Volcanoes of I27

Introduction

Galileo has returned some spectacular images so far from its three Io flybys.   None have been more spectacular than those returned from its February 22, 2000 flyby or I27.  During this flyby, Galileo imaged volcanoes and mountains that were missed due to spacecraft errors in the previous two flybys.  Galileo also imaged volcanoes to detect changes since I24 (October 11, 1999) and I25 (November 25, 1999).   Not all of the images from this flyby have been returned yet but what has been returned has been nothing short of spectacular.

Isum Patera

In July of 1999, Galileo made a full disk mosaic of Io's Antijove hemisphere at 1.3 km/pixel.  In this mosaic, images showedd that cliffs along the eastern edge of the "Isum Plateau" were sapping.  In this image and in another from C3 (November 6, 1996), this cliff was surrounded by a bright halo and it appeared that large chunks were falling down from the cliff.  It was then decided to target this cliff in I27 at an amazing 5.3 m/pixel.

The images that were returned appear to have missed the cliff but are still quite breath-taking.  The images that have been returned show a wildly varying terrain.   Some areas appear almost like the snow drifts over rocks in Antartica. Other areas appear dark and rocky.  Still others look like layers of rock or frost.  Evaporation of ices may have caused the layers and the seperation between dark and light areas.  Large boulders can be seen in this image, covered in frost.  Some boulders in frost areas have melted the frost surrounding them.

Other images show the same rock-frost mixture but this one has other elements as well.  In the right part of the is a tall cliff.  This may be the cliff they were targeting.  On the edge of the cliff is some kind of dark, bumpy material.  What is interesting about it is that its complete devoid of frost.   There is a definite line between this material and the rest of the terrain.   In fact, frost borders it.  Is this material new?  What is it made of and what is its origin? These questions will have to wait for further analysis by scientists.

Chaac Patera

Chaac Patera is a green caldera northwest of Prometheus.  Chaac is located at the bottom of a depression within a plateau.  Chaac was imaged at high-resolution (7 m/pixel) and at medium-resolution (188 m/pixel) during the flyby on February 22.  The high resolution views show many areas in and around the Chaac caldera.  The images show ridged terrain, lava flows both inactive and active, bright terrain, and the caldera wall.

The top two high resolution views show the region just outside the northwest edge of the caldera, the caldera wall, and terrain along the inside edge of the caldera. The area just outside the caldera appears to be ridged, much like that seen near Prometheus and Zamama.  The ridges themselves are bright and they all seem to face away from the caldera.  The material between the ridges is dark, much like that seen at Ot Mons.   The caldera wall is steep, measuring 2.8 km tall in some places, and have a 70° angle to it.  This is compared to the 30° angle to cliffs seen at the Grand Canyon which are 1.8 km tall.  The caldera wall is topped by a layer of SO2 frost 160 meters deep.  The top also appears to be edgy in some places. Below that is a silicate cliff that is believed to be layered.  At the bottom of the caldera wall are layers of debris that has fallen from the cliffs or possibly dried up lava.

The caldera floor itself is covered with lava flows and pyroclasic material.  The lava flows have a wide range in ages with dark new lava and bright old lava.  The lava texture is similar to that seen at Kilauea at Hawaii.  This suggests that Chaac has been been covered by a combination of lava flows and lava lakes.  The dark lava appears dark because they have not been covered by the sulfurous materials that cover most of this caldera and most of Io's surface.

The bottom Hi-res view shows thick, bright lava flows, dark lava, and other bright terrain. The dark flow is actually a long pit.  This pit was formed when lava formed a pond that crusted over and drained into the ground.  This formed a deep pit.   The bright lava flows indicate that they are no longer active.  They also appear to be 50 meters thick.  The bright terrain borders the outside of the caldera wall and is even bright in context images.   The bright terrain is flat and is crossed by dark lava cracks or fissures. The eastern part of this bright area appears stained by the green material from the caldera.

The context view of Chaac Patera (resolution: 188 m/pixel) gives us a better understanding of the high-resolution views.  Chaac is an irregularly shaped caldera 100 km by 30 km, possibly indicating that Chaac was formed by a tectonic fault.  Looking at the images, its does appear that the volcanoes western and eastern sides do match up to some degree.  The context image shows various ages to the lava with dark lava in the north and older lava to the south.  Some areas are white, meaning there might be a lack of lava in these areas.  The lava appears to stream away from a lava "pond" in the northern part of the caldera.  Also, new lava flows border the western edge of the caldera, much like was seen at Pele during I24. Cliffs seen to the west and northwest of the caldera border the depression that Chaac resides in.  A bright patch east of Chaac covers the bottom of another depression.  The bright patch is believed to be sulfur dioxide ice.  The eastern edge of the plateau that surrounds Chaac can be seen in the eastern part of the context image.

Prometheus

Prometheus has been the site of an active plume in every image that has ever been taken of it.  Images taken during I27 were designed to image the vent area at the edge of the flow front.  The images above show just some of the images that were taken.  These images have a resolution of 12 m/pixel.

The images show the flow front at the northeast edge of the Prometheus lava "pond."  Many dark areas can be seen in this image indicating active lava flows.  Extensions from the lava flow appear to have rougher terrain and lighter edges than the rest of the lava flow.  The edge appears convoluted and is determined by preexisting terrain. This preexisting terrain consists of long ridges, much like those seen at Chaac.  Unlike those seen at Chaac, the ridges here appear monotone rather than bright ridges with dark floors in between.  Also, these ridges appear to have flat tops, unlike the round ridges seen at Chaac.  The ridges have an east-west trend and may have formed by the folding of a surface layer or by deposition or erosion.   Some ridges are covered in SO2 frost, making them look like a tile roof in winter.  They frost covering is recent because they don't appear evident in I24 images.  Ridges farther away from the lava flows are more rounded than those closer to the lava flows, indicating that streams of material coming from the lava flows are wearing away the ridges there.

