"About Mars"

We earthlings have long romanticized about the heavens, attributing to the objects therein familiar human traits, as well as strange and imaginative mythologies. Perhaps no other object has been the beneficiary of so much of humanity’s collective imagination as the planet Mars. This dusty and decidedly hostile world has been the inspiration for countless literary classics, including H.G. Wells’ The War of the Worlds, and Edgar Rice Burroughs’ A Princess of Mars. And let us not forget those paragons of the cinematic craft Santa Claus Conquers the Martians, and Mars Needs Women.

Its dirty red color must have dislodged memories of blood-soaked battlefields, inspiring early observers to name the planet for the Roman god of war. Although we now know Mars’ surface to be entirely blood-free, the color of both substances can be explained by the same chemical reaction – the oxidation of iron, better known as rust. The reddish-brown dust that veils much of Mars’ rocky surface is rich in iron oxide. Areas that are comparatively free of this dust appear darker, exposing the underlying rocky terrain and creating a pattern of contrasts that changes as colossal global windstorms redistribute the dust. Particles of dust suspended in the planet’s thin atmosphere give the sky a pinkish orange glow.

How did we get from the desolate Mars of science fact to the inviting Mars of science fiction? Perhaps it’s because Mars seems to be the next most inhabitable of the nine planets. If a place is even marginally inhabitable, our imaginations eagerly take the giant leap to inhabited. The whole "Martian" thing really got started when, in 1877, the Italian astronomer Giovanni Schiaparelli announced that he had identified a pattern of intersecting straight lines on the surface of Mars. He referred to them as canali, which in Italian simply means naturally occurring waterways, as in channels. Canali was quickly and incorrectly translated into English, however, as canal, which implies the active participation of a team of civil engineers and several large construction firms. Such an implication was not Schiaparelli’s intention.

Enter amateur astronomer Percival Lowell. By 1900, Lowell had mapped out as many as 160 "canals" on the Martian surface. Their purpose, according to the wealthy, self-instructed astronomer from Boston, was to redirect the runoff from melting polar caps to hydrologically challenged cities closer to the equator. Hmmm. Of course this notion quickly permeated all aspects of popular culture, and the masses were delighted to contribute liberal profits to many a publisher’s coffers.

This case is an ironic example of the ability of science fiction to foreshadow science fact. Although Lowell’s artificial canals, as he imagined them, turned out to be basically the product of illusion and zeal, a record of flowing water is indeed preserved on the planet’s surface. Networks of natural outflow channels, carved by catastrophic flooding early in Mars’ history and far too small to have been detected by Lowell, are present in abundance. The water appears to have welled up suddenly from beneath the surface. One theory suggests that heat from an impact event, or volcanism, may have melted subterranean ice, driving the liquid water to the surface as the ground above collapsed into the resulting sinkhole. By using impact crater patterns to estimate relative ages of the outflow channels, astronomers can infer that the flooding has probably occurred episodically throughout Mars’ history.

Much of the remaining water on Mars is preserved in its polar ice caps. Because Mars’ axis of rotation is tilted about 25° with respect to the plane of its orbit, it experiences a change of seasons similar to that on Earth. The Martian polar caps expand and recede as each pole alternates between long periods of darkness and light. During Northern Hemisphere winter the northern polar cap expands as a blanket of carbon dioxide ice enshrouds the mound of water ice beneath. In the summertime the dry ice recedes, shrinking the cap and exposing the more stable water ice beneath. The ice is laid down in layers, which are separated by deposits of dust. Like sedimentation layers on Earth, the Martian polar ice layers amount to a record of climatic change reaching millions, maybe billions of years into the past.

The Northern and Southern Hemispheres are strangely dissimilar. Densely cratered and rather Moon-like, the Southern Hemisphere retains much of the impact evidence left after the period of heavy bombardment experienced by all planets early in the formation of our solar system. These features are thought to be in the range of 3.8 to 4.5 billion years old. The Northern Hemisphere, in contrast, consists primarily of smooth plains ranging in age from 500 million to 3.5 billion years old. Cratering is light, and the elevation averages about three miles lower than its southern counterpart. No certainty is, as yet, assigned to any theory explaining this mysterious dichotomy. One popular hypothesis suggests that a tremendous impact may have wiped clean the surface of Mars’ Northern Hemisphere following the period of heavy bombardment.

Don’t let the red planet’s small stature fool you - when Mars does something, it does it in a big way. Picture the state of Arizona with Rhode Island dropped smack in the middle – that’s approximately the size of Olympus Mons and its central crater, known as a caldera. This 26-km high shield volcano dwarfs Hawaii’s Mauna Loa volcano (9.1 km) by almost three to one. Both features developed over volcanic hot spots. The difference in scale is a result of plate tectonics. Since Earth’s surface plates are in motion, a chain of relatively smaller peaks, the Hawaiian Islands, was formed as the plate gradually slid across the vent. Mars’ plates are stationary, however, and a single enormous feature was allowed to develop over time. The fact that small bodies cool faster than larger bodies means that Mars probably cooled much faster than Earth; although it may have experienced active plate tectonics early in its history, the movement of its plates was slowed and eventually halted as the crust solidified.

