Preface

Planetary Geology Field Symposium, Kitakyushu, Japan, 2011:Planetary geology and terrestrial analogs

1. 2011 PERC Planetary Geology Field Symposium and thespecial issue

The 2011 PERC (The Planetary Exploration Research Center of theChiba Institute of Technology) Planetary Geology Field Symposium(see http://www.perc.it-chiba.ac.jp/meetings/pgfs2011/) was held withthe theme “Planetary Geology and Terrestrial Analogs.” The sympo-sium was hosted on 5–6 November 2011 in Kitakyushu City, KyushuIsland, Japan and was followed by a field trip to hot springs andvolcanic features in the Beppu-Shimabara Graben. This field sympo-siumwas the first of its kind held in Asia (Komatsu and Namiki, 2012).

Asia is rich and diverse in geological environments and thushas much to offer in terms of terrestrial analogs for planetarygeologic features. Geoscientists versed in relevant features in Asia,and from other regions, have a broader basis for understandinggeological processes operating on various planetary bodies. Pla-netary geology can certainly benefit from the insights and offer-ings of terrestrial geoscientists familiar with processes andfeatures that may occur in other planetary settings. This specialissue gives a taste of what is now being achieved as a result of theexpansion in diversity of planetary geology community andterrestrial expertise that is being applied to extraterrestrial geolo-gical research.

The special issue presented here consists of contributions fromAsian and non-Asian scientists who participated in the symposium.The topics of the contributions in the special issue are wide ranging:the methodological concept of planetary geology (Baker, 2014);quantitative analyses based on a new digital global geologic map ofMars (Tanaka et al., 2014); a new hypothesis for the clay mineralformation on Mars (Berger et al., 2014); tsunami wave propagation onan purported ancient ocean on Mars (Iijima et al., 2014); terrestrialanalog sites for geological (Komatsu et al., 2014; Essefi et al., 2014) andastrobiological applications (Sugawara et al., 2014; Sakakibara et al.,2014); analysis of boulder distributions on asteroid Itokawa(Miyamoto, 2014); and geological features discovered on Mercury bythe MESSENGER spacecraft (Xiao and Komatsu, 2014; published in aprevious issue and reprinted in this issue).

2. Candidate terrestrial analog sites in Kyushu, Japan

The field trip entitled “Volcanic and geothermal activities alongthe Beppu-Shimabara Graben as terrestrial analogs for comparativeplanetary geology” (Komatsu et al., 2011) was held on 7–9 November

2011 in central Kyushu Island, Japan after the 2011 PERC PlanetaryGeology Field Symposium held in Kitakyushu City (Fig. 1). Attendeesvisited sites along the Beppu-Shimabara Graben trending east–west inthe central part of Kyushu.

The Beppu-Shimabara Graben is characterized by its associatedvolcanism, which is expressed in a wide variety of eruptive styles.The most dominant volcanic structure is Aso Volcano (Fig. 1),which is one of the largest caldera structures in the world. Thecaldera formed primarily �270,000 to 90,000 years ago. Today,only some of its central, interior cones are still active. The Unzen

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Fig. 1. Location map of the field trip entitled “Volcanic and geothermal activitiesalong the Beppu-Shimabara Graben as terrestrial analogs for comparative planetarygeology” held on 7–9 November 2011 in central Kyushu Island, Japan.

http://dx.doi.org/10.1016/j.pss.2014.04.0020032-0633/& 2014 Published by Elsevier Ltd.

Planetary and Space Science 95 (2014) 1–4

Volcano (Fig. 1) is located west of Aso, and it is infamous for itspyroclastic flow events—including the one that occurred on 3 June1991 that killed a number of people including three volcanologists.All the volcanic processes and resulting landforms observable atAso, Unzen, and Kuju to the east of Aso, provide a diverse set ofvolcanic analogs for similar features on other terrestrial planets(as well as on the Moon).

A remarkable element of the volcanism associated with theBeppu-Shimabara Graben is the fact that its hydrologic interac-tions are clearly expressed in a variety of forms. Beppu Onsen(Hotspring) (Fig. 1) is well known as a tourist attraction, but it isalso scientifically important for the presence of hotspring poolswith a range of precipitated minerals. The Aso Volcano is known tohave hosted a caldera lake in its past, and it is hypothesized thatthe lake was catastrophically spilled to the west through a valley.The Mayuyama Volcano, a volcanic edifice near Unzen Volcano,collapsed suddenly into the Ariake Sea on 21 May 1792, therebyproducing a gigantic landslide and tsunami that killed more than15,000 people.

