12 Outstanding questions and future explorations Franck Marchis, John R. Spencer, and Rosaly M. C. Lopes

12.1 INTRODUCTION

A lthough o ur knowledge of Io had been significa ntl y advanced by several missions (Pioneer JO in December 1973, Pioneer J lone yea r la ter, and especially the Voyager twin probes in I 979) , the ar riva l of the Galileo spacecraft in 1995 brought unprece­dented op portu nities to observe lo. Galileo orb ited the Jovian system for 8 years, ob taini ng num ero us observat ions of lo in severa l wavelength ranges (Chapter 3). The study of these data , comp lemented by recent data from gro und -based telesco pes and the Hubble Space Te lescope (HST), has revolutioniz ed ou r under standing of the nat ur e of this mo st exot ic moo n. The previo us chapters have summarized the sta te of our knowledge of Io , from the interior to the torus , and made clear that man y key q uestions remain unanswered. After the demi se of Galileo, these questio ns must be addre ssed by continuin g work on a lready acq uired data , by new gro und-b ased and space telesco pe obse rvation s and , in the futur e, by new missions to the Jupiter system with dedicated observat ions of Io . Table 12. 1 reviews the capabil ities of severa l facilitie s (spacecraft s and telescopes) which were used or are in development to stud y thi s cap tivating sa tellite of Jupiter.

Io is a key target for future explo ration. It is a fasc ina ting wor ld in its own right and , as the most dynamic body in the Solar System , this sate llite occupies a unique place in planetar y science. It is the only plac e beyond Earth where we can watch active vo lcani sm happen on a large sca le. The int erconnect ions between orbit dynamics , in terior , tecto nics , volcanism, surfa ce chemistry, and atmos phere , and the assoc iated magneto spheric phenomena , can significantl y help o ur unde rstanding of the evol ution of planet s and sa tellite s. lo is the best target for the study of tida l heat ing, a process of fundam ental impo rtan ce to the evo lution of planetary sate llite system s, and one that may great ly expan d the habitabilit y zone for extra-terre strial life. Io 's tidal heating is intimately con nected to Europa's , which is thou ght to maintai n an ocea n of liqu id water underneath an icy crus t (e.g. , Greeley et al. , 2004).

Table 12. 1. Overview of facilit ies (spacecraft and telescope s) used or proposed to study lo .

Mission Year of fly-by Instruments Spatial resolution Main scientific resu lts

or telescope operation (performed on lo N 00 00

or schedu led) 0

Pioneer 10 December 1973 Radio , UV, IR No images recorded due s ~

photomete rs + visible to rad iation co = imaging Q. s·

Pioneer 11 December 1974 Radio. UV , IR I visib le image Characterization of the surface (IQ

photometers + visible (resolu tion of 376 km) ,.Q

= imaging seen from North Po le "' ~

Voyager I March 1979 IR spectrome ter, I km in visible Discover y of volcanic acti vity , o· =

visible camera (IRIS) 9 act ive centers. surface morpholog y, <I>

co plumes = Q.

Voyager 2 July 1979 IR spectromete r, IO km in visible Study of 8 eruptions, surface morpholog y, =' visible camera (IRI S) plumes =

Galileo December 1995- SSI , NIMS, UVS, 7 fly-bys, ~ 160 hot spots de tected, plumes. ., "'

spacecraft Septembe r 2003 PPR, fields and best visible resolution close-up of eruptions, fields and particles "' ;.(

-= particles (Chapter 3) ~6m , best IR data 0

(NIMS ) ~1 km .. ~

Cassini Janua ry 200 1 CIRS , ISS, UV IS. VIMS 60 km in visible Two giant plumes . aurora , o· =

spacecraft Pele acti vity <I>

Hubble Space Since 1990 UV- visible- N IR ~200 km in optical Plume compositions, surface change

Telescope ES0-3.6m 1996-200 1

camera /spectrometers N IR + The rmal IR ~250km in N IR Hot spot ac tivity

+ ADON IS camera Keck-LOm Since 2000 N IR + thermal IR ~JOO km in N IR Hot spot activity and tempera ture .

+ AO camera + spectro SO d istribution

Gemini+ GP! 20 10 N IR + thermal IR 65 km in R-band (visible) Hot spot acti vity and temperature ,

camera + spectro surface changes

TMT-30m April 2014 Undefined but 40km in N IR Hot spot acti vity and temperature , visible + NIR IO km in visible surface changes camera + spectro

OWL- IOOm Complete in 2020 Undefined but 10 km in N IR Hot spot acti vity and temper ature , visible + NIR surface changes and morphology camera + spectro n

New Horizon February - UV , visible, N IR 20k m High-temperature volcan ism. plume s, :='"

fly-by March 2007 camera + spectro interaction with magne tosphere, ;::; SO em ission

Sec. 12.2] Outstanding issues 289

Io ca n help us better understand the ea rly Earth, as lo provides a living examp le of how a planet responds to high heat flow analogous to the Ea rth 's heat flow at the time life began. Io 's prodigious volcan ic activ ity, and possible ultramafic compos ition of the lavas, give us the opportunity to understand the Eart h 's ear ly geo logic act ivity . Io also prov ides an ideal testbe d for understanding fund amen tal atmospheric , planetary escape, and magneto spher ic processes.

12.2 OUTSTANDING ISS UES

12.2.1 Interior structure and relationship to the heat flow

What is the composition of the core?

A precise tracking of the Galileo spacecraft during its five fly-bys of Io sa tellite has provided new constraints on its inter ior. ll is now clear that Io is a differentiated body co nsistin g of a meta llic core and a silicate man tle (Chapter 5). Gravity measurements, which were success fully co llected during four fly-b ys (Anderson et al., 200 I) indica te that lo has a large core (between 0.37 an d 0.52 times Io's rad ius, see Cha pt er 5), probably made of iron or iron su lfide as predicted by cos mo chemica l considera tion s (Conso lma gno , 198 1). Jn C hap te r 5, the author s discuss the absence of a magnet ic field signat ure, which was not detected even durin g the late Galileo fly-bys a few hundr ed kilometers above th e nor th and sout h poles (Kive lson et al., 200 I; Ki velson et al., 2004). Thi s impli es th at littl e co nvectio n is taking place in the co re of the satell ite. The compos ition and origin of the core of Io remains un cer tain . Bet ter constra int s on core comp osit ion will be d ifficult withou t in situ seismic data , but model ing of the chem istr y of Io 's volcanic gases can perhaps provid e so me constraint s (Zolo to v and Feg ley , 2000).

