Honey, I Shrunk the Moon!

Today at a press conference at NASA HQ in Washington, DC, scientists from the Lunar Reconnaissance Orbiter Camera (LROC) team announced their discovery of lobate thrust fault scarps on the surface of the Moon that indicate that not only has the Moon's radius shrunk by 100 meters, but it has done so in geologically recent times.  High-resolution images of these scarps show that they cross-cut small impact craters, which would normally be removed over hundred-million-year timescales by thermal cycling of the soil by the moon's month-long day, solid body tides from its gravitational interaction with the Earth, and micrometeorite impact gardening.

What would cause such shrinking?  When the moon was formed out of the impact of a large planetoid into the infant Earth, it initially was molten.  Over time the crust and core solidified, but its mantle still contained a large amount of molten silicate magma.  Magma from this "magma ocean" on occasion reached the Moon's surface, filling many of its large impact basins, creating the dark mare basalt regions.  Over time, the Moon's mantle cooled and solidified as a result of reduced heating from radioactive elements, conduction through the Moon's crust, and the removal of hot magma through volcanism.  Now as you may remember, for most materials, the phase change from a liquid to a solid causes a change in the density of the material.  For silicate magma, the density increases, so for a given amount amount of magma, the volume will decrease when it solidifies.  When this occurs on a planetary scale, such as a the Moon or Mercury, as the interior of the planet cools and solidifies, the planet or moon shrinks.

When this shrinking occurs, the crust of the planet must accommodate it.  After all, when the volume decreases, the surface area also decreases.  This causes compression within the crust which can be taken up by compacting loose surface material or reducing pore space, or through thrust faulting.  Thrust faults are low-angle faults that allow the surface above the fault to be pushed up on top of the rock beneath it. The surface expression of thrust fault on the Moon or Mercury is a arcuate scarp.  Examples of these scarps are shown above from the Moon (shown on the left side of the side-by-side image above from LROC data) and from Mercury (shown on the right).  The lunar scarps are smaller than their Mercurian cousins, rising only 100 meters above the surrounding terrain at most, with the majority only rising a few tens of meters.  On Mercury, like Beagle Rupes shown above cutting across the oblong Sveinsdóttir crater, these scarps are much than those found on the Moon, often rising a kilometer or above the surrounding terrain.  Their height, length, and estimated horizontal displacement suggest that Mercury may have contracted by several kilometers.  MESSENGER images will be used to date these scarps to determine when this shrinkage occurred just as the LROC group has done.

The situation is a bit different for icy bodies.  Unlike silicates, when water solidifies its density decreases and volume increases when it freezes.  So the radius of icy bodies increases rather than shrinks like rocky worlds like the Moon and Mercury.  The equivalent feature to the lobate scarps of the rocky bodies formed from this expansion are the narrow extensional fractures that criss-cross ancient worlds like Tethys, Dione, and Rhea.  The image at left, taken last Friday by Cassini, shows a pair of narrow fractures that cut across Penelope, an ancient impact basin on Saturn's moon Tethys. These fractures suggest that these worlds have expanded as a result of the solidification of water in their interior more recently than the heavy bombardment that produced the plethora of impact crater visible in scenes like the one at right.  Like the work being performed at the Moon and Mercury, finding and mapping these fractures and measuring the amount of displacement they accommodated will tells us how much these moons have expanded (and thus how much of their interiors remained molten until comparatively recently) and approximately when, based on superposition relationships with impact craters.

Now this wouldn't be an Io-centric blog without pointing out that the lobate thrust fault scarps seen on the Moon and Mercury are puny when compared to equivalent features on Io.  Now, obviously, unlike those two geologically dead(ish) worlds, Io is much more active and its mantle still has a large melt fraction.  Its volcanoes dredge up enough material every year that it could cover the entire surface of Io in a layer one centimeter thick (though the amount of resurface varies greatly from place-to-place).  These centimeters add up, and after a million years, the surface of Io is covered in approximately 10 kilometers (6.2 miles) of cooled lava, sulfur, and sulfur dioxide (though some of this material is locally recycled, so 10 km maybe an upper limit).  This has the effect, for the present-day surface, of shrinking the radius of Io by 10 kilometers in one million years.  As a result of global subsidence, thrust faults drive up massive "lobate scarps", also recognized as the mountains of Io.  Two clear examples of this are shown at left, Gish Bar Mons and Euboea Montes.  Both mountains reach 8-10 kilometers above Io's surrounding plains.  Of course, this may be an over-simplified model for how Io's mountains form, so I would recommend reading my article on various formation models for more information on the details.

