J/A+A/664/A172 Spectra of Earth-like exoplanets Ocean signatures (Trees+, 2022)
Ocean signatures in the total flux and polarization spectra of Earth-like
exoplanets.
Trees V.J.H., Stam D.M.
<Astron. Astrophys. 664, A172 (2022)>
=2022A&A...664A.172T 2022A&A...664A.172T (SIMBAD/NED BibCode)
ADC_Keywords: Models ; Exoplanets ; Polarization
Keywords: radiative transfer - polarization - techniques: spectropolarimetric -
planets and satellites: oceans -
planets and satellites: terrestrial planets
Abstract:
Numerical simulations of starlight that is reflected by Earth-like
exoplanets predict habitability signatures that can be searched for
with future telescopes.
We explore signatures of water oceans in the flux and polarization
spectra of this reflected light.
With an adding-doubling algorithm, we computed the total flux F,
polarized flux Q, and degree of polarization Ps of starlight reflected
by dry and ocean model planets with Earth-like atmospheres and patchy
clouds. The oceans consist of Fresnel reflecting surfaces with
wind-ruffled waves, foam, and wave shadows, above natural blue
seawater. Our results are presented as functions of wavelength (from
300 to 2500nm with 1nm resolution) and as functions of the planetary
phase angle from 90° to 170°.
The ocean glint increases F, |Q|, and Ps with increasing phase angle
at nonabsorbing wavelengths, and causes the spectra of F and |Q| for
the various phase angles to intersect. In the near-infrared, Q is
negative, that is, the direction of polarization is perpendicular to
the plane through the star, planet, and observer. In the Ps spectra,
the glint leaves dips (instead of peaks) in gaseous absorption bands.
All those signatures are missing in the spectra of dry planets.
The dips in Ps and the negative Q in the near-infrared can be searched
for at a phase angle of 90°, where the planet-star separation is
largest. Those ocean signatures in polarized light do not suffer from
false positive glint signals that could be due to clouds or reflecting
dry surfaces. For heavily cloudy planets, ocean detection is possible
when the glint is (partially) cloud-free. When modeling signals of
planets with oceans, using horizontally inhomogeneous cloud covers is
thus crucial. Observations spread over time would increase the
probability of catching a cloud-free glint and detecting an ocean.
Description:
We present the computed spectra of the total flux F, polarized flux Q,
and degree of polarization Ps of starlight reflected by our dry and
ocean model planets with Earth-like atmospheres that are cloud-free,
fully cloudy, or partly cloudy with patchy clouds. F and Q are
normalized w.r.t. to the geometric albedo of the planet (see Eq. 10 of
the paper) and Ps is defined by Eq. 7 of the paper). The polarization
reference plane is the planetary scattering plane, that is, the plane
through the centers of the planet, the host star and the observer.
The spectra are presented at a 1nm wavelength resolution and range
from 300 to 2500nm, and at planetary phase angles ranging from 0 to
180 degrees with a step size of 10 degrees.
The surfaces of our dry planets reflect Lambertian (fully depolarizing
and isotropically), and have a specified surface albedo. The surfaces
of our ocean planets reflect as rough ocean surfaces, including
subsurface blue ocean water, wave shadows and foam, and have a
specified wind speed.
