No. 118: Jul-Aug 1998
Now that the Mars Global Surveyor has convinced almost everyone that the famous "face" on this planet is really only an eroded mesa, we can attend to some other Martian curiosities. One of these is the so-called "stealth" region.
For some 2,000 kilometers along the Martian equator west of Arsia Mons and Pavonis Mons stretches an area that is invisible to terrestrial radar. Of course, we can see this region; but, when a 3centimeter radar is pointed at it, no detectable echoes are returned. Thus, the term "stealth," as in the F-117 Stealth aircraft,
Are the clever Martians trying to conceal something from prying earthlings, using, say, a "cloaking" device a la Star Trek? Of course not. Loose, unconsolidated sediments are poor reflectors of radar waves. Examination of Viking-Orbiter photos tell geologists that the "stealth" region is almost certainly thickly strewn with volcanic ash, which would absorb the radar waves very well.
(Edgett, Kennth S., et al; "Geologic Context of the Mars Radar "Stealth" Region in Southwestern Tharsis," Journal of Geophysical Research, 102:21,545, 1997.)