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Space Topics: MarsFacts and Pictures
Since Giovanni Schiaparelli first described the “canali” he saw on Mars in 1877, popular imagination has identified Mars as the abode of alien civilizations. Among scientists, life on Mars was thought to be a real possibility until 1965. That year, Mariner 4 passed over Mars’ southern hemisphere and saw a Moon-like cratered landscape, and hopes of finding life were dashed. But the Mariner 9 and Viking orbiters saw more of the planet in 1971 and
1976, and the images contained ancient water-carved features everywhere. It
turns out that Mars is a two-faced planet, with “southern highlands” that
are densely cratered like the Moon, and “northern lowlands” that
are smooth and flat and show few craters. The two regions are separated
by a “dichotomy boundary” that has been carved up by huge
channels, places where some fluid, most likely water, flowed from highlands
to lowlands at some time in the past. Mariner 4, as well as Mariners
6 and 7, had seen only the cratered highlands. Mariner 9 found other exciting landforms on Mars as well. Among these are Valles Marineris, the largest canyon in the solar system, named after Mariner 9. Valles Marineris, at more than 3,000 kilometers (1,800 miles) long and 8 kilometers (5 miles) deep, is four times longer and deeper than the Grand Canyon in Arizona on Earth. Mariner 9 also found truly gargantuan volcanoes. In particular, Olympus Mons is the largest volcano in the solar system, almost three times as tall and five times as wide as Earth’s biggest volcano, Mauna Loa. Mars’ lower gravity (only 1/3 that of Earth’s) is the main reason that its geologic structures can get so big.
Since the arrival of Mars Pathfinder and Mars Global Surveyor in 1997, a Renaissance in Mars exploration has rekindled the public debate about life on Mars, past or present. Mars certainly does not display any clear sign of the existence of life today. But the ubiquitous water features on its surface indicate that it was once a more geologically active place and possibly a more pleasant environment for primitive life than it is now. Whether life ever existed on Mars is an open question and will be a difficult one to answer. Instead of searching for life, the mantra for Mars exploration has become “follow the water.” To scientists, that means searching for evidence of past water on Mars, and, more important, evidence for what climatic and geologic environments prevailed when liquid water was present. Was liquid water a momentary thing, released in rare eruptions of groundwater floods or hot springs? Or did Mars have long-lasting rivers, lakes, oceans, even clouds and rain, and a stable environment for life to evolve? As more and more spacecraft visit the Red Planet, the story of past water on Mars is becoming ever more complex and nuanced. Every byte of data returned from Mars has increased our knowledge but also multiplied the unanswered questions. Mars Today
At present, Mars has a thin atmosphere consisting mostly of about 95% carbon dioxide, 1-3% each nitrogen and argon, and minor amounts of water vapor, carbon monoxide, and oxygen. The atmosphere has only 0.6% of Earth’s atmospheric pressure. Because of the thin atmosphere and lack of water, diurnal temperature variations on Mars are greater than they are on Earth. Near the equator, typical Martian temperatures are -5°C (20°F) during the day and -85°C (-120°F) at night. Occasionally, daytime temperatures do get above the melting temperature of water. But if any liquid water were present today, the low atmospheric pressure would cause the water to evaporate into the air very rapidly. As with Earth, Mars has a tilted rotational axis, which gives it seasons. However, unlike Earth, Mars has a significantly elliptical orbit (more so than any other planet except Pluto), so that the distance between Mars and the Sun varies by 20% within one Mars year. Mars happens to be closer to the Sun (and moving faster in its orbit) when it is northern winter/southern summer. So, in Mars’ northern hemisphere, the shorter winter days are mitigated by a closer (hotter) Sun. But the southern hemisphere experiences short, intense summers and long, frigid winters. Each year, during the extreme southern summer, large dust storms develop that can spread across the entire planet, obscuring the surface from view. Mariner 9, the first Mars orbiter, arrived at Mars during one such storm. These dust storms have covered almost the entire surface of the planet with a layer of dust that appears to have the same composition nearly everywhere. This dust frustrates the efforts of orbiting spacecraft to see the composition of underlying rocks, data that are critical to the understanding of Mars’ geologic history.
Mars’ Surface GeologyThe surface of a terrestrial planet like Earth, the Moon, or Mars is built up by volcanism (where new landforms are built by the outpouring of lava), tectonics (where new landforms are made by the crumpling or opening of the crust in response to great stresses), and impact cratering (which digs great holes in the surface of a planet and deposits piles of material outside the crater). As with Earth, all of these processes have taken place on Mars in the past. However, their relative importance differs. On Earth, most of the landforms that we see were created by plate tectonics and volcanism, with very few impact craters visible. On Mars, the most prominent landforms originated with giant impacts and volcanoes.
