Reflec i i y o Venus’s Dayside Disk Du ing he 2020 Obse a ion Campaign: Ou comes
and Fu u e Pe spec i es
Yeon Joo Lee
1,2
, An onio Ga cía Muñoz
3
, A sushi Yamazaki
4,5
, E ic Quéme ais
6
, S e ano Mo ola
1
,
S ephan Hellmich
1,7
, Thomas G anze
8
, Gilles Be gond
9
, Ma in Ro h
8
, Eulalia Gallego-Cano
10
, Jean-Y es Chau ay
6
,
Rozenn Robidel
6
, Go Mu akami
4
, Kei Masunaga
4
, Mu a Kaplan
11
, O han E ece
11
, Rica do Hueso
12
, Pe Kabá h
13
,
Magdaléna Špoko á
13,14
, Agus ín Sánchez-La ega
12
, Myung-Jin Kim
15
, Vale ia Mangano
16
, Kandis-Lea Jessup
17
,
Thomas Widemann
18
, Ko-ichi o Sugiyama
19
, Shige o Wa anabe
20
, Manabu Yamada
21
, Takehiko Sa oh
4
,
Masa o Nakamu a
4
, Masa aka Imai
22
, and Juan Cab e a
1
1
DLR Ins i u e o Plane a y Resea ch, Be lin, Ge many
2
Pionee Resea ch Cen e o Clima e and Ea h Science, Ins i u e o Basic Science (IBS), Daejeon 34126, Republic o Ko ea; [email p o ec ed]
3
AIM, CEA, CNRS, Uni e si é Pa is-Saclay, Uni e si é de Pa is, Gi -su -Y e e, F ance
4
Ins i u e o Space and As onau ical Science (ISAS/JAXA), Sagamiha a, Japan
5
G adua e School o Science, Uni e si y o Tokyo, Tokyo, Japan
6
LATMOS-OVSQ, Uni e si é Ve sailles Sain -Quen in, Guyancou , F ance
7
Labo a o y o as ophysics, École Poly echnique Fédé ale de Lausanne (EPFL), Obse a oi e de Sau e ny, 1290 Ve soix, Swi ze land
8
Leibniz-Ins i u e o As ophysics Po sdam (AIP), Po sdam, Ge many
9
CAHA, Alme ia, Spain
10
IAA G anada, Spain
11
Akdeniz Uni ., An alya, Tu key
12
Dp . Fśica Aplicada, Escuela de Ingenie á de Bilbao, Uni e sidad del PaśVasco UPV/EHU, Bilbao, Spain
13
As onomical Ins i u e AS CR, Ond ejo , Czech Republic
14
Masa yk Uni e si y, Depa men o heo e ical physics and as ophysics, Ko lá
ská 2, 611 37, B no, Czech Republic
15
Ko ea As onomy and Space Science Ins i u e (KASI), Daejeon, Republic o Ko ea
16
INAF-IAPS (Ins i u e o As ophysics and Plane ology om Space), Rome, I aly
17
Sou hwes Resea ch Ins i u e, Boulde , CO, USA
18
Obse a oi e de Pa is-PSL & Uni e si é Pa is-Saclay, LESIA—UMR CNRS, Meudon, F ance
19
Ma sue Na ional College o Technology, Ma sue, Japan
20
Space In o ma ion Cen e , Hokkaido In o ma ion Uni e si y, Ebe su, Japan
21
Plane a y Explo a ion Resea ch Cen e (PERC), Na ashino, Japan
22
Kyo o Sangyo Uni e si y, Kyo o, Japan
Recei ed 2022 Ap il 9; e ised 2022 July 25; accep ed 2022 July 26; published 2022 Sep embe 14
Abs ac
We pe o med a unique Venus obse a ion campaign o measu e he disk b igh ness o Venus o e a b oad ange
o wa eleng hs in 2020 Augus and Sep embe . The p ima y goal o he campaign was o in es iga e he abso p ion
p ope ies o he unknown abso be in he clouds. The seconda y goal was o ex ac a disk mean SO
2
gas
abundance, whose abso p ion spec al ea u e is en angled wi h ha o he unknown abso be a ul a iole
wa eleng hs. A o al o h ee spacec a and six g ound-based elescopes pa icipa ed in his campaign, co e ing
he 52–1700 nm wa eleng h ange. A e ca e ul e alua ion o he obse a ional da a, we ocused on he da a se s
acqui ed by ou acili ies. We accomplished ou p ima y goal by analyzing he eflec i i y spec um o he Venus
disk o e he 283–800 nm wa eleng hs. Conside able abso p ion is p esen in he 350–450 nm ange, o which we
e ie ed he co esponding op ical dep h o he unknown abso be . The esul shows he consis en wa eleng h
dependence o he ela i e op ical dep h wi h ha a low la i udes, du ing he Venus flyby by MESSENGER in
2007, which was expec ed because he o e all disk eflec i i y is domina ed by low la i udes. Las , we summa ize
he expe ience ha we ob ained du ing his fi s campaign, which should enable us o accomplish ou second goal
in u u e campaigns.
Unified As onomy Thesau us concep s: Venus (1763);A mosphe ic clouds (2180);Plane a y science (1255);
Sola sys em as onomy (1529);Plane a y a mosphe es (1244);Obse a ional as onomy (1145)
1. In oduc ion
As he hi d-b igh es objec in he sky a e he Sun and he
Moon, he scien ific obse a ions o Venus s a ed ea ly. A
cen u y ago, g ound-based obse a ions disco e ed he p e-
sence o da k pa ches in ul a iole (UV)images o he plane
(W igh 1927; Ross 1928). The chemical ha p oduces he da k
pa ches on he plane is cha ac e ized by b oad abso p ion ha
ex ends om he UV o he isible wa eleng hs. The iden i y o
such a chemical emains elusi e, and he subs ance is s ill
called he “unknown abso be ”(Ba ke e al. 1975; Pollack
e al. 1980; Zaso a e al. 1981; Mills e al. 2007; Ti o e al.
2018). Recen s udies ha e sugges ed ha he unknown
abso be may be OSSO o S
2
O, which explains he obse ed
UV spec um (Pé ez-Hoyos e al. 2018). Acco ding o
pho ochemical model calcula ions (K asnopolsky 2018)and
glo y obse a ion analysis (Pe o a 2018), he unknown
abso be could also be i on chlo ide. The e a e mo e
The Plane a y Science Jou nal, 3:209 (18pp), 2022 Sep embe h ps://doi.o g/10.3847/PSJ/ac84d1
© 2022. The Au ho (s). Published by he Ame ican As onomical Socie y.
O iginal con en om his wo k may be used unde he e ms
o he C ea i e Commons A ibu ion 4.0 licence. Any u he
dis ibu ion o his wo k mus main ain a ibu ion o he au ho (s)and he i le
o he wo k, jou nal ci a ion and DOI.
1
candida es, such as S
x
,Cl
2
, SCl
2
, e c. (Mills e al. 2007).
Recen ly, i on-bea ing mic oo ganisms ha e also been p o-
posed (Limaye e al. 2018).
