Icarus 170 (2004) 430–442 www.elsevier.com/locate/icarus Io’s volcanic control of Jupiter’s extended neutral clouds Michael Mendillo a,∗ , Jody Wilson a , John Spencer b , John Stansberry c a Center for Space Physics, Boston University, 725 Commonwealth Avenue, Boston, MA 02215, USA b Lowell Observatory, Flagstaff, AZ 86001, USA c University of Arizona, Steward Observatory, Tucson, AZ 85721, USA Received 12 December 2002; revised 3 January 2004 Available online 9 June 2004 Abstract Dramatic changes in the brightness and shape of Jupiter’s extended sodium nebula are found to be correlated with the infrared emission brightness of Io. Previous imaging and modeling studies have shown that varying appearances of the nebula correspond to changes in the rate and the type of loss mechanism for atmospheric escape from Io. Similarly, previous IR observational studies have assumed that enhancements in infrared emissions from Io correspond to increased levels of volcanic (lava flow) activity. In linking these processes observationally and statistically, we conclude that silicate volcanism on Io controls both the rate and the means by which sodium escapes from Io’s atmosphere. During active periods, molecules containing sodium become an important transient in Io’s upper atmosphere, and subsequent photochemistry and molecular-ion driven dynamics enhance the high speed sodium population, leading to the brightest nebulas observed. This is not the case during volcanically quiet times when omni-present atmospheric sputtering ejects sodium to form a modest, base-level nebula. Sodium’s role as a “trace gas” of the more abundant species of sulfur (S) and oxygen (O) is less certain during volcanic episodes. While we suggest that volcanism must also affect the escape rates of S and O, and consequently their extended neutral clouds, the different roles played by lava and plume sources for non-sodium species are far too uncertain to make definitive comparisons at this time. 2004 Elsevier Inc. All rights reserved. Keywords: Io; Jupiter, magnetosphere; Satellites, atmospheres; Volcanism 1. Introduction Jupiter’s magnetosphere offers one of the richest blends of geological, atmospheric, and space plasma physics phe- nomena in the Solar System. The heavy ions of oxygen and sulfur which dominate the mass and energy budget of the magnetosphere have their origin in the volcanoes of Jupiter’s remarkable moon, Io. From the volcanoes, the sulfur and oxygen move by steps into Io’s atmosphere and onto its sur- face, and then into large extended neutral clouds around Io and Jupiter via atmospheric escape. By subsequent ioniza- tion and pickup they populate the plasma torus, and finally they exit the jovian system altogether by outward diffusion, charge exchange reactions, or by precipitation into Jupiter’s auroral regions. Several spacecraft encounters with Jupiter—Pioneer in 1973–1974, Voyager in 1979, and Ulysses in 1992—have * Corresponding author. Fax: 617-353-6463. E-mail address: [email protected] (M. Mendillo). provided snapshots of the jovian magnetospheric environ- ment. These remarkable datasets provided insights into a complex system of volcanism, surface geology, atmospheres, charged particles and fields. On a much longer multi-year timescale, the Galileo satellite has just completed a detailed in-situ study of variations of Io, the plasma torus, and the magnetosphere as a whole. Complementing these spacecraft missions to Jupiter has been an ongoing series of observa- tions conducted from the ground and from low-Earth-orbit. We describe here a decade long view of changes in the jovian sodium nebula and contemporaneous observations of vol- canic activity on Io. The sodium clouds generated by Jupiter’s satellite Io pro- vide a window through which to examine a complex chain of processes that appear to dominate a magnetosphere in a way not found elsewhere in the Solar System. While a con- sensus model is far from complete, its potential elements have been described in several review articles (Schneider et al., 1989; Spencer and Schneider, 1996; Thomas, 1997; Bagenal, 2004). Briefly, volcanic eruptions of lava and geyser-like plumes populate its surface and atmosphere with 0019-1035/$ – see front matter 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2004.03.009
Io’s volcanic control of Jupiter’s extended neutral clouds
Michael Mendilloa,∗, Jody Wilsona, John Spencerb, John Stansberryc
a Center for Space Physics, Boston University, 725 Commonwealth Avenue, Boston, MA 02215, USAb Lowell Observatory, Flagstaff, AZ 86001, USA
c University of Arizona, Steward Observatory, Tucson, AZ 85721, USA
Received 12 December 2002; revised 3 January 2004
Available online 9 June 2004
Abstract
Dramatic changes in the brightness and shape of Jupiter’s extended sodium nebula are found to be correlated with the infrarebrightness of Io. Previous imaging and modeling studies have shown that varying appearances of the nebula correspond to changeand the type of loss mechanism for atmospheric escape from Io. Similarly, previous IR observational studies have assumed that enhancein infrared emissions from Io correspond to increased levels of volcanic (lava flow) activity. In linking these processes observatiostatistically, we conclude that silicate volcanism on Io controls both the rate and the means by which sodium escapes from Io’s atDuring active periods, molecules containing sodium become an important transient in Io’s upper atmosphere, and subsequent photochemand molecular-ion driven dynamics enhance the high speed sodium population, leading to the brightest nebulas observed. This isduring volcanically quiet times when omni-present atmospheric sputtering ejects sodium to form a modest, base-level nebula. Sodas a “trace gas” of the more abundant species of sulfur (S) and oxygen (O) is less certain during volcanic episodes. While we svolcanism must also affect the escape rates of S and O, and consequently their extended neutral clouds, the different roles playedplume sources for non-sodium species are far too uncertain to make definitive comparisons at this time. 2004 Elsevier Inc. All rights reserved.
