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Io is famous for its active volcanoes, but its vigorous tectonics, which are unlike Earth’s plate tectonics, are no less remarkable. The nature of Io’s thick, cold, brittle lithosphere has been revealed through decades of investigations. The dynamics of this system is most easily explained by considering three cycles: magmatic, tectonic, and sulfurous. The magmatic cycle transports heat by a “heat pipe” process that may have operated during the earliest histories of many of the rocky bodies in the Solar System. The subsidence of the erupted lavas drives mountain uplift in the tectonic cycle. Sulfurous fluids could have a significant impact on the movement of both heat and rock.

The Jovian moon Io features hundreds of paterae, broad depressions often filled with lava. The largest such example, Loki Patera, features a large, polygonal, and fractured island in its center. This island may reflect the fragmentation of the upper Ionian lithosphere by a shallow magma body, facilitated by a great number of shallow extensional structures. We propose a model for the Ionian lithosphere in which the upper portion is heavily deformed by joints, normal faults, and graben resulting from combined tidal, subsidence, and thermal stresses, and a lower portion rife with major thrust faults formed by horizontal compression of deeper crustal levels from the continued burial of surface units. Some of those thrusts may be reactivated normal faults. An extensionally fractured upper lithosphere accounts for the Loki Patera island, for observations of more than 175 paterae with straight edges, for the fact that almost 95% of Io’s surface is covered with volcanic flow and plains units, and for widespread instances of mass wasting within Io’s gigantic mountain blocks. Additional, high-resolution image and topographic data are required to test this hypothesized model for the moon’s lithosphere.

Wind-dominated landscapes can be found on all Solar System bodies with atmospheres. These landscapes record the point of interaction of the atmosphere with the solid surface in a narrow region that also correlates with where field geologists interact with a planetary body. The Altiplano-Puna of Argentina is a high altitude, hyperarid landscape with young and easily eroded deposits and high wind activity. These factors make the region an exceptional analogue for Mars and other planetary regions where wind erosion is dominant, which includes portions of Titan, Venus and Pluto. Our field studies inform our understanding of how wind affects planetary surfaces. Figure 1. Yardangs of the Puna, carved into ash, surrounded by gravels. With young cinder cone. Introduction Wind erosion of planetary bodies yields a variety of landforms, including yardangs (wind-carved ridges), bedrock ridges, sastrugi (ridges formed in hardened snow), with many of these features covered or surrounded by gravels or sands (Fig. 1, ) [1, 2, 3]. These landforms are also found on Mars [4, 5], Titan [6] and Venus. The regular spacing seen in many of these wind-eroded landforms is postulated to have arisen naturally out of wind-sediment relationships possibly controlled by bedrock hardness [7] and sediment supply [8] more than pre-existing fracturing or river channels, though both of those have also been implicated [9]. Our ongoing field project in the Puna of Argentina involves field operations over 5-7 days using observations and simple instrumentation. Our methods are designed to measure the effects of current and past winds, rock properties, and erosive power of sands and gravels on yardang and bedrock ridge morphologies. We investigate the proposition that the simple action of wind on solid surfaces can lead to the complex, self-organized forms observed where wind is dominant and surfaces are erodible. The Puna Field Site and Methods The Puna desert is dominated by young deposits of volcanic ash [10], evaporites and basalt cinder cones. The high elevation means air density is lower about ~1/3 compared to sea level. On Mars, the lower air density leads to large and high dust devils, and similarly large dust devils have been observed during our field campaigns in the Puna [11]. The ash deposits are widespread, and often contain up to km-scale (mega) yardangs. The youngest and softest ignimbrite, called the Campo de Piedra Pomez (CPP) [10], contains abundant medium-sized (meter-scale), mesoyardangs. We visited this region in 2015, 2018 and 2019 (Fig. 1). Fig. 2. Smoke released near a lone yardang, revealing sideways (secondary) winds. We examined field relationships such as orientations of yardangs, rock hardness and layering. We observed differences in rock colors related to intrinsic composition as well as weathering over time. We recorded relative matrix/pumice hardnesses and subsequent variations in erosional properties. We measured locations and sizes of wind indicators such as dedos (protrusions protected by harder lithics) and scours. We observed the action of wind on a single yardang over the course of a 7-day field excursion with an array of small instruments including Kestrel anemometers, simple smoke candles and a camera (Fig. 2), and a tuft net [12]. We deployed two DJI Mavic drones across the main CPP field to obtain a 3 cm DEM (Fig. 3), which also reveals parameters such as length, orientation, and spacing. Key Observations Winds over the time we observed the lone yardang (Fig. 3) in 2019 were dominantly from a direction oblique to the yardang long axis orientation and previously observed primary winds. This secondary wind may impact yardang size and shape. Preliminary results from study of the drone DEM obtained in 2019 reveal a 1:1 width/spacing relationship, similar to other studies. This is ascribed to the steady operation of wind and erosive agents over time on a relatively uniform substrate. A similar ratio was found using a DEM for a portion of the Medusae Fossae Formation (MFF) on Mars. A dark orange coating on the leeward sides of the yardangs [13] (Figs. 2 and 3) is not presently being eroded. They are likely older surfaces reflective of the slopes of the wind-eroded surfaces at the onset of yardang emergence. Fig. 3. Drone view of CPP yardang field (DEM images obtained from higher altitude). Person for scale. Summary and Conclusions Field studies in the Puna have revealed that unique landforms emerge when wind is dominant. Regularly spaced features such as yardangs or ridges emerge out of the interaction between wind and bedrock. Sizes and spacings of features likely reflect rock hardness properties. The utility of simple, reconnaissance-style field campaigns is evident in the knowledge gained through our studies of the Puna. References [1] Blackwelder (1934), Yardangs. GSA Bulletin 45. [2] Ward (1979), Yardangs on Mars. JGR 8147-8166. [3] de Silva et al. (2013) Gravel-mantled megaripples of the Puna, GSA Bulletin 125. [4] Greeley & Iverson (1985), Threshold speeds on Venus, LPSC. [5] Kerber et al. (2011), Origin of Medusa Fossae Formation Mars, Icarus 216. [6] Paillou et al. (2016), Radar scattering of linear dunes and mega-yardangs on Titan, Icarus 270. [7] de Silva S. et al. (2010), Yardangs in terrestrial ignimbrites, PSS 58. [8] Pelletier et al. (2018), Yardang development JGR 123. [9] Dong et al. (2012), Yardangs in the Kumtaugh, Geomorphology 139. [10] Baez et al. (2020), Puna flow dynamics, Bull. Volc. 82. [11] Lorenz & Radebaugh (2016), Dust devils in thin air, GRL 43. [12] Kerber et al., in prog, Puna Yardang Observations. [13] Aulinas et al., (2015), Rock Varnish in Dusty Regions, ESP 40.

