New Horizons Found Evidence of Sand Dunes Made of Methane on Pluto

Scientists say they have found evidence of dunes of frozen methane on Pluto.

The research, which is published in the journal Science, suggests that the distant world is more dynamic than previously thought.

Pluto's atmosphere was believed to be too thin to create the features familiar in deserts on Earth.

The findings come from analysis of the startling images sent back by Nasa's New Horizons mission, which flew close to Pluto in July 2015.

After an epic trek through the Solar System that took nearly a decade, New Horizons sped by at a speed of 58,536 km/h (36,373 mph), gathering data as it passed.

In their study, the researchers explain how they studied pictures of a plain known as Sputnik Planitia, parts of which are covered with what look like fields of dunes.

They are lying close to a range of mountains of water ice 5km high.

The scientists conclude that the dunes are 0.4-1km apart and that they are made up of particles of methane ice between 200-300 micrometers in diameter - roughly the size of grains of sand.

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Do Sand Dune Shapes Suggest Mars had a Thin Atmosphere by the end of the Noachian 3.7 Billion Years ago?

Large wind ripples on Mars: A record of atmospheric evolution

Authors:

Lapotre et al

Abstract:

Wind blowing over sand on Earth produces decimeter-wavelength ripples and hundred-meter– to kilometer-wavelength dunes: bedforms of two distinct size modes. Observations from the Mars Science Laboratory Curiosity rover and the Mars Reconnaissance Orbiter reveal that Mars hosts a third stable wind-driven bedform, with meter-scale wavelengths. These bedforms are spatially uniform in size and typically have asymmetric profiles with angle-of-repose lee slopes and sinuous crest lines, making them unlike terrestrial wind ripples. Rather, these structures resemble fluid-drag ripples, which on Earth include water-worked current ripples, but on Mars instead form by wind because of the higher kinematic viscosity of the low-density atmosphere. A reevaluation of the wind-deposited strata in the Burns formation (about 3.7 billion years old or younger) identifies potential wind-drag ripple stratification formed under a thin atmosphere.

Present-day formation and seasonal evolution of linear dune gullies on Mars

Present-day formation and seasonal evolution of linear dune gullies on Mars

Authors:

Pasquon et al

Abstract:

Linear dune gullies are a sub-type of martian gullies. As their name suggests they only occur on sandy substrates and comprise very long (compared to their width) straight or sinuous channels, with relatively small source areas and almost non-existent visible deposits. Linear dune gullies have never been observed on terrestrial dunes and their formation process on Mars is unclear. Here, we present the results of the first systematic survey of these features in Mars’ southern hemisphere and an in-depth study of six dunefields where repeat-imaging allows us to monitor the changes in these gullies over time. This study was undertaken with HiRISE images at 25–30 cm/pix and 1 m/pix elevation data derived from HiRISE stereo images. We find the latitudinal distribution and orientation of linear dune gullies broadly consistent with the general population of martian gullies. They occur predominantly between 36.3°S and 54.3°S, and occasionally between 64.6°S and 70.4°S. They are generally oriented toward SSW (at bearings between 150° and 260°). We find that these gullies are extremely active over the most recent 5 Martian years of images. Activity comprises: (1) appearance of new channels, (2) lengthening of existing channels, (3) complete or partial reactivation, and (4) disappearance of gullies. We find that gully channels lengthen by ∼100 m per year. The intense activity and the progressive disappearance of linear dune gullies argues against the hypothesis that these are remnant morphologies left over from previous periods of high obliquity millions of years ago. The activity of linear dune gullies reoccurs every year between the end of winter and the beginning of spring (Ls 167.4°–216.6°), and coincides with the final stages of the sublimation of annual CO₂ ice deposit. This activity often coincides spatially and temporally with the appearance of recurrent diffusing flows (RDFs)—digitate-shaped, dark patches with low relative albedo (up to 48% lower than the adjacent dune) that encompass the active site. South- and SSW-facing dune slopes are those which preferentially host CO2 frost deposits, however, it is only those with angles of ∼20° just below the crest which possess linear dune gullies, suggesting a slope-limited formation process. These observations provide a wealth of temporal and morphometric data that can be used to undertake numerical modeling, to direct future image monitoring and guide laboratory experiments that can be used to better constrain the formation process of these features.

Are Variations in Titan's Dune Orientation a Result of Orbital Forcing?

