Lunar and Planetary Science XXX 1741.pdf

THE TEMPORAL ACTIVITY OF ’S HOT SPOTS. -Gautier1, W.D. Smythe1, A.S. McEwen2, P.E. Geissler2, ,A.G. Davies1, L. Kamp1, L.A. Soderblom3, R.W. Carlson1, L. Keszthelyi2, J.R. Spencer4, and the NIMS Team. 1Jet Propulsion Laboratory, Caltech, Pasadena, CA 91109 ([email protected]..gov). 2Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona 85721. 3 U.S.Geological Survey, Flagstaff, Arizona 86001. 4 Lowell Observatory, Flagstaff, Arizona 86001.

The Galileo and Galileo Europa Mission have pro- cant variation, perhaps the clearest of which is vided an unprecedented opportunity to monitor the whether they are long-lived or short-lived. Studies activity of Io’s hot spots. The NIMS instrument has from Voyager data by McEwen and Soderblom [3], re- observed Io at least twice during every Galileo orbit evaluated by McEwen et al. [1] show that several types around Jupiter. The Solid State Imaging System on of plume are present on Io: Prometheus-type plumes Galileo has also monitored the activity of the hot spots are long-lived, small, bright, and surrounded by by observing Io in eclipse [1]. During orbits G1 (June prominent white deposits. Other plumes appear to be 1996) to E16 (July 1998), NIMS and SSI detected 52 unique in character, such as Pele, Loki, Pillan, Acala, hot spots on Io. The combination of NIMS and SSI and Ra. SSI results show that Acala is so far the only data with data from Voyager, ground-based observa- stealth-type plume [4] detected on Io, though other tions (J. Spencer et al. 1997, C. Dumas, pers. comm.) plumes show similar characteristics. In the light of and observations from HST using NICMOS (J. Galileo SSI results, we think that plumes are best clas- Goguen, pers. comm.) have revealed 74 active vol- sified as Prometheus-type, unique, short-lived, or ab- canic centers on Io (73 hot spots plus the Ra Patera sent. The short-lived category pertains to locations plume), and 23 additional sites that were identified as where SSI detects a new plume deposit but no active probable active volcanic centers. In this paper, we plume. Our joint SSI and NIMS results have shown a examine what is known about the temporal behavior correlation between active plumes detected by Voyager of hot spots and activity styles in terms of (1) duration and Galileo and persistent hot spots, with the possible of activity; (2) presence or absence of plumes, and exception of the Masubi and Ra plumes. Masubi and duration of plume activity; (3) presence or absence of Ra may also be persistent hot spots, however, both ephemeral red deposits around hot spots; (4) magma locations are poorly observed by NIMS because of the temperature; and (5) variations in power output. geometry of Galileo’s orbits so there may be an obser- vational bias in the lack of detection of activity. A The long-range monitoring by Galileo NIMS and SSI plume (but no hot spot) was detected by SSI at Ra has allowed us to identify two types of hot spot activity during the first Galileo orbit, G1 [1]. NIMS did not in terms of duration: persistent and sporadic [2]. We observe Ra at that time and it is possible that the ac- define sporadic activity as those events that have tivity stopped after the first orbit, since no hot spots or lasted under 3 months, as observed by Galileo and plume has been detected at that location in subsequent from ground-based instruments. We cannot dismiss orbits. The Masubi plume was observed by Voyager the possibility that the activity at these hot spots is in 1979. Galileo SSI detected a new plume deposit persistent for longer durations, but falls to levels be- around Masubi in orbit E11 and, in the same orbit, low the detection limits of the instruments (for a filled NIMS detected a hot spot at that location. At present pixel, 180 K for NIMS and 700 K for SSI). We define we do not have enough temporal data to assess if Ma- persistent hot spots as those observed to be active for subi is a persistent hot spot. Apart from Masubi and periods longer than one year. We have so far identi- Ra, all other plumes locations coincide with locations fied 29 persistent hot spots, several of which were of persistent hot spots. Hot spots also differ in terms observed by Voyager and from ground-based observa- of presence or absence of red deposits. The combi- tions during the years between Voyager and Galileo. nation of SSI and NIMS data shows that there is a It is possible that the activity at these hot spots lasts correlation between red deposits surrounding persis- for considerably longer than 1 year, perhaps decades. tent hot spots and the presence of plumes [e.g. 2, 5]. The persistent hot spots are particularly important for our study of Io’s activity because they most likely rep- Another possible difference between activity styles at resent the major pathways of magma to Io’s surface. hot spots is magma temperature. One of the major questions posed by the Galileo results is whether the Hot spot activity also differs in terms of presence or very high temperatures observed at Pillan Patera [6] absence of plumes. Plumes themselves present signifi- are common on Io, or whether different hot spots ex- Lunar and Planetary Science XXX 1741.pdf

