The Polar Ice Caps

The physical chemistry of H₂O molecules and CO₂ molecules are the main factors in controlling the weather on Mars.  Firstly, the behaviour at the ice caps is governed by the properties of these two substances.  It is important to realise that by ICE we mean water  (H₂O) ice AND carbon dioxide (CO₂) ice.   CO₂ ice is often called dry ice.  This in turn affects the changes in pressure over the whole planet that causes the winds and the violent planet wide dust storms.

Both the North Polar Regions and the South Polar Regions have permanent ice caps throughout the whole year.  In both cases there is a residual ice cap which is permanent and persists throughout the year and a seasonal ice cap which waxes and wanes with the seasons and reaches its greatest extent during the winter.

Like Earth, Mars is inclined on its axis.  The inclination of the axes of the two planets is similar; Earth 23.4° and Mars 25°.  As a consequence of this Mars shows seasonal variations like Earth.   By another coincidence the period of rotation of Mars is only slightly longer than ours.  The Martian day of just over 24 hours and 37 minutes has been given the name of a SOL.  The Martian year is considerably longer than that of the Earth, being 687 Earth days.   As far as the climate is concerned there is another important factor imposed of the climate of Mars and that is the fact that it has a far more elliptical orbit than the Earth.  As a result of this there is a much greater percentage difference in its distance from the Sun at perihelion (the nearest) and aphelion (the furthest) when compared to the Earth.   Mars experiences considerable differences in the amount of radiation it gets from the Sun throughout its year - it gets about 40% more at perihelion than at aphelion. This affects the climatic differences between the two hemispheres much more than it does on Earth.  The table illustrates the difference in the distances from the Sun of Mars, Earth, Venus and Mercury together with the lengths of the 'year' and the 'day' for each planet

Approximate Distances of Mercury, Venus, Earth and Mars from the Sun

The southern summers are shorter but hotter than the northern summers since at this time Mars is closest to the Sun.  On the other hand, the southern winters are far colder than the northern winters when Mars is furthest from the Sun.   The percentage difference for the Earth is quite small and anyway many other factors play important roles in seasonal differences on Earth.   However there is a big percentage difference in the planet-sun distances for Mars and the effect is very marked.

The Southern Hemisphere, particularly in the polar-regions, is extremely cold in winter.  During the winter time temperatures as low as -113°Celsius have been recorded and at these low temperatures very little if any water vapour was detected. Almost all the H2O will be in the form of water ice.  On the other hand considerable amounts of carbon dioxide sublime out to form the seasonal polar ice cap.   Despite the fact that the south polar-regions in general have warmer summers than the north but it is thought that there is so much solid CO₂ formed in the winter that it does not all vaporise in summer. Therefore, the southern residual ice is almost certainly a mixture of both water-ice and solid carbon dioxide.   The south polar cap shows a much greater variation than does the north.   In mid-winter it extends as far as 50° South: for comparison the north only extends about 65° during the northern mid-winter.

Thanks to ESA’s Mars Express, we now know that Mars has vast fields of perennial water ice, stretching out from the south pole of the Red Planet.
 
Astronomers have known for years that Mars possessed polar ice caps, but early attempts at chemical analysis suggested only that the northern cap could be composed of water ice, and the southern cap was thought to be carbon dioxide ice.

Recent space missions then suggested that the southern ice cap, existing all year round, could be a mixture of water and carbon dioxide.

Only with Mars Express have scientists been able to confirm directly for the first time that water ice is present at the south pole too. The results showed that hundreds of square kilometres of ‘permafrost’ surround the south pole. Permafrost is water ice, mixed into the soil of Mars, and frozen to the hardness of solid rock by the low Martian temperatures. This is the reason why water ice has been hidden from detection until now - because the soil with which it is mixed cannot reflect light easily and so it appears dark. OMEGA observations were made between 18 January and 11 February 2004 this year, when it was late summer for the Martian southern hemisphere and temperatures would be at their highest. Even so, that is probably only –130 degrees Celsius and the ice that Mars Express has observed is a permanent feature of this location.

OMEGA looked at the surface of the polar ice-cap with infrared eyes and, being sensitive to heat, clearly picked up the signature of water ice. The discovery hints that perhaps there are much larger quantities of water ice all over Mars than previously thought. sing this data, planetary scientists now know that the south polar region of Mars can be split into three separate parts. Part one is the bright polar cap itself, a mixture of 85% highly reflective carbon dioxide ice and 15% water ice.

The second part comprises steep slopes known as ‘scarps’, made almost entirely of water ice, that fall away from the polar cap to the surrounding plains.

The third part was unexpected and encompasses the vast permafrost fields that stretch for tens of kilometres away from the scarps

Mars Express and OMEGA will now continue looking for water ice and minerals across the surface of the planet. In May, another Mars Express instrument, the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) is alsocollecting data, looking for water underground.

Picture on right shows ice and dust in north polar regions Picture Courtesy ESA

Another Mars Express instrument, the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) has quickly detected what appeared to be water ice extending as deep as 3.5 kilometres below the southern polar and stretching 1.8 kilometres below the surface of the northern polar ice cap. Now, it has found what looks like water ice it will begin collecting data, looking for water underground.  MARSIS is looking at the south pole in particular because, once planetary scientists know how deep the ice reaches, they will be able to calculate exactly how much water there is. Knowing this is very important to understand how Mars evolved and if it ever harboured life.   Water may once have flowed freely on Mars and, like the Earth, the planet receives sunlight and has its own internal energy source. So the odds on primitive life thriving for at least some time during the planet’s history are reasonably good.

