About Mars: A Review of the Characteristics of the Martian Planet

Abstract

The colonization of Mars has been a topic of much discussion, and in recent years the media has had plenty to publish on this topic making this venture appear to be relatively easy. However, despite Mars being a neighbouring planet, it is an alien planet with an inhospitable environment. This paper — based on published research and data — highlights the major characteristics and challenges of the Martian environment as compared to Earth. Mars’ orbit is more elliptical in shape than that of Earth. In addition, the two planets are rarely in orbital positions that allow missions to launch and arrive at the red planet. These launch windows occur approximately every two Earth years. Also, Mars’ characteristics are far different from Earth’s. On the Martian surface, objects experience approximately 38% of Earth’s gravitational field. Furthermore, the Martian atmosphere is thin, nearly 100 times less than Earth’s, and the composition of the two atmospheres is quite different: Mars’ atmosphere is composed mainly of 95% carbon dioxide, while Earth’s atmosphere is largely 78% nitrogen and 21% oxygen. The thin atmosphere and the atmospheric composition make the Martian atmosphere a poor absorber and retainer of solar heat. Moreover, Mars lacks a global magnetic field to protect its surface and atmosphere from harmful radiation. The eccentric orbit and weak greenhouse effect both play a role in causing severe climate fluctuations that vary during both the diurnal cycle and the seasonal cycle. The only major similarities are the planets’ axial tilt, polar ice caps, and the duration of each planetary day. The journey to Mars and its colonization can only be possible if we can surmount the obstacles and hazards presented by this alien planet.

Introduction

The Red Planet has been the object of dreams and ambitions for many decades. It is our closest neighbouring planet in the solar system and represents the next step in mankind\’s future expansion. It is therefore important to understand the basic characteristics of Mars and how they may impact current and future plans of making it a habitable destination for humans.

As humans, we have adapted to life on Earth. Our survival, as well as the survival of all life, is dependent on the characteristics of our home planet. Earth orbits the Sun at just the right distance for us to survive. Earth’s atmosphere is composed of the right gases for trapping just enough of the Sun’s warmth and maintaining relatively stable climates. Earth’s gravity is not too strong and not too weak allowing us to stand upright, build skyscrapers, and fly aeroplanes. Its magnetic field shields us from solar radiation that would cause devastating damage.

Orbits and Rotations

Mars is the fourth planet from the Sun; it is located between Earth and the asteroid belt at an average distance of 228 million kilometres from the Sun (1.5 AU), as compared to Earth’s average distance of 149.7 million kilometres, defined as 1 astronomical unit (AU)^[1]. The orbits of these two planets are not perfectly circular, but rather elliptical in shape. Elliptical orbits are characterized by their closest distance from the Sun (perihelion), their furthest distance (aphelion), as well as their eccentricity — a property that describes the extent to which an ellipse deviates in shape from a perfect circle into an ovular or elliptical shape^[2].

Figure 1. Planets in elliptical orbits have varying distances from the Sun. The aphelion is when the planet is at its furthest distance from the Sun, while the perihelion is when it is at its closest distance from the Sun. (Illustration not drawn to scale.

 

Table 1. Distances and average speeds of Earth and Mars[3,4]
	Perihelion (AU)	Aphelion (AU)	Average distance from the Sun (AU)	Average orbital speed (km/s)
Mars	1.38	1.67	1.53	23.34
Earth	0.98	1.02	1.00	29.78

 

 

Table 2. Earth and Mars’ day and year durations
	Day	Year	Close approach cycles
Mars	24.6 hours	669.6 sols	802.8 sols
Earth	23.9 hours	365.3 days	780.0 days

 

The different average distances from the Sun, different eccentricities, and the different average orbital speeds give Earth and Mars varying orbital trajectories around the Sun, with Mars having an orbital period of 687 Earth days, which is about 1.9 times longer than that of Earth. In terms of Martian days, referred to as sols, a Martian year is 669.9 sols^[1]. These differences imply that Earth and Mars are not always in close proximity and that their location, relative to one another, changes as they move through space, making the planning of any trip between the two planets challenging; as the spacecraft must travel between two moving objects in space that are travelling at different speeds along different elliptical orbital paths. However, upon arriving at Mars, there is good news; the day on Earth is quite similar to the day on Mars (sol): 23.9 hours and 24.6 hours, respectively. This means that, from this aspect, humans can adjust to living on Mars; their natural biological clocks can integrate with that of the Martian day.

