One technique scientists use to monitor Io's active volcanism and atmospheric processes is by observing the moon while it is in eclipse. Every Io day, the satellite passes into the shadow of Jupiter for a period of 2 hours and 20 minutes, plunging the entire moon into darkness. But as first faintly seen by the
Voyager 1 spacecraft when it flew by in 1979, Io glows in the dark. These glows come from a variety of sources on Io's surface and atmosphere, such as thermal emission from lava flows and lava lakes, and aurorae from the interaction between gases in the atmosphere and Jupiter's magnetic field. In this article, we will explore what scientists can learn from observing Io while in the darkness of Jupiter's giant shadow.
A few hours after its flyby of Io on March 5, 1979,
Voyager 1 observed the moon while it was in eclipse. It was a very long exposure, wide-angle-camera image designed to capture the night-side of Io against the background of stars at the end of the encounter. The image contained multiple exposures of the night-side with a camera slew between each exposure. It (FDS 16395.39, PICNO 0376J1 + 000) revealed a number of faint glows across Io.
Cook et al. in 1981 examined this image, and found that most of the faint glows, smeared as they were by the multiple exposures, were related to the plumes detected in
Voyager 1's daylight images of the satellite, including faint flows from Pele, Marduk, Amirani-Maui, Prometheus, and Volund. The poles of the moon were also faintly illuminated, with the north pole being a bit brighter than the southern one. No thermal emission from Io's volcanic hot spots were observed on this occasion because the vidicon sensors used by the
Voyager camera system could not detect photons with wavelengths longer than the orange portion of the visible spectrum. This is much too short to detect thermal emission from all but the hottest of volcanic eruptions. Cook
et al. concluded that the glowing gases were not the result of extremely hot plasmas, but were glowing due to being excited by electrons from Jupiter's magnetosphere. They suggest, however, that the polar glows are the result of gases emitted from numerous small vents, as opposed to plumes, which as we shall see from
Galileo's results, may not be the correct interpretation.
The
Galileo spacecraft arrived at Jupiter 16 years after the
Voyager encounters, and stayed to observe Io every few months until its eventual destruction in Jupiter's atmosphere in September 2003. On
Galileo's first orbit,
it imaged Io in eclipse on June 29, 1996. These first images were taken in the Solid State Imager's clear filter, just as
Voyager 1's eclipse observation was, but unlike
Voyager's camera, the SSI was sensitive to photons from the near-infrared portion of the spectrum (0.7-1.0 microns). This allowed the SSI camera to detect volcanic hot spots with temperatures greater than 700 K. Scientists found several visible hotspots in this first observation (Pele, Reiden, Marduk, Isum, Mulungu, Fo, and Zamama), providing a proof of concept for this method, which supplemented the spectral observations of the Near-Infrared Mapping Spectrometer (NIMS). The greater sensitivity to high-temperature volcanism, and finding it all across Io's surface also provided further proof that basaltic volcanism was wide-spread. In addition to these more discreet glows,
Galileo also observed fainter glows from volcanic plumes (such as Ra Patera), similar to the
Voyager observation. The limb of Io also glowed, not just the poles
Voyager had seen, though the south pole was brighter in this case (the north pole was brighter when Voyager observed Io in eclipse).
Galileo observed Io while the moon was in eclipse during most of its nominal mission orbits and on several occasions during the extended missions. These images allowed scientists to monitor high-temperature volcanic activity on the surface. A particularly
brilliant hot spot was seen at Pillan in June 1997, providing evidence for a massive volcanic eruption there, which would be confirmed by the presence of a volcanic plume in daylight images from that orbit and a massive surface change around the hotspot seen three months later. Eclipse images even showed how Pillan's hot spot had split by September and November 1997 due to lava flowing down into Pillan Patera from the plains above, thus creating a new source of thermal emission in the near-infrared. Such small-scale details were not visible to the NIMS instrument until
Galileo's encounters with Io later in the mission. Images taken in the clear filter as well as SSI's near-infrared filters, such as the one at 1.0 micron, provided a way to crudely measure the blackbody temperature of Io's high-temperature volcanoes. This was accomplished by calculating the ratio between the clear filter radiance of a hot spot and its radiance in the one micron filter.
