J/AJ/159/129 21 saturnian small moons brightness with ISS (Hedman+, 2020)
Photometric analyses of Saturn's small moons: Aegaeon, Methone, and Pallene are
Dark; Helene and Calypso Are Bright.
Hedman M.M., Helfenstein P., Chancia R.O., Thomas P., Roussos E.,
Paranicas C., Verbiscer A.J.
<Astron. J., 159, 129 (2020)>
=2020AJ....159..129H 2020AJ....159..129H
ADC_Keywords: Photometry; Solar system
Keywords: Saturnian satellites ; Natural satellites (Solar system) ;
Photometry ; Surface photometry ; Planetary magnetosphere ;
Planetary rings
Abstract:
We examine the surface brightnesses of Saturn's smaller satellites
using a photometric model that explicitly accounts for their elongated
shapes and thus facilitates comparisons among different moons.
Analyses of Cassini imaging data with this model reveal that the moons
Aegaeon, Methone, and Pallene are darker than one would expect given
trends previously observed among the nearby mid-sized satellites. On
the other hand, the trojan moons Calypso and Helene have substantially
brighter surfaces than their co-orbital companions Tethys and Dione.
These observations are inconsistent with the moons's surface
brightnesses being entirely controlled by the local flux of E-ring
particles, and therefore strongly imply that other phenomena are
affecting their surface properties. The darkness of Aegaeon, Methone,
and Pallene is correlated with the fluxes of high-energy protons,
implying that high-energy radiation is responsible for darkening these
small moons. Meanwhile, Prometheus and Pandora appear to be brightened
by their interactions with the nearby dusty F-ring, implying that
enhanced dust fluxes are most likely responsible for Calypso's and
Helene's excess brightness. However, there are no obvious structures
in the E-ring that would preferentially brighten these two moons, so
there must either be something subtle in the E-ring particles's
orbital properties that leads to asymmetries in the relevant fluxes,
or something happened recently to temporarily increase these moons's
brightnesses.
Description:
The disk-integrated brightness estimates considered in this study are
derived from images obtained by the Narrow Angle Camera (NAC) of the
Imaging Science Subsystem (ISS) on board the Cassini Spacecraft. These
images were all calibrated using version 3.9 of the Cisscal package to
remove dark current and instrumental electronic noise, apply
flat-field corrections, and convert the raw data numbers to values of
radiance factors I/F, a standard dimensionless measure of reflectance
that is unity for an illuminated Lambertian surface viewed at normal
incidence and emission angles.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table8.dat 128 168 Brightness estimates from clear-filter images of
128 Aegaeon
table9.dat 128 167 Brightness estimates from clear-filter images of Anthe
table10.dat 128 187 Brightness estimates from clear-filter images of
128 Methone
table11.dat 128 159 Brightness estimates from clear-filter images of
128 Pallene
table12.dat 128 142 Brightness estimates from clear-filter images of
128 Telesto
table13.dat 128 157 Brightness estimates from clear-filter images of
128 Calypso
table14.dat 128 122 Brightness estimates from clear-filter images of
128 Helene
table15.dat 128 136 Brightness estimates from clear-filter images of
128 Polydeuces
table16.dat 128 41 Brightness estimates from clear-filter images of Pan
table17.dat 128 71 Brightness estimates from clear-filter images of Atlas
table18.dat 128 40 Brightness estimates from clear-filter images of
128 Prometheus
table19.dat 128 40 Brightness estimates from clear-filter images of
128 Pandora
table20.dat 128 53 Brightness estimates from clear-filter images of Janus
table21.dat 128 47 Brightness estimates from clear-filter images of
128 Epimetheus
