J/ApJ/811/67 Saturn's G and D68 rings ISS observations (Hedman+, 2015)
Saturn's G and D rings provide nearly complete measured scattering phase
functions of nearby debris disks.
Hedman M.M., Stark C.C.
<Astrophys. J., 811, 67 (2015)>
=2015ApJ...811...67H 2015ApJ...811...67H (SIMBAD/NED BibCode)
ADC_Keywords: Solar system
Keywords: circumstellar matter; planetary systems;
planets and satellites: rings ; techniques: photometric
Abstract:
The appearance of debris disks around distant stars depends upon the
scattering/phase function (SPF) of the material in the disk. However,
characterizing the SPFs of these extrasolar debris disks is
challenging because only a limited range of scattering angles are
visible to Earth-based observers. By contrast, Saturn's tenuous rings
can be observed over a much broader range of geometries, so their SPFs
can be much better constrained. Since these rings are composed of
small particles released from the surfaces of larger bodies, they are
reasonable analogs to debris disks and so their SPFs can provide
insights into the plausible scattering properties of debris disks.
This work examines two of Saturn's dusty rings: the G ring (at
167500km from Saturn's center) and the D68 ringlet (at 67600km). Using
data from the cameras on board the Cassini spacecraft, we are able to
estimate the rings' brightnesses at scattering angles ranging from
170° to 0.5°. We find that both of the rings exhibit extremely
strong forward-scattering peaks, but for scattering angles above
60° their brightnesses are nearly constant. These SPFs can be well
approximated by a linear combination of three Henyey-Greenstein
functions, and are roughly consistent with the SPFs of irregular
particles from laboratory measurements. Comparing these data to
Fraunhofer and Mie models highlights several challenges involved in
extracting information about particle compositions and size
distributions from SPFs alone. The SPFs of these rings also indicate
that the degree of forward scattering in debris disks may be greatly
underestimated.
Description:
The raw data for this investigation consists of images obtained by the
Imaging Science Subsystem (ISS) on board the Cassini Spacecraft. We
performed a comprehensive search for images of both the G ring and
D68, and then selected images with sufficient signal to noise to
detect these ring features. For the purposes of this analysis we only
considered images obtained through the clear filters of the Narrow
Angle Camera (NAC) and Wide Angle Camera (WAC) clear filters (651nm
and 634nm, respectively) or the RED (647nm) filter on WAC.
File Summary:
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FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
table1.dat 53 620 Normal equivalent width estimates for D68 derived
from entire images, almost all of which were
obtained at scattering angles above 15 deg
table2.dat 73 131 Normal equivalent width estimates for D68 ringlet
derived from observations at scattering angles
below 15 deg
table3.dat 57 135 Normal I/F contrast across the G-ring's inner edge
derived from observations at scattering angles
above 15 deg
table4.dat 41 89 Normal I/F contrast across the G-ring's inner edge
derived from observations at scattering angles
below 15 deg
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See also:
J/AJ/149/27 : Cassini ISS astrometry of Saturnian satellites (Cooper+, 2015)
J/A+A/575/A73 : Saturnian satellites astrometry (2004-12) (Tajeddine+, 2015)
J/AJ/146/12 : Occultations through Starun's C ring (Hedman+, 2013)
J/A+A/493/1183 : Saturn major satellites obs. (1874-2007) (Desmars+, 2009)
J/AJ/103/983 : Stellar occultation candidates of Saturn (Bosh+, 1992)
http://pds-rings.seti.org/cassini/ : Rings node's Cassini home page
Byte-by-byte Description of file: table1.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 11 A11 --- Image Image Name
13- 28 F16.6 s ET Ephemeris Time; seconds past J2000 epoch
30- 35 F6.2 deg theta Scattering angle at the ring (θ)
37- 42 F6.2 deg B [-43/67] Opening angle of the ring to
the spacecraft
44- 48 F5.2 m NEW-INT [-0.4/6.6] Normal equivalent width;
Integrated Method
50- 53 F4.2 m NEW-FIT [0.1/5.5] Normal equivalent width; Fit Method
--------------------------------------------------------------------------------
Byte-by-byte Description of file: table2.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 11 A11 --- Image Image Name
13- 28 F16.6 s ET Ephemeris Time; seconds past J2000 epoch
30- 34 F5.2 deg theta Scattering angle at the ring (θ)
36- 41 F6.2 deg B [-20/21] Opening angle of the ring to
the spacecraft
43- 48 F6.2 deg MinLon [-50/314] Minimum of Longitude Range
50- 55 F6.2 deg MaxLon [-45/318] Maximum of Longitude Range
57- 64 F8.2 m NEW-INT [5/10049] Normal equivalent width;
Integrated Method
66- 73 F8.2 m NEW-FIT [5.4/15363] Normal equivalent width;
Fit Method
--------------------------------------------------------------------------------
Byte-by-byte Description of file: table3.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 11 A11 --- Image Image Name
13- 28 F16.6 s ET Ephemeris Time; seconds past J2000 epoch
30- 35 F6.2 deg theta [17.7/167] Scattering angle at the ring (θ)
37- 41 F5.2 deg B [-5.1/10] Opening angle of the ring to
the spacecraft
43- 49 F7.5 10-6 BCont [0.1/3] Brightness (I/F) contrast across
the G-ring's relatively sharp inner edge
51- 57 F7.5 10-6 e_BCont [0.003/0.2] Uncertainty on Contrast estimated
from the errors from two linear fits
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Byte-by-byte Description of file: table4.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 19 A19 --- Seq Observation Sequence
21 I1 --- Quad [1/4] Quadrant
23- 27 F5.2 deg theta [0.8/13] Scattering angle at the ring (θ)
29- 35 F7.2 10-6 BCont [4.6/2047] Brightness (I/F) contrast across the
G-ring's relatively sharp inner edge
37- 41 F5.2 10-6 e_BCont [0.1/47] Uncertainty on Contrast estimated from
the errors from two linear fits
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History:
From electronic version of the journal
(End) Prepared by [AAS], Emmanuelle Perret [CDS] 24-Dec-2015