A journey between Earth and Mars should be planned and executed such that the distance travelled by spacecraft is short, the use of energy and fuel is efficient, and that the spacecraft arrives at its target destination and does not miss it; the spacecraft must arrive at the specified point in space at the precise time. The basis of this journey lies in the Hohman transfer, described by Walter Hohman in 1925. The transfer describes an efficient method of transferring directly from the orbit of one planet into the orbit of another. The trajectory of this spacecraft is elliptical in shape as well and is referred to as the transfer ellipse^[2,5].

Figure 2. A flight from Earth to Mars is essentially a transfer from Earth’s orbit to that of Mars’. This is referred to as a Hohmann transfer, and it requires the least amount of energy to travel between the two planets.

 

As the distance of each planet from the Sun varies, the distance between the two planets does as well. This implies that at some point during their orbits the two planets are closer to each other; this is referred to as “close approach”, which is approximately 54.6 million kilometres. The term “approximately” is used to indicate that not every close approach is of the same distance. However, while close approaches may vary in distance, their frequency is relatively constant at about 26 Earth months, or 780 days^[6]. Close approaches offer opportunities for travel between the two planets. Knowledge of close approaches has important implications for estimating launch windows for trips between Earth and Mars.

 

Table 3. Upcoming Dates for perihelions, aphelions and close approaches[7,8,9]
	Perihelion date	Aphelion date	Close approach date
Mars	July 21, 2022	July 12, 2021	December 8, 2022
Earth	January 4, 2022	July 5, 2021	December 8, 2022

 

Mass and Gravity

Mars\’ mass is 6.42 x 10²³ kilograms, about 10 times less than Earth; this gives the planet a weaker gravitational field^[4]. On Mars, objects experience a 3.72 m/s² gravitational field, which is about 38% of that on Earth. This is because the gravitational field induced by an object, such as a planet, is directly proportional to its mass. With Mars having a smaller mass it will have a weaker gravitational field as compared with Earth. This would have numerous implications on fields such as transportation and architecture that are heavily influenced by Earth’s gravity: rocket ships and buildings would weigh less and would probably be able to explore new and alien designs. Similarly, a human with a mass of 70 kilograms would weigh approximately 687 N (Newtons: the unit of force) on the surface of Earth and 260 N standing on Mars. In practical terms, it would be far easier to stand upright on Mars than on Earth.

 

Some Calculations The gravitational field of a spherical mass, such as a planet, as felt at its surface is described by the formula , where g is the gravitational field, G is the gravitational constant, m is the mass of the object inducing the field, and r is its radius.     Mass (kg) Radius (m) Gravitational field (m/s2) Mars 6.42 x 1023 3.39 x 106 3.72 Earth 5.97 x 1024 6.37 x 106 9.81

 

Microgravity is when an object or person feels weightless. When standing on the surface of Earth, humans feel their weight because the Earth’s surface pushes back. However, in orbit, both the astronauts and the spacecraft fall towards Earth with the same acceleration, so the spacecraft does not push back. Therefore, the astronauts appear to be weightless and experiencing microgravity^[10].

