Pluto in Glory: Discovery of Its Huge Opposition Surge B

Home , Bond albedo, Opposition surge, Palomar Observatory

Pluto in Glory: Discovery of its Huge Opposition Surge B. J. Buratti (1), M. D. Hicks (1), E. Kramer (1), J. Bauer (2), D. R. Ciardi (3), M. B. Lund (3), K. J. Lawrence (1) (1) Jet Propulsion Laboratory, California Institute of Technology; (2) University of Maryland;

Article

Key Points: Pluto reached an historically small solar phase angle that will not be repeated for 161 years and

that complements New Horizons observations. Observations with the adaptive optics system at Palomar Observatory during this time successfully captured both hemispheres of Pluto. Pluto exhibits an unusually large opposition surge that may be due to active geology and an unusual surface texture. Accepted

This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1029/2021GL092562.

This article is protected by copyright. All rights reserved. Moon revealed a similar su similar a revealed Moon 1° (Domingue et al., 1991; Brown et al., 198 were observed to exhibit a (Irvine, 1966; Hapke disappearance of mutual shadowing amongrapid particles comprising the the optical portion of the by regolith caused is it that is surge” “opposition this of explanation canonical The phase. full largenon a exhibits Moon the that known well Itis ascendingnode. ordescending Pluto’s of longitude the at also is Earth the and Pluto and Earth of planes orbital the of crossing system at true opposition: the lowest possible 2018 by observed not was that observation key One observations byLi ofPluto X puzzling the as lacking, were domains spectral and geometries viewing entire encounters, Pluto on observed the and events to seasonal the model requiredand understand, capture, is that baseline temporal long the out leaving time, in instant an represented it was, S examples two least at and transport, first al., et Grundy 2016; al., 2016 et Moore 2015; al., et (Stern time first the for focus sharp into Object the of encounter 2015 July The surface texture exhibit Charon,main its moon as well as Pluto, of hemispheres both that show Observatory Palomar at Observations a reached Pluto observed small solar phase angles The Plain Summary Language 0. geometricPluto’s inthe albedo filter system,JHK whichwe findtobe processes. geologic active and transport bycaused surface unusualtexture an to due curve phasemaybe unusual Pluto’s Objects. Belt Kuiper other and moons icy for surges of ~30 during the historically small solar phase angles in 2018 (PH Observer Resolution Angular Near Abstract olar System and that may be tied to the origin of life on Earth. But as spectacular as this flyby this as spectacular as But Earth. on life of origin the to tied be may that and System olar 39 1. - ±0.04 for Pluto, respectively,±0.04 for Pluto, and 0.68±0.0 This article is protected by copyright. All rights reserved. ThisAllarticleis rights by copyright. reserved. protected New Horizons New ever observed active glaciers outside the Earth, possible clouds, snow, haze, seasonal volatile - - a

nrrd o infrared 2022

Accepted; Gladstone et al., Article Introduction

–

- h et potnt os o ocrfraohr11 years 161 another for occur not does opportunity next the 35 due toactivityand volatile transpor srain o te lt Cao sse wr cpue wt the with captured were system Charon Pluto the of bservations n % in the last half degree of solar phase angle

itrcly ml slr hs age ht il o b repeat be not will that angle phase solar small historically

– spacecraft revealed Pluto to be a geologically active, complex world. It never never It world. complex active, geologically a be to Pluto revealed spacecraft

, 1986; 2 and that happen on 100 on happen that and

a 2016; Buratti et al.,

surge of surge n even narrower sse et al. (2017) showed. al. (2017)sse et showed. rge, suggesting that it that suggesting rge,

008

New Horizons Horizons New ). In the 1980’s and 1990’s several icy moons of the outer planets – ARO) adaptive o adaptive ARO) over

when the of disk Pluto is fully Inilluminated. 2018 and 2019,

of the dark, elusive dark, the of

3 0

hs observa These % in brightness at opposition, suggesting an unusual unusual an suggesting opposition, at brightness in % large surge in brightness at solar phase angles less than

2017 3

- solar phase angles that occur when Pluto is at a nodal ; Buratti et al. year time scales, at least. As with most other flyby flyby other most with As least. at scales, time year 6 spacecraft with Pluto brought a large Kuiper Belt Kuiper large a brought Pluto with spacecraft In addition, Pluto’s surge is exceptionally steep. steep. exceptionally is surge Pluto’s addition, In