These landforms are modified over time by weathering (the breakdown of solid rocks and minerals into grains and new minerals), erosion and transport (the motion of broken-up material from its place of origin), and deposition (the formation of new landforms from piles of weathered rock). In addition, existing landforms can be buried by volcanic eruptions or destroyed by impact craters. Modification of the geologic features of Earth is almost completely dominated by the action of water and Earth’s living creatures. In contrast, Mars has no life, as far as is known. Its surface does show signs of having been modified by liquid water in the past, but present liquid water activity is not possible. There are two main kinds of surface modification that may be taking place on Mars today: glacial activity, where rock-covered deposits of ice may slowly flow downhill, and the activity of wind. Both of these environments -- cold-based glaciers and intensely dry, cold deserts -- have analogs on Earth, rare places that scientists can visit to learn how to study Mars. Water on MarsAlthough there is no liquid water on Mars today, scientists have long suspected that a huge reservoir of ground ice lies beneath Mars’ surface. The ground ice would be close to the surface at Mars’ poles, where it is cold enough for water ice to exist above ground without evaporating. Near the equator, the ground ice would lie at a depth of 1 to 8 kilometers (0.6 to 5 miles) underground. Many unique features on Mars have been attributed to the influence of ground ice, in particular polygonal terrain in the northern polar plains. Ice fills polar craters, sometimes evaporating and reappearing every season. And many surface features on Mars look like Earth glaciers. Some of Mars’ apparent glaciers lack any craters at all, suggesting that they might even be active today.
Another line of evidence for underground water is the shape of a type of crater that is unique to Mars. Throughout the solar system, craters are usually surrounded by an “ejecta blanket,” a bouldery, hummocky spray of material surrounding the crater. The ejecta blanket is thickest near the crater and becomes thinner and more diffuse with distance from the crater, grading imperceptibly into the background. But on Mars, “rampart craters” have a very different kind of ejecta blanket. These craters are surrounded by what looks like a “splat” of thick material, ending abruptly with a steep edge. Many scientists believe that rampart craters occur where there was groundwater or ground ice underneath the impact zone, which mixed with the ejecta to make a fluid flow spreading out from the crater before freezing or evaporating. The most obvious sign of water on mars is fluid-carved features. These vary greatly in size, from the tiny “valley networks” that dissect Mars’ southern highlands to the great, wide, flat-floored “outflow channels” that drain into Mars’ northern lowlands. The presence of the valley networks would seem to indicate that Mars once had a water cycle like Earth’s, where there were oceans that evaporate to make clouds, which then condense to rain on land; rain flowing downhill collects into rivulets, which merge into streams, then rivers, and finally into the ocean, there to begin the cycle again. But Mars’ valleys are not quite like Earth’s.
Earth rivers have uncountable numbers of tributaries. If you zoom in on a tributary, it has tributaries, and that has tributaries too, and so on for as far as you can zoom in. Mars valley networks have far lower numbers of tributaries, and for the most part, they don’t seem to grade into channels too tiny to see. Instead, Mars channel tributaries begin abruptly, in what geomorphologists call “theater-shaped valley heads.” Another difference is that while Earth rivers flow for such a long time that they shape the landscape for themselves, making broad fluvial plains and complex delta deposits, valleys on Mars seem to flow in directions that are controlled by the shape of the land that existed already before the channels were there. Taken together, the differences between Mars and Earth valleys generally indicate that Mars valleys didn’t flow for very long, and that they didn’t begin with rain. Instead, they seem to start from springs, places where groundwater makes its way onto the surface. However, this is a hotly debated subject. Some scientists argue that the differences between Earth and Mars channels are because Mars’ “regolith” or surface layer of soil is very porous, so rain would sink in quickly, rather than flowing across the surface. Others point out that small tributary features that might have formed in the ancient past could have been hidden since then under windblown dust or the ejecta from impact craters. Martian outflow channels are a different story from the valley networks. The outflow channels almost certainly represent the sudden, cataclysmic release of immense quantities of groundwater from an underground source. What caused such sudden floods is not known. Outflow channels begin in bizarre terrain called “chaos,” whole areas of land that crumbled and collapsed as the groundwater emptied from below it. The channels are wide and flat with virtually no tributaries, and flowed downhill, splitting around obstacles, carving wide, flat-floored channels and streamlined islands. The water from the outflow channels eventually reached Mars’ northern lowlands. There, depending on the climate, the surface could have frozen, insulating a liquid ocean beneath it for some time. Temporary lakes could also have formed where the outflows spilled into craters. Gusev crater, Spirit’s landing site, is the site of one such paleolake. How long an ice-capped paleolake could have lasted -- and whether it existed at all -- is debated, but it could have been for a thousand or ten thousand years or even much longer. How the floods even began is also debated. Many of the theories depend upon assumptions of the state of Mars’ past climate, which is itself a subject of hot debate! |
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