Jupiter’s magnetosphere offers one of the richest bleof geological, atmospheric, and space plasma physicsnomena in the Solar System. The heavy ions of oxygensulfur which dominate the mass and energy budget ofmagnetosphere have their origin in the volcanoes of Jupiremarkable moon, Io. From the volcanoes, the sulfuroxygen move by steps into Io’s atmosphere and onto itsface, and then into large extended neutral clouds arounand Jupiter via atmospheric escape. By subsequent iotion and pickup they populate the plasma torus, and finthey exit the jovian system altogether by outward diffusicharge exchange reactions, or by precipitation into Jupiauroral regions.
Several spacecraft encounters with Jupiter—Pionee1973–1974, Voyager in 1979, and Ulysses in 1992—h
0019-1035/$ – see front matter 2004 Elsevier Inc. All rights reserved.doi:10.1016/j.icarus.2004.03.009
-
-
provided snapshots of the jovian magnetospheric enviment. These remarkable datasets provided insights incomplex system of volcanism, surface geology, atmosphcharged particles and fields. On a much longer multi-ytimescale, the Galileo satellite has just completed a detain-situ study of variations of Io, the plasma torus, andmagnetosphere as a whole. Complementing these spacmissions to Jupiter has been an ongoing series of obstions conducted from the ground and from low-Earth-orWe describe here a decade long view of changes in the joviasodium nebula and contemporaneous observations ofcanic activity on Io.
The sodium clouds generated by Jupiter’s satellite Iovide a window through which to examine a complex chof processes that appear to dominate a magnetospherway not found elsewhere in the Solar System. While a csensus model is far from complete, its potential elemhave been described in several review articles(Schneideret al., 1989; Spencer and Schneider, 1996; Thomas, 1Bagenal, 2004). Briefly, volcanic eruptions of lava angeyser-like plumes populate its surface and atmosphere
Volcanic control of Jupiter’s extended neutral clouds 431
m-thusns
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uds
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several gases (SO2, SO, S, O); a minor species is sodium, iportant for its easy detection at visible wavelengths andas a “tracer” of the more plentiful elements. Energetic iotrapped in Jupiter’s magnetic field bombard the atmospand the surface of Io, “sputtering” some of the Na atomsof Io and into orbits around Jupiter. Some of the atmosphgases still bound to Io and others in near-Io orbits abJupiter become ionized by the impact of energetic plasand solar EUV radiation. These low energy plasmas areject to Jupiter’s strong, corotating magnetic field, and tmagnetic capture (or “pick-up”) leads to the maintenanca plasma torus surrounding the planet at approximatelyradial distance of Io (5.9RJ), as well as an ionosphere on I
Recycling of plasma back into the neutral gas statevolves an equally remarkable series of steps linked to thethat three distinct populations co-exist near Io: torus platraveling at co-rotational speeds (∼ 75 km/sec), togethewith neutrals and ionospheric plasma moving with Io’sbital speed (∼ 17 km/sec). Neutralizations of the atomand molecular ions near Io or in the plasma torus resuthe release of neutral sodium at high speeds (Na∗) sufficientto escape from the magnetosphere, essentially unaffecteany other process, to form a great nebula. Within thisant structure, the slower speed, sputtered sodium (Na) fothe elongated cloud that orbits Jupiter with Io (the “banacloud).
There is a rich literature relating to the individual sodiuclouds composed of Na near Io and Na∗ forming jets,streams and nebulas. Using images of the Io sodium cloat multiple fields of view (±7RJ, ±30RJ, ±500RJ) from1990–1996,(Wilson et al., 2002)formulated a workingmodel of the overall jovian sodium budget. They determinthat various combinations of two basic atmospheric escprocesses—atmospheric sputtering of Na, and ionosphescape of an unidentified molecular sodium ion (NaX+)—are needed to explain essentially all manifestations ofsodium clouds. The pick-up of NaX+ and its subsequendestruction in the torus to form a “stream” or distributsource of Na∗ produces a bright and distinctly rectangulshaped sodium nebula when it is the dominant processmospheric sputtering may also contribute to the nebulaits efficiency in doing so depends strongly on the velocdistribution of the escaping neutrals that result. Given aficient fraction of higher-speed ejections, atmospheric stering was shown to produce a faint and somewhat diamshaped nebula around Jupiter during times when it iscompeting with NaX+ escape. Simple charge exchanonce thought to be the dominant escape mechanism, isunderstood to be only a minor exospheric process becauseis unable alone to account for the appearances seen staneously in the three fields-of-view datasets. Thus, wenow at the stage where imaging observations of the sodnebula provide a quantitative measure of both the ratesmeans of atmospheric escape from Io. The next step is tvestigate how the coupled neutral–plasma system at∼ 6RJresponds to changes in Io’s volcanism.
y
l-
2. Volcanic activity and the sodium nebula
Given the primary role of volcanic activity (lava flowand plumes) in producing Io’s atmosphere, it is reasonto expect that Io’s sodium clouds and plasma torus shbe affected by changes in Io’s volcanism. Yet, no obsetional study to date has been able to demonstrate suchlationship. We address this issue here by presenting thecomparison of neutral sodium cloud data with contemponeous measurements of volcanic activity over a nine-yeariod. The two main challenges are (1) finding measuremwhich somehow quantify “volcanic activity,” and (2) givethat sodium cloud observations and observations of volcactivity rarely, if ever, occur on the same night, finding a wof estimating volcanic activity during a sodium cloud oservation from the nearest volcanic observations availaThese are difficult issues open to varying interpretationsthus detailed discussions are required.
