Mars is the fourth planet from the Sun, with a mean orbital distance of 255 million km.  Mars is about one-third the size of Earth with a diameter of 3,396 km and a mass of 6.42 E23rd.   Mars has a weak magnetic field  and no detectable radiation belt.    It is a solid body whose fairly low average density, 3.4 grams per cubic centimeters, show that iron represents 25 per cent (by mass) of the compositions of the planet.  The corresponding per cent on Earth is 33 percent. [1]

 The morphology of the plane is interesting.  It is roughly divided into the highlands and the lowlands.  The southern hemisphere is heavily cratered, the northern has volcanic areas and plains with light cratering.  The northern hemisphere has a generally lower altitude that the south.  We will see later how important his fact becomes regarding the flow of water.  Differences in altitude on Mars are as high as 30 km.  The highest area is the Tharsis  plateau where giant volcanoes  exist at altitudes of up to 6 km. This plateau extends over 4,000 miles north to south and 3,000 miles east to west.

Mars has two moons, Phobos and Deimos.  Phobos rotates around Mars more rapidly that the planet spins.  Because of tidal forces it is slowly spiraling in toward Mars, and will crash into it in about 30 millions years.  [Cambridge p 158]  Deimos orbits Mars almost synchronously and is largely undisturbed by tidal forces.  Phobos is about 27 km long, and Deimos is 15 km, both are irregularly shaped. 

The Orbit

     The eccentricity of Mars’s orbit is much larger that Earth’s.  The Earth is only about 3% closer to the Sun at perihelion than at aphelion.  For Mars the difference is about 20%.   The means that the amount of sunlight reaching the surface of Mars is much greater at perihelion. 

The Martian southern hemisphere summer occurs when Mars is much nearer the Sun than in the winter. This creates a larger difference in the temperatures in the southern hemisphere during the change of the seasons.[2]

Crust

The process of dating surfaces by looking at the relationships between them is called stratigraphy. In general, the more craters appear on a surface, the older that surface. But like most principles in the real world, that we need to exercise judgment in its application.

Our best theory about how the planets formed is that they were accreted from smaller bodies, which were continuously impacted by other objects adding to the mass of each planet. Eventually, most of these smaller bodies had impacted the planets, and so the rate of cratering tailed off to almost zero. The largest bodies (the ones that would form the largest craters) were used up before the smaller ones, since there were fewer of the larger ones to start with. So as a rule of thumb, the larger a crater is, the older it probably is.

We can roughly divide the history of crater formation into three periods, from oldest to newest:

¨      large and small craters formed

¨      small craters only formed

¨      very few craters formed

Craters are not uniformly distributed on Mars; instead, there are a few areas with significant numbers of very large craters (greater than 300 km in diameter), most of the rest of the southern highlands have only smaller craters, which we interpret as older terrain. All of the northern lowlands have very few craters, most of which are small.

In the figure below, the regions are color-coded, red for the most heavily cratered, green for the intermediate areas, and blue for the least cratered areas.