J. J. Lopez-Moreno

Vertical distribution of Titan's atmospheric neutral constituents

The vertical distribution of Titans neutral atmosphere compounds is calculated from a new photochemical model extending from 40 to 1432 km. This model makes use of many updated reaction rates, and of the new scheme for methane photolysis proposed by Mordaunt et al. [1993]. The model also includes a realistic treatment of the dissociation of N 2 , of the deposition of water in the atmosphere from meteoritic ablation, and of condensation processes. The sensitivity of the results to the eddy diffusion coefficient profile is investigated. Fitting the methane thermospheric profile and the stratospheric abundance of the major hydrocarbons requires a methane stratospheric mixing ratio of 1.5-2% rather than 3%. Fitting the HCN stratospheric profile requires an eddy diffusion coefficient at 100-300 km that is 5-20 times larger than that necessary for the hydrocarbons. Most species are reasonably well reproduced, with the exception of CH 3 C 2 H and HC 3 N. The formation of CH 3 CN may involve the reaction of CN with either CH 4 or (preferably) C 2 H 6 . The observed CO 2 profile can be modeled by assuming an external source of water of ∼6 × 10 6 cm -2 s -1 . For a nominal CO mixing ratio of 5 × 10 -5 , the chemical loss of CO exceeds its production by ∼15%, and equilibrium is achieved for CO = 1 × 10 -5 .

DENSITY AND CHARGE of PRISTINE FLUFFY PARTICLES FROM COMET 67P/CHURYUMOV-GERASIMENKO

The Grain Impact Analyzer and Dust Accumulator (GIADA) instrument on board ESA’s Rosetta mission is constraining the origin of the dust particles detected within the coma of comet 67 P/Churyumov–Gerasimenko (67P). The collected particles belong to two families: (i) compact particles (ranging in size from 0.03 to 1 mm), witnessing the presence of materials that underwent processing within the solar nebula and (ii) fluffy aggregates (ranging in size from 0.2 to 2.5 mm) of sub-micron grains that may be a record of a primitive component, probably linked to interstellar dust. The dynamics of the fluffy aggregates constrain their equivalent bulk density to <1 kg m-3. These aggregates are charged, fragmented, and decelerated by the spacecraft negative potential and enter GIADA in showers of fragments at speeds <1 m s-1. The density of such optically thick aggregates is consistent with the low bulk density of the nucleus. The mass contribution of the fluffy aggregates to the refractory component of the nucleus is negligible and their coma brightness contribution is less than 15%.

Altitude distribution of vibrationally excited states of atmospheric hydroxyl at levels v = 2 to v = 7

Abstract An instrument for the measurement of three different infrared regions of the OH Meinel system was launched in night conditions from El Arenosillo Sounding Rocket Range (Spain). The instrument covered transitions between vibrational states from v = 2to v = 7 in the first overtone sequence. The altitude profiles of the concentration of OH ∗ in the vibrational levels 2 to 7 are deduced. The individual profiles show that there is a dependence of the characteristics of the layers (altitude of the peak and half width) on the vibrational level. The lower levels extend to lower altitude than the higher ones. The obtained profiles are used, on the basis of a photochemical model, to investigate the values for the rate constants of the production and the chemical and collisional deactivation of atmospheric OH ∗ .

THE CHARACTERISATION OF TITAN'S ATMOSPHERIC PHYSICAL PROPERTIES BY THE HUYGENS ATMOSPHERIC STRUCTURE INSTRUMENT (HASI)

The Huygens Atmospheric Structure Instrument (HASI) is a multi-sensor package which has been designed to measure the physical quantities characterising the atmosphere of Titan during the Huygens probe descent on Titan and at the surface. HASI sensors are devoted to the study of Titans atmospheric structure and electric properties, and to provide information on its surface, whether solid or liquid.

