•Applies analytic vortex model to generate encounter signatures.•Examines solution space constraints from data and fitting accuracies.•Demonstrates model with new high-quality terrestrial field ...data.•Considers optimal camera views for InSight mission.
A physically-realistic migrating vortex model is developed and applied to generate pressure and wind speed and direction histories for dust devil passage. The asymmetric character of wind histories is noted, and we examine how these combined data constrain the solution space of dust devil parameters (migration velocity, diameter and intensity), ambient wind, and miss distance. These histories are compared with a new terrestrial field dataset of high-time resolution pressure and wind measurements of over twenty dust devil encounters in New Mexico. This new dataset is made available electronically and it is found that model fits can be typically achieved with simultaneous root-mean-square errors of ∼0.05hPa (∼5–10% of the peak pressure signature), ∼20°of wind azimuth, and ∼2m/s windspeed. The fits are not unique, however, and some heuristic aspects of resolving the intrinsic degeneracies of the problem and nonideal features of real encounters are discussed. The application of this approach to the InSight lander is noted, offering the possibility of defining the context for any possible detections of electromagnetic and seismic signatures of dust devils on Mars.
Saltation modeling by Comola et al. (2022, https://doi.org/10.1029/2022GL097913) considering a higher interparticle cohesion suggests smaller sand size on Titan than previously assumed, with possibly ...frequent sand transport. These predictions are testable by the Dragonfly mission presently under development.
Plain Language Summary
The organic sand on Saturn's moon Titan may be more “sticky” than sand on Earth made from silicate rocks. A study with a novel approach to calculating how sand moves suggests that sand particles on Titan might be smaller and could move more often than previous work indicated. The Dragonfly lander will be able to study these materials and processes directly on Titan.
Key Points
Sand transport to form dunes depends on a threshold velocity, which is higher (and the sand size is larger) if grains stick together
Titan materials may me more cohesive, and so larger than previously thought
The Dragonfly mission will explore Titan's dunes
We use a simple box model to explore possible differences in the liquid composition of Titan's seas. Major variations in the abundance of involatile ethane, somewhat analogous to salinity in ...terrestrial waters, arise from the hydrological cycle, which introduces more “fresh” methane rainfall at the highest latitudes in summer. The observed composition of Ligeia Mare, flushed by methane rainfall and exporting its solutes to Kraken via a narrow labyrinth of channels, may have a methane‐rich (>~80%) composition, well out of thermodynamic equilibrium with the atmosphere, whereas the basins of Kraken are relatively well mixed and will have an ethane‐dominated (~60%) composition. These variations, analogous to Earth's salinity gradient between the Black Sea and the Mediterranean, may be detectable with Cassini measurements and are important for future exploration.
Key Points
Applies a hydrological model to disequilibrium composition of Titan's SeasLigeia Mare is 'fresh' (methane‐rich), analogous to the Baltic on EarthKraken Mare is several times richer in involatiles (e.g. ethane) than Ligeia
•Reviews near-surface wind measurements from Venus probes.•Develops analytic (Weibull) probability distribution of wind speeds.•Implies frequent motion of surface particulates.•Critical input for ...landing safety assessment on future missions.
A surface wind specification is needed for future landed missions to Venus. While sparse, there exist enough data from the limited surface and near-surface measurements to date to define a probability density function that guides expectations of winds for rational design of landing systems. Following a review of all available data (mostly from the Venera missions), a Weibull function, used previously for Mars and Titan, and widely used in terrestrial engineering applications, is proposed. Best-estimate wind measurements are reasonably described by P(>V)=exp−(V/c)k, with c=0.8m/s, k=1.9: this function yields a 95% chance of winds <1.4m/s and 99% <1.8m/s. A worst-case function, allowing the high end of Venera measurement uncertainties to force the fit, has slightly higher values (c=0.9m/s, k=1.7; 95% wind 1.7m/s; 99%, 2.2m/s). The data suggest that winds strong enough to move dust and sand on Venus are rather common (more so than is typical for Mars, Earth or Titan), a prediction testable with radar interferometry on future orbital missions and/or from landed observations. More elaborate analyses should take site-specific factors such as slope or time of day into account, but cannot be meaningfully constrained by present data.
