The effects of aliasing and lock-in processes on palaeosecular variation records from sediments

Studies of sedimentary records of palaeointensity variation report periods as long as 50 kyr. Archaeointensity data show geomagnetic periods of 2 kyr with large ampli-tudes. Sampling of the sedimentary records can be as coarse as 8 kyr, so the apparent long periods could be caused by aliasing. The sedimentary lock-in process could smooth the record and remove short periods, thereby preventing aliasing from occurring. We examine possible effects of aliasing by creating a 100-kyr-long synthetic sequence of palaeointensity variation with a similar spectrum to that of archaeomagnetic data from the last 12 kyr and resampling at longer intervals. With no lock-in smoothing,aliasing produces spurious energy in the spectra at long periods. When smoothing by the sedimentation process is applied, the amplitudes of the aliased peaks are reduced but still cause significant, spurious, long-period energy in the spectra for some sedi- mentation rates. We restrict our analysis to palaeointensity data but similar problems may also exist for coarsely sampled directional data. To avoid aliasing we recommend a maximum sampling interval of 2 kyr

Upper limits for PH3 and H2S in Titan's Atmosphere from Cassini CIRS

We have searched for the presence of simple P and S-bearing molecules in Titan's atmosphere, by looking for the characteristic signatures of phosphine and hydrogen sulfide in infrared spectra obtained by Cassini CIRS. As a result we have placed the first upper limits on the stratospheric abundances, which are 1 ppb (PH3) and 330 ppb (H2S), at the 2-sigma significance level.Comment: 12 pages text, 1 table, 2 figure

Constraints on Jupiter’s Stratospheric HCl abundance and chlorine cycle from Herschel/HIFI

Detection of HCl on Jupiter would provide insight into the chlorine cycle and external elemental fluxes on giant planets, yet so far has not been possible. Here we present the most sensitive search for Jupiter׳s stratospheric HCl to date using observations of the 625.907 and 1876.221 GHz spectral lines with Herschel׳s HIFI instrument. HCl was not detected, but we determined the most stringent upper limits so far, improving on previous studies by two orders of magnitude. If HCl is assumed to be uniformly mixed, with a constant volume mixing ratio above the 1 mbar pressure level and has zero abundance below, we obtain a 3-σ upper limit of 0.061 ppb; in contrast, if we assume uniform mixing above the 1 mbar level and allow a non-zero but downward-decreasing abundance from 1 mbar to the troposphere based on eddy diffusion, we obtain a 3-σ upper limit of 0.027 ppb. This is below the abundance expected for a solar composition cometary source and implies that upper stratospheric HCl loss processes are required for consistency with observations of the external oxygen flux. We investigated loss via aerosol scavenging using a simple diffusion model and conclude that it could be a very effective mechanism for HCl removal. Transient scavenging by stratospheric NH3 from impacts is another potentially important loss mechanism. This suggests that it is extremely unlikely that HCl is present in sufficient quantities to be detectable in the near future. An alternative explanation for our very low upper limits could be that HCl is sub-solar in comets or that cometary chlorine exists in inactive reservoirs that are not readily converted to HCl during the impact process

Hiding in the Shadows: Searching for Planets in Pre--transitional and Transitional Disks

Transitional and pre--transitional disks can be explained by a number of mechanisms. This work aims to find a single observationally detectable marker that would imply a planetary origin for the gap and, therefore, indirectly indicate the presence of a young planet. N-body simulations were conducted to investigate the effect of an embedded planet of one Jupiter mass on the production of instantaneous collisional dust derived from a background planetesimal disk. Our new model allows us to predict the dust distribution and resulting observable markers with greater accuracy than previous work. Dynamical influences from a planet on a circular orbit are shown to enhance dust production in the disk interior and exterior to the planet orbit while removing planetesimals from the the orbit itself creating a clearly defined gap. In the case of an eccentric planet the gap opened by the planet is not as clear as the circular case but there is a detectable asymmetry in the dust disk.Comment: Accepted to ApJL 25th September 2013. 4 figures, 1 tabl