The patchy cloud covers of the partly cloudy ocean planets
(0 < cloud fraction < 1) are such that the ocean glint is cloud-free
or fully covered by clouds. Similar cloud covers are used for the
partly cloudy dry planets, see also Figs. 10, 12, and 14 of the paper.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
f000a00.dat 289 2201 Total flux of dry planet, cloud fraction = 0.00,
surface albedo = 0.0
f000a01.dat 289 2201 Total flux of dry planet, cloud fraction = 0.00,
surface albedo = 0.1
f000a08.dat 289 2201 Total flux of dry planet, cloud fraction = 0.00,
surface albedo = 0.8
f000v01.dat 289 2201 Total flux of ocean planet, cloud fraction 0.00,
wind speed = 1 m/s
f000v07.dat 289 2201 Total flux of ocean planet, cloud fraction 0.00,
wind speed = 7 m/s
f000v13.dat 289 2201 Total flux of ocean planet, cloud fraction 0.00,
wind speed = 13 m/s
f025a00f.dat 289 2201 Total flux of dry planet, cloud fraction = 0.25,
surface albedo = 0.0, cloud cover similar to
cloud-free glint
f025v07c.dat 289 2201 Total flux of ocean planet, cloud fraction 0.25,
wind speed = 7 m/s, cloud-covered glint
f025v07f.dat 289 2201 Total flux of ocean planet, cloud fraction 0.25,
wind speed = 7 m/s, cloud-free glint
f050a00c.dat 289 2201 Total flux of dry planet, cloud fraction = 0.50,
surface albedo = 0.0, cloud cover similar to
cloud-covered glint
f050a00f.dat 289 2201 Total flux of dry planet, cloud fraction = 0.50,
surface albedo = 0.0, cloud cover similar to
cloud-free glint
f050v07c.dat 289 2201 Total flux of ocean planet, cloud fraction 0.50,
wind speed = 7 m/s, cloud-covered glint
f050v07f.dat 289 2201 Total flux of ocean planet, cloud fraction 0.50,
wind speed = 7 m/s, cloud-free glint
f075a00f.dat 289 2201 Total flux of dry planet, cloud fraction = 0.75,
surface albedo = 0.0, cloud cover similar to
cloud-free glint
f075v07c.dat 289 2201 Total flux of ocean planet, cloud fraction 0.75,
wind speed = 7 m/s, cloud-covered glint
f075v07f.dat 289 2201 Total flux of ocean planet, cloud fraction 0.75,
wind speed = 7 m/s, cloud-free glint
f100a00.dat 289 2201 Total flux of dry planet, cloud fraction 1.00,
surface albedo = 0.0
f100v07.dat 289 2201 Total flux of ocean planet, cloud fraction 1.00,
wind speed = 7 m/s
q000a00.dat 289 2201 Polarized flux of dry planet,
cloud fraction = 0.00, surface albedo = 0.0
q000a01.dat 289 2201 Polarized flux of dry planet,
cloud fraction = 0.00, surface albedo = 0.1
q000a08.dat 289 2201 Polarized flux of dry planet,
cloud fraction = 0.00, surface albedo = 0.8
q000v01.dat 289 2201 Polarized flux of ocean planet,
cloud fraction 0.00, wind speed = 1 m/s
q000v07.dat 289 2201 Polarized flux of ocean planet,
cloud fraction 0.00, wind speed = 7 m/s
q000v13.dat 289 2201 Polarized flux of ocean planet,
cloud fraction 0.00, wind speed = 13 m/s
q025a00f.dat 289 2201 Polarized flux of dry planet,
cloud fraction = 0.25, surface albedo = 0.0,
cloud cover similar to cloud-free glint
q025v07c.dat 289 2201 Polarized flux of ocean planet,
cloud fraction 0.25, wind speed = 7 m/s,
cloud-covered glint
q025v07f.dat 289 2201 Polarized flux of ocean planet,
cloud fraction 0.25, wind speed = 7 m/s,
cloud-free glint
q050a00c.dat 289 2201 Polarized flux of dry planet,
cloud fraction = 0.50, surface albedo = 0.0,
cloud cover similar to cloud-covered glint
q050a00f.dat 289 2201 Polarized flux of dry planet,
cloud fraction = 0.50, surface albedo = 0.0,
cloud cover similar to cloud-free glint
q050v07c.dat 289 2201 Polarized flux of ocean planet,
cloud fraction 0.50, wind speed = 7 m/s,
cloud-covered glint
q050v07f.dat 289 2201 Polarized flux of ocean planet,