Global dust storms coincide with the retreat of the southern polar cap near perihelion.  A large temperature gradient exists at perihelion between the newly exposed south circumpolar surface and the residual southern ice cap.  These factors are thought to be sufficient to generate winds fierce enough to inject large amounts of dust into the atmosphere.  Dust in its turn has a positive feed back effect since the presence of dust causes a temperature rise in the atmosphere that tends to raise more dust.  This can lead to the large global dust storms that occur in the planet's atmosphere from time to time.

In the North Polar Regions, during the northern winter, both water molecules and carbon dioxide molecules sublime out to form the seasonal polar ice cap.  However in the summer all the carbon dioxide is believed to enter the gas phase.  Therefore, the northern residual ice cap is almost certainly water ice only.  

Sand dunes and ongoing movements of sand across the planet have been observed in the North Polar Regions.   The North Polar Cap is surrounded by a zone of dark dunes, which contain fragments of rock and minerals 0.06-2.0 millimetres in diameter.  The sand appears to have been transported by wind in one of two ways - by hopping over the ground, which is called SALTATION or by rolling along the ground, a process known as TRACTION.  Some of the dunes are covered with thin bright frost, which is intersected by dark streaks.

.The Atmosphere of Mars

The present atmosphere consists mainly of carbon dioxide with traces of nitrogen, noble gases and water vapour.

There is clear evidence that, although Mars is now a cold desert with a very low atmospheric pressure, it passed through at least one warm phase in which there were large amounts of running water on the surface. This is backed up by the results of the Viking and Pathfinder landings, by successful orbiters, and by the observations of Earth-based telescopes and the Hubble Space Telescope.

The Climate of Mars

On Mars the ambient temperature can range between minus 125º Celsius and plus  27º Celsius although for almost all of the time it lies well below zero Celsius all over the planet.   However on a very warm Martian day near the equator the temperature can rise as high as 27º Celsius above zero during the early afternoon.  Even then the temperature drops very sharply at night.   Under the low conditions of temperature and humidity H₂O always sublimes directly to ice (frost) when it reaches the saturation partial pressure.  This process of sublimation is very important on Mars where the ambient pressure is very low and H₂O does not appear to exist in the liquid state anywhere on the surface.   The 'morning mists ' which are sometimes observed at dawn are due to ice crystals subliming out directly from the vapour phase.  Later in the morning the mist clears as the temperature rises and the ice changes back directly to the gas phase.

The Pathfinder Mission took place during the summer 'months' of the Northern Hemisphere of Mars.

Extensive monitoring of the weather was carried out during the Mission.  Pressure variations during the day could change considerably by as much as 0.3 millibars (4.5% of the average pressure on Mars).

Frozen water-ice clouds appeared in the morning over the Pathfinder site and evaporated as the temperature rose.

Cloud Formation on Mars

Both water and carbon dioxide form clouds.    Clouds often appear above the equatorial regions around mid-day.  At these times the warming of the air is at its greatest causing the warm air to rise so that ice crystals condense (sublime) out to form clouds.  Clouds quite often appear around the Tharsis Volcanoes, which are at a very high altitude.

Fogs, which appear to consist of both solid H₂O and CO₂, appear at sunrise particularly in the depths of the canyons.

Dust Storms on Mars.

From time to time in the thin air of the red planet vast dust storms blow across the cold dry sands obscuring its surface from outside observation.  Sometimes wind speeds up to 400 kilometres an hour are reached. Dust storms are a feature of the Martian seasonal cycle.  During 1977, the Viking Mission recorded a total of 35 storms.

In May 1999 the Hubble Space Telescope picked up evidence of a colossal storm.   The hurricane probably formed as frozen CO₂ at the North Pole sublimed rapidly to the vapour phase during the northern summer leaving water ice behind.  The temperature difference between the bright ice-cap and the warm dark sand caused a very high wind to arise.  The eye of the hurricane was about 320 kilometres in diameter and the whole system was 1,700 kilometres across (see New Scientist report 29 May 1999).

The Terraforming of Mars

This may well be a subject of great controversy of the future.  There will be some who will want to planetary engineering want to transform Mars into a second Earth by a huge effort of planetary engineering.  There will doubtless be many more against it.

Even though it is much smaller in mass and diameter the land area of Mars is almost as large as Earth because Earth's surface is covered by about three quarters water.  Even so a terraformed Mars will at the very least require large stretches of water.

The total surface area of the Earth is 510.1 million square kilometres, most of which is ocean.  The Total Land Surface Area of the Earth 149.8 million square kilometres.   Although Mars is a far smaller planet than the Earth, the land surface area of Mars is coincidentally almost the same.  The surface area is of Mars is 145.0 million square kilometres.                                                         

Any human beings and any other animals born and raised on Mars will be subjected to a lower gravity and would be unlikely ever to go to be able to tolerate the higher gravity of Earth.

One very important point about space vessels going to Mars for a return trip to bring back rock samples is the question of life.

Apart from the already acknowledged danger of contaminating Mars with earthly organisms there is the far more serious possibility of bringing back from Mars any primitive Martian organisms and contaminating Earth 

 An excellent book by Arthur C Clarke entitled  ‘The Snows of Olympus’ deals with the terraforming of Mars was published  

Mars Space Missions

The subject of Mars Space Missions is dealt with in another page of the web-site