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Full text of "Thermal Emission Variability of Zamama, Culann and Tupan on Io Using Galileo Near-Infrared Mapping Spectrometer (NIMS) Data"

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Lunar and Planetary Science XXXVI (2005) 


'^Morehead State University, Morehead, KY 40351 (; ^Jet Propulsion Laboratory-California Institute of Technology, 
ms 183-501, 4800 Oak Grove Drive, Pasadena, CA 91109-8099 (email tel: 818-393- 

Introduction: The Jovian satellite lo is the 
most volcanically active body in the Solar System. 
Previous analyses [e.g., 1-4] indicate the presence of 
high-temperature silicate volcanism on lo, similar to 
silicate volcanism occurring on Earth. Instruments 
onboard the Galileo spacecraft, especially the Near 
Infrared Mapping Spectrometer (NIMS) and the Solid 
State Imager (SSI), provided much data of lo's active 
volcanoes throughout the duration of the Galileo mis- 
sion (June 1996-September 2003). NIMS data is par- 
ticularly sensitive to thermal emission from active and 
cooling lava over cooling times of seconds to a few 
years. The objective of this ongoing study of lo's vol- 
canism is to determine the variability of thermal emis- 
sion from volcanoes on lo's surface, in order to better 
understand the styles of eruption, and to constrain the 
volumes of material erupted. Ultimately, this will help 
to constrain the contribution of active volcanism to 
lo's thermal budget. Data have been analyzed for the 
volcano Zamama, located at 173 W, 21 N, and the 
power output of Zamama, the volumes of lava being 
erupted, and the eruption rate determined. Culann and 
Tupan have also been analysed in this way. This ab- 
stract primarily concentrates on Zamama. 

NIMS Data Analysis: NIMS obtained 26 
usable observations of Zamama during the Galileo 
mission. NIMS covered a spectral range from 0.7 to 
5.2 microns. NIMS spectra consisted of up to 408 
wavelengths, constructed using 24 different grating 
positions and 17 detectors. The 5-pm thermal emis- 
sion of Zamama was charted against time. NIMS 5- 
pm data are used as, firstly, most volcanoes on lo emit 
the most thermal emission at this wavelength within 
the NIMS spectral range, and, secondly, the effects of 
daytime sunlight are minimized at this wavelength. 

Methodology: In NIMS radiance data a vol- 
canic hot spot can generally be recognized by a char- 
acteristic thermal ramp towards longer wavelengths. 
In NIMS "tube" (non-averaged) products values of 
two adjacent pixels in the direction of the mirror 
sweep are added to account for the instrument point- 
spread function. Care was taken to avoid data artifacts 
such as radiation spikes, noise, and boom hits. Data 
were converted from NIMS intensities (in units of 
pW/cm^/str/pm) to power output (W/pm) [4]. This 
conversion also removes the effect of range to target 
and emission angle to allow observation-to- 
observation comparison. 

Assumptions: A number of assumptions are 
made. It is assumed that the style of lava emplacement 
at Zamama has not fundamentally changed since the 
Gl orbit (June 1996). Temperatures representative of 
basaltic volcanism were derived from NIMS data ob- 
tained at this time [3]. The surface flows at Zamama 
seen during later orbits by SSI, and the shape of the 
thermal emission curve seen by NIMS, are characteris- 
tic of Hawaii-like pahoehoe flows. Subsequently, we 
scale the thermal emission from post-Gl orbits to the 
Gl thermal emission, using this ratio to determine 
mass eruption rate as a factor of mass eruption rate 
estimated from Gl data [5]. Having decided on an 
emplacement mechanism and composition, it is possi- 
ble to estimate volumetric eruption rate by balancing 
the needed supply of cooling lava to generate the ob- 
served thermal emission [3-6]. 

Results: Figure 1 shows the calculated erup- 
tion volumetric fluxes for Zamama against time, with 
upper and lower limits shown by the blue and pink 
curves respectively. The average volumetric rates are 
shown in red. Volcanic activity at Zamama from 28 
June 1996 to 2 May 1999 (1038 days) was as follows: 
there was a general decreasing trend after the first ob- 
servation (Gl) when Zamama was active with high- 
temperature components to the IR spectrum [3]. This 
decrease represents a lessening in effusive activity, or 
cooling of old flow surfaces. A later increase in activ- 
ity begins at the highlighted area of the curve, indicat- 
ing the beginning of an eruption. This period of time 
coincided with a plume at Zamama, seen by SSI 
(David Williams, pers. comm.). Following this epi- 
sode there is another peak in emission: however, due 
to the sparseness of the data from this point we do not 
know the subsequent nature of activity over the fol- 
lowing few months. 