Large linear dunes are found in great abundance across the equatorial regions of Saturn’s moon Titan. They are similar in width and spacing to the large dunes of the Saharan, Arabian and Namibian deserts, indicating atmospheric conditions, sand sizes and winds are comparable to those on Earth. An examination of their geomorphometric properties, such as length, width, spacing and distribution can reveal aspects of their relationship with wind strength and direction and controls by underlying topography. We traced long axes of about 70% of all measurable dunes, which involved over 20,000 measurements. We mapped all of the dunes in Shangri-La, Fensal, Aztlan, and half of the Belet Sand Sea. In addition, we measured 90,000 dune widths across Titan at 500 m intervals and fit a nonstationary statistical model with a Gaussian spatial process to determine correlations of dune spacings. Dune long axes are dominantly oriented E-W, a proxy for the sand flux and wind directions. Dunes range to over 400 km in length, with an average length of 40 km. The average length may reflect a rough spacing of obstacles, large-scale topographic variations, or the availability of sand. Dunes are directed slightly NE in the Belet Sand Sea, where dunes are especially abundant and wider. The longest dunes are also found here. Belet may thus represent a fully mature sand sea, where dunes are free to grow as long and large as possible. To the east is the Shangri-La sand sea, which is the location of the Dragonfly landing site. Shangri-La hosts dunes directed dramatically southward, especially near the Xanadu region margin. Dunes here are narrower and interdunes are clearly visible near the elevated rim of the Selk impact crater and other topographic obstacles. Sand collects most densely along the eastern boundary, at the margin of Xanadu, and at the downwind margins of all sand seas. This perhaps indicates that sand is transported until major boundaries are encountered that preclude sand movement. Dune width values can be divided into about 5 major (20 minor) regions globally within the sand seas, with widest groupings at the sand sea centers and isolated, narrower groupings at higher latitudes. The narrowest dunes appear to have the most obstacles or topographic control or be at the highest latitudes. However, within each cluster, dunes of any size within the 1-3 km width range can exist. These studies reveal that while local controls are impactful, dunes will ultimately grow to the extent possible under the conditions present, which on Titan are highly favorable for large linear dunes. Further examination of dune parameters can reveal details about the landscape, basement bedrock conditions, sand transport history and regional wind effects on the dunes of Titan.

NASA’s Dragonfly mission will send a rotorcraft lander to the surface of Titan in the mid-2030s. Dragonfly's science themes include investigation of Titan’s prebiotic chemistry, habitability, and potential chemical biosignatures from both water-based “life as we know it” (as might occur in the interior mantle ocean, potential cryovolcanic flows, and/or impact melt deposits) and potential “life, but not as we know it” that might use liquid hydrocarbons as a solvent (within Titan’s lakes, seas, and/or aquifers). Consideration of both of these solvents simultaneously led to our initial landing site in Titan’s equatorial dunes and interdunes to sample organic sediments and water ice, respectively. Ultimately, Dragonfly's traverse target is the 80 km diameter Selk Crater, at 7° N, where we seek previously liquid water that has mixed with surface organics. Our science goals include determining how far prebiotic chemistry has progressed on Titan and what molecules and elements might be available for such chemistry. We will also determine the role of Titan’s tropical deserts in the global methane cycle. We will investigate the processes and processing rates that modify Titan’s surface geology and constrain how and where organics and liquid water can mix on and within Titan. Importantly, we will search for chemical biosignatures indicative of past or extant biological processes. As such, Dragonfly, along with Perseverance, is the first NASA mission to explicitly incorporate the search for signs of life into its mission goals since the Viking landers in 1976.