Variations in Titan’s dune orientations as a result of orbital forcing

Authors:

MacDonald et al

Abstract:

Wind-blown dunes are a record of the climatic history in Titan’s equatorial region. Through modeling of the climatic conditions associated with Titan’s historical orbital configurations (arising from apsidal precessions of Saturn’s orbit), we present evidence that the orientations of the dunes are influenced by orbital forcing. Analysis of 3 Titan general circulation models (GCMs) in conjunction with a sediment transport model provides the first direct intercomparison of results from different Titan GCMs. We report variability in the dune orientations predicted for different orbital epochs of up to 70°. Although the response of the GCMs to orbital forcing varies, the orbital influence on the dune orientations is found to be significant across all models. Furthermore, there is near agreement among the two models run with surface topography, with 3 out of the 5 dune fields matching observation for the most recent orbital cycle. Through comparison with observations by Cassini, we find situations in which the observed dune orientations are in best agreement with those modeled for previous orbital configurations or combinations thereof, representing a larger portion of the cycle. We conclude that orbital forcing could be an important factor in governing the present-day dune orientations observed on Titan and should be considered when modeling dune evolution.

Radar scattering of linear dunes and mega-yardangs

Radar scattering of linear dunes and mega-yardangs: Application to Titan

Authors:

Paillou et al

Abstract:

The Ku-band (13.8 GHz – 2.2 cm) RADAR instrument onboard the Cassini-Huygens spacecraft has revealed the richness of the surface of Titan, as numerous seas, lakes, rivers, cryo-volcanic flows and vast dune fields have been discovered. Linear dunes are a major geomorphological feature present on Titan, covering up to 17% of its surface, mainly in equatorial regions. However, the resolution of the RADAR instrument is not good enough to allow a detailed study of the morphology of these features. In addition, other linear wind-related landforms, such as mega-yardangs (linear wind-abraded ridges formed in cohesive rocks), are likely to present a comparable radar signature that could be confused with the one of dunes. We conducted a comparative study of the radar radiometry of both linear dunes and mega-yardangs, based on representative terrestrial analogues: the linear dunes located in the Great Sand Sea in western Egypt and in the Namib Desert in Namibia, and the mega-yardangs observed in the Lut Desert in eastern Iran and in the Borkou Desert in northern Chad. We analysed the radar scattering of both terrestrial linear dunes and mega-yardangs, using high-resolution radar images acquired by the X-band (9.6 GHz – 3.1 cm) sensor of the TerraSAR-X satellite. Variations seen in the radar response of dunes are the result of a contrast between the dune and interdune scattering, while for mega-yardangs these variations are the result of a contrast between ridges and erosion valleys. We tested a simple surface scattering model, with parameters derived from the local topography and surface roughness estimates, to accurately reproduce the radar signal variations for both landforms. It appears that we can discriminate between two types of dunes – bare interdunes as in Egypt and sand-covered interdunes as in Namibia, and between two types of mega-yardangs – young yardangs as in Iran and older ones as in Chad. We applied our understanding of the radar scattering to the analysis of Cassini RADAR T8 acquisitions over the Belet Sand Sea on Titan, and show that the linear dunes encountered there are likely to be of both Egyptian and Namibian type. We also show that the radar-bright linear features observed in Cassini RADAR T64 and T83 acquisitions are very likely to be mega-yardangs, possible remnants of ancient lake basins at mid-latitude, formed when Titan’s climate was different.

Material transport map of Titan: The fate of dunes

Material transport map of Titan: The fate of dunes

Authors:

Malaska et al

Abstract:

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.

Compositional and spatial variations in Titan dune and interdune regions

Compositional and spatial variations in Titan dune and interdune regions from Cassini VIMS and RADAR

Authors:

Bonnefoy et al

Abstract:

Dunes cover about 15%15% of Titan’s visible surface, and represent one of the largest reservoirs of hydrocarbon solids on Titan (Rodriguez, S. et al. [2014]. Icarus 230, 168–179; Lopes, R.M.C. et al. [2016]. Icarus 270, 162–182.). Herein, we use data from the Cassini spacecraft to derive constraints on the compositional and regional variability of Titan’s dune and interdune regions by combining spectral information from the Visual and Infrared Mapping Spectrometer (VIMS) and spatial information from Synthetic Aperture RADAR (SAR) data. Using the combined datasets, we extract pure infrared spectra of dune and interdune regions by extrapolating linear correlations between VIMS reflectance and dune area fraction calculated in each VIMS footprint from SAR images. We applied the same method using the Cassini RADAR Radiometer dataset to extract the microwave surface emissivity of the dune and interdune regions.

