Prospects for the habitability of OGLE-2006-BLG-109L

The extrasolar system OGLE-2006-BLG-109L is the first multiple-planet system to be discovered by gravitational microlensing (Gaudi et al., 2008); the two large planets that have been detected have mass ratios, semimajor axis ratios, and equilibrium temperatures that are similar to those of Jupiter and Saturn; the mass of the host star is only 0.5 M_sun, and the system is more compact than our own Solar system. We find that in the habitable zone of the host star, the two detected planets resonantly excite large orbital eccentricities on a putative earth-mass planet, driving such a planet out of the habitable zone. We show that an additional inner planet of ~>0.3M_earth at <~0.1 AU would suppress the eccentricity perturbation and greatly improve the prospects for habitability of the system. Thus, the planetary architecture of a potentially habitable OGLE-2006-BLG-109L planetary system -- with two ``terrestrial'' planets and two jovian planets -- could bear very close resemblance to our own Solar system.Comment: 11 pages including 4 figures; accepted for publication in ApJ-Letter

Degradation of small simple and large complex lunar craters: Not a simple scale dependence

The crater record of a planetary surface unit is often analyzed by its cumulative size‐frequency distribution (CSFD). Measuring CSFDs involves traditional approaches, such as traditional crater counting (TCC) and buffered crater counting (BCC), as well as geometric corrections, such as nonsparseness correction (NSC) and buffered nonsparseness correction (BNSC). NSC and BNSC consider the effects of geometric crater obliteration on the CSFD. On the Moon, crater obliteration leads to two distinct states in which obtained CSFDs do not match the production CSFD—crater equilibrium and nonsparseness. Crater equilibrium occurs when each new impact erases a preexisting crater of the same size. It is clearly observed on lunar terrains dominated by small simple craters with steep‐sloped production CSFDs, such as Imbrian to Eratosthenian‐era mare units. Nonsparseness, on the other hand, is caused by the geometric overlap of preexisting craters by a new impact, which is also known as “cookie cutting.” Cookie cutting is most clearly observed on lunar terrains dominated by large craters with shallow‐sloped production CSFDs, such as the pre‐Nectarian lunar highlands. We use the Cratered Terrain Evolution Model (CTEM) to simulate the evolution of a pre‐Nectarian surface unit. The model was previously used to simulate the diffusion‐induced equilibrium for small craters of the lunar maria. We find that relative to their size, large craters contribute less to the diffusion of the surrounding landscape than small craters. Thus, a simple scale dependence cannot account for the per‐crater contribution to degradation by small simple and large complex craters

An investigation of the degradation of Fluorinated Ethylene Propylene (FEP) copolymer thermal blanketing materials aboard LDEF in the laboratory

Samples of fluorinated ethylene propylene copolymer thermal blanketing material, recovered from the Long Duration Exposure Facility (LDEF), were investigated to determine the nature and the extent of degradation due to exposure to the low-Earth-orbit environment. Samples recovered from the ram-facing direction of LDEF, which received vacuum-ultraviolet (VUV) radiation and atomic-oxygen impingement, and samples from the trailing edge, which received almost exclusively VUV exposure, were investigated by scanning electron microscopy and atomic force microscopy. The most significant result of this investigation was found on samples that received only VUV exposure. These samples possessed a hard, embrittled surface layer that was absent from the atomic-oxygen exposed sample and from unexposed control samples. This surface layer is believed to be responsible for the 'synergistic' effect between VUV and atomic oxygen. Overall, the investigation revealed dramatically different morphologies for the two samples. The sample receiving both atomic-oxygen and VUV exposure was deeply eroded and had a characteristic 'rolling' surface morphology, while the sample that received only VUV exposure showed mild erosion and a surface morphology characterized by sharp high-frequency peaks. The morphologies observed in the LDEF samples, including the embrittled surface layer, were successfully duplicated in the laboratory

