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date: 03 December 2022

Terrestrial Analogs to Planetary Volcanic Phenomenafree

Terrestrial Analogs to Planetary Volcanic Phenomenafree

  • Peter J. Mouginis-MarkPeter J. Mouginis-MarkHawaii Institute Geophysics and Planetology, University of Hawaii
  •  and Lionel WilsonLionel WilsonLancaster Environment Centre, Lancaster University

Summary

More than 50 years of solar system exploration have revealed the great diversity of volcanic landscapes beyond Earth, be they formed by molten rock, liquid water, or other volatile species. Classic examples of giant shield volcanoes, solidified lava flows, extensive ash deposits, and volcanic vents can all be identified, but except for eruptions seen on the Jovian moon Io, no planetary volcanoes have been observed in eruption. Consequently, the details of the processes that created these landscapes must be inferred from the available spacecraft data. Despite the increasing improvement in the spatial, temporal, compositional, and topographic characteristics of the data for planetary volcanoes, details of the way they formed are not clear. However, terrestrial eruptions can provide numerous insights into planetary eruptions, whether they are effusive eruptions resulting in the emplacement of lava flows or explosive eruptions due to either volatiles in the magma or the interaction between hot lava and water or ice. In recent decades, growing attention has been placed on the use of terrestrial analogs to help interpret volcanic landforms and processes on the rocky planets (Mercury, Venus, the Moon, and Mars) and in the outer solar system (the moons of Jupiter and Saturn, and the larger asteroids). In addition, terrestrial analogs not only provide insights into the geologic processes associated with volcanism but also can serve as test sites for the development of instrumentation to be sent to other worlds, as well as provide a training ground for crewed and uncrewed missions seeking to better understand volcanism throughout the solar system.

Subjects

  • Observational and Experimental Techniques
  • Planetary Science Policy and Planning
  • Planetary Surfaces

What Makes a Good Terrestrial Analog?

Analogs for geologic phenomena on the planets can take many forms, but perhaps the most valuable allow the geologist to better understand processes that operated beyond the Earth millions to billions of years ago (Baker, 2014). Analogs may focus on active terrestrial phenomena that have long since ceased elsewhere in the solar system or are currently too remote for current or near-future examination beyond the Earth. In this vein, this article focuses on using terrestrial volcanoes and volcanic processes to better understand volcanic phenomena throughout the solar system.

Analog studies are of the greatest value at a scale that is too large for direct analysis. For instance, one of the best insights into the impact cratering process came from the field investigations of the Manicouagan impact structure in Canada in the 1970s (Simonds et al., 1976). Large ejecta blocks “frozen” in quenched impact-produced melt provided new understanding of the process that would subsequently be investigated on the Moon using data from the Lunar Reconnaissance Orbiter and other spacecraft (e.g., Carter et al., 2012). Similarly, the distribution of material within giant landslides, such as the ones observed in Valles Marineris on Mars (Lucas & Mangeny, 2007; Lucchitta, 1979), is another process that is impossible to model because of scale. Thus, the Socompa landslide in Chile (Francis et al., 1985; van Wyk de Vries et al., 2001) provides a rare opportunity to observe transport mechanisms that controlled where material from different elevations within a large landslide finally came to rest. Although field observations from the NASA Mars Exploration Rover Opportunity (Crumpler et al., 2015) have yielded important insights into the subsurface structure of impact craters on Mars, it was the field observations of basal scouring of the ejecta blanket of Ries Crater in Germany (Hörz & Banholzer, 1980) that subsequently proved to be directly relevant to the emplacement of the ejecta layers around fresh Martian impact craters (Boyce & Mouginis-Mark, 2006).

The focus of this article is on volcanic processes, landforms, and related phenomena, be they ancient eruptions on airless worlds such as the Moon or Mercury, geologically more recent eruptions potentially including interactions with water or ice on Mars, or currently active volcanism on Io (a moon of Jupiter) and (almost certainly) Venus. Magmas need not be silicate melts; cryovolcanism involving liquid water instead of liquid rock is active on the Saturian moon Enceladus (Cooper et al., 2009) and probably on the Jovian moon Europa (Sparks et al., 2017). The observation that flows of water with entrained rock fragments (i.e., mud) can have similar flow properties to some types of lava flows led Wilson and Mouginis-Mark (2014) to make the caveat that similarity of morphology is not a guarantee of similar chemistry. Indeed, differences between the atmospheric pressure and the acceleration due to gravity can lead to significant differences between the morphologies of features produced by the same process on different solar system bodies (Wilson & Head, 1981a). Nevertheless, the focus of this article is on silicate volcanism, and analogs are drawn largely from basaltic volcanoes on Earth. This is because none of the other silicate planets exhibit plate tectonics. Without recycling of crustal rocks into the mantle to modify their compositions, all magmas erupted on these bodies are close compositional relatives of basalts, the primary mantle melts erupted on Earth at mid-ocean ridges and hot spots.