Wltat is Io's total lteat flow?

An accurate measu rement of the heat flow and its var iab ility with time prov ides imp ortan t cons traint s for the interior of lo. Io 's rampant volcan ism result s from the dissipation of energy pro du ced by tida l and orbita l int eractio ns with Jup ite r, Europa , and to a lesser exte nt , Gany mede (Peal e et al., 1979; Hussmann and Spo hn , 2004; see a lso Chapter 5). Severa l mea suremen ts of Io's heat flow using severa l tech niques and instrum ent s have been publi shed (Matson et al., 198 1; Veeder et al., 1994) yielding a minimum average heat flow of 2.5 W 111-

2. Veeder et al. (2004) developed a new model

(3± I Wm - 2) includin g the surprisingly warm po lar regions observed with Galileo/

ph otop olar imeter and radiomete r (PPR) (Spe ncer et al., 2000) , and att ribut ed these to an excess of end ogen ic heat due to a different style of volca nism at high lat it udes (Rathbun et al. , 2004) . Howeve r, uncertaintie s remain and most heat flow estimates are mode l dependent. To better und ersta nd the mechani sm of tidal d issipa tion, the evolution of Io int o th e Lap lace resonance , and its relation wit h Europa tidal heating , accura te measur ement s of Io's hea t flow an d its temporal va ria tion is needed. Though this task ca n be add ressed through improved models and int ensive groun d-based

290 Outstanding questions and future explorations [Ch. 12

observations, these should preferab ly be comb ined with spacecraft observation s to broad en th e wavelength and pha se angle coverage.

What is the thick11ess of the lithosphere?

Th e thickness of the lith osp here is model dep endent and severa l estimat es, ran ging from a few km to I 00 km , have been prop osed (Sch uber t el al., 2004). Howe ver, a litho sphere thicknes s less than 30 km is inconsi stent with the "'I O km height s of mountain s on lo (Carr et al., 1998). The se mountain s are evenly distributed on the sur face of Io and mo st of thos e imaged by Galileo and Voyager show tectonic orig in (Jaeger el al., 2003). A minimum thickness of 12 km is necessary to form them by compressive stress at the base of the lithosphere , a scenar io propo sed by Schenk and Bulmer ( 1998). For a thick litho sphere (>3 0 km) , magma advect ion is prob ably the main mechanism for tran spor t of heat across the litho sphere instead of heat co ndu ction . If conducti on is negligible, an important conseque nce is that the remotely meas ur ed volcanic hea t flows report ed previo usly shou ld therefore corre­spo nd to th e total hea t loss.

The origin of the moun ta ins is still unclear (Chapter 6). Schenk el al. (2001) point ed out that if they a re considered to be upthru st blocks, th e litho sphere should be a t leas t as th ick as the tallest mountain s (Boo saule M ontes has a heigh t of 17.5±3 km). In fact , the litho sphere may not be homo geneous, being thick er. for instance, where the mountains are located . High spa tial resol ution obse rva tion s of the morpholo gy of mountain s throu gh new spacecraft mission s cou ld pro vide dir ect informa tion on their formation mechani sm and furth er constrai nt s on th e litho sph eric thickness.

12.2.2 Nature of the active volcanic centers

How does volca11ism operate i11 extl'eme e11viro11me11ts?

The mod eling o f eruptio ns throu gh meas urement of th eir therma l outpu t as a function of wave length , position, and t ime (e.g ., Davie s el al., 200 I; William s et al., 200 I; Ra th bun et al., 2002) provide s co nstra int s on th e erupti ve centers (such as volumetric erup tion ra te, compo sition of the lava , type of volca nism). Simila r stud ies a re curren tly perform ed to stud y terrestrial volca noes from Ear th orb it (e.g., Ha rri s el

al., 1999). For Ear th, th e analy sis is co mplicated by the presence of a significant a tmo sphere and al so because of th e int eractio n with volati le elements (i.e. , wate r) prese nt on our planet. Fo r Jo the low spatial reso luti on of most availab le observatio ns introdu ces uncertainti es in the application of mode ls.

The key to the stud y of th e phy sica l pro cesses co ntro lling volca nism is to sepa rate the influ ences of ma gma co mpo sition and vola tiles, tecton o-physical proce sses (e.g., tidally-induced superheatin g, mantl e plum e asce nt) , and local conditi ons (e.g. , gravity ) . Because of its low gravi ty, lack of plat e tecto nics, high ma gma temperatures, very low a tmo spheric pressure, and co nstant activity , lo provid es an end -member exampl e where we can test phys ica l model s of silicate volcani sm . Ob ser­va tions at grea ter spatial and temp ora l resolution wou ld be of great value in test ing th ese mod els and further co nstraining eruption parameter s.

Sec. 12.2] Outstanding issues 29 I

What is the compositio11al ra11ge of Io 's magmas?

