Io Volcano of the Week: Zal

This month for my Io Volcano of the Week series, we will be 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.  Over the course of the next month, we shall examine such diverse volcanic edifices as Culann, Emakong, Hi'iaka, and Tvashtar.  This week, we will be taking a gander at Zal Patera, a large volcanic depression on Io's leading hemisphere.

The difference between Zal Patera and last week's volcano of the week, Pillan Patera, is like night and day.  Where Pillan experiences episodic, explosive volcanic eruptions that produce tremendous amounts of lava in a short span of time, eruptions at Zal were more frequent and consisted of smaller bursts of lava.  These eruptions also cause smaller changes to the Ionian landscape, but Galileo's observations of the region show that this was not always the case as clear evidence for much larger lava flows are visible within Zal and to its south.

Zal Patera itself is located at 40° North Latitude, 74.5° West Longitude and is 165 kilometers (103 miles) wide.  As you can see in the color image above from the C21 mammoth mosaic, this places Zal near the boundary between the red-brown, radiation-deep-fried plains of Io's irradiated North and the slightly-less irradiated, yellow-green plains of Media Regio along Io's equator.  Despite the intense radiation this far north on Io, the region around Zal is quite colorful as a result of various allotropes of sulfur, sulfurous compounds such as iron sulfide and sulfur dioxide, silicate materials like lava flows and pyroclastic deposits.  Within the patera itself, dark lava flows seem to have flowed out from the western margin of the patera toward the center of the depression.  Lava flows of differing ages are visible, slowly brightening as they age due to the persistent influx of falling sulfur and sulfur dioxide.  The volcano however, doesn't seem to be strongly segregated from the surrounding plains, with only low scarps marking its edge to the south and east.  The western margin is sharp, but it does seem to have much topography related to it, but based on measurements of the shadow of the plateau to its west, we know that the floor of Zal Patera is 0.5 kilometers (1,600 feet) lower than the narrow swath of plains material that separate the plateau from the volcano.  Immediately to the south, the terrain is painted with reddish sulfur, a deposit centered around a deep red lineament that runs north from a dark, unnamed volcanic depression (or patera) north to the southwest corner of Zal Patera.  In a bit we shall discuss the significance of these colorful patterns.  But first, let's look at the structures that surround the volcano.

Zal Patera, shown in the center of the top frame in the mosaic above, is bounded by three mountains, all collectively part of Zal Montes.  The northern component is 1.9-2.5 kilometers (6,200-8,200 feet) in height and consists of a large, heavily eroded plateau covering 15,600 square kilometers (6,000 square miles).  The erosion is the result of two forces (since of course there is no atmosphere to support physical or chemical weathering): mass wasting and sapping. The first causes portions of the mountain to move down slope, either quickly through landslides or slowly through creep, forming rough landslide lobes at the base of the mountain.  This can be most clearly seen on the southeastern edge of the plateau.  The latter, sapping, occurs when sulfur dioxide is heated up and escapes from a mountain slope.  On North Zal Montes, this creates the sawtooth pattern on its western and northern margins and the pits on top of the mountain, particularly on its southwestern end.  South Zal Montes consists of a 225-kilometer (140-mile) long, 7.4-kilometer (24,300-foot) tall ridge that runs roughly north-south.  The mountain has an asymmetric profile, like many of Io's uplifted crust blocks, with a steep sloped west margin and a shallower slope on its eastern side. On a whole, this mountain is more rugged than North Zal Montes due its more angled slopes, which encourages mass wasting.  A smaller, third mountain is visible to the east of Zal Patera.

So what does these mountains have to do with Zal itself?  Proposed by Melissa Bunte and her colleagues in their paper on a geologic map of the region, one theory for how this large volcano formed is that the three structures that make up Zal Montes were part of a single mountain in the past.  A right-lateral strike slip fault formed along the eastern margins of what are today east and north Zal Montes and along the western margin of south Zal Montes, displacing these structures due to north-south lateral slip.  An extensional fault later formed between the northern plateau and the smaller east Zal Montes and along the northern end of the older strike slip fault.  Extensional faulting pushed the smaller mountain to the east, opening up the basin that became Zal Patera.  A similar formation mechanism has been used for other volcanoes such as Monan Patera and Hi'iaka Patera.

