Mars -the Red Planet

Planet Mars is named after the Roman God of war.

Planet mars is named after the Roman God of war

It is the outermost of the four dense, rocky planets of the inner solar system.

Inner planets of the solar system including mercury earth venus and mars

Whilst Mars possesses a silicate mantle and crust…

….it is still unclear if the iron core of the planet is completely solid or if a proportion of it is liquid.

cross section of planet mars showing interior

The density of Mars, calculated by dividing total mass by total volume, is 3.93 g/cm³. This compares with Earth’s density of 5.514 g/cm³.

Cross section of earth and mars showing core, crust and mantle

It would take more that six Mars to fill the volume of just one Earth.

comparison of size of earth and mars

Mars’s axial tilt of 25° is similar to Earth’s. This means that Mars experiences seasons and temperature variations similar to that of Earth.

At ‘perihelion’, when closest to the Sun, Mars is 206,655,215 km from the Sun. At aphelion, when furthest away from the Sun, Mars is 249,232,432 km distant.

Aphelion and perihelion of Mars during orbit round sun

Mars has an orbital eccentricity of 0.0934 meaning that its orbit around the Sun is slightly elliptical. By way of explanation an orbital eccentricity of zero is perfectly circular whereas an orbital eccentricity of one is highly elliptical.

With an orbital eccentricity is 0.0167 Earth’s orbit is more circular.

One Martian year takes 686.971 Earth days or 1.88 Earth years. One sidereal rotation, the time it takes for the planet to complete a single rotation on its axis, takes 24 hours and 40 minutes. So one Martian day (or ‘sol’) is close to a single Earth day.

Mars has a mass 85% less than Earth’s. A lesser mass means that the force of gravitational attraction is weaker on Mars than on Earth. In fact the force of gravity on Mars is 62% less than the force of gravity on Earth.

gravitational force on mars compared to earth through basketball

On Mars acceleration due to gravity is 3.7 m/s2 compared to 9.8 m/s2 on Earth.

Surface gravity of Mars

The maximum (or ‘terminal’) velocity reached by a sky diver falling through Earth’s lower atmosphere in the ‘spread eagle’ position is 195 km/h (121 mp/h)….

skydiver in spread eagle position falling at terminal velocity

….whereas the terminal velocity for a skydiver on Mars would be 909 km/h (585 mph)!!

terminal velocity in lower atmosphere of Mars is 585mph

  • Given the smaller force of gravitational attraction of Mars, why is it that the terminal velocity in the lower Martian atmosphere is far greater than the terminal velocity in Earth’ s lower atmosphere?

The answer lies in the mediating effects of the atmosphere. The mean atmospheric pressure of Mars is only 0.6% of Earth’s. This means that air resistance and consequent drag force is far less on Mars than on Earth. The braking effect of the atmosphere in slowing down any intrepid Martian skydiver would be minimal!

With a smaller gravitational force, the escape velocity on Mars (the minimum speed needed for a rocket to escape the effects of gravity) is only 5.03 km/sec.

By way of comparison Earth’s escape velocity is 11.19 km/sec.

rocket blasting off from surface of Mars

Dust particles, ‘Mie scattering’ and colors of the Martian sky

Weak surface gravity, Martian winds and the planet’s dry dusty surface means that the Martian atmosphere is saturated with a huge volume of fine dust particles.

Martian hills in distance showing fine dust partially obscuring those hills

These fine dust particles can generate regional dust storms…

dust storm on Mars

…..which can sometimes blanket the whole planet and obscure the planetary surface.

dust storms on Mars can be planet wide

Dust storms are especially common during the Martian summer when heat from the Sun can trigger strong convection currents which rise into the atmosphere.

strong updrafts of winds on Mars

The predominantly red color of Martian skies is caused by the way in which sunlight collides with atmospheric particles of fine dust. After impacting with dust in the atmosphere, light of different wavelengths is deflected and scattered from its straight path.

It is the light with the longest wavelength, the color red,…

Wavelengths of visible light demonstrated

….which scatters most widely after colliding with particles of dust…

Mie scattering of light during daytime on Mars

…giving the Martian sky its red hue.

The scattering of light off large particles (ie large relative to the wavelength of light) is called ‘Mie scattering’. With this type of scattering light is scattered furthest in a forwards direction.

Mie scattering diagram

If ‘Mie scattering’ creates the red hue of Martian skies, the same phenomenon also creates amazing blue sunrises and sunsets!

  • How does this occur?

