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Lesson OneIonian Coordinate System and the Three Major Volcanoes
IntroSo you have been reading through this website and you hear place names on Io and you have no idea where they are? Well you have come to the right place! This is the first part of a 16 part series on Ionian geography. Since this is the first lesson we are going to begin with the basics. As we go along through these lessons, you will learn more about where even the tiniest volcano is and Io diverse geology. Geography can be defined as the study of where places are and how the relate to people and animals. Since there are no people and animals, yet, on Io, we are going to have to throw out the first part of the definition. And since we are not dealing with the Earth, we will call it Ionian geography or "Iography." Also one must learn the definition of feature types on Io: Catena: Chain of volcanic craters Io Coordinate System
Latitude Ionian Latitude system is set up much like Earth's. There are two poles, the north and the south, where the rotational axis meets the surface. These are marked as being at 90 degrees, 90 deg. North for the north pole and 90 deg. south for the south pole. The equator is defined as the "set of points equidistant from both the north pole and the south pole." In layman's terms, the equator is the dividing line between the northern and southern hemispheres. The equator is at 0 deg. Latitude. An important latitude to remember as far as Io is concerned is 30 deg. north and south. Most of Io's volcanoes and all but one of the long term plumes occur within 30 degrees of the equator. Volcanoes within 30 degrees of the equator also seem to be smaller in size and more consistent. The volcanoes outside of the 30 degrees north or south of the equator are larger and less consistent. Longitude Io's longitude system is similar to Earth's with a few notable differences. Like Earth, Io has 360 degrees of longitude with meridian running from pole to pole. The lines of longitude on a globe give it almost an orange slices-like look. On Earth, longitude is counted both to the east and to the west, counting up to 180 degrees. This separates Earth into the eastern and western hemispheres. However, longitude is counted to the west up to 360 degrees where it reaches the prime meridian. The prime meridian on earth is marked as a line that runs from pole to pole and goes though the Greenwich Observatory near London, England. On Io, the prime meridian is an imaginary line that runs from pole to pole and goes through the subjovian point. The subjovian point is the spot on Io surface that is at the center of the hemisphere that always faces Jupiter. Consequently, Jupiter is always directly overhead.
Because of Io's rotation is in synch with its revolution around Jupiter, Io can be separated into 4 different hemispheres. The first set of hemispheres, the Projovian and the antijovian, have to with the fact that Io keeps one face always pointed at Jupiter and the other away from Jupiter. The projovian hemisphere runs from 270 deg. to 90 deg. longitude and always faces Jupiter. Every place in this hemisphere has the center of Jupiter's disk above the horizon. The antijovian hemisphere runs from 90 deg. to 270 deg. and nearly ever place on this hemisphere never sees Jupiter. The other set of hemispheres are the leading hemisphere and the trailing hemisphere. The leading hemisphere is the hemisphere that always faces the direction of motion in Io's orbit. The leading hemisphere runs from 0 deg. longitude to 180 deg. longitude and includes the volcanoes Prometheus and Kanehekili. The trailing hemisphere is the hemisphere that always faces in the direction opposite from the motion of Io around Jupiter. This hemisphere runs from 180 deg. to 0 deg. longitude and includes the volcanoes Pele and Loki. The Three Major VolcanoesTo begin to understand Ionian geography you must first know about the four major volcanoes. They are Pele, Loki, Prometheus, and Kanehekili. Pele
Pele is an oblong lava lake located at 18.6 deg. South latitude and 257.8 deg. West longitude. This means that Pele is located in the southern, antijovian, and trailing hemispheres. The lava lake is located at the northern end of a rifted plateau called Danube Planum. The hottest lavas on Pele's lava lake occur on a line along the edge. These lavas have temperatures over 1400 Kelvin. Surrounding the lava lake are dark pyroclastic deposits made of ash and beyond that, S2 and SO2 deposits that show up as a red ring in visible images. More will be discussed about Pele in Lesson Eight. Loki
Loki is considered the largest volcano on Io if not the solar system. It is located at 12.6 deg. North Latitude and 308.8 deg. Longitude, in the northern, projovian, and trailing hemispheres. The 250-km wide main caldera is semi-circular in shape with the straight edge on the northwest side. There is a large island crisscrossed by branching lava channels in the center of the caldera giving the volcano a horse shoe shape. The island is the usual starting place of the periodic eruption that occur at Loki every 15-18 months. The lava flows flow down the slope of the island in the lava channels. It then flows across the caldera floor, going south, then east, and finally north. These eruptions last a few months. Northwest of the caldera is a long NW-SE fissure. Plumes were seen erupting from each end of the fissure in 1979. Lava flows flow from the fissure to the main caldera. More will be discussed about Loki in Lesson Seven. Prometheus
Prometheus is a small caldera located at 1.6 deg. South Latitude and 153 deg. Longitude. From that volcano emanates a 90 km long sheet lava flow. SO2 and S2 plumes erupt near where the lava breaks out on the surface and near the lava flow front. Prometheus is considered one of the most consistent volcanoes in the solar system despite the fact that the SO2 plume has shifted position 75 km and that the S2 plume in the east is now more powerful. More will be discussed about Prometheus in Lesson Nine. The Regiones
There are 9 major regions on Io: Bactria Regio, Bosphorus Regio, Chalybes Regio, Colchis Regio, Illyrikon Regio, Lerna Regio, Media Regio, Mycenae Regio, and Tarsus Regio. Most of the regiones are bright SO2 snowfields. The biggest ones are Colchis, Media, and Bosphorus Regiones. These three occur along the equator and with the exception of Media Regio, are giant snowfields. Colchis Regio is located on the antijovian hemisphere and has varying shades of white, gray, and yellow. Bosphorus Regio is a 1200 km wide snowfield/sulfur lava field located on the leading hemisphere. This region surrounds Emakong Patera, the origin of the sulfur lava flows in this region. Media Regio is a poorly defined region on the projovian hemisphere. The other regiones are also snowfields with the exception of Lerna Regio. Tarsus Regio is an SO2 snowfield centered around 40 deg. South Latitude and 60 deg. Longitude. This region surrounds the volcanoes Masubi and Laki-oi Patera. Chalybes Regio is a bright snowfield with an ethereal quality like a transparent covering of frost. It is located northwest of Zal Patera on the leading hemisphere at 45 deg. North Latitude and 80 deg. Longitude. Illyrikon Regio is located near the south pole at 70 deg. South Latitude and 160 deg. Longitude. This puts it in the Antijovian hemisphere. Several mountains are seen within Illyrikon Regio. Bactria Regio is also located with in the Antijovian hemisphere and like Chalybes Regio, also has an ethereal quality like a transparent covering of frost. Mycenae Regio is a semi-bright regio surrounding Nina Patera. This region is located at 37 deg. South Latitude and 165 deg. Longitude on the Antijovian hemisphere. Lerna Regio is believed to be either a sulfur lava flow or a sulfur snowfield. This yellow colored region is located near the south pole on the trailing hemisphere centered at 65 deg. South Latitude and 290 deg. Longitude.
In the next lesson, we will discuss the north polar region of Io.
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