Galileo's observations of Io in eclipse also allowed scientists to better understand the faint glows from the plumes and atmosphere. These glows are the result of interactions between Jupiter's magnetosphere and the atmosphere of Io. On two of Io's orbits, G8 and E15,
Galileo scientists imaged Io in color to better understand the types of emissions that make up the aurorae on Io. The best of this data set, from May 1998, is shown at left. They reveal glowing aurorae of various color across different parts of Io's surface. These can be categorized as bright blue, equatorial glows, red polar limb glows, and fainter green, anti-Jupiter hemisphere glows. When an excited atom or molecule returns to the ground state, it sends out a photon with a specific energy. This energy depends on the type of atom and on the level of excitement, and we perceive the energy of a photon as color as the energy relates to a specific wavelength on the visible spectrum. Just as red aurora on Earth are caused by atomic oxygen, the red limb auroral glows on Io are thought to be due to emission from neutral atomic oxygen at 630 and 636 nm. The oxygen in Io does not derive from organisms on the surface, but from the break-up by solar ultraviolet light of sulfur dioxide in a process called photolysis:
SO2 + hν → SO + O.
The green glows are thought to caused by excitation of neutral atomic sodium, which releases photons in the sodium D lines at 589 and 590 nm, within the bandwidth of the green filter. Atomic sodium is another photolysis product, this time deriving from sodium chloride belched from Io's volcanoes. The bright blue glows along the limb near Io's equator are thought to be due to the excitation of molecular sulfur dioxide. Most of these glows are related to various plumes visible near Io's limb at the time of the observation, including Amirani, Acala, Prometheus, Zamama, and Culann. The identification of these chemical species are supported by ground-based and Hubble observations of Io's aurorae, which revealed the presence and absence of various atomic and molecular emission lines, including sulfur and oxygen lines in the ultraviolet at 190 and 135.6 nm, respectively.
Geissler et al. (2001) explored much of the available
Galileo and
Cassini eclipse image dataset and reported on the how the intensity and position of these auroral emissions change with time. For example, across the
Galileo dataset, the red limb emissions were brightest over either Io's north or south pole. The bright equatorial glows, visible as blue glows in the E15 dataset, tend to be located either at plumes or near the sub- and anti-Jovian points on Io's surface. In the latter case, these glows at various times either appear north of the sub-Jovian point and south of the anti-Jovian point, or vice versa. Both temporal variations in the red limb glows and the blue equatorial glows are related to Io's position in the Jovian magnetosphere. Jupiter's magnetic field is tilted with respect to the orbital plane of its main satellites by 9 degrees, so at various times as Io's orbits the planet, the moon is either above or below the equatorial plane of Jupiter's magnetic field. At a system III longitude (λ
III) of 0°, Io is below the normal plane of the magnetic field (this occurs with λ
III between 290° and 110°). The ion flux from the plasma torus is greatest over the north polar region of Io, causing greater excitation and increasing the brightness of Io's red auroral glows over the north pole. Jupiter's magnetic field lines are also tilted with respect to Io's equatorial plane, so they are tangent to surface north of the equator near Io's sub-Jovian point, and south of the equator near the moon's anti-Jovian point. When the λ
III longitude is between 110° and 290°, Io's situation is reversed as it is now above the plasma torus. This time, the south pole limb emissions are brighter than the north's, and the equatorial glows (that aren't related to volcanic plumes) have switched hemisphere as they are now bright south of the sub-Jovian point and north of the anti-Jovian point. This cycle recurs every 12.95 hours, the
synodic period between Io's orbital period and the rotational period of Jupiter's magnetic field.