This study of lunar lobate scarps highlights the need for global high-resolution studies, whether we are talking about the Moon, Mars, Mercury, or Io, as LROC images allowed researchers to assess the global population of these cliffs. Once their true distribution was determined, they could then come up with a proper model of how they formed. This provided an important glimpse to the Moon's recent geologic activity, after the formation of the mare basalts.

Link: NASA's LRO Reveals 'Incredible Shrinking Moon' [www.nasa.gov]

Io Volcano of the Week: Hi'iaka

This month for my Io Volcano of the Week series, we are looking at volcanoes that were observed at moderate resolution (160-280 meters or 525-920 feet per pixel) during Galileo's I25 flyby of Io on November 26, 1999.  In the last couple of weeks we have looked at Zal and Emakong, two large paterae - volcanic depressions - on Io's leading hemisphere.  This week we turn our focus a few hundred kilometers to the east of Emakong at the strangely shaped lava flow field, Hi'iaka Patera.  It maybe pretty quiet as far as Ionian volcanoes go, but it may have had a wild past with a violent formation from the breakup of two massive mountains and lava flows that formed in the 17 years between Voyager and Galileo.

Let's get some of the basics out of the way first.  Hi'iaka Patera is located near Io's equator on the moon's leading hemisphere (3.64° South, 79.47° West) and is 128 kilometers (80 miles) wide.  The oddly shaped volcano is bounded by a set of low faults that confine a lava flow field on the eastern side of the volcanic depression.  The western edge of Hi'iaka is bounded by a 3.5-kilometer (2.2-mile) tall massif named North Hi'iaka Montes.  The southern margin of the volcano is less than 10 kilometers (6.2 miles) from the northern tip of another mountain, South Hi'iaka Montes.

Images of Hi'iaka taken during that Thanksgiving 1999 flyby were particularly useful for determining how the volcano may have formed.  Observations of the spatial relationships between these two mountains and the volcano, the comparable heights for the two mountains (save the tall peak on the northeast tip of North Hi'iaka Montes) led to the suggestion that the two mountains were once joined.  Subsequent extension and strike-slip tectonism then rifted these two mountains.  In this scenario, Hi'iaka Patera may be a pull-apart basin, which forms when there is a bend or gap in a strike-slip fault system, creating extension at the bend.  The image at left shows a schematic of these basins are formed.  Depending on the speed of the rifting, lava can exploit the faults along the margin of this basin to reach the surface and cover portions of the depression that is formed between the two transform faults [this scenario was further explained back in December 2009 and February 2009 when I  discussed work performed by Melissa Bunte and her co-authors].  Lava using extensional faults on Earth certainly isn't unheard of.  The East African Rift Valley has a number of prominent active or dormant volcanoes (*cough* Kilimanjaro), and fissure eruption are not unheard of in the last few million years.  The active lava lake, Erta'ale, is located in the Afar triangle and the triple junction between the Red Sea, the Gulf of Aden, and the East African Rift Valley. The motions of Ionian micro-plates like that suggested for Hi'iaka Patera may have resulted in the formation of several volcanoes including Zal, Monan, and Shamshu, though additional information on Io's sub-surface will be needed to pin down this theory, since later deposition of volcanic ash and sulfur has obscured most surface expressions of tectonic faults on Io's surface.