2.1. Observations of sodium nebula
The distant Na∗ atoms in the nebula were successfuimaged for the first time in December 1989(Mendilloet al., 1990; Flynn et al., 1994)and at yearly intervalssince then. The imaging technique used to observe Jupsodium nebula has been described in several previous w(Mendillo et al., 1990; Baumgardner and Mendillo, 199Baumgardner et al., 1993). The basic approach is to usesmall refractor (0.1 m aperture) to achieve a large fieldview (approximately 6◦), an interference filter (5893± 7 Å)to capture theD1 + D2 sodium lines, an occulting maskprevent the bright image of Jupiter from reaching thetector, and a CCD camera (both image-intensified andversion have been used over the past decade). On- anband sequences of on-target and off-target images are usseparate scattered light and terrestrial sodium from theian signal. Calibration to absolute brightness units is dusing either a standard source or standard stars.
Observations of the sodium nebula have been madeyear since its discovery. Optimal conditions require Jupnear opposition, a moonless sky, and extremely clearphotometric atmospheric conditions. In most cases, such lobrightness level imaging requirements occurred on oneat best a few nights) of each observing season. Hence, wthere is continuity of nebula observations from year to yshorter time resolution issues cannot be addressed witha dataset.
2.2. Observations of infrared emission
Observations of infrared (IR) emission from Io are takto be a readily available measure of hot lava flows assated with volcanic activity. These observations can be mwith groundbased telescopes, and it is even possible totinguish individual points of emission, or “hotspots,” reresenting particular volcanic features on Io. These type
432 M. Mendillo et al. / Icarus 170 (2004) 430–442
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observations have been made far more frequently than iming observations of volcanic plumes, the signature ofcanism that is a more direct injection of volcanic gasesIo’s atmosphere. Thus, regardless of whether or not infremission is the best proxy for the volcanic contributionIo’s atmosphere, it provides by far thebest possibility forcorrelation studies with the sodium clouds. A major isshowever, is that the patrol-type IR observations needemonitor Io’s volcanism are usually allotted telescope tion nights when the Moon is up, and thus the nebula andcanic datasets are not often simultaneous; we address tha careful analysis of statistical patterns and trends to aat the best possible indicator of volcanism for each neobservation.
To obtain the best statistical comparisons possible,select a dataset that (1) includes relatively frequent obsevations, (2) that is most representative of “overall volcaactivity,” and (3) that is as consistent as possible overtime period from 1990 to 1998. The disk-averaged brigness of Io’s Jupiter-facing hemisphere at 3.5 and 3.8appears to be the best measure available. Io’s Jupiter-fahemisphere has been observed more frequently thanlongitudes because it can be seen during eclipse wheis in Jupiter’s shadow, a time when sunlight does not stIo’s disk and contaminate the IR measurements. Theand 3.8-µm wavelengths are a middle-ground between2.2-µm band, whose brightness can change quickly aand which is most sensitive to small, hot volcanoes, and4.8-µm band which varies more slowly, and which is msensitive to large and relatively cool expanses of lavaIo’s surface. Indeed, there have been periods of time wthe 2.2-µm brightness of Io was many times greateraverage, while the 4.8-µm brightness remained nearly uchanged, and vice-versa. We eliminate the problem of juing the volcanic activity level from Io from such disparameasurements by using the wavelengths in-between, wthere is significant sensitivity to both extreme cases. Fofew cases where data is not available at 3.5 or 3.8 µmuse statistical arguments to estimate those brightnesses2.2- and 4.8-µm data, as discussed in the next sectionthis study we incorporate a combination of published msurements (Spencer et al., 1992, 1994, 1997; Veeder et1994; Howell and Klassen, private communication, 20Silverstone et al., 1995; Stansberry et al., 1997) and pre-viously unpublished measurements obtained at NASA’sfrared Telescope Facility using NFSCAM.
2.3. Estimating volcanic activity levels during nebulaobservations
The available IR measurements are shown inTable 1,where they have been converted to the same units forof comparison.
The entries inTable 1describe the remarkable accoplishment of “monitoring volcanism” on a tiny, remoworld. Our analysis of this chronology of episodic activ
-
y
r
e
r
e
led us to define the 3.5-µm brightness of 30 GW/(str µm)as the boundary between “bright” and “dim” values, corsponding to volcanically “active”and “quiet” states. Brightnesses tend to show periods of either slowly-changing vathat are less than this cutoff, or more variable values grethan the cutoff, consistent with our interpretation of actand quiet periods. Thus, while the 30 GW/(str µm) choiceappears to be somewhat arbitrary, our statistical anal(described below and in anAppendix A) provide support forthere being both unambiguous activity patterns per seIo’s volcanism, and that such a threshold level can beposed.
