Meet Reiden Patera. On the surface, it is an ordinary volcanic pit on Io. But in reality, it is anything but ordinary. Every few years, this particular volcano becomes just another part... of the Twilight Zone.
Okay, I am not Rod Sterling. However, today I thought I would write a little post on this curious volcano. Actually, it is quite ordinary. Our best resolution images are at only 1.5 km/pixel. It has never been the site of an outburst. However this volcano in the shadow of Pillan has gone through an interesting cycle of activity since the feature was first observed by Voyager 1 in 1979.
Reiden Patera is a 73-km wide volcanic depression located on Io's trailing hemisphere a couple hundred kilometers to the southwest of Pillan Patera. Reiden's proximity to Pillan causes occasional confusion between the two features when trying to identify the source of thermal emission in eclipse and near-infrared images of this region of Io. Reiden generally has a dark green floor with dark spots scattered around the margin of the patera. Surrounding the patera, there is normally a bright annulus, which is then surrounded by a dark annulus. The closest analog seen at high resolution by Galileo would be Camaxtli Patera, a similar-sized volcano on Io's anti-Jupiter hemisphere. Like Reiden, Camaxtli has a floor with a patchwork of bright deposits and dark lava flows, and has concentric bright and dark halos surrounding the depression. Reiden, like Camaxtli, has a roughly polygonal outline, with several straight margins, suggestive of structural control, by pre-existing tectonic faults, of the patera margin. A possible landslide deposit can be seen along Reiden's northern margin.
As you can see in the above montage, the appearance of Reiden changed during the course of the Galileo mission. During the first few orbits, several dark spots were seen along the margin, some appearing over time. This suggested on-going volcanic activity centered on the patera margin, and this is substantiated in the thermal data acquired by NIMS and SSI. The camera onboard Galileo, SSI, detected a hotspot at Reiden during the mission's first orbit in late June 1996 (G1). NIMS, the near-infrared spectrometer on Galileo, may have detected a hotspot at Reiden during the second and third orbits (early-September and early-November 1996, respectively), though Lopes et al. 1997 attributes the observed thermal emission to Pillan instead. Changes observed in images acquired in February 1997 (E6) suggests that Reiden was active until shortly after the November 1996, but SSI did show that Reiden had decreased in activity by E6 (in fact, Reiden was not visible in SSI eclipse observations like it was in G1). The lack of changes in images acquired in April 1997 (G7) provides further evidence that the eruption at Reiden had ended. By the next orbit, the outburst eruption at nearby Pillan had begun.
Over the next few years, Reiden remained an inactive volcano, and the dark lava flows seen along its margins began to turn from black to dark green. In images acquired in September 1997 (C10), even the dark halo surrounding Reiden was gone, but the inner, bright halo remained. How much topography and the nearby Pillan eruption played in this change isn't clear, but it appears that Reiden's bright halo maybe located on a low, topographic rise that surrounds Reiden as it was not covered by Pillan's pyroclastic deposits and the topographic rise acted as an impedment to the pyroclastic flow. This provides further evidence that Io's dark silicate deposits, associated with some volcanoes like Pillan, Tvashtar, Pele, and Babbar, are deposited in a process akin to terrestrial basal surges compared to the umbrella-like gas plumes Io is so famous for. As Pillan's dark deposit faded in 1998 and 1999, the dark halo seen when Reiden was active remained absent.
Reiden reactivated by late 2000 as it was seen as a hotspot by Cassini ISS during that spacecraft's distant flyby on December 30, 2000. Galileo during this time observed a darkening at Reiden, further suggesting that activity had resumed. In addition, Galileo observed fresh reddish deposits to the east and northwest of Reiden, perhaps from this new eruption. During a flyby in October 2001 (I32), Reiden was seen at higher resolution. This observation revealed fresh dark material (compared to comparable data acquired in October 1999) along most of its margin, except to the north (where there is a landslide). Reiden may have reactivated as early as October 1999 (I24), when two dark spots were observed along the margin of the patera, near its southwestern margin and along the southern part of the landslide deposit.
Reiden was also seen as active by New Horizons in LORRI imager data and was seen as a dark feature with a bright halo. During the Voyager flybys, it appeared similar to its appearance during the first few orbits of the Galileo mission, though with a dark spot along its northeastern margin, suggesting that Reiden was active during the Voyager mission.
Reiden, though named after the Japanese god of thunder (or was it the Mortal Kombat character...), has long been in the shadow of more famous volcanoes like Pillan and Pele, volcanoes that occasional affect the appearance of Reiden and its surrounding terrain. However, the history of activity at Reiden is an interesting one, where several distinct eruption cycles have been observed by multiple spacecraft. All of Reiden's activity has been confined to small effusive eruptions along the depression's margins. This would suggest that perhaps Reiden is a large lava lake, but there is no evidence of a massive crustal recycling event like those seen at Loki, a more classic example of a lava lake on Io. It is possible that magma uses the faults that bound the depression as conduits to reach the surface, explaining why flows are confined to the margins of the patera. However, a passive lava lake would explain the small eruption along the southern margin of the landslide deposit seen during C3 and I24, as the margin of the landslide would likely not be structurally controlled. It is also possible that this northern bright area is not a landslide, but a cool "island", similar to those seen at Loki and Tupan, two volcanoes thought to be lava lakes.
Hope you all enjoyed this look at Reiden Patera. I hope to post similar articles about other "forgotten" volcanoes here in the future.