Aside from Earth, Mars is the world that has been the subject of more speculation—and in modern times, more robotic exploration—than any other planet in our solar system. Although it is actually quite cold, it nevertheless lies in the outer half of the so-called “habitable zone” around the Sun. If it had a thicker atmosphere, it could conceivably be a cool, but livable world. In fact, until modern observations showed that its atmosphere was too thin and lacking in oxygen, many writers of speculative fiction assumed that it was inhabited.
The surface of Mars is largely barren and covered with rocks. Craters pock the face of the planet. Nevertheless, aside from Earth, Mars has the friendliest environment in the solar system for visiting probes.
Mars is subject to dust storms. No doubt, these dust storms account for much of the variations in the surface that were seen in telescopes before space probes ever went to investigate. And when one orbiter arrived, it found the entire planet blanketed in dust, a phenomenon previously unimagined. Only four spots showed through the haze, which turned out to be the tops of four enormous volcanoes. One of them, Olympus Mons, is the largest volcano in the solar system. None of these volcanoes appears to be active today; in fact, there does not appear to be any active volcanism on the planet. This does not mean that there is no geologic activity at all. Recently, an orbiting probe observed a large, 700 meter high avalanche.
The overall elevation of the entire northern hemisphere of Mars is notably lower than the southern hemisphere. This has recently led to the theory that an immense asteroid—or even a planetoid around the size of Pluto—once smashed into the northern polar regions, i.e. the northern hemisphere is one titanic impact crater. There are a couple of problems with this—objects in the solar system tend to orbit in the plane of the ecliptic, and if something that big really had hit Mars in the north pole, it should have knocked its orbit somewhat askew. However, it is also possible that the impact was equatorial, and that the crust of the planet gradually shifted into a more dynamically stable configuration afterward.
Scientists have long suspected that Mars once had large amounts of water, and had theorized that it still has water today, in the form of permanently frozen polar caps and permafrost. Both of the Mars Exploration Rovers, as well as the Mars Science Laboratory rover, discovered strong evidence of past water at their respective sites. In 2008, the Phoenix lander arrived in the northern polar regions and quickly found large quantities of ice only inches below the surface, thus confirming the theories about the presence of frozen water today. However, because of the extremely thin atmosphere, liquid water is impossible; it would immediately evaporate. So today Mars is drier than the driest desert on Earth. And the thin atmosphere and lack of magnetic field also provide practically no protection against solar ultraviolet radiation and cosmic rays, both of which are deadly to the complex molecular structures that make up living organisms.
Mars is an interesting place. Mars may someday be our best bet for setting up human colonies, although they would still have to be under pressurized domes that would also provide protection from radiation. Nevertheless, in its own right Mars is a dead, hostile world, without life.
Mars has two small moons, Phobos and Deimos, which are asteroidal in nature. The most common theory states that they were captured from the asteroid belt. Photos taken from orbiting probes reveal a pair of irregular, crater-pocked rocks. Both were discovered within days of each other in August of 1877 by an astronomer at the U.S. Naval Observatory in Washington,DC.
Because of its proximity to Mars, Phobos actually orbits faster than the planet rotates. The result is that Phobos rises in the west and sets in the east. It appears about one-third as wide as Earth's Moon appears in our sky. As a result, it can cause solar eclipses, but they are never total, since the angular diameter of the Sun is larger than that of Phobos. An interesting related phenomenon—one which can never occur on Earth—is a “dual lunar” eclipse, where Phobos passes in front of Deimos. Such a phenomenon has actually been observed by the Mars Science Laboratory Curiosity rover, as you can see in the image to the right of this paragraph.
Another consequence of its proximity and fast orbital speed is that the gravity of Mars is acting to slow Phobos down in its orbit, as opposed to speeding it up. The result is that the orbit is gradually becoming lower; i.e. Phobos is slowly getting closer to Mars. It is estimated that in less than 30 to 50 million years, it will approach Mars close enough that tidal forces will tear it apart. Chains of craters near the planet's equator suggest the possibility that Mars had additional moons in the past that may have met this same fate, with the result that the remnant pieces crashed onto the planet like a series of large meteors.
Like nearly all airless bodies, Phobos is covered in impact craters, with one very large crater named Stickney at one end of its oblong shape.
Due to its smaller size and greater distance, Deimos never appears larger in the Martian sky than about one-twelfth the angular diameter of Earth's Moon. Because its orbital period is not too different from Mars' rotational period, Deimos moves very slowly across the sky, taking nearly three days to go from moonrise to moonset.
The surface of Deimos is notably smoother than that of Phobos. At the time of this article, only two craters have actually been given names; they are Swift and Voltaire, named after writers who speculated about the existence of two Martian moons well before their actual discovery.
In a naming connection with one of the aforementioned authors, Jonathan Swift, a sample return probe to Deimos was proposed in 2010, with the name Gulliver.
The ancient Babylonians called Mars Nergal, after their god of fire, war and destruction. The Greeks picked up on that and called it Ares, after their war god, whose Roman equivalent was Mars. To the Hindus it was Mangala, and to the Hebrews it was Ma'adim, “the one who blushes”.
Early telescopic observations showed what appeared to be a series of straight lines across the surface of Mars, and the Italian astronomer Giovanni Schiaparelli called them canali, which can be translated either as “channels” or “canals”. It was the latter translation that fired the imaginations of those who believed Mars to be inhabited by advanced creatures. Additionally, surface features were seen to vary, including the so-called “summer darkening” that occurred fairly regularly, which added fuel to the idea that there was at least some kind of vegetation present. Only when spacecraft began actually taking close-up pictures did astronomers finally realize that both were illusory.
Martian Sunset seen by the Spirit rover
Mars has been extensively visited by space probes from the United States, the Soviet Union/Russia, the European Union and India. Japan also sent a single orbiter, but it failed, as have nearly two thirds of all probes sent to the red planet. However, enough craft have arrived safely and either gone into orbit or else landed to make Mars the most studied of all the planets in our solar system, aside from Earth itself.
In 1976, Viking 1 sent back the first pictures from the surface of Mars. It was quickly followed by Viking 2, and then many years passed before the next lander was sent. A number of orbiters, such as Mars Global Surveyor and Phobos 2 were sent in the 80's and 90's. Finally, in '97 Pathfinder landed, which carried a small prototype rover called Sojourner, which paved the way for the Mars Exploration Rovers Spirit and Opportunity, both of which landed in 2004.
A few years later came the Phoenix lander, which set down in the northern polar regions to look for water ice. It also carried a microscope capable of resolving objects a thousandth of the width of a human hair. Due to the harsh polar nights and increasingly shorter days, Phoenix only lasted a few months before going silent. But they were very fruitful months.
In August of 2012 the Mars Science Laboratory rover, named Curiosity, survived a hair-raising new method of landing, involving a parachute/retrorocket sequence that included lowering the car-sized rover to the surface from a hovering sky crane. It arrived with very little damage and quickly began returning scientific results. It is currently making its way up large mountain in the center of a large crater.
Currently, in addition to the one still-functioning rover (Spirit went silent in 2010, and Opportunity died in 2018), there are six orbiters in operation: Mars Odyssey (US), Mars Express (EU), Mars Reconnaissance Orbiter (US), Mars Orbiter Mission (India), MAVEN (US) and ExoMars Trace Gas Explorer (EU, Russia). A number of future missions are also planned.
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