New Horizons New ). ). The “dwarf planet” is a dynamic body with the for Charon inthe for J

is characteristic of rocky bodies as well as icy as well as bodies rocky of characteristic is t. - linear increase in increase linear ptics

- 2019. Both objects exhibit large opposition in eal acrt dtriain of determination accurate enable tions system on the 200 the on system material that is ubiquitous in the outer the in ubiquitous is that material ,1992 types of geologic processes that were that processes geologic of types , which is

and that can only be observed in in observed be only can that and ). Clementine - filter.

brightness as it reaches this reachesthis it as brightness among the largest 0.8 6

– ±0.04

- is the Pluto the is inch Hale telescope telescope Hale inch

observ d o 11 years. 161 for ed seasonal volatileseasonal , 0.59 aoa High Palomar ations of the ±0.05 observed observed - Charon , and - ray ray Table 1 atvisible Palomar the by set is which observable, ( and parameters and the results dome. illuminated an with filters. all integration time utilized for for observations. Flatfields in each of seconds the J, 80 were times integration angle phase solar observati good but issue, equipmentone and seeing poor and weather by plagued were nights the of five nights; half seven receivedwe In 2019 seeing.poor of because bodies two the resolve to able not werewe 29 arcseconds. 40 the imaging mode and resolution set to 40 200 Hale the in 2019, and the opposite hemisphere that was poorly imaged by corresponding were Data observed ~30 corresp 2013 al., et Resolution Observer (PHARO both observations successful on reports paper This observed opposition surge of 0.20 magnitudes in the last 0.5 degree o geometric Ozrin, 2009; the backscatter direction to cause an increase in brightness (Hapke, 1990, 2009; Mishche which multiply scattered photons following the same but reversed paths interfere constructively in ones 0.0084 20190713 5:40 20180731 6:18 20180713 (UT) Date

20180712 7:02 2. - α=average solar phase angle for each night.) each for angle phase solar α=average

35 - 7:32 9:20 (Buratti et al., 1996) al., et (Buratti eipee of hemispheres 9:23

Accepted Article % ond Observations and Observations and ° –

0.01 on July 12,201 on July 0.02° anglephase of Summary ofobservations over the last half last the over

albedo albedo ing to 1.25, 1.65 1.25, to ing ) 33.84 33.61 33.60 33.60 (AU) r ahrd vr ngt sann te oet oa pae nls n 08 n 2019, and 2018 in angles phase solar lowest the spanning nights 5 over gathered

- s 1 inch Telescope’s PHARO adaptive optics system on the nights listed in Table in listed nights the on system optics adaptive PHARO Telescope’s inch 92. Neptune’s 1992). ystem on the 200 the on ystem to the two hemispheres of Pluto: Pluto: of hemispheres two the to

°) we We were assigned four half nights in nights half four assigned were We

32.82 32.65 32.58 (AU) Δ 32.58 Tomba covering Pluto of hemisphere albedo higher the for only of ons were obtained on July 12 and 13, when the system was at its historic low historic its at was system the when 13, and 12 July on obtained were ons

nearly

0.77 0.77 lt ad hrn ih the with Charon and Pluto

during 0.046 0.542 0.021 0.010 (°) α 8. .

. This sharp peak was explained in terms of coherent backscatter, in in backscatter, coherent of terms in explained was peak sharp This .

(The geometric parameters are r= Data Analysis - Table n i ms lkl a captured a likely most is and

reach degree of solar phase angle phase solar of degree , and 2.22 µm. Both µm. 2.22 and ,

; Hayward et al. 2001 (Buratti et al., 2011) daylight hours. 287 54.5 059 54.9 342 54.9 39 (°) Latitude Long

- angular inch Hale telescope Hale inch moon Triton, which is similar to Pluto in size size in Pluto to similar is which Triton, moon

itude

summarizes the summarizes

the theoretical limit of the smallest

radius radius 20/22/27 10/10/9 22/16/15 18/12/7 (J/H/K) # Images

milliarcseconds

during the historically small solar phase angles phase solar small historically the during

of the Sun at the target to be 0.0 be to target the at Sun the of

the bright “hea bright the

On the On Pluto Pluto

13.96±0.024 (13.96) 13.98±0.030 (13.82) 13.78±0.02 (13.65) 13.66±0.05 mag brightness J Pluto . near )

successful e band atm dr fae at frames dark daytime obtained We behind the P3K .