The abso p ion spec um o he unknown abso be was
epo ed o ha e i s maximum a 340 nm, wi h an FWHM o
140 nm, acco ding o he MESSENGER/MASCS da a (Pé ez-
Hoyos e al. 2018). Bu conside ing he limi ed spec al ange
o he MESSENGER/MASCS da a—300–1500 nm— he spec-
al p ope ies o he unknown abso be a λ<300 nm we e no
accessed, emaining undefined. Spec al da a a such sho
wa eleng hs we e acqui ed by he SPICAV spec ome e on
boa d Venus Exp ess, co e ing he 170–320 nm ange wi h i s
UV channel. In o de o explain he da a aken by SPICAV’s
UV channel, Ma cq e al. (2011,2020)pos ula ed he p esence
o an unknown abso be in he o m o a cloud ae osol, in
addi ion o a pu e sul u ic acid ae osol. The pu a i e abso be
would explain he abso p ion sho wa d o 300 nm. These
p e ious s udies sugges ha he unknown abso be emains
e ec i e a wa eleng hs om ∼200 nm (Ma cq e al. 2020) o
∼600 nm (Pé ez-Hoyos e al. 2018). These obse a ions we e
done a di e en imes, and wi h di e en iewing geome ies,
so hei da a canno be di ec ly combined o unde s and he
spec al p ope ies o he unknown abso be o e he en i e UV
− isible wa eleng h ange. To elucida e such p ope ies, i is
clea ha addi ional obse a ions should be made o e a
b oade ange o wa eleng hs, such as hose done by he STIS
spec ome e on boa d he Hubble Space Telescope o e
200–600 nm (Jessup e al. 2020).
The UV obse a ions a e also use ul o e ie ing
abundances o ace gases nea he cloud- op le el. Fo
example, SO
2
bands a e loca ed nea 215 and 280 nm, he
SO band nea 215 nm, and he O
3
band nea 250 nm (Esposi o
e al. 1988; Na e al. 1990; Belyae e al. 2012; Jessup e al.
2015; Ma cq e al. 2019,2020). Thei abundances and
a ia ions a e impo an o unde s anding pho ochemical
p ocesses in he a mosphe e (Mills e al. 2007; Ti o e al.
2018), including hei in e ac ion wi h he unknown abso be
(Ma cq e al. 2013,2020; Lee e al. 2015a,2019). Howe e ,
wi hou high spec al esolu ion, he in e p e a ion is compli-
ca ed by he o e lap o he bands and by he abso p ion o he
unknown abso be . A u he complica ion would be ep e-
sen ed by he p esence o an addi ional species, H
2
S, nea he
cloud- op le el, as sugges ed by Bie son & Zhang (2020). This
con ibu ion, no conside ed in p e ious s udies (Na e al. 1990;
Belyae e al. 2012; Jessup e al. 2015), is cha ac e ized by a
UV band nea 215 nm ha o e laps hose o he SO and SO
2
gases.
Significan empo al a ia ions o he unknown abso be and
SO
2
gas abundance ha e been epo ed o e bo h sho - and
long- e m pe iods (Del Genio & Rossow 1982; Esposi o e al.
1988; Del Genio & Rossow 1990; Ma cq e al. 2013,2020; Lee
e al. 2015a,2019,2020; Imai e al. 2019). In e ms o disk-
in eg a ed UV b igh ness, sho - e m a ia ions indica e he
p esence o global-scale a mosphe ic wa es wi h a pe iodici y
o 4–5 days (Del Genio & Rossow 1982; Lee e al. 2020),
whose ampli udes a e changing wi h ime (Del Genio &
Rossow 1990; Imai e al. 2019; Lee e al. 2020). Changes in he
disk-in eg a ed UV b igh ness o e imescales o decades can
impac he sola ene gy deposi ion in he a mosphe e, because
almos hal o he sola hea ing a he cloud- op a mosphe e is
caused by he unknown abso be (C isp 1986; Lee e al.
2015b). The la e can lead o conside able changes in global-
scale ci cula ion and zonal wind speeds (Lee e al. 2019).
In iguingly, he UV b igh ness a ia ions a e co ela ed wi h
he SO
2
gas abundance nea he cloud- op le el (Lee e al.
2015a,2019; Ma cq e al. 2020). Tha connec ion is key o
unde s anding he pho ochemical p ocesses ha a ec cloud
o ma ion (Mills e al. 2007)and he impac o possible
olcanic ou gassing on he a mosphe e. We need u he da a o
in es iga e he ela ionship be ween he sul u - ela ed gaseous
abundance and he unknown abso be . Tha was he main
mo i a ion o he Venus dayside obse a ion campaign ha we
pe o med in 2020.
As ou campaign measu es he disk-in eg a ed spec al
b igh ness, he esul s will be use ul o compa ison wi h
spa ially un esol ed da a acqui ed by u u e exoplane imaging
in es iga ions. Fo example, we now know ha measu ing he
plane ’s b igh ness a mo e han one phase angle could be a
aluable s a egy o iden i ying Venus-like clouds a exopla-
ne s, i hey exis , wi h u u e di ec imaging elescopes
(Ca ión-González e al. 2020,2021). In his manusc ip , we
desc ibe he campaign (Sec ion 2), explain he da a educ ion
(Sec ion 3), he a mosphe ic modeling (Sec ion 4), and he da a
analysis (Sec ion 5), and o e ou lessons lea ned o he
pu pose o planning u u e campaigns (Sec ion 6).
2. Obse a ions
In 2020 Augus and Sep embe , we pe o med he Venus
dayside obse a ion campaign om h ee loca ions in he sola
sys em: he Aka suki Venus o bi e , he BepiColombo Me cu y
o bi e , on i s c uise phase owa d Me cu y, and he Ea h ( ia
he Ea h-o bi ing Hisaki spacec a and g ound-based ele-
scopes; Figu e 1(a)). JAXA’s Venus o bi e Aka suki ope a es
om a highly ellip ical equa o ial o bi . The onboa d UV
came a (Ul aViole Image : UVI)has moni o ed Venus since
he o bi inse ion in 2015 Decembe (Nakamu a e al. 2016).
ESA-JAXA’s BepiColombo conduc ed a away Venus obse -
a ions om a dis ance o 0.3 au in he pe iod 2020 Augus 28–
Sep embe 2, when Venus was wi hin he field o iew (FOV)
o he onboa d UV spec ome e (PHEBUS; Mangano e al.
2021). While hese wo spacec a we e ope a ing, g ound-
based elescopes we e in a good posi ion o obse e Venus o
mo e han an hou igh be o e sun ise. Th ee elescopes o he
Cala Al o obse a o y (CAHA)joined he campaign and
conduc ed he Venus obse a ions: he CAHA 1.23 m DLR-
MKIII CCD came a,
23
he CAHA 2.2 m Plane Cam came a
(Mendikoa e al. 2016), and he CAHA 3.5 m Po sdam Mul i-
Ape u e Spec opho ome e (PMAS; Ro h e al. 2005).
TÜBİTAK Na ional Obse a o y’s T100 CCD came a
24
and
he STELLA 1.2 m elescope’s Wide-Field STELLA Imaging
Pho ome e (WiFSIP; S assmeie e al. 2010)acqui ed
images, and he Pe ek elescope’s Ond
ejo Echelle
Spec og aph (OES; Kabá h e al. 2020)acqui ed spec a.
