ASPERA: Energetic Neutral Atoms Analyser
How the solar wind erodes the Martian atmosphere.
The Martian atmosphere is a shadow of its former self. Four billion years ago it was thick, similar to the Earth's. Today, atmospheric pressure at the planet's surface is only about 7 mbar, or 0.6% of that on the Earth. Where did the atmospheric gases go? One of the main tasks of the Energetic Neutral Atoms Analyser (ENA) on board Mars Express is to find out.
The Earth's magnetic field channels the solar wind away from our atmosphere. But Mars no longer has a magnetic field and the charged particles that make up the solar wind are free to interact unhindered with the outermost atmospheric gases. Atoms of gas that are ionised (electrically charged) by this interaction, or by the action of sunlight, get picked up by the solar wind and swept out to space. This is almost certainly a mechanism by which Mars loses atmospheric mass. But could it account for all the loss over four billion years? In particular, could it account for the loss of Mars's water? When water evaporates from the surface, the vapour eventually reaches the upper atmosphere where sunlight can break up the molecules into hydrogen and oxygen, which can be ionised and swept away by the solar wind. How much water has Mars lost via this route?
Although the loss is of ionised gas, ASPERA will explore the answers to these questions mainly by detecting energetic neutral atoms (ENAs). An ion formed by interaction with the solar wind can become a neutral atom again by stripping an electron off an atom of neutral background gas. As the ion was energetic, the resulting neutral atom is too. Neutral atoms are undeflected by electromagnetic fields and so travel in a straight line from the site of their formation. "Some of the energetic neutral atoms pass through the ionosphere into the atmosphere below, where they deposit their energy. We think that at times this energy deposition is as high as that from Extreme UV radiation from the Sun," says Rickard Lundin, Principal Investigator for ASPERA from the Swedish Institute of Space Physics in Kiruna.
By detecting the ENAs and their direction of travel, ASPERA will build up a global image of the region in the upper atmosphere that is interacting with the solar wind, or plasma. The neutral particle imager, one of the four sensors on ASPERA, will be responsible for generating this image. "Remote sensing of the Earth's atmosphere has revealed that ENAs have a far greater impact on the neutral upper atmosphere than anyone had anticipated. We expect this will be the case for Mars" says Lundin, When it arrives at the red planet at the end of 2003, ASPERA will make the first ever measurement of ENAs at another planet. "From a position close to apocentre (the point on Mars Express' orbit most distant from Mars), the neutral particle imager will generate instantaneous images of the whole planet that reflect the density of the plasma. We will be able to see the plasma escaping the planet."
|Simulated ENA image of escaping oxygen.|
"The neutral particle detector can distinguish between only two ENAs, hydrogen and oxygen, but these are the most important atoms for analysing water loss," says Stas Barabash who works as a co-PI with Lundin on the instrument. An earlier, simpler version of ASPERA, flown on the Phobos mission to the moon of Mars in 1988, suggested that the solar wind does indeed blow away some part of the atmospheric gases. "The mechanism seemed to be so effective that it could have been responsible for evacuating up to 30 m of Martian water," says Barabash. Estimates put the depth of a layer of water covering the entire surface of early Mars around 100 m. The neutral particle detector on ASPERA will add far more precise information to this earlier experiment to confirm or deny its findings.
Ions and electrons are measured, too
To understand just how the solar wind interacts with the upper atmosphere, ASPERA will measure ions and electrons when Mars Express passes close to the red planet in the region where the solar wind and Martian atmosphere interact. An ion mass analyser will measure the flux and mass of ions coming from any direction. As the solar wind consists mostly of protons (1 atomic mass unit) and Martian ions are mainly oxygen (16 atomic mass units), the origin of the ions can be determined from their masses. An electron spectrometer, perhaps the smallest ever built, will measure electron fluxes in the energy range of a few to 20 000 eV.
When sunlight ionises an atom in the upper atmosphere, the electron produced carries information about the kind of atom ionised. As these photoelectrons can propagate upwards from low altitudes, their measurement can provide information on what is happening well below the orbit of the spacecraft. The ion and electron measurements will give a snapshot of a particular location in the Martian atmosphere at a particular time. They will therefore complement the neutral particle measurements, which will provide information on the interaction region of the whole atmosphere at the same time.
|Nozomi, the Japanese Space Agency's mission, which was due to arrive at Mars shortly after Mars Express.|
Mars Express was to have collaborated with Nozomi, the Japanese spacecraft that was due to arrive at the red planet at the end of 2003. Attempts to insert Nozomi into orbit around Mars failed and the spacecraft passed the planet at a distance of about 1000 km to enter a heliocentric orbit. The ASPERA team would have been the only team to have an instrument on both spacecraft: it was to have flown an ion mass analyser on Nozomi. Most of Nozomi's fourteen instruments were aimed at investigating the Martian ionosphere and atmospheric loss, hence the spacecraft's overall objectives coincided most closely with those of ASPERA out of all Mars Express's instruments.
Nozomi would have followed an equatorial orbit, making its measurements complementary to those taken by Mars Express from its polar orbit.
Principal Investigator: Dr. Rickard Lundin, Swedish Institute of Space Physics, Kiruna, Sweden
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