Groundbased observations 1988 – 2001 and post-2003 –Two distinct populations of brightness –Brightenings occur periodically Between 2001 and 2003 –Single brightness population –Lower maximum brightness

October 1999 February 2000 Galileo Voyager Propagation direction

Quantification of model Unroll to make a simple rectangular lava lake Age determined as function of time and length Temperature calculated using cooling model of Davies, 2005 Total brightness calculated assuming blackbody emission Two input parameters: –Raft size Doesn’t affect results –Propagation speed, which influences Duration of event Maximum brightness reached: 3.5 micron brightness (in GW/μm/str) = speed (in km/day) x km 390 km raft WE

Model results What if Loki was a sulfur lake? –Used analytic cooling model of Howell (1997) altering the parameters for Sulfur –Doesn’t match A typical event lasts ~225 days To last 225 days, propagation speed must be 1.7 km/day The model predicts an average active brightness of 55 GW/μm/str Matches observed average active brightness of ~ 60 GW/μm/str

Matching data from Best three- year period of data Matched by simple variations of velocity with time

1998 position data Brightness and position of hot spots on 7/12 and 8/4 measured by MacIntosh et al. (2003) using speckle imaging Ran previous model and calculated 2.2 micron brightness – matches measurements Used velocity as a function of time to predict position of hot front during observations – matches measurements

data Remained at approximately average brightness (~35 GW/micron/str) for days A speed of 0.9 km/day gives –maximum brightness of 29 GW/μm/str –takes ~450 days to overturn entire patera Adaptive optics brightness measurements at 2.2, 3.8, and 4.3 microns (Marchis et al., 2005) –Data taken close together in time (December 18 th, 20 th, and 28 th ) –Velocity of 0.5 km/day matches the average data value With a velocity of 0.5 km/day, it takes 780 days to overturn the entire patera If the westernmost rafts begin to overturn after 540 days, 2 fronts will be present for part of the time Maximum brightness ~ 33 GW/μm/str –Compare to 17 GW/μm/str

Conclusions Model can match all data with simple changes in velocity of overturn propagation Changes in velocity imply that the age at which the rafts sinks also changes over time Age of raft when it sinks d epends on 1.The density of the magma 2.Initial density (especially porosity) of the crust 3.Other factors (e.g. the behavior of neighboring slabs) We use porosity profile from Peck et al. and a simple Stefan model of solidification to calculate the density of the solidified crust as a function of time Density remains remarkably constant between ~400 and 800 days (~1% difference) Small differences in porosity profile used will similarly lead to large differences in age at which the raft sinks Small changes in magma volatile content can produce large variations in sinking time (via effects 1 and 2) and thus propagation speed of the sinking front