Answer to Question 1
When the crust of a planet is strained, it may break, producing faults and rift valleys. Near the Tharsis region on Mars is a great valley, Valles Marineris, named after the Mariner 9 spacecraft that first photographed it. The valley is a block of crust that has dropped downward along parallel faults. Erosion and landslides have further modified the valley into a great canyon. It is four times deeper, nearly ten times wider, and more than ten times longer than the Grand Canyon. The number of craters in the valley indicates that it is 1 to 2 billion years old, placing its origin sometime before the end of the most active volcanism in the Tharsis region.
Answer to Question 2
Unlike Mercury or Venus, Mars has moons. Small and irregular in shape, Phobos (28 23 20 km in diameter) and Deimos (16 12 10 km) are almost certainly captured asteroids. These moons are so small they cannot pull themselves into spherical shape. Phobos and Deimos are not just small; they are tiny. An athletic astronaut who could jump 2 m (6 ft) high on Earth could jump almost 3 km (2 mi) on Phobos. These moons are so small that any interior heat would have leaked away very quickly, and there is no evidence of any internally driven geologic activity on either object.
The tenuous atmosphere of Mars, visible on the horizon in this low-orbit photo
Phobos transits the Sun, as seen by Mars Rover Opportunity on March 10, 2004
the comet Encke riding along its pebbly trail of debris (long diagonal line) between the orbits of M
Paths of Mars
A Piece of Mars on Earth
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