Globally the dune spectra show little variation, but we find that the interdune spectra exhibit several different behaviors. Similarly, we extract from passive radiometry a mean dune emissivity of 0.98 ±± 0.01, while interdune emissivity varies from 0.86 to 0.98. We find that the interdune regions are often spectrally similar to other Titan terrain units, namely Caladan Planitia, the Adiri Mountains, and Sinlap crater, while the dunes are spectrally distinct from all terrain units. Around Sinlap crater, the interdune regions correspond to the dark blue VIMS unit: the dunes could be forming on top of the ejecta, or the material corresponding to the blue unit could be depositing preferentially in the interdunes areas. There was one region in the Belet sand sea where we were unable to extract the dune and interdune spectra and emissivities in spite of high-quality data, which we interpret to result from a thick sand cover in the interdune regions, implying inactive or saturated dune fields. However, the fact that we were able to extract distinct dune and interdune spectra and emissivities in most of Titan’s dune fields makes a strong case for sand-free interdune areas of varying composition in these regions, on depths from micrometers to decimeters. This would imply that the sand dunes have been active recently on geologic timescales.

The Black Dunes of Mars

Curiosity Reaches Martian Sand Dunes

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Curiosity Headed to Active Sand Dunes on Mars

On its way to higher layers of the mountain where it is investigating how Mars' environment changed billions of years ago, NASA's Curiosity Mars rover will take advantage of a chance to study some modern Martian activity at mobile sand dunes.

n the next few days, the rover will get its first close-up look at these dark dunes, called the "Bagnold Dunes," which skirt the northwestern flank of Mount Sharp. No Mars rover has previously visited a sand dune, as opposed to smaller sand ripples or drifts. One dune Curiosity will investigate is as tall as a two-story building and as broad as a football field. The Bagnold Dunes are active: Images from orbit indicate some of them are migrating as much as about 3 feet (1 meter) per Earth year. No active dunes have been visited anywhere in the solar system besides Earth.

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Differentiating Linear Dunes and Mega-yardangs on Titan

Radar scattering of linear dunes and mega-yardangs: Application to Titan

Authors:

Paillou et al

Abstract:

The Ku-band (13.8 GHz - 2.2 cm) RADAR instrument onboard the Cassini-Huygens spacecraft has revealed the richness of the surface of Titan, as numerous seas, lakes, rivers, cryo-volcanic flows and vast dune fields have been discovered. Linear dunes are a major geomorphological feature present on Titan, covering up to 17% of its surface, mainly in equatorial regions. However, the resolution of the RADAR instrument is not good enough to allow a detailed study of the morphology of these features. In addition, other linear wind-related landforms, such as mega-yardangs (linear wind-abraded ridges formed in cohesive rocks), are likely to present a comparable radar signature that could be confused with the one of dunes. We conducted a comparative study of the radar radiometry of both linear dunes and mega-yardangs, based on representative terrestrial analogues: the linear dunes located in the Great Sand Sea in western Egypt and in the Namib Desert in Namibia, and the mega-yardangs observed in the Lut Desert in eastern Iran and in the Borkou Desert in northern Chad. We analysed the radar scattering of both terrestrial linear dunes and mega-yardangs, using high-resolution radar images acquired by the X-band (9.6 GHz - 3.1 cm) sensor of the TerraSAR-X satellite. Variations seen in the radar response of dunes are the result of a contrast between the dune and interdune scattering, while for mega-yardangs these variations are the result of a contrast between ridges and erosion valleys. We tested a simple surface scattering model, with parameters derived from the local topography and surface roughness estimates, to accurately reproduce the radar signal variations for both landforms. It appears that we can discriminate between two types of dunes - bare interdunes as in Egypt and sand-covered interdunes as in Namibia, and between two types of mega-yardangs - young yardangs...

How Sand Moves Across the Face of Mars

Unsteady saltation on Mars

Authors:

Wang et al

Abstract:

Saltation is an important process on Mars, as it contributes to dust raising, bedform dynamics, and aeolian abrasion. Lander measurements and mesoscale meteorological models suggest that winds in the Martian atmosphere rarely exceed the fluid threshold value that is necessary to aerodynamically initiate saltation, a fact in stark contrast to the existence of dunes and ripples on the planet, many of which are in an active state of migration. In an attempt to reconcile these observations, we perform an unsteady simulation with a simple turbulence model to calculate the saltation transport rate. Sinusoidal wind variations are imposed on the saltation layer. The numerical simulations verify that gusty transport is one of the main manifestations of Martian sediment transport events. A formula for the saltation transport rate is reported, Qm∼(u∗-u∗it)pQm∼(u∗-u∗it)p, where u∗u∗ and u∗itu∗it are the friction velocity and impact threshold friction velocity. The power p varies in the range of 0.7–1.8 on Mars and ∼1.5 on Earth, depending on the period and amplitude of the gusty inflow wind. Our results show that the law of Martian and terrestrial transport rate are not universal, and hence one should be cautious when trying to extrapolate existing terrestrial results to Mars.