 

Note The ISS and its occupants, although in free-fall, do not actually fall to Earth; they stay in orbit. This is because of their momentum, which has two components: radial and tangential. The radial component is equal in magnitude and opposite in direction to the pull of Earth’s gravity; the two cancel each other out, so the ISS and its occupants do not fall to Earth or fly off into space. They are left travelling in an orbit driven by the tangential component of their momentum that keeps them moving along a circular path around Earth. Credit: Image was adapted from Dreamstime[31]

 

So far, humans have not experienced Martian gravity, but they have experienced microgravity while on the International Space Station (ISS) in Earth’s orbit. From experience aboard the ISS, it has been established that prolonged exposure to microgravity has damaging effects on the human body. The effects include: loss of calcium from bones leading to weaker bones, smaller and weaker muscles, a decrease in blood plasma volume, and deconditioning of the heart and blood vessels that can lead to an inability to tolerate changes in body position^[11]. This implies that plans for travelling to and inhabiting Mars must take into account protecting human biology from any effects of the microgravity of interplanetary space travel and possibly the lower Martian gravity experienced while living there.

Atmospheres and Greenhouse effects

The Martian atmosphere is very different from that of Earth. On Mars, the atmospheric surface pressure is about 0.01 bar which is 100 times less than Earth’s 1 bar^[12]. Earth’s atmosphere is composed mainly of nitrogen and oxygen having 78% and 21%, respectively; it also contains trace amounts of other gases such as argon, carbon dioxide, and water vapour^[13]. The Martian atmosphere is currently mainly composed of 95% carbon dioxide; it also contains 2.8% nitrogen, 2% oxygen, 2% argon, and trace amounts of carbon monoxide, and water vapour^[14].

The greenhouse effect is the trapping of solar heat by gases in the atmosphere; the greater the amount of certain gases present in the atmosphere, the more heat is trapped. Both Mars and Earth’s atmospheres display greenhouse effects. Carbon dioxide acts as a greenhouse gas absorbing about 20% of heat radiating from the Martian surface. However, due to the absence of significant amounts of other greenhouse gases, such as water and methane, the Martian greenhouse effect is weaker than that of Earth’s, with net global warming of around 5-6K on Mars and 30K on Earth^[15].

It is important to note that the content of carbon dioxide in the Martian atmosphere is about 37 times more than that of Earth’s. When content is measured as a vertical column of atmosphere, the Martian atmosphere contains 150 kg/m² as compared with just 4 kg/m² in the atmosphere of Earth. However, Earth has 400 to 4,000 times more water in its atmosphere than does Mars. In addition to the greater content of water, water as a molecule acts as a better absorber of infrared radiation than carbon dioxide. Both the greater content of water and its property as a stronger greenhouse gas makes the greenhouse properties of Earth’s atmosphere greater than that of Mars^[15].

 

Table 4. Atmospheric content of carbon dioxide and water
	Vertical Column Mass of CO2 (kg/m2)	Vertical Column Mass of H2O (kg/m2)
Mars	150	0.01 – 0.1
Earth	4	40

 

The Martian atmosphere will have a significant impact on any plans to inhabit the planet. The lack of adequate levels of oxygen and its weak atmospheric pressure makes it necessary for equipment to create and maintain artificial atmospheres within living habitats for both humans and plants, that replicate the oxygen-carbon dioxide cycle of life that exists on Earth. This technology might even take advantage of having large amounts of carbon dioxide present in the atmosphere. Technology may even advance to the point where humans are able to terraform Mars completely and create a hospitable atmosphere.

Magnetic field and Magnetosphere

Radiation is a general term that encompasses a wide spectrum of energy and particles naturally present in space. There are two main types of radiation: non-ionizing radiation and ionizing radiation. Ionizing radiation, or high energy radiation, is in the form of alpha particles, beta particles, gamma rays, X-rays, and galactic cosmic radiation; this form of radiation consists of particles that have high enough energy to remove electrons from the atoms and molecules creating positively charged ions. Non-ionizing radiation, or low energy radiation, is in the form of radio waves, microwaves, infrared, visible light, and ultraviolet light; non-ionizing radiation consists of charged particles that do not have enough energy to strip electrons from the atoms and molecules it encounters^[16].