Very high magma temperatures measured by Galileo at Pillan (Chap ter 7) suggest ul tramafic composi tions, at least for one hot spot. This raise s the question of how relative ly primitive ultramafic volcan ism (typica lly associated with th e ancient Archea n (3.8- 2.5 Ga) on Eart h) could persist on such a dynamically ac tive body as Io where extre me differentiation sho uld be expec ted (Keszt helyi and McEwe n, 1997) . Either the curr ent sty le of volca nic activity is a geolog ica lly recent phenomenon (i.e., Io has on ly recent ly atta ined its resona nt orbit with resulting tidal heating) , or remixing of cru stal material back int o the mantle has prevented differentiation (see Keszt helyi et al. (2004) fo r one theory). Co ntinued m eas urement s of Io 's volcanic thermal emissio n are needed to determine whether o th er hot spots exhibit simila rly high temperatures that could impl y that ultramafic volca nism is wid espread . Several simila r erupti ons character ized by a very high temperature of magma and a brief short life (ca lled Pillania n erup tions, see Chapte r 7) have been observed with vario us techniqu es, such as Tvas hta r with Galileo (e.g., Williams et al., 200 1; Mila zzo et al., 2005), and Surt with adaptive optics (AO) from the gro und (Marchis et al., 2002). These erup tions cor respo nd to the out bursts seen by photometric measure­ment s from the grou nd (Stansbe rr y et al .. 1997; Howell er al., 200 I). Beca use they are extre mely bright and energetic, with relatively large areas of hot materials exposed , these erup t ions give us the best op portu nity to extract magma temperatures throu gh spectrosco pic measurement s, befor e a significa nt amou nt of coo ling has taken place. Such meas urements are needed to constrai n the composit ion of the magma that will provide a vita l wind ow into Jo's interior composition and struc tu re, and to provide an estimate of the highest lava tempera tu res. Th ese meas urements will be a h igh priority for future spacecraf t exp lorat ion , th ough this question can a lso be add ressed from the ground give n eno ugh telescope time to a llow detection of th e brightest and hotte st eruptio ns, whic h a re quite rare.

Ho w do Io 's very large ,,ofca11oes work?

The mo st widespread volca nic landforms on lo are paterae , in wh ich erupt ions are mostly confi ned with in a ca ldera -like depressio n (Radebaugh et al. , 200 I; Lop es et al., 2004). A study by Jaeger et al. (2003) showed that 4 1 % of the mou ntains on Io ha ve one or more paterae adjacent to th em, indicating a genetic link between mounta ins and paterae. Since Voyager , over 160 active volca nic centers have been observed (Lopes et al ., 2004) . Their distributio n suggests the absence of large-sca le plate tectonics on lo. At lea st 45 of these hot spot s were seen act ive mo re th an 4 time s. Many highly pers istent hot spots a re fou nd inside pat erae, notably Lok i and Pele. How do these eruptio ns behave? Are they lava lakes , perhap s constantly overturning (e.g. , Rat hbun et al., 2002), or a re they temp orary lava lakes, formed by a pro cess aki n to mid- ocea n spreading centers on Ea rth (Gregg and Lope s, 2004)? How is the ma gma supp lied to rep leni sh these eruptions. which last for years or eve n decades? The prese nce of multi-kilometer-high eruption plume s that produce vario us co lored pyr­oclastic deposits on the surfa ce suggests complex int erac tion s between magma and multiple volatile reservo irs, includin g su lfur , sulfur dioxide, and pos sib ly halides and

292 Outstand ing questions and future explorations [Ch. 12

other s. Are ma gma- volatile interact ions restricted to the sha llow crust, or is ther e deeper assimilation and injecti on of volati les by ascend ing magma s? Close-up spa ce­craft ob servation s at ult rav iolet, visible, and infrar ed wavelengths, with much better temporal and spatial coverage than Galileo, provid e the best way to answer these qu estion s, but continued ground-based monitor ing of thermal emissio ns may also provid e valuabl e insight s.

Does sulji,r volca11ism exist 011 lo?

Sulfur is widespread on Io 's surface , and sulfur flows may have been empl aced a t variou s hot spot s such as Ra Pat era and Emakong (Chapter 7). Sulfur volca nism may play a secondary but import ant ro le in Io's resurfacing , but so far we have no proof that sulfur-dom inated volcanic flows exist. T his q uestion can probably only be answered by high-re solution spacecraft ob servation s capable of measur ing the peak temperature s of act ive flows, or reso lving the likely distincti ve morpho logy of sulfur flows, as it is difficult to distingui sh sulfur flows from sulfur-coated silica te flows using reflectance spectroscopy.

12.2.3 Io's young surface

Composition of smface

The composition of Io's surfa ce is still not well und erstood. S0 2 frost is ubiquito us, cover ing most of the surfa ce, and identifying oth er compound s has been difficult given the spa tial resolution of the avai lable observations (Chapter 9) . The relat ion ship between surface co lors and composition is not straig htforward, although the distribu­tion of color unit s was well character ized by Galileo's observa tions (Ge issler et al., l 999; see a lso Chap ter 9). Co mpo sition of the lavas on lo has only been dete rmin ed indirectly through temperature and the relative ro les of basa ltic vs. ultram afic co m­positions have not been estab lished (Chapter 7). Unfortunat ely, the near-infrar ed mappin g spectro meter (NIMS) grating sto pped mo ving when the instrument started observin g Io a t high spat ial resolutio n (Chapter 3), therefo re, the opportunity to spectrall y cha rac ter ize different geo logic units a t high spa tial and spectra l reso lution was lost. New spectroscop ic observat ions from spacecraft are needed to determine the compo sitio n of lavas and the detailed distribut ion of S0 2 and other compounds a round Io 's hot spots.

Res urfaci ng rate

Io's prodigious volcanic rate means tha t, even on timesca les of month s, sur face changes ca n be identified. The lack of impact crate rs on the sur face has long been attributed to the rapid resurfacing ra te, howe ver, the nature of the pro cess is still unknown : does the resurfacing happen most ly because of volcanic products such as lava flows or , as suggested by Geissler et al. (2004), are plum e depo sits largely responsible? The fact that mo st ac tivity on lo seems to be co nfined inside pat erae, possibly as lava lak es (Lopes et al., 2004) , and tha t relatively few large lava flows a re

Sec. 12.2] Outstanding issues 293

seen on the surfa ce ha s chall enged the ear ly ass umption that la vas were responsible for the burial of craters. T emp ora l observations of Io 's act ivit y (p lum e vs. flows and lava la kes) wo uld better co nstra in the dominant resurfac ing proc ess .