JAXA’s Ea h-o bi ing Hisaki space elescope also ob ained
Venus da a in he ex eme UV (EUV) ange, wi h he EXCEED
spec ome e (Yoshikawa e al. 2014), which has been used o
de ec he ai glow o Venus (Na a e al. 2018). The EUV da a
can help o examine possible ain dayside eflec ion by he
uppe haze o Venus, hanks o hei long exposu e ime o e
23
h p://www.caha.es/CAHA/Ins umen s/IA123/DLR_Obse a ion_
guide_ 1.11.pd
24
h ps:// ug. ubi ak.go . /en/ eleskopla / 100- elescope
2
The Plane a y Science Jou nal, 3:209 (18pp), 2022 Sep embe Lee e al.
10 days. Table 1shows he comple e lis o acili ies, wi h
obse a ion da es and wa eleng h anges.
The uniqueness o his campaign is he b oad spec al
co e age o obse a ions o he Venus disk, which ex ends
om 52 o 1700 nm, and which canno be acqui ed by a single
ins umen . We ook ad an age o he spec al o e lap be ween
he ins umen s, which could be used o combine indi idual
spec al pieces o he b igh ness. Fo example, EXCEED and
PHEBUS o e lap a 145–148 nm; PHEBUS and UVI a
283 nm; UVI, he g ound-based Uband, and PMAS a
365 nm; and he g ound-based Bband and PMAS a 445 nm.
Hal o he acili ies acqui ed da a o su ficien quali y o
scien ific analysis, bu no he o he s (Table 1). The e we e ou
p oblems o he la e . (1)The fi s p oblem was he
Figu e 1. O e iew o he campaign obse a ions. (a)The ela i e loca ions o he obse a ion acili ies om Venus. The g een a ows indica e he di ec ion owa d
Ea h om he spacec a . The g ay cu e o BepiColombo indica es he ajec o y om 2020 Augus o No embe , and he blue cu e highligh s he loca ion du ing
he campaign pe iod (Augus 28–Sep embe 2). Aka suki’s ajec o y be ween Augus 28 and Sep embe 2 is shown in he enla ged ed box. The ed do s a e he
loca ions o he spacec a on Augus 28 a 04:30 UT. (b)Viewing geome ies o Venus om Aka suki, BepiColombo, and Ea h on Augus 30. The day/nigh a eas
a e indica ed wi h he whi e/g ay a eas o e he disk. The sola phase angle (α), he appa en size o Venus, and he dis ance be ween he plane and obse e s a e
lis ed a he bo om. The subsola and he subobse e poin s a e indica ed wi h he yellow and g een do s, espec i ely.
3
The Plane a y Science Jou nal, 3:209 (18pp), 2022 Sep embe Lee e al.
unce ain ies in he poin ing ha occu ed du ing he da a
acquisi ion o PHEBUS and EXCEED. Na ow-sli spec o-
me e s equi e a high accu acy o spacec a a i ude con ol.
The Venus obse a ions by BepiColombo we e in ac pa o
he pe o mance es s on he c uise phase, and i u ned ou ha
he poin ing accu acy was no always as good as planned.
Hisaki g adually saw such con ol de e io a ing wi h aging.
Rega dless o his p oblem, bo h he PHEBUS and he
EXCEED da a could ha e been su ficien o ela i e spec al
analysis. Bu he PHEBUS da a had an addi ional issue; hei
e ec i e a ea u ned ou no o be well defined o scien ific
analysis (Sec ion 3.2). Consequen ly, he EXCEED da a could
no be used, as a da a compa ison was no possible a he
o e lapping wa eleng hs (145–148 nm). Also, spec al com-
pa ison o he eflec ed dayligh be ween EXCEED and
PHEBUS may no be possible e en in u u e, because he
exposu e ime o PHEBUS canno be as long as ha o
EXCEED. (2)The second p oblem was he pho ome ic
calib a ion o he g ound-based measu emen s, which equi ed
pa icula ca e. To define he ellu ic ex inc ion coe ficien s
accu a ely, he CAHA1.23 DLR-MKIII came a equen ly
in e spe sed measu emen s o e e ence s a s wi h hose o
Venus (Sec ion 3.3), and he STELLA WiFSIP measu ed a
e e ence s a con inuously un il Venus ose su ficien ly high o
acqui e da a (Sec ion 3.4). Howe e , his was no he case o
he o he acili ies, such as he CAHA2.2 Plane Cam, whose
equency o e e ence s a obse a ions u ned ou o be
insu ficien o pho ome ic analysis unde a iable sky
condi ions. (3)The hi d p oblem was caused by he di ficul ies
in defining an op imal ape u e size in he CCD ape u e
pho ome y analysis. The TUG T100 da a su e ed om his
p oblem, which may ha e been exace ba ed by he b igh ness
o Venus. (4)Finally, he Pe ek OES measu emen s a e no
used in his s udy, because i s 2″wid h sli is likely on he
mo ning e mina o ( he cen e o he 20″diame e Venus disk).
The Venus obse a ions we e conduc ed in h ee sola phase
angle (α) anges, as shown in Figu e 1(b). Nea he end o
Augus , αwas 60° o PHEBUS, 0°–40° o UVI, and ∼80°
om he Ea h. In his manusc ip , we in es iga e he spec al
ea u es o he en i e Venus disk a α=80°. To ha end, we
app oxima ely co ec ed all he obse a ions a o he phase
angles o o m equi alen obse a ions a α=80°. In he
u u e, we plan o in es iga e he sola phase angle dependence
o Venus’s b igh ness (Lee e al. 2021)o e a b oad spec al
ange by epea ing simila campaigns a mul iple epochs.
3. Da a
De ails o he da a acquisi ion and calib a ions a e desc ibed
in his sec ion o each ins umen .
3.1. Aka suki/UVI
UVI has wo fil e s, cen e ed a 283 and 365 nm (Yamazaki
e al. 2018). The 365 nm wa eleng h is o de ec he abso p ion
by he unknown abso be , and he 283 nm wa eleng h is
loca ed nea he cen e o a SO
2
band. In he egula
obse a ion mode, UVI ob ains Venus images ia he wo
fil e s, e e y 2 h , om a highly ellip ical equa o ial o bi . We
selec ed images wi h comple e co e age o he Venus dayside
be ween 2015 Decembe 7 and 2021 Ma ch 31. Some known
a i ac images a e excluded om he da a se .
In his analysis, we used wo fla fields; he fi s fla field was
measu ed in a labo a o y, be o e he launch (Yamazaki e al.
2018), and a second, new, fla field was p epa ed wi h he
di use images acqui ed in 2020–2021. The fi s fla field was
applied o he images be o e 2019 Sep embe 17, and he new
fla field was applied o images om 2019 Sep embe 17. Bo h
fla fields a e publicly a ailable in he calib di ec o y o
DARTS da a se s.