Blue Martian sunrise and sunset are examples of Mie scattering

At sunrise and sunset, when the Sun is low on the horizon, light has to travel greater distances through the atmosphere before reaching the ground. Being transported greater distances means that visible sunlight collides with greater amounts of atmospheric dust, leading to a greater amount of light scattering.

Mis scattering on Mars at sunrise showing scattering of red wavelength of light

Blue light, with the shortest wavelength, is deflected the least. It is the blue light, able to penetrate the atmosphere to ground level, which gives the sky its blue appearance. The other colors of light, including red light which predominates during the day, are scattered away.

Perhaps future colonists on Mars will one day be able to admire some amazing blue sunsets!

Blue sunrise and sunset on mars caused by Mie scattering©

Whereas on Mars it is atmospheric dust that largely drives the color of the Martian skies, on Earth it is the large volume of atmospheric gas that determines the color of the skies through the phenomenon known as ‘Rayleigh scattering’.

Rayleigh scattering on Earth gives blue skies and red sunsets

On Earth during the day it is the shortest wavelength of visible light, the blue light, which scatters most widely off molecules of atmospheric gas. It is the widely scattered blue light which gives our sky its blue coloration.rayleigh scattering showing direction of incident light

At sunrise and sunset, when light travels longer distances through the atmosphere before reaching the ground, all wavelengths of light (apart from red light) are scattered away. It is red light, which has not been scattered away, that appears visible from Earth’s surface when in the line of sight of the Sun.

sunrise showing Rayleigh scattering of red light

Martian atmosphere, sublimation, photodissociation and the magnetosphere

The tiny amounts of gas in the Martian atmosphere provides the planet with an average surface pressure of 0.087 psi (6.0 mbars) or only 0.6% of Earth’s mean sea level pressure of 14.69 psi (1,013 mbars).

The insignificant Martian atmosphere is overwhelmingly composed of carbon dioxide.


The thin Martian atmosphere does little to prevent the loss of heat and solar radiation into space.

The mean temperature on Mars is well below freezing everywhere all of the time. The average surface temperature is -63 degrees C (-81 degrees F); night time temperatures on Mars can plunge to -110 degrees C (-170 degrees F).

Faculty of Pure and Applied Science, York University

It is so cold at the polar ice caps during the Martian winter that carbon dioxide condenses to become dry ice. This dry ice forms a solid layer of CO2 ice on top of existing water ice caps.

North and south poles of mars

In Spring and Summer the CO‘sublimates’ (changes from a solid to a gaseous state without becoming a liquid) off the ice cap. The change of CO2 into a gaseous state increases the atmospheric mass by tens of percent during the course of a single Martian Summer.

electromagnetic radiation on Mars

CO2 ice completely vanishes from the northern ice cap during the Summer, exposing the H2O ice cap. In contrast during the Summer at the South Pole a small COice cap survives on top of H2O ice.

  • So what happens to the sublimated CO2 and H2O?

CO2 and H2O clouds are often formed. The process by which H2O clouds are formed is explained below:

Molecules of water in the form of water vapour (H2O in a gaseous state) are transported by winds to higher altitudes where, in the presence of dust aerosols (atmospheric particles of dust), they condense…

Water Vapour Molecules condense into clouds

…into clouds.

clouds on planet mars

Where there are too few dust aerosols, condensation no longer takes place, with the result that substantial amounts of water vapour remains in the atmosphere.

‘Supersaturated’ water vapour (atmospheric water vapour which remains in a gaseous state) is transported substantial distances across the planet by winds or carried higher into the atmosphere.

photodissociation of water vapour in atmosphere of Mars
In the upper atmosphere the supersaturated water vapour is affected by ‘photodissociation’.  Photodissociation occurs when solar radiation splits the water vapour molecules into hydrogen and oxygen atoms….

Photodissociation of H2O in Martian atmosphere

….following on from which the separated hydrogen and oxygen atoms escape into space.

photodissociation of h2o

  • Why does photodissociation, and the consequent loss of hydrogen and oxygen molecules into space, occur on such a large scale?

It is the lack of a comprehensive magnetosphere, which would shield the atmosphere from lethal solar radiation, that prevents Mars retaining its atmospheric gases.

Mars’s current magnetic field  has a strength of 1500 nanotesla. Earth’s, by comparison, reaches a maximum of 65000 nanotesla, or a magnetic field more than 40 times stronger than Mars’s.

magnetospheres of earth and mars compared

In times past Mars did have a far more extensive global magnetosphere generated by an internal liquid iron core. This protected the planet from harmful solar radiation and enabled the presence of shallow oceans of water.

