The red wanderer in our sky, Mars - once an omen of strife and misfortune due to its color, now a beacon to the hopes and dreams of mankind - holds the promise of a second home, a "boundless" frontier, and a new Earth if we dare to make one. Mars is exotic enough to be our greatest challenge yet, but not so hostile to make our aspirations on the Red Planet foolhardy. It is the testing ground, where humanity emerges both physically and psychologically into its role as a species committed to spreading itself into the entire solar system. While it may seem a relatively cold, frozen, dead world at first glance, look more closely and you will see something completely different: A world yet to be born.
The progress of our adventure so far (current in bold):
1. The Sun
2. Mercury
3. Venus
4. Earth (Vol. 1)
5. Earth (Vol. 2)
6. Earth (Vol. 3)
7. Earth (Vol. 4)
8. Earth (Vol. 5)
9. Earth (Vol. 6)
10. Luna
11. Mars (Vol. 1)
12. Mars (Vol. 2)
13. Mars (Vol. 3)
14. Phobos & Deimos
15. Asteroids
16. Ceres
17. Jupiter
18. Io
19. Europa
20. Ganymede
21. Callisto
22. Saturn
23. Mimas
24. Enceladus
25. Tethys, Dione, and Rhea
26. Titan
27. Iapetus
28. Rings & Minor Moons of Saturn
29. Uranus
30. Moons of Uranus
31. Neptune
32. Triton
33. The Kuiper Belt & Scattered Disk
34. Comets
35. The Stellar Neighborhood
In sections III and IV of the Mars sub-series, we examine the planet's properties and briefly review its orbital environment.
I. Context
II. History
III. Properties
IV. Natural & Artificial Satellites
V. Past Relevance to Humanity
VI. Modern Relevance to Humanity
VII. Future Relevance to Humanity
VIII. Future of Mars
IX. Catalog of Exploration
III.
Properties
1. Orbital and Rotational Features
At slightly less than 25 hours, the Martian rotational period is similar to Earth and is called a sol. Earth-based crews that remotely operate spacecraft on the Martian surface adjust their sleep schedules to be available when the probes are in daylight (and thus have access to solar energy), meaning they have to live on a 25-hour schedule along with the probes they operate and monitor. A Martian year is somewhat less than twice a terrestrial year (1 year, 320 days), and the planet has a similarly pronounced axial tilt at 25° compared with Earth's 23.5°. As with most planets, Mars rotates counterclockwise relative to solar North as per the conditions of its formation in the protoplanetary disk.
Apart from seasonal changes due to its axial tilt, Mars also experiences significant climatic effects due to the eccentricity of its orbit - i.e., the relatively large difference between its closest approach (perihelion) and furthest distance (aphelion) to the Sun. A change of roughly 43 million km occurs between perihelion and aphelion - a good 19% of the planet's semi-major axis (i.e., its longest orbital distance). As noted in Volume 1 of this sub-series, the amount of sunlight reaching Mars differs substantially between perihelion and aphelion: 713 W/m2 and 490 W/m2, respectively, alternately intensifying or moderating the effects of axial tilt on climate. Since hemispheric seasons don't perfectly align with perihelion/aphelion (known collectively as apses), the coldest winters occur when they coincide with an aphelion, just as the warmest summers coincide with a perihelion. Apsis effects are far less pronounced on Earth due to the relative circularity of its solar orbit, but Mars is somewhat more eccentric.
As Mars is not a particularly massive body, and has no large satellites to balance its rotation, its axis can wander periodically over millions of years by about ten degrees in either direction from its current tilt - a phenomenon known as nutation. During the periods where the tilt is more extreme the planet spends half its year under radicalized conditions with extended days and nights, and only has relatively normal conditions closer to equinoxes. Under these conditions, the extremes do not balance toward a "happy medium" - rather, the atmosphere likely freezes on to the surface on the months-long shadowed side, thinning the air all over the planet while building massive ice caps. About 40 such ice ages are estimated to have occurred in the past 5 million years. Mars is currently thought to be in a warming period. Via JPL:
An animation illustrating why rotational axes move over time:
Because Mars is outward from the Sun relative to our orbit and travels more slowly, the planet exhibits apparent "retrograde" motion from our location - i.e., it appears to move backwards in the sky when Earth passes it. This is because we are moving ahead of it, not because it is moving backwards, but against the background of the stars the apparent motion is the same. This behavior puzzled ancient astronomers, and contributed to Mars and similar objects being named Πλανήτης (planetes - "wanderers") since their motion was distinct from the stars.