25
Using s a obse a ions be ween 2010 and
2020, he calib a ion co ec ion ac o s (β)we e calcula ed. The
a e aged βa e 1.533 ±0.208 a 365 nm and 1.991 ±0.279 a
283 nm. These βa e close o he alues epo ed in Yamazaki
e al. (2018). We no ice a weak sensi i i y change wi h ime a
283 nm, bu his is no e iden a 365 nm. S a obse a ions by
UVI will con inue, so we will examine possible sensi i i y
changes in mo e de ail in he nea u u e. In his s udy, we ook
he a e aged β
λ
o each channel (λ).
We calcula ed he disk-in eg a ed flux o Venus, F
Venus
in
[Wm
−2
μm
−1
], as ollows:
() () ()
å
al b=´W
l
<
F Ixy,, , , 1
Venus pix
o
whe e αis he phase angle, λis he wa eleng h, is he
obse a ion ime, Iis he measu ed adiance a (x,y)pixel
Table 1
Summa y o he Campaign Obse a ions
Loca ion Facili y/Ins . Type
a
Spec al Range (nm)Da e S a us and Sec ion
(1)(2)(3)(4)(5)(6)
Space (Venus o bi )Aka suki/UVI I 283, 365 Regula moni o ing Success, Sec ion 3.1
Space (in e plane a y)BepiColombo/PHEBUS S 145–315, 402, 423 Aug 28–Sep 2 Insu ficien o analysis, Sec ion 3.2 (no used)
Space (Ea h o bi )Hisaki/EXCEED S 52–148 Aug 21–Sep 3 Success o ela i e analysis (no used)
Spain CAHA1.23/DLR-MKIII I BVRI bands Aug 21–28 Success, Sec ion 3.3
Spain CAHA2.2/Plane Cam I 380–1700 Aug 28–31 Insu ficien o analysis (no used)
Spain CAHA3.5/PMAS I and S 364–457 (dλ=0.28 nm)Aug 27–30 Success o ela i e analysis, Sec ion 3.5
Tu key TUG/T100 I UBV bands Aug 25–Sep 2 Insu ficien o analysis (no used)
Spain (Tene i e)STELLA/WiFSIP I Uband Aug–No Success, Sec ion 3.4
Czech Republic Pe ek elescope/OES S 375.3–919.5 Aug 21–Sep 2 Insu ficien o analysis (no used)
No e. Lis o ac onyms—UVI: Ul aViole Image ; PHEBUS: P obing o He mean Exosphe e By Ul a iole Spec oscopy; EXCEED: EX eme ul a iole
spec osCope o ExosphE ic Dynamics; CAHA: Cala Al o Obse a o y; PMAS: Po sdam Mul i-Ape u e Spec opho ome e ; TUG: TÜBİTAK Na ional
Obse a o y; WiFSIP: Wide-Field STELLA Imaging Pho ome e ; OES: Ond
ejo Echelle Spec og aph.
a
I: Image; S: Spec um.
25
h ps://da s.isas.jaxa.jp/doi/ co/ co-00016.h ml
4
The Plane a y Science Jou nal, 3:209 (18pp), 2022 Sep embe Lee e al.
loca ions on an image, Ω
pix
is he solid angle o one pixel, and
is he dis ance o (x,y) om he Venus disk cen e .
o
is he
limi ing dis ance o in eg a ion, which includes he Venus
adius in pixels and he poin -sp ead unc ion (se en pixels).So
<
o
defines an a ea o flux in eg a ion om he plane cen e
( =0) o
o
. Then, we sub ac ed he mean backg ound noise
pe pixel. The solid angle o Venus, Ω
Venus
( ), was calcula ed
as
() () ()pW=
-
R
d
a csin , 2
Venus Venus
V obs
2
⎜⎟
⎜⎟
⎛
⎝⎛
⎝⎞
⎠⎞
⎠
whe e R
Venus
is he adius o Venus and d
V−obs
is he dis ance
o he spacec a om Venus in km a he ime o obse a ion .
Fo R
Venus
, we conside ed he cloud- op al i ude om he cen e
o he plane (6052 +70 km).
We calcula ed he disk-in eg a ed albedo A
disk−in
, as he
ollowing (S omo sky e al. 2001):
() ()
() ( )
() ()
al pal
l
=W
-
-
A
d F
S
,, ,, ,3
disk in
Venus
VS 2Venus
whe e d
V−S
( )is he dis ance be ween Venus and he Sun [au]
a he ime o obse a ion ,Ω
Venus
( )is he solid angle o
Venus as iewed om Aka suki, and S
e
(λ)is he sola
i adiance a 1 au [Wm
−2
μm
−1
](see Sec ion 3.6), calcula ed
o he ansmi ance unc ions o each fil e . A
disk−in
is simila
in meaning o he adiance ac o (Hapke 2012) ha can be
applied o spa ially esol ed images. A
disk−in
(α=0°,λ)is he
“geome ic albedo”a wa eleng h λ.
Figu e 2shows he mean phase cu es a he wo channels
be ween 2015 and 2021 (g ay lines). The colo ed ci cles
indica e he da a be ween 2020 Augus and No embe , when
ou g ound-based U-band obse a ions we e conduc ed (see
Sec ion 3.4 o de ails). The symbols show consis en phase
angle dependence wi hin he s anda d de ia ions o he mean
phase cu e ( he ligh g ay a ea). The g ound-based Uband is
wide (34 nm) han he UVI band (14 nm), which may be he
eason o he sys ema ic o se . P e iously epo ed mean
phase cu es o he Uband a e compa ed in he same plo . The
I ine e al. (1968)Uband has he la ges bandwid h (116 nm).
Mallama e al. (2017)adop ed he phase angle dependence o
he Bband o he Uband, and adjus ed he geome ic albedo o
ma ch p e ious obse a ions. De ails o hese Ubands a e
p o ided in Sec ion 3.4.
3.2. BepiColombo/PHEBUS
BepiColombo was launched in 2018 Oc obe , and is on i s
way o Me cu y (a i al in 2025). BepiColombo is composed
o wo spacec a : he Me cu y Plane a y O bi e (MPO)and
he Me cu y Magne osphe ic O bi e . PHEBUS is he UV
spec ome e on boa d MPO. BepiColombo made wo Venus
flybys in 2020 Oc obe and 2021 Augus , which became
oppo uni ies o close-up obse a ions o Venus (Mangano
e al. 2021). Du ing he Venus flybys, PHEBUS acqui ed da a
o e he nigh side and limb, because he dayside o Venus was
oo b igh o he PHEBUS senso , which is designed o de ec
ain UV emissions om he a mosphe ic gases o Me cu y and
he nigh side albedo o Me cu y (Quéme ais e al. 2020).
The obse a ions o he Venus dayside used he e we e
ob ained om a long dis ance, when he iny plane a y disk
en e ed he sli o PHEBUS. Be ween 2020 Augus 28 and
Sep embe 2, he e we e such oppo uni ies: he 66″appa en
size o Venus was wi hin he 2°×0.2°FOV (Figu e 1), and he
PHEBUS eam made he fi s Venus a away obse a ions.
180–181 images we e acqui ed daily o e he consecu i e six
days wi h he a -UV (FUV; 145–315 nm)and wo nea -UV
(404 and 422 nm)de ec o s. The da a acquisi ion was done a
4550V o he Mic ochannel Pla e In ensifie , which al e s he
gain (Chassefiè e e al. 2010). Da k and e ec i e a eas a e also
measu ed a 4550V in fligh .