13 pages, white paper submitted to the NRC Decadal Survey for Planetary Science and Astrobiology; International audience; We outline a flagship-class mission concept focused on studying Titan as a global system, with particular emphasis on the polar regions. Investigating Titan from the unique standpoint of a polar orbit would enable comprehensive global maps to uncover the physics and chemistry of the atmosphere, and the topography and geophysical environment of the surface and subsurface. The mission includes two key elements: (1) an orbiter spacecraft, which also acts as a data relay, and (2) one or more small probes to directly investigate Titan's seas and make the first direct measurements of their liquid composition and physical environment. The orbiter would carry a sophisticated remote sensing payload, including a novel topographic lidar, a long-wavelength surface-penetrating radar, a sub-millimeter sounder for winds and for mesospheric/thermospheric composition, and a camera and near-infrared spectrometer. An instrument suite to analyze particles and fields would include a mass spectrometer to focus on the interactions between Titan's escaping upper atmosphere and the solar wind and Saturnian magnetosphere. The orbiter would enter a stable polar orbit around 1500 to 1800 km, from which vantage point it would make global maps of the atmosphere and surface. One or more probes, released from the orbiter, would investigate Titan's seas in situ, including possible differences in composition between higher and lower latitude seas, as well as the atmosphere during the parachute descent. The number of probes, as well as the instrument complement on the orbiter and probe, remain to be finalized during a mission study that we recommend to NASA as part of the NRC Decadal Survey for Planetary Science now underway, with the goal of an overall mission cost in the "small flagship" category of ~$2 bn. International partnerships, similar to Cassini-Huygens, may also be included for consideration.

Thanks to the Cassini-Huygens mission, Titan, the pale orange dot of Pioneer and Voyager encounters has been revealed to be a dynamic, hydrologically-shaped, organic-rich ocean world offering unparalleled opportunities to explore prebiotic chemistry. And while Cassini-Huygens revolutionized our understanding of each of the three layers of Titan--the atmosphere, the surface, and the interior--we are only beginning to hypothesize how these realms interact. In this paper, we summarize the current state of Titan knowledge and discuss how future exploration of Titan would address some of the next decade's most compelling planetary science questions. We also demonstrate why exploring Titan, both with and beyond the Dragonfly New Frontiers mission, is a necessary and complementary component of an Ocean Worlds Program that seeks to understand whether habitable environments exist elsewhere in our solar system., Submitted to the PSJ Focus Issue on Ocean World Exploration

In response to ESA Voyage 2050 announcement of opportunity, we propose an ambitious L-class mission to explore one of the most exciting bodies in the Solar System, Saturn largest moon Titan. Titan, a "world with two oceans", is an organic-rich body with interior-surface-atmosphere interactions that are comparable in complexity to the Earth. Titan is also one of the few places in the Solar System with habitability potential. Titan remarkable nature was only partly revealed by the Cassini-Huygens mission and still holds mysteries requiring a complete exploration using a variety of vehicles and instruments. The proposed mission concept POSEIDON (Titan POlar Scout/orbitEr and In situ lake lander DrONe explorer) would perform joint orbital and in situ investigations of Titan. It is designed to build on and exceed the scope and scientific/technological accomplishments of Cassini-Huygens, exploring Titan in ways that were not previously possible, in particular through full close-up and in situ coverage over long periods of time. In the proposed mission architecture, POSEIDON consists of two major elements: a spacecraft with a large set of instruments that would orbit Titan, preferably in a low-eccentricity polar orbit, and a suite of in situ investigation components, i.e. a lake lander, a "heavy" drone (possibly amphibious) and/or a fleet of mini-drones, dedicated to the exploration of the polar regions. The ideal arrival time at Titan would be slightly before the next northern Spring equinox (2039), as equinoxes are the most active periods to monitor still largely unknown atmospheric and surface seasonal changes. The exploration of Titan northern latitudes with an orbiter and in situ element(s) would be highly complementary with the upcoming NASA New Frontiers Dragonfly mission that will provide in situ exploration of Titan equatorial regions in the mid-2030s., Comment: arXiv admin note: substantial text overlap with arXiv:1908.01374

We outline a flagship-class mission concept focused on studying Titan as a global system, with particular emphasis on the polar regions. Investigating Titan from the unique standpoint of a polar orbit would enable comprehensive global maps to uncover the physics and chemistry of the atmosphere, and the topography and geophysical environment of the surface and subsurface. The mission includes two key elements: (1) an orbiter spacecraft, which also acts as a data relay, and (2) one or more small probes to directly investigate Titan's seas and make the first direct measurements of their liquid composition and physical environment. The orbiter would carry a sophisticated remote sensing payload, including a novel topographic lidar, a long-wavelength surface-penetrating radar, a sub-millimeter sounder for winds and for mesospheric/thermospheric composition, and a camera and near-infrared spectrometer. An instrument suite to analyze particles and fields would include a mass spectrometer to focus on the interactions between Titan's escaping upper atmosphere and the solar wind and Saturnian magnetosphere. The orbiter would enter a stable polar orbit around 1500 to 1800 km, from which vantage point it would make global maps of the atmosphere and surface. One or more probes, released from the orbiter, would investigate Titan's seas in situ, including possible differences in composition between higher and lower latitude seas, as well as the atmosphere during the parachute descent. The number of probes, as well as the instrument complement on the orbiter and probe, remain to be finalized during a mission study that we recommend to NASA as part of the NRC Decadal Survey for Planetary Science now underway, with the goal of an overall mission cost in the "small flagship" category of ~$2 bn. International partnerships, similar to Cassini-Huygens, may also be included for consideration., 13 pages, white paper submitted to the NRC Decadal Survey for Planetary Science and Astrobiology