The Earth is shielded from radiation by both its atmosphere and its magnetic field. Earth has a global magnetic field that surrounds it and extends thousands of kilometres into space^[17]. Earth’s electrically charged internal liquid core, made of mainly iron and nickel, spins to generate its magnetic field; this is called the dynamo effect.

According to recent data, Mars’ core — consisting primarily of iron, nickel, and sulfur — is only partially liquid^[18]. Consequently, it cannot spin and therefore cannot create a magnetic field.

However, Mars still has some magnetic field referred to as the induced magnetosphere. Solar winds collide with the upper atmosphere of Mars. As a result, electric currents pile up in the ionosphere creating the induced magnetosphere^[19].

However, this process is gradually depleting the planet of its atmosphere and is likely to have played a role in the transformation of Mars into the red desert it is now^[19]. After a Martian year of monitoring the red planet, the MAVEN (Mars Atmosphere and Volatile EvolutioN) orbiter recorded an average of 1-2 kg/s of gas lost to space^[21].

In 2001, while in orbit around Mars, the Odyssey conducted the Mars Radiation Environment Experiment, MARIE. It detected radiation levels of about three times greater than those aboard the ISS. These elevated levels of radiation were attributed to Mars’ lack of a protective magnetic field^[20].

 

Credit: NASA/JPL/Johnson Space Center[20]

 

Both the depleting atmosphere, not to mention the lack of oxygen, and the absence of a global magnetic field pose significant obstacles that must be circumvented to make Mars a habitable planet. How heavily will these impact the design of habitats? Will countermeasures against radiation limit the architectural freedom given by the low gravity?

Climate and Cycles

Understanding the climate on Mars plays a significant role in determining the strategies and plans required to make life for humans on the planet feasible. Based on the observations and data collected during the various missions to Mars, scientists have learned that the climate on Mars is regulated to a certain extent by the carbon dioxide, water, and dust cycles along with other processes^[22].

Mars orbits the sun with a similar axial tilt to Earth: 25.2° for Mars as compared to 23.4° for Earth. This gives Mars a similar succession of seasons. However each Martian season is approximately twice the duration of Earth\’s seasons due to its larger orbit, and the seasons on Mars are not of relatively equal duration given the greater elliptical eccentricity of the Martian orbit. Like Earth, Mars’ northern hemisphere experiences opposite seasons relative to its southern hemisphere^[23]. But due to its axial tilt, and elliptical orbit this gives the red planet’s northern hemisphere colder and longer summers and shorter and warmer winters than the southern hemisphere. This seasonal variation affects the temperature of the surface and atmosphere; it results in three major observed cycles on Mars: the carbon dioxide cycle, the water cycle, and the dust cycle.

 

The seasons on Mars are divided based on solar longitudes (Ls). In the northern hemisphere, the spring equinox is at Ls = 0°, the summer solstice is at Ls = 90°, the autumn equinox is at Ls = 180°, and the winter solstice is at Ls = 270°. Credit: NASA/JPL

 

Throughout a Martian year, about 30% of carbon dioxide is cycled through the polar caps, which act as reservoirs for carbon dioxide. During the winter of each pole, the lower temperatures allow carbon dioxide in the atmosphere to freeze and deposit as seasonal ice caps on top of the existing residual ice caps of frozen water. This layer of dry ice covers a large area at each pole. In the northern hemisphere, it can extend to reach 60° latitude. The northern ice cap is larger than that of the south; their respective diameters are about 1,100 km and 400 km. During summer, the seasonal ice cap sublimates in the now-higher temperatures, leaving the residual ice cap that is made of water ice^[24]. As carbon dioxide is being vaporized at one pole it is being frozen at the other; this cycling allows the atmospheric pressure to remain roughly constant over the year.

