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Artigo em Inglês | MEDLINE | ID: mdl-31783433


RATIONALE: Quadrupole mass spectrometers equipped with an Electron Ionization (EI) sources have been widely used in space exploration to investigate the composition of planetary surfaces and atmospheres. However, the complexity of the samples and the minimal calibration for the fragmentation of molecules in the ionization chambers have prevented the deconvolution of the majority of the mass spectra obtained at different targets, thus limiting determination of the exact composition of the samples analyzed. We propose a Monte-Carlo approach to solve this issue mathematically. METHODS: We have decomposed simulated mass spectra of mixtures acquired with unit resolving power mass spectrometers and EI sources into the sum of the single components fragmentation patterns weighted by their relative concentration using interior-point least-square fitting. To fit compounds with poorly known fragmentation patterns, we use a Monte-Carlo method to vary the intensity of individual fragment ions. We then decompose the spectrum thousands of times to obtain a statistical distribution. RESULTS: By performing the deconvolution on a mixture of seven different molecules with interfering fragmentation patterns (H2 O, O2 , CH4 , Ar, N2 , C2 H4 and C2 H6 ) we show that this approach retrieves the mixing ratio of the individual components more accurately than regular mass spectra decomposition methods that rely on fragmentation patterns from general databases. It also provides the probability density function for each species's mixing ratio. CONCLUSIONS: By removing the solution degeneracy in the decomposition of mass spectra, the method described herein could significantly increase the scientific retrieval from archived space flight mass spectrometry data, where calibration of the ionization source is no longer an option.

J Geophys Res Planets ; 121(9): 1814-1826, 2016 Sep.
Artigo em Inglês | MEDLINE | ID: mdl-29629249


We use observations from the Imaging Science Subsystem on Cassini to create maps of Saturn's Northern Hemisphere (NH) from 2008 to 2015, a time period including a seasonal transition (i.e., Spring Equinox in 2009) and the 2010 giant storm. The processed maps are used to investigate vortices in the NH during the period of 2008-2015. All recorded vortices have diameters (east-west) smaller than 6000 km except for the largest vortex that developed from the 2010 giant storm. The largest vortex decreased its diameter from ~11000 km in 2011 to ~5000 km in 2015, and its average diameter is ~6500 km during the period of 2011-2015. The largest vortex lasts at least 4 years, which is much longer than the lifetimes of most vortices (less than 1 year). The largest vortex drifts to north, which can be explained by the beta drift effect. The number of vortices displays varying behaviors in the meridional direction, in which the 2010 giant storm significantly affects the generation and development of vortices in the middle latitudes (25-45°N). In the higher latitudes (45-90°N), the number of vortices also displays strong temporal variations. The solar flux and the internal heat do not directly contribute to the vortex activities, leaving the temporal variations of vortices in the higher latitudes (45-90°N) unexplained.