International audience; Titan has an active methane-based hydrologic cycle1 that has shaped a complex geologic landscape2, making its surface one of most geologically diverse in the Solar System. Despite the differences in materials, temperatures and gravity fields between Earth and Titan, many of their surface features are similar and can be interpreted as products of the same geologic processes3. However, Titan’s thick and hazy atmosphere has hindered the identification of its geologic features at visible wavelengths and the study of its surface composition4. Here we identify and map the major geological units on Titan’s surface using radar and infrared data from the Cassini orbiter spacecraft. Correlations between datasets enabled us to produce a global map even where datasets were incomplete. The spatial and superposition relations between major geological units reveals the likely temporal evolution of the landscape and provides insight into the interacting processes driving its evolution. We extract the relative dating of the various geological units by observing their spatial superposition in order to get information on the temporal evolution of the landscape. The dunes and lakes are relatively young, whereas the hummocky or mountainous terrains are the oldest on Titan. Our results also show that Titan’s surface is dominated by sedimentary or depositional processes with a clear latitudinal variation, with dunes at the equator, plains at mid-latitudes and labyrinth terrains and lakes at the poles.

Planetary spatial data returned by spacecraft, including images and higher-order products such as mosaics, controlled basemaps, and digital elevation models (DEMs), are of critical importance to NASA, its commercial partners and other space agencies. Planetary spatial data are an essential component of basic scientific research and sustained planetary exploration and operations. The Planetary Data System (PDS) is performing the essential job of archiving and serving these data, mostly in raw or calibrated form, with less support for higher-order, more ready-to-use products. However, many planetary spatial data remain not readily accessible to and/or usable by the general science user because particular skills and tools are necessary to process and interpret them from the raw initial state. There is a critical need for planetary spatial data to be more accessible and usable to researchers and stakeholders. A Planetary Spatial Data Infrastructure (PSDI) is a collection of data, tools, standards, policies, and the people that use and engage with them. A PSDI comprises an overarching support system for planetary spatial data. PSDIs (1) establish effective plans for data acquisition; (2) create and make available higher-order products; and (3) consider long-term planning for correct data acquisition, processing and serving (including funding). We recommend that Planetary Spatial Data Infrastructures be created for all bodies and key regions in the Solar System. NASA, with guidance from the planetary science community, should follow established data format standards to build foundational and framework products and use those to build and apply PDSIs to all bodies. Establishment of PSDIs is critical in the coming decade for several locations under active or imminent exploration, and for all others for future planning and current scientific analysis., Comment: 8 pages, 0 figures. White paper submitted to the Planetary Science and Astrobiology Decadal Survey 2023-2032

Solid CO2 surface deposits were reported in Enceladus’ South Polar Region by Brown et al. (2006). They noted that such volatile deposits are temporary and posited ongoing replenishment. We present a model for this replenishment by expanding on the Matson et al. (2012) model of subsurface heat and chemical transport in Enceladus. Our model explains the distributions of both CO2 frost and complexed CO2 clathrate hydrate as seen in the Cassini Visual and Infrared Mapping Spectrometer (VIMS) data. We trace the journey of CO2 from a subsurface ocean. The ocean-water circulation model of Matson et al. (2012) brings water up to near the surface where gas exsolves to form bubbles. Some of the CO2 bubbles are trapped and form pockets of gas in recesses at the bottom of the uppermost ice layer. When fissures break open these pockets, the CO2 gas is vented. Gas pocket venting is episodic compared to the more or less continuous eruptive plumes, emanating from the “tiger stripes”, that are supported by plume chambers. Two styles of gas pocket venting are considered: (1) seeps, and (2) blowouts. The presence of CO2 frost patches suggests that the pocket gas slowly seeped through fractured, cold ice and when some of the gas reached the surface it was cold enough to condense (i.e., T ∼70 to ∼119 K). If the fissure opening is large, a blowout occurs. The rapid escape of gas and drop in pocket pressure causes water in the pocket to boil and create many small aerosol droplets of seawater. These may be carried along by the erupting gas. Electrically charged droplets can couple to the magnetosphere, and be dragged away from Enceladus. Most of the CO2 blowout gas escapes from Enceladus and the remainder is distributed globally. However, CO2 trapped in a clathrate structure does not escape. It is much heavier and slower moving than the CO2 gas. Its motion is ballistic and has an average range of about 17 km. Thus, it contributes to deposits in the vicinity of the vent. Local heat flow indicates that gas pockets can be located as deep as several tens of meters below the surface. Gas pockets can be reused, and we explore their life cycle.