Water follows a similar annual cycle as carbon dioxide. Both polar ice caps act as reservoirs for water and carbon dioxide and are affected by seasonal changes in solar insolation and surface temperatures. As a result, during the winter seasons at each pole, the poles experience low temperatures allowing both water and carbon dioxide to condense forming a residual icecap. The reverse process occurs in their summers: as temperatures rise, water and carbon dioxide sublimate and enter the atmosphere. In addition to the poles, surface regolith — a blanket of particles made up of silicate dust and sandy materials — at lower latitudes can absorb and release water due to its porous nature. This results in seasonal changes in surface relative humidity as observed by the Mars Science Laboratory (MSL), the Curiosity rover. The relative humidity is higher and more variable during the summer; it peaks at about L_s = 90° with values of up to 70% recorded. In the winter, relative humidity is lower and more stable with values recorded below 20%^[25].

Water and carbon dioxide can both form ice clouds in the atmosphere. They mainly form over the polar regions in the winter and autumn seasons, forming a cloud coverage at the poles called the Polar Hood. Water and carbon dioxide clouds also form at lower latitudes when atmospheric conditions permit. Water ice has been observed to form early morning fog and is commonly associated with seasonal and topographic features, such as at the Tharsis Plateau during aphelion when the atmosphere is cooler and richer in water^[15].

The Mars dust cycle has also been observed to play a role in the Martian climate. The surface of the planet is essentially a global desert of rock and regolith, which is composed of fine particulate material easily picked up by wind currents in one location, transported, and deposited at another. Not very different from Earth which has an average wind speed of about 11.84 km/h, wind currents near the surface of Mars have speeds typically between 16 and 32 km/h^[26,30]. Wind speed during dust storms can reach 100 km/h. Low atmospheric density means that there are not many particles of air within a certain volume. As a result, even such high wind speeds are only enough to kick up dust and particles on the surface^[27].

The radiative heat from the sun is what causes nearly all wind and airflow on Mars, including dust storms. The surface regolith of Mars has low thermal inertia; it absorbs heat from the Sun easily. Heating of the surface regolith can cause air currents by heating the atmosphere around it and causing it to rise. Then, cooler air gets pulled in to replace it. This motion produces winds and airflow, which can also cause whirlwinds called dust devils^[27]. Larger dust storms typically happen in summer in Mars’ Southern Hemisphere, when the temperatures are at their yearly peak^[26,27]. Larger storms sometimes converge into global storms that can last for several weeks or months^[28].

Dust, in general, is a problem on Mars. As on the Moon, dust is slightly electrostatic, so it sticks to surfaces it comes in contact with. Its grit can cause damage to the moving parts of machinery, and it can be a big problem for solar panels by blocking out sunlight and significantly reducing the amount of solar energy being harnessed^[28].

Temperature Variations

Mars experiences a wide variation in temperature during a day, throughout the year, and across latitudes. From the equator to the poles and from day to night, Martian surface temperature varies significantly. The temperature on Mars ranges between -140℃ and 30℃, with an average of -63℃^[14].

While Mars exhibits near-surface temperature differences between latitudes and altitudes, these factors are not enough to explain the wide fluctuations. Three other factors can explain the wide fluctuations of temperatures observed near the surface of Mars: a greater orbital eccentricity, a thin atmosphere with a limited greenhouse effect, and the lack of large bodies of liquid water, such as oceans.

On Earth, the majority of the surface is covered by liquid water. Water is known to have higher specific heat capacity than sands, soils, and other minerals. This means that water is harder to heat and once heated it releases its heat more gradually than the surface of the land. During the day — periods of insolation — both land and water absorb heat, but land absorbs more solar heat than the water and quickly releases it into the surrounding atmosphere. On the other hand, water absorbs less solar heat, but it retains it for longer periods releasing gradually through the night hours. This is why areas nearer to oceans, seas, and lakes, often experience less diurnal temperature fluctuations than the dryer, more inland areas, such as deserts. Mars has no oceans, it is a desert and behaves as such exhibiting diurnal cycles of rapid heating and cooling. These fluctuations are further intensified by the thinness of the atmosphere and its lack of a greenhouse effect which would have assisted in trapping more heating and limiting temperature fluctuations^[29].

