We present ground-based observations of Io during Spring 2014, contributing to decadal timelines of individual hot spots' volcanic activity

de Kleer, K.

de Pater, I.

Gutierrez, S.

Rathbun, J.

Caltech Authors - Main

OCCULTATION LIGHT CURVES OF IO’S HOT SPOTS IN 2014.  S. Gutierrez1, K. de Kleer2, I. de Pater1,3, J. 
Rathbun4, 1Astronomy Department, UC Berkeley, Berkeley, CA (s.gutierrez@berkeley.edu); 2Division of 
Geological and Planetary Science, California Institute of Technology, Pasadena, CA; 3Institute of Earth Observation 
and Space Systems, Delft University of Technology, Delft, The Netherlands; 4Physics Department, University of 
Redlands, Redlands, CA. 
 
 
Introduction:  Groundbased observations in the 
infrared complement spacecraft images of Io. Until 
adaptive optics was developed, occultation photometry 
was the only technique to resolve individual hot spots 
with groundbased observations. First applied to Io in 
1990, the method uses the regular occultation of Io by 
Jupiter and light curves to find candidates for active 
hot spots [7, 9, 11,12]. This method measures Io's total 
brightness and uses known hot spot positions and 
occultation models to determine the occultation phase 
for individual hotspots [11]. The steps in the 
lightcurves correspond to the reappearance or 
disappearance of a hotspot. Our eight nights of data are 
all in-eclipse occultation reappearances. With the 
known occultation phases for Loki Patera and Janus 
Patera/Kanehekili Fluctus, we have deduced the 
individual brightnesses of these hot spots over Spring 
2014, and compare them to Gemini N observations 
taken at the same time period [2]. This analysis 
contributes to the decadal timelines of Loki Patera's 
and Janus Patera/Kanehekili Fluctus's volcanic 
variability.    
Loki Patera, Io's most powerful volcano, has been 
observed for 30 years, during which time it regularly 
has undergone brightening events [6]. Rathbun and 
Spencer [9, 10] proposed an overturning lava lake with 
varying overturn propagation speeds. de Kleer and de 
Pater [1] find that a modified version of the Matson et 
al.  [8] overturning lake model fits all three decades of 
reported data. Recent mapping of Loki Patera indicates 
that this resurfacing process has multiple phases as 
different velocity waves move around an anchored 
island and converge in the eastern part of the lake [3]. 
Janus Patera and Kanehekili Fluctus are close 
together on Io's surface, and cannot be separated by 
occultation observations such as those presented here. 
Their combined radiance has been recorded since the 
1990's, while their individual timelines have been 
measured over time from spatially-resolved adaptive 
optics observations [4]. Observations of Janus Patera 
over the last two decades demonstrate relatively steady 
radiant fluxes between 2 and 8 Gw/sr/micron; the 
stable fluxes along with the spectrum shape support the 
presence of an active lava lake [5]. Kanehekili Fluctus' 
thermal emissions are more variable and are consistent 
with insulating crust lava flows [5]. 
 
Figure 1: Lightcurve of the 02/26/2014 occultation 
reappearance. Flux is plotted as a function of Occultation 
Reappearance Phase, defining the beginning of the 
reappearance as 0 and Io completely coming into view as 
1.0. Every step signifies at minimum one active hot spot 
appearing at the reappearance phase. Every night of 
observation has an active hot spot consistently at the 
occultation phases .27 and .93, most likely being Janus 
Patera /Kanehekili Fluctus and Loki Patera respectively.  
Methods:  All of the observations were taken with the 
CSHELL CCD camera at NASA's Infrared Telescope 
Facility. Images of Io were taken during reappearance 
from Jupiter’s occultation into eclipse, therefore the 
derived fluxes originate entirely from hot spot 
volcanism. We plot the total observed radiance of Io as 
it reappears from behind Jupiter, as a function of 
reappearance phase for each date (Figure 1.) Since Io 
is in Jupiter's shadow the entire time, all of the 
radiance measured is directly from its hot spots, not 
reflected radiance from the Sun.  
Results and Discussion:  Loki Patera. After 
determining the individual hot spot fluxes from the 
lightcurves, we construct time lines for the individual 
hot spots. Figure 3 plots Loki Patera's eight radiance 
values (circles) from our analysis and five radiance 
values (crosses) from [2]. Although, it may appear that 
we observe a relative brightening event in February, in 
the context of Loki Patera's periodic time line, our 
points are steady and at a low-activity period [6]. The 
Gemini N. and Keck data also depict this trend.  
 