A word should be said about one of the mysteries regarding
Galileo and
New Horizons eclipse observations of Io: scattered around the sub-Jovian and anti-Jovian points on the satellite are a great number of glowing spots. These spots appear similar to hot spots in size and intensity, but their concentration is much greater than the rest of the satellite. Each of these spots can be correlated with a volcanic feature in Voyager images of this region. Other infrared observations of the satellite suggest that thermal emission is not higher in these regions compared to the rest of Io, so the spots are probably not thermal emission from high-temperature lava. As you can see in the image at left, the glows seem to be concentrated north of the equator near the sub-Jovian point (the white lines in the image are the equator [horizontal line] and the prime meridian [curved vertical line]), consistent with the tangent point of the Jovian magnetic field lines at the time of this observation. From this one could surmise that these spots are the result of glowing gases rather than thermal emission. But what would concentrate these gases into discreet spots that "look" like hot spots? One theory that I favor suggests that these spots are related to gases being slowly leaked from Io's interior at cooler volcanoes. Not enough gas is released at these volcanoes to form plumes, but there is enough of sulfur dioxide that their auroral emissions are brighter than the surrounding regions. They show up near the sub- and anti-Jovian points and not in other regions, not because there are more volcanoes there, but because Jupiter's magnetic field lines, and the electrons that are transported along them, connect to Io in these two regions. This increased electron flux increases the brightness of the auroral emissions.
Future missions to Io will certainly use this technique to monitor changes in Io's volcanic activity and atmospheric emissions. Narrow-band filters on future cameras would allow researchers to focus on specific line emissions from various chemical species such as atomic oxygen, sulfur, and sodium. They would also allow for improved measurements of hot spot temperatures. With increased bandwidth, more images using different filters can be taken during each eclipse, allowing for temporal variability of auroral and volcanic thermal emission to be monitored. Finally, these measurements will allow researchers to determine the contribution outgassing from smaller volcanoes provides to Io's atmosphere. Previously modeling focused on the contribution a few volcanic plumes provide, but not weaker outgassing from cooler, but more numerous, volcanoes.
References:
Cook, A. F.;
et al. (1981). "
Volcanic Origin of the Eruptive Plumes on Io".
Science 211 (4489): 1,419–1,422.
McEwen, A. S.;
et al. (1997). "
High-temperature hot spots on Io as seen by the Galileo solid state imaging (SSI) experiment".
Geophysical Research Letters 24 (20): 2443–2446.
McEwen, A. S.;
et al. (1998). "
High-Temperature Silicate Volcanism on Jupiter's Moon Io".
Science 280 (5373): 87–90.
Geissler, P.;
et al. (1999). "
Galileo Imaging of Atmospheric Emissions from Io".
Science 285 (5429): 870–874.
Geissler, P.;
et al. (2001). "
Morphology and time variability of Io's visible aurora".
Journal of Geophysical Research 106 (A11): 26,137–26,146.
The Galileo spacecraft arrived at Jupiter 16 years after the Voyager encounters, and stayed to observe Io every few months until its eventual destruction in Jupiter's atmosphere in September 2003. On Galileo's first orbit, it imaged Io in eclipse on June 29, 1996. These first images were taken in the Solid State Imager's clear filter, just as Voyager 1's eclipse observation was, but unlike Voyager's camera, the SSI was sensitive to photons from the near-infrared portion of the spectrum (0.7-1.0 microns). This allowed the SSI camera to detect volcanic hot spots with temperatures greater than 700 K. Scientists found several visible hotspots in this first observation (Pele, Reiden, Marduk, Isum, Mulungu, Fo, and Zamama), providing a proof of concept for this method, which supplemented the spectral observations of the Near-Infrared Mapping Spectrometer (NIMS). The greater sensitivity to high-temperature volcanism, and finding it all across Io's surface also provided further proof that basaltic volcanism was wide-spread. In addition to these more discreet glows, Galileo also observed fainter glows from volcanic plumes (such as Ra Patera), similar to the Voyager observation. The limb of Io also glowed, not just the poles Voyager had seen, though the south pole was brighter in this case (the north pole was brighter when Voyager observed Io in eclipse).