In term of recent activity, Hi'iaka is a relatively quiescent.  While there are dark lava flows with different albedos (and presumably ages) covering much of the eastern half of the depression, no high-temperature eruptions have been observed at the site.  Galileo's Near-Infrared Spectrometer observed thermal emission from the volcano on six occasions during the Galileo Nominal Mission, but it was never seen by camera system during an eclipse, even though the nearby volcano, Tawhaki Patera, was.  This suggested that low levels of effusive activity were present at Hi'iaka during the Galileo mission, which is consistent with the morphology of the dark lavas seen in the high-resolution Galileo images.  Bunte et al. 2010 suggested that Hi'iaka Patera might be a lava lake, or in the process of forming one.  At present, however, the morphology of the dark lava at Hi'iaka, with its multi-lobed structure and suggestions of a series of overlapping flows, is more consistent with an inflated flow field that is built up by a series of thin lava breakouts.  Similar eruptions, though more vigorous, are seen across Io at volcanoes like Zamama, Prometheus, Marduk, and Amirani.

That's not to say that more vigorous eruptions are not possible.  A faint, reddish plume deposit was seen surrounding Hi'iaka when Galileo first started imaging Io in June 1996 that slowly faded as the mission progressed.  Even more curiously, Hi'iaka is barely visible in Voyager images of the region taken in 1979.  This suggestions that the lava flows Galileo and later New Horizons saw at Hi'iaka formed between 1979 and 1996, in same time frame as Zamama and the new flow fields at Prometheus and Culann.  Using the Pillan eruption as a possible template, perhaps it is possible that these flow fields built up quickly, on the scale of only a few months.  As the eruptions died down, activity at these volcanoes may have transitioned to from a violent, fire-fountaining, outburst eruption to a more quiescent insulated lava flow.  However, given the number of years between Voyager and Galileo, there is no reason to require an initial outburst at these flow fields, and they may have built up gradually over the 16 year time span.  However, I will point out that only minor expansions of the extent of these flow fields were ever seen during 4 years of the Galileo mission at Io.  Several outbursts were known to have occurred in Hi'iaka's region (or at least its hemisphere) during this interval that may have resulted in the initial creation of the Hi'iaka flow field, including a bright outburst seen in 1986 that provided evidence for silicate volcanism on Io.

That's it for this week's star volcano, Hi'iaka Patera.  Come back next week when we will profile the colorful Culann.

References: 
Bunte, M.; et al. (2010). "Geologic mapping of the Hi’iaka and Shamshu regions of Io". Icarus 207: 868–886.
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.
Johnson, T. V.; et al. (1988). "Io: Evidence for Silicate Volcanism in 1986". Science 242 (4883): 1280–1283.

New Videos Showing Topography in the Jupiter System

Paul Schenk has posted some new, high-definition videos on Youtube showing the topography of various surface features across the four Galilean satellites.  These videos were created for a conference/celebration going on in Padua starting today, the IAU Symposium 269: Galileo's Medicean Moons - Their Impact on 400 Years of Discovery.  Schenk's videos can be found online at his Youtube channel, GalSat400.  These videos are based on photoclinometry and stereo imaging derived from Galileo images.  Photoclinometry uses changes in the topographic shading of a features to determine local slopes, and thus topography.  Stereo imaging uses pairs of images taken at different viewing angles to determine topography.  The latter example are often depicted as stereo pairs or anaglyphs.  For Io, these videos include flyovers of Tohil Mons based in part on a high-resolution mosaic of mountain from Galileo's flyby of Io during orbit I32, Hi'iaka Montes from I25 data, and the Pillan flow field from I24 data (not posted yet but I definitely look forward to Paul getting that online).  For your convenience, those videos are also below.  Paul has also post videos using high-resolution Galileo mosaics for the other Galileans.  Of these other videos, I kinda like the flyover of Manannan crater on Europa.  This relatively small impact crater looks crater-ish from a distance, but up close, when you look a the topography of the feature, you can see that the thin ice shell and ductile layers of that shell have mangled that crater out of all recognition.

Definite check these videos out!