Evolution of the Dust Size Distribution of Comet 67P/Churyumov–Gerasimenko from 2.2 au to Perihelion

The Rosetta probe, orbiting Jupiter-family comet 67P/Churyumov–Gerasimenko, has been detecting individual dust particles of mass larger than 10−10 kg by means of the GIADA dust collector and the OSIRIS Wide Angle Camera and Narrow Angle Camera since 2014 August and will continue until 2016 September. Detections of single dust particles allow us to estimate the anisotropic dust flux from 67P, infer the dust loss rate and size distribution at the surface of the sunlit nucleus, and see whether the dust size distribution of 67P evolves in time. The velocity of the Rosetta orbiter, relative to 67P, is much lower than the dust velocity measured by GIADA, thus dust counts when GIADA is nadir-pointing will directly provide the dust flux. In OSIRIS observations, the dust flux is derived from the measurement of the dust space density close to the spacecraft. Under the assumption of radial expansion of the dust, observations in the nadir direction provide the distance of the particles by measuring their trail length, with a parallax baseline determined by the motion of the spacecraft. The dust size distribution at sizes >1 mm observed by OSIRIS is consistent with a differential power index of −4, which was derived from models of 67Ps trail. At sizes <1 mm, the size distribution observed by GIADA shows a strong time evolution, with a differential power index drifting from −2 beyond 2 au to −3.7 at perihelion, in agreement with the evolution derived from coma and tail models based on ground-based data. The refractory-to-water mass ratio of the nucleus is close to six during the entire inbound orbit and at perihelion.

Neutral atmospheric composition between 60 and 220 km: A theoretical model for mid-latitudes

Abstract A new theoretical model for mid-latitude mesosphere and lower thermosphere at equinox conditions has been developed by including new values of various characteristic atmospheric parameters. A value of the eddy diffusion coefficient was deduced compatible with others reported in the literature and we discuss its effect upon the concentration of the different constituents. Photodissociation processes were handled with a detailed division of the different regions of the solar spectrum, mainly in the Schumann-Runge bands, allowing an exact computation of the diurnal O(1D) and O 2 ( 1 Δ g ) altitude profiles produced by solar radiation. The model includes CO, CO2, N2 and oxygen and hydrogen compounds. Our results compare well with recent experimental altitude profiles of minor compounds at mesospheric levels. The altitude profiles have been tested to reproduce various nightglow features, such as the Infrared Atmospheric System of O 2 ( 1 Δ g ) in 1.27 μm and the atomic oxygen green line. The resulting profiles are in line with results obtained using rocket borne instrumentation. In the thermosphere, the profiles agree with other more elaborate models restricted to this region and with mass spectrometer measurements. Diurnal variation of the concentrations is shown and explained in terms of photochemical and transport processes. Carbon compounds have been included to obtain a comprehensive model that can be used subsequently to obtain an interpretation of the effect of minor compounds on the population ofdifierent CO2 excited levels giving rise to infrared emission at 4.3 and 15 μm.

GIADA: shining a light on the monitoring of the comet dust production from the nucleus of 67P/Churyumov-Gerasimenko

During the period between 15 September 2014 and 4 February 2015, the Rosetta spacecraft accomplished the circular orbit phase around the nucleus of comet 67P/Churyumov-Gerasimenko (67P). The Grain Impact Analyzer and Dust Accumulator (GIADA) onboard Rosetta moni- tored the 67P coma dust environment for the entire period. Aims. We aim to describe the dust spatial distribution in the coma of comet 67P by means of in situ measurements. We determine dynamical and physical properties of cometary dust particles to support the study of the production process and dust environment modification. Methods. We analyzed GIADA data with respect to the observation geometry and heliocentric distance to describe the coma dust spatial distribu- tion of 67P, to monitor its activity, and to retrieve information on active areas present on its nucleus. We combined GIADA detection information with calibration activity to distinguish different types of particles that populate the coma of 67P: compact particles and fluffy porous aggregates. By means of particle dynamical parameters measured by GIADA, we studied the dust acceleration region. Results. GIADA was able to distinguish different types of particles populating the coma of 67P: compact particles and fluffy porous aggregates. Most of the compact particle detections occurred at latitudes and longitudes where the spacecraft was in view of the comet’s neck region of the nucleus, the so-called Hapi region. This resulted in an oscillation of the compact particle abundance with respect to the spacecraft position and a global increase as the comet moved from 3.36 to 2.43 AU heliocentric distance. The speed of these particles, having masses from 10−10 to 10−7 kg, ranged from 0.3 to 12.2 m s−1 . The variation of particle mass and speed distribution with respect to the distance from the nucleus gave indications of the dust acceleration region. The influence of solar radiation pressure on micron and submicron particles was studied. The integrated dust mass flux collected from the Sun direction, that is, particles reflected by solar radiation pressure, was three times higher than the flux coming directly from the comet nucleus. The awakening 67P comet shows a strong dust flux anisotropy, confirming what was suggested by on-ground dust coma observations performed in 2008.

