National Aeronautics and Space Administration

Planets orbiting M-dwarf stars are prime targets in the search for rocky exoplanet atmospheres. The small size of M dwarfs renders their planets exceptional targets for transmission spectroscopy, facilitating atmospheric characterization. However, it remains unknown whether their host stars' highly variable extreme-UV radiation environments allow atmospheres to persist. With JWST, we have begun to determine whether or not the most favorable rocky worlds orbiting M dwarfs have detectable atmospheres. Here, we present a 2.8-5.2 micron JWST NIRSpec/G395H transmission spectrum of the warm (700 K, 40.3x Earth's insolation) super-Earth GJ 486b (1.3 R$_{\oplus}$ and 3.0 M$_{\oplus}$). The measured spectrum from our two transits of GJ 486b deviates from a flat line at 2.2 - 3.3 $\sigma$, based on three independent reductions. Through a combination of forward and retrieval models, we determine that GJ 486b either has a water-rich atmosphere (with the most stringent constraint on the retrieved water abundance of H2O > 10% to 2$\sigma$) or the transmission spectrum is contaminated by water present in cool unocculted starspots. We also find that the measured stellar spectrum is best fit by a stellar model with cool starspots and hot faculae. While both retrieval scenarios provide equal quality fits ($\chi^2_\nu$ = 1.0) to our NIRSpec/G395H observations, shorter wavelength observations can break this degeneracy and reveal if GJ 486b sustains a water-rich atmosphere.

This study presents the flow condensation heat transfer results of the Flow Boiling and Condensation Experiment (FBCE). The primary goal of FBCE is to obtain fundamental flow boiling and condensation heat transfer data in microgravity (mu ge) through experiments onboard the International Space Station. Experiments were performed with the Condensation Module for Heat Transfer (CM-HT), which is a tube-in-tube counterflow heat exchanger. Condensing nPFH flows through a stainless steel tube with an inner diameter of 7.24 mm and rejects heat to cooling water flowing in an annular channel (with inner and outer gap diameter of 7.94 and 12.70 mm, respectively) surrounding the tube. Experiments tested a broad range of nPFH mass velocities, G = 72.8 - 291.5 kg/m2s, inlet thermodynamic equilibrium qualities, xe,in = 0.28 - 1.19, inlet pressures, pin = 103.9 - 160.2 kPa, and water mass velocities, Gw = 129.4 - 324.7 kg/m2s. A parametric investigation shows local condensation heat transfer coefficient, h, is primarily dependent on G and local xe, which can be represented by the two-phase mixture Reynolds number, Retp. Channel averaged heat transfer coefficient in the saturated two-phase region, htp, increases with increasing G and xe,in. However, increasing inlet superheat does not affect htp, but does increase the heat transfer coefficient averaged over the entire channel, h. In the present experiments, G is sufficient to mitigate the effects of gravity, and htp in mu ge aligns with those for vertical down flow and horizontal flow in Earth gravity. Various correlations for htp were assessed, and the best performing correlation with a Mean Absolute Error (MAE) of 7.1% was that by Dorao and Fernandino, which is a function of Retp. Some correlations were shown to be overly dependent on the effect of gravity and were not applicable for the present mu ge database. A Separated Flow Model for annular condensation was employed to predict htp. The model's physical basis makes it seamlessly adaptable for mu ge, and it resulted in a MAE of 32.3%.

A series of far-field high-energy X-Ray diffraction microscopy (ff-HEDM) measurements gathered at discrete temporal points from the illuminated gage section of a Ti-7Al coupon subjected to creep revealed significant stress relaxation in soft grains neighboring a hard grain. A crystal plasticity finite element (CPFE) framework was used to perform a creep simulation on the tessellated volume of the gage section of the coupon based on the grain centroid data obtained from ff-HEDM. For a contiguous hard and soft grain pair, a correlation was found between grain-average stress relaxation (measured from experiments) and total accumulated slip (estimated from CPFE simulations). However, the magnitude of stress relaxation from CPFE simulation was significantly underpredicted due to the inherent homogeneous nature of the crystal plasticity framework which cannot resolve the mechanisms underpinning events (e.g., intermittent motion of dislocations, slip band formation, precipitate shearing, etc.) leading to stress relaxation in grains.

Europa's surface composition and physical characteristics are commonly constrained using spectral deconvolution through linear mixture (LM) modeling and radiative transfer-based (RT) intimate mixture modeling. Here, I compared the results of these two spectral modeling- LM versus RT- against laboratory spectra of water (H2O) ice and sulfuric acid octahydrate (SAO; H2SO48H(2)O) mixtures measured at near-infrared wavelengths (similar to 1.2-2.5 mu m) with grain sizes of 90-106 mu m (Hayes and Li, 2025). The modeled abundances indicate that the RT more closely reproduces the laboratory abundances, with deviations within +/- 5 % for both H2O ice and H2SO48H(2)O with similar to 100 mu m grains. In contrast, the LM shows slightly larger discrepancies, typically ranging from +/- 5-15 % from the true abundances. Interestingly, both LM and RT tend to consistently overestimate the abundance of H2SO48H(2)O and underestimate H2O ice across all mixtures. Nonetheless, when H2SO48H(2)O either dominates (>80 % as observed on Europa's trailing hemisphere; Carlson et al., 2005) or is present only in trace amounts (similar to 10 % on areas in Europa's leading hemisphere; Dalton et al., 2013; Ligier et al., 2016), both the LM and RT render acceptable results within +/- 10 % uncertainty. Thus, spectral modeling using the RT is preferred for constraining the surface composition across Europa, although the LM remains viable in specific compositional regimes.