2 2018 and obtained data on all four; on July July on four; all on data obtained and 2018 ,3 This AO system covers the J, H, K filters filters K H, J, the covers system AO This

, with a minimum phase angle of 0.0 of angle phase minimum a with , -

and Charon and nrrd camera infrared

night night , giving a field of view of approximately

f its solar phase curve

upr et bet ehbt a vis a exhibits Object, Belt Kuiper

heliocentric range 13.81±0.022 (14.02) 14.02±0.025 (13.74) 13.71 (13.78) 13.77 mag brightness H Pluto observations along with geometric with along observations of the minimum so minimum the of rt” of Pluto Pluto of rt” New Horizons 2 ,3 ±0.02 ±0.09

adaptive o

exhibit

and

effect

Palomar 14.23±0.025 (14.38) 14.38±0.029 (14.24) 14.20±0.03 (14.12) 14.1 mag brightness K Pluto K filters were obtained

– opposition surges opposition ive

2 Tombaugh Regio Tombaugh ±0.1 , ugh Regio ugh ,3

ptics (AO , in 2 Δ=observer Δ=observer range

, with a minimum

solar phase angle 079

lar phase angle angle phase lar ih a with

High Angular Angular High

018

° , which was was which , 15.38±0.03 15.71±0.04 15.46±0.03 15.35±0.04 mag brightness J Charon nko, 1992, . . ; Dekany We We used Typical Typical 2 ,3 visible

08 each each 1

ual

of of of

– ° ,

standard star in our field. standardfield. star inour stars. dimmer the for large especially are errors J the photometric uncertainties of the cataloged in on only sufficient was Charon for show a trend in brightness; the l filter a were objects both for filter the 2MASS cataloged in magnitudes, each added listed in quadrature. Finally,errors the the and night), per filter per measurements 10+ (typically frames individual for thec https://vizier.u tool: access catalogue bin/VizieR) and are (VizieRfrom the 2MASS Survey (Skrutskie et al. 2006; see Catalog Table 2). The error VizieR budget the using found aperture non under performed we magnitudes calibrated into photometry of the brightest background field stars in our frames, magnitudes which enabled a calibration even instrumental our convert To changes on withposition the standard star and Pluto Note thata point a with fluxes Charon an i and Pluto the measure to used photometric aperture of 30 pixels in radius and a was sky annulus with an outer radius of 60 pixels and images four these for pixel. photometry this at component symmetric as radiallydefined is image symmetric the defined list this of value pi each between distance the measured and this frame, radius was used to given identify a list of a pixels that fell along for this radius. The median Pluto of image anti radially anti radially a and ring convolv still were af arcsec approximately0.5 J typical a flat after shows frame Pluto/Charon 1 Figure frames. science the with ratioed and normalized, constructed, hot of majority the removed which images, 199 frames of al. appropriate expo et (Tody environment analysis IRAF the using began reduction Data 4 3 2 longitudes. 1

Pluto’s “Heart Pluto’s rotati are parenthesis Numbers in correction. angle phase solar No AU). Δ=38.482 AU, (r=39.482 distance opposition mean to Reduced sub Charon locked: Tidally 7:05 20190714 7:02 nner 30pixels. radius of - - - This article is protected by copyright. All rights reserved. ThisAllarticleis rights by copyright. reserved. protected masking, masking, a total of four images were generated from each science frame: a radially symmet 9:20 9:22

Acceptedlist is Article alibrated magnitudes for Pluto and Charon listed in and listed for Charon magnitudes alibrated Pluto Table 1included the dispersionthe in

- photometric ed 33.84 - symmetric image with Charon as the centroid. To generate the radially symmetric radially the generate To centroid. the as Charon with image symmetric -

s in Table 1. Table in haped” feature (Tombaugh Regio) visible. Regio) (Tombaugh feature haped”

- spread function fitting technique was used not for tworeasons: thebrightness of 32.82 ed. We employed “ring masking” “ring employed We ed. - symmetric image with Pluto as the centroid, and a radially symmetric and a a and symmetric radially a and centroid, the as Pluto with image symmetric

