The Radar Search For Martian Water

Until the last few years, Mars has been regarded as a cold, arid world that lost most of its water long ago. However, recent observations by NASA’s Mars Global Surveyor and Mars Odyssey spacecraft have provided tantalising evidence that huge amounts of water may be hidden just below the surface.

Now, a powerful new instrument is poised to probe the Martian soil, using an advanced radar system to penetrate the rust-red desert. On Friday 11 April, Professor Iwan Williams (Queen Mary) will explain to the UK/Ireland National Astronomy Meeting how the MARSIS experiment on board the European Space Agency’s Mars Express mission will search for the elusive water several kilometres beneath the planet’s surface.

MARSIS is a type of ground penetrating radar. On Earth, such radar is typically operated from the ground or from aircraft to prospect for water or man-made objects a few tens of metres below ground. On Mars, it will search for water up to 5 km below ground from its vantage-point on board Mars Express, 250-300 km above the planet.

This technique has only been tried once before on a space mission, in a successful experiment during one of the Apollo lunar missions. However, MARSIS will be the first such radar to look for underground water on another planet.

The entire instrument, including antenna and data processing unit, weighs about 12 kg. It works by sending long wavelength, low frequency radio waves (1.3-5.5 MHz) towards the planet from a 40 metre long antenna, which will be unfurled after Mars Express goes into a near-polar orbit. Such a long antenna is needed to work at long wavelengths, which are able to penetrate Mars to a depth of a few kilometres.

The radio waves will be reflected from any surface they encounter. Most of them will bounce off the rough surface of Mars. But because of the low frequency, a significant fraction will travel through the crust to encounter further interfaces between layers of different material. If there is a layer containing liquid water, it should generate a radar echo. The presence of weaker signals after the first strong surface return will allow the detection of subsurface interfaces, while the time delay between the two signals will give a measure of the depth of the interfaces.

By sending two different frequencies at the same time and analysing the echoes generated, MARSIS will be able to extract information on the electrical properties of the reflecting surface and hence its composition. An underground zone of liquid water will have very different electrical properties from the surrounding rocks and will reflect very strongly. The top of a liquid zone somewhere in the upper 2-3 km should be seen fairly easily. If other conditions are favourable, the surface may be ‘seen’ even at depths of 5 km or more.

“The radio waves will be reflected at any interface, not just that between rock and water,” said Professor Williams, “so MARSIS should also reveal much about the composition of the top 5 km of crust in general. It should, for example, pick out layers of rock interspersed with ice, which are more likely to exist close to the Martian surface than liquid water.”