Bubble streams in Titan's seas as product of liquid N2-CH4-C2H6 cryogenic mixture [EPA]


Titan, Saturn’s largest moon, is the only extraterrestrial body known to support stable liquid on its surface, in the form of seas and lakes that dot the polar regions. Many indications suggest that the liquid should be composed of a mixture of N2, CH4 and C2H6. Recent RADAR observations of Titan’s large sea, called “Ligeia Mare”, have shown unexplained and ephemeral bright features, possibly due to rising bubbles. Here we report that our numerical model, when combined with experimental data found in the literature, shows that Ligeia Mare’s bed is a favourable place for nitrogen exsolution. This process could produce centimeter-sized and radar-detectable bubbles.

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D. Cordier, F. Garcia-Sanchez, D. Justo-Garcia, et. al.
Tue, 25 Apr 17

Comments: Accepted in Nature Astronomy

Efficiency of thermal relaxation by radiative processes in protoplanetary discs: constraints on hydrodynamic turbulence [EPA]


Hydrodynamic, non-magnetic instabilities can provide turbulent stress in the regions of protoplanetary discs, where the MRI can not develop. The induced motions influence the grain growth, from which formation of planetesimals begins. Thermal relaxation of the gas constrains origins of the identified hydrodynamic sources of turbulence in discs.
We estimate the radiative relaxation timescale of temperature perturbations and study the dependence of this timescale on the perturbation wavelength, the location within the disc, the disc mass, and the dust-to-gas mass ratio. We then apply thermal relaxation criteria to localise modes of the convective overstability, the vertical shear instability, and the zombie vortex instability.
Our calculations employed the latest tabulated dust and gas mean opacities and we account for the collisional coupling to the emitting species.
The relaxation criterion defines the bulk of a typical T Tauri disc as unstable to the development of linear hydrodynamic instabilities. The midplane is unstable to the convective overstability from at most $2\mbox{ au}$ and up to $40\mbox{ au}$, as well as beyond $140\mbox{ au}$. The vertical shear instability can develop between $15\mbox{ au}$ and $180\mbox{ au}$. The successive generation of (zombie) vortices from a seeded noise can work within the inner $0{.}8\mbox{ au}$.
Dynamic disc modelling with the evolution of dust and gas opacities is required to clearly localise the hydrodynamic turbulence, and especially its non-linear phase.

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M. Malygin, H. Klahr, D. Semenov, et. al.
Tue, 25 Apr 17

Comments: 13 pages, 8 figures

Cataclysm no more: New views on the timing and delivery of lunar impactors [EPA]


If properly interpreted, the impact record of the Moon, Earth’s nearest neighbour, can be used to gain insights into how the Earth has been influenced by impacting events since its formation ~4.5 billion years (Ga) ago. However, the nature and timing of the lunar impactors – and indeed the lunar impact record itself – are not well understood. Of particular interest are the ages of lunar impact basins and what they tell us about the proposed “lunar cataclysm” and/or the late heavy bombardment (LHB), and how this impact episode may have affected early life on Earth or other planets. Investigations of the lunar impactor population over time have been undertaken and include analyses of orbital data and images; lunar, terrestrial, and other planetary sample data; and dynamical modelling. Here, the existing information regarding the nature of the lunar impact record is reviewed and new interpretations are presented. Importantly, it is demonstrated that most evidence supports a prolonged lunar (and thus, terrestrial) bombardment from ~4.2 to 3.4 Ga and not a cataclysmic spike at ~3.9 Ga. Implications for the conditions required for the origin of life are addressed.

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N. Zellner
Tue, 25 Apr 17

Comments: 32 pages, 3 figures, 1 table

Prior Indigenous Technological Species [EPA]


One of the primary open questions of astrobiology is whether there is extant or extinct life elsewhere the Solar System. Implicit in much of this work is that we are looking for microbial or, at best, unintelligent life, even though technological artifacts might be much easier to find. SETI work on searches for alien artifacts in the Solar System typically presumes that such artifacts would be of extrasolar origin, even though life is known to have existed in the Solar System, on Earth, for eons. But if a prior technological, perhaps spacefaring, species ever arose in the Solar System, it might have produced artifacts or other technosignatures that have survived to present day, meaning Solar System artifact SETI provides a potential path to resolving astrobiology’s question. Here, I discuss the origins and possible locations for technosignatures of such a $prior$ $indigenous$ $technological$ $species$, which might have arisen on ancient Earth or another body, such as a pre-greenhouse Venus or a wet Mars. In the case of Venus, the arrival of its global greenhouse and potential resurfacing might have erased all evidence of its existence on the Venusian surface. In the case of Earth, erosion and, ultimately, plate tectonics may have erased most such evidence if the species lived Gyr ago. Remaining indigenous technosignatures might be expected to be extremely old, limiting the places they might still be found to beneath the surfaces of Mars and the Moon, or in the outer Solar System.