Today is the 31st Anniversary of the Voyager 1 spacecraft flyby of Io. Last year, I wrote a retrospective series on this encounter, the images acquired, and the discovery of active volcanism on Io:
Thirty years ago today, on July 9, 1979, the Voyager 2 spacecraft encountered Jupiter from a distance of 650,000 kilometers about its cloudtops, marking the second Voyager project flyby of the planet. The encounter provided an opportunity to see the anti-Jupiter hemispheres of Ganymede and Callisto, to monitor changes on Jupiter and Io since the Voyager 1 encounter four months earlier, and to observe Europa up close for the first time.
For Io, the second Voyager encounter did not provide the same revolution in our understanding of that world; the clear star of the July 9 encounter was the cracked world of Europa. While Voyager 1 flew within 20,000 kilometers of Io on March 5, 1979, Voyager 2's trajectory kept the spacecraft outside Europa's orbit and, with Io on the other side of Jupiter during closest approach, Voyager 2 never came close than 1,128,000 kilometers of Io. However, the discovery of active volcanism on Io by Voyager 1 necessitated a change in the schedule of observations for the second encounter, including a 8-hour long sequence of images of a narrowing crescent Io as Voyager 2 receded from Jupiter. This prolonged observation sequence allows Voyager 2 to monitor volcanic plumes along Io's limb, including those at Amirani, Maui, and Loki. From observations such as these, Voyager 2 found that most of the plumes first observed by Voyager 1 were still active during the July 1979 encounter. Only Pele appeared to have shut down, though later observations by Hubble, Galileo, and other spacecraft seem to suggest that the Pele plume is intermittently active. Volund was not observed near the limb of Io during the Voyager 2 encounter, so it could not be determined if that plume was still active.
Earlier, Voyager 2 had observed Io while it was more illuminated by the Sun, like the image at left. The goal of these observation was to search for surface changes on Io as the result of volcanic activity between the two Voyager encounters. Among the changes observed were two new plume deposits surrounding Surt (probably active during a short, intense eruption in June 1979, but Surt was not viewed near the limb by Voyager 2 to see if it was still active) and Aten Patera. The presence of these changes from large plumes but without the observation of the plumes associated with them have led to the conclusion that large, Pele-type plumes tend to be short-lived compared to more persistent, smaller dust plumes like at Prometheus. Additional surface changes included a larger dark area within Loki Patera, the result of overturning of more surface area of the lava lake that covers much of Loki, and a change in the shape of the Pele plume deposit, from a heart shape to an oval. Subtle changes in the shape and intensity of the Pele plume deposit were also observed by Galileo in the late 1990s and early 2000s.
Of course, Io wasn't the only world Voyager 2 observed during its encounter 30 years ago. The star of the show was Europa. Europa was poorly observed during the previous Voyager encounter so this one really provided a great leap in our understanding of this icy world. Like Ganymede, Europa's surface was dominated by tectonic structures, ridges and dark, linear bands that criss-cross the surface. Unlike Ganymede, Europa's surface was found to be quite young with very few impact craters, though enough to show that the satellite's surface was much older than Io's. Spectral measurements suggesting a water ice surface, mass estimates between that of Io and Ganymede, and youth surface age soon led to the suggest that just beneath Europa's ice shell lay a liquid water ocean that today is a main focal point for exploration in the Jupiter system.
