The US and ESA satellites orbiting Mars have so far deployed remote sensing instruments that detect visible to thermal infrared radiation from the planet’s surface. Ultimately the energy involved is from the Sun: these are passive instruments. Engrossing as they are, images from these sensors reveal only details of surface mineralogy and the Martian topography. So far, virtually nothing is known about what lies buried beneath it, apart from inferences about ground ice. The ESA Mars Express has one last imaging trick up its sleeve, which uses energy generated on board and beamed obliquely down to the surface. This is the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS). Radar remote sensing on Earth generally uses high-frequency microwaves in the wavelength range from 0.01 to 0.1 metres, and the images produced show how much energy is scattered by surfaces of varying roughness, to be received by antennae deployed from an aircraft or satellite. The longer the wavelength the greater the height of small-scale surface irregularities that cause scattering and therefore a received signal. Smooth perfectly surfaces reflect all the energy away from the antennae, like a mirror, so no energy returns to be sensed. How microwaves interact with the Earth’s surface depends on the electrical properties of the materials. Good electrical conductors, such as metals and liquid water are extremely efficient reflectors, whereas minerals are poor conductors and tend to absorb microwaves to some extent. If soils are extremely dry, with less than 1% moisture content, as in some deserts, some of the absorbed energy is scattered by materials below the surface and images show subsurface features. This lies behind the principle of ground penetrating radar, but since many soils are damp, only radar waves generated at the surface give good signals in most areas, to be exploited by civil engineers and archaeologists. Ice is very different from liquid water, being so poorly conductive that it is almost transparent to microwaves. Consequently it has proved possible to sound the depth of glaciers and ice sheets using ground penetrating radar deployed from aircraft. The depth of penetration, and of course that involves energy returning to the surface in order to get a signal, is governed by the radar wavelength. For instance, unknown former courses of the River Nile’s tributaries have been detected by 0.25 m radar waves beneath the hyperarid eastern Sahara through about 3 metres of dry sand.
MARSIS can transmit microwaves with 4 wavelengths 170 , 100 , 80 and 60 m. Given rocks and soils free of liquid water, which comprise most of Mars’s surface, or ice, it can penetrate as deep as almost 5 km. The multi-wavelength arrangement can also potentially discriminate water ice from rock and soil. A great deal of speculation and some evidence suggest that parts of Mars may be underlain by permafrost, that is melted only under unusual conditions, such as after meteorite impacts. There are also suggestions that glaciogenic-like landforms may still be underlain by ice, and bizarrely that there are frozen seas (see The triumph of the old on Mars in April 2005 EPN). MARSIS may well throw Mars investigations into a turmoil, but maybe not. The delay in sparking it up has been caused by fears that deploying its antennae might damage the whole spacecraft, and the first attempt seems to have got stuck. It’s other drawback is limited power so that horizontal resolution will be between 5 to 10 km and vertically only 100 m, so results may be so blurred as to be inconclusive. NASA plans a similar device aboard its Mars Reconnaissance Orbiter (launch date August 2005). The Shallow Subsurface Radar (SHARAD) will use microwaves with 12 to 20 m wavelengths that give penetration to 1 km, but horizontal and vertical resolutions of 300 and 15 metres.
See: Reichhardt, T. 2005. Going underground. Nature, v. 435, p. 266-267.