Above-mentioned sites along the Beppu-Shimabara Graben canbe regarded as terrestrial analog sites for comparative planetarygeology. The terrestrial analog aspect of these sites would beuseful also in the technological development for planetaryexploration. Indeed, rover testing was conducted during and afterthe symposium at Sunasenrigahama on Aso Volcano. The follow-ing subsections describe particularly outstanding examples of theterrestrial analogs.

2.1. Calderas and associated volcanic features in the solar system

Volcanism occurs on almost all planetary bodies, because it isdriven by internal heat loss. Landforms identified as calderas arewidely observed, for example, on Mars and the Galilean satellite Io.This fact indicates that effusive and explosive volcanism fromshallow magmatic reservoirs operate in the Solar System, andmechanisms for producing conditions sufficient to cause large-scale eruption and subsequent collapse exist not only on Earth.Particularly large summit calderas occur on gigantic shield volca-noes on Mars and Venus that were likely formed by non-explosivesubsidence due to draining of huge volumes of lava. Scoria (orcinder) cones are relatively small volcanic edifices made ofpyroclastic materials, and they seem to occur on planetary surfacesincluding the ones associated with large shield volcanoes on Mars.The Aso Volcano’s caldera, suite of scoria cones and otherassociated volcanic landforms are excellent terrestrial analogs forplanetary volcanism due to their pristine preservation states andeasy access (Fig. 2a, b, and d).

2.2. Extensional faulting and volcanism in the solar system

The east–west trending Beppu-Shimabara Graben in centralKyushu (Figs. 1 and 2e) provides an example in which extensionalfaulting controls the location of associated volcanism. The east–west aligning Kuju, Aso and Unzen Volcanoes occur along graben-bounding normal faults.

Normal faulting represents the brittle failure of crustal materi-als in response to extensional stress fields, and it appears to bevery common on planetary surfaces. Such faults are often accom-panied by volcanism. For example, large volcanic features arefound to straddle over rift systems on Venus. On Mars, lava flowsemanating from grabens are observed.

2.3. Geothermal processes and astrobiology

Geothermal processes involving water have been debated exten-sively for their possible roles on Mars, which include astrobiological

implications. The proposed evidence for ancient hydrothermal systemshas been linked to the presence of certain landforms such as valleynetworks, impact craters, and volcanoes where hydrothermal activitieswere hypothesized to have occurred. An example of the latter is theDao Vallis outflow channel, which originates from the pervasivelydissected Hadriacus Mons volcano have also been speculated to be thesites of hydrothermal activities.

The hot springs at Beppu provide an excellent opportunity toobserve springs or mud pools where a variety of mineralsprecipitate (Fig. 2f–h). Silica is particularly relevant to Mars as itsdiscovery in certain locales on the Red Planet has been hypothe-sized to be linked to the activities of ancient hydrothermalsystems. Iron oxides have also been found on Mars, and theirdeposition is explained by various mechanisms—among themhydrothermal systems.

2.4. Crater lakes and outflow events on Mars

The presence of paleo-crater lakes has been hypothesized onMars for many years. In the case of Mars, the craters hosting thepaleo-lakes are most likely of impact origin but there areproposed, ancient volcanic craters that have been dissected byvalleys and also may have hosted paleo-lakes. Some such paleo-crater lakes may have drained out through spillways breachedthrough the crater rims. The Aso caldera paleo-lake and itsproposed catastrophic drainage may be excellent examples forcomparing with similarly breached Martian impact and volcaniccraters.

2.5. Pyroclastic flows in the solar system

Pyroclastic flows (or ash falls) may be common on planetarybodies. For example, some localized geological observationsindicating ancient pyroclastic flow events have been reportedon Mars. Mars is known for its ubiquitous occurrence of layeredmaterials identified on its surface from orbit, and a possibilityexists that some of them resulted from pyroclastic activities.Recognition of pyroclastic deposits on planetary surfaces is notan easy task. Without outcrop examinations, layered depositsresulted from pyroclastic activities could be indistinguishablefrom stratifications produced by deposition of non-volcanicmaterials such as eolian and authigenic sediments. The wide-spread pyroclastic deposits in central Kyushu (e.g., Fig. 2c)provide insightful examples useful for learning what to lookfor in the identification of pyroclastic deposits on planetarysurfaces.

