MARS

Mars is the fourth planet from the Sun and also the most Earth-like. It is one of the brightest objects in the sky and therefore has been known since the earliest antiquity. Because of its distinct red color, strong brightness and strange movement in the sky, it has often been associated with death or war by the oldest civilizations; its modern name is actually that of the Roman god of war. The red color is due to the presence of Iron Oxide prevalent on its surface. Mars’ radius is only half that of the Earth, it is significantly colder and has a much thinner atmosphere. However, the Martian environment is the most similar to the terrestrial one in the solar system: most atmospheric and geologic processes have their equivalent on our planet, for instance the global atmospheric circulation and the volcanic processes. One of the major differences is related to the absence of liquid water at the surface in the present conditions, although there is much evidence that liquid water existed in the past.

The physical study of Mars began with the invention of the telescope at the beginning of the 17th century. Galileo first demonstrated the spherical shape of the planet from his telescopic observations. Some 70 yr later, Christian Huygens and G D Cassini had recognized the main characteristics of the planet. The study of Mars improved very quickly at the end of the 19th century, when the progress of optical instruments made it possible to observe the high variability of the surface and atmosphere.

Mars, like the other terrestrial planets, is divided into a crust, mantle and core. The thickness and density of the core can be constrained from the knowledge of mass, radius and moment of inertia of the planet, with a rather large uncertainty. Although Mars Pathfinder measured the moment of inertia with a sufficient accuracy in 1997, two models are compatible with the current data: Mars could possess either a small core of high density (a core of iron would represent 15% of the planet’s mass for a radius of 1300 km), or a large low-density core (a mixture of sulfur and iron representing 25% of the planet’s mass for a radius of 2000 km).

The two satellites of Mars, Phobos and Deimos are also interesting objects. They were discovered only in the late 19th century because of their very small size and proximity to Mars. Both satellites are very dark and have a very low density, indicating that they are most probably captured asteroids

Historically, the observation of Mars has often triggered important astronomical progress. Mars has been the most widely studied plants in the entire solar system. The reasons for this situation are not only that Mars is the most accessible planet but also that it has the most Earth-like environment and is probably the most favorable location for the development of life in the solar system: although the current conditions at the surface are actually very hostile to life, this was almost certainly different in a distant past. Mars is therefore likely to become the first planet where humans will go, although this still appears a rather distant issue. Although the idea that some form of life probably existed on Mars was widely accepted at the end of the 19th century, this view changed with the first studies of the atmosphere, which demonstrated that Mars is a cold planet with a rather unfavorable environment.

Questions to Ponder:

• What color would Mars be if you were standing on it?
• Do you think Martians exist?