m Photometry: Spring 1997 Loki Brightening
Thermal Models and 1-5 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 m flux from Loki
was extremely low (R. Lopes-Gautier, personal communication).
Our photometry at 3.5 m shows that 20 days later the flux from Loki
had increased dramatically to 
GW m 
str
(completely dominating the emission from Io at 3.5 m) and that it
monotonically brightened over the March - April period, with the flux on
April 29 being 
GW m str . In contrast,
disk-integrated 2.3:3.5 m color temperatures, from the March 19
- April 29 period decline monotonically, from 
K to 
K, suggesting that we were seeing spreading and cooling lava flows.
This color temperature, 
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 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 
K. On April 29 we measured
a disk-integrated 1.7:2.3 m color temperature of 
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 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 
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.