Mars is an icy planet










Series: From the acid clouds of Venus through the hurricane hell of Neptune to the icy Pluto desert

This magnificent photo of our neighboring red planet was taken on June 26, 2001 by the Hubble Space Telescope. It is considered to be the sharpest photo of Mars ever taken from Earth. You can see two storm systems brewing together: One is revealed by the white cloud formations in the north polar region, the other by the yellow-brown dust veil on the right edge of the southern hemisphere of the planet. A few weeks later, the dense dust clouds of a global mega-hurricane enveloped the entire planetary disk in a cloudy, reddish light.


The world of lost water

Hardly any other planet in our solar system is as similar to Earth as Mars. With a diameter of almost 6,800 kilometers, our neighbor is significantly smaller than the earth, but it also has a shell-shaped, inner structure with a solid core, a partly glowing mantle and a solid rock crust with a rocky surface. On the red planet there are strongly structured landscapes with partly volcanic mountains, valleys, basins and wide plains. In addition, there are seasons and changing weather processes with clouds, wind and even water ice. The most important building block of life as we know it, liquid water, was largely lost to our red neighbor in space a long time ago.


Mighty volcanoes once provided an abundance of water


The largest volcano in our solar system, the Martian volcano Olympus Mons, as seen by the Viking 1 space probe on June 22, 1978. The giant has a height of 26.4 kilometers from the base and a base diameter of almost 600 kilometers. - The huge eruptions of volcanoes like this once brought water to the red planet.

In the beginning there was the water. Even on Mars. Water flowed over its surface, in rivers and lakes, yes, even filled entire ocean basins - planetary researchers now have little doubt about that. Because countless traces of erosion point to the existence of a humid age in Mars history shortly after the formation of the planet. The warm and humid climate regime presumably lasted for several hundred million years, but came to an abrupt end around 4 billion years ago. Although there are indications that in the more recent history of Mars, at least intermittently and regionally, water has flowed again and again or has led to the formation of glaciers in the form of snow, but such temporary events are likely to be related to the primordial, planet-wide, humid climate never have come back.


The perspective false-color image of the high-resolution stereo camera HRSC on board the European space probe Mars-Express suggests that this winding valley, cut deep into the surface of Mars, was once formed by flowing water.

In those days, huge volcanoes, in addition to lava and dust, also spat vast amounts of carbon dioxide and water vapor into the then dense and warm Martian atmosphere. When the planet had cooled down sufficiently, the water vapor condensed into clouds like on Earth, and rivers fed by heavy rains formed valleys, ravines and basins in the surface of the young planet. For several hundred million years there was an abundance of water. Snowfalls caused glaciers to form in the polar regions and on the flanks of the mountain ranges.


The NASA simulation shows Mars with the glaciers of a prehistoric ice age that have advanced far towards the equator. Researchers assume that ice ages have occurred several times in the history of the planet because the inclination of its axis of rotation fluctuates with respect to the plane of the orbit. At present, this inclination of around 25 degrees is comparable to that of Earth, but Mars spins like an unstable top through space. Its axis inclination fluctuates between 0 and up to 60 degrees, a fact that, depending on the axis position in relation to the sun, causes enormous climatic fluctuations. - The earth was spared such a fate only because our moon has a stabilizing effect on the earth's axis.


A fatal chain reaction made the water disappear

Unlike the more massive Earth, however, the young Mars was unable to maintain its atmosphere, which was fed by volcanic eruptions and asteroid impacts, so that part of its gas envelope was permanently escaping into space. When the volcanism subsided and the supply of gases and water came to a standstill, a chain reaction set in with devastating consequences: the atmosphere became thinner and more and more water began to evaporate.

With the inexorably escaping atmosphere, the greater part of the water vapor gas was gradually lost and the warming greenhouse effect weakened more and more. This in turn meant that it got colder and colder until the last remnants of water had frozen to ice and the planet finally turned into the dry dusty desert that we know it today.


At the bottom of a crater about 35 kilometers in diameter near the North Pole of Mars, pale water ice stands out clearly. The ice layer is only a few decimeters thick, but it bears witness to the damp past of our neighboring planet. The photo was taken by the high-resolution stereo camera HRSC on board the Mars Express space probe, but was processed to enhance contrast. In reality, the visible ice cover should appear less blue and more beige, as it is covered by a layer of dust. - The frozen crater lake probably owes its survival to this day only to this dust and the annually recurring winter preservation of the surface by dry ice.

However, the last remnants of that former water resource can still be found today on and above all under the surface of Mars. Some of them have entered into chemical compounds, but others are also hidden under the surface of the planet in the form of embedded layers of ice, where they are well preserved covered by dust and debris and largely protected from evaporation through sublimation.


The flow pattern of this old terrain formation, called "hourglass crater" because of its shape resembling an hourglass, shows that glaciers and debris once flowed from the mountain range at the top left into the upper small impact crater until it overflowed. Then the ice pushed further down into the larger crater 500 meters below.