The time sequence of volcanic activity on Io can bescribed as follows: The large volcano Loki erupted on a 5day cycle during our years of study, with average eruptlasting 230 days, and quiet periods lasting at least 150(Rathburn et al., 2002). Loki eruptions do not appear to binterrupted by any temporary quiet periods, nor are thein-between eruptions interrupted by short-term Loki activThus, Io is undoubtedly volcanically active on any day ding a Loki eruption. The nebula observations of 1990, 19August 1997, and 1998 (both) were made during Loki ertions.
On top of the Loki eruption cycle, the remaindervolcanic activity on Io is characterized by relatively shoduration eruptions separated by longer periods of qucence. Except for Loki, Io is volcanically quiet for the mjority of the time, as monitored by the IR techniques. ThIR observations are critical during Loki lulls to determithe activity levels of the remaining volcanoes. The nebul1995 was observed during the quiet phase of the Loki cybut it also occurred during an eruption of one of the mivolcanoes, Tiermes Patera, which lasted between 60 andays. IR observations showing high activity were made bbefore and after each of these nebula observations, methere is little doubt thatthe nebulas represent high volcanactivity levels.
Non-Loki eruptions have durations which are mushorter than a Loki eruption, anywhere from a few monto less than a day, so accurate determinations of non-activity require frequent measurements. Thus, during Llulls, our knowledge of the total volcanic activity level ofduring a nebula observation is only as good as the aable IR measurements taken on the same day or nedays. Five nebulas were observed during lulls in the Lcycle along with accompanying IR observations showquiet levels. However, in most cases there are gaps oferal days between the nebula and IR measurements, meshort-term eruptions could have occurred during the neobservations without being seen in IR. We therefore uselarge dataset of IR observations to statistically characize non-Loki eruptions, and then calculate the probabilitof eruptions happening during these nebula observatWhenever possible, we make assumptions that maximthe possibility of volcanic activity during these nebula oservations in the Loki lulls, making it as difficult as possib
Volcanic control of Jupiter’s extended neutral clouds 433
Table 1Infrared observations of Io’sJupiter-facing hemisphere
Date IR brightness (GW/(str µm)) Ref.a Long. Active volc.
a V: Veeder et al. (1994). H: Howell and Klassen (private communication, 2002). S94:Spencer et al. (1994). S92:Spencer et al. (1992). S95:Silverstoneet al. (1995). S97a:Spencer et al. (1997). S97b:Stansberry et al. (1997). Unreferenced: Previously unpublished data obtained at Nasa’s Infared TeleFacility using NFSCAM. L:Lopes-Gautier (1997). T = volcano Tiermes Patera. U= volcano Ukko Patera. TU= volcanoes Tiermes and Ukko simultaneuos“Loki +” = possible short, unidentified eruption in addition to Loki eruption.
b-ct
t de-velytionrupratepre-
erup-ith-.up-
ays,b-
re ina ob-
to categorize Io’s activity level as quiet. As a result we otain a strong lower limit to the probability that Io was in favolcanically quiet during these nebula observations.
2.3.1. Long-term non-Loki eruptionsIn 1995, Tiermes Patera and Ukko Patera erupted a
tectable levels for 60–80 days and 30–70 days, respectiTogether, these represent 14 of the 23 non-Loki erupmeasurements in the total dataset of 120 nights. (The etions overlapped in time, but we treat them as two sepaeruptions for our statistics.) These two volcanoes thus re
.
-
sent the majority of non-Loki activity in our data; howevtheir long duration also makes it less likely that similar ertions could have occurred during a nebula observation wout being detected in an earlier or later IR measurement
We estimate that one or more Tiermes/Ukko-style ertions are occurring on Io’s Jupiter-facing hemisphere∼ 8%of the time (seeAppendix A), or 15% of the time glob-ally. Assuming these types of eruptions last at least 30 dthe only opportunities for similar eruptions to affect the oserved nebulas without being detected in the IR data a1993 and 1994. There is a 5-day gap between the nebul
436 M. Mendillo et al. / Icarus 170 (2004) 430–442
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servation and IR observation in 1993, so a 60-day eruphas a 92% chance of being detected by the IR observif it occurs during the nebula observation. Thus, the chaof Tiermes/Ukko-type activity during the 1993 nebula obsvation is∼ 0.7% on the Jupiter-facing hemisphere of Io,∼ 9% globally. (The other hemisphere is never measurThe 22-day gap between IR and nebula data in 1994 cspondingly makes for a 3% chance of an eruption occuron the sub-jovian hemisphere during the nebula observaor 11% globally.
2.3.2. Mid-range non-Loki eruptionsTwo eruptions in the data lasted between 16 and 30 d
9503B and 9812A. We estimate these types of eruptionspen∼ 4% of the time on Io’s Jupiter-facing hemisphere (Appendix A), or 8% globally. As with the Tiermes and Ukkeruptions, the only significant opportunities for similar eruptions to affect nebula observations and still be undetectein 1993 and 1994. A volcanic event of this type is most likto have occurred during these nebula observations if wesume the minimum duration of 16 days. The result forJupiter-facing hemisphere of Io is a 1% chance of an etion during the 1993 nebula and a 4% chance of an erupfor the 1994 nebula, or 5 and 8% globally, respectively.
