The first ever underground investigation of another planet has been performed by a radar antenna aboard Europe鈥檚 Mars Express spacecraft. The instrument probed two kilometres below the Martian surface and found tantalising hints of liquid water pooling in a buried impact crater.
The MARSIS antenna was deployed successfully in June 2005 after a series of glitches. It works by sending radio pulses towards the Red Planet and then analysing the time delay and strength of the pulses that bounce back. The radio waves that penetrate the surface rebound when they encounter a sub-surface boundary between materials with different electrical properties 鈥� such as rock and water.
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But aside from one Apollo 17 radar experiment on the Moon in 1972 鈥� which yielded mixed results 鈥� the technique had never been tested.
The most exciting part of this experiment is simply 鈥渢hat it works鈥�, says MARSIS co-leader Jeff Plaut of NASA鈥檚 Jet Propulsion Laboratory in Pasadena, California, US.
William T K Johnson, MARSIS manager at NASA鈥檚 Jet Propulsion Laboratory in Pasadena, California, US, agrees. 鈥淭his is very experimental,鈥� he says. 鈥淲e wondered 鈥� can we see anything in the subsurface? The answer to that is yes.鈥�
Ice bowl
Johnson and colleagues have now revealed subsurface measurements of two regions in the planet鈥檚 northern hemisphere 鈥� the mid-latitude lowlands called Chryse Planitia and the northern polar cap.
They believe a 250-kilometre-wide circular structure that lies between 1.5 and 2.5 kilometres below the surface of Chryse Planitia is an impact crater that was buried with volcanic ash or soil several billion years ago. The team sees no radar boundaries in material that fills the bowl of the crater and the radar signals lose little strength when passing through it. That suggests the infill must contain a large proportion of ice, which is nearly transparent to radar.
Substantial amounts of ice in the soil would make sense given the crater鈥檚 location in what appears to be a basin where ancient rivers once converged. 鈥淚f the water could be captured in a basin and preserved for several billion years, it may still be there,鈥� says Plaut.
Intriguingly, the signal reflected from the bottom of the crater is so strong and appears so flat that it may be liquid water. 鈥淚f you put water there, that鈥檚 what the signal might look like,鈥� Johnson told New 杏吧原创. But he cautions the data is based on only one pass over the region and could be caused by another material.
Rare pass
MARSIS also studied the northern polar cap and found nearly pure water ice stretching down 1.8 kilometres below the surface, with an icy layer of sand underneath.
The researchers are encouraged that such interesting features have emerged from only three data-gathering passes. MARSIS has only been able to make this small number of observations because the subsurface results can only be obtained under special circumstances.
It can best study the subsurface when it is closest to Mars 鈥� just 26 minutes of each 7-hour orbit 鈥� and when it is also on the planet鈥檚 鈥渘ight鈥� side. That is because energetic electrons in the sunlit portions of the planet鈥檚 outer atmosphere, or ionosphere, block the radar鈥檚 longest, ground-penetrating wavelengths.
For the last several months, these conditions have not existed at all. But, the conditions are now right again and will remain so until May 2006. The next study regions are in the southern hemisphere, including the south pole.
But gathering the data is only the first step 鈥� it then has to be interpreted, which can take scientists months. That is because radar signals travel at different speeds through the ionosphere depending on their wavelength, and the ionosphere itself varies in size depending on the Sun鈥檚 activity.
鈥淭he ionosphere is always around pestering us,鈥� says Johnson. He adds that so far the ionosphere has prevented the instrument鈥檚 longest wavelengths 鈥� which could reach down as far as five kilometres 鈥� from returning data.
Journal reference: Science (DOI: 10.1126/science.1122165)
