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. Phobos & Deimos
14. Asteroids
15. Ceres
16. Jupiter
17. Io
18. Europa
19. Ganymede
20. Callisto
21. Saturn
22. Mimas
23. Enceladus
24. Tethys, Dione, and Rhea
25. Titan
26. Iapetus
27. Rings & Minor Moons of Saturn
28. Uranus
29. Moons of Uranus
30. Neptune
31. Triton
32. The Kuiper Belt & Scattered Disk
33. Comets
34. The Stellar Neighborhood
Mars in true-color, as our eyes would see it from high Martian orbit:
I have decided that the material for Mars is lengthy enough to divide into volumes, so the topics addressed by the current volume are in bold:
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
I.
Context
Mars is the third-largest and third-most-massive rocky body in the solar system after Earth and Venus, and its orbit is the fourth closest to the Sun among planets. Its general region marks the outer boundary of the solar system's terrestrial zone, where large, rocky planets are able to form independently without either being captured as moons, swallowed, or in other ways disrupted by gas giants. Mars is also well within the Sun's habitable zone - the region where the right atmospheric chemistry can sustain temperatures and pressures suitable for liquid water. As a result, these conditions have been observed transiently on the Martian surface on a seasonal basis, with small trickles of water remaining liquid long enough to flow for a few seconds on the surface before boiling in the thin air. Diagrams of Mars' orbital location and path of motion, from a receding perspective:
The orbital eccentricity of Mars - its deviation from perfect circularity - is second only to Mercury among solar system planets, and contributes substantially to seasonal climate variations. Its perihelion (closest approach to the Sun) is about 42.5 million kilometers closer than at aphelion (the furthest point in its orbit) - a distance great enough to have a significant impact on the amount of sunlight reaching the surface. Solar flux - the amount of light reaching the vicinity of Mars per unit area per unit time - is about 713 W/m2 at perihelion and 490 W/m2 at aphelion, as compared to the average flux at Earth of about 1361 W/m2, which only changes a few percent over the course of its orbit. As a result of reduced sunlight, solar panels on Mars probes have to be as efficient as possible. A diagram comparing solar flux for the four inner planets:
As you can see, the intensity of sunlight falls off pretty rapidly as we explore further from the Sun, making nuclear power sources increasingly attractive as opposed to solar panels. Mars, however, is close enough that solar remains practical as a power source for spacecraft and landers, and is often a good bargain in exchange for much lower development and regulatory costs. One visual consequence of distance is that the Sun appears somewhat smaller on Mars than on Earth:
Thanks to the ongoing Mars Exploration Rovers (MER) program, there are a few images and videos of the Sun from Mars that give us a clear idea of how it looks under diverse conditions:
Something I find interesting is that in the second video, the Sun is a clear disk until it disappears behind dust near the horizon, but in the third video it begins as diffuse and only becomes clear as it approaches the horizon. There must be quite a bit of variability in the density of atmospheric dust at any given time and location. Another interesting thing is that the Sun doesn't seem to have glare at this distance, but is a distinct disk of light. A human could probably look directly at it for several seconds through UV-polarized glass without damaging their eyesight.
From the surface of Earth, Mars is a bright red dot that can just barely be distinguished as a disk through high-end amateur telescopes. However, its brilliance, unusual color, unusual motion relative to the stars, and its modern significance as a planet of hope and future adventure make it a popular target of amateur astronomy. Here is an image of the Moon, Venus, and Mars in the terrestrial sky, with Mars being the humbler object to the right:
Earth is likewise merely a dot in the Martian sky, contrary to much woefully inaccurate and fanciful space art. It is, however, close enough to vaguely discern two lobes to the bluish-white "star" - one being the Moon:
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II. History
Like the rest of the inner solar system, Mars formed by a process of accretion about 4.5 billion years ago (commonly denoted Gya, or giga-years ago). Hot, gaseous metals in the Sun's protoplanetary disk cooled sufficiently to form liquid droplets - metallic "rain" - that collided and coalesced into larger blobs, which subsequently cooled further and became solid dust particles. Those particles then collided and stuck together, forming larger and larger bodies until single bodies dominated the orbits now associated with each inner planet. The transition of protoplanetary gaseous metals into liquid and then solid is known as precipitation, and is more or less the same phenomenon as water rain on Earth - i.e., declining temperatures causing a phase change.