The mountains of Jupiter's volcanic moon Io are tall, steep, and tectonic in origin, yet their precise modes of formation and their associations with volcanic paterae are not fully understood. Global spatial statistics of paterae and mountains and their associated lineations reveal that both types of features are more common at low latitudes and tectonic lineations have preferred orientations, whereas straight patera margins are randomly oriented. Additionally, structurally controlled lineations tend to cluster with each other, and in areas of high concentrations these tectonic lineations are shorter in length than their global average. These results indicate that global-scale (rather than local or regional) processes are involved in forming Io's tectonic structures, but that the diversity of mountain characteristics and the collapse of paterae adjacent to mountain complexes are more locally controlled. Regional structural mapping of the Hi'iaka, Shamshu, Tohil, and Zal regions reveals Io's mountains reside in large, fault-bounded crustal blocks that have undergone modification through local responses of subsurface structures to variable stresses. Strike-slip motion along reactivated faults led to the formation of transpressional and transtensional features, creating tall peaks and low basins, some of which are now occupied by paterae. We propose Io's mountains result from a combination of crustal stresses involving global and local-scale processes, dominantly volcanic loading and tidal flexing. These stresses sometimes are oriented at oblique angles to pre-existing faults, reactivating them as reverse, normal, or strike-slip faults, modifying the large, cohesive crustal blocks that many of Io's mountains reside in. Further degradation of mountains and burial of faults has occurred from extensive volcanism, mass wasting, gravitational collapse, and erosion by sublimation and sapping of sulfur-rich layers. This model of fault-bounded blocks being modified by global stresses and local structural response accounts for the variation and patterns of mountain sizes, shapes, and orientations, along with their isolation and interactions with other features. It also provides a context for the operation and extent of global and regional stresses in shaping Io's surface.

Achieving our goals of advancing science and benefitting society depends on ideological diversity. Cultivating relationships and dialogue with ideological opponents creates compassion and progress.

International audience; Titan was a mostly unknown world prior to the Cassini spacecraft’s arrival in July 2004. We review the major scientific advances made by Cassini’s Titan Radar Mapper (RADAR) during 13 years of Cassini’s exploration of Saturn and its moons. RADAR measurements revealed Titan’s surface geology, observed lakes and seas of mostly liquid methane in the polar regions, measured the depth of several lakes and seas, detected temporal changes on its surface, and provided key evidence that Titan contains an interior ocean. As a result of the Cassini mission, Titan has gone from an uncharted world to one that exhibits a variety of Earth-like geologic processes and surface-atmosphere interactions. Titan has also joined the ranks of “ocean worlds” along with Enceladus and Europa, which are prime targets for astrobiological research.

Persephone is a NASA concept mission study that addresses key questions raised by New Horizons’ encounters with Kuiper Belt objects (KBOs), with arguably the most important being, “Does Pluto have a subsurface ocean?” More broadly, Persephone would answer four significant science questions: (1) What are the internal structures of Pluto and Charon? (2) How have the surfaces and atmospheres in the Pluto system evolved? (3) How has the KBO population evolved? (4) What are the particles and magnetic field environments of the Kuiper Belt? To answer these questions, Persephone has a comprehensive payload, and it would both orbit within the Pluto system and encounter other KBOs. The nominal mission is 30.7 yr long, with launch in 2031 on a Space Launch System Block 2 rocket with a Centaur kick stage, followed by a 27.6 yr cruise powered by existing radioisotope electric propulsion and a Jupiter gravity assist to reach Pluto in 2058. En route to Pluto, Persephone would have one 50–100 km class KBO encounter before starting a 3.1-Earth-year orbital campaign of the Pluto system. The mission also includes the potential for an 8 yr extended mission, which would enable the exploration of another KBO in the 100–150 km size class. The mission payload includes 11 instruments: Panchromatic and Color High-Resolution Imager, Low-Light Camera, Ultra-Violet Spectrometer, Near-Infrared (IR) Spectrometer, Thermal IR Camera, Radio Frequency Spectrometer, Mass Spectrometer, Altimeter, Sounding Radar, Magnetometer, and Plasma Spectrometer. The nominal cost of this mission is $3.0 billion, making it a large strategic science mission.

