QUESTION:
How big is the canyon on Mars?
 
ANSWER from Jeff Plescia on April 7, 1997:
The largest canyon on Mars is "Valles Marineris" (names
after the Mariner 9 spacecraft that discovered it). It has
a length of more than 4000 kilometers (2500 miles) and
would stretch from Boston to Los Angeles if it were on
Earth. It has a width of hundreds of kilometers in places
and depths of 10 km in places. The Grand Canyon of Arizona
would be a small tributary to Valles Merineris.

Valles Marineris is believed to have been formed by a large
block down dropped between two parallel faults (a graben).
The walls of the canyon have been heavily modified by
slumping and erosion. Some of most spectacular landslides
in the solar system occur off the wall and across the floor
of Valles Marineris. There has also been some minor volcanism
along the faults and some people believe parts of the canyon
were once filled with water.

ANSWER from Jeff Plescia on May 22, 1997:
The largest canyon system on Mars is Valles Marineris.
The system of troughs extends east west across the equatorial
area of Mars for about 4000 km (2500 miles) - about the
distance across the U. S.

Individual troughs are as much as 100 km (60 miles) wide
but they form groups as much as 600 km (375 miles) wide.
Depths are 8-10 km (5-6 miles). In comparison the Grand
Canyon is only 1 mile deep. The troughs are interconnected
and open into catastrophic flood channels to the east.
The features seem to have been formed by faults and then
been heavily modified by erosion (landslides and running
water).