2.3.3. Short-term non-Loki eruptionsThree eruptions in the IR data lasted less than 2 we
Karei in 1996 lasted 9–14 days; 9610A lasted 1–7 days;9706A lasted 1–14 days. Other eruptions with these dtions could have occurred without detection during anythe nebula observations—most importantly, during theriods when Loki was not active. Nebula observations wmade in IR data gaps of 7 days in 1992, 9 days in 1996,16 days in 1997.
As there are more opportunities to miss such short etions in the IR data gaps, we can approximately estimtheir occurrence rate as the detection rate in these data.tal the three short eruptions represent 5 detections, or 4the IR observation days. We will therefore assume the wcase for our level of certainty—that such short-term ertions are happening 4% of the time on Io’s Jupiter-fachemisphere, regardless of any IR observations made onvious or subsequent nights. This implies eruptions 8% oftime globally.
2.3.4. OutburstsOutbursts are bright and short eruptions perhaps las
less than one day. They are estimated to occur 3% otime globally(Spencer and Schneider, 1996). The short-termeruptions considered above may or may not be outburstassuming the maximum amount of doubt, we must addbursts as a 4th type of eruption that may occur in-betwIR observations. That means that, taken together, short-eruptions and outbursts are occurring 11% of the time gally, regardless of IR data 1 day earlier or later.
-
,
:-
-
-f
-
Table 2Total volcanically active contribution to “quiet” nebulas
Year Total prob. (%) Short+ outburst (%) Mid (%) Long (%)
The total probability of an eruption happening includes one or more etion types happening simultaneously. The probability of the opposite—eruption happening—is the product of the probabilities of no eruption foeach type of eruption. For example, in 1992 the probability of no eruptio0.89×0.96×0.92= 0.78, meaning a 22% chance of one or more eruptihappening.
To summarize the above, there is ample evidenccharacterize a nebula observation as being made dvolcanically-active periods. For those made during lullsmajor activity, but possibly near a time of a short-term ertion, we must turn to statistical treatments to characteri“quite-time” nebula, and these cases are summarized inTa-ble 2.
2.4. Observational trend
In Fig. 1we display all of the nebula images by year agroup them according to their volcanic activity. Since the IRactivity level crossed the active/quiet threshold during1997 observation period, the nebula of August 2–3 is plain the active group, and the nebula of September 26 is plin the quiet group. The nebula observed in-between thdates (August 29 to September 2) is not shown inFig. 1sincethe IR brightness changed from “quiet” to “active” duringthe 5-night observing sequence, meaning that nebulanot belong in either group.
There are significant differences in the brightnessshape of the sodium nebula images inFig. 1 that appeato be correlated with the volcanic activity on Io. Jupitesodium nebula was on average∼ 3 times brighter duringvolcanically active periodsthan during quiet periods. Although the nebulas from 1990 (active), 1991 (active), 1(quiet), and 1997 (both) were very similar in brightnethere was no volcanically quietnebula that appeared brightthan any volcanically active nebula. The nebula was brighest in 1995 and 1998, which were both volcanically acperiods, while it was dimmest in 1992 and 1993, whwere both volcanically quiet periods. AsFig. 1 shows, thenebula’s brightness contours during the volcanically acperiods of 1991, 1995, and 1998 were distinctly rectangin shape and, with the exception of the very dim appearancin 1992, the nebula lacked this rectangular shape duringcanically quiet periods.
Our attempts to correlate in a rigorously quantitative wthe dual characteristics (brightness and shape) of the newith Io’s volcanic IR brightness levels met with mixed sucess. InFig. 2, we display the sodium brightness at a fix
Volcanic control of Jupiter’s extended neutral clouds 437
uri
e
Fig. 1. Summary of Jupiter’s sodium nebula and Io’s volcanic activity analyses for the 9-year period 1990–1998. (Top panel) Image from 1995 taken dngan eruption of Tiermes Patera. (Middle panels) Remaining nebula images sorted by Loki’s volcanic activity level. Occulting masks of various sizes were usedto block Jupiter and its inner moon (and especially so in the 1994 panel).
Fig. 3. (Top panels) Average of nebula images for the active (left) and quiet (right) Loki periods shown inFig. 1. (Bottom panels) Models of the averagsodium nebulas for volcanically active (left) and quiet periods (right).
438 M. Mendillo et al. / Icarus 170 (2004) 430–442
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Fig. 2. Plot of Io’s sub-jovian hemispheric brightness at 3.5 µm vs. tbrightness of the sodium nebula at 200RJ from Jupiter. Uncertainty barindicate the range of IR brightness measured before and after each nobservation. The IR values and their uncertainty bars do not take intcount the anti-jovian hemisphere, i.e., the possible 25% chance of aneruption during the “quiet” nebula observations of 1992, 1993, 1994, 1and 1997 (dimmest value).
radial distance (200RJ) with the hemispheric brightness vaues “at the time of the observations” (as described aboThe trend noted inFig. 1is again evident here. Less succeful were attempts to quantify the spatial morphology of enebula by simple parameterizations of rectangular-shavs. diamond-shaped brightness contour features. Thustrends shown inFig. 1 and the population of only two ofour possible quadrants inFig. 2 remain as the best speifications we can offer of Io’s volcanic control of Jupitegreat sodium nebula.
