Cold, arid Mars, where we'll land next.  Polar caps, volcanoes, the biggest canyon ever seen - Mars probably once had oceans.
 

Outline:

Mars is similar to Earth in that it has an atmosphere, a tilted axis, seasons and a day of twenty-four-and-a-half hours.  But there are crucial differences.  Mars has a sub-zero climate, global dust storms and an atmosphere 150 times weaker than Earth's.

Space missions have revealed much of Mars - like the Viking Orbiter images of the planet's surface and the search for life in the planet's red soil by the Viking Landers, albeit unsuccessful.

A Martian guided tour takes in Mariner Valley, the 4000-kilometre long rift cutting across the planet's equatorial regions - plus the great shield volcanoes, such as Olympus Mons, the tallest volcanic peak in the Solar System.

Volcanic activity may once have been important - not only in triggering massive flash floods, but in giving Mars a respectable atmosphere and surface temperatures.  But when the volcanoes shut down there were no eruptions to replenish the atmosphere, which leaked into space because of the weak Martian gravity.

Recent missions to the Red Planet include the Mars Pathfinder landing and Mars Global Surveyor's evidence for ancient Martian oceans and a weak magnetic field.  Future expeditions are previewed, together with the continuing search for microbial life.
 
 

Sub-chapters:
 

The Red Planet

*  Mars as the "dusty Red Planet" and the relevance of Martian exploration in the light of the  planet's similarities to Earth.
 

Seasons and Dust Storms

*  Bitterly cold temperatures because Mars is so much farther from the Sun.

*  Two tiny Martian moons.

*  How tilted axis and elliptical orbit of Mars cause and distort the Martian seasons.  The  differences between the seasons in the northern and southern hemispheres.   How the polar caps expand and contract.

*  First pictures of Mars from early Mariner spacecraft - leading to a steady growth in our knowledge of the planet's surface.

*  Dust storms that can envelop the entire planet.
 

The Vikings

*  The Viking Orbiters revolutionised our understanding of the Red Planet.  Their pictures hinted at the presence of water in the form of sub-surface ice, and to past flash floods.

*  Frosts and early morning fog.

*  Touchdown of two Viking landers which dig for signs of life.
 

Giant Volcanoes

*  Mars is small.  North America would fit across the planet.

*  AmMartian fly-over - Mariner Valley (the biggest rift), and the great volcanic peaks, including Olympus Mons, the tallest volcano in the Solar System.
 

Pathfinder & Global Surveyor

*  The theory of oceans forming on early Mars.  The importance of great volcanic eruptions and climatic instabilities in explaining why Mars doesn't spark life.

*  Volcanic shut-down - main reason for today thin Martian atmosphere.

*  Touchdown of Mars Pathfinder and the work of its six-wheeled rover in analysing the composition of the rocks - with results that may support the existence of ancient oceans.

*  Supporting evidence from Mars Global Surveyor in orbit, which finds a huge flat region.  Surveyor also measures the weak Martian magnetic field.
 

Future Mars Missions

*  With the success of Pathfinder and Global Surveyor, further missions are planned, each with a lander and an orbiter.

*  The aim of the Mars Surveyor program is to understand the history of water on the Red Planet.  Landers will continue the hunt for signs of life.
 
 

Background:

Seasons of Mars

The rotation axis of Mars is tilted by 24 degrees to the vertical, very similar to the axial tilt of Earth.  Mars, therefore, has seasons in much the same way as Earth.  The orbit of Mars around the Sun, however, takes 687 days compared with Earth's 365.  So Martian seasons are correspondingly longer than Earth's.

The highly elliptical orbit of Mars has a noticeable effect on the seasons.  When Mars is closest to the Sun (and travelling more quickly) the southern hemisphere is tilted towards the Sun.  As a result, southern spring lasts only 143 Martian days, and southern summer 154 days.  In addition, because the planet is closest to the Sun, the southern Martian summer is hotter and shorter than the northern summer.  Northern spring lasts 194 days and summer, 178 days.  Northern winter is as long as southern summer, and the other seasons are similarly reversed.  Southern winter, when Mars is farthest from the Sun, is colder and longer than northern winter.  Climate in the southern hemisphere of Mars, therefore, displays a wider range of temperatures than in the north.
 

The Mystery of the Missing Water of Mars

When measurements by the US Viking landers confirmed low atmospheric pressure and low  temperatures on Mars, it became obvious that the Red Planet was extremely dry - a "freeze-dried desert".  Water can only exist as a liquid over a certain range of pressure and temperature.  The conditions on Mars today would result in any liquid water boiling violently and rapidly evaporating into the thin Martian atmosphere.  Liquid water, therefore, cannot be present on present-day Mars.

