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At least 343 records · Page 19
All sourcesNASA NTRSDOE OSTI
NASA NTRS2021

Airborne Dust Plumes Lofted by Dislodged Ice Blocks at Russell Crater, Mars

Linear dune gullies on poleward-facing Martian slopes are enigmatic. Formation by CO2-ice block or snow cornice falls has been proposed based on optical imagery of bright, high-albedo features inside gully channels. Because these features often resemble patchy frost residue rather than three-dimensional blocks, more evidence is needed to support the ice-block formation mechanism. Satellite imagery captured two simultaneous airborne plumes with in-channel sources at the Russell crater megadune, thrust up, and dispersed outward along the path of linear dune gullies. We use spectral data analyses, climatic analyses of bolometric temperatures, and thermal modeling to further develop the mechanistic framework for linear dune gully development. Basal sublimation and CO2 gas venting likely cause CO2-ice-block detachment and falls from gully alcoves in southern early spring, accompanied by ice-block off-gassing and saltation of sands and coarse silts that are redeposited around gully channels, and lofting of sublimation lag (coarse dust/silt) into airborne plumes.

Cynthia Dinwiddie↗
NASA NTRS2023

Gas Sources from the Coma and Nucleus of Comet 46P/Wirtanen Observed Using ALMA

Gas-phase molecules in cometary atmospheres (comae) originate primarily from (1) outgassing by the nucleus, (2) sublimation of icy grains in the near-nucleus coma, and (3) coma (photo)chemical processes. However, the majority of cometary gases observed at radio wavelengths have yet to be mapped, so their production/release mechanisms remain uncertain. Here we present observations of six molecular species toward comet 46P/Wirtanen, obtained using the Atacama Large Millimeter/submillimeter Array during the comet's unusually close (∼0.1 au) approach to Earth in 2018 December. Interferometric maps of HCN, CH 3 OH, CH 3 CN, H 2 CO, CS, and HNC were obtained at an unprecedented sky-projected spatial resolution of up to 25 km, enabling the nucleus and coma sources of these molecules to be accurately quantified. The HCN, CH 3 OH, and CH 3 CN spatial distributions are consistent with production by direct outgassing from (or very close to) the nucleus, with a significant proportion of the observed CH 3 OH originating from sublimation of icy grains in the near-nucleus coma (at a scale length L p = 36 ± 7 km). On the other hand, H 2 CO, CS, and HNC originate primarily from distributed coma sources (with L p values in the range 550–16,000 km), the identities of which remain to be established. The HCN, CH 3 OH, and HNC abundances in 46P are consistent with the average values previously observed in comets, whereas the H 2 CO, CH 3 CN, and CS abundances are relatively low.

Short period comets↗
NASA NTRS2021

Modelling the Water and Carbon Dioxide Production Rates of Comet 67P/Churyumov-Gerasimenko

The European Space Agency Rosetta/Philae mission to Comet 67P/Churyumov–Gerasimenko in 2014–2016 is the most complete and diverse investigation of a comet carried out thus far. Yet, many physical and chemical properties of the comet remain uncertain or unknown, and cometary activity is still not a well–understood phenomenon. We here attempt to place constraints on the nucleus abundances and sublimation front depths of H 2 O and CO 2 ice, and to reconstruct how the nucleus evolved throughout the perihelion passage. We employ the thermophysical modelling code ‘Numerical Icy Minor Body evolUtion Simulator’, or NIMBUS, to search for conditions under which the observed H 2 O and CO 2 production rates are simultaneously reproduced before and after perihelion. We find that the refractories to water–ice mass ratio of relatively pristine nucleus material is μ ≈ 1, that airfall material has μ ≈ 2, and that the molar abundance of CO 2 relative H 2 O is near 30 per cent. The dust mantle thickness is typically ≲ 2 cm. The average CO 2 sublimation front depths near aphelion were ~ 3.8 m and ~ 1.9 m on the northern and southern hemispheres, respectively, but varied substantially with time. We propose that airfall material is subjected to substantial fragmentation and pulverisation due to thermal fatigue during the aphelion passage. Sub–surface compaction of material due to CO 2 activity near perihelion seems to have reduced the diffusivity in a measurable way.

67P/Churyumov–Gerasimenko↗
NASA NTRS2021

Widespread Megaripple Activity Across the North Polar Ergs of Mars

The most expansive dune fields on Mars surround the northern polar cap where various aeolian bedform classes are modified by wind and ice. The morphology and dynamics of these ripples, intermediate-scale bedforms (termed megaripples and transverse aeolian ridges (TARs)), and sand dunes reflect information regarding regional boundary conditions. We found that populations of polar megaripples and larger TARs are distinct in terms of their morphology, spatial distribution, and mobility. Whereas regionally-restricted TARs appeared degraded and static in long-baseline observations, polar megaripples were not only widespread but migrating at relatively high rates (0.13± 0.03 m/Earth year) and possibly more active than other regions on Mars. This high level of activity is somewhat surprising since there is limited seasonality for aeolian transport due to surficial frost and ice during the latter half of the martian year. A comprehensive analysis of an Olympia Cavi dune field estimated that the advancement of megaripples, ripples, and dunes avalanches accounted for ~1%, ~10%, and ~100%, respectively, of the total aeolian system’s sand fluxes. This included dark-toned ripples that migrated the average equivalent of 9.6±6 m/yr over just 22 days in northern summer - unprecedented rates for Mars. While bedform transport rates are some of the highest yet reported on Mars, the sand flux contribution between the different bedforms does not substantially vary from equatorial sites with lower rates. Seasonal off-cap sublimation winds and summer-time polar storms are attributed as the cause for the elevated activity, rather than cryospheric processes. Plain Language Summary “Megaripples” are distinct wind-driven bedforms that occur on the surface of Earth and Mars, often with sizes between that of smaller ripples and larger dunes. Recent work has found the thin martian atmosphere can mobilize some coarse-grained megaripples, overturning prior notions that these were static relic landforms from a past climate. We mapped megaripples and adjacent bedforms across the north polar sand seas, the most expansive collection of dune fields on Mars. Megaripples were found to be widespread across the region and migrating at relatively high rates relative to other sites on Mars that are at lower latitudes. This enhanced activity is likely related to the greater sand fluxes found for neighboring dunes which are driven by summer-time seasonal winds when polar ice is sublimating. In contrast, other megaripples appear to be stabilized, a likely result of inter-granular ice within low wind areas.