It cannot be completely ruled out that somewhere deep under the surface of Mars, in addition to layers of materials containing water ice, veins of liquid water could also survive. The radar probes by Mars Express and the Mars Reconnaissance Orbiter could soon provide clarity about the structure of the deeper Martian soil layers.


The artist's impression shows the NASA probe "Odyssey" which, with the help of a gamma spectrometer, detected thin layers with 20 to 50 percent water ice (light deposit) in the Martian soil close to the Martian poles in 2001.

Regardless of its dramatic climatic history, which has transformed the planet into a hostile dust desert after losing almost all of its water, some climatic cycles and weather processes that we also know on earth were able to assert themselves on Mars. There, as here, there are changing seasons, which can be seen in the advancement and retreat of the polar ice caps, which are covered by both dry ice and remnants of water ice.


Seasons and weather processes like on earth

The cause of the seasons on Mars is the already mentioned inclination of its axis to the orbit plane by currently around 25 degrees, so that in the polar regions of the two hemispheres - as on Earth - the midnight sun and polar night alternate. If summer is in the northern hemisphere, it is winter in the southern hemisphere and vice versa. And because Mars needs almost two earth years to orbit the sun, its seasons are almost twice as long as on earth: One Mars year corresponds to 687 earth days and thus about 22 ½ months.


Spring storms on the edge of the northern polar region: near the poles, due to large temperature differences - just like on Earth - storms with cloud fronts can brew together, separating cold from milder air masses. These impressive weather systems are driven solely by seasonal air pressure fluctuations. Without the change of seasons, there would be no significant weather processes on Mars.


Icy cold due to the lack of a greenhouse effect

The Martian temperatures are anything but comfortable. The annual mean temperature is minus 55 ° C, which is around 70 ° below that of our earth. The reason for such low values ​​is the almost missing greenhouse effect. The thin atmosphere consists of 97 percent carbon dioxide, but the small, absolute amount of CO2 can only raise Mars temperatures by 5 ° C. On the other hand, the greenhouse effect is currently causing a temperature increase of 35 ° C. Without greenhouse gases, the mean temperature of the earth would only be minus 20 ° C, which would freeze the oceans and endanger the most important prerequisites for life.

While Mars passes through its orbit area closest to the sun, the air near the equator heats up to around plus 20 ° C in the afternoon, in the best case it is enough for plus 25 ° degrees or a little more. If, on the other hand, it is far from the sun or if dust clouds reduce solar radiation, there is also frost all day at the equator. At night the temperatures always drop well below freezing point, in low latitudes to minus 30 to minus 70 °, in higher latitudes to minus 70 to minus 100 ° and in the polar night regions in winter even to values ​​close to minus 140 ° C, so that carbon dioxide gas then precipitates in layers from the thin atmosphere onto the polar ice caps, which are otherwise mainly composed of dust-covered water ice.


Thick layers of water ice and dust shape the landscape at the north pole of Mars. The cliffs are almost two kilometers high. The dark material in the crater-like structures and dune fields could be volcanic ash.


Air pressure and wind on the red planet

Of all meteorological phenomena, the wind is by far the most important, because wind, in interaction with received solar energy, is the engine of all weather events on all planets that have an atmosphere. On Mars, too, it compensates for differences in air pressure and temperature caused by different levels of solar radiation, thus ensuring a constant change in the state of the atmosphere. Depending on regional conditions and the season of the year, it only blows as a gentle, barely perceptible breath or it chases away as a thundering hurricane and can change entire landscapes through the dust that is always carried along.


From the plateau surrounding the caldera of the 4,500 meter high volcano "Albor Tholus", an impressive "dustfall" pours into the three-kilometer-deep crater. The plume of dust indicates a strong wind in the summit area of ​​the volcano.

Under the thin Martian atmosphere, the friction of the wind with the surface of the planet is significantly less than on Earth and due to the low force of gravity, which is only about a third of the earth's gravity, dust particles can be blown up by the wind to great heights and transported over long distances . Even with longer periods of calm weather, the finest dust particles always float in the atmosphere and give the marsh sky its typical, dull reddish shimmer even in completely cloudless weather.

The air pressure on Mars is more than a hundred times lower than on Earth. On average, it is only slightly more than 6 hectopascals (hPa), with the range of fluctuation between about 1 hPa at the summit level of the 26 km high Olympus Mons volcano and up to around 11 hPa in the deepest depressions and basins on the planet. Since there are no seas on Mars (any more), the mean altitude of the atmospheric pressure surface of 6.1 hPa was set as the reference point for the altitude "normal zero" (NN). All altitude data on the planet Mars therefore relate to this freely defined zero point. - The fluctuation range of air pressure caused by meteorological factors can also reach around 25 percent everywhere on Mars.


