Gaston Kockarts

The study of the martian atmosphere from top to bottom with SPICAM light on mars express

Abstract SPICAM Light is a small UV-IR instrument selected for Mars Express to recover most of the science that was lost with the demise of Mars 96, where the SPICAM set of sensors was dedicated to the study of the atmosphere of Mars (Spectroscopy for the investigation of the characteristics of the atmosphere of mars). The new configuration of SPICAM Light includes optical sensors and an electronics block. A UV spectrometer (118–320 nm, resolution 0.8 nm) is dedicated to Nadir viewing, limb viewing and vertical profiling by stellar occultation (3.8 kg). It addresses key issues about ozone, its coupling with H2O, aerosols, atmospheric vertical temperature structure and ionospheric studies. An IR spectrometer (1.2– 4.8 μm , resolution 0.4–1 nm) is dedicated to vertical profiling during solar occultation of H2O, CO2, CO, aerosols and exploration of carbon compounds (3.5 kg). A nadir looking sensor for H2O abundances (1.0– 1.7 μm , resolution 0.8 nm) is recently included in the package (0.8 kg). A simple data processing unit (DPU, 0.9 kg) provides the interface of these sensors with the spacecraft. In nadir orientation, SPICAM UV is essentially an ozone detector, measuring the strongest O3 absorption band at 250 nm in the spectrum of the solar light scattered back from the ground. In the stellar occultation mode the UV Sensor will measure the vertical profiles of CO2, temperature, O3, clouds and aerosols. The density/temperature profiles obtained with SPICAM Light will constrain and aid in the development of the meteorological and dynamical atmospheric models, from the surface to 160 km in the atmosphere. This is essential for future missions that will rely on aerocapture and aerobraking. UV observations of the upper atmosphere will allow study of the ionosphere through the emissions of CO, CO+, and CO2+, and its direct interaction with the solar wind. Also, it will allow a better understanding of escape mechanisms and estimates of their magnitude, crucial for insight into the long-term evolution of the atmosphere. The SPICAM Light IR sensor is inherited from the IR solar part of the SPICAM solar occultation instrument of Mars 96. Its main scientific objective is the global mapping of the vertical structure of H2O, CO2, CO, HDO, aerosols, atmospheric density, and temperature by the solar occultation. The wide spectral range of the IR spectrometer and its high spectral resolution allow an exploratory investigation addressing fundamental question of the possible presence of carbon compounds in the Martian atmosphere. Because of severe mass constraints this channel is still optional. An additional nadir near IR channel that employs a pioneering technology acousto-optical tuneable filter (AOTF) is dedicated to the measurement of water vapour column abundance in the IR simultaneously with ozone measured in the UV. It will be done at much lower telemetry budget compared to the other instrument of the mission, planetary fourier spectrometer (PFS).

Simple parameterization of the absorption of the solar Lyman‐alpha line

The absorption of the solar Lyman-alpha line by the terrestrial atmosphere is calculated, taking in account the wavelength variations of the emission line and of the O2 cross-section, as well as the temperature dependence of the cross-section. A new parameterization is developed to reproduce in atmospheric models the results of this high-resolution calculation, up to an attenuation of 1010 for the incident solar radiation. The error made in most of existing models when computing the Lyman-alpha contribution to photo dissociation rates in the middle atmosphere, using a constant O2 cross-section of 10-20 cm2, is shown to be important and this can affect the loss rate of mesospheric constituents such as H2O or CH4. Copyright 1997 by the American Geophysical Union.

Observations of atomic deuterium in the mesosphere from ATLAS 1 with ALAE Instrument

During the first ATLAS mission, the ALAE Lyman α spectrophotometer collected various measurements of hydrogen and deuterium atoms, from the mesosphere, the thermosphere, the exosphere and the interplanetary medium. In this paper is presented a preliminary analysis of some observations of atomic deuterium, which Lyman α emission is excited by resonance scattering of solar photons. Nadir measurements along the sunlit Earth part of the orbit show that the emission changes as a function of solar zenith angle. Comparison with a simple model shows that, from the shuttle altitude of 300 km and at low solar zenith angles, the line-of-sight probes atomic deuterium down to 80 km of altitude (where O2 absorption is complete), whereas at angles from 60° to 90°, the mesospheric part of the emission progressively vanishes. Then, the remaining emission mainly consist of the thermospheric part (z ≥ 100 km). This type of observations provides a sounding of atomic deuterium at its peak production and concentration, and D atoms can be used as a proxy to H atoms (which cannot be observed from a satellite) in this particularly active region of the mesosphere.