This article first considers how observations of eruptions on Earth can provide information about the temperature distribution within an active lava flow (which has direct relevance to observation of ongoing activity on Io) as well as among airborne pyroclasts from explosive eruptions. Lava–water or lava–ice interactions have been proposed for Mars, but only on Earth can the details of the process be observed. Because the fine-scale morphology is rapidly eroded from lava flows on Mars, a comparison with the equivalent fresh lava flows on Earth is sometimes needed to refine ideas on the mode of emplacement. The article also considers factors controlling the lengths of lava flows, aspects of explosive volcanism that may pertain to the formation of the Medusae Fossae Formation on Mars, and the internal structures of planetary volcanoes.

The article concludes with the recognition that terrestrial analogs have additional value for the analysis of planetary volcanic landforms. Frequently, instruments that are subsequently flown to the planets are first tested in Earth orbit. Synthetic Aperture Radar is one such example that first flew in Earth orbit with the Seasat mission in 1978; comparable systems were subsequently used to study volcanoes on Venus (Campbell et al., 1993). A second application of analogs is the need to gain information on the potential difficulties in moving across a planetary landscape; such procedures need to be established and practiced for either robotic or human exploration of other planetary bodies (Yingst et al., 2013), particularly as astronauts prepare to return to the Moon later in this decade.

Lava Temperatures

Since the Voyager 1 encounter with Jupiter in 1979 (Morabito et al., 1979), the active volcanism on the moon Io has generated considerable debate about the magma chemistry and the styles of eruption. Early interpretations of lava flow chemistry included possible sulfur flows (Pieri & Baloga, 1984), but as subsequent spacecraft made higher resolution observations of Io, the notion arose that high-temperature magmas (perhaps as hot as 1,700oC) could be erupted (McEwen et al., 1998). If this were the case, these lavas are most likely to be ultramafic (magnesium-rich) silicates and offer an insight into a type of eruption not seen on Earth for billions of years (Mole et al., 2014). However, these conclusions were based on data collected using multispectral imagers with broad spectral coverage so that the peak temperatures were challenging to determine.

Because Io can be observed telescopically from Earth, time-series data for specific features on Io, such as the giant lava lake Loki (figure 1), can be utilized to build models for the rate of overturning of the lake surface (Lopes et al., 2004, 2018). For example, the lava lake at Pele on Io is ~30 km in diameter, and it almost certainly contains a persistent lava fountain. Escaping gas probably disrupts the crust of the lava lake and exposes large expanses at or near eruption temperature (Davies et al., 2011). Lava temperatures for Io eruptions have frequently been given as peak temperatures but rarely as a combination of temperature and area. In comparison, terrestrial studies have evolved from two-band estimates by Dozier (1981) to hyperspectral measurements of active lava flows at night (Flynn & Mouginis-Mark, 1992, 1994).

Figure 1. Active lava lake at Loki volcano on Io (13.0oN, 303.8oW). Top two images are a Voyager 1 ISS view of Loki in 1979. The white box in the top image shows the area of coverage of the middle image. Loki Patera (delineated by arrows) is ~200 km in diameter. Bottom image shows two observations made by the Galileo NIMS instrument spaced 4½ months apart in 1999 and 2000 that illustrate the dynamic nature of this feature, which is interpreted to be an active lava lake. Temperature contours in the lower image are in Kelvin.

Sources: The top two images are Jet Propulsion Laboratory (JPL) Photojournal frame PIA00320, and the bottom image is JPL Photojournal frame PIA02549.

Field spectroradiometer data collected in the wavelength range 0.4–2.5 μ‎m and at a spectral resolution of 1–5 nm were first used by Flynn et al. (1993) to compute the radiative temperature of the surface of an active lava lake at Kīlauea volcano, Hawai‘i (figure 2). They found that fresh magma radiating at 1, 100oC formed up to 29% of the flow area, while cooling from magmatic temperatures to newly formed crust at 790oC took place in a matter of seconds. During quiescent periods, the lake was covered by a thick (a few centimeters?) crust that had a temperature of 80–345oC and indicated that temporal variability of the thermal output of the lava lake occurred on the timescale of seconds to minutes. Such dynamic changes over short periods of time (possibly seconds to tens of seconds) are unlikely to be true for Io because of the greater size of the features, but nevertheless these high-spectral resolution observations imply that caution is needed when trying to interpret a few broadband spectral measurements obtained with limited temporal resolution during a flyby encounter.

Figure 2. Hyperspectral data collected on an active lava flow (left) and the Kupaianaha lava pond in Hawai‘i (right) allowed the determination of temperature as a function of fractions of the area, which is essential for the determination of the style of activity of lava flows on Io.

Source: Both images were taken by P. J. Mouginis-Mark.

Ejecta Temperatures

On all volcanically active rocky bodies lacking atmospheres (i.e., the Moon, Mercury, Io, and certain early forming differentiated asteroids), the zero pressure to which erupting magma is exposed would maximize both the release of volatiles from the liquid rock and the subsequent expansion of those volatiles. This would lead to the expectation that essentially all such eruptions will be explosive, with extremely efficient magma fragmentation into small, largely sub-millimeter, droplets (Morgan et al., 2021; Wilson & Head, 1981b, 2017). The small droplet size commonly causes the resulting lava fountains to be optically dense, with only droplets at the very outermost part of the fountain being able to radiate their heat into space, thus ensuring that most of the ejecta will still be hot when it reaches the ground.