- observer longitude = Pluto sub =Pluto longitude observer conditions 0.020

the

The excursion in solar phase angle was not sufficient on any night to to night any on sufficient not was angle phase solar in excursion The sure times were coadded and subtracted from the domeflats and science was ter ter

image veraged on fielding. With a plate a With fielding.

the 231 55.9 55.8 argest span was on July 12, 2018, from 0.011 to 0.0084. greatly different on most nights; and with adaptive optics, the the original image minus the radially symmetric image. symmetric radially the minus image original the - corrected using the the using corrected 4 . AO correction, the point the correction, AO

and throughout the night.and throughout The JHK magnitudes from background reference stars were were stars reference background from magnitudes JHK The

on a given night given a on 17/13/18 - itr o osrc a oa pae uv. h listed The curve. phase solar a construct to filter - chip reference stars dominated our error budget ies Flat pixels.

In no case was there more than one than more there was case no In lightcurve lightcurve

- (13.84) 13.80±0.05 (13.96) observer longitude +180°. observerlongitude - scale of 0.04 arcsec/pixel and with seeing of of seeing with and arcsec/pixel 0.04 of scale

to separate Pluto and Charon fluxes. With fluxes. Charon and Pluto separate to ; the number of images images of number the ;

of Kosiarek (2016) of Kosiarek

- ils t h JK aeegh were wavelengths JHK the at fields spread functions of Pluto and Charon and Pluto of functions spread

(13.73) 13.70±0.07 (13.85) e ad lt’ cnri was centroid Pluto’s and xel

resulting magnitudes in each

Longitudes are eastern eastern are Longitudes , as explained in the text. the in asexplained , (14.37) 14.35±0.09 (14.28)

3 h rdal anti radially The averaged for each each for averaged

). Daytime dark dark Daytime ). -

strasbg.fr/viz photometric

15.43±0.05 The signal

Aperture Aperture - band ; t PSF ric ric he

- -

Charon (which isolates the flux of Pluto) and the radially anti next two components in the upper panel show the radially anti upper The functions. 1. Figure 20190714 135 XPM 20190713 135 XPM 20180731 136 XPM 20180713 136 XPM 20180712 136 XPM Table 2 UT H CatalogJ Date K ID

Background reference stars Efficacy of ring of Efficacy panel shows a typical J typical a shows panel ------1363813 15.448±0.052 14.782±0.091 14.878±0.068 1364022 12.593±0.023 1290301 12.686±0.030 12.027±0.027 12.106±0.025 1293908 12.922±0.023 12.289±0.027 12.334±0.022 1294179 15.232±0.046 14.858±0.109 14.943±0.090 masking in the deconvolution of the Pluto and Charon point Charon and Pluto the of deconvolution the in masking

- band image, obtained 2018 07 12 7: 12 07 2018 obtained image, band - - symmetric image centroided symmetric image centroided around 12.248±0.022 12.189±0.024 12.248±0.022 30 UT. The UT. 30

- around around spread spread

the J the (2016) ha that data Figure 2 decreases at longer 2019). al., et Buratti 2017; al. rotational lightcurve is due mostly to differences in water ice abundance on the surface measurements based The center of this filter’s bandpass is at Horizons light curve in the near 2018 case the in 2065 to 2015 years 3 Figure from are corrections magnitude each under the large; not is which the of use make to able were we However, curve. phase rotational a determine to enough extensive not are observations (e.g., longitude subobserver of function a as rotational solar phase We also made for about comparison. Pluto, centered profiles radial the shows panel bottom The Charon). of flux the isolates (which Pluto This article is protected by copyright. All rights reserved. ThisAllarticleis rights by copyright. reserved. protected - - filter data for Charon. for data filter Accepted Article2019, as these amplitudes change very slowly. . Figures A, B, and C show the phase curves for Pluto; D and E show colors; and F shows shows F and colors; show E and D Pluto; for curves phase the show C and B, A, Figures .