Tohil Mons, Io (high-resolution)


Liftoff from Hi'iaka Montes

Link: Youtube - GalSat400's channel [www.youtube.com]
Link: Galileo and Padova - 1610-2010 Celebration (New Videos) [stereomoons.blogspot.com]

Paper: Geomorphologic Mapping of Hi'iaka and Shamshu Regions

On Friday, a new paper was published in press in the journal Icarus titled, "Geologic mapping of the Hi’iaka and Shamshu regions of Io".  In press articles are those that have been reviewed and approved for publication, but have not yet been published in the print journal.  This paper, by Melissa Bunte, David Williams, Ronald Greeley, and Windy Jaeger.  This paper discusses the results of a geologic mapping project based on the 25ISTERM__01 mosaic of the Hi'iaka region and the 27ISSHMSHU01 mosaic of Shamshu region [I've also uploaded labeled versions of these two mosaics, Hi'iaka and Shamshu, to help people with this discussion].  These two regions are dominated by a patera with a floor partially covered in dark lava flows surrounded by several large mountains.  This paper is part of a series of geomorphologic mapping projects using medium-resolution Galileo mosaics of Io for their basis.  We previously covered papers discussing maps of Prometheus and Zal.  Other regions mapped by the ASU group include: Chaac-Camaxtli, Culann-Tohil, Zamama-Thor, and Amirani-Skythia-Gish Bar.  In March, I covered this group's LPSC abstract covering the mapping of Hi'iaka and Shamshu, so bare with me if I repeat some things from that post..

For this paper, the authors created two geomorphologic maps of the regions surrounding Hi'iaka and Shamshu Paterae, two volcanoes near Io's equator on the leading hemisphere.  These maps were based primarily on two SSI mosaics from Galileo's I25 and I27 encounters with Io in November 1999 and February 2000, respectively.  Color information and low phase brightness information was taken from the global color mosaic produced by the USGS.  Geomorphologic maps are a type of geologic maps where different surface units are identified and mapped so that relationships between these different units can be identified.

In the case of the Hi'iaka and Shamshu regions, four basic units types were identified: plains material, mountain/plateau materials, patera floor materials, and lava flow materials.  These units were also identified in the mapping of other regions on Io, though diffuse material, recent surface coatings that are derived from volcanic or sapping processing and identified in other mapped regions, was not identified in the Hi'iaka and Shamshu regions.  It should be noted though that diffuse materials are often transient; red diffuse material was observed in a faint partial ring of material surrounding Hi'iaka during Galileo's first orbit of Jupiter, suggesting the presence of a plume at Hi'iaka shortly before June 1996.

These four basic units were further sub-divided into different sub-units based on their albedo, color, surface texture, or structural contacts.  For example, mountain units are divided into lineated (ridged/grooved material, generally found at higher altitudes, bounded with plains by scarps), mottled (hummocky, often with lobed margins), undivided (intermediate in texture and altitude between the mottled and lineated types), and plateau (flat-lying terrain with smooth or hummocky textures) types.  Based on the spatial relationships between these units, the authors theorize that these types represent different stages of degradation on Io's mountains.  Lineated mountain units tend to be higher standing and have sharper unit contacts with the surrounding plains, suggesting that terrains of this unit have undergone the least amount of degradation.  The ridges and grooves, such as those seen above at north Hi'iaka Montes, are the result of down slope slumping of a mobile surface layer atop the mountain (see Jeff Moore's 2001 paper on slope degradation on Io).  Remember that mountains are thought to be tilted crustal blocks, the ridged and grooved top surface of the mountain is the result of the upper few kilometers of the mountains, which is a mix of basalt and sulfurous materials that was once part of the upper few kilometers of Io's flat plains.  Once that upper layer sloughs off, the unit transitions to the undivided or mottled types (though the mottled types are likely a mix of old mountain and the mass wasted materials surrounding the mountain).

A similar age progression in sub-units was also noted in patera floor and lava flow materials based on color and albedo (for dark flow/patera floor materials, darker equals younger).  The paper only mentions briefly the debate whether some bright or yellowish lava flows are primary or secondary sulfur flows, or silicate flows with chemical altered surfaces.  The authors (and I agree) that these flows are likely old silicate flows.  This supported less in this region but at Thor, where an outburst eruption generated lava flows that overlapped those of an older, yellowish flow.  Exceptions to this could include a bright white flow to the south of Shamshu Patera and the orange floors of "Mekala Patera" and western Hi'iaka Patera, which could be due to mobilized sulfur dioxide and sulfur, respectively.