Lava fountains on Earth involve larger magma droplets and are opaque to translucent (figure 3) so that while a hot lava pond may accumulate in the middle of the fountain, the pond will be surrounded by a spatter/cinder cone where the ejecta falls back to the surface having cooled significantly. Lava fountains on the Moon were mostly opaque due to the small pyroclast sizes combined with high mass eruption rates, leading to hot lava ponds forming the sources of lava flows and sinuous rilles (Hurwitz et al., 2012). Numerous “dark mantle deposits” interpreted to be pyroclastic deposits have been identified on both the Moon (Gaddis et al., 1985, 2003; Weitz et al., 1998) and Mercury (Goudge et al., 2014), and the orange glass beads discovered by the Apollo 17 astronauts have been shown to have formed during explosive volcanic activity (Arndt & von Engelhart, 1987) (figure 4). Silicate and/or sulfur eruption plumes on Io (Cataldo et al., 2002; Kieffer, 1982), as well as water ice plumes from vents on the Saturnian moon Enceladus (Kite & Rubin, 2016), are almost completely transparent (figure 5) because although their pyroclasts have small sizes, the volatile contents are so large and the gravity so small that the maximum clast ranges are large and so the inter-clast spacings are also large.

Figure 3. Studying the shapes and trajectories of pyroclasts, as well as their rate of cooling, holds importance for the modeling of explosive eruptions on airless bodies, as well as the analysis of volatile-rich eruptions on Mars and the Moon (and possibly Venus). (Left) An explosive eruption in April 1975 at Stromboli volcano, Eolian Islands. Note how the individual hot lithics separate with greater range from the vent. (Right) Details of the edge of a lava fountain from the Pu‘u ‘O‘o cone, Hawai‘i, in August 1984. Here, the color (i.e., temperature) of this plume changes from the vent (at left) to the edge of the plume (at right).

Source: Both images were taken by P. J. Mouginis-Mark.

Figure 4. Pyroclastic deposits on the Moon, as seen at the Apollo 17 Taurus Littrow landing site (left) and at Alphonsus crater (119 km in diameter; 13.4oS, 2.8oW), where several dark mantle deposits are arrowed (right).

Sources: (Left) Apollo image AS17-137-20990 and (right) Lunar Reconnaissance Orbiter Wide Angle Camera image.

Figure 5. Plumes on Io (left) and Enceladus (right). Pyroclastic eruptions on both moons will be very different from those on Earth because they are under vacuum conditions, especially in a low-gravity environment.

Sources: (Left) Jet Propulsion Laboratory Photojournal frame PIA09248 and (right) Jet Propulsion Laboratory Photojournal frame PIA17184.

This thermal argument implies that there would be much less localized accumulation of cool spatter and cinders around vents on these airless bodies, explaining the rarity of cinder and spatter cones on the Moon and Mercury (Broz et al., 2018) and the general difficulty of identifying vent locations (Head & Wilson, 2017). In the case of even smaller planetary bodies, such as the asteroid 4 Vesta, it is estimated (Wilson & Keil, 1997) that the lava fountains would have been extremely optically dense so that typically <1% of the pyroclasts would have sufficiently cooled to form recognizable fall deposits. For even smaller asteroids (<200 km diameter), such as the aubrite parent body, explosive activity produced by even a very small volatile content of the magma would cause the expansion of the melt into a spray of droplets moving faster than the local escape velocity (Wilson & Keil, 1991) so that all the pyroclasts would be lost into space rather than create a deposit around the vent.

Lava–Water or Lava–Ice Interactions

Several geologic studies of Mars suggest that there has been extensive lava–water or lava–ice interactions. Hamilton et al. (2011) proposed that the tens of thousands of pitted cones in the Tartarus Colles region formed as rootless vents due to the emplacement of lava over a shallow layer of ground ice that became volatilized. The large outflow channel Hrad Vallis may have originated from the interaction of a dike or sill from Elysium Mons with permafrost (Hamilton et al., 2018; Wilson & Mouginis-Mark, 2003). Explosive activity on Earth is often associated with water becoming trapped in active lava tubes at the coast (Mattox & Mangan, 1997), producing abundant fragmented material that is sufficiently cold on landing that it builds a cinder cone rather than a lava flow (figure 6, left). Water–lava interactions have also been documented at the 2014–2015 Holuhraun eruption in Iceland, where cooling of the lava flow by a proglacial river system produced an ephemeral hydrothermal system (Dundas et al., 2020). Dundas et al. (2020) also made the Earth–Mars connection and suggested that cooling of Martian lava flows by melted ground ice could have influenced the thermal history of the flows.

Figure 6. Two views of lava reaching the ocean at Kīlauea volcano, Hawai‘i. (Left) The airborne fragments of lava are the result of wave action flooding an active lava tube with the resultant explosive steam eruption fragmenting the lava and producing a cinder cone. The largest particles are estimated to be ~1 m in size. (Right) An air photo of active lava flows (at right in image) entering the ocean (at left) that produce extensive vapor plumes. These plumes contain not only steam but also gases such as HCl and HF produced by the interaction of hot rock and ocean water.

Source: Both images were taken by P. J. Mouginis-Mark.