Over the last few days, we've been taking a look back at the Voyager 1 encounter with Jupiter and its volcanic moon, Io, that took place 30 years ago on March 5, 1979. For those of you who are just joining us, here are some quick links to those posts:
In one model, all the volcanism on Io could be explained with sulfur. In this model, Io was overlain with a thick layer of sulfur and sulfur dioxide. The top layer of this sulfur would be solid, but it liquefied the further down you went, until you had a nearly global sulfur ocean underlying a solid sulfur crust. Imagine Europa with its water ice shell and water ocean underneath, but with sulfur instead of water. The multi-colored flows Voyager observed were explained as being the result of sulfur and its various allotropes flowing out across the surface; the dark volcanic pits were filled with boiling, pitch black sulfur. Underlying this sulfur shell was the silicate lithosphere. This lithosphere had to be active itself, as the mountains on Io had to consist primarily of silicates in order to sustain the heights seen by Voyager. In these cases, the mountains consist of parts of the silicate crust poking up above the sulfur shell. Silicate volcanism was thought possible beneath this shell, as silicate magma reaches up to the level of the sulfur-silicate interface before forming a sill and heating the sulfur above it. This model of primarily sulfur volcanism was supported not only by Io's multi-colored surface, but also by the temperatures seen at Io's volcanoes by the IRIS instrument on Voyager.
In the other model of Io's interior and volcanism, Io's crust consisted primarily of silicate rock with only a thin veneer of sulfur and sulfur dioxide. In this case, Io's lavas consisted of silicate rock, like volcanism on Earth (with some exceptions), with a sulfur enrichment. At the time of the Voyager flybys, this model was supported by the topographic structures visible on Io, which were not thought to be supportable by a crust that consisted primarily of sulfur. However, the thermal data to back it up didn't really come until 1986 when an eruption on Io's leading hemisphere was observed from Earth, and the blackbody radiation temperatures of the eruption were too high to be explained by sulfur. Other eruptions, including an eruption at Surt in June 1979, also had temperatures too high to be explained by sulfur. The lower temperatures seen by Voyager could be explained by the fact the IRIS instrument's wavelength band-passes were in the mid-infrared, wavelengths too long to sense the high-temperatures consistent with silicate volcanism. The discoveries made by Galileo in the 1990s and early 2000s seemed to put the nail in the coffin for the sulfur volcanism model as high-temperature volcanism (> 1000 K) was observed at many active volcanoes on Io and absorption bands consistent with orthopyroxene were found in some of Io's pyroclastic deposits.
The current consensus view of Io's volcanism, interior, and geology is in many ways seems like a merging of these two models. This consensus view was published in 2004 by Keszthelyi et al.. In this model, Io's crust consists primarily of silicate rock. As depth decreases, or as your approach the surface from Io's interior, the amount of sulfur in the lithosphere increases. This sulfur and sulfur dioxide is constantly being brought back up the surface, or recycled, by silicate volcanism. However, as silicates rise through the lithosphere from deeper magma reservoirs, they stall out as they become neutrally buoyant and form sills. These sills heat up the sulfur near it, which then melt, forming a depression on the surface and leading to sulfur volcanism (and some silicate volcanism as well). Over time, this depression deepens, allowing more silicates to reach the surface until the sill is unroofed and the bottom of the depression, or patera, becomes a silicate lava lake. I posted a more detailed explanation of this process last week.
Today I got finished processing three additional mosaics from Voyager 1 images. The first two use narrow-angle camera images acquired shortly before the encounter, and the other uses wide-angle camera images during the spacecraft's closest approach to Io on March 5, 1979.
The second mosaic uses images that were part of a high-resolution mosaic design over the south polar region of Io. Click here to see this ISS-NA mosaic at full-resolution The terrain here is much closer to the terminator, so more topographic features are clearly visible. For example, several tall mountains are visible along the terminator, the line between day and night, including Haemus Montes at bottom left. Some of the low plateaus visible, such as Echo Mensa (the rounded rectangular mesa at center bottom) and the western part of Nemea Planum (to the northeast of Echo) appear eroded, potentially from sapping of Sulfur dioxide, though no evidence of recent sapping, like bright material along the base of the low cliffs at these features, is visible. Such evidence can be more clearly seen at the eastern end of Nemea and in a fracture to the west of Echo.
