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MaRS: Mars Radio Science Experiment
Science from data transmission
The radio science experiment on board Mars Express (MaRS) makes a virtue out of a necessity by using the radio signals that convey data and instructions between the spacecraft and Earth to probe the planet's ionosphere, atmosphere, surface and even interior. "This is almost like getting science for free," says Martin Pätzold from Köln University, Germany who is principal investigator on the experiment.
The two radio downlinks on Mars Express, one in the
Mars Express will differ from the other spacecraft that have performed similar experiments (Mariner 4 in 1965 and Mars Global Surveyor now) by using the S- as well as the X-band. "The S-band is sensitive to plasma (ionised gas) densities," says Pätzold, which makes it particularly useful for studying the ionosphere, the ionised, outer part of the atmosphere.
One of the outstanding questions about Mars is how it came to lose most of its atmosphere. One possibility is that the solar wind, the stream of charged particles flowing out from the Sun, has gradually stripped the atmosphere away through interactions with the ionosphere. The radio science experiment will measure density profiles in the ionosphere, which will help determine whether, how and when this happened.
The speed of a spacecraft relative to the ground station can be measured with an accuracy of "less than one tenth the speed of a snail at full pace," according to Pätzold. Such high precision is achieved by measuring the frequency change in the radio signal caused when the spacecraft moves towards or away from the ground station. (The phenomenon, called the Doppler shift, is responsible for the familiar change in pitch or frequency in the sound of an approaching or retreating siren.)
Variations in the gravitational field of Mars will cause slight changes in the speed of the spacecraft relative to the ground station as it travels along its orbit. Doppler measurements therefore provide a way of mapping the planet's gravity field, which can then be compared with a map of the topography. Gravitational anomalies occur when the gravity field is not what you would expect from the topography - for example, when there is an increase in the gravity field over flat, featureless ground (from the gravity data you would expect a mountain to be present).
NASA's Mars Global Surveyor has recently identified gravitational anomalies in the red planet's northern hemisphere (see North-South dichotomy). "We want to investigate these and other local gravity anomalies and correlate them with 3D topographic models derived from images sent back by the HRSC (the Mars Express camera)," says Pätzold. The HRSC will produce a topographical map with higher resolution than any previous Mars mission, which will allow the gravitational anomalies to be mapped more accurately than before.
"If you point the spacecraft's high-gain antenna at the surface of Mars and transmit a radio signal in the X-band, its reflection from the surface can be detected by the ground station on Earth and used to determine the surface roughness," says Pätzold. The rougher the surface is, the broader the spread of frequencies will be in the reflected signal. The plan is to compare the surface roughness data with data on the surface composition recorded by OMEGA, PFS and the HRSC's images.
Mars Express will find itself directly opposite the Sun from the Earth - a position known as a solar conjunction - in autumn 2004 and autumn 2006. "For eight weeks the planet Mars and the orbiting spacecraft will be behind the Sun, so the radio signal will have to propagate through the solar corona (the Sun's atmosphere). Any frequency or phase changes due to the Martian atmosphere and/or gravity field will be masked by changes as the radio signal propagates through the solar corona. So we will seize the opportunity and really do solar science for free," says Pätzold. He hopes to study the coronal density distribution, the solar wind in its source regions and coronal mass ejections - and to compare the results with measurements recorded at the same time by solar observatories.
Principal Investigator Dr. Martin Pätzold, Universität Köln, Cologne, Germany
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