Atmospheric Organic Polymers may be Raining out on Titan, Contributing to Titan's Dunes

Electron-molecule chemistry and charging processes on organic ices and Titan’s icy aerosol surrogates

Authors:

Pirim et al

Abstract:

Electron-induced polymerization processes and charging events that can occur within Titan’s atmosphere or on its surface were simulated using electron irradiation and dissociative electron attachment (DEA) studies of nitrogen-containing organic condensates. The DEA studies probe the desorption of H− from hydrogen cyanide (HCN), acetonitrile (CH3CN), and aminoacetonitrile (NH2CH2CN) ices, as well as from synthesized tholin materials condensed or deposited onto a graphite substrate maintained at low temperature (90–130 K). The peak cross sections for H− desorption during low-energy (3–15 eV) electron irradiation were measured and range from 3 × 10−21 to 2 × 10−18 cm2. Chemical and structural transformations of HCN ice upon 2 keV electron irradiation were investigated using X-ray photoelectron and Fourier-transform infrared spectroscopy techniques. The electron-beam processed materials displayed optical properties very similar to tholins produced by conventional discharge methods. Electron and negative ion trapping lead to 1011 charges cm−2 on a flat surface which, assuming a radius of 0.05 μm for Titan aerosols, is ∼628 charges/radius (in μm). The facile charge trapping indicates that electron interactions with nitriles and complex tholin-like molecules could affect the conductivity of Titan’s atmosphere due to the formation of large negative ion complexes. These negatively charged complexes can also precipitate onto Titan’s surface and possibly contribute to surface reactions and the formation of dunes.

Toarcian Jurassic Gregarious Tritylodontid (?) Therapsid Dunefield Trackways Suggest Colonial, Prairie Dog-like Lifestyle

TRACKWAYS OF A GREGARIOUS, DUNEFIELD-DWELLING, EARLY JURASSIC THERAPSID IN THE AZTEC SANDSTONE OF SOUTHERN NEVADA

Authors:

Rowland et al

Abstract:

We describe and interpret a tracksite in the Lower Jurassic Aztec Sandstone in Valley of Fire State Park, southern Nevada. The site contains approximately one hundred tracks of the ichnogenus Brasilichnium, arranged in twelve, subparallel trackways, all on the same foreset bedding plane. The Brasilichnium trackmaker was most probably a fossorial, tritylodontid therapsid. Sedimentologicial analyses indicate that the trackway surface is a wind-ripple horizon with a primary dip of about 25°, and that the animals climbed straight up the slip face of the dune. A combination of features leads us to conclude that the footprints were impressed into a crust of moist, cohesive sand, leaving two modes of preserved tracks: (1) shallow, well-defined tracks without associated sand crescents, and (2) deeper, less well defined tracks with associated sand crescents. We interpret this assemblage of tracks to record gregarious behavior in a mixed-age group of tritylodontid therapsids. In the correlative Navajo Sandstone, other researchers have documented the presence of complex networks of burrows concentrated in elevated mounds, reminiscent of colonies of North American prairie dogs. The Brasilichnium trackmaker is a good candidate to have excavated the burrows. Although we cannot directly associate the Brasilichnium trackmaker with the burrow complexes, we hypothesize that these gregarious, fossorial animals lived in prairie-dog–town–like colonies. This study supports the aridity food-distribution hypothesis, which posits that the patchy distribution of food resources in arid environments creates selective pressure for colonial behavior.

Streaks on Martian Dune Slopes Might be Thin Sheets of Water

Viscous liquid flow on Martian dune slopes

Authors:

Dobrovolskis

Abstract:

The observed temporary dark streaks on some dune slopes on Mars may be due to thin sheets of water (or some other liquid) trickling downhill. This note corrects conceptual errors in a previous paper (M\"{o}hlmann and Kereszturi 2010, Icarus 207, 654-658) which affect the velocity profile of such flows, and produce over-estimates of their depths and mass fluxes by factors of almost two.

