Bell, S. K.

Bickel, V. T.

Czaplinski, E. C.

Farrant, B. E.

Harrington, E. M.

Honniball, C. I.

Kring, D. A.

Martinez, S. N.

Rogaski, A.

Sargeant, Hannah

Tolometti, G. D.

English

Open Research Online

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsDetermining the Bearing Capacity of PermanentlyShadowed Regions of the Moon using Boulder TracksConference or Workshop ItemHow to cite:Sargeant, H. M.; Bickel, V. T.; Honniball, C. I.; Martinez, S. N.; Rogaski, A.; Bell, S. K.; Czaplinski, E. C.;Farrant, B. E.; Harrington, E. M.; Tolometti, G. D. and Kring, D. A. (2019). Determining the Bearing Capacity ofPermanently Shadowed Regions of the Moon using Boulder Tracks. In: 50th Lunar and Planetary Science Conference2019, 18-22 Mar 2019, Houston, TX.For guidance on citations see FAQs.c© [not recorded]Version: Version of RecordLink(s) to article on publisher’s website:https://www.hou.usra.edu/meetings/lpsc2019/pdf/1792.pdfCopyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.ukDETERMINING THE BEARING CAPACITY OF PERMANENTLY SHADOWED REGIONS OF THE MOON USING BOULDER TRACKS. H. M. Sargeant1, V. T. Bickel2,3, C. I. Honniball4, S. N. Martinez5, A. Ro-gaski6, S. K. Bell7, E. C. Czaplinski8, B. E. Farrant7, E. M. Harrington9, G. D. Tolometti9, and D. A. Kring10. 1The Open University, United Kingdom (Email: hannah.sargeant@open.ac.uk), 2Max Planck Institute for Solar System Research, Germany,3Swiss Federal Institute of Technology, Switzerland, 4University of Hawai’i, United States, 5Tulane University, United States, 6South Dakota School of Mines & Technology, United States, 7University of Manchester, United Kingdom, 8University of Arkansas, United States, 9University of Western Ontario, Canada, 10Lunar and Planetary Institute, Universities Space Research Association, United States,11NASA Solar System Ex-ploration Research Virtual Institute, United States.   Introduction:  Permanently Shadowed Regions (PSRs) are areas that do not experience direct sunlight and are commonly found in the polar regions of the Moon. PSRs can reach temperatures as low as 30 K and are of interest because there is direct and indirect evidence to suggest the presence of water (H2O/OH-) inside most PSRs [1,2].  Several missions to explore PSRs have been pro-posed, but little is known about the strength of regolith near potential landing sites. Hints about regolith poros-ity have been extracted: The LCROSS impactor into a lunar PSR resulted in ejecta angles and flashes ex-pected in highly porous material of ~70 % to equiva-lent depths of 2-3 m [3]; LRO DIVINER low thermal inertia values suggest the upper few centimeters of high-latitude regions of the Moon are highly porous [4]; and laboratory studies suggest material experienc-ing low thermal cycling, such as that in PSRs, will be more porous [5]. If such high porosity soils truly exist, they may not be able to sustain loads as well as that in areas already explored on the Moon, requiring signifi-cant changes to landing pad and rover wheel designs. To test those findings, we analyze boulder tracks to determine the bearing capacity of lunar soils in lunar south polar PSRs.  Those results are then used to eval-uate the trafficability of these regions. Lunar boulder tracks: Rockfalls and their associ-ated boulder tracks are abundant on the Moon [6,7]. The dimensions of tracks with respect to associated boulders can be used to infer soil strength using bear-ing capacity theory. This work uses a variation of Han-sen’s formula [8]:  qf=cNcscdcicbcgc+qoNqsqdqiqbqgq+0.5γBHNγsγdγiγbγgγ (1)   where c is the cohesion of the soil, q0 is the vertical stress within the soil, γ is the unit weight of the soil, B is the width of footing, N(c,q,γ) are the bearing capacity factors, d(c,q,γ) are the depth factors, s(c,q,γ) are the shape factors, g(c,q,γ) are the local slope inclination factors, and i(c,q,γ) and  b(c,q,g) are the load and foundation incli-nation factors, respectively.  Hansen’s formula consid-ers slope angles and rectangular shaped boulders. This was deemed suitable for application to the generally non-spherical boulders found on the Moon and their boulder tracks which are generally formed on crater walls and slopes [8]. Methods: Representative PSRs were selected from a recent map of PSRs [9]. Narrow Angle Camera (NAC) images of those areas were stretched by enhanc-ing contrast and brightness to identify boulder tracks in shadowed areas. Only areas with secondary sunlight, diffusely reflected from crater walls into PSRs, were used in this initial study as a minimum amount of illu-mination was required to identify boulders.  