As I mentioned in my post from March on the researchers' LPSC abstract on this subject, the authors examined the hypothesis that the three mountains that surround Hi'iaka Patera had at one point been part of a single structure that had split apart as a result of strike-slip and extensional faulting.  This fault would have also resulted in the creation of pull-apart basins that would become Hi'iaka Patera and orange-color patera to the lower left of north Hi'iaka Montes (named Mekala Patera in the paper).  The authors note that the freshest lava flows on the floor of Hi'iaka appear to emanate from the eastern and southern margins of the patera, a pattern consistent with this break-up scenario.  This model of regional-scale plate tectonics was also applied to the mountains in the Shamshu region (with Shamshu Patera being presented as a pull-apart basin) with less convincing results, but it does suggest a new line of investigation to see just how prevalent this style of plate tectonics is on Io with mountains forming, breaking apart, and shifting around across Io's surface.  How many other close groupings of mountains were the result of a single mountain being broken apart by strike-slip and extensional faulting?  I should note that several Ionain mountains have been observed with canyons running down the centers as a result of extensional faulting, like Ionian Mons (upper left), Mongibello Mons, and Danube Planum.

This paper summarizes the results of geomorphologic mapping of the Hi'iaka and Shamshu regions on Io based on regional-scale imaging by Galileo, finding units consistent with those found in other regional mosaics of the satellite.  The geologic history of these regions based on their mapping supports earlier research into this area by authors like Turtle et al. 2001 and Jaeger et al. 2001 that Hi'iaka Patera formed as the result of Hi'iaka Montes breaking apart by strike-slip and extensional faulting.  Magma then ascending to the surface using these same faults that bounded the pull-apart basin.  At this point, the volcanic activity at both Hi'iaka and Shamshu appear to be dominated by compound lava flows, unlike the lava lakes seen at Pele and Loki.  The mountains were then further degraded by gravitational mass wasting, SO₂ sapping from ice layers within the mountain, and thermal erosion from nearby volcanoes.  Thermal erosion by flowing lava is thought to be responsible for Tawhaki Vallis, a channel observed to the right side of the image at top, as proposed by Schenk and Williams 2004.  Each of mountains in the region exhibit signs of degradation, from the ridges and grooves atop north Hi'iaka Montes and north Shamshu Montes, indicative of a slumping mobile surface layer, to hummocky-textured and lobe-edged landslide deposits, indicative of gravitationally- or sapping-induced mass wasting.

I think that leaves one more region to map, Tvashtar.

Link: Geologic mapping of the Hi’iaka and Shamshu regions of Io [dx.doi.org]

Paper: Formation of Mountains on Io

The second Io paper posted Saturday on the journal Icarus's Articles in Press page is titled, "Formation of mountains on Io: Variable volcanism and thermal stresses". The authors for this paper are Michelle Kirchoff and Bill McKinnon from the Lunar and Planetary Institute in Houston and Washington University in St. Louis. This paper takes a look at the geophysics behind the formation of mountains on Io and why there is a global scale anti-correlation between mountain and volcanic paterae. In a nutshell, they find that variations in volcanic activity can vary the level of thermal stresses in Io's lithosphere, which in concert with subsidence stress (a compressive stress that increases with depth resulting from the high resurfacing rate), leads to the formation of thrust faulting and mountains if that volcanic activity decreases.

