J/A+A/674/A82 Lunar swirls Reiner Gamma images (Bhatt+, 2023)
Unique regolith characteristics of the lunar swirl Reiner Gamma as
revealed by imaging polarimetry at large phase angles.
Bhatt M., Woehler C., Rogall J., Aravind K., Ganesh S., Bhardwaj A.
<Astron. Astrophys. 674, A82 (2023)>
=2023A&A...674A..82B 2023A&A...674A..82B (SIMBAD/NED BibCode)
ADC_Keywords: Solar system
Keywords: Moon - methods: data analysis - techniques: polarimetric
Abstract:
Lunar swirls are high-albedo irregular markings that are gene rally
associated with prominent magnetic anomalies. The formation of swirls
is still unknown. Near-infrared spacecraft-based imaging suggests
reduced space weathering at the locations of swirls. However, the
reduced space weathering alone cannot explain the observed spectral
properties.
We provide detailed physical characteristics of the regolith at the
Reiner Gamma swirl. For the first time, systematic telescopic
observations in a range of phase angles are used to derive the surface
roughness, opposition effect strength, and grain size distribution at
a spatial resolution of 1km.
Imaging polarimetric observations of Reiner Gamma were obtained at the
Mount Abu IR Observatory between January and March, 2021. These
observations were collected with the two narrow-band continuum
filters, GC (green) and RC (red), in a range of phase angles. The
georeferenced polarimetric images were used to derive the
single-scattering albedo, photometric roughness, and amplitude of the
opposition effect by adopting the Hapke reflectance model. We further
computed median regolith grain size maps of Reiner Gamma using the
derived photometric roughness, albedo, and degree of polarization.
A comparison of the polarization properties of Reiner Gamma swirl with
the craters Kepler and Aristarchus suggests grain size variations
within the swirl structure. The Hapke modeling of the Reiner Gamma
swirl suggests significant changes in the opposition effect strength
at the central oval, but only marginal differences in surface
roughness from its surroundings. Within the swirl, the median grain
size varies significantly in comparison to the background mare grain
size of ∼45um.
Our results confirm the occurrence of surface alteration processes
that might have disrupted the regolith microstructure in the Reiner
Gamma swirl. These findings are consistent with an external mechanism
of swirl formation, by considering interaction between the regolith
and cometary gas. Subsequent to its formation, the swirl structure was
preserved due to shielding by crustal magnetic field.
Description:
The data were acquired between January 5th and March 5th, 2021, wit h
the 1.2m telescope of Mount Abu Observatory in Rajasthan, India,
using the CCD-based imaging polarimeter EMPOL. The image resolution is
1/3arcsec/pixel, corresponding to about 0.6km/pixel on the lunar
surface. Scale of the maps in pixels per degree is 30. The
georeferenced maps are provided in simple cylindrical projection. The
latitude and longitude range of all maps are:
latitude: 0°-20°N, longitude: 70°W-45°W. Further
details about the individual observations can be found in Table B.1 of
the above mentioned paper.
The data are provided in FITS format. Each folder corresponds to one
individual observation. The folder contains five files with the
following name style:
RGXXsYYMMDDV.fits
Where,
RG: Reiner Gamma
XX: filter used; gc (green continuum) or rc (red continuum)
s: set of observation (example s,s1,s2,s3).
The files with no set number indicate only a single set observed.
YYMMDD: year,month, and date of observation (example: 20210205)
V: map of-
F: unpolarized flux (intensity) [DN];
A: amplitude of cosine function fitted to polarization angle specific
pixel intensities [DN];
E: root-mean-square residual error of fitted cosine function,
normalized to intensity;
P: fraction of linearly polarized light; W: relative direction of
the plane of polarization
File Summary:
--------------------------------------------------------------------------------
FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
tableb1.dat 73 38 Summary of the observations and the
relative blur parameters
files/* . 38 Individual folders
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See also:
https://zenodo.org/record/7774449#.ZC_0CY7P1-V : Files in matlab format
Byte-by-byte Description of file: tableb1.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 10 A10 "date" Date Observation date
12- 17 F6.2 deg Angle Phase angle
19- 20 A2 --- Filter [RV GC] Filter used, GC for green continuum
or RC for red continuum
22- 23 A2 --- Set [s s1 s2 s3 s4] set of observation
25- 32 F8.5 --- Blur Relative blur
34- 39 F6.4 ---- NSR Noise to signal ratio
41- 47 F7.5 --- Kb K coefficient for blurred image
49- 55 F7.5 --- Ks K coefficient for sharpened image
57- 73 A17 --- Rep Folder containing the 5 fits files
in subdirectory file (1)
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Note (1): All files have name as RGXXsYYMMDDV.fits, with, for "V":
F = unpolarized flux (intensity) [DN]
A = amplitude of cosine function fitted to polarization angle specific pixel
intensities [DN]
E = root-mean-square residual error of fitted cosine function,
normalized to intensity
P = fraction of linearly polarized light
W = relative direction of the plane of polarization
--------------------------------------------------------------------------------
Acknowledgements:
Megha Bhatt, mubhatt19(at)gmail.com
(End) Patricia Vannier [CDS] 07-Apr-2023