A second potential consequence of lava–water interactions on Mars may also be illustrated by volcanic activity in Hawai‘i, where flowing lava entering the ocean creates giant stream plumes (figure 6, right). Kullman et al. (1994) and Heggie (2009) showed that these plumes, which have the local name “laze, ” contain quantifiable concentrations of hydrochloric acid (HCl) and hydrofluoric acid (HF). Airborne particulates were composed largely of chloride salts (predominately sodium chloride). Such a situation seems plausible within Cerberus Fossae on Mars (figure 7), where there may have been alternating episodes of large fluvial events followed by the eruption of fluid lavas (Berman & Hartmann, 2002; Plescia, 2003). Had these events occurred relatively close in time, the lava could have encountered standing bodies of water (or meltwater generated by the lava flowing over ice layers at a shallow depth). It is possible that volcanic laze plumes could have been generated on Mars, with HCl being one of the products. If this were the case, then the high chlorine levels detected in this region by the Gamma Ray Spectrometer onboard the Mars Odyssey spacecraft (Diez et al., 2009; Keller et al., 2006) could be due to surficial deposits from this relatively recent volcanism rather than the reflection of unusually chlorine-rich parental magmas.

Figure 7. Thermal Emission Imaging System (THEMIS) daytime infrared (IR) (top) and THEMIS nighttime IR (bottom) mosaics of Cerberus Fossae (10oN, 158oE), Mars, where both water and lava appear to have come out of fracture that cross the terrain from top left to lower right. The bright surfaces in the nighttime data are blockier than the surrounding units. Many authors infer that there have been large volumes of water, interspersed with large volumes of lava, released from the young graben at Cerberus Fossae. Did these eruptions occur sufficiently close in time to the water release for the two to interact?

Source: NASA/JPL-Caltech/Arizona State University.

The Lengths of Lava Flows

It has been proposed for Earth that some of the Deccan and Rajahmundry Trap lavas in India are the longest and largest flows, potentially up to ~1,000 km in length (Self et al., 2008). The Moon, Mars, and Venus all have many examples of lava flows that are hundreds of kilometers in length (figure 8). Zimbelman (1998) recognized that on the Moon, the longest flows in Mare Imbrium may extend ~400 km from the probable vents, whereas some Martian flows may be ~500 km long, and on Venus flows can be at least 180 km in length. It is tempting to use this observation to infer information about eruption conditions and mantle processes on these planets (Wilson et al., 1992), but clearly the eruption of a large volume of magma requires conditions that are different from those seen on terrestrial basaltic volcanoes. However, there are many problems when using comparisons of the lengths (and widths and thicknesses) of lava flows on multiple planets to infer eruption conditions (e.g., Glaze & Baloga, 2006). Visible lava flows are the most recent ones on a given terrain. One long or wide lava flow may bury many smaller flows, so there may be a bias toward observing areally extensive flows. Also, there is a general tendency among all geophysical phenomena for large-scale events to occur less frequently than small-scale events. This means that on Earth, where rain and wind cause rapid rates of erosion, the deposits from infrequent large-volume events such as flood basalt eruptions are not well preserved, whereas on bodies without atmospheres, despite the effects of meteoroid bombardment, they are generally readily identified.

Figure 8. Long lava flows within Mare Imbrium, just south of Mons La Hire (27.66oN, 25.51oW), on the Moon (left), southwest of Arsia Mons (17.68oS, 235.5oE) on Mars (center), and north of Sif Mons volcano (24oN, 352oE) on Venus (right) are all much longer than terrestrial flows. The Venus image is a Magellan radar backscatter image, wherein rough surfaces appear bright and smooth surfaces are dark.

Sources: From left to right, images are, respectively, Apollo AS15-M-1557, THEMIS Visible Imaging System mosaic, and Jet Propulsion Laboratory Photojournal image PIA00471.

Despite these sources of uncertainty, there seems little doubt that where very large numbers of long and voluminous lava flows are observed, there is an implication for the conditions in the mantle and crust that are producing the magmas and controlling their transfer to the surface. This topic is discussed further in the section on “Lava Textures, ” but note here that whereas the volumes of lava flows have implications for the melting processes in the mantle, the length of a flow is more closely related to the rate at which lava is erupted from the vent feeding the flow, with cooling setting the maximum distance to which a flow can travel (Pinkerton & Wilson, 1994; Rowland & Harris, 2004; Self et al., 2008). But even here there is a source of uncertainty. In addition to long lava flows, there are numerous volcanic features on the Moon (with some also on Mars) called sinuous rilles (figure 9). These are meandering channels typically ~1 km wide incised into the surface for many tens of kilometers and are interpreted to be the result of lava flows removing the surface immediately beneath them by a mixture of thermal and mechanical erosion (Carr, 1974; Fagents & Greeley, 2001). Even longer (>6, 000 km) channels called canali (see figure 9) exist on Venus, and they too may have formed as lava channels (Gregg & Greeley, 1993).

Figure 9. Sinuous channels, also called sinuous rilles, can be found on the Moon (left), Mars (middle), and Venus (right, arrows). The lunar example is Schröter’s Valley (26.2oN, 50.8oW), with the 35-km diameter Herodotus crater near the source. The Martian example is located on the southern flank of Arsia Mons volcano (12.18oS, 238.79oE). The Venus image shows a canali (49oN, 165oE) and is a Magellan radar backscatter image.

