Last week, a new paper was published in press (the paper has been approved for publication, but hasn't found a slot in the dead-tree version of the journal yet) in the journal Icarus discussing spectroscopic observation of Io as it emerged from the shadow of Jupiter. The paper is titled "Eclipse reappearances of Io: Time-resolved spectroscopy" and was written by Dale Cruikshank, Josh Emery, Katherine Kornei, Giancarlo Bellucci, and Emiliano d'Aversa.
In this new paper, the authors discuss spectroscopic observation of Io acquired using NASA's Infrared Telescope Facility (IRTF) in Hawaii during five eclipse reappearances in April, May, and June 2004. These observations were intended as a follow-up to results from Cassini VIMS observations in Bellucci et al. 2004 taken during that spacecraft's Jupiter flyby during New Year's 2001 that showed a brightening of Io's surface in the near-infrared and a deepening of several strong sulfur dioxide absorption bands following Io's emergence from Jupiter's shadow. This result continues a 40-year-long mystery concerning the interaction between Io's atmosphere and its surface during and after an eclipse by Jupiter.
Unlike lunar eclipses, when the Earth passes between the Sun and our Moon and which happen about once a year, or every 13 orbits of the Moon around the Earth, eclipses of Io by Jupiter occur about once each Ionian day. This is due the large size of Jupiter compared to Earth and the much lower axial tilt of Jupiter and its main satellite system. Each Ionian lunar eclipse lasts about 2 hours and 22 minutes. During this time, the temperature of Io's surface cools due to the sudden lack of sunlight. As Io cools down as the eclipse progresses, atmospheric Sulfur dioxide (SO2) condenses onto the surface. Check out a post I wrote earlier this year on another paper for more details on this process.
Depending on the amount of SO2 that condenses onto the surface, the fresh frost should be visible shortly after Io emerges from behind Jupiter's shadow as a brightening of Io's surface compared to its appearance prior to being eclipsed, and it should quickly dim as the frost sublimates from the surface now that the Sun is able to heat it up. In addition, the strong SO2 absorption bands at 3.56 μm, 3.78 μm, 4.07 μm, and 4.37 μm would be deeper than they were prior to the eclipse and should become shallower during the first 60-90 minutes after each eclipse and particularly in the first 15 minutes as the fresh, fine-grained SO2 frost sublimates back into the atmosphere. Results from multiple studies using ground-based and spacecraft observations over the last 40 years, since Binder and Cruikshank 1964 revealed a brightening of Io of 10 percent following an eclipse by Jupiter, have been inconsistent with some showing such a brightening, and others showing none. As explained in this new paper, Nelson et al. 1993 found that post-eclipse brightenings are likely to be rare as a fresh SO2 frost layer several millimeters thick would be required to explaining the magnitude of the brightenings that were seen, and it would take longer than 15 minutes to sublimate that layer away. In addition, modeling of Io's atmosphere during an Io eclipse by Moore et al. 2009 suggests that SO2 condensation onto the surface would be curtailed to some degree by atmospheric heating by the Io plasma torus and by non-condensable species like Sulfur monoxide preventing SO2 in Io's upper atmosphere from condensing.
Cruikshank et al. examined their observations taken at IRTF and found no evidence of changes in Io's albedo or the area of three SO2 absorption bands at 3.56 μm, 3.78 μm, and 4.07 μm. What changes were observed were either the result of the rotation of Io during the 60-90 minutes of each observation run, were found in one absorption band but not in the other three, or were the result of observation noise or the thick airmass of Earth's atmosphere. Therefore, the authors were not able to confirm the VIMS results published by Bellucci et al. 2004. The authors suggested that the two conflicting results could be due to the background frost coverage in the area observed by the two groups of researchers. VIMS observed Io's trailing hemisphere which is thought to have the least abundant SO2 frost coverage while the Cruikshank et al. group observed the sub-Jupiter hemisphere, SO2 abundance is higher. The lower SO2 abundance would have made condensed SO2, even if in a very thin layer, more noticeable compared to the sub-Jupiter hemisphere.
In other results, Cruikshank et al. observed additional SO2 absorption bands between 2.11 and 2.24 μm, including a faint one at 2.198 μm that the authors thought they were first to see in Io's near-infrared spectrum. Another weak absorption band at 2.1255 μm was mapped by Laver and de Pater and the results of that study were published earlier this year and discussed on this blog. Cruikshank et al. also observed Io's emission spectrum while the satellite was still in the shadow of Jupiter during the observation run on June 22, 2004. They did not find convincing evidence for condensed SO2 in Io's atmosphere, which would be expected in Io's volcanic plumes. This negative result could be the result of the temporal variability of Io's plumes.
Finally, the table of contents for the October 2009 issue of Icarus has been published online. No Io-related papers in this issue, but there are a series of papers covering Jupiter's Oval BA, also known as Red Spot Jr.
Link: Eclipse reappearances of Io: Time-resolved spectroscopy (1.9-4.2 μm) [dx.doi.org]