Higher-than-predicted saltation threshold wind speeds on Titan.

Titan, the largest satellite of Saturn, exhibits extensive aeolian, that is, wind-formed, dunes, features previously identified exclusively on Earth, Mars and Venus. Wind tunnel data collected under ambient and planetary-analogue conditions inform our models of aeolian processes on the terrestrial planets. However, the accuracy of these widely used formulations in predicting the threshold wind speeds required to move sand by saltation, or by short bounces, has not been tested under conditions relevant for non-terrestrial planets. Here we derive saltation threshold wind speeds under the thick-atmosphere, low-gravity and low-sediment-density conditions on Titan, using a high-pressure wind tunnel refurbished to simulate the appropriate kinematic viscosity for the near-surface atmosphere of Titan. The experimentally derived saltation threshold wind speeds are higher than those predicted by models based on terrestrial-analogue experiments, indicating the limitations of these models for such extreme conditions. The models can be reconciled with the experimental results by inclusion of the extremely low ratio of particle density to fluid density on Titan. Whereas the density ratio term enables accurate modelling of aeolian entrainment in thick atmospheres, such as those inferred for some extrasolar planets, our results also indicate that for environments with high density ratios, such as in jets on icy satellites or in tenuous atmospheres or exospheres, the correction for low-density-ratio conditions is not required.

Fluvial geomorphology on Earth-like planetary surfaces: A review.

Morphological evidence for ancient channelized flows (fluvial and fluvial-like landforms) exists on the surfaces of all of the inner planets and on some of the satellites of the Solar System. In some cases, the relevant fluid flows are related to a planetary evolution that involves the global cycling of a volatile component (water for Earth and Mars; methane for Saturn's moon Titan). In other cases, as on Mercury, Venus, Earth's moon, and Jupiter's moon Io, the flows were of highly fluid lava. The discovery, in 1972, of what are now known to be fluvial channels and valleys on Mars sparked a major controversy over the role of water in shaping the surface of that planet. The recognition of the fluvial character of these features has opened unresolved fundamental questions about the geological history of water on Mars, including the presence of an ancient ocean and the operation of a hydrological cycle during the earliest phases of planetary history. Other fundamental questions posed by fluvial and fluvial-like features on planetary bodies include the possible erosive action of large-scale outpourings of very fluid lavas, such as those that may have produced the remarkable canali forms on Venus; the ability of exotic fluids, such as methane, to create fluvial-like landforms, as observed on Saturn's moon, Titan; and the nature of sedimentation and erosion under different conditions of planetary surface gravity. Planetary fluvial geomorphology also illustrates fundamental epistemological and methodological issues, including the role of analogy in geomorphological/geological inquiry.

Sieved mass and shape data from simulated fluvial transport of icy clasts in the Titan Tumbler.

Data in this article are related to the research article "Rapid rounding of icy clasts during simulated fluvial transport in the Titan Tumbler". Whereas that research focused on low-temperature ice abrasion in the context of Saturn's moon Titan, the full dataset on experiments testing the breakdown of water ice under a variety of tested conditions is reported in this article. Following the work of previous terrestrial studies, these experiments utilize tumblers that produce collisions to simulate some aspects of mechanical weathering during fluvial transport. Data files publicly available on Mendeley Data include measures of mass and roundness of clasts of specific grain sizes as well as raw images, videos, and the MATLAB script used for analysis. In this article, the varying conditions of temperature, initial clast size, shape, ice type, number of clasts for each of the 42 experiments are reported, along with best-fit models of abrasion typically applied in terrestrial tumbler studies. This text describes the methodology, including the development of icy clasts, operation of the tumblers, measurement of clast properties, calculation of derived parameters, and application of abrasion models. Exploration of various approaches to tumbler development and data acquisition are reported to benefit future researchers in this area. Experiments on the abrasion of different materials benefit from cross-comparison, which is also a fundamental aspect of planetary science.