12.2.4 Atmosphere and interaction with Jovian magnetosphere

Plumes: 11ature a11d for111atio11

Io 's volca nic plumes (Chapter 8) were no t well studi ed by Galileo, because the low data rate prevented good spat ial and tempor al coverage, and only one plume source, Prometheus, was imag ed at high enough spa tial reso lution in da ylight to pro vide deta iled insights into the plum e generation mech anism . The Prom etheu s data sugges t that the smaller " Prom et heus-type" plume s are generated not by direct volc anic venting but by hot la vas vo lat ilizing the S0 2-rich sub strate (Kieffer et al., 2000; Mil azzo et al., 2001). We do not know if all small plume s are generated this way , a nd we do not under stand how thi s mechani sm generates sta ble, long-lived , often symm etrical plumes. Additi onal high-r eso lution imaging of plum e so ur ces from future mission s is need ed to fully und ersta nd them , but in the mea ntime pro gress ca n be made in under standing existing dat a by the further deve lopment of numer ical model s o f plum e beha vior. More deta iled analysis of the composition of Prometh eus ­type plum es, by spectros cop y from future Earth-o rbiting telesco pes o r missions to th e Jupiter syste m are needed to further constrain the compos ition of the surface vo latiles. The co mp os ition o f the lar ger, more variable " Pele-type " plum es, which appea r to result from dir ect vo lca nic venting , pro vide s informati on on th e chemistry of the ma gma s with wh ich they ha ve equi librated in the vent, so these plume s will al so be impor tant ta rgets for futur e co mpo sit ional studi es.

Characteriza tio11 of the atmmphere

Io 's tenuou s atmo sp here , dominated by sulfur and oxyge n co mp ound s, was di scov ­ered by Pioneer 10 spacecra ft (K liore et al., 1974). Sinc e then it has been intensive ly studi ed in various wave length ranges from millim eter to ultra violet. How eve r, a full a tm osph er ic mod el. takin g into acco unt the full co mple xity o f this atm osp here (includin g th e effect of pla sma hea tin g and ot her factors) is no t yet ava ila ble (Chapter I 0) a nd the at mo sphere is still no t well und ersto od . We do not yet kno w wheth er the atmosphere is pr ima rily support ed by sublimation of Sun-warmed sur face vo lati les, o r by volca nic venting , so we do not kn ow whether th e atmosphe re collapses at night. More data on the variation in a tmo sph eric density with surfa ce locat ion , tim e of day , a nd helioc entri c di stan ce is needed to answer th is basic question. Non-vo lat ile spec ies such as the recentl y di scove red NaC l, which ca nn ot be supported by sublim a­tion , should be trac ers of dir ect volcanic input , allowing us to di stingu ish volca nic and subli ma tion sourc es if we co uld map at mos pheric co mp os ition at high spa tial reso­luti on. In addition , the at mosp heric temperature and its vertic a l va riab ility is still poorly co nstr a ined , complicating reco nciliati on o f the disparat e so ur ce of data on the atmosphere from ultra violet , infr ared, a nd millimeter wavelengths.

294 Outstanding questions and future explorations [Ch. 12

Plasma transport and formation of the torus

The Galileo and Cassini fly-bys have pro vided much new informa tion on th e inter­action between Io and th e Jo vian magneto spher e, pa rticularl y rega rdin g the flux tub e tha t a llows th e tran sfer of charged particl es between th e two bodie s . But we still do not und ersta nd ho w vari a tions in Io's volcano es cont ro l the suppl y of plasma to the to rus, mechanisms of du st form at ion, or man y other deta ils of the flow of ma tter and energy between Io, Jupit er, and the magnetos pheric plasma (see Chapt er 11 for more details).

12.3 GRO UN D-BASED TELESCOPES AND NEAR - EARTH TELESCOPES

Until the a rriva l of a new mission in the Jo vian system with dedi ca ted lo observa tion s, the futur e explorati on of Io and stud y of its volcani sm lies for th e next several years largely in th e han ds of ground -based ob servers.

12.3.1 The promise of ground-based telescope contributions

With the development o f space-born e observa tories, th e gro und- base d obse rvation s of the Solar System objects have been relega ted to a seco ndar y place in th e last two decades. Despite the lar ge apertures of th e recent ly developed telesco pes, th e effect o f a tmosp heric turbul ences is still prepo nder ant. Th e a ngula r resolution on th e ima ges is limit ed to seeing (i.e., rv0.7 11 in visible light from a very good site such as on the top of Mauna Kea in H awa ii) qui te close to the a ngular diamet er of lo (rv l. 2" ) at its oppos ition. In th e near -infr a red lo can be rou tinely reso lved by direct imag ing with usefu l spa tia l reso lution app roac hing 0.3" (Spe ncer et al., 1990). To break this "see ing bar rier" and access the d iffract ion-limited reso luti ons of cu rrent tele­sco pes (0.040" in the nea r-infrare d on a 10-m telesco pe), severa l tech niq ues, which take advantage of th e develo pment of severa l tech nologies, have been develope d. Th e co ncept of adap tive op tics (AO) was proposed by Babcock (1953), but it was necessa ry to wait unt il th e end of th e 1980s before the firs t pro tot ypes (Star fire and Co me-on) were develo ped ind epe nd ently by several gro ups based in the U.S .A. and F rance. T he AO systems pro vide in real time an image with an ang ula r resolut ion close to the d iffract ion limit of th e telescope . Beca use o f techn olog ica l limit at ions, linked to the way the wave fro nt is an alyzed, most of the AO systems proc ure a co rr ect ion that is part ia l and slight ly var iable in time in the near- in frare d ( 1- 5 µm ). These systems were made ava ilable to the astro nom ica l comm unity on 4-m class te lescopes less than 10 years ago. Mar chis et al. (2000, 200 I) have used th e ADON IS AO syste m on the 3.6-m ESO (Euro pea n So uth ern Observa tory) telescope to mo nitor Jo volcanic activ ity ove r a per iod of 4 years a t a wave length of 3.8 ~Lm. The spat ial reso lution obtai ned in these obse rvat ions was 0 .15", or ,.._,500 km on lo . At these long wave lengt hs, only the bright est hot spo ts co uld be seen aga inst th e sunlit d isk of Io (rv4 per hemisphere) and the meas ureme nt of th eir indi vidual flux was compli ca ted by the limited ang ular resolution . Wi th th e adve nt of the 8-m class telesco pes, the detectabi lity of hot spo ts on lo increased d rast ica lly. Approx imately 5- 8 ac tive sources were detec ted on one