The high resolution images acquired by Galileo during the November 26, 1999 and February 22, 2000 encounters, the color data taken in July 1999, and near-infrared thermal data from Galileo's Near-infrared Mapping Spectrometer (NIMS) can be combined to create a picture of the style of volcanic activity that occurs at Zal.  Zal was seen as an active volcano, based on its emission of near-infrared light, on several, intermittent occasions during the Galileo mission (June 1996, April-June 1997, September 1997, May 1998, and August-October 2001) and during the New Horizons flyby in February 2007.  High-resolution thermal data from August and October 2001 provide a clue for the type of activity that occurs at Zal.  In these observations, a hotspot was seen lying along the western margin of north Zal Montes (in October 2001, much of this hotspot was blocked from view by the plateau).  Combined with the color data which shows a deep red lineament running from a small patera (called "Rustam Patera" in the Bunte et al. paper) along the western margin of south Zal Montes north to southwestern Zal Patera, a lineament which continues north as the western margin, and apparent source of lava flows at the volcano, of Zal Patera, a possible scenario emerges.  The original source for lava at Zal Patera is to its south at "Rustam Patera".  Lava emerging from this volcano flows northward via lava tubes that run along a fault that may have once broken up the Zal Montes complex.  Skylight and the occasional breakout of lava along the 300-kilometer (186-mile) long system allow for the release of gas from the lava, forming deep red deposits along the fault, and fainter red deposits distally, including along the western slope of south Zal Montes.  Galileo's SSI camera saw hotspots in eclipse along this fault/lava tube system on several occasions, particularly in May 1998.  This system of tubes eventually ends along the western margin of Zal Patera where the lavas finally reach their nadir and flow out on to the surface, likely as part of an inflated flow field, as suggested by the NIMS data which suggest that active lava flows at Zal are less than a meter thick and certainly don't release tremendous amounts of energy like those at Pillan or Surt.  Alternatively, the lava flows seen within Zal may form as a result of magma exploiting the fault that split up Zal Montes and formed the depression that became Zal Patera.  This exploitation created "Rustam Patera" at the southern end of the fault, allowed sulfurous gases to escape along the southern half of it, and lava to flow out onto Io's surface along the fault's northern half.  Future higher resolution observations will be required to distinguish between the two theories, but the predominance of thermal emission at one half of the fault or the other at any given time (rather than both at the same time) may lend support for the latter theory.

Eruptions at Zal Patera are certainly not as explosive as those seen at the last volcano of the week, Pillan, though there was a small plume observed at the volcano during the New Horizons flyby in 2007.  However, what it lacks in explosiveness, it makes up for in persistence, providing an outlet for Ionian magma on a regular basis since at least the start of the Galileo mission.  Imaging of the region as a whole does show that volcanism along the fault that forms the western margin of Zal Patera can form more voluminous lava flows, as evidenced by the radiating set of bright flows (older silicate lava that have since been coated in sulfur frost) centered around "Rustam Patera".  Perhaps one day this volcanic system will reawaken, and fresh lavas will flow along the earlier ones, much like Thor in 2001.  The formation mechanism suggested for Zal Patera provides a glimpse at how at least a few of Io's volcanoes may have formed, through the actions of regional plate tectonics.

Next week, we will look at another of the volcanoes Galileo examined during the troubled I25 encounter, Emakong Patera.  Until then, I hope you've enjoyed this look at this fascinating volcano.

References:
Schenk, P.; et al. (2001). "The Mountains of Io: Global and Geological Perspectives from Voyager and Galileo". Journal of Geophysical Research 106 (E12): 33201–33222.
Bunte, M. K.; et al. (2008). "Geologic mapping of the Zal region of Io". Icarus 197: 354–367.
Spencer, J. R.; et al. (2007). "Io Volcanism Seen by New Horizons: A Major Eruption of the Tvashtar Volcano". Science 318 (5848): 240–243.
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.
Davies, A. G.; et al. (2005). "Post-solidification cooling and the ages of Io's lava flows". Icarus 176: 123–137.

Geologic Mapping of the Zal region of Io

There is a new article in press in the journal Icarus titled, "Geologic Mapping of the Zal region of Io," by Melissa Bunte, David Williams, and Ron Greeley. A summary of their results was presented in March at the Lunar and Planetary Sciences Conference and was reported on here. Now the full paper is available on the Icarus website (subscription required to view paper). This paper is based on imagery acquired during the I25 and I27 Galileo encounters with Io.

Like many of the regions mapped by the ASU previously, such as near Camaxtli Patera, Tohil Mons, Amirani, and Thor, the authors mapped 5 basic units in the region: mountains/plateaus, smooth/layered plains, patera floor material, flow material, and diffuse materials. The flow features in this area appear to be generated from a small patera lying near the western margin of South Zal Montes (they propose the name "Rustam Patera" for this volcano) or from a fissure that runs north from "Rustam" along the western margin of South Zal Montes and the eastern margin of North Zal Montes. The flows include bright flows (possibly of sulfurous composition) radiating out from "Rustam" and dark flows which flow east across part of Zal Patera from the northern part of the fissure. Additional flow features are also seen within Zal Patera, but these appear to be older in age based on their brighter appearance.

