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 X-band (8.4 GHz) the other in the S-band (2.3 GHz), will be used for four different types of investigation, each of which makes use of the fact that changes to the radio signal's frequency or phase and amplitude can say much about the physical mechanisms that brought the changes about.

Probing the neutral atmosphere and ionosphere

On many orbits around Mars, the spacecraft will go behind the planet as seen from Earth. Just before it disappears or emerges from behind the planetary disc, signals sent between the spacecraft and the ground station will travel through the Martian atmosphere. "Before the spacecraft disappears behind the planet, the radio signal will propagate through the ionosphere and then the neutral atmosphere. This will produce changes in the frequency or phase shift of the signal," says Pätzold. Careful analysis of the changes back at the ground station will reveal how the temperature, pressure and density of the atmosphere change with height above the Martian surface.

Mars Express occultation in the plane of the sky. The high gain antenna is pointing toward the Earth (toward the viewer).The spacecraft will first go behind the ionosphere (the yellow ring), then the neutral atmosphere (the blue ring) and then the planetary disc. The radio signal propagates from the spacecraft first through the ionosphere and later through the atmosphere towards the Earth.

Timing and duration of planetary occultations during Mars Express's nominal and extended missions.

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.

Gravity

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.

Bistatic radar

"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.

Looking through the solar corona

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.

Solar conjunction as seen from a position above the plane of the Sun and planets. The Sun-Earth line is fixed in this coordinate system and Mars moves behind the Sun as seen from Earth. The shaded area marks an angle of 10° at each limb of the Sun. The Rosetta spacecraft also undergoes a solar conjunction at the same time.

Solar conjunction of Mars as seen from Earth (the viewer). The dots mark the apparent position of Mars in the plane of the sky when the planet is closer than 40 solar radii from the Sun, which corresponds to the 10° elongation in the previous figure. Each dot represents the position of Mars at 00.00 UT on each day.

Principal Investigator Dr. Martin Pätzold, Universität Köln, Cologne, Germany

	HRSC: High/Super Resolution Stereo Colour Camera
	MARSIS: Subsurface Sounding Radar/Altimeter