More on the recent Keck Observations of Io

On Friday, I posted a note about Franck Marchis' observations of Io using the Keck Telescope's adaptive optics system on June 28, 2010.  These observations allow us to sneak a peek at the ongoing volcanic activity on Io's anti-Jovian hemisphere.  Their images revealed hot lava at several volcanoes like Pillan Patera (the most intense hotspot seen on that date), Isum Patera, Marduk, Prometheus, and Volund (not Zamama as previously suspected).  These hotspots were seen in both 3.8 μm and 4.7 μm wavelength images, in the near infrared.  Hotspots were seen in only the 4.7 μm wavelength image, indicative of more cooled lava, at Rata Patera, Culann Patera, near Kurdalagon Patera, Tupan Patera, near a patera at 42.5 South Latitude, 172.5 West Longitude, and at a patera located at 6 degrees South, 190 degrees West.  All but the last of these hotspots was seen before by either Galileo or New Horizons as active volcanoes.  None of these hotspots were seen in the 2.1 μm image, suggesting that none of these volcanoes were vigorously active on June 28.  The 2.1 μm instead showed Io's surface in reflected sunlight.  Silicate materials show up as dark in the 2.1 μm image, while sulfurous materials show up as bright.

Now as Franck pointed out, this isn't the most exciting set of results.  Save one new hotspot, all the others were seen as active before and none are currently in an outburst phase.  But regardless, it is important to observe Io more often to understand what is typical in terms of volcanic activity on the satellite.  How often do outbursts occur?  You are not going to know that if some observing days reveal none.  Exciting, okay, maybe not, but VERY useful.

Anyways, I was a bit curious about the one exception to this "boring" fest, the new hotspot at 6 degrees South, 190 degrees West.  The patera this hotspot seems to be associated with is 60 kilometers by 45 kilometers in size.  The image at left shows various views taken by Galileo during its mission between November 1996 and October 1999.  As you can see, no changes were observed at this volcano during the Galileo mission, nor did the volcano look any different in New Horizons images taken in February 2007. While the 2.1 μm image from Keck has a very low resolution (150 km or so), there does seem to be dark spot associated with this volcano that would indicate the emplacement of dark lava, whereas before, the floor of the patera was covered with sulfurous materials.

Very intriguing stuff that at least one volcano on Io seems to have reawakened.

Link: Keck AO Observations: Io Volcanism - “Mornes plaines” [www.cosmicdiary.org]

New Io Data! Hallelujah, praise Jebus!

Believe it or not, but public information on recent volcanic activity on Io has been pretty limited.  In fact the last report on Io's volcanic activity that is available to discuss here came during the New Horizons encounter back in late February 2007, more than three years ago.  Thankfully, that information drought has finally, mercifully come to an end! Franck Marchis posted on his blog at Cosmic Diary results and images he and one of his undergraduate assistants, Keaton Burns, of Io on June 28.

Using the adaptive optics system on the W.M. Keck II telescope on the Big Island of Hawai'i, Marchis was able to image Io at several near-infrared wavelengths during a break between searching various asteroids for satellites (a project Marchis has previously been quite successful with).  On his blog, Marchis posted a set of images taken at 2.1 μm (Kp), 3.8 μm (Lp) and 4.7 μm (Ms) of Io's anti-Jovian hemisphere. The 2.1 μm image shows mostly reflected sunlight, but the other two reveal thermal emission from Io's volcanoes.  More volcanoes are visible in the 4.7 μm image because cooler (read older) lava can be detected at longer wavelengths.   Both the 3.8 μm and the 4.7 μm images revealed emission at the usual suspects like Marduk, Prometheus, and Zamama, with the most intense hotspot located at Pillan.  Looking at the image myself, you can also see additional, fainter hotspots can be seen at Rata, Culann, and Isum, as well as a new hotspot near a volcano at 6 degrees South, 190 degrees West.

I need to head out the door right now, so I will definitely have more when I get back.

Link: Keck AO Observations: Io Volcanism - “Mornes plaines” [www.cosmicdiary.org]

New Paper: Distribution of SO2 Ice on Io

A new Io-related paper was posted online on the journal Icarus's Article-in-Press page, "The global distribution of Sulfur Dioxide ice on Io, observed with OSIRIS on the W.M. Keck telescope" by Conor Laver and Imke de Pater. This paper covers K-band (1.97-2.38 μm), high-spatial and spectral resolution observations of Io's leading and trailing hemisphere acquired by the OSIRIS near-infrared spectrometer at Mauna Kea's Keck II telescope. This is the third in a series of papers by Laver and de Pater based on observations acquired in April and June 2006. The first paper, published in Icarus in 2007, covered an outburst eruption at Tvashtar Patera in April 2006. The second paper, published in Icarus in 2008, covered observations acquired in April 2006 of SO₂ frost distribution across Io's leading hemisphere.