Sec. 12.3) Ground-based telescopes and near-Earth telescopes 295

3-µm image using the AO system ava ilable on the Keck l 0-m telescope which pro vides an angu la r resolutio n of0.0 5" a t 2.2 ~Lm (i.e., 130 km on lo at its oppos ition ). Because the hot spots can be also seen at longer wave lengths (5 µm) , their temp eratur e (between 500 and 1,000 K) and emission area can be also estimated (Marchis et al. , 2005). Additionally to these faint act ive centers, Marchis et al. (2002, 2005) repo rts the detec tion of several active cent ers at a short er wave leng th rang e ( <2.5 ~Lm) which ha ve highe r temp erat ures (T > I ,300 K) and a re more energet ic. Because these hot spots are det ected in a large wavelength range (dow n to I ~un on several occas ions), it has been possib le to constrain the type of activity using a basaltic lava coo ling mod el (Davies , 1996). Surt -200 l , the largest eruption ever witnessed in the Solar System, was lucki ly detected by Marchis' gro up at its beginning in February 200 l with the Keck AO system. The int ensity profile indic ates the presence of a vigoro us, high-temp erat ure volca nic eruption (T > 1,400 K), consistent with eit her a basaltic or ultramafi c eru pti on. Th e type of eru pti on that produces this therma l signature (Pillanian) is thou ght to have incandescent fire fountains of mo lten lava which can be several kilometers high, propelled at great speed out of the ground by expa ndin g gases , accom panied by extensive lava flows on the surface . Th e integrat ed the rmal output of thi s eruption was clo se to the tot a l est ima ted output of Io ("' IO 14 W, Veeder el a{, 1994).

Several AO sys tem s a re or will be soo n ava ilable on 8-m-cl ass telescopes (M MT , GranteCan , LBT) and the AO technique s have become mor e reliab le and accessib le to a wider community. Th e extended temporal baseline of ground -based observation s is highly complementary to the intensive but short-b aseline coverage provided by space­craft. Cu rrent AO syste ms provide data with the same or better spat ial reso luti on as most of the glob al Galileo/N IMS obse rvations taken durin g the Galileo tour (i.e. "' 200- 300km) (Lope s-Gau tier et al ., 1999; Dout e et al., 200 1). A new generatio n of integral field spectrograph s is now com missioned on severa l 8-m-cl ass telescopes , such as SPJFFI for the YL T 8-m telescope and OSIRIS for the Keck 10-m telescope. By obtaining 2-D spatial coverage and spectral coverage simultaneou sly, they will give the oppor tuni ty to record in ha lf an hour a spectral cube of Io 's surface with a much better spectral reso luti on (R = I ,000- 10,000) than Galileo/ NIMS (R "'3 00) between 0.9 and 2.5 ~Lm, helpin g to cha rac ter ize the comp osition of the sur face and the ac tive volca nic centers. Moreove r, after a radiation -induced anoma ly sto pped the movem ent of the gra ting, NIMS spectral samplin g decrea sed to 12 wave length s in the 1.0- 4. 7-µm ran ge (Chapt er 3). Despite this limited spectral coverage , Doute el al . (2004) ob tain ed high spatial reso lution maps ofS0 2 ab undan ce on Io 's surfa ce and propo sed different mechanisms of formation for these areas linked with therm odynamic and volcanic processes. The spa tial reso lution of the data provided by the gro und -based spectro­imager instrument s will not be as high as the best regional map s from the Galileo/ NIMS instrument ("' 150 km per pixel compare d with 7- 25 km per pixel) but they will help to better character ize change s in the comp osition of the surface due to volcani sm (plume depos its, lava flow fields), as well as charac terizing the tempera tur e of the bright hot spot s.

The next generat ion of AO system s is currently und er development. Th ese new "ex treme AO " systems , built mos tly to image plane ts aro und nearby star s, will

296 O utstanding questions and future ex plorations (Ch. 12

Figure 12. 1. Observation s of lo in H-band ( 1.6 µm) with several AO systems. Three AO system performance s were simulated. T he spa tial reso lution with the Keck AO is est imate d to be 160 km on the center of the disk. An extreme AO system (ExAO) wou ld provide a full correction of the wavefront for such a brigh t ta rget, provid ing a sharper image, with a spatia l reso lutio n of 120 km on Io. Because of its larger aper ture , the spatia l resolution of 45 km , atta inable on lo with the Th irty Meter Telescope (TMT), would be competitive with most of the global observations recorde d by the Galileo spacecra ft. On rare occasions the thermal output of hot spo ts is large enough at H-band to be de tected in sunlit observations such as these. Thank s to the sta bility provided by ExAO , hot spot A is detected with thi s system. Hot spot B, with an inten sity 12 times lower than the Keck AO limit of detec tion is clearly visib le in the TMT simu lation. More of these high-temperature eruptive cente rs could be studi ed with thos e new instrumen ts help ing to bette r co nstra in the composition of the magma . No a posteriori data proce ssing to enhance the sha rpness of the images (such as deco nvolutio n) was app lied to these simulati ons. (See al so color section.)

