Using Ground-penetrating Radar at Io

Yesterday we took a more detailed look at the types of observations that are being planned for the Europa/Jupiter System Mission (if it's selected as the next flagship mission). The Final Report for the Jupiter Europa Orbiter also provided us with details of the encounters planned in the current reference trajectory for the mission. One of the types of observations that I want to talk about today is sub-surface sounding using the Ice Penetrating Radar (IPR). This is a type of observation that has never been done at Io and I have become curious as to what IPR (or a similar instrument) could see if it were turned at Io.

The Ice Penetrating Radar would work in a similar fashion to the MARSIS and SHARAD instruments currently at Mars on board Mars Express and Mars Reconnaissance Orbiter, respectively. Using a pair of antennae, IPR would emit pulses of high-power radio waves at the surface of the target body. These waves would first bounce off the surface and then structures in the sub-surface which become more difficult to detect the deeper you get as the signal returned becomes fainter. Basically, when the radio waves first encounter the surface, some of the waves are bounced back to the spacecraft, while others bounce down into the sub-surface, where it will then encounter another reflecting surface. This layer (or fault plane) will then bounce radio waves upward back toward the spacecraft or further into the sub-surface. This process continues until the amount of signal from a sub-surface layer doesn't rise above the noise level or is interfered with by other signals coming back.

The IPR will have two frequency modes: a 5-MHz (60-m wavelength) deep mode and a 50-MHz (6-m wavelength) shallow mode. When JEO is orbiting Europa, the shallow mode would be used to identify near-surface pockets of water while the deep mode would be used to search for the interface between the ice crust and the liquid water ocean beneath. At Europa, the IPR is expected to be able to sense layers as deep as 3 km using the shallow mode and 30 km using the deep mode. There are trade-offs to using each mode. While the 5-MHz mode can sense much deeper into the Europan sub-surface, it has a lower vertical resolution, 100 meters. The 50-MHz mode can not penetrate as deeply, but the vertical resolution in this mode is 10x better, 10 meters. So the shallow mode would be able to over thinner layers and smaller structures than the deep mode could. So the mode used on a particular orbit of Europa would need to be chosen based on what the scientists are trying to look for: small pockets of water near the surface or a broader perspective on the thickness of the water-ice crust. IPR also has a raw data mode, which would record the high-bit rate, unprocessed data for processing on the ground (rather than in the instrument electronics). This mode would be used for high-science value targets but the JEO final report doesn't provide information on which wavelengths would be used (at the moment I presume both).

The IPR would be used during two of the Io flybys, Io-1 and Io-4 - the two low-altitude encounters currently planned. For both encounters, the raw data mode would be used for the two minutes surrounding closest approach, providing a 1000-km long swath across Io's surface. During these encounters, the Laser Altimeter would also be used to help verify the surface heights measured by IPR. Now, obviously, near-surface properties of Io and Europa's crust are quite different. Io's near-surface should consist of a mix of basaltic rock and sulfur and sulfur dioxide ices. These differences in the material properties of their lithosphere should result in differences in the depth IPR can penetrate into Io's crust compared to the values given in the Final Report for Europa orbital science. Now, the best figures I could find for radio sounding penetration depths in basalt suggests that IPR should be able to sense layers as deep as 50-wavelengths below the surface. This would translate to 300 meters for the 50-MHz mode and 3 km for the 5-MHz mode. I am not sure if the vertical resolution also scales (1/10th of the ice penetration depths). For this discussion, I will presume that it doesn't. If any of these figures seem incorrect, please let me know so I can post a correction.

Now with those depths and vertical resolutions, what kinds of structures would IPR see? Let's take a look at two potential swaths, shown at left. During the two minutes surrounding closest approach during the July 2026 Io-1 encounter (300 km at C/A), the JEO spacecraft would pass just north of an unnamed active volcano, then travel southeast across the Maui portion of the large Amirani flow field, then across an active patera thought to be the source vent for Amirani, and then over the older flows at Amirani as well as the plume deposit from the volcano. After passing over Amirani, JEO would continue to travel southeast, finally passing over the 6-km tall mountain Monan Mons. During the two minutes surrounding closest approach during the December 2026 Io-4 encounter (75 km at C/A), JEO would pass over four paterae, including two active volcanoes: Malik Patera and Altjirra Patera (the two dark patera under the flight path, left and right respectively). No topographic structures beyond these paterae are under the flight path. The flight path would also cross a couple of old flow lobes associated with Arusha Patera.

Using the 5-MHz band, assuming a penetration depth of 3 km and a vertical resolution of 100 meters, IPR could look at tectonic structures in the upper crust as well as search for shallow magma reservoirs. As far as tectonic structures go, IPR could look at the faults that underlie the mountain Monan Mons. This mountain is thought to have been uplifted by imbricate thrust faulting, and IPR could try to look for these faults to test this hypothesis. IPR could also look at the connection between tectonism and volcanism by look at how deeply paterae bounding faults penetrate. Do these faults reach as far down as the shallow magma reservoirs? It should be pointed out that most of these reservoirs are expected to be located at depths between 4 and 10 km, so they maybe too deep for IPR to detect. However, Leone et al. 2008 reported that Prometheus' shallow reservoir could have a rough as shallow as 3 km below the surface, so it might be possible for IPR to detect magma bodies below the surface of Io.