In their most recent paper, Laver and de Pater explore data acquired on June 1 and 2, 2006 to look at SO₂ frost/ice distribution across the leading and trailing hemisphere of Io. In particular, they look at two weak SO₂ absorption bands at 1.981 μm (3ν₁ + ν₃ vibrational mode) and 2.126 μm (ν₁ + 3ν₃). In addition to including coverage over the trailing hemisphere, the data from June have a higher spatial resolution (65 km at Io's equator) than the April data that was the subject of last year's paper. Like the NIMS and VIMS instruments on Galileo and Cassini, respectively, the OSIRIS instrument produces a reflectance spectrum for each of the nearly 2000 pixels that cover the disk of Io. From these reflectance spectra and following some calibration procedures described in the paper, the authors calculated the equivalent width (a method of both measuring the strength of an absorption band and normalizing it to the surrounding continuum) and the Full-width at half-maxium (FWHM; basically the wavelength width of the absorption band in nanometers) for each of the two absorption bands. Based on these calculations, the authors can then measure the fractional coverage of SO₂ for each pixel in their map based on the equivalent width and a lower-limit grain size of 700 μm. Theoretically, differences in the FWHM can be used the look at differences in grain size across Io, but the spectral resolution was not high enough to explore this. Based on the observed FWHM and the depths of these bands, a grain size lower limit of 700 μm was determined.

The authors found an enhancement of SO₂ frost within Io's bright equatorial regions, such as Bosphorus Regio, Colchis Regio, Tarsus Regio, and Media Regio and a significant deficiency in SO₂ frost west and southwest of Pele on Io's trailing hemisphere. The authors compared these results to similar studies conducted using Voyager ISS data (McEwen 1988) and Galileo NIMS data (Carlson et al. 1997 and Douté et al. 2001). The maps created by Laver and de Pater agree fairly well with McEwen and Carlson's weaker band results, but disagree strongly with Douté and Carlson's stronger band results. The differences observed are likely due to the latter maps showing the distribution of small grained SO2 while the results from weaker bands, like those observed by Laver and de Pater, are sensitive to larger grained ice.

The maps by Douté, for example, show higher concentrations of small-grained SO2 at mid-latitudes, particularly north and south of Pele. Laver and de Pater theorize that this difference in the distribution of small- and large-grained SO2 frost results from frost grains annealing in the warmer, equatorial regions (similar to aging snowfields on Earth, just imagine the difference between walking on freshly fallen snow and snow that's been on the ground for a few days, though some of that is due to shrinking pore space between ice grains and diurnal freeze/thaw cycles). At mid-latitudes, frost grains are cooler, and don't anneal to the degree they do at the equator.

Overall, an interesting paper that was quite easy to follow, despite being a spectroscopy paper ;) The authors hope to acquire additional observations of these regions to improve the signal-to-noise ratio of their maps.

Link: The global distribution of Sulfur Dioxide ice on Io, observed with OSIRIS on the W.M. Keck telescope [dx.doi.org]

Red-Blue Jupiter

Adaptive optics observations of Jupiter, acquired from Chile's European South Observatory, were released on Thursday. These images show Jupiter as it appears in the near-infrared near 2 microns, beyond the light that humans can see. So clearly, these are false color images that were released. But the images still are very cool, showing how the recent reshaping of Jupiter's cloud belts have affected its haze layers higher up in the atmosphere. For example, the brightest part of the haze layer over the equatorial belt has switched from the northern to the southern hemisphere. The observations resulted from a new system that allows the adaptive optics system to use moving reference targets for a longer period of time, allowing astronomers to use the AO system over a larger field of view.

They still need to look at Io more often ;-)

Speaking of ground-based observations of the Jupiter system, amateur astronomer Paul Haese has set up an online gallery showing the observations he has acquired of Jupiter during the last three years. These images have high enough resolution to pick up some of the larger scale albedo markings on Io, like the difference in brightness and color between the polar and equatorial regions. Definitely worth checking out.

Link: Sharpening Up Jupiter [www.eso.org]