SPICAM: UV and IR Atmospheric Spectrometer
Why is the Martian atmosphere so oxidising?
If life ever existed on Mars, its traces have almost certainly long since vanished from the surface of the planet. Something in the Martian atmosphere interacts aggressively with surface materials to strip them of electrons. This process gives the surface of Mars its reddish hue by making iron in the soil turn rusty. Such oxidation would rapidly destroy organic molecules derived from life. But precisely what makes the Martian atmosphere so oxidising remains a mystery. Ozone (O3) is thought to be implicated, so, too, is UV light. Just how they reap their devastating effects is one topic to be addressed by SPICAM, the UV and IR atmospheric spectrometer on Mars Express.
The UV sensor will take measurements in three different modes. In the first mode, called nadir pointing, the spacecraft will point directly at the centre of Mars and the UV sensor will measure sunlight which has travelled through the atmosphere after being reflected from the Martian surface. In the second mode, called stellar or solar occultation, the spacecraft will point across the atmosphere towards a star, or the Sun, seen emerging from behind the disc of the planet. From this position, the UV sensor will measure the light absorbed by the atmosphere directly from starlight or sunlight. In the third mode, called limb pointing, the sensor will point across the atmosphere, as during stellar occultation, but without a star being present. In this position, the UV sensor will measure the "glow" given off by the atmosphere. The IR sensor will be used in nadir mode only.
Measuring ozone and water vapour
In nadir pointing mode, the UV sensor will measure ozone, which absorbs 250 nm light, while the IR sensor measures water vapour, which absorbs 1.38 micron light, along the track of the orbit. These measurements will give the total amount of ozone and water vapour in columns of atmosphere with a 10 km² cross section. "Over the lifetime of the mission, we should be able to build up measurements of ozone and water vapour over the total surface of the planet for the different seasons," says Jean-Loup Bertaux from the Service d'Aeronomie du CNRS, Verrihres-le-Buisson, France and Principal Investigator for SPICAM. "Our computer models of the chemical composition of the Martian atmosphere predict that ozone and water are strongly coupled. When you have ozone there's no water and vice versa. If we don't see this anti-correlation, we'll need to revise our model. If we see it, then the model will be validated".
The model is crucial to understanding the oxidising nature of the Martian atmosphere. "The model can compute some things that can't be measured at all, such as the OH radical," says Bertaux. The OH radical, which is created when UV light splits water vapour (H2O) into H and OH, could be the major oxidising component in the atmosphere. It is an aggressive molecule which combines with most other molecules to alter them completely. If it exists in the Martian atmosphere, it is unlikely that organic molecules, which form the basis of living matter, remain unaltered on the surface of the planet.
Stellar and solar occultation measurements with the UV sensor will be used to derive the vertical distribution of ozone and also carbon dioxide (CO2). As CO2 is the predominant atmospheric gas, its distribution will be used to derive the vertical density and temperature profile of the neutral atmosphere. Understanding the density profile in the upper atmosphere is important for future missions which might want to use friction with the atmosphere to brake a spacecraft gradually into a desired orbit (aerobraking), or even capture it from its interplanetary flight from Earth (aerocapture). "Measurements taken by Mars Global Surveyor, NASA's spacecraft now in orbit around Mars, seem to be suggesting that the atmospheric density is highly variable. We need to understand this variability to make predictions for future aerobraking or aerocapture," says Bertaux.
SPICAM's vertical profile measurements for carbon dioxide and along track water measurements will complement those of another Mars Express instrument, the Planetary Fourier Spectrometer (PFS). "PFS is interested in the lower atmosphere, whereas SPICAM is measuring the density of carbon dioxide at altitudes of
When sunlight hits the upper atmosphere, it strips electrons from atoms and molecules to leave a layer of electrically-charged atmospheric gases known as the ionosphere. Electrically-charged molecules (ions) of carbon dioxide and carbon monoxide (CO) emit UV light at very distinct and characteristic wavelengths
The measurements will be taken across the limb during daytime when the emission is more intense. "We'll be monitoring the ionosphere by remote sensing. We'll measure the quantity of light in each spectral line and deduce how many ions there are in the line of sight," says Bertaux. The measurements will complement those taken by another Mars Express instrument, ASPERA, which has as one of its objectives to record the density of ions as the spacecraft flies through the ionosphere. "These measurements should allow us to make a diagnosis of the state of the upper atmosphere and its coupling with the ionosphere," says Bertaux. And that should help to answer another outstanding question about Mars - how was the dense, wet atmosphere of early times lost to space?
Principal Investigator Dr. Jean-Loup Bertaux, Service d'Aéronomie, Verrières-le-Buisson, France
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