The total power output observed at Zamama 
from 28 June 1996 to 2 May 1999 was 1.25 x 10^^ J, 
with an average power output of 139.8 GW. The total 
volume erupted from this hot spot throughout the time 
of observation, a total of 1038 days, is estimated at 3.5 
± 1.4 km^, with an average volumetric flux of 39.4 ± 
15.5 m^/s. 

The highlighted area under the curve covers 
274 days of increased thermal activity suggesting a 
volcanic eruption from 8 June 1997 to 29 March 1998. 
This eruption episode yielded a total power output of 
3.95 X 10^^ J and an average power output of 167 GW. 
The total volume erupted was 1.1 ± .45 km^ the aver- 

Lunar and Planetary Science XXXVI (2005) 


age volume rate was 47 ± 18.5 m /s. The peak volu- 
metric rate on 16 December 1997 was 101.1 ± 39.8 
m^/s. A plume was seen by SSI above Zamama on 7 
November 1997 (marked with an arrow in Figure 1). 
Figure 2 shows 5 pm emission from Zamama com- 
pared with Culann, Tupan and Prometheus, volcanoes 
that show similar styles of thermal emission [7]. 

Discussions: Having determined estimates of 
volumetric rate it is now possible to make volcano-to- 
volcano comparisons with other lo volcanoes and ter- 
restrial volcanoes (Table 1). The average Zamama 
eruption rate falls into the same range as other lo vol- 
canoes of the same eruption style, apparently pahoe- 
hoe-like eruptions which resulting in an insulated flow 
field. From Table 1 it is evident that Zamama has 
lower volumetric emission rates when compared to 
some other styles of eruption seen on lo. Neverthe- 
less, Zamama is much more powerful than its terres- 
trial volcanic counterparts, such as Kilauea, Hawaii. 
This, once again, highlights the main difference be- 
tween Ionian and terrestrial eruptions of the same 
style: lo eruptions have larger volumetric fluxes, and 
have much larger active areas. Despite the increased 
thermal activity seen by NIMS from 8 June 1997 to 29 
March 1998, SSI did not see large new lava flows re- 
sulting from this activity. This implies that any new 
surface flows were emplaced on older flows, as seen at 
the other lo volcano Prometheus [8]. 

and L. Keszthelyi (2005) LPSC XXXVI abstract. [8] 
Davies A. G. et al. (2005) Icarus, in press. 

180 n 



-♦— Davies et al. 2000 
-■-Davies, 2003 
A Average 




« 100 - 





g en 

1 « 

^ 4 




n - 




200 400 600 800 
Days since 28 June 1996 

Figure 1 Calculated volumetric eruption rate (m^/s) 
for Zamama plotted against number of days since the 
first usable observation on 28 June 1996 until 20 July 
1998. The arrows denote the eruption peaking 16 Dec 

References: [1] McEwen A. S. et al. (1998) Icarus, 
135, 181-219. [2] Davies, A.G. et al. (2000) Icarus 
148, 211-225. [3] Davies, A.G., (2003) JGR, 108, 
5106, doi: 10.1029 2001JE001509. [4] Davies, A.G., 
et al, JGR, 106, 33,079-33,103, 2001. [5] Davies, A. 
G. (2003) LPSC XXXIV, Abstract 1445. [6] Harris, A. 
et al. (1998) Bull. Vole, 60, 52-71. [7] Davies A. G. 



-*— Zamama 



30 - 









1 20 - 




1 1^- 









\ ^>v 



5 - 




28-Oct-95 ll-Mar-97 24-JUI-98 6-Dec-99 19-Apr-Ol l-Sep-02 


Figure 2. 5-pm variability of Zamama, Tupan, and 
Culann as seen by NIMS. The waxing and waning of 
individual eruptions are seen at all of these volcanoes, 
allowing mass eruption rates to be calculated. 

Table 1 Comparison of volumetric eruption rate with 
other styles of eruptions on lo and on Earth assuming 
basaltic composition^ 

Eruption (style) and Volumetric Eruption Rate, m^/s [3] 

Jan 1990 (outburst) 10^ to lO'^ 

Pillan (open channel flow) 10^ to lO" 

Pele (lava lake) 
Zamama (peak of activity) 
Amirani (average: insulated flow field) 
Zamama (insulated flow field) 
Prometheus (average: insulated flow field) 


Laki, Iceland (open channel flow) 
Mauna Loa, HI (a-a and channel flows) 
Kilauea Iki, HI (fire fountain) 
Kupaianaha, HI (lava lake) 
Kilauea, HI (pahoehoe flow field) 

-250 to 350 




8700 (max) 
10 to 1000 
typically ~2 to 5 

Acknowledgements: This work was carried 
out at the Jet Propulsion Laboratory-California Insti- 
tute of Technology, under contract to NASA. AGD is 
supported by grants from the NASA PG&G and OPR 
Programs. During Summer 2004 Megan Ennis was 
supported by the Planetary Geology and Geophysics 
Undergraduate Research Program.