IOS HOT SPOTS: R. Lopes-Gautier et al.

hibit temperatures in the “basaltic” range and in the know that even more dramatic variations (outbursts) “ultramafic” range, possibly implying different occur at Loki and other hot spots [7]. magma compositions. Alternatively, the magmas in the “ultramafic” range may be superheated basalts. Data from NIMS, SSI, Voyager, ground-based tele- How common the high temperature magmas are on Io scopes, and HST form a powerful combination for is a particularly difficult problem to assess since the assessing the different styles of activity on Io and high temperature eruptions such as that at Pillan may finding correlations between the factors identified in not happen often, or cover sufficiently large areas to Table 1. Other factors that we will consider in future make their thermal emission detectable by Galileo at studies include hot spot albedo, temporal changes of the current spacecraft ranges. Data from the close Io low-albedo material and depth of the 0.9-micron ab- fly-bys late this year are needed to assess how wide- sorption feature identified by SSI [5]. In summary, we spread the very high temperatures observed at Pillan find: (1) A correlation between persistent activity and are on Io. presence of plumes and red deposits; (2) A concentra- tion of plumes and persistent hot spots at latitudes Variations in activity of hot spots in terms of power lower than 30 degrees, which is more consistent with output have been studied from ground-based observa- tidal heating occurring in the asthenosphere than in tions prior to Galileo. It has long been known that the deep mantle [2, see also abstract by Smythe et al., Loki undergoes periods of brightening and has been this volume]; (3) A variation in the power output of the site of giant outbursts and other outburst sites have persistent hot spots from NIMS data showing steady been detected from ground-based observations [see 7 output (Pele, Prometheus, Maui, Gish Bar), brighten- for a review]. Galileo data showed brightenings at ing (Malik, Tupan, Shamash), and fading (Amirani, Loki, Malik [2] and Pillan [1, 6]. Current data from Altjirra). These and other correlations between the NIMS and SSI indicate that Pele may have a fairly factors in Table 1 are important for undertanding Io’s constant brightness [see abstract by Davies et al., this surface (eruption styles, surface deposits), interior volume]. An important question is whether Io’s hot (tidal dissipation models), and atmosphere (plume spots present different, long-term activity styles rang- activity). ing from “steady” (e.g. Pele) to “outburst” (e.g. Loki) and whether these may be correlated with other varia- TABLE 1: Differences in activity at Io’s hot spots tions in style of activity. Duration Variation in Plume Temperature Red deposits power output activity range The large quantity and temporal coverage of NIMS ______data can help us assess variations in power output of Persistent Outburst Absent “ultramafic” Prominent persistent hot spots. We use the NIMS spectrum for Sporadic Brightening Prometheus-type “basaltic” Faint Fading Short-lived Absent each hot spot (wavelength range 0.7 to 5.2 microns) to Steady Unique obtain a best-fit temperature and area. From these, we ______calculate the total power output of hot spots to study their activity style. The observations used for this References: [1] McEwen, A.S. et al. 1998, preliminary study are from orbits G1, G2, G7, C9, 135, 181-219. [2] Lopes-Gautier et al. 1999, Icarus, E11, E15, and E16. All the hot spots were observed in press. [3] McEwen, A.S. and L. A. Soderblom by NIMS in darkness, with the exception of 5 hot 1983, Icarus 55, 191-217 [4] Johnson, T.V. et al. spots observed in orbit G2 in a sunlit observation. 1995, GRL 22, 3293-3296. [5] Geissler, P.E. et al. The reflected sunlight and thermal components of 1999, Icarus, in press. [6] McEwen, A.S. et al. 1998, radiation were separated in this observation [see ab- Science 281, 87-90. [7] Spencer, J.R., and N. Schnei- stract by Soderblom et al., this volume]. Our results der 1996, Annu. Rev. Earth Planet. Sci., 24, 125-190. show that, during the period observed by NIMS, some [8] Spencer, J.R. et al. 1997, GRL 24, 2451-2454. [9] hot spots remained fairly steady in terms of power Stansberry, J. et al. 1997, GRL 24, 2455-2458. output (Pele in G2, E15, and E16; Gish Bar in G1 and E11, Prometheus in G1 and C9, and Maui in G1 and C9). Other hot spots showed significant variations, including brightening (Malik and Tupan from G1 to C9 to E11, Shamash from G1 to C9) and fading (Alt- jirra and Amirani from G1 to E11). Variations in the activity of Loki were recorded by NIMS and ground- based data [2, 8, 9]. From ground-based data, we