table22.dat 128 76 Brightness estimates from clear-filter images of Mimas
table23.dat 128 82 Brightness estimates from clear-filter images of
128 Enceladus
table24.dat 128 67 Brightness estimates from clear-filter images of
128 Tethys
table25.dat 128 67 Brightness estimates from clear-filter images of Dione
table26.dat 128 74 Brightness estimates from clear-filter images of Rhea
table27.dat 87 18 Brightness estimates from color images of Aegaeon
table28.dat 87 14 Brightness estimates from color images of Methone
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See also:
J/A+AS/136/257 : Saturn's satellites in 1995/97 (Harper+ 1999)
J/A+A/383/296 : Saturnian Satellites positions (1996-2000) (Peng+, 2002)
J/A+A/422/377 : 1997-2000 Saturn's satellites astrometry (Qiao+, 2004)
J/A+A/485/293 : Events of Saturn satellites during 2009 equinox (Arlot+, 2008)
J/AJ/136/2214 : Positions of Saturn & its satellites in 2002-2006 (Peng+,2008)
J/A+A/493/1183 : Saturn major satellites obs. (1874-2007) (Desmars+, 2009)
J/other/SoSyR/45.523 : CCD obs. of saturnian satellites (Grosheva+, 2011)
J/A+A/572/A43 : Saturnian satellites Cassini ISS astrometry (Cooper+, 2014)
J/A+A/575/A73 : Astrometry of Saturnian satellites 2004-12 (Tajeddine+, 2015)
J/A+A/582/A8 : Astrometry of the main satellites of Uranus (Camargo+, 2015)
J/AJ/149/27 : Cassini ISS astrometry of Saturnian satellites (Cooper+, 2015)
J/ApJ/811/67 : Saturn's G and D68 rings ISS observations (Hedman+, 2015)
J/other/KFNT/33.70 : Topocentric positions Saturn's moons (Yizhakevych+, 2017)
J/A+A/610/A2 : Saturnian satellites Cassini ISS astrometry (Cooper+, 2018)
J/A+A/614/A15 : Mars Express astrometric obs. of Martian moons (Ziese+, 2018)
Byte-by-byte Description of file: table[89].dat table[12][0-6].dat
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Bytes Format Units Label Explanations
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1-11 A11 --- File Image Filename
13-21 F9.1 km Rad Distance between spacecraft and body center
23-26 F4.1 deg Phase Phase Angle
28-32 F5.1 deg S/CLat Sub-spacecraft planetocentric latitude
34-38 F5.1 deg S/CLon Sub-spacecraft west longitude
40-44 F5.1 deg SolLat Sub-solar planetocentric latitude
46-50 F5.1 deg SolLon Sub-solar west longitude
52-63 F12.5 km2 Aeff Effective area value
65-76 F12.5 km2 e_Aeff ? Statistical uncertainty in Aeff (G1)
78-89 F12.4 km2 Aphys ? Geometric cross section
91-102 F12.4 km2 aPre1 ? Predicted scaled effective area (G2)
104-115 F12.4 km2 aPre0.75 ? Predicted scaled effective area (G2)
117-128 F12.4 km2 aPre0.5 ? Predicted scaled effective area (G2)
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Byte-by-byte Description of file: table2[78].dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1-11 A11 --- File Image Filename
13-15 A3 --- Filt Filter
17-23 F7.1 km Rad Distance between spacecraft and body center
25-28 F4.1 deg Phase Phase Angle
30-34 F5.1 deg S/CLat Sub-spacecraft planetocentric latitude
36-40 F5.1 deg S/CLon Sub-spacecraft west longitude
42-45 F4.1 deg SolLat Sub-solar planetocentric latitude
47-51 F5.1 deg SolLon Sub-solar west longitude
53-59 F7.5 km2 Aeff Effective area value
61-66 F6.4 km2 Aphys ? Geometric cross section
68-73 F6.4 km2 aPre1 ? Predicted scaled effective area (G2)
75-80 F6.4 km2 aPre0.75 ? Predicted scaled effective area (G2)
82-87 F6.4 km2 aPre0.5 ? Predicted scaled effective area (G2)
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Global Note:
Note (G1): Error estimate is based on scatter of brightness values in
regions near the moon.
Note (G2): Predicted scaled effective area
computed assuming Minnaert k=1 (aPre1), k=0.75 (aPre0.75), and k=0.5
(aPre0.5) scattering law.
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History:
From electronic version of the journal
(End) Prepared by [AAS], Coralie Fix [CDS], 29-Jun-2020