The final mosaic consists of two, violet-filter wide-angle camera images acquired during Voyager 1's closest approach to Io. Click here to see this ISS-WA mosaic at full-resolution. These two images show similar terrain to the other mosaics I've posted here in the last few days, but what makes these images exciting to me is what you can see along the limb on the left side of the mosaic. This region shows feature that I had thought had only been seen at much lower resolution. These images have a pixel scale of 2 km/pixel, whereas the previous best I could find has a resolution of around 9.5 km/pixel. So obviously, this appears to be a significant improvement. Among the features visible is the Masubi flow field (at lower left, also see the cropped image at the top of this post), a semi-persistent volcanic plume source. This feature is notorious for having plumes that jump around along the length of the flow field as well as having plume with multiple source vents (such as during the Voyager 1 and New Horizons encounters). In this mosaic, you can see a plume deposit around the main vent, a V-shaped flow field (not quite sure where the source patera, but I have an idea). There appears to be a disconnect between the flow to the south and the main V-shaped field, not sure how real that is, or if the flow connecting the two was quite narrow, akin to the connection between the Amirani and Maui flows fields. These WACs seem to close the notorious gap between the Voyager and Galileo high-resolution coverage that existed between 40 and 60 West, at least south of the equator, which I am excited to see.
Imagine you were a scientist on the Voyager Imaging Team 30 years ago today. Only days before, Voyager 1 had passed through the Jovian system, returning thousands of images of Jupiter and its moons. The Galilean moons Voyager revealed had strange and fascinating vistas: cratered Callisto, grooved Ganymede, streaked Europa. Perhaps the strangest of these four worlds was Io. It was clear from the images that were coming down in the days following Voyager's close pass that Io was a world that was constant renewing its surface. Instead of impact craters that were prevalent on most worlds, Io had none and was covered in apparent volcanic features. Given the lack of impact features on the surface, Io was likely even active today.
The discovery of active volcanism on Io was announced at a press conference on Monday, March 12, 1979. By then more plumes had been found as earlier images of Io were re-examined and areas of high thermal emission were being found by Voyager's Infrared Interferometer Spectrometer (IRIS) . For example, in the mosaic I posted earlier today (and if you haven't checked that out, I couldn't recommend to do so more!), two plumes are visible along the limb: Masubi and Pele. I've created a special version showing these plumes in the mosaic. Masubi is at lower left and Pele is at right. These two plumes show the two main types of plumes on Io. Masubi is a smaller, dust plume, also known as a Prometheus-type plume. It has an umbrella-like shape, with a dense central column (in this case TWO central columns) and a bright shock canopy. Pele is the archtype of the Pele-type or gas plumes. In this case, there is no central, dense, eruption column and the shock canopy has a filamentary structure.
It only took a year but I finally finished the huge, southern hemisphere mosaic of Io using images acquired by Voyager 1 in March 1979.
I found a copy online of the NASA publication, "Voyage to Jupiter," by David Morrison and Jane Samz. This publication provides some great insights into the Voyager encounters with Jupiter in 1979, the science investigations the two spacecraft performed and the new knowledge they obtained. The document also has a day-by-day account of the activities of the spacecraft and the science and engineering teams for both Voyager encounters. Below is an excerpt covering the Io encounter by Voyager 1, 30 years ago today. Don't forget to check out the post immediately below this one for some of my own thoughts and a nice animation of the encounter with Io. Later this week, I will post more about the discovery of active volcanism on Io, which occurred a few days after the flyby, and on how scientists viewed Io and its geology following the encounter.
30 years ago today, the Voyager 1 spacecraft encountered the first planet in the Voyager programs decade-long epoch tour of the outer solar system. While Pioneer 10 and 11 were the first space probes to reach Jupiter, proving passage through the asteroid belt was possible, Voyager 1 opened up the worlds of Jupiter system to humanity, turning these moons from mere points of light into fully-realized worlds.