While Venus was success ully cap u ed by PHEBUS o six
consecu i e days, we aced h ee p oblems. (1)The fi s
p oblem was he un ealis ic fluc ua ions in pho on coun s,
which a ied day o day. These fluc ua ions we e la e ound o
Figu e 2. Obse ed disk-in eg a ed albedo a (a)365 nm and (b)283 nm as a
unc ion o phase angle. The g ay solid lines a e he mean phase cu e o
Aka suki/UVI, and he ligh g ay filled a eas a e he s anda d de ia ions. The
ci cles a e he Aka suki/UVI da a and he o ange diamonds a e he g ound-
based U-band da a (Sec ion 3.4). The colo s o he filled symbols indica e he
obse a ion da es, be ween 2020 Augus 1 and No embe 30, as shown in he
colo ba . The ed cu e in panel (a)is aken om Mallama e al. (2017),
con e ed om magni ude o albedo. The blue cu e is aken om I ine e al.
(1968), also con e ed om magni ude o albedo. Thei o iginal magni udes a e
shown in Figu e 4.
5
The Plane a y Science Jou nal, 3:209 (18pp), 2022 Sep embe Lee e al.
be caused by he poin ing accu acy. The obse a ions aimed o
pu he disk a he cen e o he FOV, bu he spacec a ’s
a i ude could no pu Venus a he cen e , as planned. Ins ead,
Venus was some imes loca ed nea he bounda y o he FOV,
acco ding o he la e examina ion, esul ing in a significan
educ ion in he pho on coun s. This p oblem p e en s he
absolu e flux analysis, bu i should be fine o ela i e spec al
analysis. (2)The second p oblem was he da k coun
es ima ion. The da k measu emen (deep space imaging)a
4550V was done a mon h ea lie . As he da k coun a e
changes wi h he empe a u e o he de ec o , he ime
di e ence caused insu ficien da k sub ac ion om he Venus
images. The PHEBUS eam he e o e ied o es ima e he da k
cu en , using he pho on coun s o e he deep space pixels
ou side he Venus illumina ing a ea. We confi med consis en
day- o-day pa e ns, al hough his may ha e in oduced
addi ional small e o s. (3)The hi d p oblem was he e ec i e
a ea e ie al a 4550V, which was de e mined wi h he
obse a ions o Spica on 2020 Feb ua y 4. The e ie ed
e ec i e a ea was as expec ed a wa eleng hs sho e han
270 nm, bu a longe wa eleng hs i u ned ou o be
insu ficien o ob aining eliable esul s. This hi d p oblem
became c i ical, as i mean ha we could no compa e he
b igh ness wi h he UVI da a a 283 nm, and we could no
quan i y ei he he ela i e abso p ion by he SO
2
gas o e he
240–315 nm wa eleng h ange (see Sec ion 6 o de ails).
A e he examina ion explained abo e, we excluded he
PHEBUS da a om he scien ific analysis in his pape .
Looking in o he u u e, PHEBUS should p o ide aluable
in o ma ion o e ie ing he disk mean SO
2
gas abundance,
and o unde s and he unknown abso be in he FUV spec al
ange, which a e he main goals o he campaign. Fu u e
PHEBUS obse a ions will esol e he h ee p oblems ha we
ha e iden ified du ing his campaign.
3.3. CAHA1.23/DLR-MKIII
The DLR-MKIII CCD came a ins alled a he CAHA 1.23 m
elescope pe o med Venus obse a ions in he Johnson–
Cousins BVRI bands. F om Augus 22 o 28 UTC, Venus
was isible igh be o e he sun ise. Venus’s appa en diame e
changed om 22″ o 20″du ing his pe iod. HR2208 was
selec ed as a sola -like e e ence s a ; i s spec al ype is G2V
(S epien & Geye 1996)–G5V (G ay e al. 2003), and i was
su ficien ly b igh nea Venus, a he same ai mass ange as
Venus. The pho ome ic a iabili y o he s a is epo ed o be
0.03 and 0.035 mag a Vand B, espec i ely, wi h a 7.8 day
pe iod (S epien & Geye 1996). This le el o a ia ion has a
negligible impac on his s udy, as ou accu acy does no each
such a le el; his is compa able o he daily s anda d de ia ions
o ou measu emen s.
The Venus images we e aken unde s ongly de ocused
condi ions (Gillon e al. 2009; Sou hwo h e al. 2009), o
sp ead pho ons o Venus o e he wide FOV o he CCD
came a. This success ully p e en ed he sa u a ion o he Venus
images, wi hou a neu al densi y fil e . This benefi s accu a e
flux measu emen s o Venus. S a obse a ions we e done a
he no mal ocus posi ion. One obse a ion cycle was
composed o Venus and he s a imaging a he ou fil e s
(a leas ou images pe fil e pe objec ), and his cycle was
epea ed h ee o ou imes each nigh .
Usual ape u e pho ome y was used o de e mine he
ape u e sizes o in eg a ing he fluxes o Venus and he s a ,
and we calcula ed he signal- o-noise a io (S/N)a he
co esponding ape u e sizes wi h he CCD equa ion. The
ypical S/N o Venus is ∼10
5
and ha o he s a is 1000–2000.
A mosphe ic ex inc ion coe ficien s we e de e mined a each
fil e , by using a linea eg ession be ween he ins umen al
magni ude o he s a and he ai mass. The ai mass anges we e
1.6–2.1 each nigh o bo h Venus and he s a . The
a mosphe ic ex inc ion coe ficien s we e consis en o he fi s
fi e nigh s. Du ing he las wo nigh s, pa ial clouds en e ed
he iew, esul ing in empo ally a iable ellu ic opaci y. Since
he ins umen al magni ude a ze o ai mass is known o be
s able o he CAHA1.23 DLR-MKIII came a, we could also
compu e he ins an aneous ex inc ion coe ficien a he ime o
he Venus obse a ions by in e pola ion du ing nonpho ome ic
nigh s, hanks o he epea ed cycles be ween Venus and he
s a . The appa en magni ude o Venus was calcula ed wi h he
in e pola ed a mosphe ic ex inc ion coe ficien and he known
magni udes o he s a (Table 2).
The appa en magni ude o Venus was con e ed o he
educed magni ude, which is he b igh ness a 1 au om bo h
he Sun and he Ea h. The dis ances be ween he Sun and
Venus, and he Ea h and Venus, a he ime o imaging we e
calcula ed using he JPL SPICE oolki (Ac on 1996). He e-
a e , magni ude e e s o he educed magni ude, and he
esul s a e shown in Figu e 3. The compa ison wi h he
b igh ness epo ed by Mallama e al. (2017)shows a good
ag eemen wi h he expec ed b igh ness a α∼80°a he ou
bands ( o he daily a ia ion, see Figu e 8).