Crustal subsidence stress from volcanic resurfacing is one of the dominant compressive stressors on the Ionian lithosphere, but subsidence stress is insufficient to produce thrust faults that reach the surface as each time a fault is formed and moves, the subsidence stress global is that much more reduced. In order for a mountain or a cluster of mountains to be formed at a particular location, a "focusing" mechanism is required. Three hypotheses have been put forth for these mechanisms:

• Jaeger et al. 2003 suggested that mantle plumes impinging on the base of the lithosphere could locally increase the compressive stress at a particular location, resulting in thrust faulting and mountain formation. Kirchoff and McKinnon call this hypothesis "plume-modified subsidence."
• Tackley et al. 2001 suggested that global mantle upwellings and downwellings produce tensile and compressive stresses in the crust. Upwellings results in tensile (extentional) stress on the lithosphere due to crustal thinning and stretching, producing increased volcanic activity. Downwellings result in compressive stress on the lithosphere, producing increased mountain formation. This model was developed in response to the obsevation that while some paterae abut mountains and thus there maybe some local correlations between mountains and volcanic pits, globally these features are anti-correlated. The authors of this paper call this hypothesis "Convection-modified subsidence".
• McKinnon et al. 2001 suggested that decreases in volcanic activity on a regional scale (and thus decreasing the transport of heat from the interior to the surface), could result in heat building up at the base of the lithosphere, causing it to melt. The increase in thermal stresses caused by this melting result in the propagation of thrust faults closer to the surface and would thus support mountain formation (also known as orogenesis). Kirchoff and McKinnon call this hypothesis "thermal-stress-modified subsidence".

The authors examined the viability of the "thermal-stress-modified subsidence" hypothesis by modeling temperature and stress levels within the lithosphere, first assuming a steady-state resurfacing rate (which, according to the O'Reilly and Davies 1981 heat pipe model, is directly related to heat flow; the advective escape of heat from Io's interior). They then increased or decreased that resurfacing rate by 50% and 100% to see how this affected the temperature and stress levels. Kirchoff and McKinnon looked at both cases where the asthenospheric heating rate and resurfacing rate are coupled and uncoupled. In the coupled case, a decrease in resurfacing rate is the result of a decrease in the local asthenospheric heating rate. In the uncoupled case, the local asthenospheric heating rate stays the same while the amount of heat that is advected to the surface (through volcanism) is varied.

• In the coupled case, the thickness of the lithosphere is maintained. Temperatures in the lithosphere increase, causing the region in compressive failure (where stresses surpass Byerlee's Rule) to become larger and more shallow. Eventually the temperatures and stress levels reach a steady state.
• In the uncoupled case, decreasing the resurfacing rate increased heating at the base of the lithosphere, leading it to melt and increasing temperatures throughout the lithosphere (though convection all the way to the surface remains negligible). Over 0.5-1 million years, the crust thins. The resulting thermal stress decreases the depth at which thrust faults could form and propagate. The more the resurfacing rate is reduced, the greater the effect. Increasing the resurfacing rate causes cooling at the base of the lithosphere, thickening it. No steady state is achieved, eventually something has to give, like an increase in volcanic activity.
• The authors also looked at a case where the lithosphere is 50 km thick, instead of 25 km. This just increases the time it takes for the lithosphere to thin enough to bring the brittle compressive zone close enough to the surface to support orogenesis.

Kirchoff and McKinnon ignored the effects of warm intrusions (which are likely important in formation of paterae) and assume that old lava flows completely cool before the next is laid down and the presence of a partially molten asthenosphere. The thermal effects of the first two are likely negligible as they don't seem to be a major component of the heat flow (heat flow estimates at longer and shorter wavelengths are consistent). The third is consistent with most modern tidal heating models for Io given volcano distribution. They also found that changing crustal rheologies (dry basalt, peridotite, or "komatiite") doesn't greatly change the modeling results. Eruption temperature estimates are consistent with dry basalt with greater iron and magnesium (olivine) content.

The authors then compared their results to the other models. For the plume-modified subsidence hypothesis, stress caused by an impinging mantle plume was not significant compared to crustal stresses. Because tidal heating is focused in the asthenosphere (or upper mantle), heat "plumes" should be downwellings, not upwellings, according to Tackley et al. 2001. For the convection-modified subsidence hypothesis, compressive stress on the lithosphere from mantle downwellings are too small to focus mountain formation. Maximum lithospheric stress are only a few kPa, compared to hundreds of MPa needed for compressive failure.