Precipitation occurred first on the outer reaches of the disk, well beyond the current planetary region, and swept inward as it cooled, so the inner planets formed a bit later than the solid bodies of the outer solar system, while the gas giants had formed even earlier via a different process than accretion. The outer solar system was so rich in mass, and became cool enough quickly enough, that solid cores accreted very rapidly and were able to cause their region of the disk to gravitationally collapse around them, forming massive gaseous envelopes. This is different from accretion because the gases never precipitated or coalesced, but remained in a free gaseous state that is bound only by gravity rather than structure. I will deal with gravitational collapse in more detail in the gas giant entries in this series.
The inner disk, however, never cooled enough for gravitational collapse to occur: The vast majority of the hydrogen and helium that originally existed was too energetic to stay bound to the weak gravity of the planetary cores, and was blown away by the solar wind into cooler, more distant regions. As a result, the inner planets have a comparatively thin and relatively slow-moving "skin" of heavier gases above solid surfaces (particularly CO2, although in Earth's case the dominant chemistry is N2 and O2) that their weaker gravities are able to keep.
About 90% of the mass of the inner solar system ended up accreting into Earth and Venus, with the remainder divided unequally between Mercury and Mars on the inner and outer edges of the region, respectively. Mars may have gotten the better end of the deal due to being closer to the Main Belt asteroids, and thus having access to more accretion mass (i.e., impactors) over its entire history than Mercury.
Mars may also have benefited from having only about half the average orbital speed of Mercury (24.1 km/s compared to 47.9 km/s): This would reduce the kinetic energy of typical impacts, thus increasing the proportion of an impactor's mass that is kept by the planet rather than being blown off, and also reduce the chances of actually losing mass in the process. There is also the fact that Mars is much further from the Sun than Mercury, and thus objects entering its orbit from further out in the solar system would not have picked up nearly as much kinetic energy from the solar gravity well (see the Mercury diary for a more detailed explanation).
Martian geologic time is divided into four high-level epochs: Pre-Noachian (formation - 4.1 Gya), Noachian (4.1 - 3.7 Gya), Hesperian (3.7 - 3.0 Gya), and Amazonian (3.0 Gya - present). The most significant post-formation event of the Pre-Noachian epoch was the creation of the Martian dichotomy - the radical differentiation of terrain between the Northern and Southern hemispheres, with the North being dominated by comparatively smooth, lowland plains and the South rugged, heavily-cratered highlands. This is similar to the dichotomy between the lunar Near and Far Sides, and both cases are thought to have been the result of giant, transformative impacts thinning the crust in one hemisphere enough to allow lava plains to dominate the terrain. If Mars is ever terraformed, the dichotomy will have major consequences for the coastlines of future Martian oceans. An elevation map illustrating the feature:
From the above map, you can also see the legacy of the Pre-Noachian period's terminating event - the formation of Hellas Planitia or the Hellas Impact Basin 4.1 to 3.8 Gya: The huge purple splotch in the Southern hemisphere. It is thought to have formed during the Late Heavy Bombardment period of the solar system, which also accounts for a number of features on other bodies. Just to be clear, this impact is unrelated to the dichotomy, and believed to have occurred long after its formation.
The Hellas Basin is the largest visible crater in the solar system, and one of the largest impact features known to exist (a few others are bigger whose existence is deduced rather than being visually obvious). Despite being in the midst of the Southern highlands, the floor of the basin is 7 kilometers beneath mean altitude (the "sea level" used for bodies without oceans), and close to 8 kilometers beneath its surrounding rim. If Mount Everest were placed in Hellas Basin, its summit would just barely peek over the rim.