An analytic prediction of the frictional temperature rise of materials in pneumatic transfer systems is developed, with the aim of anticipating from first principles the onset of problematic ...formation of melt structures such as ‘angel hair’. Specifically, particulates of size d passing at velocity V through a 90° bend of radius R experience a maximum surface temperature rise ΔT = μdV2(Vρ/kcR)0.5, where μ,ρ,k,c are the particle friction coefficient, density, thermal conductivity and heat capacity respectively. Limits of applicability of this idealized (but conservative) model are discussed, and simple corrections for the mitigating effects of pipe texture and gas cooling are discussed. The expression predicts temperature increments of many tens of Kelvin for 5 mm polyethylene granules at speeds of >40 m/s, but only a few K for 1 mm organic grains at 30 m/s in a planned pneumatic sampling system for a planetary exploration mission.
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•Simple formula predicts heating in pneumatic transport.•Allows prediction of angel hair formation.•Elaborations and limitations of the theory are discussed.•Expression is applied to industrial and exploration settings.
•Considers spacecraft sampling of putative plumes on Europa.•Determines 2mm maximum particle size to 2km altitude.•Evaluates organic and cell inventory for Earth ocean analog.•Proposes log-uniform ...metric for relative science merit for uncertain target.
The anticipated material captured from a flythrough of Europa’s putative plumes is investigated using a simple model. With parameters appropriate to observed constraints (plume height 100–200km, column mass 1E20 H2O molecules/m2, originating in a liquid water exposure), bacterial cells of ∼10μm could be lofted even to the plume tops, but no particles larger than 2mm will be lofted above ∼2km, a likely lower limit on feasible altitude. Intercepted mass densities of 1E−5 to 1E−3kg/m2 are calculated. With a small in-situ sampler at the lowest altitudes a few hundred cells might be captured if the liquid is as abundant in biota as the richest environments on Earth, but statistically less than 1 cell for Vostok waters, a Europa analog. The imperative for a large collection area is noted. The likelihood of capturing at least a single cell, with a log-uniform prior of cell abundances, is proposed as a science value metric for different flyby altitudes.
Abstract
Observations with sensitive photodiode detectors on the Perseverance rover (Hueso et al., 2023,
https://doi.org/10.1029/2022JE007516
; Vicente‐Retortillo et al., 2023,
...https://doi.org/10.1029/2022je007672
) to detect dust devils and track formation, and movies of the Ingenuity helicopter's downwash impingement on the Martian surface (Lemmon et al., 2022a,
https://doi.org/10.1029/2022je007605
; Lemmon et al., 2022b,
https://doi.org/10.1029/2022gl100126
), together with in‐situ meteorological data, give new insights into the important problem of dust‐lifting on Mars, a phenomenon which influenced the lifetime of recent rovers and landers. These results, together with new low‐gravity wind tunnel experiments on parabolic flights and interpretation of the large blast pattern from lander retrorockets, indicate that particle motion and visible darkening on Mars can result from aerodynamic pressures of only 1–5 Pa, considerably less than previously thought.
Plain Language Summary
Dust is important on Mars, not least in that dust falling on solar panels limits the life of spacecraft. New measurements from the Perseverance rover and Ingenuity helicopter show that dust may be lifted off the ground more easily than had been thought.
Key Points
New data show the dust‐lifting threshold may be lower than had been assumed
The dust‐lifting threshold may help interpret features such as dust devil tracks and impact blast waves
Observation of dust devil tracks from orbit may help diagnose the rate of solar array cleaning
Spirit rover solar array data, which if publicly-archived would provide a useful window on Mars meteorology, shows dust-clearing events coinciding with the onset of dust devil season in three Mars ...years. The recurrence interval of 100–700days is consistent with the extrapolation of Pathfinder and Phoenix vortex encounters indicated by pressure drops of ∼6–40Pa (similar to laboratory measurements of dust lifting threshold) and with observed areas and rates of generation of dust devil tracks on Mars.
We present a map of the orientation and extent of Titan's sand dunes derived from the complete radar imaging dataset from the Cassini prime mission. The 16,000 dune segments we have mapped cover ∼8% ...of the Titan's surface (suggesting a total coverage of ∼20%), are confined within 30° of the equator, and show local and regional deviations of dune orientation of up to about 40° from due Eastwards. There is no obvious global longitudinal pattern, although some divergence with latitude is apparent. The most striking pattern is one of collimation by and divergence around bright and/or high terrain. Obstacles 100–300 m high obstruct dunes when the local slope is 1/50 or steeper, while slopes 1/200 or shallower cause dunes to thin out as they climb, or to deviate around the obstacles.
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