cloud fraction 0.50, wind speed = 7 m/s,
cloud-free glint
q075a00f.dat 289 2201 Polarized flux of dry planet,
cloud fraction = 0.75, surface albedo = 0.0,
cloud cover similar to cloud-free glint
q075v07c.dat 289 2201 Polarized flux of ocean planet,
cloud fraction 0.75, wind speed = 7 m/s,
cloud-covered glint
q075v07f.dat 289 2201 Polarized flux of ocean planet,
cloud fraction 0.75, wind speed = 7 m/s,
cloud-free glint
q100a00.dat 289 2201 Polarized flux of dry planet,
cloud fraction 1.00, surface albedo = 0.0
q100v07.dat 289 2201 Polarized flux of ocean planet,
cloud fraction 1.00, wind speed = 7 m/s
p000a00.dat 208 2201 Degree of polarization of dry planet,
cloud fraction = 0.00, surface albedo = 0.0
p000a01.dat 208 2201 Degree of polarization of dry planet,
cloud fraction = 0.00, surface albedo = 0.1
p000a08.dat 208 2201 Degree of polarization of dry planet,
cloud fraction = 0.00, surface albedo = 0.8
p000v01.dat 208 2201 Degree of polarization of ocean planet,
cloud fraction 0.00, wind speed = 1 m/s
p000v07.dat 208 2201 Degree of polarization of ocean planet,
cloud fraction 0.00, wind speed = 7 m/s
p000v13.dat 208 2201 Degree of polarization of ocean planet,
cloud fraction 0.00, wind speed = 13 m/s
p025a00f.dat 208 2201 Degree of polarization of dry planet,
cloud fraction = 0.25, surface albedo = 0.0,
cloud cover similar to cloud-free glint
p025v07c.dat 208 2201 Degree of polarization of ocean planet,
cloud fraction 0.25, wind speed = 7 m/s,
cloud-covered glint
p025v07f.dat 208 2201 Degree of polarization of ocean planet,
cloud fraction 0.25, wind speed = 7 m/s,
cloud-free glint
p050a00c.dat 208 2201 Degree of polarization of dry planet,
cloud fraction = 0.50, surface albedo = 0.0,
cloud cover similar to cloud-covered glint
p050a00f.dat 208 2201 Degree of polarization of dry planet,
cloud fraction = 0.50, surface albedo = 0.0,
cloud cover similar to cloud-free glint
p050v07c.dat 208 2201 Degree of polarization of ocean planet,
cloud fraction 0.50, wind speed = 7 m/s,
cloud-covered glint
p050v07f.dat 208 2201 Degree of polarization of ocean planet,
cloud fraction 0.50, wind speed = 7 m/s,
cloud-free glint
p075a00f.dat 208 2201 Degree of polarization of dry planet,
cloud fraction = 0.75, surface albedo = 0.0,
cloud cover similar to cloud-free glint
p075v07c.dat 208 2201 Degree of polarization of ocean planet,
cloud fraction 0.75, wind speed = 7 m/s,
cloud-covered glint
p075v07f.dat 208 2201 Degree of polarization of ocean planet,
cloud fraction 0.75, wind speed = 7 m/s,
cloud-free glint
p100a00.dat 208 2201 Degree of polarization of dry planet,
cloud fraction 1.00, surface albedo = 0.0
p100v07.dat 208 2201 Degree of polarization of ocean planet,
cloud fraction 1.00, wind speed = 7 m/s
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Byte-by-byte Description of file: f*.dat
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Bytes Format Units Label Explanations
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1- 4 I4 nm lambda Wavelength
8- 19 E12.6 --- F(0) Total flux, alpha = 0 degrees
23- 34 E12.6 --- F(10) Total flux, alpha = 10 degrees
38- 49 E12.6 --- F(20) Total flux, alpha = 20 degrees
53- 64 E12.6 --- F(30) Total flux, alpha = 30 degrees
68- 79 E12.6 --- F(40) Total flux, alpha = 40 degrees
83- 94 E12.6 --- F(50) Total flux, alpha = 50 degrees
98-109 E12.6 --- F(60) Total flux, alpha = 60 degrees
113-124 E12.6 --- F(70) Total flux, alpha = 70 degrees
128-139 E12.6 --- F(80) Total flux, alpha = 80 degrees
143-154 E12.6 --- F(90) Total flux, alpha = 90 degrees
158-169 E12.6 --- F(100) Total flux, alpha = 100 degrees