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J. Wright
Tue, 25 Apr 17

Comments: 11pp, no figures. Accepted for publication in the International Journal of Astrobiology. Manuscript refused by Astriobology without review as “outside of the purview” the journal

Constraints on the Magnetic Field Strength of HAT-P-7 b and other Hot Giant Exoplanets [EPA]


Observations of infrared and optical light curves of hot Jupiters have demonstrated that the peak brightness is generally offset eastward from the substellar point [1,2]. This observation is consistent with hydrodynamic numerical simulations that produce fast, eastward directed winds which advect the hottest point in the atmosphere eastward of the substellar point [3,4]. However, recent continuous Kepler measurements of HAT-P-7 b show that its peak brightness offset varies significantly in time, with excursions such that the brightest point is sometimes westward of the substellar point [5]. These variations in brightness offset require wind variability, with or without the presence of clouds. While such wind variability has not been seen in hydrodynamic simulations of hot Jupiter atmospheres, it has been seen in magnetohydrodynamic (MHD) simulations [6]. Here we show that MHD simulations of HAT-P-7 b indeed display variable winds and corresponding variability in the position of the hottest point in the atmosphere. Assuming the observed variability in HAT-P-7 b is due to magnetism we constrain its minimum magnetic field strength to be 6\,G. Similar observations of wind variability on hot giant exoplanets, or lack thereof, could help constrain their magnetic field strengths. Since dynamo simulations of these planets do not exist and theoretical scaling relations [7] may not apply, such observational constraints could prove immensely useful.

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T. Rogers
Mon, 24 Apr 17

Comments: 8 pages, 3 figures, Accepted at Nature Astronomy

Spectra and physical properties of Taurid meteoroids [EPA]


Taurids are an extensive stream of particles produced by comet 2P/Encke, which can be observed mainly in October and November as a series of meteor showers rich in bright fireballs. Several near-Earth asteroids have also been linked with the meteoroid complex, and recently the orbits of two carbonaceous meteorites were proposed to be related to the stream, raising interesting questions about the origin of the complex and the composition of 2P/Encke. Our aim is to investigate the nature and diversity of Taurid meteoroids by studying their spectral, orbital, and physical properties determined from video meteor observations. Here we analyze 33 Taurid meteor spectra captured during the predicted outburst in November 2015 by stations in Slovakia and Chile, including 14 multi-station observations for which the orbital elements, material strength parameters, dynamic pressures, and mineralogical densities were determined. It was found that while orbits of the 2015 Taurids show similarities with several associated asteroids, the obtained spectral and physical characteristics point towards cometary origin with highly heterogeneous content. Observed spectra exhibited large dispersion of iron content and significant Na intensity in all cases. The determined material strengths are typically cometary in the $K_B$ classification, while $P_E$ criterion is on average close to values characteristic for carbonaceous bodies. The studied meteoroids were found to break up under low dynamic pressures of 0.02 – 0.10 MPa, and were characterized by low mineralogical densities of 1.3 – 2.5 g cm$^{-3}$. The widest spectral classification of Taurid meteors to date is presented.

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P. Matlovic, J. Toth, R. Rudawska, et. al.
Mon, 24 Apr 17

Comments: N/A

An Optical/near-infrared investigation of HD 100546 b with the Gemini Planet Imager and MagAO [EPA]


We present H band spectroscopic and Halpha photometric observations of HD 100546 obtained with GPI and MagAO. We detect H band emission at the location of the protoplanet HD 100546b, but show that choice of data processing parameters strongly affects the morphology of this source. It appears point-like in some aggressive reductions, but rejoins an extended disk structure in the majority of the others. Furthermore, we demonstrate that this emission appears stationary on a timescale of 4.6 yrs, inconsistent at the 2sigma level with a Keplerian clockwise orbit at 59 au in the disk plane. The H band spectrum of the emission is inconsistent with any type of low effective temperature object or accreting protoplanetary disk. It strongly suggests a scattered light origin, as it is consistent with the spectrum of the star and the spectra extracted at other locations in the disk. A non detection at the 5sigma level of HD 100546b in differential Halpha imaging places an upper limit, assuming the protoplanet lies in a gap free of extinction, on the accretion luminosity and accretion rate of 1.7E-4 Lsun and MMdot<6.4E-7Mjup^2/yr for 1Rjup. These limits are comparable to the accretion luminosity and rate of TTauri-stars or LkCa 15b. Taken together, these lines of evidence suggest that the H band source at the location of HD 100546b is not emitted by a planetary photosphere or an accreting circumplanetary disk but is a disk feature enhanced by the PSF subtraction process. This non-detection is consistent with the non-detection in the K band reported in an earlier study but does not exclude the possibility that HD 100546b is deeply embedded.

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J. Rameau, K. Follette, L. Pueyo, et. al.
Mon, 24 Apr 17

Comments: 10 pages, 5 figures, 1 table