Thermal Models and 1-5 tex2html_wrap_inline31 m Photometry: Spring 1997 Loki Brightening

J.A. Stansberry (Lowell Obs.), J.R. Spencer (Lowell Obs.), R.R. Howell (U. Wyoming), C. Dumas (U. Hawaii)

Observations from 1.7 - 4.8 microns obtained between March 12 and May 31 1997 from the IRTF in Hawaii and from the 72'' Perkins Telescope in Flagstaff, AZ, reveal that Io's volcano Loki was then active. Measurements from the Galileo NIMS instrument at the E6 encounter on February 20, 1997, show that the 3.5 tex2html_wrap_inline33 m flux from Loki was extremely low (R. Lopes-Gautier, personal communication). Our photometry at 3.5 tex2html_wrap_inline33 m shows that 20 days later the flux from Loki had increased dramatically to tex2html_wrap_inline37 GW tex2html_wrap_inline33 m tex2html_wrap_inline41 str tex2html_wrap_inline43 (completely dominating the emission from Io at 3.5 tex2html_wrap_inline33 m) and that it monotonically brightened over the March - April period, with the flux on April 29 being tex2html_wrap_inline47 GW tex2html_wrap_inline33 m tex2html_wrap_inline41 str tex2html_wrap_inline43 . In contrast, disk-integrated 2.3:3.5 tex2html_wrap_inline33 m color temperatures, from the March 19 - April 29 period decline monotonically, from tex2html_wrap_inline57 K to tex2html_wrap_inline59 K, suggesting that we were seeing spreading and cooling lava flows. This color temperature, tex2html_wrap_inline61 K, is remarkable only in that it is not elevated relative to the color temperature we find for Io when there are no bright hotspots (Spencer et al., 1997; Stansberry et al., 1997). Because most of the 3.5 tex2html_wrap_inline33 m flux from Io during this period was originating from Loki, the implication is that nearly all of the newly erupted material there had a temperature tex2html_wrap_inline65 K. On April 29 we measured a disk-integrated 1.7:2.3 tex2html_wrap_inline33 m color temperature of tex2html_wrap_inline69 K. This color temperature also is not greater than values we obtain for a quiescent Io, suggesting a lack of high-temperature material at Loki.

Models of 1-5 tex2html_wrap_inline33 m emission from lava flows (Howell, 1997; Stansberry et al., 1997) will be compared with these data and any additional observations obtained in June and July in order to explore in detail the temperature of the Loki lavas, the extent of the flows, limits on the area at Loki that might be occupied by lavas with tex2html_wrap_inline73 K, and the eruption rates associated with this brightening.

Howell, R.R. (1997). Thermal emission from lava flows on Io. Icarus, In Press. Spencer, J., et al. (1997). A history of high-temperature Io volcanism at the start of the Galileo tour. Submitted to GRL. Stansberry, J., et al. (1997). Violent silicate volcanism on Io in 1996. Submitted to GRL.