Ground penetrating radar analyses of the Sahara has showed ancient river channels covered with sand. It has also alllowed geologists to track ancient plate tectonic movements obscured by the sand:

FROM ANCIENT EARTH TO MODERN FLOODS, SPACE RADAR FINDINGS
OFFER NEW INSIGHTS ON THE CHANGING FACE OF OUR HOME PLANET.
NASA HQ Public Affairs Office (NASANews@luna.osf.hq.nasa.gov)
Mon, 6 Nov 1995 16:10:53 -0500
"One of the most astonishing results of the 1981 mission was the discovery of ancient river beds under the sands of
the Sahara Desert in North Africa. The 1984 mission enabled explorers to find the Lost City of Ubar in Oman."
...
"Hundreds of millions of years before the first humans were born, and even millions of years before the dinosaurs reigned, the Earth was dominated by one giant landmass that formed when all of the present-day continents crashed together. Now, scientists using spaceborne radar to look beneath the sands of Africa's Sahara Desert have discovered where these ancient continents collided more than 650
million years ago.
"The formation of this 'supercontinent' resulted in a massive ice age that covered the land with glaciers and set
the stage for the evolution of the first complex animals. Finding the location of this collision zone is fundamental
to understanding how this ancient supercontinent was formed," said Dr. Robert Stern, a member of the radar team from the University of Texas at Dallas."
http://www.qadas.com/qadas/nasa/nasa-hm/0329.html

The lack of observed complex distributary channel systems in Mars late history had led to the conclusion that the infrequent late channels seen on Mars must be due to ground-water sapping, not precipation under a thick atmosphere. Ground penetration radar may allow these complex systems to be seen if they exist under the sedimentary cover known to exist in the northern plains of Mars.
Also, plate tectonics was believed not to have occurred on Mars. However, the magnetic anomalies seen by MGS have led to the possibity of plate tectonics occurring in Mars earlier history to be reconsidered:

Plate Tectonics . . . on Mars
Magnetic map reveals ancient activity on the Red Planet
Science News, Volume 155, Number 18 (May 1, 1999)
http://www.sciencenews.org/sn_arc99/5_1_99/bob2.htm

Ground penetrating radar may also reveal the evidence of such ancient tectonic motions on Mars, under the sedimentary cover of the martian northern plains.
Mars Express 2003 and Mars Reconnaissance Orbiter 2005 are both planned to carry ground penetrating radar.

Bob Clark