>>>No probe that has ever been sent to Mars was ever designed for serious geo-study. There is not even an example of a vessel scraping the surface to reveal what might be underneath. All our knowledge of Mars is from surface imaging, and mostly in still frames.

Hope is on the way! Mars Express in '03, will carry a wonderful (but not very hi-res) radar that will map to a depth of... can't remember. Seem to recall it was in the kilometers range. It will be able to distinguish between soil and H2O/CO2, but not very well between H2O and CO2.

Netlander (in '05 or so?) will be able to get a great deal of internal exploration done.

Further in the future another RADAR similar to the one on Mars Express but with much higher res (but less penetration) was scheduled to fly on a NASA mission (in '05?). Hope it doesn't get budgeted out of existence.