The morphologies of craters on planetary surfaces reveal clues about the geologic mechanisms by which they originate and subsequently evolve, as well as the materials and physical variables inherent to the environment in which they formed. We carried out a quantitative multivariate analysis of shape descriptors derived from the outlines of craters formed by volcanic processes on Mars, Io, and Earth and by impact cratering on the Moon using elliptic Fourier analysis (EFA) and the Zahn-Roskies (Z-R) shape function. Canonical variate analysis (CVA) was used to construct a statistical model of differences between the crater groups to classify craters produced by various volcanic and impact processes. The classification model from canonical variate analysis of EFA shape descriptors yielded a 90% rate of success for the assignment of group membership among 406 examined craters. It correctly classified 138 of 154 (90%) ionian paterae,154 of 155 (99%) lunar impact craters, 31 of 35 (89%) terrestrial basaltic shield calderas, 32 of 38 (84%) terrestrial ash-flow calderas, and 12 of 24 (50%) martian basaltic shield calderas. The classification model from canonical variate analysis of Z-R shape function descriptors classified 84% of the total population of the examined craters correctly. The analysis correctly classified 96% of ionian paterae, 100% lunar impact craters, 51% terrestrial basaltic shield calderas, and 63% martian calderas, but only 16% of the terrestrial ash-flow calderas were correctly classified. Canonical variate analysis of EFA and Z-R results shows that the shapes of ash-flow calderas and paterae on Io differ the least of all groups included in this study, and basaltic shield calderas and martian calderas analyzed together also have few differences. The Z-R model successfully classifies more ionian patera and impact craters than the EFA classification model but performs poorly at classifying the other crater groups. This result shows that the descriptors convey different shape information. The Z-R model is robust in its ability to classify end-member differences in complexity while the EFA model is robust in its ability to reliably classify among more groups. These differences and similarities in shape confirm previously understood commonalities related to the origin and evolution of various types of craters. In general, basalt shield calderas on Earth and Mars are morphologically similar and are thought to have similar origins; this study confirms that the 2-D shapes of their craters are quantitatively correlated. Similarities have been noted between terrestrial ash-flow calderas and paterae on Io, principally in their large sizes, shallow magma chambers and complex evolution; this study confirms their shapes are also similar. Impact craters and ionian paterae are most dissimilar, as are their evolutions. This study demonstrates rigorous landform shape analysis can greatly increase our understanding of the diversity in craters and the processes involved in their formation.

We describe how near-field audio recording using a pocket digital sound recorder can usefully document volcanic activity, demonstrating the approach at Yasur, Vanuatu in May 2014. Prominent emissions peak at 263 Hz, interpreted as an organ-pipe mode. High-pass filtering was found to usefully discriminate volcano vent noise from wind noise, and autocorrelation of the high pass acoustic power reveals a prominent peak in exhalation intervals of ~ 2.5, 4 and 8 s, with a number of larger explosive events at ~ 200 s intervals. We suggest that this compact and inexpensive audio instrumentation can usefully supplement other field monitoring such as seismic or infrasound. A simple estimate of acoustic power interpreted with a dipole jet noise model yielded vent velocities too low to be compatible with pyroclast emission, suggesting difficulties with this approach at audio frequencies (perhaps due to acoustic absorption by volcanic gases).

We documented eruption activity at three primary vents at Yasur volcano, Tanna Island, Vanuatu using portable instrumentation in the field over a period of 5 h on 21 May 2014, and acquired aerial images of the craters and vents on 22 May 2014. Although limited in duration, our observations of eruption intervals, durations, temperatures, and speeds of ejected material illustrate the characteristics of the activity at the time at each of the primary vents, providing a useful snapshot of eruption behavior and revealing continued variability at Yasur in comparison to other observation campaigns. Hand-held, high-resolution, near-infrared observations of one of the vents gave peak temperatures of 850 °C to 930 °C for ejected clasts, with a maximum temperature of 1033 °C. These temperatures are significantly higher than previous measurements because exposed lavas could be resolved at timescales less than a second. Our aerial near-infrared images allowed us to estimate the combined area of the active vents within the crater to be ~ 150 m 2 , and comparison to MODIS radiance measurements in the same time frame yields temperatures, averaged over the combined vent area, of 530–730 °C. In the context of previous observations at Yasur, the activity in May 2014 exhibited lower overall intensity, as well as differences in the nature of the eruptions at the various vents, providing insight regarding the temporal variability of Yasur's activity.

Observations from field remote sensing of the morphology, kinematics and temperature of the Marum/Mbwelesu lava lake in the Vanuatu archipelago in 2014 reveal a highly active, vigorously erupting lava lake. Active degassing and fountaining observed at the ~ 50 m lava lake led to large areas of fully exposed lavas and rapid (~ 5 m/s) movement of lava from the centers of upwelling outwards to the lake margins. These rapid lava speeds precluded the formation of thick crust; there was never more than 30% non-translucent crust. The lava lake was observed with several portable, handheld, low-cost, near-infrared imagers, all of which measured temperatures near 1000 °C and one as high as 1022 °C, consistent with basaltic temperatures. Fine-scale structure in the lava fountains and cooled crust was visible in the near infrared at ~ 5 cm/pixel from 300 m above the lake surface. The temperature distribution across the lake surface is much broader than at more quiescent lava lakes, peaking ~ 850 °C, and is attributed to the highly exposed nature of the rapidly circulating lake. This lava lake has many characteristics in common with other active lava lakes, such as Erta Ale in Ethiopia, being confined, persistent and high-temperature; however it was much more active than is typical for Erta Ale, which often has > 90% crust. Furthermore, it is a good analogue for the persistent, high-temperature lava lakes contained within volcanic depressions on Jupiter's moon Io, such as Pele, also believed from spacecraft and ground-based observations to exhibit similar behavior of gas emission, rapid overturn and fountaining.