173-184 E12.6 --- F(110) Total flux, alpha = 110 degrees
188-199 E12.6 --- F(120) Total flux, alpha = 120 degrees
203-214 E12.6 --- F(130) Total flux, alpha = 130 degrees
218-229 E12.6 --- F(140) Total flux, alpha = 140 degrees
233-244 E12.6 --- F(150) Total flux, alpha = 150 degrees
248-259 E12.6 --- F(160) Total flux, alpha = 160 degrees
263-274 E12.6 --- F(170) Total flux, alpha = 170 degrees
278-289 E12.6 --- F(180) Total flux, alpha = 180 degrees
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Byte-by-byte Description of file: q*.dat
-------------------------------------------------------------------------------
Bytes Format Units Label Explanations
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1- 4 I4 nm lambda Wavelength
7- 19 E13.6 --- Q(0) Polarized flux, alpha = 0 degrees
22- 34 E13.6 --- Q(10) Polarized flux, alpha = 10 degrees
37- 49 E13.6 --- Q(20) Polarized flux, alpha = 20 degrees
52- 64 E13.6 --- Q(30) Polarized flux, alpha = 30 degrees
67- 79 E13.6 --- Q(40) Polarized flux, alpha = 40 degrees
82- 94 E13.6 --- Q(50) Polarized flux, alpha = 50 degrees
97-109 E13.6 --- Q(60) Polarized flux, alpha = 60 degrees
112-124 E13.6 --- Q(70) Polarized flux, alpha = 70 degrees
127-139 E13.6 --- Q(80) Polarized flux, alpha = 80 degrees
142-154 E13.6 --- Q(90) Polarized flux, alpha = 90 degrees
157-169 E13.6 --- Q(100) Polarized flux, alpha = 100 degrees
172-184 E13.6 --- Q(110) Polarized flux, alpha = 110 degrees
187-199 E13.6 --- Q(120) Polarized flux, alpha = 120 degrees
202-214 E13.6 --- Q(130) Polarized flux, alpha = 130 degrees
217-229 E13.6 --- Q(140) Polarized flux, alpha = 140 degrees
232-244 E13.6 --- Q(150) Polarized flux, alpha = 150 degrees
247-259 E13.6 --- Q(160) Polarized flux, alpha = 160 degrees
262-274 E13.6 --- Q(170) Polarized flux, alpha = 170 degrees
278-289 E12.6 --- Q(180) Polarized flux, alpha = 180 degrees
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Byte-by-byte Description of file: p*.dat
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Bytes Format Units Label Explanations
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1- 4 I4 nm lambda Wavelength
7- 15 F9.6 --- Ps(0) Degree of polarization, alpha = 0 degrees
18- 26 F9.6 --- Ps(10) Degree of polarization, alpha = 10 degrees
30- 37 F8.6 --- Ps(20) Degree of polarization, alpha = 20 degrees
41- 48 F8.6 --- Ps(30) Degree of polarization, alpha = 30 degrees
52- 59 F8.6 --- Ps(40) Degree of polarization, alpha = 40 degrees
62- 70 F9.6 --- Ps(50) Degree of polarization, alpha = 50 degrees
73- 81 F9.6 --- Ps(60) Degree of polarization, alpha = 60 degrees
84- 92 F9.6 --- Ps(70) Degree of polarization, alpha = 70 degrees
95-103 F9.6 --- Ps(80) Degree of polarization, alpha = 80 degrees
106-114 F9.6 --- Ps(90) Degree of polarization, alpha = 90 degrees
117-125 F9.6 --- Ps(100) Degree of polarization, alpha = 100 degrees
128-136 F9.6 --- Ps(110) Degree of polarization, alpha = 110 degrees
139-147 F9.6 --- Ps(120) Degree of polarization, alpha = 120 degrees
150-158 F9.6 --- Ps(130) Degree of polarization, alpha = 130 degrees
161-169 F9.6 --- Ps(140) Degree of polarization, alpha = 140 degrees
172-180 F9.6 --- Ps(150) Degree of polarization, alpha = 150 degrees
183-191 F9.6 --- Ps(160) Degree of polarization, alpha = 160 degrees
194-202 F9.6 --- Ps(170) Degree of polarization, alpha = 170 degrees
206-208 A3 --- Ps(180) [nan] Degree of polarization,
alpha = 180 degrees
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Acknowledgements:
Victor Trees, v.j.h.trees(at)tudelft.nl
(End) Patricia Vannier [CDS] 06-Jun-2022