA, designated SitesA, B, C, D, and E, are highlighted in orange boxes.

Pipeline [7,8], giving reliable results for the topographyof features on the tens of meters scale. Roughness heightdeviation is produced by subtracting the mean elevationin a surrounding 50 meter wide square cell from eachDTM pixel.

We then used the roughness height values to calculatethe expected transmission losses in a subsurface return asit passes through the surface, following the methods of[9]:

σφ =4πσhλ

(√�− 1)

ρ = e−σ2φI20 (

σ2φ2)

Where σφ is the rms phase delay in the SHARAD sig-nal calculated from the roughness height σh, SHARADwavelength λ (15 m), and the dielectric constant of thedebris layer �. The signal loss ρ is then calculated fromσφ using the zeroth-order Bessel function of the firstkind, I0.

Results: SHARAD Reflection Amplitudes: Typicalvalues for strong, shallow basal reflections and surfacereflections from DCG are -25 to -30 dB. SHARAD’sSNR performance is nominally -50 dB relative to a spec-ular reflection from a � = 3 surface [11], and in practiceis roughly -40 to -45 dB. Observed SHARAD reflectionson DCG are thus roughly 10-30 dB above SNR. Atten-uation values calculated at each of Sites A (-25 dB/km),B (11 dB/km), C (-16 dB/km), and E (-27 dB/km) wereroughly similar to values found by [2,3] (-9-18 dB/km),notably with the inclusion of Site C. Site C is an anoma-lously thick DCG deposit, exhibiting SHARAD reflec-tions of comparable amplitude to the other sites fadingout to SNR at a depth of 1.2 km. Site D exhibits no re-flectors and thus is not included in this analysis.

HiRISE Geomorphology: Geomorphic characteriza-

6084.pdfSixth Mars Polar Science Conference (2016)

mailto:eric_petersen@utexas.edu

2

Figure 2: Roughness derived on fields of brain terrain inHiRISE DTMs produced for Sites A, C, and D.Hillshades display the faithfulness of the DTMs toobserved topography, with lighting from the NW. Allimages are at the same scale.

tion revealed the following: Site A is dominated by brainterrain and altered by ejecta from a nearby synglacialcrater, Site B is dominated by polygonal mantle deposits,Site C is dominated by brain terrain arranged in flow lin-eations, Site D is dominated by linear brain terrain, flowlineations and deflated troughs, and Site E is dominatedby polygons punctuated by large fields of brain terrain.

All of the sites exhibited brain terrain. The percent-age of DCG surface area covered by brain terrain corre-lated poorly with radar results. In most cases brain ter-rain was randomly oriented, with the exception of SiteD. At Site D brain terrain is typically linearly orientedsub-orthogonal to inferred glacial flow directions, andexhibits larger scales of roughness.

HiRISE DTM Roughness: Roughness values calcu-lated for the HiRISE DTMs were similar between SitesA and C, with 7.4% of the surface at roughness values of>2 m. The DTM at Site D was much rougher, with 35%of the surface at roughness values of >2 m.

Theoretical transmission losses can thus be quite sig-nificant for SHARAD (Fig. 3B) at Site D. If we assumea debris layer formed of basaltic clasts (� = 9) with aporosity of 30%, we can estimate the effective dielectricconstant to be � = 5.7 from the Maxwell-Garnett mixingformula [10]. Transmission losses for this value are cal-

Figure 3: (A) Roughness heights and (B) subsequentSHARAD signal loss as a function of dielectric constantcalculated from HiRISE DTMs on Sites A, C, and D.The black dashed line in (B) corresponds to thedielectric constant expected for a debris layer composedof basalt clasts with 30% porosity (see text).

culated at 40 dB for Site D, while Sites A and C exhibitmodest losses at 8-10 dB.

Discussion: The presence and extent of brain terrainon DCG surfaces does not correlate with SHARAD per-formance, which would seem to indicate that it is not aprimary control. However, differences in brain terrainbutte heights play a much stronger role than extent.

At Site C, signal loss is achieved at depth through at-tenuation typical of pure DCG ice, similar to other sites.At Site D, higher roughness brain terrain induces moresignificant signal losses, providing an additional -30 dBsignal loss over other sites at an assumed debris layerdielectric constant of � ≥ 5.5. The higher roughnessbrain terrain at Site D, along with its orientation sub-orthogonal to flow, may be the result of ice crevassingdominating over thermal contraction-cracking in seed-ing brain terrain formation. This provides us insight intovarying conditions in the ancient flow state and debrislayer emplacement of DCGs.

These findings support the hypothesis that all studysite DCG are composed of pure ice overlain by a debrislayer composed of bedrock clasts, and that debris layerproperties as well as DCG thickness are responsible forvaried radar properties.

References: [1] Levy, J., et al., 2014, JGR: Plan-ets, 119(10), [2] Holt, J., et al. (2008), Science, 322,1235-1238. [3] Plaut, J., et al. (2009), GRL, 36(2). [4]Parsons, R., et al. (2011), Icarus, 214(1), 246-257. [5]Levy, J., et al. (2009), Icarus, 202(2), 462-476. [6] Head,J., et al. (2003), Nature, 426(6968), 797-802. [7] Brox-ton, M., L. Edwards (2008), LPSC 39, 2419, [8] Moratto,Z., et al. (2010), LPSC 41, 2364. [9] Schroeder, D., etal. (2016), Geophysics, 81(1). [10] Maxwell-Garnett, J.(1904), Philos. Trans. R. Soc. London, 203(385), [11]Nunes and Phillips (2006), JGR, 111(E6).

6084.pdfSixth Mars Polar Science Conference (2016)