T MVIC NIR MVIC he reduced magnitudes as a function of solar phase angle, with red points representing redangle, representing solar points a with phase functionreduced as of he magnitudes ve

corrections our to observations account to ofPluto for changes inbrightness due to been corrected for rotational phase variations using the light curve of Kosiarek of curve light the using variations phase rotational for corrected been

wavelengths: variations. variations. Pluto, and to a less cannot be cannot - - IR, the rotational effect should be filter has a half a has filter itd n al 1 n son n iue 2 Figure in shown and 1 Table in listed rotation JHK JHK In the top three cells, the open point shows data shows point open the cells, three top the In

- rotational lightcurve of lightcurve rotational 5 in Kosiarek (2016), which gives JHK model lightcurves for the the for lightcurves model JHK gives which (2016), Kosiarek in 5 -

of a static frost model for Pluto. Pluto. for model frost static a of made made e for magnitude corrected

The the rotational correction pcrl otat ewe oau mtra ad ae ic water and material opaque between contrast spectral - directly directly ~0.94 ~0.94 amplitude of about 0.03 magnitudes (Buratti et al., 2019). al., et (Buratti magnitudes 0.03 about of amplitude µ Buie et al., 20 al., et Buie m, so corrections from from er

For Charon, which does not have a published

data collected with collected data extent extent Charon, exhibit changes in brightness Kosiarek (2016) to correct for this effect, this for correct to (2016) Kosiarek even s

for ach data point is listed in parentheses in listed is point data ach 1 small Charon 0; to our longer Buratti et al., 2015 al., et Buratti We used the same numbers for for numbers same the used We

y h rd aa points data red the by er , as lightcurveits in the

would would it. However, the visible visible the However, it. - thus wavelength ground that were gathered gathered were that be at most ~3%. ; 2019 ;

(Howett (Howett et . ). Our Our ). These These New e -

wavelength range of of range wavelength or small body was obtained by the opposit visible opposition a with Ariel moon Uranian 0.89 at between 0.5° and 0.01° solar phase angle of 0.10, and none showed al. et Rabinowitz System. Solar the in largest measur spectral range, it is difficult to make comparisons the observations our Because from particles haze and volatiles in bri of deposits ongoing atmosphere, are global. include Although data for Charon are noisier, it exhibits a 0.36 possiblymagnitude increase which surge, hemisphere albedo lower opposite, Pluto of clear color dependence on solar phase angle or longitude. Notably, Tombaugh Regio asurgeexhibits of years. Data from the PHARO adaptive optics system on the angle phase solar successfullyWe a uniqueopportunitycaptured toobserve thePluto smaller errorsresult. with asolid more the fortunately, but nights, prime two these on field the in whereerfilters, K stars. These errors are reflected in the data points at the two lowest solar phase angles in the H and As stated above, our errors are dominated by the errors in the photometric accuracy of the standard larger(the phaseangles maximum solar June and October observations Further shape. broader possibly a with magnitudes), 0.28 mags, respectively, H the in Observations brightness. in the In Pluto. for J the in Charon for and Pluto, for filters three all for results the shows 2 Figure are muchlarger notapparent phaseangles et until (Hillier al.,2021). extr so is atmosphere hazy Pluto’s Because ahugedata are appearsexhibit noisy, also Charon to (Figure surge 2F). the Although surge. opposition the to dependence color discernable no is there show 2E and 2D Tom with circles in shown is bottom), the at scale the (with angle phase solar the while diamonds in shown is top) the at scale the (with longitude H showing graphs the For visible. was Regio Tombaugh when 4. 3. This article is protected by copyright. All rights reserved. ThisAllarticleis rights by copyright. reserved. protected ghtness lasthalf inits Accepted Article ements, our data show that Pluto and perhaps Charon exhibit surges that are among the the among are that surges exhibit Charon perhaps and Pluto that show data our ements, Summary, discussion and future Summary, and discussion Results µm (Buratti et al., 2011). al., et (Buratti µm -

exhibits t exhibits (Karkoschka, 2001). The (Karkoschka,2001).

- 2021 will clarify visible from Palomar Observatory Palomar from visible rors of ~0.10 are stated. Unfortunately arestated. ~0.10 of rors filter, Pluto’s solar phase curve increases by 0.3 by increases curve phase solar Pluto’s filter, over he same he 0.3

(Hicks et al., 2005) 0.3 magnitudes filter0.3 intheJHK although the data are more - 5.1 µm 5.1

of the Pluto system’s solar phase curve phase solar system’s Pluto the of - degree phaseangle. ofsolar

surge

(

- Brown et al., 2004). 2004). al., et Brown Cassini Perhaps the Perhaps (if (if successful) amplitude .