Matthew Chojnacki↗
NASA NTRS2026

CH3OH and HCN in Interstellar Comet 3I/ATLAS Mapped with the ALMA Atacama Compact Array: Distinct Outgassing Behaviors and a Remarkably High CH3OH/HCN Production Rate Ratio

We report the detection of methanol (CH3OH) toward interstellar comet 3I/ATLAS using the At-acama Compact Array of the Atacama Large Millimeter/Submillimeter Array (ALMA) on UT 2025 August 28, September 18 and 22, and October 1, and of hydrogen cyanide (HCN) on September 12 and 15. These observations spanned pre-perihelion heliocentric distances (r H ) of 2.6 – 1.7 au. The molecules showed outgassing patterns distinct from one another, with HCN production being depleted in the sunward hemisphere of the coma, whereas CH3OH was enhanced in that direction. Statistical analysis of molecular scale lengths in 3I/ATLAS indicated that CH3OH included production from coma sources at Lp > 258 km at 99% confidence. However low signal-to-noise on long baselines, which sample emission on small spatial scales closest to the nucleus, prevented definitively ruling out CH3OH as purely a parent species. In contrast, HCN production was indistinguishable from direct nucleus sublimation. The CH3OH production rate increased sharply from August through October, including an uptick near the inner edge of the H2O sublimation zone at rH = 2 au. Compared to comets measured to date at radio wavelengths, the derived CH3OH/HCN ratios in 3I/ATLAS of 124+30−34 and 79+11−14on September 12 and 15, respectively, are among the most enriched values measured in any comet, surpassed only by anomalous solar system comet C/2016 R2 (PanSTARRS)

Comae↗
NASA NTRS1962

Numerical Analysis of the Transient Response of Advanced Thermal Protection Systems for Atmospheric Entry

Equations for the transfer of heat through thermal protection shields are derived in finite difference form. These equations are applicable to charring ablators, impregnated ceramics, subliming ablators, heat sinks, and insulating materials and have been programed for solution on a high-speed digital computer. In the program, thermal properties can be functions of temperature. Provision is made for analysis of heat shields subjected to simultaneous convective and radiative heat inputs. Some typical results are presented. Limited comparisons with experimental results are made.

THERMAL PROTECTION↗
NASA NTRS1962

RE-ENTRY MATERIALS

The major requirement for a re-entry material is that it protect the vehicle from the aerodynamic heating associated with re-entry. A relatively short time ago, the term re-entry brought to mind a ballistic type trajectory and its characteristic heat pulse. Today, however, a variety of thermal re-entry conditions are common due to the various types of vehicles currently of interest. Extremes of re-entry heating-time conditions vary from the relatively low heat flux-long time pulse associated with glide and shallow orbital re-entries through high flux-short time ballistic re-entry to the very high heat flux-moderate time pulse associated with super orbital re-entry. Materials for providing thermal protection, therefore, vary considerably depending upon the type of reentry heat-time pulse. From a functional standpoint, thermal protection materials may be divided into two types; absorptive and radiative. The absorptive material dissipates heat by storage (heat sink) or mass transfer and chemical change (ablation, transpiration or sublimation).

Thermal protection↗
NASA NTRS1958

Boundary-Layer-Transition Measurements in Full-Scale Flight

Chemical sublimation has been employed for boundary-layer-flow visualization on the wings of a supersonic fighter airplane in level flight at speeds near a Mach number of 2.0. The tests have shown that laminar flow can be obtained over extensive areas of the wing with practical wing-surface conditions. In addition to the flow visualization tests, a method of continuously monitoring the conditions of the boundary layer has been applied to flight testing, using heated temperature resistance gages installed in a Fiberglas "glove" installation on one wing. Tests were conducted at speeds from a Mach number of 1.2 to a Mach number of 2.0, at altitudes from 35,000 feet to 56,000 feet. Data obtained at all angles of attack, from near 0 deg to near 10 deg, have shown that the maximum transition Reynolds number on the upper surface of the wing varies from about 2.5 x 10(exp 6) at a Mach number of 1.2 to about 4 x 10(exp 6) at a Mach number of 2.0. On the lower surface, the maximum transition Reynolds number varies from about 2 x 10(exp 6) at a Mach number of 1.2 to about 8 x 10(exp 6) at a Mach number of 2.0.

SUPERSONIC FLOW↗