3.4. STELLA/WiFSIP
The WiFSIP wide-field image ins alled a he STELLA
1.2 m obo ic elescope conduc ed Venus imaging a he U
band. The pe iod o obse a ions con inued be ween 2020
Augus 11 and No embe 8, excep he ime when a Saha a
dus s o m a ec ed he elescope’s si e in Tene i e. The
obse a ions we e conduc ed be o e sun ise e e y day. Fo
abou 30 minu es, a b igh sola -like e e ence s a nea Venus
was con inuously obse ed o define he ellu ic ex inc ion
coe ficien . Then he Venus imaging ollowed immedia ely,
when Venus ose high in he da k sky. Typically, 15 Venus
images we e acqui ed each nigh (excep Augus 12, when fi e
images we e acqui ed). The ai mass o Venus changed wi h he
ime o he obse a ions: 1.6–1.7 om Augus 11 o Sep embe
14, 1.7–1.8 om Sep embe 20 o Oc obe 2, 1.8–2.0 om
Oc obe 3 o 27, and 2.0–2.4 un il No embe 8.
Ape u e pho ome y was applied o de e mine he size o he
a ea o in eg a ing he Venus flux and e e ence s a s. The
ypical S/N o Venus is 6000–8000, and hose o s a s ange
om ∼500 o ∼3000, depending on he s a s. Following he
loca ions o Venus on he sky, ou e e ence s a s changed wi h
ime (Table 2). No e ha κGemini has an accompanying s a ,
and i s co esponding pixels we e excluded om he ape u e
pho ome y. The anges o s a ai mass a ied wi h ime, e.g.,
1.9–2.2 on Augus 11, 1.4–1.9 on Sep embe 14, 1.5–2.0 on
Oc obe 10, and 1.6–1.75 on No embe 8. Daily ex inc ion
coe ficien s we e moni o ed, and we excluded da es o
abno mal beha io compa ed o he o he da es. Ou a e aged
ex inc ion coe ficien o he Uband is 0.485 ±0.093. The
appa en magni ude o Venus was calcula ed wi h he daily
a mosphe ic ex inc ion coe ficien and he known magni udes
o each s a (Table 2). The appa en magni ude was con e ed
o he educed magni ude, as desc ibed in Sec ion 3.3.
6
The Plane a y Science Jou nal, 3:209 (18pp), 2022 Sep embe Lee e al.
Ou STELLA U-band magni ude measu emen s a e, o he
bes o ou knowledge, he fi s a e I ine e al. (1968). The
compa ison o hese da a se s is shown in Figu e 4.Asa
e e ence, wo mo e da a se s a e shown oge he : he oldes
measu emen (Knuckles e al. 1961)and a ecen es ima ion
(Mallama e al. 2017). The compa ison o ou da a wi h I ine
e al. (1968)shows a consis en magni ude, bu i is in ac an
inadequa e compa ison, conside ing he la ge bandwid h o
I ine e al. (1968; 116 nm) han o STELLA (34 nm). Mallama
e al. (2017)es ima ed he U-band phase cu e ha ollows he
phase angle dependence in he Bband and has he geome ic
albedo, o be consis en wi h he wo olde U-band obse a-
ions. Knuckles e al. (1961)show a much b igh e Venus
magni ude, and i is di ficul o unde s and he cause o such a
di e ence. In his s udy, we ha e adop ed he I ine e al.
(1968)phase cu e as a e e ence phase cu e a U o co ec
he phase angle dependence o he STELLA da a (Equa ion (8)
in Sec ion 5).
The fluc ua ion o STELLA’sUband is no iceable in
Figu e 4. These may be eal sho - e m fluc ua ions, as epo ed
in a ecen s udy o Venus’s disk-in eg a ed albedo (Lee e al.
2020; see Sec ion 5.1). The Venus moni o ing by STELLA will
con inue, and we should be able o cons uc he ue mean
phase cu e a Uand ex ac accu a e empo al a ia ions in he
nea u u e.
3.5. CAHA3.5/PMAS
PMAS is ins alled a he CAHA 3.5 m elescope (Ro h e al.
2005), and i acqui ed Venus da a om 2020 Augus 26 o 29.
The Venus obse a ions we e conduc ed wi h he ba e fibe
bundle in eg al field uni (PPAK), which has a wide hexagonal
FOV o 65″×74″, as shown in Figu e 5(a). A o al o 331
fibe s ob ained scien ific da a wi hin he FOV, and an
addi ional 36 fibe s simul aneously acqui ed sky da a a 72″
away om he cen e o he FOV. The wide FOV is su ficien
o cap u ing he en i e Venus disk, which had an appa en
diame e o ∼20″.
PMAS is op imized o obse e ain objec s. In ac , Venus is
oo b igh o PMAS in i s no mal ope a ion mode, so he
Venus obse a ion was conduc ed wi h special ca e; only one
pe al o he mi o co e was open, o educe he pho on flux.
Ou a ge ed spec al ange o he obse a ion, om UV o blue
colo , also helped o educe he pho on flux o Venus due o
ellu ic ex inc ion. We acqui ed high–spec al esolu ion da a,
dλ=0.28 nm, which e ec i ely sp ead he pho ons be ween
326.1 and 478.3 nm, using he U1200 g a ing.
The Venus da a we e acqui ed a he end o each nigh , o
abou an hou , wi h a 0.4 s exposu e ime, esul ing in ∼100
images pe nigh . The ai mass o Venus anged ypically
be ween 1.5 and 2.5 each nigh , and only he da a wi h ai mass
close o ha o ou e e ence s a obse a ions we e selec ed o
he analysis. Re e ence s a obse a ions we e conduc ed a he
beginning o nigh , in he middle, and igh be o e he Venus
obse a ions. 10Lac, Vega, and eps Aq we e obse ed each
nigh , and we e en ually used 10Lac (CALSPEC da abase;
26
Bohlin e al. 2014) o cons uc he ellu ic ansmi ance
unc ion, which makes use o PMAS’s high spec al esolu ion.
Two se s o 10Lac obse a ions we e conduc ed each nigh ,
be ween 1.3 and 1.7 ai mass.
The P3D e sion 2.7 package was used o he da a
educ ion (Sandin e al. 2010), which includes c ea ing a
mas e bias image, acing spec a, finding spec al posi-
ions, and gene a ing a fla field, and ul ima ely p oduces a
educed spec al image. We in eg a ed he flux o he
a ge sas ollows(Rosales-O ega e al. 2010).We
sub ac ed he median sky spec um om he scien ific
da a in each image. F om he cen e o Venus o he
e e ence s a (e.g., he black “X”in Figu e 5(a)),we
inc eased he a ea o flux summa ion, un il he o al flux did
no inc ease u he (<1%)o e he en i e wa eleng h
ange. Such adii we e 16″ o Venus and 12″ o 10Lac.
The selec ed fibe s o he flux in eg a ion a ea a e ma ked
wi h whi e ci cles in Figu e 5(a), and he o al flux spec um
o Venus is shown in Figu e 5(b).
We gene a ed a e e ence ellu ic ansmi ance unc ion
T
e
(λ)each nigh (Wy enbach e al. 2015),as
() ( ) ()l=l
TEsexp , 4
e e
whe e λis he wa eleng h, E
λ
is he ellu ic op ical dep h a
zeni h, and s
e
is he mean ai mass o he e e ence s a
obse a ions. The di e ence om Wy enbach e al. (2015)is
ha he ansmi ance is no a he uni y ai mass (zeni h), bu a
s
e
.