- most comparable published comparable most This result suggests that the surface processes causing this this causing processes surface the that suggests result This filter and K and filter

attainable

. Triton work Visual Infrared Mapping Spectrometer aordinarily forward aordinarily s uch uch a large surge.

o sre f 03 antds ewe 0 between magnitudes ~0.3 of surge ion in J and H, and within the error bars for K for bars error the within and H, and J in (2007) most similar extant data are data extant similar most

- the the shape of both surges and extend the curve to with other objects ’s surge over the same range is ~ 0.2 baugh Regio again with open symbols. open with again Regio baugh

filter show similar increases of increases similar show filter currently

noisy. , which will not be repeated for another 161 161 another for repeated be not will which ,

observed 10 KBOs and Centaurs below a below Centaurs and KBOs 10 observed

, s E

ystem were starsstandard relativelydim these Charon’s surge is xtensive observations of the moons of of moons the of observations xtensive stars’ measurements in the in measurements stars’ 200

is 1.6

- - Varuna increas . Our observations sofar show. Our no observations scattering, its photometric effects effects photometric its scattering, inch inch in the J - J and K and J data set of an outer planet outer an of set data 7

are among the first the among are assigned or requested between requested or assigned °) 1 magnitudes, a 3 a magnitudes, 1 - . When compared with visible Charon systemCharon

- J colors (D and E), the the E), and (D colors J slightly higher (~0. -

filter show that Pluto those that show the the show that those es 0.16 magnitudes - filter, and colors and filter, (VIMS 0.32 mags and and mags 0.32 at – 3

magnitudes

its smallestits % in the JHK the in J the the “heart” - filter had filter ), with a increase .5 Figures ,

as the as ° and and ° moon 3 6

be producedfrom the data were data these LEISA, near angles phase solar large 2021. June until again COVID for down shut Observatory data. coherent backscatter. opposition near surge large very ground from compiled backs coherent Hapke’s to required angle) and 1.67° (the maximum solar phase angle observable from Earth in the next two years) are 0.54° of angles phase solar between observations additionalAlthough particleson theatmospheric are based small models. 2016) al. et Moore 2015; al., et (Stern atmosphere its sized, as do Gladstone et al. (2016) 10 of range the in sizes the of Estimates a phenomenon similar beoccurringsurfaces. could ontheir micron to effect this attributed They moons. (Grundy et al., a an Such regolith. textured uniquely a provide to atmosphere Pluto’s from particles accrete may processes, or even snow. Charon, which currently has no atmosphere or ongoing geologic activity sizes grain smaller of comprised un something be to appears there Thus on Pluto/Charon. with 0.20 mag increase in the last half of those with line in more are curves phase solar their phase Bu brightness surge. larger a to contribute may Sun the of size angular smaller the of because achievable target. the of distance the at Sun the of size angular the is attainable angle phase solar minimum the limits that factor One from Rhea with those of Pluto although etal., angle 0.05(Buratti 2009). only was theminimum degree, 0.5 last the in magnitudes 0.20 about of increases showed tour orbital the during Saturn ccumulation - This article is protected by copyright. All rights reserved. ThisAllarticleis rights by copyright. reserved. protected R oa pae uvs o te Pluto the for curves phase solar IR

Accepted( Article angles and Nereid, less thanthe 0.10°, moonofNep h adtoa osrain rqie fr modelin for required observations additional The

gather t o t

etr oe te poiin surge opposition the model better pposition solar phase curves of curves phase solar pposition - ed 2016

with subsequent photolysis subsequent with to understand the composition of the system’s bodies, a solar phase curve could could curve phase solar a bodies, system’s the of composition the understand to

sizes of Pluto’s surface particles var particles surface Pluto’s of sizes near Furthermore, Pluto’s surge, and possibl b ). This This -

infrared infrared - Additional observations are are observations Additional Verbiscer et al. (2007) observed large surges in the visible for the Saturnian - based and based 1000 µm. Hillier et al. (2021) model atmospheric aerosols to be micron be to aerosols atmospheric model (2021) al. et Hillier µm. 1000 catter model catter are being are

correlation onlyasthe is Hillier et. notional, almodelwas visible fitto to augment the observations presentedto augment theobservations here. It is possibl is It

in the J data data at mid

New Horizons New comparable to the one observed one the to comparable . The surface of Pluto appears to be covere u t atv saoa vltl tasot ogig geological ongoing transport, volatile seasonal active to due requested - 19 . - - filter degree A radiative tra radiative A e that the geometric effect of a smaller solar phase angle angle phase solar smaller a of effect geometric the that e

during Pluto’s entire 2020 apparition; it is not obs not is it apparition; 2020 entire Pluto’s during usual - - hrn ytm b system, Charon