But when orbiting spacecraft obtained detailed photographs of the Martian surface, features which resembled dried-up riverbeds were apparent.  There were also erosion features that  looked like flash-flood plains.  This supported the theory that liquid water once flowed across the surface.  Scientists, therefore, were faced with the problem of the planet's missing water.  Part of this water undoubtedly resides as ice in the polar caps of Mars.  With arrival of summer, the upper layer of frozen carbon dioxide (dry ice) in the ice caps rapidly evaporates.  This exposes a layer of water ice beneath that survives the chilly Martian summers.  The thickness of this water ice, however, has not yet been determined.

The flash-floods on Mars seem to have gushed from regions of collapsed, jumbled terrain.  This indicates that the water released came from a layer of melted permafrost beneath the Martian surface - identical to that found in the tundra in far northern parts of Earth.  If heat from volcanic activity occasionally melted this sub-surface ice, the ground would collapse as vast quantities of water were forced to the surface, producing a brief flash-flood as the water boiled away.  From photographs, scientists have estimated the depths and widths of the Martian flash-flood channels.  Results suggest the Martian flash-floods must have been at least 100 times greater than anything known on Earth.

Scientists disagree about the amount of water lying frozen in the polar ice and as sub-surface ice and permafrost, but some believe that if all this water melted, it would be sufficient to produce a shallow ocean, 200 metres deep, across the surface of Mars.
 

The Volcanoes of Mars

The Martian volcanoes were first photographed by the US spacecraft Mariner 9 which entered orbit around the planet in 1971.  The largest Martian volcano, Olympus Mons, rises 27 kilometres above the surrounding plains, making it the highest known volcano in the Solar System.  Olympus Mons is three times higher than Earth's highest mountain, Mount Everest.  The highest volcano on Earth is Mauna Loa in the Hawaiian Islands, but it is largely submerged in the waters of the Pacific Ocean.  Its summit rises eight kilometres above the ocean floor.  The base of Olympus Mons is ringed with cliffs reaching six kilometres in height and its diameter is nearly 600 kilometres.  Only two of the volcanoes on Venus, Rhea Mons and Theia Mons, cover a larger area - and they are only six kilometres high.  At the summit of Olympus Mons, there is a complex caldera, approximately 80 kilometres across, containing a number of overlapping volcanic craters.

The colossal size of Olympus Mons strongly suggests that there is no plate tectonic activity  on Mars - as there is on Earth.  A "hot spot" in the mantle of Mars keeps pumping molten rock upwards through the same vent for millions of years.  This produces one giant volcano, rather than a chain of volcanoes like the Hawaiian Islands on Earth.  Olympus Mons is one of four large volcanoes clustered together on the Tharsis bulge, a region 2500 km across, lying just north of the Martian equator.  The other three volcanoes of the Tharsis group lie in a rough line.  They are called Ascraeus Mons, Pavonis Mons, and Arsia Mons.  A separate, smaller group of Martian volcanoes is located almost on the opposite side of the planet.  None of the Martian volcanoes appear to be active today, but it is not known whether they are truly extinct or merely dormant.
 

Links for Further Information:

General information about Mars.
http:seds.lpl.arizona.edu/nineplanets/nineplanets/mars.html

An atlas of Mars, with answers to frequently asked questions.
http://ic-www.arc.nasa.gov/ic/projects/bayers-group/Atlas/Mars

A compilation of facts about Mars, including arguments about the possibility of life on the planet, with comparative images of Mars and Earth.
http://cass.jsc.nasa.gov/publications/slidesets/marslife.html

A chronology of space exploration to Mars.
http://www.star.le.ac.uk/edu/solar/craft2.html#mars

'Earth invades Mars' - a full account of the Mars Pathfinder and Mars Global Surveyor missions.
http://www.spacer.com/mars/mars-prev.html

Mars Pathfinder page.  Comprehensive image gallery, with accompanying text, plus links to other images, animations and the Mars Sunset movie.
http://www.brandx.net/dbajot/mpf/

Mars Pathfinder images from NASA.
http://www.in-search-of.com/frames/events/mpf_archive.shtml

Mars Global Surveyor - Welcome to Mars!  Comprehensive site with what's new, latest images, directory of all mgs links, and weekly status overview.
http://mpfwww.arc.nasa.gov/mgs/
 

Questions and Activities for the Curious:

1.  Compare and contrast the seasons in the northern and southern hemispheres of Mars with those of Earth.

2.  Describe the evidence from spacecraft which has led scientists to believe that liquid water once flowed on the surface of Mars.

3.  Research and describe the similarities and differences between volcanoes on Earth and Mars.  Why are volcanoes on Mars larger than those on Earth?

4.  Explain why Earth's sky is blue, whereas on Mars it is pinkish in colour?

5.  Carbon dioxide is almost as abundant in the atmosphere of Mars as it is in the Venusian atmosphere.  But why is there no runaway greenhouse effect on Mars?

6.  Mars is often described as "the planet which is the least unlike Earth".  Justify this statement.

7.  Research and report on the current status of the debate concerning life on Mars.  Do you think Mars could be as barren and sterile as the Moon?

8.  Imagine you are an astronaut living on Mars and you have a motorised vehicle for exploring the planet.  Where would you like to go, and why?