Detailed analyses of slopes and arcuate planform morphologies of Titan’s equatorial mountain ridge belts are consistent with formation by contractional tectonism. However, contractional structures in ice require large stresses (4–10 MPa), the sources of which are not likely to exist on Titan. Cassini spacecraft imagery reveals a methane-based hydrological cycle on Titan that likely includes movement of fluids through the subsurface. These crustal liquids may enable contractional tectonic features to form as groundwater has for thrust belts on Earth. In this study, we show that liquid hydrocarbons in Titan’s near subsurface can lead to fluid overpressures that facilitate contractional deformation at smaller stresses (

Using SAR data from Cassini’s RADAR instrument, we examined the orientations of three terrain units on Titan, bright lineated plains, streak-like plains, and linear dunes. From the overall integrated pattern of their orientation, we were able to determine Titan’s global material transport vectors. The analysis indicates that, in both the northern and southern hemispheres, materials from 0 to 35 deg latitude are transported poleward to a belt centred at roughly 35 deg. Materials from 60 to 35 deg latitude are transported equatorward to the belt at roughly 35 deg. Comparison with the global topographical gradient (Lorenz, R.D. et al. [2013]. Icarus 225, 367–377) suggests that fluvial transport is not the dominant process for material transport on Titan, or that it is at least overprinted with another transport mechanism. Our results are consistent with aeolian transport being the dominant mechanism in the equatorial and mid-latitude zones. The zone at 35 deg is thus the ultimate sink for materials from the equator to low polar latitudes; materials making up the equatorial dunes will be transported to the latitude 35-deg belts. Only plains units are observed at latitudes of ∼35 deg; dunes and materials with the spectral characteristics of dunes are not observed at these latitudes. This observation suggests that either dune materials are converted or modified into plains units or that the margins of dunes are transport limited.

Titan is a hydrologically active world, with dozens of alluvial fans that are evidence of sediment transport from high to low elevations. However, the distribution and requirements for the formation of fans on Titan are not well understood. We performed the first global survey of alluvial fans on Titan using Cassini Synthetic Aperture Radar (SAR) data, which cover 61% of Titan’s surface. We identified 82 fans with areas ranging from 28 km2 to 27,000 km2. A significant fraction (∼60%) of the fans are restricted to latitudes of ±50–80°, suggesting that fluvial sediment transport may have been concentrated in the near-polar terrains in the geologically recent past. The density of fans is also found to be correlated with the latitudes predicted to have the highest precipitation rates by Titan Global Circulation Models. In equatorial regions, observable fans are not generally found in proximity to dune fields. Such observations suggest that sediment transport in these areas is dominated by aeolian transport mechanisms, though with some degree of recent equatorial fluvial activity. The fan area-drainage area relationship on Titan is more similar to that on Earth than on Mars, suggesting that the fans on Titan are smaller than what may be expected, and that the transport of bedload sediment is limited. We hypothesize that this has led to the development of a coarse gravel-lag deposit over much of Titan’s surface. Such a model explains both the morphology of the fans and their latitudinal concentration, yielding insight into the sediment transport regimes that operate across Titan today.

The Undifferentiated Plains on Titan, first mapped by Lopes et al. (Lopes, R.M.C. et al., 2010. Icarus, 205, 540–588), are vast expanses of terrains that appear radar-dark and fairly uniform in Cassini Synthetic Aperture Radar (SAR) images. As a result, these terrains are often referred to as “blandlands”. While the interpretation of several other geologic units on Titan – such as dunes, lakes, and well-preserved impact craters – has been relatively straightforward, the origin of the Undifferentiated Plains has remained elusive. SAR images show that these “blandlands” are mostly found at mid-latitudes and appear relatively featureless at radar wavelengths, with no major topographic features. Their gradational boundaries and paucity of recognizable features in SAR data make geologic interpretation particularly challenging. We have mapped the distribution of these terrains using SAR swaths up to flyby T92 (July 2013), which cover >50% of Titan’s surface. We compared SAR images with other data sets where available, including topography derived from the SARTopo method and stereo DEMs, the response from RADAR radiometry, hyperspectral imaging data from Cassini’s Visual and Infrared Mapping Spectrometer (VIMS), and near infrared imaging from the Imaging Science Subsystem (ISS). We examined and evaluated different formation mechanisms, including (i) cryovolcanic origin, consisting of overlapping flows of low relief or (ii) sedimentary origins, resulting from fluvial/lacustrine or aeolian deposition, or accumulation of photolysis products created in the atmosphere. Our analysis indicates that the Undifferentiated Plains unit is consistent with a composition predominantly containing organic rather than icy materials and formed by depositional and/or sedimentary processes. We conclude that aeolian processes played a major part in the formation of the Undifferentiated Plains; however, other processes (fluvial, deposition of photolysis products) are likely to have contributed, possibly in differing proportions depending on location.

We report in situ measurements and field observations of dust devils at ~3800–4200 m elevation in the Argentine desert plateau (Puna). These first quantitative data at elevations where the ambient atmospheric pressure is only ~600–700 mbar support the notion that large and strong dust devils may be systematically more common in low-pressure conditions, although regional meteorology and/or surface thermophysical properties may also contribute.