A color mosaic of Prometheus was taken at 170 meters per pixel.  Only one of the green filter images has been returned.  This image shows many changes since the I24 flyby in October 1999.  When scientists compared this image to those taken in October 1999, they found that 60 sq. km worth of fresh lava had been emplaced since that time. This yields an average flow rate of .45 sq. km per day.  This is ten times higher than that ever seen at Kilauea in Hawaii.  The most of the fresh lava appear to be extensions of the lava flows seen in I24.  Also note the numerous bright streaks that were seen upclose in the images above.

The mesa seen in I24 can also be seen in this image. It appears to actully surround the caldera on all sides, not just the eastern side as was thought after seeing the I24 images.  The bright band down the middle of the image is the plume deposit of the main plume.  It formed when the secondary plume off to the east of the image, increased in strength.  This pushed the main plume to the west.  The lava breakout to the north of the lava chanel has faded since it was first imaged in I24.

Tvashtar Catena

Tvashtar Catena was imaged in color at 315 m/pixel.  This images shows changes that have occured at Tvashtar since a lava curtain was seen here in I25.  The lava fountain has shut down, replaced with a line of lava at 600 K.  Also, there is an extension of lava on the eastern end of the fissure, just where scientist believed there might be one after I25 images.  There is a new pyroclastic deposit but it does not appear associated with the I25 fissure.  Instead, it is associated with a hot lava flow in the western section of the catena.  Hot lava is flowing north two active toes that are glowing white.  The outer edge of the hot lava flow in the south is older and cooler than the interior of the lava flows.  This eruption has produced a large pyroclastic deposit that also covers the fissure. In addition to this activity, a caldera north of the catena has darkened since July 1999 when it was last imaged.

See my notes on the Tvashtar from I25 for more information.

Zal Patera

Zal Montes was imaged in I27 at 335 m/pixel. These observations were meant to detect changes since I25 and to get stereo coverage over some areas.  Some of the images have been returned including an image of the top portion of Zal Montes "Plateau" and an image of Zal Montes "south."

Zal Montes South is a two lobed mountain with a thin sectoin connecting the two.  Ridges cover the east facing slope of the mountain and steep cliffs line the western side.  The east slope is covered in long, N-S trending slopes.  A lava flow runs along the base of the northern part of the cliff and to a volcano southwest of the northern lobe.  This cliff may be associated with the mountains formation.  Red material streams from this cliff indicating that the mountain may still be forming.  The red material is believed to be S3 or S4.  If this is true, this means that mountain formation is a long process, not something that happens overnight.  A note: there is no red material coming from the south lobe.  This may mean that the north lobe is still forming while the south mountain is already finished forming or has stopped for the time being.

The latest image of the Zal Montes Plateau did not show the crater but it has yielded some new information.  First, the plateau is not the western boundary for Zal Patera.  Instead a fissurel binds the western edge of the Zal caldera.  Additionally, lava flows appear to stream away from this fissure.  This is similar to lava flows seen at the edges of calderas at Chaac and Pele.  Also, new lava appears to be covering old lava at Zal Patera.  Zal Montes Plateau also appears to have disrupted terrain in the Southeast part that may have been caused by a landslide.   The northwest side of this plateau appears to have been modified by sapping. Debris from the sapping process van be seen on the plains below the cliff.

For more information on Zal, go to my information page on I25 geology.

Shamshu Patera

Shamshu Patera was imaged at 342 m/pixel during I27.  This mosaic of two of the planned images shows Shamshu Patera to the right and Shamshu Mons to the left.

Shamshu Patera is a caldera with dark lava flows on its floor.  The caldera is heart shaped with a extrusion to the east and to the west.   The dark flows do not cover the entire caldera, only about 60% of it.  Dark pyroclastic material has been deposited to the northwest and southwest of the caldera.  Bright sulfur flows run in a semi-circle west of the caldera.  Shamshu Patera appears to have cut away from the mountain to its northeast.  This indicates that the volcano is younger than the mountain.  Shamshu Mons is an oval shaped plateau with a scarp to its northwest seperated by a ten-kilometer wide canyon and a thick, debris apron to its Southeast.  The canyon that seperates the scarp and the main mountain appears to have had material flow through it.  The debris apron was created when a massive landslide occured at Shamshu Mons.  The northwest face of Shamshu Mons appears scalloped due to erosion.

Two previously unknown mountains are seen northeast and south east of Shamshu Mons.   The mountains appear very rugged indicating they are young. Both mountains, though, do show signs of mass wasting.  Two possible craters can be seen at the southeastern mountain.

Telegonus Mensa

Telegonus Mensa was imaged at 350 m/pixel during I27.  The observation was designed to look at a sapping cliff beleived to be in this region.

In this image, bright white material thought to be SO2 frost surrounds the cliffs. These frost were formed when SO2 sapped from the cliff, becoming a gas, then condensing onto the surface as a frost.  This process is very dramatic due to the lower gravitation acceleration compared to the earth and Io's very low atmospheric pressure.   In fact, it is now believed that sapping may be the dominate erosional process on Io.

The mountain in the left part of the image is Telegonus Mensa.  This mountain exhibits numerous ridges parallel to its margins indicating that this mountain is quite old.