surpa ss the performan ce of existin g AO syste ms by two orders of mag nitude in contrast - see, for instance , the Gem ini Planet lmager instrument for the Ge mini telescope (Macintosh et al., 2004). To achieve such nearly per fect and stab le correc ­tion , the reference targets mu st be br ighter than 8th magni tude in I-band. Extreme AO system s wou ld therefore also have the capabil ities to ima ge lo with a quasi-perfect angular resolut ion (very close to the diffrac tion limit of the telescope) between 0.6 and 5 µm , corresponding to 45 and 340 km on Io's surfac e at its oppos ition. Figure 12. 1 illustrat es the gain in angula r resolu tion , comparing an extreme AO ima ge taken in H­ban d ( 1.6 ~un) of lo mo unted on the Keck 10-m telescope and the same Jupiter-facing hem isphere observed with the current AO system. In H-band , even after the image was degraded to the reso lution prov ided by the AO system , most of the fea ture s detectab le are albedo ma rking such as the dark paterae (Loki pat era is discernab le at the center of Io) and bright regions covered by S0 2 frost. On ra re occasions , when the tem ­perat ure and surface covered by an act ive eruption is large enough (typ ically T > 1,000 K for S > 2 km 2

), its therm al output is sufficiently contrasted to be detect ed on the sun lit background. On the initial simulat ed image , we artific ia lly added an eruption north of Loki (in Amatera su Pa tera) with inten sity four times lower

Sec. 12.3] Ground-based telescop es and near-Earth telescopes 297

than the limit of detection of Keck AO. Because of the better stab ility of the ext reme AO system, thi s hot spot is detected on the image provided. Th erefo re, the study of Jo's volcan ism (monitoring in particular) will benefi t from this planet finder instrument. Infrar ed or laser-guide-star AO systems ha ve the potential to a llow diffract ion-limit ed imagi ng of volca nic thermal emission in Jupiter eclipse, wher e high-temperature volca nism can be studied without compe tition from sunli ght. Th e further promise of obtai ning high angu lar reso lution images sho rtwa rd of 1 µm will be crucial to measuring high ma gma temperatures , and for imaging Io's plume s. Even at IO ~1111, a useful spatial reso lution of 600 km is possible on 10-m-cla ss telescopes, allowing the major therma l sources to be det ected and their contribution to the tota l output of Io better estima ted .

The instrum ents mounted on a grou nd -base d te lescope are not restrai ned in size and weight as are tho se on board space vehicles. Th ey can also be easily updated taki ng advantage of the most recent technology. For instance, high- resol ution spec­trographs (R = 50,000) in the mid-infr ared (5- 25 ~tm), such as TEX ES (the T exas Echelon Cross Echelle Spectrograph ; Lacy et al., 2003), can be used to detect mol ecu les in Io's atmo sphe re (Spencer et al., 2005), and on large telescope s should a llow mapping of the spatial di stributi o n of the atmosphere.

Further advances in gro und -based astronomy will come with the likely develop­ment in the next two decades of giant segmente d mirror telescopes (G SMT ). Several comp et itive proj ects are under study, such as the Thirty Meter Telescope (TMT) which sho uld be built in partnership between the U.S.A. and Canadian institutes (F igure 12.2), the Eu ro50 (50 m) telescope, a project estab lished between scientist s in Finland , Ireland , Spain , Sweden , and the U. K ., and finall y the OverWhelmingly Large (OWL I 00-m) telescope under study by the Europea n Southern Observatory (ESO). The designs, under stud y, face several major challenges, such as the de sign of their enclosu re, their weigh t, and their cost. The se futur e telesco pes will be eq uipped with AO syste ms (R ussell et al., 2004) to allow diffrac tion -limit perfo rman ces (i.e., the y will pro vide images with an angular reso lution up to the milli-arcsec in the op tica l). On Figure 12. 1, a simulated obse rvation with the T MT 30-m of Io's Jupiter-facing hemi sp here in H-band is given for comp ariso n. The gain in angula r resolu tion is sufficient to detect small albedo features at a spatial reso lution of 30 km at 1 µm. Another hot spo t whose int ensity on the initial image was chosen to be 12 times lowe r than the dete ction limit of the Keck AO is clearl y visible between Loki and Pele (labe led B).

12.3.2 Airborne telescopes

T he SOF IA airbo rn e observatory (Erickson, 2005) has the potential to make va luable co nt ribut ions to the study of Io's at mosp here. Amon g the instrum ent s being deve loped for SOFIA is EXES , a high- reso lut ion, mid-infrared spec trograp h that will be ab le to detect atmosp heric absorpt ions fro m S0 2 and other species at wave ­length s, such as the 7-~tm region , where obse rvations from the gro und are difficult du e to at mo spheric H20 abso rpti on.

298 Outstanding questions and future ex plorations [Ch. 12

Figure 12.2. Art ist's rende rings of the TMT and com parison with the Paloma r 5-m Hale telescope. Thi s telesco pe, develo ped in par tnership betwee n the U.S.A. and Canad ian institutes should be avai lable in 20 14. Because o f the la rge size of its ape rture. co mbined with the cap a bilities of AO systems. it will provide an unprecedent ed spat ia l resolution of lo, better tha n mo st of the Galileo spacecraf t infra red observat ion s (co ur tesy Ca liforni a Institut e of Tec hnology) . (See a lso co lor sectio n.)

12.3.3 Ultraviolet-dedicated telescopes

Th e ult rav io let reg io n is lar gely ina ccessible to gro und-b a sed obse rvers a nd has bee n ex plo ited very rece ntl y, mostly with th e I-IST. Fo r instanc e, the first spectro scop ic detectio n o f S0 2 ga s (Balles ter et al., 1994) in this wave length ra nge led to nu mero us wor ks at temp t in g to estim ate the d istribution o f this gas in the th in at mo sphe re o f lo (Jessup el al., 2004 a nd C hapt er 10) a nd its link with volca nic ac tivity, such as the density o f du st pa rti cle s and gas in th e Pele plume. T he ult ravio let ligh t is a n in teresting wa velen gth ra nge to study th e int era ctions betwee n the su rfa ce o f lo , its at mosp here , a nd plasma sur roun d ing the sate llite . Fo r insta nce , fa r-ultr av io let images of Jupit er fro m I-IST revea l po lar a urora l emissions (Rego el al., 200 1 ), and d iscr ete emissio n from Io's magnet ic footpr int (C lar ke el al., 2004) . Ult ra vio le t emission o f a to ms ca n a lso be used to detect the neutra l clo ud mad e o f mater ia ls leav ing lo (Cha p ter 11) th ro ugh severa l p rocesses . 0 and S emi ssio ns were d etec ted in Int ern ationa l Ultrav io le t Exp lorer (I UE) spec tr a in the coro na (a t a di stance < 500 R1)

Sec. 12.4) Future space missions 299

and extended cloud s, which populate s aft er ionization of the plasma toru s. Thi s atmosphere-clouds - plasma - magne to sphere in teraction is central to the und ers tand­ing of the system, but it is as difficult to model as it is to observe (Sp encer and Schneider, 1996).