International audience; Fans, landforms that record the storage and transport of sediment from uplands to depositional basins, are found on Saturn's moon Titan, a body of significantly different process rates and material compositions from Earth. Images obtained by the Cassini spacecraft's synthetic aperture radar reveal morphologies, roughness, textural patterns and other properties consistent with fan analogues on Earth also viewed by synthetic aperture radar. The observed fan characteristics on Titan reveal some regions of high relative relief and others with gentle slopes over hundreds of kilometres, exposing topographic variations and influences on fan formation. There is evidence for a range of particle sizes across proximal to distal fan regions, from c. 2 cm or more to fine-grained, which can provide details on sedimentary processes. Some features are best described as alluvial fans, which implies their proximity to high-relief source areas, while others are more likely to be fluvial fans, drawing from larger catchment areas and frequently characterized by more prolonged runoff events. The presence of fans corroborates the vast liquid storage capacity of the atmosphere and the resultant episodic behaviour. Fans join the growing list of landforms on Titan derived from atmospheric and fluvial processes similar to those on Earth, strengthening comparisons between these two planetary bodies.

International audience; The Cassini/Huygens mission revealed a terrain type on Saturn's moon Titan of dissected, elevated plateaux with a high density of valleys named labyrinth terrain. We define four subtypes of labyrinth terrains: valleyed, polygonal, finely-dissected, and the possible outlier Kronin Labyrinth. We mapped the locations of all labyrinths imaged by Cassini and found they are distributed preferentially at high latitudes. We characterize the labyrinths by morphometric parameters such as intervalley width, valley width, and percent valleys. We find many labyrinths contain closed valleys, which constrains their formation and evolution. We also examine their low microwave emissivity spectral characteristics and find that the labyrinths are consistent with a bulk composition of dominantly organic materials, with some component of water ice – characteristics similar to Titan's undifferentiated plains. Our analyses show that labyrinths are ancient terrains – only the mountains and hummocky terrains are older. This implies that significant organic production occurred early in Titan's history. The organic inventory represented by the labyrinths is estimated to be 15–42% of the solid organic inventory of Titan (or 14–35% of the total surface organics, if the hydrocarbons of the lakes and seas are also included). Our preferred formation of the labyrinth terrains is erosion through dissolution and fluvial processes that dissect the plateau in a manner similar to dissolution geology (karst) on Earth. This scenario requires that the organics that make up the labyrinth terrain be soluble in methane and/or ethane liquids. It also suggests that the origin of the plateaux may have derived from Titan's past chemical production and subsequent depositional record.

International audience; Some of Titan's small northern hemisphere lakes show raised rampart features (which are distinct from raised rims), and appear as SAR-bright mound-like annuli extending away from the lake for up to tens of kilometers from the shoreline. We investigate the infrared and microwave characteristics of these features using Cassini Visual and Infrared Mapping Spectrometer (VIMS) and RADAR data. A spectral comparative analysis is performed among the lakes, their ramparts, and the surrounding regions. We overcome the profound difference in spatial resolution between VIMS and SAR data by using a method that provides overlays between the spectral images and SAR, thus enabling the correct selection of VIMS pixels. The surface properties of the selected areas are obtained using a radiative transfer analysis on the selected VIMS pixels, in addition to emissivity obtained from the RADAR in radiometry mode. Analysis of these combined and co-registered data provides constraints for the formation mechanism(s) of raised ramparts. The results show that the emissivity of the raised ramparts is close to that of Titan's labyrinthic terrains and to that of empty lake floors in the northern polar regions. This is confirmed by the VIMS analysis that also shows that the infrared spectral response of the raised ramparts is very similar to that of some empty lake floors. This suggests that both areas are made from or are covered by a similar material. In addition, two out of the eight lakes with raised ramparts show spectral differences at three specific wavelengths, 1.6, 2.0, and 5.0 μm, between the ramparts and the surrounding terrain. We hypothesize that this could be due to some component, or mixture of components in the ramparts that is less absorbent at these specific wavelengths, or it could be an effect of different grain sizes. These observations provide first insights into the possible mechanisms leading to the formation of the raised ramparts that are discussed here.

Active lava lakes are rare on Earth, with only ten documented examples, all formed by lavas of basaltic composition and housed inside summit craters or calderas. The existence of lava lakes on other planetary bodies may imply similarities in either composition or volcano-tectonic settings, and so has important implications for understanding the link between melt production and volcanism. We review lava lakes on Earth and other planets, particularly active caldera-like features interpreted to be currently containing active lava lakes on Jupiter's moon Io, and features interpreted as remnants of lava lakes on Venus and Mars. Mercury and the Moon do not boast the major calderas or shield volcanoes of their larger rocky neighbors, partly due to a horizontally compressive tectonic regime arising from global contraction that makes it difficult to move melts through the upper crust. We discuss the evidence for active lava lakes on Io and show how modeling based on terrestrial lava lakes can reveal how these phenomena differ on both bodies; the superficial similarities do not necessarily imply that the plumbing is similar. Observations of the largest lava lake on Io, Loki Patera, provide insight into the nature of Ionian lava lakes in general, which may be more similar to eruptive episodes on the East Pacific Rise on Earth, which lead to temporary lava lakes. Although temporal data for Io's lava lake activity are scarce, studies of temporal variability of lava lakes on Earth are useful for providing ground truth for comparisons. Future studies of Earth by remote sensing and field observations, and of Io by both ground-based observations and future missions, are needed to answer many questions, including why lava lakes, rare on Earth, appear to be common on Io.