J/AJ/160/238 Reflectivity of 4 Galilean satellites with ISS (Mayorga+, 2020)
Reflected light observations of the Galilean satellites from Cassini: a test
bed for cold terrestrial exoplanets.
Mayorga L.C., Charbonneau D., Thorngren D.P.
<Astron. J., 160, 238 (2020)>
=2020AJ....160..238M 2020AJ....160..238M
ADC_Keywords: Solar system; Planets; Photometry; Optical
Keywords: Galilean satellites ; Exoplanet surface characteristics ;
Exoplanet atmospheres
Abstract:
For terrestrial exoplanets with thin or no atmospheres, the surface
contributes light to the reflected light signal of the planet.
Measurement of the variety of disk-integrated brightnesses of bodies
in the solar system and the variation with illumination and wavelength
is essential for both planning imaging observations of directly imaged
exoplanets and interpreting the eventual data sets. Here we measure
the change in brightness of the Galilean satellites as a function of
planetocentric longitude, illumination phase angle, and wavelength.
The data span a range of wavelengths from 400 to 950nm and
predominantly phase angles from 0° to 25°, with some
constraining observations near 60°-140°. Despite the
similarity in size and density between the moons, surface
inhomogeneities result in significant changes in the disk-integrated
reflectivity with planetocentric longitude and phase angle. We find
that these changes are sufficient to determine the rotational periods
of the moon. We also find that at low phase angles, the surface can
produce reflectivity variations of 8%-36%, and the limited high phase
angle observations suggest variations will have proportionally larger
amplitudes at higher phase angles. Additionally, all of the Galilean
satellites are darker than predicted by an idealized Lambertian model
at the phases most likely to be observed by direct imaging missions.
If Earth-sized exoplanets have surfaces similar to that of the
Galilean moons, we find that future direct imaging missions will need
to achieve precisions of less than 0.1ppb. Should the necessary
precision be achieved, future exoplanet observations could exploit
similar observation schemes to deduce surface variations, determine
rotation periods, and potentially infer surface composition.
Description:
While en route to Saturn, Cassini/ISS took tens of thousands of images
of Jupiter during a flyby spanning from 2000 October to 2001 March.
The two cameras, the Wide Angle Camera (WAC) and the Narrow Angle
Camera (NAC), had identical 1024 by 1024 pixel CCD detectors, but the
resulting fields of view (FOVs) for the WAC and NAC were 3.5° and
0.35°, respectively. The data span a range of wavelengths from 400
to 950nm.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table3.dat 99 8000 Tabulated reflectivity as a function of moon,
filter, phase angle, and planetocentric longitude
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See also:
J/A+A/532/A36 : Galilean satellites mutual events in 2009 (Zhang, 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/580/A76 : Positions of satellites of giant planets (Gomes-Junior+, 2015)
J/A+A/582/A8 : Astrometry of the main satellites of Uranus (Camargo+, 2015)
J/other/SoSyR/49.383 : Galilean satellites, Jupiter position (Narizhnaya, 2015)
J/AJ/153/147 : The orbits of Jupiter's irregular satellites (Brozovic+, 2017)
J/other/KFNT/33.70 : Topocentric positions Saturn moons (Yizhakevych+, 2017)
J/A+A/610/A2 : Saturnian satellites Cassini ISS astrometry (Cooper+, 2018)
J/other/SoSyR/53.368 : Observations of the Galilean moons (Narizhnaya+, 2019)
J/AJ/159/129 : 21 saturnian small moons brightness with ISS (Hedman+, 2020)
Byte-by-byte Description of file: table3.dat
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Bytes Format Units Label Explanations
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1- 8 A8 --- Moon Galilean Satellite identification; Callisto,
Europa, Ganymede and Io
10- 15 A6 --- Filt Cassini/ISS Filter (1)
17- 36 F20.16 deg PA [1.02/141] Phase angle
38- 59 F22.17 deg LON [-180/180] Planetocentric Longitude
61- 81 F21.19 --- IF [0.0003/0.75] Reflectivity (2)
83- 99 A17 --- Image Image name
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Note (1): Filters as follows:
CL1VIO = centered at 420nm (2177 occurrences)
CL1GRN = centered at 568nm (2368 occurrences)
CL1RED = centered at 648nm (2101 occurrences)
CB2CL2 = centered at 752nm (964 occurrences)
CB3CL2 = centered at 938 nm (390 occurrences)
Note (2): Reflectivity, I/F is the the intensity of the detected light, I,
normalized to the incident solar flux, F.
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History:
From electronic version of the journal
(End) Prepared by [AAS], Coralie Fix [CDS], 27-Jan-2021