High-Resolution Mystery in Bulicame Regio on Io

On February 22, 2000, Galileo flew close to Io for its 4th targeted flyby of the satellite with a altitude of 198 kilometers (a flyby known as I27).  Right at closest approach, Galileo turned its suite of remote sensing instruments at the southeastern edge of a low plateau east of the active volcano Isum Patera, on the southern end of Bulicame Regio.  The camera was pointed significantly off nadir and was instead looking much closer to the local horizon at an emission angle (an angular measure of how far a point on the surface is from nadir: looking straight down is 0° and looking at the horizon is 90°) was   The observation, 25ISSAPPNG01, was designed to look for evidence of sapping and other slope degradation processes on the margin of this plateau.  Sapping, a geologic process by which sulfur dioxide in gas or liquid form escapes from a mountain slope, undermining the slopes strength and causing it to downslope movement.  Four images were planned for this observation and three were eventually played back, though only 2/3rds of the lines of each frame (525 out of 800) were played back due to the limited amount of downlink available since the camera and spacecraft worked perfectly during this encounter, in contrast to the camera and spectrometer anomalies experienced during I24 and the spacecraft safing event on I25 that led to loss of data.  However, in the 10 years since these images were sent back from Galileo, they have become a bit of a mystery for planetary scientists, as interpreting the geology of the terrain seen in the images of 25ISSAPPNG01 has been difficult.

I, for the bazillionth time I swear, have processed the data into a mosaic that you can see above, though you can check out a full-resolution version here.  Like my version of the Chaac Patera mosaic taken a few minutes later, the images have been reprojected into a point-perspective map projection.  This approximates the view Galileo had at the time.  North is just to the left of up in this mosaic, as it has been rotated so north is up.  The perspective is from the point when the middle image was acquired (though I had to adjust the center latitude and longitude to eliminate distortions in the middle image, compared to the sub-spacecraft point reported in the PDS).  Since Galileo was moving rapidly over Io's surface during the observation, the first and third images are a bit distorted compared to the original data in order to match the perspective of the second, with the first images becoming stretched and the third becoming squished.  However, the distortion is not so bad that we can't examine the features seen in the images.

The first problem we face however in interpreting the surface features seen in this mosaic is that we lack proper context.  The images in this mosaic have an effective resolution of approximately 11.6 meters per pixel when you take into account that we are looking toward the horizon rather than straight down at Io.  This more than two orders of a magnitude better than the next best data over this area, shown at left.  This is a clear filter mosaic from October 1999, blown up about three times its original size.  With such a disparity in resolution between the two observations, it becomes difficult to correlate structures seen in both since each of the three I27 frames are 6 pixels across in the I24 data.  The other complication this disparity presents is that we can't tell exactly where the I27 images are in the I24 data.  The pointing was more than a entire SSI field-of-view off from the planned pointing.  From there we can only correct the pointing relative to each other, rather than to any basemap since the next best data doesn't even show the scale of features we seen in the I27 images.

Basically, we don't know how the features relate to the regional geologic context.  We know some basics.  We know that the region we are looking at is a few kilometers southeast of the base of a scarp that bounds a low mesa.  In lower resolution images, this area appears to be a patch work of bright and dark material on the edge of the scarp, perhaps the result of sulfur dioxide-modified lava flows.  At that scale, this imaged area was mapped as bright flows by Williams et al. 2010.  Finally, we also know that there has been recent volcanic activity in this area.

Let's take a look at the features we can observe in this mosaic.   The most prominent feature is a 400-meter-tall mesa at the bottom of the middle frame.  There doesn't appear to be much evidence for layering along the scarp face of this mesa, except for  a 50-meter-thick cap rock.  Surrounding the mesa is a low plain marked by swirls of bright and dark terrain.  Beyond that is region of smoother dark terrain that is marked by large boulders and narrow channels.  Finally, beyond that in the third frame, there is a region of bright and dark layered terrain.

This is certainly quite an enigmatic landscape.  One interpretation provided by Moore et al. 2001 suggests that the mesa was once much larger, but over time sapping has undermined the edge of the mesa, causing it to shrink.  The mottled terrain that surrounds the mesa, filled with bright and dark swirls maybe the area the mesa retreated from.  However, note that this is not the main plateau, which is to the north west of the area covered in these images, and that the mesa seen in the high-res data was not seen in the lower resolution images, suggesting that it isn't much bigger than the what we see here.  I don't think this necessarily changes the story.  On Earth, headward sapping erosion of plateaus have been known to cause portions to be cut off, forming small mesas.  The bright and dark swirls may result from sulfur dioxide filling low areas as it flowed out from the base of the cliff and from fumaroles with the adjacent dark terrain (seen in the middle frame).  Like the water in the gullies on Mars, the sulfur dioxide is boiling as it flows across Io's surface, either escaping to the atmosphere as a gas or freezing as a bright frost in low regions.  The bright/dark mottled terrain appears to follow the shape of the mesa, further suggesting that it is the result of scarp retreat.  As for where the material that made up this scarp went, it is thought that the upper 1-2 kilometers of Io's crust is made up largely of sulfur and sulfur dioxide with cooled silicate existing as layers in the near-surface.  This scarp, and others like it, may then retreat more akin to the polar ice caps on Mars than mesas on Earth, leaving behind only a trace amount of talus near the slope.

Alternatively, the features seen here could be explained by volcanic processes.  After all, this area was mapped as a bright flow from global-scale images.  One of the reasons I wanted to write up this article is that the bright/dark patchwork landscape bears a resemblance to other sulfur-modified lava flow fields seen on the floor of Chaac Patera and to the southeast of Emakong Patera, the latter of which I discussed a couple of weeks ago.  This mottled terrain result again from a similar process as I discussed above: fumaroles on the surface of a cooling lava flow bring up liquid sulfur dioxide where it then freezes in low areas of the flow surface.  However, there don't seem to be any indications of flow lobes in this areas, which you would expect to see if this were once a compound flow field.  This terrain also hugs the base of the mesa, which would you would not normally see since the base of the cliff should be higher topographically that the surrounding plains.  However, the irregular pattern of the mottled terrain at the base of the mesa and the fracture that runs across the middle frame could be the result of the interaction between hot silicate lava and volatile sulfur dioxide.

In February 2000, Galileo imaged a very enigmatic landscape at high resolution.  The terrain seen in these images has been difficult to interpret due to the lack of proper context imaging of this region.  The difficulty in interpreting this data forced a change in the imaging strategy for later flybys as super high-resolution observations were dropped and other sequences with resolutions similar to this mosaic were accompanied by lower-resolution images taken at later in the encounters to help interpret small-scale features.  However, this lack of regional context doesn't make the features seen here any more interesting or make us less capable to at least speculate as to what caused this mess, whether it was erosion from sulfur dioxide sapping, volcanic lava flows, or fumaroles leaving behind frozen sulfur dioxide.

References:
Turtle, E. P.; et al. (2001). "Mountains on Io: High-resolution Galileo observations, initial interpretations, and formation models". Journal of Geophysical Research 106 (E12): 33175–33199.
Moore, J.; et al. (2001). "Landform degradation and slope processes on Io: The Galileo view". Journal of Geophysical Research 106 (E12): 33223–33240.