3. Discussion
The observed trend for the nebula to be brightrectangular-shaped during volcanically active periodsdim and/or non-rectangular during volcanically quiet peods is most likely explained by variations in the degreetorus mass-loading by molecular ions (NaX+). That is, theescape rate of NaX+ from Io’s atmosphere (and the resultirate of its neutralization in the plasma torus) is higher ding volcanically active periodsthan during volcanically quieperiods.Wilson et al. (2002)showed quantitatively how thshape and brightness of the Na nebula are, in fact, dmined by the mechanism (and rate) for Na escape fromIn Table 3we show the results ofWilson et al. (2002), inwhich the same nebula images from 1990–1996 werelyzed with a neutral cloud model; also included inTable 3are estimates of the escape rates for 1997 and 1998 basthe same model results, along with the averages for thestates of volcanic activity. The results indicate that thecape rate of NaX+ from Io is more than four times greatduring volcanically active periods than during quiet perioimplying that volcanic activity is the most important factdetermining the NaX+ escape rate. The rate of atmosphesputtering of Na, on the other hand, does not vary as gre
a
e
-
-
n
,
Table 3Sodium escape rates from Ioa (1026 atoms/sec)
a Rates for 1990–1996 are taken from analyses of the same imagWilson et al. (2002).
b Nebula in 1997 was similar in appearance to 1990 and 1996, saverage of 1990 and 1996 is assumed.
c Nebula in 1998 was similar to that in 1995, so 1995 escape rateassumed.
and may actually be smaller during volcanically activeriods. In this context, it is important to keep in mind thatmospheric sputtering rates determined from nebula imare far less certain, given the strong dependence of nebrightness on the ejection speed of Na.
It is now possible to unify independent observationsthe nebula and of volcanic activity with Monte Carlo siulations results to portray the fundamental differencenebula brightness and appearance during volcanically activeand quiet periods. We display inFig. 3 (top panels) the averages of the observed nebula images during Loki erupand during quiet periods. The lower panels show the nula model images corresponding to the average activeaverage quiet conditions, obtained from the simulationWilson et al. (2002). Given the many uncertainties in hoobservational components of a highly coupled system reto parameterized simulations of it, the strong agreementplayed inFig. 3 is both surprising and instructive.
One possible explanation for the volcanic control ofNaX+ escape rate is that eruptions supply to Io’s atmospa molecular sodium species (NaX) which is otherwisesent. NaX+ can then be created by photoionization, chaexchange with torus ions (S+, S++, O+, etc.), or electronimpact ionization. These new ions can then flow out ofionosphere in a current, be picked up by Jupiter’s magtosphere, and later dissociated in the torus to producejet and stream(Wilson and Shneider, 1994, 1999)and theaccompanying rectangular Na nebula(Wilson et al., 2002).Without NaX in Io’s atmosphere, atomic Na+ pickup ionscan still be created, but these ions cannot form the Na strfeature due to their much longer recombination lifetimes
Another possible explanation is that eruptions increthe density of Io’s SO2 atmosphere. NaX+ can then be created by chemical reactions and charge exchange ofions with an Io atmosphere of enriched SO2 and atomic Na(Johnson, 1994); the rates of these reactions should depstrongly on the density of Io’s atmosphere and the cosponding neutral column abundances along the paths oftraveling through the atmosphere(Wilson, 1996).
Volcanic control of Jupiter’s extended neutral clouds 439
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4. Summary and conclusions
We have demonstrated that Io’s silicate-lava eruptihave a controlling effect on the sodium in Io’s atmosphthat leads to the extended neutral Na clouds. While it is gerally agreed that Io’s volcanic activity is the ultimate souof Io’s atmosphere, neutral clouds, and plasma torus,firmness of our results are nonetheless somewhat unapated. Infrared emission from Io is a reasonable measulava eruptions, but we have only considered the IR brigness levels observed from half of Io’s surface; we havenored the anti-jovian hemisphere altogether (except fortistical arguments). It is the dominance of Loki, both in itsbrightness levels and apparently strong NaX output, oveof the other volcanoes that makes this possible. Loki, locon the sub-jovian hemisphere, is active approximately 5of the time; we estimate the entire anti-jovian hemispherbe active at most only 16% of the time. Hemispheric dference in surface and atmospheric properties are cleimportant for diurnal photochemistry effects(Moses et al.,2002a, 2002b), and also in models of plume gas dynam(Zhang et al., 2003). Such hemispheric characteristics apossible differences need to be included in future nebsource modeling.