We calcula ed E
λ
s
e
as ollows. The loga i hm o he
measu ed s a fluxes F
obs,s a
(λ)has a linea ela ionship wi h
he ai mass s(Lange eld e al. 2021),as
(()) ()l=+
l
FEscln , 5
obs,s a
whe e cis a cons an . As he s anda d s a flux spec um F
s d
(λ)
is a s=0, we can con e he equa ion abo e as
( ( )) ( ( )) ( )ll-=+
l
FFEscln ln , 6
obs,s a s d 0
whe e c
0
is a cons an ha makes he esul ze o a 445 nm,
which is he cen e o he Johnson Bband. We no malized he
ansmi ance unc ions o hose a 445 nm, and a e aged wo
Table 2
Re e ence S a s o Imaging Obse a ions
S a Spec al Type Magni ude Re e ence o Magni ude Da es o Obse a ion
UBVR I
(1)(2)(3)(4)(5)(6)(7)(8)(9)
HR2208 G2V–G5V 7.317 7.131 6.456 6.087 5.740 S epien & Geye (1996)Aug 11–Sep 4
κGemini G8III–IIIb 5.19 4.49 3.57 2.86 2.41 Duca i (2002)Sep 6–13
mu.02 Cnc G1IVb 6.14 5.93 5.30 Duca i (2002)Sep 14–30
35 Leo G1.5IV–V 6.85 6.64 Duca i (2002)Oc 2–10
HD88725 G3/5V 8.34 8.33 7.73 7.24 6.89 Duca i (2002)Oc 14–24
HD92719 G1.5V 7.519 7.406 6.767 6.42 6.083 Koen e al. (2010)Oc 27–No 8
26
h ps://a chi e.s sci.edu/hlsps/ e e ence-a lases/cdbs/cu en _calspec/
10lac_mod_003.fi s
7
The Plane a y Science Jou nal, 3:209 (18pp), 2022 Sep embe Lee e al.
se s o no malized ansmi ance unc ions. The T
e
(λ)on
Augus 26 is shown in Figu e 5(c)(black cu e). Fine emission
lines o he s a a e excluded om his p ocess ( ed in e als).
We e ie ed ela i e flux spec a o Venus F
Venus
(λ)using
T
e
(λ); a e selec ing he obse ed Venus fluxes F
obs, enus
(λ)
ha we e acqui ed a snea he s a obse a ions (ds <0.1)
each nigh , we di ided hese Venus fluxes by T
e
(λ)o he
same nigh :
() ()
() ()ll
l
=FF
T.7
Venus
obs, enus
e
An example o he ela i e Venus flux spec um on
Augus 26 is shown in Figu e 5(d)( he blue cu e, which
almos o e laps wi h he ed cu e). A compa ison be ween
he Venus flux and he sola i adiance (Sec ion 3.6)is
shown in he same figu e. Using he selec ed spec al
ea u es o he sola e e ence ( he ci cle symbols),we
sligh ly adjus ed he spec al loca ion o he Venus
spec um, as shown in he same plo , be o e (blue)and
a e ( ed) he adjus men . Such spec al loca ion adjus -
men s we e done be ween −4and+3Å, depending on he
da es o he obse a ions and he wa eleng hs. This las
p ocess shows only mino changes, bu helps o emo e
un ealis ic humps om he eflec i i y spec um. We
epea ed he same p ocedu e o he da a o each nigh , o
gene a e daily mean Venus spec a.
Figu e 3. Obse ed magni udes o Venus in he Johnson–Cousins BVRI bands be ween 2020 Augus 22 and 28 by he CAHA1.23/DLR-MKIII came a ( ed do s).(a)
Phase cu es o e he ange om 0° o 150°.(b)The same as (a), bu close-up ep esen a ions o he obse ed da a poin s o he campaign. The phase cu es a each
band a e shown wi h he g ay dashed lines (Mallama e al. 2017).
Figu e 4. Obse ed magni ude o Venus in he Johnson Uband by STELLA/
WiFSIP in 2020 Augus –No embe (o ange diamonds). The phase cu es
epo ed in p e ious s udies a e compa ed: Knuckles e al. (1961; g een), I ine
e al. (1968; blue), and Mallama e al. (2017; ed).
8
The Plane a y Science Jou nal, 3:209 (18pp), 2022 Sep embe Lee e al.
3.6. Sola I adiance Da a
We con e ed he obse a ion da a o eflec i i y using he
e e ence sola i adiance spec um. We used he obse ed sola
i adiance da a om TSIS-1 SIM (Ve sion 6, Le el 3, daily
da a)
27
o e he 200–2400 nm wa eleng h ange. Fo he
campaign da a o his pape , we a e aged he TSIS-1 SIM da a
om 2020 Augus o Sep embe . This mean sola spec um was
used o calcula e he sola i adiance a 283 and 365 nm o he
Aka suki da a. We calcula ed he sola magni ude in he Uband
using he e ec i e ansmi ance unc ion o STELLA,
ollowing he desc ip ion in Willme (2018) o ake in o
accoun i s small bandwid h (FWHM =34 nm). Fo he BVRI
b oad bands, we ook he alues gi en in Willme (2018). The
sola magni udes in each band a e lis ed in Table 3. Fo he
high- esolu ion spec al g ids o he PMAS da a, he TSIS-1
SIM da a we e no su ficien (5nm a λ∼400 nm),
28
so we
ook he SAO2010 sola e e ence spec um, whose spec al
esolu ion is 0.04 nm (FWHM; Chance & Ku ucz 2010).We
Figu e 5. PMAS obse a ion and calib a ion examples o he da a acqui ed on 2020 Augus 26 (image numbe 2005).(a)An image slice om PPAK a 365 nm a e
sky sub ac ion. The s ong signals show whe e he Venus dayside is loca ed. The mean o he s ong signal loca ions is ma ked wi h he black “X,” om which a 16″
adius ci cula a ea is selec ed o flux in eg a ion ( he whi e “◦”symbols).(b)In eg a ed flux spec um o Venus be o e he ellu ic ex inc ion co ec ion. (c)Rela i e
ellu ic ansmi ance unc ion T
e
(λ)o he same nigh (see he ex o he de ails). The s a (10Lac)was obse ed a he ai mass so 1.30 and 1.59 (g ay cu es). The
mean ansmi ance is used o define T
e
(λ)(black), excep fine emissions o he s a ( ed anges).(d)Rela i e flux spec um o Venus F
Venus
(λ), a e he ellu ic
ex inc ion co ec ion (blue). Rela i e sola i adiance is shown (black). Some ea u es a e selec ed (black ci cles)as spec al e e ences o adjus he spec al loca ions
o he Venus flux (blue disks). A e his fine spec al loca ion co ec ion, he final Venus spec um is shown as he ed cu e, which almos o e laps wi h he blue
cu e.
27
h ps://lasp.colo ado.edu/home/ sis/da a/
28
h ps://lasp.colo ado.edu/home/ sis/ins umen s/sim-spec al-i adiance-
moni o /
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The Plane a y Science Jou nal, 3:209 (18pp), 2022 Sep embe Lee e al.