2954.pdf49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083)
 
Figure 2: Radiance values (circles) of the individual hot spot 
Loki Patera over the eight nights of observations in Spring 
2014. The Gemini N data from [2] are plotted as stars for 
comparison. During this time, Loki Patera was fading 
following a brightening event in Fall 2013. All radiance 
values were obtained in L' band (3.8 micron). 
These observations occurred during a dim period 
between brightening events measured by [2] and 
indicate that the overturning lava lake is thickening 
and cooling in this time period. In August 2013 Loki 
Patera reached radiance values of 136 ± 20  
GW/sr/micron and then gradually decreased to a range 
of 6 to 15 Gw/sr/micron, which we observed [2]. In 
October 2014 another brightening event occurred at 
128 ± 19 Gw/sr/micron [2]. The observed flux of this 
dim period are similar in range for dim periods 
observed in Spring 2015, between 6 to 11 
Gw/sr/micron.  
 
 
 
Figure 3: Radiance values (circles) of the volcanoes Janus 
Patera/ Kanehekili Fluctus. The Gemini N data from [2] are 
plotted as crosses for comparison. 
 
Janus Patera/ Kanehekili Fluctus. The eight 
radiance values (circles) of Janus Patera/ Kanehekili 
Fluctus are plotted along with two data points (crosses) 
from [2] however the Gemini N points are only from 
Janus Patera (Figure 3). The two observations from [2] 
did not detect any radiance from Kanehekili Fluctus, 
constraining its thermal emission to at or below 3 
Gw/sr/micron at a wavelength of 3.8 micron, the 
detection limit of the Gemini N observations. With our 
error bars, the observed radiances are relatively regular 
and within the range that Janus Patera has been 
observed at for decades [4]. More recently, in Fall 
2013 Janus Patera's radiant flux ranged from 3 to 10 
Gw/sr/micron, and in Fall 2014 throughout 2015 it was 
also consistently between 2 to 9 Gw/sr/micron at 3.8 
micron [2]. These observations support the active lava 
lake model for Janus Patera [4].  
References: [1] de Kleer, K., de Pater, I., (2017) 
Icarus, 289, 181-198. [2] de Kleer, K. , de Pater, I., 
(2016) Icarus, 280, 378-404. [3] de Kleer, K. et al., 
(2017) Nature, 545, 199-202. [4] de Pater, I. et al., 
(2004) Icarus, 169, 250-263. [5] de Pater, I. et al., 
(2014b) Icarus, 242, 379–395. [6] de Pater, I.  et al.,  
(2017) Icarus 297, 265-281. [7] Howell, R. R. et al., 
2001. J. Geophys. Res., 106, 33129–33139. [8] 
Matson, D.L. et al., (2006) J. Geophys. Res.,  
doi:10.1029/2006JE002703. [9] Rathbun, J.A. et al., 
(2003) 34th LPSC, abstract no. 1375. [10] Rathbun, 
J.A., Spencer, J.R., (2006) Geophys. Res. Lett. 33, 
L17201. doi:10.1029/2006GL026844. [11] Rathbun, 
J., Spencer, J.R., (2010). Icarus 290, 625-630. [12] 
Spencer, J.R. et al., (1990). Nature 348, 618-621. 
 
2954.pdf49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083)


Occultation Light Curves of Io's Hot Spots in 2014

https://authors.library.caltech.edu/94954/1/2954.pdf

Occultation Light Curves of Io's Hot Spots in 2014

Abstract

Similar works

Full text

Available Versions

Caltech Authors - Main