Planet mars of 4 billion years ago showing shallow ocean in northern hemisphere

About 4.3 billion years ago, Mars had enough water to cover its entire surface in a layer averaging 450 feet (137 meters) deep. The water would have formed an ocean occupying almost half of Mars’ northern hemisphere, reaching depths greater than a mile (1.6 kilometers) in places.

However about 3.9 billion years ago the liquid iron core cooled, shutting off the magnetic field. Without any protection afforded by an effective magnetosphere, harmful solar radiation stripped away the existing atmosphere and the extensive shallow seas disappeared.

Solar winds strip away martian atmosphere

Terrain of Mars

Following the evolution of the Solar System some 4 billion years ago, Mars and Earth both experienced intense periods of bombardment by meteorites. However only about 190 impact craters have been identified across the surface of the Earth, whereas on Mars there are more than 635,000 impact craters at least 0.6 miles (1 kilometer) wide.

Impact craters of Mars

  • So why don’t we see large numbers of impact craters on Earth?

The Earth’s thick atmosphere burns up most meteorites before they reach our planetary surface. If meteorites do manage to impact Earth’s surface erosional processes, in the form of flowing water and rain, rapidly remove any evidence that impact craters ever existed. Over time volcanic activity and plate tectonics also play their part in erasing  impact craters.

On Mars there is a marked difference between the northern and southern hemispheres. The northern hemisphere is largely made up of rolling volcanic lava plains formed by extensive volcanic eruptions.

Mars Northern and Southern Hemispheres

The northern hemisphere is cratered far less than the south suggesting that the northern surface is younger at around 3 billion years old compared to 4 billion in the south.

Northern hemisphere of Mars

Although Mars and Earth are very different when it comes to temperature, size and atmosphere, the geological processes shaping the surface features of the two planets are surprisingly similar.

On Mars we see volcanoes, valleys and river channels.

River channel on Mars

However the sheer size of some land forms on Mars dwarfs comparable features on Earth. For example Olympus Mons, a ‘shield’ volcano,  is not only the largest volcano on Mars but also the largest known volcano in the Solar System.

Olympus Mons on Mars

Towering 26 km (16 miles) high, Olympus Mons dwarfs all volcanoes on Earth such Mauna Kea in Hawaii and Mount Everest in Nepal.

Olympus Mons compared with mT Everest and Mauna Kea

  • So why is Olympus Mons so massive?

A historical absence of active plate tectonics on Mars meant that rising magma erupted onto the same part of the planet’s crust above a volcanic ‘hotspot’ for millions of years, leading to the formation of a very large volcano.

Olympus Mons forming above a static volcanic hotspot

Higher eruption rates and lower surface gravity, facilitating the flow of erupting lava,  also played their part in building this massive volcano.

By way of comparison, on Earth active plate tectonics limits the size…

volcano forms above hotspot on Earth and moves at same speed as tectonic plate

… that volcanoes can grow to.

Volcano becomes dormant after moving beyond volcanic hotspot

In addition to a massive volcano Mars also boasts a canyon system, the Valles Marineris, that extends for a distance of 4000 km (2500 miles).

Valles Marineris on Mars

At some points the canyon is 125 miles (200 km) wide and reaches depths of 6 miles (10 km).

Valles Marineris on Mars compared with Grand Canyon in United States

If the Valles Marineris canyon were located on Earth it would extend the width of the United States.

Valles Marineris on Mars as seen from space

It is now believed that the massive canyon system was borne out of volcanic activity 3.5 billion years ago that took place in the nearby Tharsis region of the planet.

Valles marineris borne out of volcanic activity in Tharsis region of Mars

As molten rock pushed through the crust to form three massive volcanoes, the crust was thrust upwards. The upwards pressure on the crust caused it to crack, forming large fractures and  creating the Valles Marineris system of canyons.

The canyon system was further enlarged by subsurface water which escaped through these fractures and flowed along the canyon, widening the canyon walls and deepening the canyon floor.

Fog in valles Marineris

Fog in Valles Marineris

Moons of Mars

The planet has two small moons called Phobos and Deimos.

Moons of Mars Phobos and Deimos

Following a collision with a primordial body one-third its size, 100 to 800 million years after the planet’s formation…..

Mars protoplanet collision led to formation of Phobos and Deimos

… a large debris disk formed around the planet.

Several moons grew out of this debris. The gravitational pull of Mars brought most of these satellites back down onto the planet. Only the two most distant moons, Phobos and Deimos, now remain.

Phobos and Deimos in orbit around Mars

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