Advancing to Lunar Lava Tube Sensing: A New Radar Perspective of Philolaus Skylight Candidates

Shashwat Shukla1,2, Shashi Kumar1 and Valentyn A. Tolpekin2

1Indian Institute of Remote Sensing, ISRO, India | 2Faculty ITC, University of Twente, The Netherlands

Rationale

A new radar perspective enables plausible identification of

water/ice associated with lunar lava tube and skylight

candidates of the Philolaus crater.

Polar selenographic location urges the need for further exploration

into the floor through dedicated rover mission.

LRO NAC images provide evident clues of skylight candidates,

suggestive of possible lava tube in the nearby location [1]-[3].

Incorporating S-band MiniRF data to understand the behavior of an

electromagnetic wave with the regolith.

Higher penetration of the radar wave allows delineating subsurface

features, not observable in optical images.

Philolaus Skylights

Propinquity to the North pole.

Crisscrossing sinuous rilles over the

impact melt deposits on the floor.

Among youngest lunar lava flows.

Uncollapsed sections of rilles exhibit

rimless circular depressions (or pits)

called lava tube skylights [4], [5].

Potential source of volatiles within

subsurface cavities, an entrance to

the lunar attic.

Acknowledgements & ReferencesThe authors are thankful to the Lunar PDS Geoscience Node for providing free and open access to LRO S-band Mini-RF CTLR and LRO Narrow Angle Camera (NAC) data. We would also like to acknowledge Indian Institute of

Remote Sensing, ISRO, Dehradun, India and Faculty of Geo-information Science and Earth Observation (ITC), University of Twente, Enschede, The Netherlands for providing necessary means to carry out this research work.

References: [1] Wagner R. V. & Robinson M. S. (2014). Icarus, 237, 52-60. [2] Lee P. (2018) LPS XLIX, Abstract #2982. [3] Robinson M. S. et al. (2012), Planet. Space Sci., 69, 18-27. [4] Haruyama J. et al. (2009), Geophys. Res. Lett., 36, L21206. [5] Kauahikaua J. et al. (1998), J. Geophys. Res., 103(B11), 27303–27323.

Abstract #2283

Proposed Framework

MiniRF CTLR data are converted to the Stokes parameters after extracting horizontal and vertical polarization channels. The scattering mechanisms are

retrieved through m-Chi decomposition of the received Jones matrix. The data analysis part involves analyzing volumetric scattering powers in conjunction

with CPR measurements for the search of water/ice. NAC images are utilized for closely observing the terrain, avoiding any misinterpretations.

FindingsConfirmation of plausible water/ice near prospective skylight candidates.

Identification of buried lava tube (as nonlinear feature) in the vicinity of

skylights from m-Chi decomposition image.

Enhanced volumetric scattering powers and CPR values from the lava

pits, attributing to the mixture of dipole-like particles with regolith.

Future scope involves deriving petrophysical properties of the Philolaus

crater floor using physics-based backscattering inversion models.

Spectroscopic data for assessing the contribution of space weathering

processes to the detected water/ice signatures.

Shows the potential of radar imaging in lava tube sensing for subsequent

detection of subsurface features associated with buried volatiles.

Tracking of Philolaus skylights from the North pole mosaic of the Moon

(bottom left) to the zoomed version of the crater (top left). Red box

represents the location of the zoomed feature in each instance. Right

image depicts the skylight candidates, marked in red circles, with dotted

white line as buried lava tube connecting the subsurface.

50th Lunar and Planetary Science Conference, The Woodlands, Texas, United States

© 2019 Indian Institute of Remote Sensing, ISRO and Faculty ITC, University of Twente

3D visualization of m-Chi decomposition (left) and CPR image (right). Mixed scattering mechanisms retrieved from populated microcraters on the floor. Enhanced volume scattering power observed from regions

near prospective skylights. Black arrows in the SC and OC image track the plausible buried nonlinear inclusion, while white arrow locates the skylights. Evidence of water/ice in the proximal regions of skylights

attributes to higher proportions of dipole-like features. This exhibits dominant volume scattering pattern and CPR value greater than 1. Source of water/ice is, however, not confirmed yet.

LRO NAC image (left) is used to particularly mark prospective skylights (red circle). White dotted line

delineates plausible buried lava tube geometry. Two white boxes relate the regions in both NAC image and m-

Chi decomposition image (right). High resolution observations reconfirm the water/ice hypothesis.

m-Chi Decomposition Image CPR Image

SC Image

OC Image