2.6. Failures of volcanic edifices on planetary surfaces

Evidence for volcanic edifice failures has been noted onplanetary surfaces such as on Venus and Mars. Such failuresindicate that volcanic edifices are inherently unstable, and theycould collapse by a variety of triggering mechanisms. They are aneffective way to contribute to the degradation of volcanic edifices.The failure of the Mayuyama Volcano occurred relatively recently(21 May 1792), and the landforms resulting from the failure eventare well preserved in both the volcano edifice and on the sea floorin front of the edifice.

2.7. Tsunami occurrences in the solar system?

Powerful tsunami waves such as the ones caused by theMayuyama Volcano failures or by other mechanisms are thesubject of interest for planets or satellites with bodies of water(or other liquids) today or in the past. In the present-day, the

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only Solar System body with a certain presence of liquid bodieson the surface besides Earth is Titan, an icy satellite of Saturn.And tsunami may have occurred in lakes (or seas) of thesatellite. Many observations are pointing to the past existenceof bodies of water up to the size of large lakes and even oceanson Mars. Therefore, the prospect of tsunami occurrence onancient Mars should not be neglected. Possible modes oftsunami wave generation could be diverse ranging from Marsquakes, landslides to impact cratering, and their morphologicaland sedimentological influences on shoreline zones could havebeen significant.

References

Baker, V.R., 2014. Terrestrial analogs, planetary geology, and the nature ofgeological reasoning. Planet. Space Sci. 95, 5–10, http://dx.doi.org/10.1016/j.pss.2012.10.008.

Berger, G., Meunier, A., Beaufort, D., 2014. Clay mineral formation on Mars:Chemical constraints and possible contribution of basalt out-gassing. Planet.Space Sci. 95, 25–32, http://dx.doi.org/10.1016/j.pss.2013.05.024.

Essefi, E., Komatsu, G., Fairén, A.G., Chan, M.A., Yaich, C., 2014. Groundwaterinfluence on the aeolian sequence stratigraphy of the Mechertate-Chrita-SidiEl Hani system, Tunisian Sahel: Analogies to the wet-dry aeolian sequencestratigraphy at Meridiani Planum, Terby crater, and Gale crater, Mars. Planet.Space Sci. 95, 56–78, http://dx.doi.org/10.1016/j.pss.2013.05.010.

Fig. 2. Representative geological sites visited during the field trip. These sites were chosen for visits because of their significance for planetary geology: (a) view of the Asocaldera and post-caldera central cones; (b) Sunasenrigahama on Aso Volcano, where rover testing was conducted during and after the symposium; (c) pyroclastic flowdeposit emanating from Unzen Volcano; (d) the Kamikomezuka scoria cone (cross section) in the Aso caldera; (e) normal fault (on the left) of the Beppu-Shimabara Graben;and (f–h) Beppu hotspring pools of various chemical and mineralogical compositions, resulting in color variations.

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Iijima, Y., Goto, K., Minoura, K., Komatsu, G., Imamura, F., 2014. Hydrodynamics ofimpact-induced tsunami over the Martian ocean. Planet. Space Sci. 95, 33–44,http://dx.doi.org/10.1016/j.pss.2013.09.014.

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Goro Komatsu n

International Research School of Planetary Sciences, Universitàd’Annunzio, Viale Pindaro 42, 65127 Pescara, Italy

E-mail address: goro@irsps.unich.it

Kazuhisa GotoInternational Research Institute of Disaster Science, Tohoku Univer-sity, Aoba 6-6-11-1106, Aramaki, Aoba-ku, Sendai 980-8579, Japan

Planetary Exploration Research Center, Chiba Institute of Technology,2-17-1 Tsudanuma, Narashino-shi, Chiba 275-0016, Japan

Kenneth L. TanakaAstrogeology Science Center, U.S. Geological Survey, 2255 N. Gemini

Dr., Flagstaff, AZ 86001, USA

n Corresponding author. Tel.: þ39 085 453 7884; fax: þ39 085 453 7545.

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