Using the 50-MHz band, assuming a penetration depth of 300 meters and a vertical resolution of 10 meters, IPR could examine near-surface layering of sulfur and sulfur dioxide with basaltic lava flows, comparing the depth of these layers with evidence for sapping. A 10-meter vertical resolution might be enough to resolve layering from individual eruptions, though it would be insufficient to resolve individual flow lobes, which would be on the order of one-meter thick. The 50-MHz band could be useful for looking at layering in the plains of Io, which would allow scientists to better understand how they've been built up over time.

As you can see, both IPR modes could be very useful for examining some important science questions for Io, particularly the origin of mountains and paterae by examining sub-surface structures hidden by the surface layer of sulfur and sulfur dioxide frosts and ices. IPR can also examine how fractured Io's crust really is. If a choice had to be made between the shallow and deep modes on each encounter (meaning the raw data mode doesn't use both bands), I think it would be better to use the deep mode during the first encounter over Amirani, due to its usefulness for exploring deep structures such as mountain-forming faults and shallow magma reservoirs. For the other encounter, both modes could be used, but I think the lack of mountains, and the use of deep mode to explore the deeper structure under Amirani, would make the shallow mode a bit more useful for exploring how the plains are built up and for looking at the shallow sub-surface of the volcanoes in this region. Regardless of which mode is used, it would be very useful if context images from the wide- or medium-angle camera were acquired during closest approach so that structures observed by IPR and the Laser altimeter can be correlated with structures on the surface.

Amirani mosaic from I27

Okay, I got one mosaic done tonight. I wasn't planning on posting this tonight but decided to stay up and watch the season premiere of Battlestar Galactica that I missed last night as well as the last couple of episodes from last season (hey, it has been a year since last season).

This mosaic is from Galileo's I27 flyby of Io that took place on February 22, 2000. This mosaic shows the Amirani lava flow at 207 meters (680 feet) per pixel. This mosaic was released at the time as PIA02567. I have created three separate versions of this mosaic. The first is a three-frame, green-filter mosaic covering the entire main flow field except the extension out west for the old Voyager-era Maui plume. The second is a single-frame, three-color (violet, green, and IR-756) composite over the center of the flow field. The third is a composite of the green-filter mosaic, the three-color composite, and low-resolution color data from C21 (shown at left).

The Amirani flow field is a composite basaltic flow field. Basically, the flow field is a build-up of smaller flow breakouts. You can see here the main flow field in green with recent (as of February 2000) flows in darker greens and black. The green color is the result of a chemical interaction between condensing sulfur and the cooling lava flows. As the flows cool, more sulfur and sulfur dioxide can condense on them, brightening the flow field as it cools and ages. The lava at Amirani starts out at a patera near lower left and is then funneled east via a narrow lava channel or lava tube. The plume at Amirani erupts from the dark spot near the boundary between the middle and bottom frames in the mosaic.

Link: Galileo I27 Images [pirlwww.lpl.arizona.edu]

Amirani-Skythia-Gish Bar mosaic from I24

Just finished up with this mosaic from Galileo's I24 flyby, 24ISAMSKGI01. This mosaic was released as PIA02526, a combination of high resolution clear filter data from this mosaic and lower resolution false-color from C21.

A lot of interesting geologic features can be seen in this mosaic, including several mountains, volcanic depressions, and lava flows. The name sake for this website, Gish Bar Patera and Gish Bar Mons, can be see on the right side of the mosaic. Io's most powerful lava flow field, Amirani, can be seen just to the right of the large gap in the mosaic. Monan Patera, Monan Mons, Skythia Mons, and Ah Peku Patera can be seen between Gish Bar and Amirani.

One more mosaic left to go for I24...

Link: Galileo I24 Images [pirlwww.lpl.arizona.edu]

LPSC 2008: Heat Flow from Dark Volcanic Fields

I've covered all the Io-related abstracts in tomorrow night's (wait, now tonight's) poster session covering the Galilean Satellites. Yeah, Io got lumped in with *shiver* Europa (in my mind, there are United Nations peace keepers between the Io and Europa poster boards), Ganymede (everyone's second favorite schizophrenic moon), and Callisto (thanks to the discovery of rings around Rhea, Callisto now possesses the sole title of most boring moon ever). So, if you are at LPSC, be sure to visit the poor Ionians as they will most likely shoved along the back wall like they were last year. Please, show them some love.

Anyways, I am rambling on here. There is an additional Io-related abstract submitted to the LPSC conference, a print-only abstracted by Glenn Veeder, Dennis Matson, Ashley Davies, and Torrence Johnson titled, "Io: Heat Flow from Dark Volcanic Fields." The authors examined the distribution of dark flow fields on Io and examine their contribution to Io's high heat flow (the total amount of heat released in a given time period from the interior). As a print-only abstract, the results presented in this abstract will not be presented at a talk or a poster at LPSC.