: not only is the latter’s surge much larger, it is sharper. and high - -

sized particles from Saturn’s E Saturn’s from particles sized ; they do not show as st in August and September 2021 September and August in several Kuiper Belt Objects were captured at solar at captured were Objects Belt Kuiper several

about Pluto. Perhaps its optically active regolith is regolith active optically its Perhaps Pluto. about was hypothesized to explain its reddish polar cap cap polar reddish its explain to hypothesized was or nta rsls per o e vldto of validation a be to appear results initial our ,

data (Hill data assigned in summer 2021 and observations at observations and 2021 summer in assigned -

nsfer model was fit to a solar phase curve curve phase solar a to fit was model nsfer solar anglphase y , and it is reasonable to assume that these these that assume to reasonable is it and ,

most of most widely. Protopapa et al. ( al. et Protopapa widely. y wr pand o 22, u Palomar but 2020, for planned were g

tune was observed astune was lowas 0.0 observed Charon’s ier et al., 202 al., et ier

t n ntuet on instrument an ut the icy moons icy the Figure 3 , based on the importance of of importance the on based , , rong a surge as that observed

is unusually sharply peaked. es. Although the purpose of

(our maximum maximum (our

1 ). The model predicts model The ). compares - d with particles from ring, which suggests which ring, . )

These are the first the are These and small bodies small and e Horizons, New 2017) estimate 2017)

obs solar phasesolar ervations ervations ervable 1 ° , and , and

a - , ,

Buie, Science Reviews ( Brown, H, R. brightnessopposition surges. H., R. Brown, References Charon’s is at albedo0.61µm surge opposition the if ~1.0±0.1 as high as or 2015), curve rotation Charon’s in the J 0.8 (2015) al. et that exhibits p the with combined fundamental measure of energy balance, which is an important parameter to measure when for an object body: planetary a on transport energy understanding for important is quantity This area. projected same the of disk Lambert diffuse, a to compared opposition at planetarybody a of brightness integralthe is which the of calculation accurate an enable angles phase small very at Observations al (2021) measurements. withvisible largest in the Solar it System, appears instrument 3. Figure 2004 6 ±0.04, 0.59±0.05, and 0. and 0.59±0.05, ±0.04, This article is protected by copyright. All rights reserved. ThisAllarticleis rights by copyright. reserved. protected

) AcceptedM. Article . The visualandinfrared Cassini (VIMS) mapping spectrometer investigation. - filter is 0.68±0.0

W., Our

at the same wavelength same the at

seasonal

Grundy, Baines, K. H., Bellucci,Baines,K. J. G.,Bibring, w Pluto J Pluto &

t te oain orcin o Ksae (2016) Kosiarek of corrections rotation the ith 115, 111 Cruikshank

- volatile transport and activegeology. Equation Making (2) use inBuratti of W filter data filter ) 6, without rotation corrections (half our error is due to .

M., - For comparison, the visible albedo of Pluto is 0.59±0.04 (Buratti et al., et (Buratti 0.59±0.04 is Pluto of albedo visible the comparison, For 168.

39 Young, Icarus D P. D. , 0.41±0.01 (Buratti etal., 2017).

±0.04 in the JHK filters, respectively. Charon’s geometric albedo geometric Charon’s respectively. filters, JHK the in ±0.04

compared with a phase curve of Rhea from the from Rhea of curve phase a with compared

55, 83 (Buratti et al., 2009). al., et (Buratti

E. to

( 1983

F., have have a -

Young, 92. ) Te rna stlie: ufc cmoiin and compositions Surface satellites: Uranian The .

sharper sharper ae nerl t ils h Bn abd, the albedo, Bond the yields it integral hase

L. - modeled by Hillier et al. et Hillier by modeled P., Buratti,P., B.

A., peak

Not only is Pluto’s surge among the the among surge Pluto’s is only Not

, similar that to modeled by Hillier et Stern,

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