Evidence of Titan's Climate History from Evaporite Distribution

Water-ice-poor, 5-$\mu$m-bright material on Saturn's moon Titan has previously been geomorphologically identified as evaporitic. Here we present a global distribution of the occurrences of the 5-$\mu$m-bright spectral unit, identified with Cassini's Visual Infrared Mapping Spectrometer (VIMS) and examined with RADAR when possible. We explore the possibility that each of these occurrences are evaporite deposits. The 5-$\mu$m-bright material covers 1\% of Titan's surface and is not limited to the poles (the only regions with extensive, long-lived surface liquid). We find the greatest areal concentration to be in the equatorial basins Tui Regio and Hotei Regio. Our interpretations, based on the correlation between 5-$\mu$m-bright material and lakebeds, imply that there was enough liquid present at some time to create the observed 5-$\mu$m-bright material. We address the climate implications surrounding a lack of evaporitic material at the south polar basins: if the south pole basins were filled at some point in the past, then where is the evaporite

The Age of the Milky Way Inner Halo

The Milky Way galaxy is observed to have multiple components with distinct properties, such as the bulge, disk, and halo. Unraveling the assembly history of these populations provides a powerful test to the theory of galaxy formation and evolution, but is often restricted due to difficulties in measuring accurate stellar ages for low mass, hydrogen-burning stars. Unlike these progenitors, the "cinders" of stellar evolution, white dwarf stars, are remarkably simple objects and their fundamental properties can be measured with little ambiguity from spectroscopy. Here I report observations and analysis of newly formed white dwarf stars in the halo of the Milky Way, and a comparison to published analysis of white dwarfs in the well-studied 12.5 billion-year-old globular cluster Messier 4. From this, I measure the mass distribution of the remnants and invert the stellar evolution process to develop a new relation that links this final stellar mass to the mass of their immediate progenitors, and therefore to the age of the parent population. By applying this technique to a small sample of four nearby and kinematically-confirmed halo white dwarfs, I measure the age of local field halo stars to be 11.4 +/- 0.7 billion years. This age is directly tied to the globular cluster age scale, on which the oldest clusters formed 13.5 billion years ago. Future (spectroscopic) observations of newly formed white dwarfs in the Milky Way halo can be used to reduce the present uncertainty, and to probe relative differences between the formation time of the last clusters and the inner halo.Comment: Published in Nature, 2012, 486, 90. Second version corrects a missing reference (#10) in the third paragraph and Figure 1 captio

Ruprecht 147: The oldest nearby open cluster as a new benchmark for stellar astrophysics

Ruprecht 147 is a hitherto unappreciated open cluster that holds great promise as a standard in fundamental stellar astrophysics. We have conducted a radial velocity survey of astrometric candidates with Lick, Palomar, and MMT observatories and have identified over 100 members, including 5 blue stragglers, 11 red giants, and 5 double-lined spectroscopic binaries (SB2s). We estimate the cluster metallicity from spectroscopic analysis, using Spectroscopy Made Easy (SME), and find it to be [M/H] = +0.07 \pm 0.03. We have obtained deep CFHT/MegaCam g'r'i' photometry and fit Padova isochrones to the (g' - i') and 2MASS (J - K) CMDs using the \tau^2 maximum-likelihood procedure of Naylor (2009), and an alternative method using 2D cross-correlations developed in this work. We find best fits for isochrones at age t = 2.5 \pm 0.25 Gyr, m - M = 7.35 \pm 0.1, and A_V = 0.25 \pm 0.05, with additional uncertainty from the unresolved binary population and possibility of differential extinction across this large cluster. The inferred age is heavily dependent by our choice of stellar evolution model: fitting Dartmouth and PARSEC models yield age parameters of 3 Gyr and 3.25 Gyr respectively. At approximately 300 pc and 3 Gyr, Ruprecht 147 is by far the oldest nearby star cluster.Comment: 31 pages, 21 figures, 6 tables. Comments welcom

Floating Liquid Droplets on the Surface of Cryogenic Liquids: Implications for Titan Rain

