Io Volcano of the Week series, we are looking at volcanoes that were observed at moderate resolution (160-280 meters or 525-920 feet per pixel) during Galileo's I25 flyby of Io on November 26, 1999. Last week, we examined Zal Patera, a large volcano on Io's northern hemisphere that has been the site of large lava flows and a small volcanic plume. This week we take a look at Emakong Patera, a large lava lake smack dab in the middle of Bosphorus Regio. While a largely inactive lava lake during the Galileo mission, high-resolution observation of this volcano by the camera and near-infrared spectrometer on Galileo have relaunched the debate over the predominance of sulfur and silicate volcanism on Io.
Emakong Patera, like Zal Patera described last week, is a larger than average, roughly heart-shaped patera, or volcanic depression, being 79 kilometers (49 miles) long north to south and 72 kilometers (45 miles) wide west to west. The name of the volcano is derived from the mythology of the Sulka people of the southeast coast of the island of New Britain in Papua New Guinea. In the myth, Emakong dives into a stream to retrieve an ornament he dropped. Upon reaching the bottom, he found that he was in the yard in front of a house. The people from this house allowed him to stay for the night around the hearth fire, both alien concepts to Emakong. The next morning, Emakong was given night and fire as gifts to bring back to his own people. Unlike another volcano named for a mythical fire bringer, Prometheus, Emakong had never been seen as a very active volcano, despite its dark surface and numerous surrounding lava flows.
Alternatively, the flow may have originated as a silicate flow that was over time covered more and more by sulfur. Such as process is seen at Chaac Patera, where brighter sulfur ponds in the valleys within the silicate lava that covers the patera. Over time, this slowly brightens the lava flow. Like Chaac, Emakong also has a greenish color, thought to form from the interaction between cooling, iron-rich silicate lavas and sulfur. Personally, I prefer this theory over one that suggests that this is a sulfur flow. During the Galileo and New Horizons, several examples were observed where bright yellow flows were covered over by basaltic lava during more recent high-temperature eruptions. This provides a potential connection between current activity on Io and earlier activity, though there are examples on Earth where the same volcanic vent has been known to release both sulfur and silicate lava. While Emakong has been generally inactive in the current epoch, it is always possible the volcano may later re-activate these flows, forming broad silicate lava flows to cover the older sulfur-coated flows.
The mix of bright and dark material in the plains southwest of Emakong Patera is difficult to assess. The lack of clear topographic shading makes it difficult to even determine whether bright material lies on top of dark, or vice versa, which is needed to determine stratigraphy of these lavas. The shading nearer the lava channel does suggest that lava over flowing the channel is initially dark before becoming bright, which could occur if hot sulfur is quenched near the channel, while cooler sulfur is quenched more distally. Strangely, the interplay between bright and dark does appear similar to another Galileo observation, a 5.5-meter (18-foot) per pixel mosaic east of Isum Patera taken in February 2000. In both cases, the complexity of the scene makes it difficult to assess the relationship between bright and dark material. In both cases, a bright lava flow is covered at high-resolution. Discussion of that observation may have to wait for another day.
While the debate between the predominance of sulfur or silicate volcanism has been settled for much of Io in favor of silicate volcanism, Emakong is one of several locations where uncertainty remains. Regardless of whether sulfur or silicate volcanism dominates at Emakong, it has been years since it was last active beyond thermal emission from warm lava leaking out through cracks in the crust of the Emakong lava lake, unless the sulfur is highly impure and forms a low-temperature eutectic. The large bright lava flows surrounding Emakong tell the tale of past glory for the volcano, which may one day erupt again to flood hundreds of square kilometers in hot silicate or sulfur lava.
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