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Register a new userComplex Water Ice Mixtures on NII Nereid: Constraints from NIR Reflectance
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Nereid, Neptunes third largest satellite, lies in an irregular orbit and is the only outer satellite in the system (apart from Triton) that can be spectroscopically characterized with the current generation of Earth-based telescopes. We report our results on spectral characterization of Nereid using its reflectance spectrum from 0.8-2.4 $mu m$, providing the first measurements over the range of 0.8-1.4 $mu m$. We detect spectral absorption features of crystalline water ice in close agreement with previous measurements. We show that model fits of simple intimate mixtures including water ice do not provide simultaneous matches to absorption band depths at 1.5 and 2.0 $mu m$ when accounting for the spectral continuum. Possible solutions include invoking a more complex continuum, including both crystalline and amorphous water ice, and allowing for sub-micron sized grains. We show that mixtures including magnetite and the CM2 chondrite Murchison provide a flexible framework for interpreting spectral variation of bodies with neutral-sloped spectra like that of Nereid. Magnetite in particular provides a good match to the spectral continuum without requiring the presence of Tholin-like organics. We note that carbonaceous chondrites and their components may be useful analogs for the non-ice components of outer solar system bodies, consistent with recent findings by Fraser et al. (2019). Comparison to spectra of large TNOs and satellites of Uranus show that Nereids low albedo, deep water bands, and neutral color is distinct from many other icy objects, but such comparisons are limited by incomplete understanding of spectral variability among $sim$100km-sized icy bodies.
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The gas-driven dust activity of comets is still an unresolved question in cometary science. In the past, it was believed that comets are dirty snowballs and that the dust is ejected when the ice retreats. However, thanks to the various space missions to comets, it has become evident that comets have a much higher dust-to-ice ratio than previously thought and that most of the dust mass is ejected in large particles. Here we report on new comet-simulation experiments dedicated to the study of the ejection of dust aggregates caused by the sublimation of solid water ice. We find that dust ejection exactly occurs when the pressure of the water vapor above the ice surface exceeds the tensile strength plus the gravitational load of the covering dust layer. Furthermore, we observed the ejection of clusters of dust aggregates, whose sizes increase with increasing thickness of the ice-covering dust-aggregate layer. In addition, the trajectories of the ejected aggregates suggest that most of the aggregates obtained a non-vanishing initial velocity from the ejection event.
Comet 49P/ Arend-Rigaux, thought to be a low activity comet since the 1980s was found to be active in its recent apparitions. Recent analysis of the data obtained from Spitzer observation of the comet in 2006 compared with laboratory spectra has revealed amorphous water ice on the surface. In addition, in 2012 a jet was found to appear during its subsequent perihelion passage as witnessed during an observation carried out on 26th March 2012 using the PRL telescope at Mt. Abu. This confirms recent activity of Comet 49P/Arend-Rigaux due to the volatile subsurface materials exposed after several passages close to the Sun. Our result confirms the subsurface ices on cometary nuclei and insists for more observations for a better understanding.
Using the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM), we map the temporal variability of water ice absorption bands over the near-polar ice mound in Louth crater, Mars. The absorption band depth of water ice at 1.5 microns can be used as a proxy for ice grain size and so sudden reductions can time any switches from ablation to condensation. A short period of deposition on the outer edge of the ice mound during late spring coincides with the disappearance of seasonal water frost from the surrounding regolith suggesting that this deposition is locally sourced. The outer unit at Louth ice mound differs from its central regions by being rough, finely layered, and lacking wind-blown sastrugi. This suggests we are observing a new stabilizing effect wherein the outer unit is being seasonally replenished with water ice from the surrounding regolith during spring. We observe the transport distance for water migration at Louth crater to be ~4km, and we use this new finding to address why no water ice mounds are observed in craters <9km in diameter.
Water ice is one of the most abundant materials in dense molecular clouds and in the outer reaches of protoplanetary disks. In contrast to other materials (e.g., silicates) water ice is assumed to be stickier due to its higher specific surface energy, leading to faster or more efficient growth in mutual collisions. However, experiments investigating the stickiness of water ice have been scarce, particularly in the astrophysically relevant micrometer-size region and at low temperatures. In this work, we present an experimental setup to grow aggregates composed of $mathrm{mu}$m-sized water-ice particles, which we used to measure the sticking and erosion thresholds of the ice particles at different temperatures between $114 , mathrm{K}$ and $260 , mathrm{K}$. We show with our experiments that for low temperatures (below $sim 210 , mathrm{K}$), $mathrm{mu}$m-sized water-ice particles stick below a threshold velocity of $9.6 , mathrm{m , s^{-1}}$, which is approximately ten times higher than the sticking threshold of $mathrm{mu}$m-sized silica particles. Furthermore, erosion of the grown ice aggregates is observed for velocities above $15.3 , mathrm{m , s^{-1}}$. A comparison of the experimentally derived sticking threshold with model predictions is performed to determine important material properties of water ice, i.e., the specific surface energy and the viscous relaxation time. Our experimental results indicate that the presence of water ice in the outer reaches of protoplanetary disks can enhance the growth of planetesimals by direct sticking of particles.
We present the first measurements of Charons far-ultraviolet surface reflectance, obtained by the Alice spectrograph on New Horizons. We find no measurable flux shortward of 1650 A, and Charons geometric albedo is $<0.019$ ($3sigma$) at 1600 A. From 1650--1725 A Charons geometric albedo increases to $0.166pm0.068$, and remains nearly constant until 1850 A. As this spectral shape is characteristic of H$_2$O ice absorption, Charon is the first Kuiper belt object with a H$_2$O ice surface to be detected in the far-ultraviolet. Charons geometric albedo is $sim3.7$ times lower than Enceladus at these wavelengths, but has a very similar spectral shape. We attribute this to similarities in their surface compositions, and the difference in absolute reflectivity to a high concentration or more-absorbing contaminants on Charons surface. Finally, we find that Charon has different solar phase behavior in the FUV than Enceladus, Mimas, Tethys, and Dione, with a stronger opposition surge than Enceladus and a shallower decline at intermediate solar phase angles than any of these Saturnian satellites.
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