Finally, the authors examine a potential local cycle of mountain and volcano formation. First, greater volcanic activity increases the resurfacing rate (and thus the subsidence rate). This causes greater compression at depth, which constrains the deep conduits between volcanoes and their deep magma reservoirs, lowering the level of volcanic activity. This does nothing to the level of asthenospheric heat and temperatures increase throughout the lithosphere, leading to melting at its base. This melting causes thermal stresses to propagate to shallower depths, causing compressive failure between 10-20 km, where thrust faults could propagate to the surface, forming mountains. The formation of these faults and mountains relieves the subsidence stresses and thermal stresses in the lithosphere, leading to extension. This then allows magma to ascend to the surface through newly opened conduits as well as form batholiths which can helps deform mountains, such as by fracturing them. Increasing volcanic activity leads to a thickening of the crust back to its normal level. Unlike the view from McKinnon et al. 2001, volcanism does not need to complete shut down for this model to work.

One potential consequence of this cycle is that areas with high numbers of mountain should be beginning to increase their level of volcanic activity, perhaps increase the number of active paterae that abut mountains compared to areas with more paterae but fewer mountains.

Link: Formation of mountains on Io: Variable volcanism and thermal stresses [dx.doi.org]

LPSC 2009: Insights from Global Geologic Mapping of Io

The other mapping poster that will be presented at LPSC this year is by Dave William et al. and is titled, "Volcanism on Io: Insights from Global Geologic Mapping". This poster will present the status of the global geologic map project as well as present examples of results derived from the map. The authors' LPSC 2008 update was discussed on this blog last year.

The geologic mapping group finished the global map last year in ArcGIS. They are now working on a database to incorporate information about surface changes observed by Galileo, Voyager, and New Horizons.

This abstract two examples of statistical analyses that have been performed on the geologic map. The first compares the height and area of various mountain units. The authors find that lineated mountains tend to be taller than mottled mountains. This supports the hypothesis that mottled mountains are more degraded, and thus older, version of the younger lineated mountain unit. However, based on the plot of mountain height-vs-area, they may be dealing with statistics of small numbers as only 4 mottled moutains are plotted. The layered plains unit, thought to be an even more degraded mountain unit, has a similar area distribution as the other mountains units, but tend to be restricted to under six kilometers in height, further supporting formation from degradation.

The rest of the abstract is spent looking at the temporal correlation of mapped units. Williams et al. propose that mountain units are the oldest geologic unit, forming over a period from several millenia ago to perhaps several million years ago. This is based on the observed pattern of degradation at the various mountains mapped on Io (lineated mountains -> undivided mountains -> mottled mountains -> layered plains) and the superposition of other units on mountains such as diffuse volcanic deposits and paterae (seen at some mountains to be "eating" into them, such as at Tohil Mons and Gish Bar Mons). Volcanic units have formed much more recently, with diffuse deposits and dark and bright flows (both on the plains and in paterae) forming during the period of spacecraft observation. Plains units fill in the gap between volcanic units and mountain units, the result of the buildup volcanic deposits and mountain mass wasting that have since become homogenized and can not be split into different units (like undivided flows).

Again, another interesting paper, but I am curious as to when or if the geologic map will be available online as a downloadable product like some of the older Galilean satellites maps or the Venus geologic maps.

Link: Volcanism on Io: Insights from Global Geologic Mapping [www.lpi.usra.edu]

Common Features Among Io's mountains

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.

Perhaps the most common attribute of all mountains on Io are that they have experienced some form of degradation. In other words, no mountains appear prestine, despite the relatively young age of Io's mountains. Assuming an average deposition rate across Io surface of one cm per year, every part of Io's surface should be covered in ten km of material in one million years. Ten km is more than the average height of mountains on Io, so most mountains are probably younger than that. Degradation of mountains comes in the form of mass wasting which can take several forms. First, slumping on steep slopes can dislodge large blocks down slope or form thick, fan-shaped deposits. The latter case was mentioned before at Boösaule Montes "South". Slumping on shallower slopes, like on the top of Hi'iaka Montes "North", can form ridges perpendicular to the direction of slumping. The ridges are crumpled up material from the upper layers of the plateau. In other cases, long-runout landslides can result from more rapid mass movement of material. These can form deposits a few-hundred meters thick that can reach as far as a couple hundred km from the mountain.