The mighty Olympus Mons, and several other large volcanoes, inhabits an enormous surface bulge that straddles the equator, reaching across both Martian hemispheres. The elevation in this area, known as the Tharsis Rise, averages about five to ten kilometers above the average Martian ground level. It encompasses an area approximately 2500 kilometers in diameter. Here again, its origin is unclear, but there are theories. Lighter materials may have risen through denser materials in the mantle, forcing the crust to bulge upward; or it may simply be a great big pile of volcanic ejecta.

Another example of Mars’ grand scale divides the Northern and Southern Hemispheres along nearly a fifth of its total circumference. Valles Marineris is a colossal canyon system that would stretch across the U.S from coast to coast, making the Grand Canyon look like a crack in the sidewalk. The 3,100-km long feature is thought to be a rift resulting from the fracture of the crust due to internal stress. The effect is similar to painting a balloon, and attempting to further inflate it once the paint has dried. Looking much like the f-hole of a violin, Valles Marineris is about 100-km wide in places, and runs as much as 8 km deep.

Today’s Martian atmosphere is very different from that which might have supported liquid water on the planet’s surface billions of years ago. At present the atmospheric pressure on Mars is less than one percent of Earth’s, and the average temperature is well below freezing. If the features we interpret as evidence of flowing water are what we think they are, Mars’ atmosphere had to have been much denser and warmer in the past. The most likely scenario would be a greenhouse effect based on carbon dioxide, but at atmospheric pressures closer to those found on Earth – about 1,000 millibars.

It may have happened something like this. Mars and Earth both owe the composition of their early atmospheres to volcanic outgasing. The most abundant products of this process are water vapor, carbon dioxide, and nitrogen. Since carbon dioxide dissolves in water, the rains would tend to scrub the CO₂from the free air, the CO₂ contained in the falling rain would then react with rocks, and the chemical residue would eventually drain off and settle at the bottom of the oceans. Layers of limestone and other carbon-rich rocks would thus be formed. Both Mars and Earth likely experienced rains sufficient to begin this process.

Here’s where it gets interesting. Earth’s dynamic system of plate tectonics recycles these trapped gases from time to time, returning some of that CO₂ to the atmosphere by melting down old rock and releasing the trapped CO₂. Because Mars is smaller than Earth, it cooled faster; volcanic activity became infrequent, and plate tectonics came grinding to a halt. Mars would therefore have had no mechanism for returning chemically bound CO₂ to its atmosphere. In other words, once its volcanoes and plate tectonics quieted down, Mars’ CO₂ levels, and therefore its atmospheric density, may have plummeted below that required to maintain a greenhouse effect, and most of its water simply dissipated into space. Some water vapor remains, about 0.00006 of the total atmospheric volume, and more has settled into a state of permafrost. All of the H₂O remaining on Mars, in any state, would not fill the smallest of the Great Lakes.

Mars has two moons, Phobos (fear), and Deimos (panic). Named for the horses that attended the chariot of Mars by their nineteenth-century discoverer, Asaph Hall, they are thought to be captured asteroids. Although our own moon is large enough for gravity to compress into a tidy sphere, Phobos and Deimos are much smaller, and therefore destined to live out their existence as potato-shaped blobs. Phobos measures out at about 28x23x20 km, Deimos at roughly 16x12x10 km. An interesting similarity between our moon and those of Mars is that they all display a property known as synchronous rotation. The gravitational force of the host planet grabs onto an area of comparatively greater density, causing the satellite to spin on its axis at the same rate that it orbits the host planet. Once locked into this pattern, the moon will always show the same face toward the host planet.

The meteorite known as ALH 84001 became the most controversial rock of the twentieth century when, in 1997, NASA scientists announced finding possible evidence of Martian life in its matrix. Collected on the Antarctic surface in 1984, investigators cited a harmony of features in ALH 84001 that included tiny, elongated shapes - shapes that could be interpreted as fossilized bacteria. Processes unrelated to life just as easily explain each of the features described by the investigating team; the coincidence of finding them all together in one rock of Martian origin is the strongest evidence offered by the meteor’s proponents. Well, any scientist worth his Pyrex will tell you that extraordinary claims require extraordinary evidence. Although it certainly has provoked some lively conversation on the issue of extraterrestrial life, few scientists consider ALH 84001 to be any sort of proof.

Has life ever existed on Mars? At this time there is no irrefutable evidence that it has. The idea that Mars might once have been warmer, wetter, and more like the good Earth is perhaps the most encouraging piece of evidence yet collected. What can we learn from the arduous pursuit of Martian life? If events can alter the course of life on one planet, it can do the same on another. The more we understand of Mars’ dramatic climatic history, and the interdependency of climate and even the most basic of organisms, the better equipped we are to preserve the beautiful blue lifeboat that has been such a generous and nurturing host to us these 4.5 billion years.

Bruce R. Mattson is the Science and Technology Specialist at the McAuliffe/Challenger Center