The ult ra violet spectrum shortward of 3,000 A contains va luabl e information about Io 's atmosphere and its int eract ion with the Jovian magnet osphere , but can only be observed from spa ce. fo ha s been explored in thi s region by a succession of Ea rth-orbitin g spacecra ft , parti cularl y the HST , though its ability to stud y lo in the ultraviolet ha s bee n seve rely ham pered by the summer 2004 failure of the Space Telescope imaging spectro gra ph (STIS), which provided high spat ial reso lution inform a tion (about 20 pixel s across Io 's disk) on both ab sorption s by the molecula r atm osph ere and emission from atomic species. A replacement ultra violet spectrogr a ph , Cos mic Ori gin s Spectrog raph (COS) , may be installed on HST if a servicing mission is eve ntu a lly flown , but its low spa tial reso lution will limit its ca pabilit ies for st ud ying Io. Much mor e could be don e to und erstand Io's atmosp here a nd toru s in teractions with future diffraction-lim ited ultraviolet telescopes of even mode st aperture ( 1-m aperture co uld still pro vide 20 pixels acro ss Jo at 2,500 A), if placed above the Earth 's atmosphere and equipped with high-effici ency ultra vio let instrument ation .

12.3.4 James Webb Space Telescope

The James Webb Sp ace Telescope (JWST) is a la rge infrar ed-optimiz ed telescope schedul ed for launch in Augu st 20 13 (Sabeiha us et al., 2005) . Th e tele scope will be equipped with a 6.5 m diameter primary mir ro r pr ovid ing, th erefor e, ima ges with an a ngular reso luti on no better than the cu rrent I 0-m class, gro und- base d telesco pes equipp ed with AO systems. The instrumentation of this a mbiti ous project is limited to the infrared range , since mos t of the drivin g scien tific themes (t he stud y of ga laxies with reds hift z > I, imaging of circumstellar disks, .. . ) mos tly re ly on thi s wave length domain. However, JWST' s high sensitivity is not an adva nta ge for such a bright target as Io, and it will not ca rr y inst rum ent ation suitab le for stud y of the a tm osph ere , for instance , so its usefu lness for lo studies is probab ly limit ed.

12.4 FUTURE SPACE MISSIONS

Visits of spa ce veh icles into the Jov ian syste m are unfortun ate ly rare. The Pioneer JO a nd// mission vehicles were th e first spacecraft to ever visitJupit er in December 1973 and Decemb er 1974 , but it was only when the spacec ra ft Voyager J in March 1979 crossed into the Jovian moon system that the vo lca nic act ivity of lo was finally revea led (Morabito et al., 1979; see a lso Chap ter 2), opening up a com plete ly new world for planeta ry scienti sts. The Galileo mission to Jupit er (C hapt er 3) was co nceived to follow on the stud y of th e Jovian system. Th e spacecra ft ar rived in Decemb er 1995 an d was p laced into elonga ted orbi ts arou nd Jupiter , which were designed for close-u p fly-b ys of th e Ga lilea n moo ns. Althou gh seve ra l problems

300 Outstanding questions and future explorations [Ch. 12

happened during the handful of fly-bys ded icated to lo , the instrume nt s on board the spacecraft obtained unprecedented observations of lo that brought key discoveries , out lined in earlier chapters. The a rriva l of the Cassini mission in December 2000 while Galileo was orbiting in the inner part of the Jovian system gave a unique oppo rtunit y for mul ti-spacecraft ob servations of the Jovian system . However , Galileo was funda­mentally limited in its abi lity to characteri ze Io 's dynamic volcani sm by its low data rate (i.e. , man y maj or eruption s that occurred during the mission were seen only as isolated snap shot s). With the demise of the Galileo spacecraft in 2003, further observations of lo are restrict ed to telescopic observations from Ea rth, apart from observations that will be made from the New Horizons spacecraft in ea rly 2007 , during its Jupiter fly-by. Even with the fast progress made in the capabilities of ground-based instrum ents , long-term advances in under stand ing Io will require addit ion al close-up studies by spacecraft.

12.4.1 New Horizo11s fly-by

The Pluto-bound New Horizons spacecraft (e.g ., Stern and Spencer, 2003) will fly past Jupit er on 28 February 2007, and will obtain valuabl e observat ions of lo (Figur e 12.3). Its closest approac h distan ce of 2.2 million kilomet ers is ove r four times clos er than that of Cassini , and its ultraviolet and near-infrared instru mentatio n surpas ses that of both Galileo and Cassini in several respect s. New Hori ::ons will obtain global panchromatic imaging of lo with resolution of 20 km or bette r, to study surface chan ges since Galileo , tho ugh color imaging will be restricted to Jup iter-s hin e imagi ng of the Jupit er-facing hemisphe re due to saturation on the day side. Volcanic plumes will be inventoried over several orbits , providing time variabil ity information and perh aps revealing corre lations with du st streams that may be detected by the New Hor izons dust instrum ent. There will also be global 0.8-2.5 µm imagin g of night side volcan ic thermal emission with a spatia l resolut ion of bett er than 200 km , al lowing a detai led study of high-t emperature volcanic thermal emission. Numero us di sk integrated 500-1 ,800 A spect ra will docum ent the a tomi c em ission s from Io's a tmo sphere as a function of orbital and ma gneto spheric longit ude, as well as toru s emiss ions . Ob servations of severa l lo eclipses will docum ent the respon se of the ultravio let atmospheric emissio ns to eclipse, and will provide unpreced ented spa tial reso luti on on the 1.7-µm SO emissions detec ted m ground -base d obse rvat ions (de Pat er et al. , 2002).