Sources: From left to right, images are, respectively, Apollo 15 image AS15-M-2611, THEMIS VIS mosaic, and Jet Propulsion Laboratory Photojournal image PIA00245.

It is strongly suggestive that the lava producing these channels must have flowed in a turbulent manner to maximize heat transfer to the ground (Fagents & Greeley, 2001), and to make this possible, the volume of lava erupted per second must have been very large. But if lava travels in a turbulent manner, all the lava is stirred up to the surface and can cool by radiation and, if there is an atmosphere, by convection. In contrast, if lava travels in a smooth, laminar fashion, only the top and sides of the flow are exposed to the surroundings to lose heat, and the interior of the flow is insulated from heat loss (Fagents & Greeley, 2001; Rowland & Harris, 2004). Thus, a turbulent flow loses heat and cools at a much greater rate than a laminar flow. Of course, the rapid heat loss has consequences: The viscosity of the lava increases as it cools and mineral crystals grow within it, introducing a non-Newtonian component to its rheology and further impeding its ability to flow so that its motion eventually changes from turbulent to laminar. But by then, the bulk of the lava has been significantly cooled, whereas a flow that never had a turbulent phase will have a cool skin but a very hot interior and may flow farther than an initially turbulent flow. Large turbulent flows on Earth all occurred long ago and involved very low-viscosity lavas called komatiites (Huppert et al., 1984; Williams et al., 1998). Despite their relatively poor preservation, these, along with flood basalts (Courtillot et al., 1999; Jerram & Widdowson, 2005), may be the best analogs of many long flows on the Moon, Mars, and Venus. However, remaining uncertainties about the details of their emplacement warrant further research (e.g., Katona et al., 2021).

Lava Textures

One of the most enigmatic lava textures on the planets occurs within the Cerberus Fossae region of Mars (figure 10). Since Plescia (1990) first identified this as the site of some of the youngest lava flows on Mars, the plate-like structure of the surface has attracted attention. The structure of these flows is also of interest because of the potential interaction between the erupting lava and large volumes of surface water (Burr et al., 2002; Jaeger et al., 2007). In places, the surface of the flow had been fragmented in such a way that it appears almost as a jig-saw puzzle that could be reassembled into a single surface. This led Murray et al. (2005) to propose that this was an ice sheet, like sea ice on Earth. The proposed “frozen lake” was estimated to be ~800 × 900 km in size and to have an approximate age of 5 ± 2 million years, and thus had important implications for the geologically recent volatile history of the planet. However, the Shallow Radar on the Mars Reconnaissance Orbiter proved that the dielectric properties of the material precluded ice near the surface (Alberti et al., 2012). Crater counts (Page, 2010), which apparently showed that the surfaces of these plates had a drastically younger age compared with the intervening layers, were also challenging to interpret, and they drew into doubt the volcanic origin of these flows. The mode of formation of these “lava plates” was therefore originally quite enigmatic.

Figure 10. Example of the lava plates within the Cerberus region of Mars. Murray et al. (2005) argued that these plates are due to drifting ice on the surface of a freeing body of water, whereas Page (2010) found that the plates appeared to have an older age than the intervening smoother segments. Black lines connect parts of the plates that appear to have moved apart.

Source: High Resolution Imaging Science Experiment image ESP_045464_1860, centered at 5.87oN, 150.14oE.

The origin of the plates was resolved via use of an analog in Iceland, where it was determined by Keszthelyi et al. (2004) that platy-ridged lavas are associated with high-flux eruptions and brecciated flow tops (rubbly pahoehoe). Comparable features have also been observed on the surface of the December 1974 lava flow in the Ka‘u Desert of Hawai‘i (figure 11) (Hamilton et al., 2015), confirming the trapping of lava in local depressions so that an initial solidified surface formed prior to the subsurface, still fluid, part of the flow draining downslope. This drainage resulted in the rupturing of the solidified surface to produce the “jigsaw puzzle” pattern that remains. Thus, in the context of the Cerberus Fossae flows, the most likely interpretation is that these were highly fluid lava flows moving across undulating terrain. The flows temporarily ponded in shallow topographic lows until the high discharge of the lava caused sufficient erosion of these confining depressions to allow drainage downslope. During the period of ponding, the flows developed a thin cooled skin that was then disrupted by the subsequent drainage downslope of the flow.

Figure 11. Two views of the lava plates on the surface of the December 1974 lava flow in Hawai‘i. Top image is an air photo (direction of flow was from lower right to top left). The flow is ~225 m wide at the narrowest point in this image. The image at bottom is a ground view. A glove rests on the squeeze-up lava that forms the darker unit. The lava plates are the lighter gray-colored materials. This flow is an excellent analog to platey flows seen within Cerberus Fossae on Mars.

Source: Both images were taken by P. J. Mouginis-Mark.

