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This Week on Galileo?
February 22, 2000

Galileo Passes Within 200 Kilometers of Io's Volcanic Surface

Galileo flies past Io today at a distance seldom ventured in the history of the space age. At 5:47 a.m PST (6:32 am PST-ERT, see Note 1), the spacecraft passes within 200 kilometers (124 miles) of Io's volcanic surface. By comparison, on previous orbits Galileo has seen volcanic plumes reaching altitudes of 140 kilometers (87 miles), with less dense particle effects up to about 800 kilometers (497 miles). The spacecraft could find itself flying through the very diffuse parts of a volcanic plume! These diffuse regions of the plume, however, are very much still a hard vacuum by any terrestrial standards. At closest approach, Galileo will be traveling at 7.3 kilometers per second (26,280 kilometers per hour, or 16,330 miles per hour) relative to Io. One hour and 17 minutes prior to the Io flyby, the spacecraft flies through closest approach to Jupiter at a distance of 4.9 Jupiter Radii (347,000 kilometers, or 216,000 miles) above Jupiter's cloud tops.

The day's activities, save two, are solely dedicated to making observations of Io and its interaction with Jupiter's magnetosphere. The first of the two non-Io observations is performed by the Solid-State Imaging camera (SSI). The observation is designed to capture full-disk views of Europa in three different colors, and will fill gaps in existing color maps that provide context for higher resolution images taken in previous orbits. The second is performed by the Plasma Wave instrument in conjunction with instruments on the Cassini spacecraft, which is approaching Jupiter enroute to arrival at Saturn in 2004. The joint observation is designed to study the properties of radio-frequency emissions from Jupiter.

The Fields and Particles instruments begin the Io-related observations. In a 2-3/4 hour high resolution recording, initiated 3-1/2 hours prior to closest approach to Io, the instruments collect measurements of the plasma, dust, and electric and magnetic fields within the Io torus. The torus is a ring-shaped region of intense plasma and radiation activity that is maintained by Jupiter's strong electric and magnetic fields and Io's constant supply of volcanic particles. The observation will make measurements down through closest approach to Jupiter, and will be used to increase the knowledge of the structure and dynamics of the fields and particles of the torus region. The data will also be important for understanding the overall dynamics of the Jovian magnetosphere.

The Photopolarimeter Radiometer (PPR) starts the Io remote sensing campaign for today with two observations of Io's dark side. The first captures the Loki hot spot measuring temperatures of the active hot spot and surrounding regions in which no volcanism is currently active. The comparison of the measurements from the different regions will allow scientists to study heat flow on Io's surface. In the second observation, PPR measures the temperature of sulfur frost in a region known as Daedalus Patera.

The Fields and Particles instruments return to the observing schedule with an 82-minute high resolution recording performed starting 43-minutes prior to closest approach to Io. As they did during the Io torus recording, the instruments make measurements of the plasma, dust, and electric and magnetic fields surrounding Io. The data will allow scientists to better understand the interaction of Io and its torus with the Jovian magnetosphere. During this period, the Fields and Particles instruments share use of the tape recorder with the remote sensing instruments as Galileo passes over some of the highest-priority targets of this encounter.

PPR continues the observing campaign with two more observations of the Loki hot spot and is followed by the first observation performed by the Near-Infrared Mapping Spectrometer (NIMS). NIMS takes a look at the region containing the Pele volcano. The observation captures a view of Pele while it is on Io's night side and is designed to map thermal emissions from the hot spot at very high spatial resolutions, approximately 1 kilometer (0.6 miles) per NIMS element. SSI also participates in the viewing of the Pele by attempting to capture several images of the hot glowing lava in the caldera.

PPR then performs a polarimetry observation of a region of Io known as Mulungu Patera. The polarimetric measurements will provide scientists with information describing the region's surface texture and thermal properties. From this observation, scientists are hoping to learn more about sulfur frost particle sizes and composition.

SSI follows by looking at a feature that appeared to have been affected by "sapping" in an observation that was made in June 1999. Sapping is the natural process of erosion along the base of a cliff by which soft layers are worn away. The erosion removes the support for the upper part of the cliff which then breaks off in large blocks and falls from the cliff face. SSI's next target is the Chaac Patera region. A new hot spot, Chaac Patera was identified in data acquired during Galileo's November 1999 flyby of Io. The observation will provide very high resolution views of the region, 9 meters (30 ft) per picture element. Prometheus is next to be observed by SSI. At a resolution of approximately 13 meters (43 ft) per picture element, the observation captures Prometheus' plume source and active lava flow.

NIMS returns with three observations. In the first, the instrument looks at the Chaac Patera region, complementing the data captured by SSI. The Chaac region includes one of the areas on Io that are informally known as "golf courses" on account of their greenish color and similar shape (I don't think Tiger Woods would care to play on it due to the sweltering heat, and extreme radiation!). The second observation is a mosaic of several volcanic regions of Io, while the third captures the Prometheus volcanic vent.

SSI then takes a four-image mosaic of the Tohil Mons region, which will be combined with an observation taken in October 1999 in order to produce stereo views of the region. Tohil Mons is one of the mountains of Io, whose geological structure, origin and history are presently not well known. A second observation of the Prometheus region, this one in color, is next on SSI's schedule, followed by a joint NIMS and SSI observation of the Camaxtli Patera hot spot. This observation, when combined with SSI's earlier observation, will provide stereo coverage of the Chaac Patera region.

The NIMS/SSI collaborations continue with the next observation, which captures a view of the Amirani region. The instrument pair then turn their attention to Tvashtar Catena, a chain of giant calderas found in Io's northern hemisphere. One of these calderas was seen to be erupting a curtain of lava 1.5 kilometers (0.9 miles) high and 20 kilometers (12.4 miles) long during an observation made in November 1999. The SSI component of the Tvashtar Catena observations will provide images in 5 colors. SSI continues on its own with observations of the Zal and Shamshu volcanic regions. These observations are made while the regions are near Io's terminator (the imaginary line dividing day from night). The oblique lighting near the terminator provides conditions that are optimal for studying the topography of the regions. Zal and Shamshu are followed by an observation of Io's south pole region. NIMS then performs a regional scan of Io's surface to provide context information for previous higher-resolution observations.

PPR takes another look at Io with two observations. In the first, PPR looks at the Shakuru region of Io searching again for temperatures of sulfur frost. The second PPR observation is a day side thermal map of Io. It is designed to provide information on the thermal properties of Io's surface in the presence of sunlight. NIMS and SSI complete today's observing schedule by taking independent global looks at Io.

Come back tomorrow to learn what lies in store for the rest of the week!

Note 1. Pacific Standard Time (PST) is 8 hours behind Greenwich Mean Time (GMT). The time when an event occurs at the spacecraft is known as Spacecraft Event Time (SCET). The time at which radio signals reach Earth indicating that an event has occured is known as Earth Received Time (ERT). Currently, it takes Galileo's radio signals 45 minutes to travel between the spacecraft and Earth.

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