273 research outputs found
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 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
Seasonal Variations in Titan's Stratosphere Observed with Cassini/CIRS: Temperature, Trace Molecular Gas and Aerosol Mixing Ratio Profiles
Titan's northern spring equinox occurred in August 2009. General Circulation Models (e.g. Lebonnois et al., 2012) predict strong modifications of the global circulation in this period, with formation of two circulation cells instead of the pole-to-pole cell that occurred during northern winter. This winter single cell, which had its descending branch at the north pole, was at the origin of the enrichment of molecular abundances and high stratopause temperatures observed by Cassini/CIRS at high northern latitudes (e.g. Achterberg et al., 2011, Coustenis et al., 2010, Teanby et al., 2008, Vinatier et al., 2010). The predicted dynamical seasonal variations after the equinox have strong impact on the spatial distributions of trace gas, temperature and aerosol abundances. We will present here an analysis of CIRS limb-geometry datasets acquired in 2010 and 2011 that we used to monitor the seasonal evolution of the vertical profiles of temperature, molecular (C2H2, C2H6, HCN, ..) and aerosol abundances
Abundance Measurements of Titan's Stratospheric HCN, HCN, CH, and CHCN from ALMA Observations
Previous investigations have employed more than 100 close observations of
Titan by the Cassini orbiter to elucidate connections between the production
and distribution of Titan's vast, organic-rich chemical inventory and its
atmospheric dynamics. However, as Titan transitions into northern summer, the
lack of incoming data from the Cassini orbiter presents a potential barrier to
the continued study of seasonal changes in Titan's atmosphere. In our previous
work (Thelen et al., 2018), we demonstrated that the Atacama Large
Millimeter/submillimeter Array (ALMA) is well suited for measurements of
Titan's atmosphere in the stratosphere and lower mesosphere (~100-500 km)
through the use of spatially resolved (beam sizes <1'') flux calibration
observations of Titan. Here, we derive vertical abundance profiles of four of
Titan's trace atmospheric species from the same 3 independent spatial regions
across Titan's disk during the same epoch (2012 to 2015): HCN, HCN,
CH, and CHCN. We find that Titan's minor constituents exhibit large
latitudinal variations, with enhanced abundances at high latitudes compared to
equatorial measurements; this includes CHCN, which eluded previous
detection by Cassini in the stratosphere, and thus spatially resolved abundance
measurements were unattainable. Even over the short 3-year period, vertical
profiles and integrated emission maps of these molecules allow us to observe
temporal changes in Titan's atmospheric circulation during northern spring. Our
derived abundance profiles are comparable to contemporary measurements from
Cassini infrared observations, and we find additional evidence for subsidence
of enriched air onto Titan's south pole during this time period. Continued
observations of Titan with ALMA beyond the summer solstice will enable further
study of how Titan's atmospheric composition and dynamics respond to seasonal
changes.Comment: 15 pages, 16 figures, 2 tables. Accepted for publication in Icarus,
September 201
Near-Field Seismic Propagation and Coupling Through Marsâ Regolith:Implications for the InSight Mission
NASAâs InSight Mission will deploy two three-component seismometers on Mars in 2018. These short period and very broadband seismometers will be mounted on a three-legged levelling system, which will sit directly on the sandy regolith some 2â3 meters from the lander. Although the deployment will be covered by a wind and thermal shield, atmospheric noise is still expected to couple to the seismometers through the regolith. Seismic activity on Mars is expected to be significantly lower than on Earth, so a characterisation of the extent of coupling to noise and seismic signals is an important step towards maximising scientific return.
In this study, we conduct field testing on a simplified model of the seismometer assembly. We constrain the transfer function between the wind and thermal shield and tripod-mounted seismometers over a range of frequencies (1â40 Hz) relevant to the deployment on Mars. At 1â20 Hz the displacement amplitude ratio is approximately constant, with a value that depends on the site (0.03â0.06). The value of the ratio in this range is 25â50% of the value expected from the deformation of a homogeneous isotropic elastic halfspace. At 20â40 Hz, the ratio increases as a result of resonance between the tripod mass and regolith. We predict that mounting the InSight instruments on a tripod will not adversely affect the recorded amplitudes of vertical seismic energy, although particle motions will be more complex than observed in recordings generated by more conventional buried deployments. Higher frequency signals will be amplified by tripod-regolith resonance, probably reaching peak-amplification at âŒ50
Hz. The tripod deployment will lose sensitivity at frequencies >50 Hz as a result of the tripod mass and compliant regolith.
We also investigate the attenuation of seismic energy within the shallow regolith covering the range of seismometer deployment distances
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