Explosive Volcanism

The deposits associated with the plumes on Io and Enceladus are readily recognized in images (see figure 5), and many lunar samples returned by the Apollo missions contain sub-millimeter-sized glass droplets that are clearly pyroclasts (Delano, 1986). However, there are few clear examples of deposits formed by explosive volcanism on Mars or Venus. This may be explained by the high atmospheric pressure on Venus suppressing the release of magmatic volatiles (Head & Wilson, 1986), but the low atmospheric pressure on Mars should encourage explosive activity (Wilson & Head, 1994). One potential example is the enigmatic Medusae Fossae Formation (MFF) on Mars (figure 12, top). However, even after more than 40 years of investigation, it is not clear if the MFF is an ignimbrite (Bradley et al., 2002; de Silva et al., 2010; Scott & Tanaka, 1982), an air-fall deposit (Kerber et al., 2011, 2012), or something else (Mouginis-Mark & Zimbelman, 2020).

Terrestrial analogs may help resolve the origin of the MFF from the style of erosion of the deposit. For example, the June 15, 1991, eruption of Mt. Pinatubo in the Philippines produced >6 km3 of pyroclastic materials that were deposited on the flanks of the volcano (figure 12, bottom left) to a depth exceeding 200 m in places where preexisting valleys were filled (Scott et al., 1996). This material was hot on emplacement (60–80oC) but at an insufficient temperature to fuse the deposit together and thus it is easily eroded by subsequent rainfall. In contrast, some of the large-volume (>600 km3; de Silva & Francis, 1989) ignimbrites in northeast Chile (figure 12, bottom right) were emplaced at sufficiently high temperatures for vapor-phase alteration pipes to form in association with locally derived water (Wright et al., 2011). Observations of the internal structure of the MFF, either from high-resolution images (Zimbelman & Griffin, 2010) or from sounding radars in Mars orbit (Carter et al., 2009; Ojha & Lewis, 2018), may therefore help resolve the origin of the MFF if it had an explosive volcanic origin.

Figure 12. (Top) A computer-generated oblique view of the Medusae Fossae Formation on Mars (10oS, 190oE). Is this an air fall, an ignimbrite, or something else (Mouginis-Mark & Zimbelman, 2020)? Maximum height change in this view is ~1,600 m. This is a Digital Elevation Model (DEM) produced from Context Camera images D20_035124_1696 and D21_035546_1696. (Bottom left) Steve Self and Ronnie Torres admire the June 15, 1991, air-fall deposit from Mt. Pinatubo, the Philippines. This was unwelded (but still hot!) in November 1999. (Bottom right) Peter Francis is staring at some vapor-phase alteration pipes in one of the ignimbrites in northeast Chile. These ignimbrites were welded on deposition.

Sources: (Top) Harold Garbeil (University of Hawaii) produced the DEM and P.J. Mouginis-Mark created the image. (Bottom) Both field photographs were taken by P. J. Mouginis-Mark.

Internal Structure of Volcanoes

The internal structure of a planetary volcano cannot be confidently determined from satellite images, and yet gaining such an understanding is one of the high priorities for studying volcanoes on Mars (Mouginis-Mark et al., in press). Although volcanoes on Venus and Mars show clear signs of preferential activity either at the summit of the volcano or along one of the flanks, how the magma rising from depth ultimately reached the surface is not clear. There are many issues here. Earth has a crust consisting of a diverse mixture of volcanic, sedimentary, and metamorphic rocks with a significant range of densities. In contrast, the Moon may have a unique low-density crust resulting from flotation of anorthosite on a deep magma ocean (Nemchin et al., 2009; Wood et al., 1970). Mars and Venus appear to have crusts dominated by volcanic rocks, although the situation for Mars appears much more complicated in the highland areas of the planet due to protracted impact cratering and the role of volatiles. A major issue is whether Venus could have the remains of a lunar-type low-density crust from an early magma ocean hidden under extensive later volcanics (Hashimoto et al., 2008). In contrast, compared with the Moon, the relatively thin dense crust of Mercury allows for the possibility that major impact events (such as the one that formed the Caloris basin) may have excavated material from Mercury’s mantle (Padovan et al., 2015; Sori, 2018).

Eroded volcanoes on Earth therefore present the best opportunities to study the internal structures of volcanoes and then draw comparisons with their planetary equivalents. Ship Rock, New Mexico (figure 13), is one of the classic examples of an exposed dike system associated with a volcanic plug, the remains of a small magma chamber where magma accumulated. Delaney and Pollard (1981) studied the northeastern dike from Ship Rock, which has an outcrop length of ~2,900 m, an average thickness of 2.3 m, and a maximum thickness of ~7 m. They concluded that this type of dike propagation was the dominant mechanism for creating conduits for magma ascent where the host rock was brittle and elastic. Seven dikes form a radial pattern centered on Ship Rock, which may be analogous to volcanoes such as Maat Mons on Venus (Mouginis-Mark, 2016). In addition, the giant dike swarms in ancient, heavily eroded continental crustal rocks on Earth must surely be analogs for the giant graben swarms preserved on Mars (Ernst et al., 2001; Pedersen et al., 2010; Rivas-Dorado et al., 2021) and Venus (Ernst et al., 1995). However, there are only very rare examples of exposed dikes on Mars (figure 14); no doubt, this reflects the lack of substantial erosion experienced by the large volcanic constructs, which have relatively young ages (Amazonian to Late Hesperian; Werner, 2009) and were formed after the earlier era when erosion by rain and water was possible.