12.4.2 Future planned missions

Only one Jupiter mission is currently und er developm ent - the Juno Jupit er orbiter which is designed for deta iled studies of Jupi ter's interior , aurorae , and inner ma gne­to sphere. Jun o' s studi es of the Jovi an pla sma may illumin ate some aspects of Io 's interaction with the ma gneto sphere , but Jun o will not carry instrum entation for useful ob servat ion s of lo itself. Variou s incarna tions of a Europa or biter mission hav e been studied over the past decad e, and such a mission is likely to carry remot e-sensing instrumentation that will also provide valuable information on lo, even though

Sec. 12.4] future space missions 30 I

Figure 12.3. Artistic vision of the Plut o-bound New Horizons spacecraft flying past the Jovian system at the end of February 2007. Multi-wavelength observations (from ultraviolet to near­infr ared) of Io's surface, plumes , and atmosphere will be recorded . (See a lso color section.)

rad ia tion concerns would probably preclude close approac hes to Io. Howeve r, until a Europa mission is funded and its payload is cho sen, its potentia l co ntribution to Jo science remains unknown.

12.4.3 A dedicated Io mission?

In 2001, at the request of the U.S. Office of Management and Budget , the National Academy of Sciences commissioned the Nat ional Research Counc il to do a stud y to assess the highest priority objectives in Solar System explorat ion for the next decade, 2003- 2013. This st udy published in 2002 and commonly referred to as the Pla netary Decadal Survey (Space Studies Board , 2003), solic ited input from throughout the planetary science comm unit y and the general public. A series of white papers were submitted to the Survey Panel on many of the objects in the Solar System and why they were good candida tes for future exploration (Sykes , 2002). One such pape r was subm itt ed on the future of lo exploration (Spencer et al., 2002). Though an Io­dedicated spacecraft mission did not mak e the cut in terms of highes t priority missions in the next decade (partly due to engineering and technology development issues), a future Jo-d edicated mission was encouraged for the following decade (2013-2023) after certa in technologie s (e.g ., radiation -harde ned circuitry , advanced propu l­sion and comm unica tions) are deve loped for other missions. As we outlined in the lo

302 Outstanding questions and future explorations [Ch. 12

wh ite paper (Spence r et al., 2002) , we env ision a Jovicentric orbiter with an eccentric orbit with a perijo ve nea r lo and an orb ital per iod or rv I month. Based on ou r experie nce with Galileo, such an orbit er cou ld survive the heavy radiation enviro n­ment near Io for 4 years or more with rv50 monthl y close lo fly-bys. This would enable repeated fly-bys of specific hem ispheres or regio ns or lo wit h simila r lightin g geome­tries to emphas ize study or time-variab le phenomena , which is necessary for studies of active volcanis m. Data downl ink and distant obse rvat ions wo uld occur during more distant parts of the orbit. Usef ul instruments to be carr ied on such an or biter includ e: ult raviolet spectro mete r for at mo spheric studies , high-reso lution visible camera (1-10 m per pixel loca l imag ing, IOOm per pixel globa l ima ging), 1- 5 -µm spectrometer with I-km resolu tion , 10 and 20-mm imager with 10-km resolutio n , laser alt imeter, mas s spectrome ter, and fields and particles instrum ents (Spencer et al., 2002). Fu rther ­more, the spacecraft wou ld idea lly carry several (rv3) penetrat ors wit h 20-h r or better lifet imes to per form in sit u stud ies of Io's surface. These penetrators would carry seismometers to reveal Io 's internal struct ure, mass spect rom eters to determ ine atmo­sph eric compos ition during descent , and a surface compos ition instrument package. More limited mission s a long these lines have been previously propose d to NASA's Discovery Program (i.e., sma ll, focused p laneta ry mis sions) in the late- l 990s/early-2000s (e.g., Smyt he er al., 1998; Esper et al., 2003)

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Figure 12.1. Observations of Io in H-band (1.6 µm) with several AO systems. Three AO system performances were simulated. The spatial resolution with the Keck AO is estimated to be 160 km on the center of the disk. An extreme AO system (ExAO) would provide a full correction of the wavefront for such a bright target, providing a sharper image, with a spatia l resolution of 120 km on Io. Because of its larger aperture , the spatial resolution of 45 km, attainable on Jo with the Thirty Meter Telescope (TMT). would be competitive with most of the global observations recorded by the Galileo spacecraft. On rare occasions the thermal output of hot spots is la rge enough at H-band to be detected in sun lit observations such as these. Thank s to the stabi lity provided by ExAO, hot spot A is detected with this system. Hot spot B. with an intensity 12 times lower than the Keck AO limit of detection is clearly visible in the TMT simula tion. More of these high -temperature eruptive centers could be studied with those new instruments helping to better constrain the composition of the magma. No a posteriori data processing to enhance the sharpness of the images (such as deconvo lution) was applied to these simulations.

Figure 12.2. Arti st's render ings of the TMT a nd co mp arison with the Palo mar 5-m Hal e telescope. Thi s telescope , deve loped in par tn ership be tween th e U.S .A. and Ca nadian insti tut es, shou ld be availab le in 2014. Beca use of th e large size of its apertur e, combined with the capabiliti es of AO system s. it will pro vide an unpr ecedent ed spatial resol ut ion of lo, better than mos t of th e Galileo spacec ra ft infrared observatio ns (co urt esy California Institut e of Techn ology).

Figure l2.3. Art istic vision of the Plu to-bound New Hori=ons spacec raft flying past the Jo vian system at the end of February 2007. Mu lti-wave lengt h observat ions (from ultrav iolet to nea r­infrare d) of lo·s surface, plumes, a nd atmosphere will be reco rded .