An orbital image of Hellas Planitia taken by one of the Viking orbiters, followed by closeup elevation maps:
The Noachian epoch from 4.1 to 3.7 Gya is when the Martian surface became generally stable enough for craters to survive into the present, but remained dynamic enough to produce features that only rarely formed afterward: E.g., features associated with large bodies of liquid water, rivers, and highly active volcanoes. This is not to say these processes weren't also at work in the Pre-Noachian, but conditions were so violent that their products generally did not survive. If microbial life exists or previously existed on Mars, its heyday would most likely have been in the Noachian epoch. An artist's conception of Mars at the time, by Daein Ballard:
The epoch receives its name from Noachis Terra - a large region to the West of Hellas with many water-related features such as river deltas, gullies, and eroded valleys. Although the Southern hemisphere would have been less affected by water than the lower-elevation North, ironically the South is where most Noachian features are found because they were less likely to be buried beneath lava plains or sand dunes. An image of Noachis Terra via the European Space Agency's (ESA) Mars Express orbiter:
Examples of water features associated with the Noachian epoch - gullies, delta fans, and river valley networks (first image is true-color):
Of all Noachian features, the Tharsis Bulge is most prominent. It is a huge volcanic upland, and can be seen in the first topographic map shown above as the vaguely triangular, red region near the equator with a string of volcanic mountains to the North. The region is thought to be the surface manifestation of a large magma plume (see Volume 2 of the Earth sub-series for a discussion of magma plumes). As it occurred in the absence of plate tectonics, pressure from the plume built up in one region rather than migrating over time as in the Hawaiian islands on Earth, causing the crust to bulge outward. The bulge weighs so heavily on the crust that surrounding regions sag along its borders. Topographic maps of Tharsis:
However, the major volcanoes associated with Tharsis had not yet formed in the Noachian. As Mars transitioned into the Hesperian period between 3.7 and 3.0 Gya, the magma plume beneath the region began to cool, causing the crust to thicken and forcing magma still under pressure to gradually burst through in several massive shield volcanoes along its periphery. These are the Northwestern volcanoes seen in the above Tharsis maps, with the three that form a line known collectively as the Tharsis Montes (Montes is the plural of Mons): From Southwest to Northeast, Arsia Mons (16 km above mean altitude), Pavonis Mons (14 km), and Ascraeus Mons (18 km). Olympus Mons (21 km) - the highest mountain in the solar system - is to the Northwest of the Montes, and separated from the main bulge of Tharsis by midlands. Olympus Mons is more than twice the height of Mount Everest, and its caldera is more or less outside the Martian atmosphere. Closeups of the Tharsis Montes and its constituents, including Olympus Mons:
The largest and most distinctive feature of the Martian surface was also formed in the Hesperian epoch, and also resulted from the geology of Tharsis: The Valles Marineris - one of the longest and deepest systems of canyons in the entire solar system, running as a 4,000-kilometer-long scar across the Northeastern corner of Tharsis, with canyon walls as much as 200 km apart and 7 km deep. It is visible in the above topographic maps, and is the most prominent feature in the full-globe image of Mars at the top of this discussion. If one were to stand on the edge of Valles Marineris, the other side would be well beyond the horizon. Current theories about its formation hold that as the crust over Tharsis thickened, continued upward pressure caused it to rupture in the Northeast and form a rift valley rather than filling with lava and forming a midland plain as may have happened when the crust was thinner. A false-color orbital image, followed by topographic and sub-feature maps:
Hesperian features are strongly associated with volcanism and crustal fracturing such as wrinkle ridges, since the period was characterized by internal cooling (and thus, ironically, more shield volcanoes as remaining heat has to force its way through thicker crust), cooling of the climate, thinning of the atmosphere, and water becoming a much rarer influence. The period is named for Hesperia Planum, a wide lava plain to the Northeast of Hellas characterized by volcanic geology - smooth high-level terrain, ridges formed by internal collapse, relatively young crater populations (because older ones were smoothed over), and so on. A Viking image of Hesperia, followed by a labeled topographic map and an image of characteristic wrinkle ridges:
By the end of the Hesperian and the beginning of the current Amazonian period about 3.0 Gya, the large-scale features of Mars were essentially in place. However, it wasn't until the early Amazonian that Mars actually turned red - the iron oxides that give it its rust coloration had not yet accumulated, and the oxygen that would later combine with iron in the rocks was still bound in atmospheric gases. It would probably have been a relatively bland-looking mixture of grey, brown, and beige rock and sand, much like some areas of the planet still look that have less than average quantities of oxidized dust. But as Mars lost any counterbalancing chemical process - i.e., as volcanism died down, the rate of impacts declined, and water boiled off or froze - oxides in the atmosphere gradually migrated into the rocks, turning them rusty while at the same time thinning the air.
Volcanism, glaciation, and features associated with running liquid water continued to occur in the Amazonian, but very slowly and on much smaller scales. It is named for Amazonis Planitia - an unusually smooth and young lava plain to the West of Olympus Mons. Although the region itself is only thought to be about 100 million years old, other regions very similar to it go back much further and thus its namesake period is billions of years long. A Viking orbital image of Amazonis with thin cloud cover, and a topographic map:
Tune in next time for Volume 2, when we delve into the in-depth properties of the planet and explore its surface.