Figure 13. Ship Rock in New Mexico (36.67oN, 108.83oW) displays a rare example of blades of dikes that have been exposed at the surface through erosion. The width of the dike is ~2 m, and the main volcanic plug (in the distance, at right) is ~500 m wide and stands ~600 m above the surrounding plain.

Source: Both images were taken by P. J. Mouginis-Mark.

Figure 14. A rare example of an exposed dike on Mars where it intersects a possible table mountain called Galaxias Mons (Mouginis-Mark & Wilson, 2016). (Left) A dike on the surrounding terrain and the probable trace of the dike through Galaxias Mons (34.75oN, 141.22oE). The area shown at right is denoted by the white box. (Right) A higher resolution view of the dike (illumination from the left).

Source: Both images are part of Context Camera frame G19_025816_2145.

A clearer picture of the importance of the subsurface structure of volcanoes along their rift zones may be seen at the Ko‘olau volcano on O‘ahu, Hawai‘i (figure 15). Along the rift zone of this volcano, dikes can comprise as much as 60% by volume of the rock body (Knight & Walker, 1988). Individual dikes are seen to widen and thicken over distances of a few tens of meters and “meander” toward the now-eroded summit. Such observations were used by Wilson and Head (1992) to make inferences about the geometry of the conduit system along the East Rift Zone of Kīlauea volcano.

Figure 15. Dike swarm exposed at Kapaa Quarry, O‘ahu, showing multiple dikes with variable thickness and orientations. Depth beneath the original surface of the Ko‘olau Volcano may be ~600–700 m. These dikes may provide information for giant dikes on Mars, such as the ones from Arsia Mons. The large 10-wheeled truck and the two people (circled) provide the scale.

Source: Image taken by P. J. Mouginis-Mark.

Care is needed in using observations of the internal structures of volcanoes on Earth to make inferences about the structures of volcanoes elsewhere. This is because both the locations and the sizes of magma reservoirs depend not only on the relative densities of the magma and the rocks surrounding it but also on the variation, with depth below the surface, of the pressure in the rocks and magma (Huber et al., 2019). The pressure at a given depth within a planet is the product of the average density of the rocks above that depth, the depth itself, and the acceleration due to gravity. When the gravity is less than that on Earth (which is the case on all the other rocky bodies in the solar system), the pressure is smaller at a given depth than on Earth and the rate of change of the pressure is also less. The net effect of this is that the centers of magma reservoirs tend to be deeper on other bodies and the vertical extents of the reservoirs tend to be greater (Head & Wilson, 1992). The effect of gravity on the horizontal extents of reservoirs is less clear, but even so, the consequence of these factors is that magma reservoirs on other planets are modeled to both contain and erupt greater volumes of magma than on Earth. This gravity scaling appears to be at least a first-order explanation of the sizes of shield volcanoes on Mars compared with the largest on Earth. It does not, however, explain the sizes of the largest shields on Venus, which are up to twice as wide but approximately three times lower than those on Earth (Head & Wilson, 1992), presumable because of still poorly understood differences in the structure of the crust.

Understanding New Instruments

The instrumentation carried to the planets by spacecraft will continue to become more sophisticated with time, and hence there will continue to be a need to test techniques as well as understand data. This is particularly true for the analysis of the diversity of volcanic phenomena on the planets. One of the earliest analog experiments was conducted with imaging radars flown either on satellites in Earth orbit or on aircraft to obtain data on lava flows that were subsequently used to understand the radar backscatter characteristics of lava flows on Venus (figure 16), as revealed by the Magellan spacecraft (Campbell et al., 1993). The choice of radar wavelength, polarization, and incidence angle in the analysis of lava flows on Earth has been explored by MacKay and Mouginis-Mark (1997). A further example was the characterization of geologic surfaces on Earth using laser altimeters (e.g., Morris et al., 2008), enabling terrain analysis of Martian and lunar surfaces by orbiting altimeters (Head et al., 2002; Kreslavsky et al., 2013).

Figure 16. Use of synthetic aperture radar to study the distribution of lava flow types on Earth helps in the interpretation of lava flow morphologies on Venus. (Left) An L-band (24-cm wavelength, VV-polarization) radar image of the Ka‘u Desert in Hawai‘i showing (just below image center) the December 1974 lava flow as a bright flow that erupted to the right in this image. (Right) A series of radar-bright and -dark lava flows are shown encountering a north-trending ridge belt in the Lada region (47oS, 25oE) on Venus.

Sources: (Left) NASA/Jet Propulsion Laboratory image and (right) Jet Propulsion Laboratory Photojournal image PIA00486.

What has been learned from the use of radar interferometry for terrestrial volcanoes could also be applied to Venus to search for active volcanism. The deformation studies of Amelung et al. (2000) for volcanoes in the Galapagos Islands and Jung et al. (2011) for Kīlauea might enable ground deformation on Venusian volcanoes to be detected. The decorrelation of radar interferograms might also enable the effusion rate of a volcano to be studied (Zebker et al., 1996). Improved, higher spatial resolution, topographic data could also be produced for Venus volcanoes using the same approach that was employed during the Shuttle Radar Topography Mission (Farr et al., 2007). This approach will be taken with the radars on the VERITAS (Venus Emissivity, Radio Science, InSAR, Topography And Spectroscopy) mission, planned for a 2027 launch, and EnVision (Ghail et al., 2018), which is targeted for launch in the early 2030s.

