85 research outputs found
Evidence of Titan's Climate History from Evaporite Distribution
Water-ice-poor, 5-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-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-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-m-bright material and lakebeds, imply
that there was enough liquid present at some time to create the observed
5-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
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
Recommended from our members
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 quasiâcircular mass anomalies, 26â300âkm 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 850âșÂłâ°â°ââââ kg mâ»Âł relative to the surrounding crust. The presence of a large population of volcanically buried craters with minimal topographic expression and diameters up to 300âkm requires an average nearside mare thickness of at least 1.5âkm and local lenses of mare basalt as thick as ~7âkm
Cassini Bistatic Radar Observations of Titan's Seas: Results about Dielectric Properties and Capillary Waves Detection
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
Topographically Influenced Evolution of Large-scale Changes in Comet 67P/Churyumov-Gerasimenko's Imhotep Region
Large portions of comet 67P/ChuryumovâGerasimenkoâs northern hemisphere are blanketed by fallback material consisting of centimeter-sized particles termed the smooth terrains. Observations from the Rosetta mission show that the most drastic transient changes during 67Pâs 2015 perihelion passage occurred within a subset of these deposits. However, we still do not understand the processes driving these changes, limiting our overall understanding of how comets evolve over both seasonal and multiorbit timescales. Herein we provide a complete documentation of scarp-driven activity on 67Pâs largest smooth terrain deposit, a highly active portion of the Imhotep region that is the southernmost of all smooth terrain basins on 67P. We also present a thermal model that accurately predicts when and where scarps originate during the course of the observed activity. Assuming a uniform surface composition, our model shows that activity is heavily controlled by local topography rather than the presence of ice-enhanced hot spots on the surface. Scarps within the smooth terrain deposits in central Imhotep also exhibit a peculiar behavior, where three scarps originate from the same location but at different times and migrate in opposite directions. This behavior indicates that the landscape retains a memory of previous cycles of erosion and deposition, reflected by the depth of the volatile-rich layer. Future work will need to couple our thermal model with a landscape evolution model in order to explain the complete dynamic evolution of these terrains
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
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