Images were processed with two customized spatial filters to remove excess noise that was amplified during NAC contrast and brightness adaptation (Fig. 1). Boul-der and track dimensions, and the associated shadows produced, were recorded for each of 13 boulder tracks identified in 5 PSRs. The illumination angle of the sec-ondary light was estimated by determining the mid-point of the illuminated slope and its height above the boulder and its track. The illumination angle could then be used with track measurements to estimate the track depth.    Fig. 1 Pre- and post- image stretching and filtering. A boulder track can be seen entering a shadowed region.   All measurements and soil properties from the liter-ature were then input into Hansen’s formula (1) to es-timate the bearing capacity. All results were compared with values obtained in previous work from this team [8] for highland, mare, and pyroclastic regions on the Moon.  1792.pdf50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132)Results: The analysis indicates boulder tracks formed in PSRs have qualitatively similar morpholo-gies to those formed in highland, mare, and pyroclastic regions (Fig. 2).    Fig. 2 Left – boulder track in Aristarchus (highland slope). Right - boulder track near Schrödinger (PSR slope).  Calculated bearing capacities increase with depth for all terrains, although PSR and pyroclastic regolith is generally stronger than highland and mare regolith at equivalent depths (Fig. 3).   Fig. 3 Bearing capacity as a function of depth for all location types on all slopes.  Calculated bearing capacities and interpolation of PSR data also indicate steeper slopes cannot support boulders as well as flatter slopes (Fig. 4), a result of the reduced soil volume bearing the boulder [10]. PSRs are significantly stronger than mare regions in the upper 0.28 to 1 m of regolith and at slopes of 0°, with esti-mated bearing capacities of 123±18 kNm-2 and 93±23 kNm-2, respectively. PSRs are statistically similar to pyroclastic deposits which have calculated bearing capacities of 131±21 kNm-2 [8].  There was insufficient data in the upper 1 m of highland regolith for a statisti-cal comparison.   Fig. 4 Bearing capacity as a function of slope for a range of boulder track depths for PSRs.  It should be noted that this technique is limited by the minimum depth of measured boulder tracks (≳28 cm in PSRs). The minimum track depth measured is constrained by the minimum track shadow length measured, which in turn is constrained by the resolu-tion of the available NAC imagery (~0.5 m/pixel).  Discussion and Conclusions: This boulder track study suggests that regolith in PSRs is significantly stronger than mare regolith at depths ≳28 cm, poten-tially contradicting reports of very high porosity in PSRs [3,4,5] or restricting those proposed high porosi-ty conditions to the uppermost 28 cm.  In either case, trafficability of PSRs may be possible with wheel di-ameters ≳56 cm. In situ analyses are still required to verify these results and to establish the strength of the uppermost 28 cm of regolith.  Acknowledgements: We thank USRA-LPI, CLSE, and NASA SSERVI for support.  References: [1] Aye K-M. et al. (2014) LPSC XLV, Abstract #2893. [2] Li S. et al. (2018) PNAS, 115, 8907–8912. [3] Schultz P.H. et al. (2010) Science, 330, 468-472. [4] Hayne P.O. et al. (2017) JGR: Planets, 122, 2371-2400. [5] Metzger P.T. et al. (2018) 16th Biennial Int. Conf. on Eng., Sci., Construction, and Ops. in Challenging Environments. [6] Grolier M. J. et al. (1968) Langley Working Paper 506, 143-154. [7] Kumar P. et al. (2016) JGR: Planets, 121, 147-179.[8] Bickel V. T. et al. (2018) JGR (sub-mitted). [9] McGovern J.A. et al. (2013) Icarus, 223, 566–581. [10] Meyerhof G. G. (1957) Proc. 4th Int. Conf. on Soil Mechanics and Foundation Engineering. 384-386. 1792.pdf50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132)

Determining the Bearing Capacity of Permanently Shadowed Regions of the Moon using Boulder Tracks

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Determining the Bearing Capacity of Permanently Shadowed Regions of the Moon using Boulder Tracks

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