Saturn’s moon, Titan, has a hydrocarbon-based hydrologic cycle with methane and ethane rainfall. Because of Titan’s low gravity, “floating liquid droplets” (coherent droplets of liquid hydrocarbons that float upon a liquid surface) may form on the surface of Titan’s hydrocarbon lakes and seas during rainfall. Floating liquid droplets, however, have not been investigated in the laboratory under conditions appropriate for the surface of Titan (cryogenic, hydrocarbon, liquids). We conducted a set of experiments to simulate methane and ethane rainfall under Titan surface conditions (89–94 K, 1.5 bar nitrogen atmosphere) and find that floating ethane droplets form in a wide range of bulk liquid compositions, yet floating methane droplets only form in a narrow compositional range and impact velocity. We find droplet formation is independent of the liquid density and hypothesize that dissolved atmospheric nitrogen in the bulk liquid may repel liquid ethane droplets at the surface. We propose that liquid droplets will form in Titan’s methane-rich lakes and seas during ethane rainfall with a droplet radius of ≤3 mm and an impact velocity of ≤0.7 m/s. The presence of these droplets on Titan’s lakes may result in a liquid surface layer that is dominated in rainfall composition

Global Lunar Crater Density Using Buffered Nonsparseness Correction

The density of craters on a planetary surface directly relates to the age of the surface. As the surface ages, however, craters can be erased by subsequent large impacts via direct overprinting, known as geometric crater obliteration. Such counts become increasingly limited as surfaces become more heavily cratered. Techniques to infer the statistics of the regions obliterated by craters were developed in the past decade. Such techniques, however, have only been used for regional studies. Herein, we present a study of the global density of lunar impact craters ≥20 km in diameter using both traditional crater-counting and buffered nonsparseness correction (BNSC) crater-counting techniques. By comparing the measurements, we quantify the influence of geometric crater obliteration on the visible lunar crater record. Our results reveal that geometric crater obliteration erased up to three-fifths of craters ≥20 km in diameter that formed on the most ancient lunar terrains, whereas younger surfaces, like the Procellarum KREEP Terrane, show little to no evidence of such crater obliteration. The differences in derived crater densities highlight ancient surfaces in which the effects of geometric crater obliteration must be considered to characterize their cratering histories. Furthermore, our results identify the most heavily cratered area on the Moon, a region of the lunar highlands between Smythii basin and the South Pole–Aitken (SPA) basin (Smythii–SPA–Highlands); the number of impacts revealed by the BNSC technique for this region is consistent with estimates derived from the abundance of highly siderophile elements and from modeling crustal porosity

The NASA Roadmap to Ocean Worlds

In this article, we summarize the work of the NASA Outer Planets Assessment Group (OPAG) Roadmaps to Ocean Worlds (ROW) group. The aim of this group is to assemble the scientific framework that will guide the exploration of ocean worlds, and to identify and prioritize science objectives for ocean worlds over the next several decades. The overarching goal of an Ocean Worlds exploration program as defined by ROW is to identify ocean worlds, characterize their oceans, evaluate their habitability, search for life, and ultimately understand any life we find. The ROW team supports the creation of an exploration program that studies the full spectrum of ocean worlds, that is, not just the exploration of known ocean worlds such as Europa but candidate ocean worlds such as Triton as well. The ROW team finds that the confirmed ocean worlds Enceladus, Titan, and Europa are the highest priority bodies to target in the near term to address ROW goals. Triton is the highest priority candidate ocean world to target in the near term. A major finding of this study is that, to map out a coherent Ocean Worlds Program, significant input is required from studies here on Earth; rigorous Research and Analysis studies are called for to enable some future ocean worlds missions to be thoughtfully planned and undertaken. A second finding is that progress needs to be made in the area of collaborations between Earth ocean scientists and extraterrestrial ocean scientists

Identification of buried lunar impact craters from GRAIL data and implications for the nearside maria

Gravity observations from the dual Gravity Recovery and Interior Laboratory (GRAIL) spacecraft have revealed more than 100 quasi‐circular mass anomalies, 26–300 km in diameter, on the lunar nearside. These anomalies are interpreted to be impact craters filled primarily by mare deposits, and their characteristics are consistent with those of impact structures that formed prior to, and during, intervals of flooding of feldspathic terrane by mare basalt lavas. We determine that mare deposits have an average density contrast of 850⁺³⁰⁰₋₂₀₀ kg m⁻³ relative to the surrounding crust. The presence of a large population of volcanically buried craters with minimal topographic expression and diameters up to 300 km requires an average nearside mare thickness of at least 1.5 km and local lenses of mare basalt as thick as ~7 km