Lunar and Planetary Science XXXV (2004) 2071.pdf
CHARACTERIZATION OF ACTIVITY AT LOKI FROM GALILEO AND GROUND-BASED OBSERVATIONS. R. R. Howell1 and R. M. Lopes2, 1Dept. of Geology & Geophysics, University of Wyoming, Laramie, WY 82071
Introduction: While Loki is the most active vol- colored first and the small group points last, with later canic center on Io, major questions remain concerning pixel colors overwriting earlier ones. At least three the nature of that activity. Rathbun et al. [1] showed spectrally and spatially distinct groups are evident. that the activity was semi-periodic, and suggested it The green band in Figure 1a corresponds to the was due to a resurfacing wave which swept across a main body of the patera and shows a smooth variation lava lake as the crust cooled and become unstable. in intensity (labeled by different green hues) with posi- However in 2001 new observations [2] showed that an tion. In the resurfacing wave model that variation cor- intermediate level, less periodic mode of activity had responds to surface age. While there is clearly a single apparently begun. Galileo-NIMS observations of Loki parameter related to age which controls the two meas- [3,4] clearly show that the highest temperatures are ured intensities, a model with uniform (i.e. fully filled) found near the edge of the patera, consistent with dis- cooling pixels is too simple. The brightest pixels cor- ruption of a lava lake at the margins. NIMS observa- respond to a temperature of ~400K filling ~5% of the tions also show gradients in temperature across the pixel while the faintest pixels correspond to a tempera- patera which, when modeled in terms of lava cooling ture of ~350K filling ~40% of the pixel. It is possible models, are generally consistent with ages expected for to model the surface with two components, for exam- the resurfacing wave [5] but may also be consistent ple a slightly cooler temperature filling most of the with spreading flows [6]. We present a further analy- pixel plus a hot but small crack fraction. However sis of NIMS data from I24 and I32 which help define both components must then be made to fade in a the nature of the temperature variations present in Loki closely coupled way to produce the one-dimensional patera, along with Galileo-SSI images from the G1 – band. There may be alternative models which rely on I32 flybys which show albedo changes apparently cor- small amounts of volatiles to transfer heat and ho- related with the “periodic” activity measured from mogenize the surface as it cools, We are currently ground-based observations. analyzing a range of such models. NIMS Temperature Structure: During the I32 To test repeatability between brightening events encounter the NIMS instrument obtained observations the tube points from an I24 NIMS scan across the of the southern 1/3 of Loki Patera. The instrument patera are also presented, labeled in figure 1a with operated in a mode which simultaneously measures magenta. Although a brightening was underway dur- intensities at 15 wavelengths while scanning spatially. ing I24 it had not yet reached the part of the patera By analyzing the data products called NIMS “tubes” scanned, so the I24 points represent surface created which preserve the original simultaneously measured during a previous brightening. The fact that those intensities we limit confusion due to pointing and edge points clearly fall at the bottom end of the green band effects present in the spatially reprocessed and coad- show that whatever process produced the I24 surface ded “cubes”. Figure 1a is a two wavelength intensity repeated quite precisely for I32. (Magenta points near plot of each point from the I32 tube. The simplest the origin of figure 1a are over the cool island or out- model (solid and dashed lines) represents each pixel side the patera.) as a warm component of a certain temperature occupy- At least two distinctly different high temperature ing a certain fraction of a pixel, with the remainder of components are also present. The highest temperature the pixel cold and dark. For orientation figure 1b red points (typically ~550K) are all confined to the shows a 4.7-µm spatial intensity map, with the south- western margin of the lake. Intermediate temperature ern part of the patera with the central “island” and the yellow and aqua points (typically 400-500K) are con- western and southern margins also visible. fined to the inner and outer margins. In an intensity In figure 1a the points clearly fall into groups and plot like this, spatial mixing produces linear mixing to understand the associated spatial structure we lines so the diagonal bands (red, yellow, and aqua) can “manually” label the points with different colors, then be produced by spatial mixing between the hot mar- simultaneously color the corresponding pixels in the gins and the cold (zero intensity) region outside the spatial map (Figure 1c). There are many more tube patera. However a clear gap exists between the red elements than map pixels, so the large group points are (hotter) and the yellow plus aqua (cooler) points. While the higher temperatures along the inner and Lunar and Planetary Science XXXV (2004) 2071.pdf
outer margins may well be due to crust breakup along lyzed, and correlated with the ground-based observa- the margins of a lake, the higher temperature in the tions [2]. western edge, and perhaps the curvature of the right Acknowledgements: This work was supported by part of red band, requires an additional physical proc- the NASA-ASEE Summer Faculty Fellowship pro- ess. One candidate might be introduction of new fresh gram, and by NASA PG&G grant NAG5-11730.. lava to the hypothetical lake at that margin. References: [1] Rathbun, J. A. et al. (2002) Geo- SSI Images: Clear differences were evident be- phys. Res. Lett. 29, 84-1. DOI 10.1029/2002- tween the Voyager I and II images of Loki, and despite GL014747. [2] Rathbun, J. A. et al. (2003) LPS their relatively low resolution, Galileo-SSI global XXXIV, Abstract #1375. [3] Lopes, R. M. (2004) monitoring images also show changes. For example in Icarus, in press. [4] Lopes, R. et al. (2002) LPS figure 2 the southwest region of the patera darkens XXXIII, Abstract #1793. [5] Lopes, R. M. C. et al. between C21 and I24. Ground-based data show a (2003) DPS meeting #35, #02.02. [6] Davies, A. G. brightening began shortly before I24, and Galileo-PPR (2003) Geophys. Res. Lett. 30, PLA 3-1 DOI data [7] show this southwest region was the initial site 10.1029/2003GL018371. [7] Spencer, J. R. et al. of the activity. Additional images are now being ana- (2000) Science, 288, 1198-1201.
b
c
Figure 1. A) NIMS measurements of the 3.00-µm flux vs. 4.70 µm flux for each point in the I32 “tube” of Loki. Groups of points have been “manually” labeled with different colors. Solid lines of constant temperature, and dashed lines of constant pixel fill fraction are also shown. B) A 4.7-µm intensity map for orientation. C) The in- tensity map with pixels labeled by the same colors used in panel A.
Figure 2. C21 and I24 violet images of Loki patera, showing the darkening in the southwest corner corresponding to the beginning of the I24 brightening.