6. Summa y and Pe spec i e on Fu u e Campaigns
Ou dayside obse a ion campaign was conduc ed wi h he
PHEBUS spec ome e on boa d BepiColombo and he UVI
came a on boa d Aka suki o be e unde s and he UV
abso be s in he Venusian clouds. Ou campaign was designed
o co e a b oad wa eleng h ange, om 52 o 1700 nm, hanks
o Ea h-bound obse a ion acili ies. Despi e he ac ha ou
da a analysis could e en ually only u ilize he da a be ween 283
and 800 nm (Sec ion 2), we achie ed he ollowing goals and
insigh s:
1. We success ully accomplished he Venus obse a ion
campaign using mul iple g ound- and space-based
acili ies almos simul aneously.
2. Despi e he challenging b igh ness o he a ge ( oo
b igh ), we managed o acqui e high-quali y da a.
3. The PHEBUS eam was able o es ablish a obus
obse a ion s a egy o making success ul Venus obse a-
ions a u u e oppo uni ies, e.g., in 2022 June and July.
4. Using he campaign da a in he 283–800 nm ange, we
e ie ed he ela i e op ical dep h o he unknown abso be
on he mo ning-side disk. Ou esul is consis en wi h he
p e ious epo using he da a acqui ed in 2007 o e he
a e noon equa o ial egion (Pé ez-Hoyos e al. 2018).
5. We plan u u e campaigns o e ie e bo h he SO
2
gas
abundance and he abso p ion by he unknown abso be
in he 180–450 nm ange, using da a acqui ed by
PHEBUS (180–320 nm), UVI (283 and 365 nm), and
g ound-based elescopes (350–800 nm).
Figu e 10. Rela ions be ween he eflec i i y o e he wa eleng hs o he Iband and he cloud- op al i ude (Z
cloud
).(a)Va ia ions o he calcula ed eflec i i y
acco ding o Z
cloud
ha is changed be ween 60 and 75 km. (b)χ
2
o fi he obse ed I-band eflec i i y as a unc ion o Z
cloud
.
Figu e 11. Simula ed eflec i i y a α=80°wi h a ange o R
UA
om 0.0 o
0.3 (see Sec ion 4.2). This assumes he bes fi o he cloud op in Figu e 10(b)
(64 km). The unknown abso be is assumed o be wi hin he 6 km hickness
laye , whose middle is loca ed 3 km below he cloud- op le el (Lee e al. 2021),
i.e., 58–64 km. Wi hin his laye , he SSA o he cloud ae osols is educed
by R
UA
.
Figu e 12. Rela i e op ical dep h o he unknown abso be (no malized o he
maximum in he 350–500 nm ange). The equi ed abso p ion o ma ch he
PMAS da a is shown wi h he g ay line (Sec ion 5.3), and i s con olu ion
(FWHM =1nm)is shown wi h he black line. A p e ious obse a ional da a
analysis (Pé ez-Hoyos e al. 2018)is shown o compa ison (blue ci cles).
Assump ions ha we e applied o he sola hea ing a e calcula ions (upwa d/
downwa d iangles; C isp 1986; Haus e al. 2016)a e shown oge he .
16
The Plane a y Science Jou nal, 3:209 (18pp), 2022 Sep embe Lee e al.
6. We es ablished ha flux measu emen s a he VRI bands
can p o ide a cons ain on he cloud configu a ion o
gene a e he simula ed eflec i i y. We will con inue VRI
imaging in u u e campaigns.
7. The U-band phase cu e o Venus is poo ly defined. We
plan o con inue he U-band imaging o define a mean
phase cu e.
8. Th ough he g ound-based U-band measu emen s, i may
be possible o ack he empo al a iabili y o Venus’s
eflec i i y, in addi ion o space-based measu emen s.
9. Aka suki’s UV imaging is an excellen e e ence o
compa ing sho - e m a ia ions.
10. PMAS obse a ion and flux measu emen s in he B-band
will be epea ed in ou u u e campaigns o unde s and
he possible empo al a ia ions o he unknown abso be .
This esea ch used da a collec ed a he Cen o As onómico
Hispano-Alemán (CAHA)a Cala Al o, ope a ed join ly by Jun a
de Andalucía and Consejo Supe io de In es igaciones Cien íficas
(IAA-CSIC). This esea ch has made use o he in eg al field
spec oscopy da a educ ion ool p3d, which is p o ided by he
Leibniz-Ins i u ü As ophysik Po sdam (AIP). Aka suki/UVI
da a a e publicly a ailable a he JAXA a chi e websi e, DARTS
(h p://da s.isas.jaxa.jp/), and he NASA a chi e websi e, PDS
(h ps://pds.nasa.go /). UVI Le el 3 p oduc s (l3bx)we e used
in his s udy (Mu akami e al. 2018). This s udy used he TSIS-1
SIM da a (Ve sion 06, doi:10.25810/y9 8- 85).M.K.andO.E.
hank he TÜBİTAK Na ional Obse a o y o pa ial suppo in
using he T100 elescope, wi h p ojec numbe 20CT100-1688. R.
H. and A.S.L. ha e been suppo ed by he Spanish p ojec
PID2019-109467GB-I00 (MINECO/FEDER, UE)and G upos
Gobie no Vasco IT-1366-19. P.K. and M.S. acknowledge suppo
om g an LTT-20015.
Appendix
Spec al Signa u e o Me hane (CH
4
)
A fi s , we assumed possible me hane gas in he model
calcula ions (Sec ion 4.1). Bu la e , we ound ha i s spec al
signa u e should be clea o de ec wi h emo e obse a ions
(Figu e 13). We excluded me hane om he esul s in his
manusc ip (Sec ion 5). The confi ma ion o he possible
me hane may be a subjec o u u e obse a ion p ojec s.
ORCID iDs
Yeon Joo Lee h ps://o cid.o g/0000-0002-4571-0669
An onio Ga cía Muñoz h ps://o cid.o g/0000-0003-
1756-4825
A sushi Yamazaki h ps://o cid.o g/0000-0001-6468-6812
E ic Quéme ais h ps://o cid.o g/0000-0001-5376-2242
S e ano Mo ola h ps://o cid.o g/0000-0002-0457-3872
S ephan Hellmich h ps://o cid.o g/0000-0003-3997-3363
Ma in Ro h h ps://o cid.o g/0000-0003-2451-739X
Eulalia Gallego-Cano h ps://o cid.o g/0000-0002-
7452-1496
Kei Masunaga h ps://o cid.o g/0000-0001-9704-6993
Rica do Hueso h ps://o cid.o g/0000-0003-0169-123X
Pe Kabá h h ps://o cid.o g/0000-0002-1623-5352
Agus ín Sánchez-La ega h ps://o cid.o g/0000-0001-
7234-7634
Myung-Jin Kim h ps://o cid.o g/0000-0002-4787-6769
Vale ia Mangano h ps://o cid.o g/0000-0002-9903-4053
Shige o Wa anabe h ps://o cid.o g/0000-0002-3058-0689
Manabu Yamada h ps://o cid.o g/0000-0003-0726-6592
Takehiko Sa oh h ps://o cid.o g/0000-0001-9071-5808
Masa aka Imai h ps://o cid.o g/0000-0001-8543-6556
Juan Cab e a h ps://o cid.o g/0000-0001-6653-5487
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