Upper limits for undetected trace species in the stratosphere of Titan

In this paper we describe a first quantitative search for several molecules in Titan's stratosphere in Cassini CIRS infrared spectra. These are: ammonia (NH3), methanol (CH3OH), formaldehyde (H2CO), and acetonitrile (CH3CN), all of which are predicted by photochemical models but only the last of which observed, and not in the infrared. We find non-detections in all cases, but derive upper limits on the abundances from low-noise observations at 25{\deg}S and 75{\deg}N. Comparing these constraints to model predictions, we conclude that CIRS is highly unlikely to see NH3 or CH3OH emissions. However, CH3CN and H2CO are closer to CIRS detectability, and we suggest ways in which the sensitivity threshold may be lowered towards this goal.Comment: 11 pages plus 6 figure file

Titan's Prolific Propane: The Cassini CIRS Perspective

In this paper we select large spectral averages of data from the Cassini Composite Infrared Spectrometer (CIRS) obtained in limb-viewing mode at low latitudes (30S--30N), greatly increasing the path length and hence signal-to-noise ratio for optically thin trace species such as propane. By modeling and subtracting the emissions of other gas species, we demonstrate that at least six infrared bands of propane are detected by CIRS, including two not previously identified in Titan spectra. Using a new line list for the range 1300-1400cm -1, along with an existing GEISA list, we retrieve propane abundances from two bands at 748 and 1376 cm-1. At 748 cm-1 we retrieve 4.2 +/- 0.5 x 10(-7) (1-sigma error) at 2 mbar, in good agreement with previous studies, although lack of hotbands in the present spectral atlas remains a problem. We also determine 5.7 +/- 0.8 x 10(-7) at 2 mbar from the 1376 cm-1 band - a value that is probably affected by systematic errors including continuum gradients due to haze and also an imperfect model of the n6 band of ethane. This study clearly shows for the first time the ubiquity of propane's emission bands across the thermal infrared spectrum of Titan, and points to an urgent need for further laboratory spectroscopy work, both to provide the line positions and intensities needed to model these bands, and also to further characterize haze spectral opacity. The present lack of accurate modeling capability for propane is an impediment not only for the measurement of propane itself, but also for the search for the emissions of new molecules in many spectral regions.Comment: 7 Figures, 3 Tables. Typeset in Latex with elsart.cls. In press for Planetary and Space Scienc

Uranus’ stratospheric HCl upper limit from Herschel/SPIRE

Herschel/SPIRE observations of Uranus are used to search for stratospheric hydrogen chloride (HCl) emission at 41.74 cm$^{-1}$. HCl was not detected and instead 3$\sigma$ upper limits were determined; $<$6.2 ppb ($<$2.0$\times$10$^{14}$ molecules/cm$^{2}$) for a 0.1 mbar step profile and $<$0.40 ppb ($<$1.2$\times$10$^{14}$ molecules/cm$^{2}$) for a 1 mbar step profile. HCl is expected to have an external source and these upper limits are consistent with abundances of other external species (CO, H$_2$O, CO$_2$) and a solar composition source.Comment: 4 pages, 1 figure, accepted for publication in RNAAS 202

Hiding in the Shadows II: Collisional Dust as Exoplanet Markers

Observations of the youngest planets ($\sim$1-10 Myr for a transitional disk) will increase the accuracy of our planet formation models. Unfortunately, observations of such planets are challenging and time-consuming to undertake even in ideal circumstances. Therefore, we propose the determination of a set of markers that can pre-select promising exoplanet-hosting candidate disks. To this end, N-body simulations were conducted to investigate the effect of an embedded Jupiter mass planet on the dynamics of the surrounding planetesimal disk and the resulting creation of second generation collisional dust. We use a new collision model that allows fragmentation and erosion of planetesimals, and dust-sized fragments are simulated in a post process step including non-gravitational forces due to stellar radiation and a gaseous protoplanetary disk. Synthetic images from our numerical simulations show a bright double ring at 850 $\mu$m for a low eccentricity planet, whereas a high eccentricity planet would produce a characteristic inner ring with asymmetries in the disk. In the presence of first generation primordial dust these markers would be difficult to detect far from the orbit of the embedded planet, but would be detectable inside a gap of planetary origin in a transitional disk.Comment: Accepted for publication in Ap