Slide #1: Mars and Earth Size Comparison.

Mars is the fourth planet from the sun.  Mars takes 687 Earth days 
to orbit the sun (that is, one Mars year is 687 Earth days long).  
Because the planet's axis is tilted 25.2 degrees (Earth's is 23.5 
degrees), Mars experiences a changing of seasons similar to those 
on Earth.  The air on Mars, composed of 95 % carbon dioxide, is so 
thin (only 7 thousandths, 0.007, times the Earth's atmospheric 
pressure at sea level), that liquid water cannot exist on the 
surface.  Typical summer temperatures are usually less than 0 C 
(32 F), and in winter it is cold enough to freeze carbon dioxide 
at the poles (- 125 C; -193 F).  Mars has a diameter of 6,780 km 
(4,213 mi), compared to Earth's, 12,756 km (7,926 mi).  The surface 
area of Mars is about the same as all the dry land surfaces of Earth.  
Martian gravity is about 40% that of Earth's (Mars' gravitational 
acceleration is 3.73 meters per second squared).  

Figure from p. 10 in Viking Orbiter Views of Mars, NASA SP-441, U.S. Government Printing Office, 1980.

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