Download Loki Mondel - Julie Rathbun

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
February 2000
Galileo
October 1999
Propagation direction
Voyager
Quantification of model
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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 W
reached: 3.5 micron
brightness (in
GW/μm/str) =
speed (in km/day) x 32
55
km
E
390 km
raft
Model results
• A typical event lasts ~225 • What if Loki was a sulfur lake?
days
– Used analytic cooling model of
Howell (1997) altering the
• To last 225 days,
parameters for Sulfur
propagation speed must be
– Doesn’t match
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 1997-2000
• Best
threeyear
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
2001-2003 data
• Remained at approximately average
brightness (~35 GW/micron/str) for
500-900 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 18th, 20th, and 28th)
– 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
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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.
2.
3.
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The density of the magma
Initial density (especially porosity) of the crust
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