One interesting hypothesis made in this paper is that the various components of Zal Montes, which surround Zal Patera to the west, east, and south, were originally part of a single structure. This feature then broke-up due to strike-slip then extensional faulting, opening up Zal Patera. Similar plate tectonics-in-miniature is theorized for formation of Hi'iaka Patera. The paper goes on to describe the degradational processes that have occurred at the mountains in the region.

One feature I wished the paper expounded on further is a small volcano west of North Zal Montes, which they suggest the name "At'am Patera" for. What makes the volcano interesting is that it appears to be one of a very rare breed of explosive Ionian volcano. "At'am" erupted between late-June and mid-September 1997, producing a white, Sulfur dioxide-rich plume deposit and a dark-green pyroclastic deposit with a digitate margin. Some of both materials was deposited on North Zal Montes. The digitate appearance is due to the interaction between the pyroclastic flow and the arcuate margin of the western part of North Zal Montes. This morphology may provide clues on how these pyroclastic deposits are formed on Io. Oddly, for an Ionian eruption, no lava flows or thermal emission were observed at this volcano. Also, the central vent is among the smallest paterae found on Io. It is possible that the 1997 eruption could have been the result of an intrusive event, where magma ascends from a deeper chamber, but fails to reach the surface. However, volatiles and other materials, being more buoyant, do make it to the surface.

The paper does touch a bit on the plume seen at Zal last year by New Horizons. This plume is centered on Zal Patera (unlike the plume deposit seen by Galileo starting in Sept. 1997 which surrounds "At'am Patera"). Zal Patera is also the site of fresh surface changes, which include a new dark plume deposit and fresh dark lava flows.

Link: Geologic Mapping of the Zal region of Io [dx.doi.org] (subscription required to view paper)

LPSC 2008: Geologic Mapping of the Zal Region of Io

The Lunar and Planetary Sciences Conference is coming up in a couple weeks. I unfortunately will not be attending but the abstracts for the conference, which are almost mini-papers in length, have been online for a few weeks now. Nearly all the Io-related abstracts are for posters to be presented on the evening of Tuesday, March 11. In this post, and in blogs to be posted over the next week or so, I will cover one of the abstracts for this conference.

The first abstract I'm going to discuss is "Geologic Mapping of the Zal Region of Io" by Melissa Bunte, David Williams, and Ron Greeley. This abstract covers work done as a part of series of geologic mapping work performed by the Planetary Geology group at Arizona State University on the mosaics Galileo acquired of Io during its seven encounters. This time they cover the area around Zal Patera. This volcano on Io's northern Leading hemisphere is bounded to the west and south by the two-part Zal Montes.

Like most places the group has mapped, they found 5 basic terrain types: plains, mountains, patera floor materials, flow materials, and diffuse materials. Their mapping allowed the authors to determine the age relationships between the different flow units within Zal Patera based on the brightness of the flows. Basically, as a flow ages, it cools (obviously) and as it cools, more volatiles can condense on it. Color-wise, the progression at this latitude is (generally now) from black, to green (when you got your sulfur mixing with your iron), to yellow (thanks to S₈), and then to red-brown when radiation starts to work on the sulfur and break it down (anyone know how long that roughly takes, by the way?). The group also mapped some very extensive, old flows that they attribute to Rustam Patera, the proposed name for an active volcano on the western margin of South Zal Montes.

Following the work they have done previously in the Amirani and Camaxtli regions on Io, they will like publish this work sometime in the next year or so. Certainly will be a paper to look for. I would be interested in how they handle the connection between Rustam Patera and the lava channel that runs north from it up through to what they identify as a fissure along the western margin of Zal Patera. Another interesting feature in this region (or actually just off the western edge of their mapped segment) is a small, explosive eruption from the summer of 1997 that produced a small dark pyroclastic deposit that partly overlaps onto the western margin of the North Zal Montes plateau, which is identified as a flow (?) in the map included in the abstract, and a bright ring around that pyroclastic deposit. Strangely enough, no obvious effusive materials from that eruption.

The ASU group also has another poster at the conference where they present the work they have done so far on producing a global geologic map. I will discuss that abstract in a later post.

Link: Geologic Mapping of the Zal Region of Io [www.lpi.usra.edu][pdf file]