Sesquinary Catastrophe For Close-In Moons with Dynamically Excited Orbits

We identify a new mechanism that can lead to the destruction of small, close-in planetary satellites. If a small moon close to the planet has a sizable eccentricity and inclination, its ejecta that escape to planetocentric orbit would often re-impact with much higher velocity due to the satellite's and the fragment's orbits precessing out of alignment. If the impacts of returning ejecta result in net erosion, a runaway process can occur which may end in disruption of the satellite, and we term this process ``sesquinary catastrophe''. We expect the moon to re-accrete, but on an orbit with significantly lower eccentricity and inclination. We find that the large majority of small close-in moons in the Solar System, have orbits that are immune to sesquinary catastrophe. The exceptions include a number of resonant moonlets of Saturn for which resonances may affect the velocities of re-impact of their own debris. Additionally, we find that Neptune's moon Naiad (and to a lesser degree, Jupiter's Thebe) must have substantial internal strength, in line with prior estimates based on Roche limit stability. We also find that sesquinary instability puts important constraints on the plausible past orbits of Phobos and Deimos or their progenitors.Comment: Accepted for Ap

Secular resonance sweeping of the main asteroid belt during planet migration

We calculate the eccentricity excitation of asteroids produced by the sweeping $\nu_6$ secular resonance during the epoch of planetesimal-driven giant planet migration in the early history of the solar system. We derive analytical expressions for the magnitude of the eccentricity change and its dependence on the sweep rate and on planetary parameters; the $\nu_6$ sweeping leads to either an increase or a decrease of eccentricity depending on an asteroid's initial orbit. Based on the slowest rate of $\nu_6$ sweeping that allows a remnant asteroid belt to survive, we derive a lower limit on Saturn's migration speed of \sim0.15\AU\My^{-1} during the era that the $\nu_6$ resonance swept through the inner asteroid belt (semimajor axis range 2.1--2.8\AU). This rate limit is for Saturn's current eccentricity, and scales with the square of Saturn's eccentricity; the limit on Saturn's migration rate could be lower if Saturn's eccentricity were lower during its migration. Applied to an ensemble of fictitious asteroids, our calculations show that a prior single-peaked distribution of asteroid eccentricities would be transformed into a double-peaked distribution due to the sweeping of the $\nu_6$. Examination of the orbital data of main belt asteroids reveals that the proper eccentricities of the known bright ($H \leq10.8$) asteroids may be consistent with a double-peaked distribution. If so, our theoretical analysis then yields two possible solutions for the migration rate of Saturn and for the dynamical states of the pre-migration asteroid belt: a dynamically cold state (single-peaked eccentricity distribution with mean of $\sim0.05$) linked with Saturn's migration speed \sim 4\AU\My^{-1}, or a dynamically hot state (single-peaked eccentricity distribution with mean of $\sim0.3$) linked with Saturn's migration speed \sim 0.8\AU\My^{-1}.Comment: 32 pages, 7 figures. Accepted for publication in ApJ on Mar. 1, 201

Relationship between stream velocity & depth and snail size distribution & density of the balcones elimia, elimia comalensis (pilsbry, 1890) (gastropoda: pleuroceridae) in comal springs, texas

Pleurocerid snails are important components of aquatic ecosystems and the majority of species are threatened or endangered. This study describes aspects of the life-history of Elimia comalensis, specifically population density in relationship to water velocity and depth, and seasonal change. Also examined are spatial segregation of different E. comalensis age groups and the relationship of snail size (proxy for age) to flow, depth, and seasonal change. The study was carried out in the lotic portion of spring run 3, Comal Springs, New Braunfels, Texas, by quadrat sampling at 10 m intervals from the spring head to Landa Lake in Fall, Winter, and Spring 2005-2006. The length of snails was strongly influenced by physical characteristics of the stream with stream depth, water velocity, and distance from the spring head accounting for -43% of the individual variability for shell length (