Characterizing Io’s Pele, Tvashtar and Pillan plumes: Lessons learned from Hubble

Abstract Hubble Space Telescope/Wide Field and Planetary Camera 2 (HST/WFPC2) images of Io obtained between 1995 and 2007 between 0.24 and 0.42 μm led to the detection of the Pele plume in reflected sunlight in 1995 and 1999; imaging of the Pele plume via absorption of jovian light in 1996 and 1999; detection of the Prometheus-type Pillan plume in reflected sunlight in 1997; and detection of the 2007 Pele-type Tvashtar plume eruption in reflected sunlight and via absorption of jovian light. Based on a detailed analysis of these observations we characterize and compare the gas and dust properties of each of the detected plumes. In each case, the brightness of the plumes in reflected sunlight is less at 0.26 μm than at 0.33 μm. Mie scattering analysis of the wavelength dependence of each plume’s reflectance signature suggests that range of particle sizes within the plumes is quite narrow. Assuming a normal distribution of particle sizes, the range of mean particle sizes is ∼0.035–0.12 μm for the 1997 Pillan eruption, ∼0.05–0.08 μm for the 1999 Pele and 2007 Tvasthar plumes, and ∼0.05–0.11 μm for the 1995 Pele plume, and in each case the standard deviation in the particle size distribution is 9 g of sulfur dust, the 1999 Pele eruption released ∼10 9 g of SO 2 dust, the 1997 Pillan eruption released ∼10 10 g of SO 2 dust, and the 1995 Pele plume may have released ∼10 10 g of SO 2 dust. Analysis of the plume absorption signatures recorded in the F255W filter bandpass (0.24–0.28 μm) indicates that the opacity of the 2007 Tvashtar plume was 2× that of the 1996 and 1999 Pele plume eruptions. While the sulfur dust density estimated for the Tvashtar from the reflected sunlight data could have produced 61% of the observed plume opacity, 2 dust detected in the eruption. Accounting for the remaining F255W opacity level of the Pele and Tvasthar plumes based on SO 2 and S 2 gas absorption, the SO 2 and S 2 gas density inferred for each plume is almost equivalent corresponding to ∼2–6 × 10 16 cm −2 and 3–5 × 10 15 cm −2 , respectively, producing SO 2 and S 2 gas resurfacing rates ∼0.04–0.2 cm yr −1 and 0.007–0.01 cm yr −1 ; and SO 2 and S 2 gas masses ∼1–4 × 10 10 g and ∼2–3 × 10 9 g; for a total dust to gas ratio in the plumes ∼10 −1 –10 −2 . The 2007 Tvashtar plume was detected by HST at ∼380 ± 40 km in both reflected sunlight and absorbed jovian light; in 1999, the detected Pele plume altitude was 500 km in absorbed jovian light, but in reflected sunlight the detected height was ∼2× lower. Thus, for the 1999 Pele plume, similar to the 1979 Voyager Pele plume observations, the most efficient dust reflections occurred in the region closest to the plume vent. The 0.33–0.42 μm brightness of the 1997 Pillan plume was 10–20× greater than the Pele or Tvashtar plumes, exceeding by a factor of 3 the average brightness levels observed within 200 km of 1979 Loki eruption vent. But, the 0.26 μm brightness of the 1997 Pillan plume in reflected sunlight was significantly lower than would be predicted by the dust scattering model. Presuming that the 0.26 μm brightness of the 1997 Pillan plume was attenuated by the eruption plume’s gas component, then an SO 2 gas density ∼3–6 × 10 18 cm −2 is inferred from the data (for S 2 /SO 2 ratios ⩽4%), comparable to the 0.3–2 × 10 18 cm −2 SO 2 density detected at Loki in 1979 (Pearl, J.C. et al. [1979]. Nature 280, 755; Lellouch et al., 1992), and producing an SO 2 gas mass ∼3–8 × 10 11 g and an SO 2 resurfacing rate ∼8–23 cm yr −1 . These results confirm the connection between high (⩾10 17 cm −2 ) SO 2 gas content and plumes that scatter strongly at nearly blue wavelengths, and it validates the occurrence of high density SO 2 gas eruptions on Io. Noting that the SO 2 gas content inferred from a spectrum of the 2003 Pillan plume was significantly lower ∼2 × 10 16 cm −2 (Jessup, K.L., Spencer, J., Yelle, R. [2007]. Icarus 192, 24–40); and that the Pillan caldera was flooded with fresh SO 2 frost/slush just prior to the 1997 Pillan plume eruption (Geissler, P., McEwen, A., Phillips, C., Keszthelyi, L., Spencer, J. [2004a]. Icarus 169, 29–64; Phillips, C.B. [2000]. Voyager and Galileo SSI Views of Volcanic Resurfacing on Io and the Search for Geologic Activity at Europa. Ph.D. Thesis, Univ. of Ariz., Tucson); we propose that the density of SO 2 gas released by this volcano is directly linked to the local SO 2 frost abundance at the time of eruption.