The authors focused on dark flow fields, areas where lava has flowed across Io's flat(ish) plains, rather than those flows within the topographic confines of a patera. It is thought that these flows are compound pahoehoe silicate lava flows, built up by small outbreaks on top of older flows punctuated by period of high eruption rates that rapidly grow the lava (akin to flood basalts on Earth). They determined that dark flow fields are not distributed evenly across all longitudes with a peak near the center of the anti-Jovian hemisphere (in the abstract, the anti-Loki hemisphere). Flows seen near this peak include Prometheus, Zamama, Thor, Culann, Volund, and Mycenae Regio. This correlates well with a peak in the distribution of volcanic centers and paterae. The authors note that a correlation is not seen at the other peak in volcanic centers and paterae near 325° West, which includes Loki.

The authors then examined the contribution these lava flows make to Io's total heat flow. In the abstract, they focused on two prominent flow fields: Lei-Kung Fluctus (shown above, big flow field on Io's northern trailing hemisphere) and Amirani. Using NIMS and PPR data from Galileo, the authors calculated that that the two lava flows contribute 4.5x10¹¹ W and 1.5x10¹² W to Io's total heat flow, which is on the order of 10¹⁴ W. In total, the 24 flow fields the authors examined contribute approximately 10% to Io's total heat flow, equivalent to Loki Patera. It should be noted that the authors mapped about 3x10⁵ km² worth of dark flows. This is about 25% of the total amount of dark lava flows covering Io's surface according to the mapping done by Williams et al., so that 10% figure maybe an underestimate. I can't tell, but it also seems like they assumed an effective temperature (basically an average temperature for the entire flow field) to come up with their heat flow numbers for at least some of the fields they mapped.

An interesting abstract. I will be interested in seeing how their dark flow mapping compares to what Williams et al. has done (this also sounds like the kind of project that global geologic mapping is suited for), particularly since Williams et al. mapped a factor of 4 more dark flows material than Veeder et al. did.

Link: Io: Heat Flow from Dark Volcanic Fields [www.lpi.usra.edu]

LPSC 2008: Links between Volcanism and Tectonism on Io

Continuing my series of posts on the Io-related posters and abstracts for next week's Lunar and Planetary Sciences Conference, we look at Giovanni Leone et al.'s poster "Links between Volcanism and Tectonism on Io: A Comparative Study of Monan Patera, Amirani, and Prometheus". In their abstract, the authors developed models for the subsurface structures underneath Monan, Amirani, and Prometheus by looking at the tectonic structures on the surface, such as mountains and fractures, and the shapes of the paterae.

The central thesis of their work is that the same faults that act as zones of weakness that leading to the thrusting upward of the many mountains that dot the ionian landscape are also used as conduits for magma in deep-seated reservoirs to reach the surface. They used images Galileo took of these three volcanic center to model the relationship between faults, subsurface magma reservoirs, and volcanic features on the surface.

For Prometheus, their subsurface model includes three magma reservoir: a primary reservoir in the asthenosphere, a secondary reservoir at around 17 km below the surface in the middle of the lithosphere, and a shallow reservoir between 3 and 5 km below the surface. They suggest that the three reservoirs maybe connected by a thrust fault that bounds the western margin of Prometheus Mensa, a low, lumpy mesa to the east of the Prometheus volcano, and the eastern margin of Prometheus Patera. This thrust fault was the source of the N-S lava flow seen by Voyager at Prometheus and the 75-km long lava flow that developed between Voyager and Galileo. The authors suggest, based on superposition relationships between individual lava flows in the main flow field, that this thrust fault became clogged with cooled magma and a new conduit was activated on the western side of the flow field, feeding the flows that generate the main Prometheus plume. It should be noted though that thermal emission was seen by NIMS at the thrust fault as well as the western end of the flow field, so lava is exposed at the surface in both areas (at least it was eight years ago). There is also a small plume deposit reddish material emanating from the thrust fault, suggesting that it is still active as a primary vent.

Their analysis of the images of the Amirani and Monan volcanoes suggests that those two volcanoes, as well as Maui Patera, are fed by the same magma reservoir systems. Their model suggests that branching conduits from the shallow magma reservoir underneath the Amirani vent feed Maui Patera (to the southwest of the Amirani vent) and Monan Patera (to the southeast of the Amirani vent). Diapirs in the asthenosphere imping on the base of the lithosphere cause the uplifting of nearby mountains Monan Mons and an unnamed mountain to the southwest of Maui Patera. They also suggest the presence of a cold pluton (basically a solidified magma reservoir, though the term applies to any body of magma that has solidified below the surface).

A very interesting abstract. I particularly like the discussion on the subsurface connection between different volcanoes like Amirani and Monan. Such connections can be inferred in other regions as well, particularly where nearby, normally quiescent, volcanoes erupt in a relatively short span of time. Interesting examples include the volcanoes of the Tvashtar Paterae (basically four separate volcanoes partially surrounded by two mesas) and Thor.

Link: Links between Volcanism and Tectonism on Io: A Comparative Study of Monan Patera, Amirani, and Prometheus [www.lpi.usra.edu]