While we retained the notation “NaX+” to specify theunknown molecular ion containing sodium, there is conserable evidence for calling it NaCl+. Observations of Cl+ inthe plasma torus(Kuppers and Schneider, 2000), direct de-tection of NaCl in Io’s atmosphere(Lellouch et al., 2003),and a recent set of atmospheric models for volcanic cotions offer ample evidence to base further discussions uNaCl being the parent molecule of relevance (Moses et al.,2002a, 2002b, and references therein). Atoms can bejected into Io’s atmosphere bydirect volcanic eruptionsby hot lava flowing over the surface causing evaporaof surface material, and by the sputtering of surfaceterial by magnetospheric plasma. Yet, as argued byMoseset al. (2002b), molecules such as NaCl enter Io’s uppatmosphere only as an unambiguous signature of verycent volcanic activity. This is a rather important resultsodium cloud physics at Jupiter, as discussed in detaMoses et al. (2002b). While their modeling was not globaor dynamical, and only specific (“Pele-type eruption”) snarios were considered, the photochemistry pointed unbiguously towards NaCl being the dominant transientIo’s upper atmosphere following large plumes. Moreovthey pointed out that NaCl is expected to be an imptant species for other types of plumes(Zolotov and Fegley2000)and that “deposits of NaCl could be vaporized by laflowing across the surface”(Moses et al., 2002b, p. 123.The findings in our study are surely consistent with tview.
While the correlation found here applies to sodiumthe atmosphere and neutral clouds, we have no observarelevant to the contributions of these lava eruptions tomore general mass budgets of S and O in the atmosp
-f
s
e
and plasma torus. However, there is observational evidthat the sodium escape rate is indicative of S escape ratethe only study of its kind,Brown and Bouchez (1997)mon-itored the brightness of the Na clouds and of the S+ torusaround Jupiter for nine months, and observed a month-brightening in Na followed by a longer and more gradbrightening of S+ in the torus. Unfortunately, their observtional method was not able to separate the specific sodfeatures being monitored (i.e., “banana cloud,” jet andstream components) and there were no data on volcanitivity during the brightening. Thus, while their increaseS+ brightness does not necessarily represent an increathe rate of S+ being added to the torus, the relative timinof the Na and S+ brightness curves are suggestive of the+mass budget in the torus being somehow related to theincrease in Na escape from Io. If the sequence observeBrown and Bouchez is typical (S escape rates paralleescape rates), and if lava eruptions are the dominant mof controlling the Na escape rate from Io (meaning Broand Bouchez’s observed Na brightening was volcanically induced), then lava eruptions should affect the more genmass budget of S and O in Io’s atmosphere and the platorus. Indeed, the recent simulations for S and O in Io’svolcanically driven atmosphere(Moses et al., 2002a)are ex-tremely interesting in this context. The dramatic differencebetween S/O ratios for quiescent and active times unscore the complexity of such a highly coupled and strondriven neutral–plasma system. The recent analysis bySauret al. (1999)for ion mass loading of the torus underscothe complexity of this process, pointing again to the nof pursuing hemispheric and time-dependent mechaniboth observationally and via modeling. Temporal patteand episodic morphologies are the least sampled aspecthe Jupiter–Io system. While our results are the first tolate Io’s atmosphere escape and neutral cloud productiovolcanic activity on Io, our limited dataset of sodium imagrestricts our discussion to mostly annual time scales.Brownand Bouchez (1997)were able to study shorter time scawith data on neutral and plasma conditions covering dozof nights within a one-year period, but without the benefihaving volcanic activity indicators. It would be of great inteest to obtain coordinated observations of the neutral clothe plasma torus, and volcanic activity on multiple nigover a span of time much less than a year.
Finally, the idea of lava flows on Io’s surface havingdominant effect on Io’s atmosphere is somewhat contto the more accepted assumption that gaseous plumesanating from fissures on the surface are the ultimatemospheric source. Plume eruptions do not necessarilysult in increased IR flux from Io, and thus we must assuthat the changing fluxes observed here are due to lava flon the surface. It is difficult to argue against the imptance of plumes to the atmosphere given their impressize and compositional similarity to Io’s atmosphere athe plasma torus. Indeed, the model results ofMoses et al.(2002a, 2002b)which we have referred to repeatedly in th
440 M. Mendillo et al. / Icarus 170 (2004) 430–442
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work are intended for Pele-type plumes. Thus, despiteempirical evidence here linking lava flows to atmospheescape, there is still a theoretical concern that our obseIR signatures of lava flows should have such a significanfect on the state of Io’s atmosphere. However, the eruptof Pillan in 1997 and Tvashtar in 1999/2000 have shothat lava flows and plume activity are at least sometimecorrelated. This all suggests that two topics in need ofther investigation are: (1) models of lava flows as a souof NaCl and other species for Io’s atmosphere, and (2)servations of the sodium clouds, torus, and volcanoesub-annual time scales. The merging of such observawith full-system simulations will refine our understandiof how volcanoes drive the Io/Jupiter neutral cloudssubsequently all plasma populations throughout the jomagnetosphere.
Acknowledgments
At Boston University, this work was supported, in part,grants from the Magnetospheric Physics Program at NAand by seed research funds provided to the Center for SPhysics. M. Mendillo, J. Wilson, and J. Baumgardner,guest observers at the McDonald Observatory and acknedge the continued assistance of its director and staff. Wat Lowell is supported by grants NAG5-9004 and NAG10497 from the NASA Planetary Astronomy and PlanetGeology and Geophysics programs. Spencer and Stansare visiting astronomers at the Infrared Telescope Facwhich is operated by the University of Hawaii under agrment with NASA.