Moreover, the eccentric orbit of Mars plays a role in causing large seasonal temperature changes. At the perihelion (L_s = 270°), the Martian atmosphere is exposed to the greatest amount of solar insolation, as compared to its aphelion (L_s = 90°) where solar insolation is less. This results in greater temperature variations between seasons across the globe.

 

Credit: Martinez, 2017[22]

 

The Mars Science Lab (MSL) rover, Curiosity, given its location in the southern hemisphere, presented temperature measurements and an annual cycle that are reversed as compared with the Viking Lander rovers located in the northern hemisphere. During aphelion, daily mean air temperatures measured at different latitudes are closer in value than those measured during perihelion when temperature variability is accentuated. Also, the daily temperature variation at the Viking Lander 2 becomes more pronounced during perihelion as compared to measurements obtained by MSL at a more equatorial latitude^[22].

 

Table 5. The variation in near-surface temperature between the aphelion and perihelion of Mars’ orbit as recorded by three different landers at different latitudes.
	Aphelion Estimated Near-Surface Temperatures Ls = 90° (K)	Perihelion Estimated Near-Surface Temperatures Ls = 270° (K)	Temperature difference between aphelion and perihelion (K)
Mars Science Lab (4.6°S)	210	230	20
Viking Lander 1 (22.4°N)	215	195	20
Viking Lander 2 (47.9°N)	220	165	55
Note – The temperature values used in this table are visually estimated from the chart above.

 

Diurnal and seasonal temperature changes are important to consider when deciding on locations for establishing habitats, the life-support system of habitats which includes insulation, heating and cooling, the design of spacesuits, and the planning of outdoor operations.

Conclusion

Not only will Mars colonization drive unfathomable scientific and technological breakthroughs throughout numerous fields but also will drive human societies to adapt, expand, and mature. A few centuries ago, travelling beyond the horizon was thought impossible. Nevertheless, through ingenuity, dedication, and an undying curiosity, humans developed technologies that enabled them to traverse and explore the horizons of Earth. At the current rate of development, it won’t be long before humankind is travelling beyond new horizons.

Inhabiting Mars will not be an easy undertaking. It is an alien environment to which life must adapt. As this paper has demonstrated, nearly all aspects of Mars and its environment pose challenges to humans and Earth-based lifeforms. Moreover, the distance between the planets, the varying elliptical orbits, and planetary orbital speeds pose a challenge to getting to Mars and back. Developing frequent, low-cost, low-energy transit between Earth and Mars’ orbits will be a critical first step towards colonization. Once there, people will need to adapt to the low gravity. This will affect many aspects of their biology, daily activities, and even the structures they will build. They will also be exposed to a thin atmosphere lacking in oxygen and a desert environment unsupportive of liquid water. Finding a sustainable and scalable source for each will be vital to survival. Moreover, due to the lack of a strong magnetic field, the surface of the Red Planet is soaked with harmful radiation. Shielding from ionizing radiation and cosmic rays must be included in the designing of spacesuits and habitats. Seasonal changes are more extreme due to the greater eccentricity of the planet’s orbit and the lack of a strong atmospheric greenhouse effect. Equipment, machines, habitats, and people, will have to contend with extreme temperature fluctuations and dust storms; this will require specialized insulation and protection.

So, how will humans create for themselves a new home on Mars? Will humans develop the technology to terraform Mars, changing it entirely to suit their needs and wants? Will life itself change and adapt to the Martian environment, becoming more radiation-tolerant, less in need of oxygen and water, and better adapted to weaker gravity? Or will something entirely different happen?

References

Remark: References with multiple numbers within the brackets — example: [2,5] — imply that both references were used as sources of information for the preceding section.

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About the Author

Elias is a Mars enthusiast who has always dreamed about going to space and playing his part in developing the human presence in space. He is a high school student who enjoys learning about science-related topics, reading, drawing, and cooking.