2. Size and Mass Characteristics
Mars is half the size of Earth, but has only about 11% of its mass. Its surface gravity is roughly 38% of terrestrial (0.38 g), meaning that a 150 lb person would weigh 57 lbs on the Red Planet. It has ten times the mass of the Moon and is about twice as big with a little more than twice the gravity. Size comparisons with other solar system bodies in its class:
3. Temperatures
Temperatures vary widely between season, latitude, and altitude, with the warmest temperatures occurring near the ground in Summer at lower latitudes. These can be as high as −5 °C (23 °F), and may transiently rise above freezing, resulting in partial meltwater and seepage out of the ground in some places. However, just a meter above the ground the temperature may be substantially colder due to the thinness of the air and its inability to trap ground heat. Lows are about −87 °C (−125 °F) during polar winter, during which atmospheric carbon dioxide deposits on to the ice caps and forms dry ice layers that will later sublimate when conditions warm again. Varying states of the polar caps across seasons:
Temperatures also depend to some extent on the level of dust in the atmosphere:
4. Surface Features
Note that circular or flower-petal shape abnormalities on the surfaces of rocks are due to drilling by a rover's Rock Abrasion Tool or RAT to wipe off the surface layer of dust and examine the underlying rock. Most of these images are very high-definition, so you might see quite a lot more clicking on them and looking at bigger images - some are quite breathtaking. We see how diverse Mars is, both in the orbital and ground-based images: Boulder fields, dunefields, craters, pits, gently sloping hills, cliffs, rough sand or gravel, and varying colors and shades of rock.
Something very awesome: Avalanches in progress from orbit below. This is impressive because at Mars' relatively low gravity, they probably occur on walls that are steeper and couldn't be supported under Earth gravity. These avalanches occur mostly on ice walls retreating with the change of seasons:
5. Atmosphere
Atmospheric pressure on Mars is about 0.6% (6 thousandths) of sea level on Earth, and corresponds to what we would experience at a height of 35 km on this planet. Not only would it be insufficient to breathe even if the right gases were present, but the pressure is low enough that an exposed human would die quickly from explosive decompression - internal embolisms and hemmorhages. Sound could be vaguely heard on the Martian surface, but it would seem muted, and would be inaudible altogether over relatively short distances except for low-frequency transmission through the ground. Hurricane-speed winds occur during dust storms, but there is so little mass of air involved that a spacesuited human would only feel them as a modest breeze. The thin air means it heats up quickly at dawn and cools rapidly at sunset. More than 95% of the air is CO2 with most of the remainder being nitrogen and argon.
The Martian atmosphere is relatively simple compared to that of Earth or Venus, with little differentiation:
A little bit of water ice occurs in the atmosphere and more occurs underground, but there is no consistently running surface water both due to temperature and low pressure preventing a sustainable liquid state. Also, as the planet lacks a magnetosphere, over the long-term the solar wind has been desiccating the planet of water in a similar process to that described in the Venus diary: I.e., hydrogen is broken off of water molecules and blasted into interplanetary space, with oxygen left to recombine and form other compounds.
Contrary to how it is often depicted, the clear Martian sky is gray, but is tinted pink, yellow, or orange depending on the amount of dust in the atmosphere and the time of day. In a thick dust storm it turns as dark as wine, and surface lighting becomes very dim. Observe:
Due both to the thinness of the atmosphere and the fineness of Martian dust, mere solar heating is sometimes enough to keep dust particles suspended in the atmosphere for extended periods of time, and this can lead to worldwide dust cover on occasion:
One fascinating phenomenon that is sometimes captured by probe spacecraft are dust devils - whirlwinds that pull dust into a vortex pattern.
Color-enhanced view of cloud motion from Martian surface:
6. Possible Biosphere
Scientists remain divided on the likelihood of native microbial life on Mars, but while the evidence is not yet solid, most agree that the key factors for simple life as we know it have been observed on Mars: Liquid water, at least transiently; temperatures at which basic biochemistry is known to be sustainable; and ample mineral nutrients. The Martian-origin meteorite ALH 84001 contained what some scientifists considered sufficient evidence of microbial fossils having been brought to Earth from Mars, although doubt was later cast on the strength of this conclusion (though not on the credibility of Martian life as one explanation).
Since then, anomalous seasonally-associated fluctuatations in atmospheric methane have been observed on Mars that strengthened interest in the question of a Martian biosphere. Methane is quickly destroyed in the Martian atmosphere, so the only way it could be detected in significant concentrations over the long-term would be if an ongoing process is replenishing it. Such a process could be biological, or it could be geological, or both. Given that the methane releases peak in Summer, there are questions as to why that would be felt so strongly on a geological basis rather than a process whose chemistry is profoundly linked to the degree of available energy.
It seems reasonable that life would occur wherever it can sustain itself, so I would not be the least bit shocked if that turned out to be the explanation. I would, however, hope that such a discovery would not place Mars off limits to human settlement.
7. Magnetic Field
Mars lacks a magnetosphere, so its atmosphere interacts directly with the solar wind and is capable of losing water through hydrogen loss. However, like other bodies lacking a global magnetic field, there is localized magnetic activity due to heterogeneities in its crust and mantle which are left over from more active periods in its history.
---
IV. Natural and Artificial Satellites
Mars has two very small natural satellites, Phobos and Deimos, which orbit close to the planet and were either captured asteroids or chunks blown off the planet itself.
Comparative sizes:
The low gravity and closeness to Mars have caused some visionaries to see Phobos as an ideal staging area for operations on the Martian surface. A Russian probe intended for Phobos recently failed to leave Earth orbit and was declared a loss. There are about five or six artificial satellites currently in Mars orbit, some of which act as relay stations for ground craft.