Terrestrial Analogs for Human Exploration

Finding terrestrial analogs for planetary volcanic features is also beneficial because they can help evaluate new operational procedures for future rovers and human exploration (Marquez & Newman, 2006; McBarron, 1994). During training for the Apollo missions in the late 1960s (Lofgren et al., 2011; Messeri, 2014; Schindler & Sheehan, 2017), the volcanic terrains of Hawai‘i and northern Arizona were frequently used to test field methods as well as prototypes of the equipment to be used on the lunar surface (figure 17). This analog fieldwork not only helped the astronauts become familiar with the tools that they would use on the Moon but also gave them practice in selecting the most informative rocks to study in the field.

Figure 17. (Top left) Alan B. Shepard, Jr., prime crew commander of the Apollo 14 mission, uses a trenching tool during a simulation of a traverse on the lunar surface. Members of the Apollo 14 crew trained in Hawai‘i for the extravehicular activity of their upcoming mission. A modular equipment transporter is behind Shepard, and a gnomon (one of the Apollo lunar hand tools) is at extreme left. (Top right) Apollo 13 astronauts James Lovell, Jr. (left), and Fred Haise, Jr. (right), carry out a simulation of a lunar traverse at Kīlauea, Hawai‘i, site. Lovell holds a scoop for the Apollo Lunar Hand Tools and a gnomon for size and color calibration of rocks on the lunar surface. (Bottom left) An early version of the lunar rover being tested on simulated impact craters near Flagstaff, Arizona. (Bottom right) Apollo 17 astronauts Harrison Schmidt (left) and Eugene Cernan (right) drive an analog for the lunar rover; Sunset Crater lava flows, near Flagstaff Arizona, are in the background.

Sources: (Top left) NASA photoS70-34415; (top right) NASA photo S70-20253; (bottom left) U.S.G.S. photo P4741, F16924CPR; (bottom right) NASA photo S-72-54903.

Young et al, (2013) described how some of the issues, such as astronaut mobility, the functionality of sampling tools, the need for clear and precise communications, and the physical effort required to traverse various landscapes, can be addressed with terrestrial analog studies (figure 18). A combination of field geologists and a science “backroom” with which to support the crew appears to be the most productive way to proceed (Yingst et al., 2013). However, some of the field experience may return little new volcanological insight; for instance, concerning the exploration of lava tubes on Earth, the amount of new volcanology science to be gleaned from examination of the tube is quite low. The walls are crusted with lava erupted during the last phases of activity, and the floor is often partially buried by blocks that have fallen from the roof. But when considering the human exploration of, for example, a lunar or Martian lava tube, learning about the degree of difficulty for ingress/egress from a tube as a “safe place” for astronauts during a solar flare is important (Blair et al., 2017; deAngelis et al., 2002; Hörz, 1985).

Figure 18. Planetary analogs may also be important as training grounds for future planetary exploration in comparable terrains. (Left) P. J. Mouginis-Mark acts as a simulated astronaut conducting an extravehicular activity on Mauna Loa volcano, Hawai‘i, where participants learn how difficult it might be to conduct fieldwork in a bulky spacesuit. (Right) Two “pseudo-robots” carry equipment to discover how mission control could conduct field-based science remotely with a limited-mobility planetary rover.

Sources: Left image taken by David Ma, University of Hawai‘i. Right image taken by P. J. Mouginis-Mark.

The need for terrestrial analog work will be more pressing when future lunar exploration is considered. With plans to send astronauts to the South Pole of the Moon in the 2026–2028 timeframe, new challenges emerge because of the very low solar incidence angle and variable solar azimuth angle that will exist during a mission that may last 2 Earth weeks (Mouginis-Mark & Boyce, 2020). Although the lunar South Pole is not specifically of importance for volcanological studies, a possible direction that this analog fieldwork might take could involve terrestrial studies at night, with artificial lights simulating the illumination conditions to be encountered on the Moon. A return to some of the old Apollo training sites, such as the cinder cones in northern Arizona, would meet this need. In this instance, the recognition of lithological boundaries as well as priority science targets under challenging illumination conditions may become the main goals for astronaut training. Such nighttime fieldwork would have the advantage that astronaut interpretations could be tested against daytime observations, thereby demonstrating the ease (or difficulty) of working near the lunar South Pole.

Conclusion

There are many reasons for planetary volcanologists to gain experience in the interpretation of geological processes by conducting fieldwork. Indeed, Rowland et al. (2011) described a series of field workshops specifically designed to introduce students to volcanic landforms in Hawai‘i as an aid to subsequently studying volcanoes on the planets. In several instances, such as measuring lava temperatures or the internal structure of a volcano, there is no option but to study landforms on Earth. But Earth analogs may in some instances lead investigators astray, such as the case of distinguishing between lava and mud when it is the rheology of the flow that can be identified from planetary images (Wilson & Mouginis-Mark, 2014). Field analogs also serve to stimulate the development of new instrumentation and procedures that may be employed on other planetary surfaces, which will become increasingly important as astronauts return to the Moon. It is therefore clear that the symbiotic relationship between understanding volcanism on Earth and on the other planets will surely become even more important than in the past. An exciting future awaits!

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