A non-LTE radiative transfer model for infrared bands in the middle atmosphere. II. CO2 (2.7 and 4.3 μm) and water vapour (6.3 μm) bands and N2(1) and O2(1) vibrational levels

Abstract A non-equilibrium radiative transfer model has been applied to the 2.7 and isotopic 4.3 μm bands of CO 2 , together with that of H 2 O at 6.3 μm and the first excited vibrational levels of the nitrogen and oxygen molecules. The radiative transfer has been treated by a modified Curtis matrix method that allows inclusion of non-thermal sources, such as vibrational-vibrational energy exchange. The populations of the different levels have been obtained by solving a system of coupled equations, including those for the 15 μm bands. Results from the model are compared to previously reported results. A detailed study of where solar energy initially absorbed by CO 2 bands finally goes is also presented.

Chemistry of the galactic cosmic ray induced ionosphere of Titan

Titans lower ionosphere (from 1 to 400 km) has been studied with a one-dimensional ion-neutral model. In this region of the atmosphere, galactic cosmic rays (GCRs) are the main ionization source. They penetrate to the deeper atmosphere and ionize the neutral constituents of Titans atmosphere (mainly N2, CH4, Ar, H2, and CO) to produce N2+, N+, Ar+, CH4+, CH3+, CH2+, H2+, H+, and CO+. Fast reactions with the neutrals convert these ions into ions such as CH5+, C2H5+, and N2H+. Different pathways are proposed to obtain the ion and electron densities. The most abundant ions are cluster ions, like CH5+·CH4, HCO+·H2, and HCNH+·C2H4, and long chain hydrocarbon ions. In atmospheres very rich in N2, such as Titans, ions like H4C7N+ and CH3CNH+ also represent an important contribution to the total positive ion density. Three-body reactions may play an important role in the dense atmosphere of Titan, and special attention is devoted to them. The calculated electron density in the lower atmosphere reaches a peak of ≈ 2150 cm−3 at an altitude of 90 km.

Variability of the neutral mesospheric and lower thermospheric composition in the diurnal cycle

Abstract A non-steady one-dimensional model atmosphere, including N2, oxygen, hydrogen and carbon compounds, suitable for the interpretation of the processes occurring during twilight, has been developed for mid-latitude and equinox conditions at altitudes from 60 to 220 km. The temporal dependence of the photodissociation coefficients has been calculated in detail, paying special attention to twilight conditions. Photochemical processes for the different compounds are considered using the latest reaction rate coefficients. Vertical transport effects are also included, considering molecular diffusion and a semiempirical eddy diffusion coefficient profile. The Variability of the different compounds throughout a complete day is obtained and compared with other theoretical models and experimental measurements. Special emphasis is paid to the study of the rapid changes occurring during morning and evening twilight times in the concentration of various minor compounds. The results obtained have been applied to the study of the diurnal evolution of the O 2 ( 1 Δ g ) and OH emissions derived from appropriate photochemical schemes. A reasonable agreement is achieved between different measurements and our predictions and this may be taken to validate the model.