Voyager 1's highest-resolution images covered the southern pro-jovian hemisphere of Io, an area not seen again at high resolution (Galileo's best images of the region were on the order of 10-20 km/pixel). You can see my attempt to piece together Voyager 1's highest resolution mosaic, though I can't seem to find where I put the original project files... These images revealed numerous large, volcanic pits, low scarps that cross Io's generally flat terrain, and large mountains. But not a single impact crater, unlike any other solid-body world in the solar system imaged to that point.
Voyager 1 provided a wealth of information about not just Io, but the other moons in the Jovian system as well as Jupiter itself. Images revealed three additional small moons of Jupiter (later named Metis, Adrastea, and Thebe) orbiting near Amalthea, as well as narrow ring of material confirming a prediction made by Acuña and Ness 1976 based on Pioneer charged particle data. Voyager 1's cameras also revealed in detail the cratered and grooved surface of Ganymede and the ancient surface of Callisto. The spacecraft never came very close to Europa and could only hint at the amazing surface features seen by Voyager 2 four months later and Galileo 16 years later. Voyager imaging scientists did note a lack of obvious impact features as well as dark, linear streaks criss-crossing Europa's surface.
After this week's Boösaule Montes post, I thought it would be good to do a more general mountain post. I actually think I will write up a few such posts. This first one will discuss common features seen among Io's more than 150 positive-relief features. This post is based on discussion of this topic in "The mountains of Io: Global and geological perspectives from Voyager and Galileo" by Paul Schenk, Henrik Hargitai, Ronda Wilson, Alfred McEwen, and Peter Thomas. This paper was published in the Journal of Geophysical Research in December 2001.
A third form of degradation is sapping. Sapping a geologic process by which volatiles exit a slope, usually as a groundwater spring on Earth. On Io, because there is no air pressure to speak of, this process is much more explosive. Sapping is thought to create the spur-and-gully morphology seen on many Ionian plateaus, as seen at Tvashtar Mensae at left. While the sulfur dioxide sublimates away during the explosion, more solid materials is transported downslope, forming deposits a couple hundred meters thick. Again, you can see the leading edge of this deposit at Tvashtar along the bottom right side of the image. At some mountains, sulfur dioxide is released without apparent erosion but instead leaves a bright deposit of frost. This is particularly common at mountains in Io's polar regions, like Haemus Montes, but has been seen in action closer to the equator, such as at Pillan Mons.
Certain structural features are commonly seen on Io's mountains. For example, striations, representing either pre-existing layers in Io's crust or fault lines from imbricate-style faulting produced during the mountain's uplift, are seen at several mountains such Haemus Montes. Another common feature is central rifting. In this case, the mountain appears as if it has been split apart with a canyon running down the long axis of the feature. This was seen at Boösaule Montes "North" in the earlier post but can also be seen at other mountains such as Pan Mensa and Danube Planum.
A final common trait among Ionian mountains is their proximity to volcanic depressions, known as paterae, despite not being volcanoes themselves (with a few rare exceptions). An example of this is Gish Bar Mons, whose southern flank is also the northern rim of Gish Bar Patera. According to Schenk et al. 2001, 30% of mountains on Io have flanks within 5 km of paterae. 10% of paterae, according to Radebaugh et al. 2001, lie next mountains. One reason for this correlation may be that magma exploit weaknesses in the crust, including faults associated with mountain building. This process then leads to paterae formation, which if Keszthelyi et al. 2004 is correct, would involve the formation of volcanic intrusions in the upper layers of the crust which then drive off volatiles in the crust above the intrusion and melt the rest, until a depression is formed down to the level of the sill.
Welcome to The Gish Bar Times, the only blog dedicated to Jupiter's volcanic moon, Io!
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