Planet-wide spring and autumn storms

Despite the low temperature level, the differences between mild, cold and extremely cold at the end of the long winter on one hemisphere and at the end of summer on the other are enormous. In addition, the seasonal carbon dioxide sublimation reaches its maximum at this time and also creates large differences in air pressure. This creates ideal conditions for huge balancing storms to form, which can affect the entire planet. The masses of dust thrown up by such mega-storms then often cover the entire Martian sphere with dense, reddish clouds for weeks, sometimes even months.



© by NASAJ. Bell, M. Wolff and STScI / AURA


Every few years, small-scale dust storms form global hurricanes that can cover the entire planet in dense swaths of reddish dust for weeks. The time difference between the two pictures above is only 10 weeks.

The starting regions for such gigantic storms are the areas near the poles, in which, as the autumn cooling begins, carbon dioxide sublimes from the atmosphere and precipitates as dry ice on the water ice caps. As a result, the atmosphere contracts, the air pressure drops by more than 25 percent and a huge, atmospheric low pressure area is created. - Exactly the opposite processes take place at the opposite pole: Here the winter dry ice sublimates back into the atmosphere with the spring warming, whereby the air envelope expands and the air pressure balance compared to the warmer, equatorial areas is out of balance due to a pressure increase of more than 25 percent device. This creates a huge high pressure area over the South Pole region, at the edges of which cold air begins to flow out.

In both cases, the atmosphere tries to compensate for the pressure and temperature differences that arise compared to lower latitudes, and violent storms are set in motion. The polar regions of Mars can therefore be described as the driving wheels of planetary weather. In the southern hemisphere, the 8 km deep "Hellas Basin" plays an important role in the formation of storms, because nowhere else on Mars are the differences in air pressure and temperature as great as between this comparatively warm depression and the southern one adjacent, subpolar and polar regions of Mars. This fact acts on emerging storms like a huge catalyst ignited by orography.


The animation of the north polar region in summer extends over several consecutive days. Although the individual images are not taken at regular intervals, they still show the atmospheric turbulence on the edge of the ice cap very nicely. The trigger for this is the very different warming of the region: While the light ice surfaces reflect the sunlight and therefore remain cold, the darker rubble and sand surfaces absorb the radiation and warm up significantly in the process. The temperature differences then set violent winds in motion, which can be easily observed in the swirling cloud structures.

The pressure equalization storms that develop in the course of the seasonal carbon dioxide sublimation in high latitudes are also subject to the distracting influence of the Coriolis force on Mars, as a result of which the air mass boundaries form into wide curved streaks and frontal arches with elongated cloud bands. These weather fronts, characterized by clouds of dust and ice, move - like on Earth - in the northern hemisphere against and in the southern hemisphere clockwise around the controlling low.


As with low pressure areas on Earth, cloud systems also swirl on Mars, where cold and milder air masses come too close. The sequence of images shows a spring storm on the edge of the still icy North Pole Cap.

If the seasonal storm formations coincide with particularly intense radiation weather in the equatorial zones when the planet is close to the sun, the increased radiation supply further intensifies the global air pressure gradient. As a result, the initially only regional storm zones at higher latitudes can easily penetrate the planetary wind system and build up into a global mega-storm. At the end of such a development, there are pressure equalization storms that last for weeks every few years, the dust clouds of which reduce solar radiation on the entire planet until the air pressure and temperature structure has stabilized again.

The wind speeds of the most violent storms are likely to reach full hurricane strengths of more than 120 km / h on the surface at higher latitudes, because there are hardly any friction losses due to the thin air.In the free atmosphere, the wind peaks may even be over 200 km / h. These figures are of course only estimates; more precise information about the dynamics of the storm systems of Mars can only be obtained from precise meteorological research.


The comparison shows impressively how similar the processes in dust storms on Mars and Earth are: The top picture shows a regional dust storm on the Red Planet, the bottom picture a sandstorm over the Canary Islands.


Weather phenomena on Earth and Mars in comparison


Weather parameters

Temperature range
Temperature mean
Fluctuation range

Air pressure range
Air pressure medium
Fluctuation range

Wind systems
Storm systems
Dust devil

Types of clouds
Cloud shapes
Cloud fronts

Precipitation deposited
Falling precipitation
thunderstorm

earth

- 89.2 to + 57.3 ° C
+ 15 ° C
146.5 K

870 to 1083.8 hPa
1013.25 hPa
214 hPa

global and regional
spatially limited
local, rather seldom

Water and ice clouds
stratiform and convective
constant, very common

Dew and frost
Rain, snow, hail
very common

Mars

- 143 to + 27 ° C
- 55 ° C
170 K

1 to 11 hPa
6.1 hPa
10 hPa

global and regional
global storms possible
common and frequent

Ice clouds and ice fog
only known stratiform
rarely, only near the pole

some frost possible
not known
not known


The temperature and air pressure data only relate to the surfaces of the two planets, the other parameters also relate to the free atmosphere.




Join us in the second part of our Mars weather report on the "Dance of the Dust Devils". Find out everything about the clouds in the Martian sky, why dawn on the red planet is not a messenger of bad but beautiful weather and read a typical weather report for Mars


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