Appendix A. Patterns of volcanic activity on Io
A.1. Temporal cycles
The cycle of activity associated with the prominent vcano Loki is a well established pattern during the decof interest to this study(Rathburn et al., 2002). Of centralimportance to this study, therefore, are the non-Loki evand, in particular, quiet periods. In order to rule out thecurrence of a non-Loki eruption on any particular day,observations both preceding and following that day arequired at intervals which are smaller than the duration ofputative eruption. For instance, a series of IR observatmade at 5-day intervals would detect any eruption lastindays or more, but might miss eruptions lasting less thadays, with smaller detection probabilities for shorter ertions.
We assume here that the durations of the known erupin the available IR datasets are representative of the totatribution of eruption durations on Io, and we then idenperiods of time in these datasets when such eruptions c
e
y
-
e
Table A.1Non-Loki eruption measurements in the IR dataset
Name # Detections Duration (days) Assumed duration (days
Table A.2Observations that would have seen Tiermes or Ukko activity
Contiguous data windows Tiermes startdetectable Ukko start detectab
90 days 1991–1992 160 days 140 days270 days in 1995 340 days 320 days270 days in 1996 340 days 320 days240 days in 1997 310 days 290 days300 days in 1998 370 days 350 daysTotal 1520 days 1420 daysActual eruption time 70 days 50 daysEruption fraction 4.5% of the time 3.5% of the tim
ruled out. Thus, the estimated fraction of time when a pticular type of eruption occurs, in general, is approximathe duration of the representative eruption divided by thetal amount of time in the data when such an eruption cohave been detected.
IR data from 1990 and early 1991 are not useful foranalysis, since Loki was active most of the time, and thwere no regular groundbased or spacebased measureof individual hotspots that could distinguish non-Loki emsion from the main Loki eruptions. The multi-wavelengdata from 1992 are useful since any eruption would beily detected during this Loki-quiet period. Only one mulwavelength observation is available for 1994. From 1onward, there are large periods of time with frequent msurements at multiple wavelengths, accompanied by obvations of specific hotspots, so all of these data are usefuthe series of tables below, we summarize these findings
A.1.1. Non-Loki eventsSeeTable A.1.
A.1.2. Long-term eruptions of Tiermes (70 days) and Ukko(50 days)
Based on the above, we conclude that these types of etions are happening 8% of the time on one hemispher15% (1.0 − [0.92 × 0.92]) of the time globally. SeeTa-ble A.2.
A.1.3. Medium-duration eruptions 9503B and 9812Aassuming 16-day eruptions
Assuming that eruptions 9503B and 9812A lasted 16 deach, for a total of 32 days oferuptions, we conclude fromTable A.3that these types of eruptions occur 3% of the tion one hemisphere or 6% of the time globally.
Volcanic control of Jupiter’s extended neutral clouds 441
ays
imes a16-
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eed
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Table A.3Observations that would have seen 16-day eruptions
Contiguous data windows 16-day start detectable
7 days in Nov 1991 23 days7 days in Dec 1991 2316 days in 1992 321 day in 1994 177 days in Feb 1995 2340 days in Mar 1995 5660 days in May 1995 7655 days in Aug 1995 711 day in Nov 1995 1723 days in Feb 1996 3933 days in Mar 1996 4930 days in May 1996 4690 days at end of 1996 106140 days at start of 1997 15625 days in Aug 1997 4136 days in Oct 1997 521 day in Dec 1997 17110 days at start of 1998 1269 days in Aug 1998 255 days in Sep 1998 211 day in Nov 1998 1716 days in Dec 1998 3216 days in 1999 32Total 1097 daysActual eruption time 32 days (16 days×2)Eruption fraction 3% of the time
Fig. A.1. Summary of correlations of IR emissions at 2.3, 3.8, and 4.9 µmwith the 3.5-µm emission.
Table A.4Observations that would have seen 30-day eruptions
Contiguous data windows 30-day start detectable
7 days in Nov 1991 37 days7 days in Dec 1991 3716 days in 1992 461 day in 1994 31270 days in 1995 300140 days in early 1996 17090 days in late 1996 120240 days for most of 1997 2701 day near end of 1997 3170 days at start of 1998 10065 days in mid 1998 95130 days in late 1998/1999 160Total 1397Actual eruption time 60 days (30 days×2)Fraction 4% of the time
A.1.4. Medium-duration eruptions 9503B and 9812Aassuming 30-day eruptions
Assuming that eruptions 9503B and 9812A lasted 30 deach, for a total of 60 days of eruptions,Table A.4suggeststhat medium-range eruptions are occurring 4% of the ton one hemisphere, or 8% of the time globally. This ihigher probability, meaning more uncertainty, than theday assumption, so we adopt the 30-day value.
A.2. Correlation relationships between multiple IRwavelengths
In the our main text, the summary of IR data presenin Table 1included observations in four distinct wavelengbands. Most of our characterizations of volcanic activityused observations in the 3.5 µm band, with occasional nto use other bands when these were not available. InFig. A.1below, we show the correlations between the 3.5-µm briness levels with those in the 2.3-, 3.8-, and 4.9-µm bandthe full volcanic data set. Whilethe closely-spaced measurments at 3.5 and 3.8 µm give the best scaling patternsrelationships with 2.3 and 4.9 µm are still useful in estimaof the 3.5-µm behavior.
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