Comparing Tunisian Sahel to Mars

Groundwater influence on the aeolian sequence stratigraphy of the Mechertate–Chrita–Sidi El Hani system, Tunisian Sahel: Analogies to the wet–dry aeolian sequence stratigraphy at Meridiani Planum, Terby crater, and Gale crater, Mars

Authors:

Essesfi et al

Abstract:

A multidisciplinary study of the watershed and depressions of the Mechertate–Chrita–Sidi El Hani (MCSH) system in eastern Tunisia shows that groundwater upwelling and/or seepage toward the modern surface is important in the shaping of its geomorphologic features and sediment outcrops. Along the watershed of the system, groundwater is downward enriched with evaporitic minerals. These minerals precipitate as cement and protect the sediment outcrops from aeolian erosion. The water table is the limiting control on erosion and deposition, and also influences the succession of sediment along the system. The water table further determines the local base level, which controls the deposition within depressions. With increasing humidity at the limit of the capillary fringe, the landscape of the evaporative system is organized according to three sedimentary types: (1) unconsolidated sediment of aqueous and/or aeolian origin that is eroded and transported toward depressions (away from groundwater interactions), (2) consolidated sediment that is also aqueous and/or aeolian in origin and is protected from aeolian erosion by groundwater influence, and (3) sedimentary filling of depressions located within accumulation zones. These sediments are organized along a lateral, basinward profile. Here we show that during periods of relative water table fall, sediments from the watershed prograde to cover the sabkha basin fill. The rise and fall of the water table and the connected base level result in the deposition of genetically-related progradational and retrogradational sequences. We propose that these genetic sequences can be useful to interpret the sequence stratigraphy at three locations on Mars where sedimentary formations were probably controlled by direct groundwater influence: Meridiani Planum, Terby crater, and Gale crater. At Meridiani Planum, the exposed stratigraphic sequence of the Burns formation starts with deposition of dry aeolian sediment derived from a former watershed. Then, due to the rise of the water table, wet sediments of a sabkha rest atop the dry aeolian cycle to comprise a retrogradational sequence. At Terby and Gale craters, an opposite stratigraphic sequence starts with the wet deposition of the sabkha fill. Then, due to the fall of the water table, the dry aeolian sedimentation progrades atop to the sabkha fill to comprise a progradational sequence. We conclude that the various stratigraphic sequences at the MCSH system, described here, represent different possible analog scenarios for diverse depositional sequences on Mars, in all cases involving groundwater activity.

Sinking Sand Dunes From Orosirian PaleoProterozoic India Hint at Warm Climate

Palaeoproterozoic fluvio-aeolian deposits from the lower Gulcheru Formation, Cuddapah Basin, India

Authors:

Basu et al

Abstract:

An analysis of facies was done to understand the depositional environment and the palaeoclimate of the sedimentary succession from the lower part of the Palaeoproterozoic (∼2.0 Ga) Gulcheru Formation exposed along the southwestern margin of the Cuddapah Basin. Twelve distinct sedimentary facies were identified and grouped into three main facies associations - wadi fan, ephemeral fluvial and aeolian. Identification of the fluvial and the aeolian facies allowed a more elaborate interpretation of the depositional environment and its palaeoclimate. Facies characteristics indicated that the sediments in the beginning were deposited in a dominantly aeolian realm, under warm and semiarid climatic condition. Translatent strata, pin stripe lamination, zibars, high-index granule ripples, sand sheet deposits, grainflow cross-strata and grainfall laminae, asymptotically down-lapping cross-strata often with erosional lower bounding surface and massive sand-bodies with bimodal fabric, the unambiguous evidences of aeolian depositional regime led to this conclusion. However, the aeolian regime was often punctuated temporarily by fluvial input from ephemeral streams during sudden rainstorm. Depending upon the size, character and availability of sediments, relief difference and the sediment/water ratio cohesionless debris flow, hyperconcentrated flood flow and sheetflood deposits were formed near the basin margin, whereas, coarse-load braided channel deposits were laid further inside the basin. Ephemeral lakes/ponds were formed due to stagnation of floodwater in normally dry interdune lows. Overbank-interdune sediments were deposited in those ephemeral lakes/ponds. Amongst the aeolian facies, translatent strata and sand sheet dominate in the west, whereas, massive beds and dunes with well-developed slipfaces dominate in the eastern part. The spatial distribution of the aeolian bedforms suggests development of erg apron to the west and dune field (erg) to the east. The aeolian sediments identified in the Gulcheru Formation may be considered to be amongst the oldest Palaeoproterozoic aeolian sediments of the world.