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.

Many mountains have structural similarities. There are the low plateaus: relatively flat mesas with smooth or rough top surfaces and steep basal scarps (that are often arcuate as a result of sapping). These plateaus are often two to four km in height. Another interesting mountain type is the double ridge. These mountains have two parallel ridges with a central valley in between. Two examples of an Ionian double ridge include Ionian Mons and Mongibello Mons. Another mountain type is the flatiron, as seen at Gish Bar Mons above. Flatiron mountains have an asymmetric profile, with a relatively shallow slope on one side and steeper, more rugged slopes on the other sides. Schenk et al. surmise that these mountains are produced by thrust faulting with the smooth slope representing the formerly flat surface and the steeper slopes revealing a cross-section of the upper lithosphere of Io. These are the tilted crustal blocks highlighted in Schenk and Bulmer 1998.

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.

In the next Mountains post, which I will try to post sometime later this week, we will look at what the distribution of mountains on Io can tell us about Io's crust.

Boösaule Montes

By popular demand, I thought I would write a blog post about the tallest mountain on Io, Boösaule Montes. Not sure why it is so popular, but you got to give the people what they want...

Boösaule Montes was best seen during the 1979 Voyager 1 encounter with Io but it can also be seen in several global views acquired by Galileo and New Horizons. The image at right is part of a mosaic acquired by Voyager 1 and is the highest resolution images of this feature at around 0.9 km/pixel.

Boösaule Montes, located just to the northwest of the Pele plume deposit, actually consists of not one, but three separate mountains connected by a raised plain. Each mountain displays a unique morphology.

Boösaule Montes "South", seen near the bottom of the image at right, is the tallest mountain on Io. Based on stereo work performed by Paul Schenk et al. 2001, this massif is at least 17.5 kilometers in height. This is more than four kilometers greater than the next tallest mountain on Io, Euboea Montes. The mountain has an irregular morphology, with a relatively gentle slope throughout much of the mountain except for an abrupt scarp on the peak's southeastern margin. According to Schenk et al. 2001, this scarp has a height of approximately 15 km and a slope of 40°. Such a sharp scarp would suggest that a landslide transported some of the mountain's material downslope, but unlike most landslides on Io, the material doesn't seem to have been transported very far from the mountain. Instead, a textured deposit ~3-5 km thick can be seen at the base of the scarp. Based on this morphology, Schenk et al. interprets this sharp slope as a slump scarp.

The mountain is surrounded by a broad plateau that likely represent mass-wasted material, but again, this is oddly absent on the mountain's southwestern margin. On potential sink for material seen at other mountains on Io, paterae, is absent from the immediate surroundings of Boösaule Montes "South". However, to the southwest of Boösaule "South", an old lava flow follows the basal scarp of the mountain. The flow originates from the southern end of a patera (seen at center left in the above image) to the northwest of Boösaule "South". I'm not sure if that's why there is no long-runout landslide deposit in that direction.

Boösaule Montes "North", seen above center in the image at top, is an 8.5-km-tall, dome-shaped mountain with a large fracture running north-south through it. This type of morphology is akin to lava domes on Earth. One possible formation mechanism for this feature is that this mountain sits atop a diapir, created from rising magma in Io's lithosphere. A plateau, perhaps formed by landslide debris, can be seen between both mountains.

The last component, Boösaule Montes "East", seen near upper right in the image at top, is a 7-km tall plateau with two mophologic sections. The western part has a rugged surface with several lineaments running down the length of the mountain. Evidence for mass wasting can be seen along this sections southwestern margin. The eastern part of this mountain has a lower elevation and has a plateau-like morphology. Parts of this plateau have a scalloped margin, suggestive of sapping. Such activity would suggest the presence of sulfur dioxide within some of the layers in the mountain.

Coming up later this week, I will address Io's mountains more generally by looking at some features many of these mountains have in common.