a story about mars

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Astronomy astrobiology science


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    Chapter 6


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    With the end of autumn, the Sun has bobbed along the horizon for many weeks. Sunsets on Mars are blue, and these winter days see more and more blue skies. Nights are growing longer than days, and new weather encroaches it is beginning to snow. Dry-ice frost forms on hexagonal ridges stretched over nearby ground, telltale signs of subsurface permafrost. Ice crystals grow on metallic landing legs and bloom over mirrored solar panels, sprouting

    The Noachian epoch saw a nightmarish hail of rock and metal from the sky and volcanism from below ( Michael Carroll)

    M. Carroll, Drifting on Alien Winds: Exploring the Skies and Weather of Other Worlds, DOI 10.1007/978-1-4419-6917-0_6, Springer Science+Business Media, LLC 2011

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    like crystalline ferns. Soon, the Phoenix lander will be encased in a coffin of carbon dioxide ice. Winter has come to northern Mars. But Martian winters have not always been this way.

    the story of Marss atmosphere is one of remarkable transformation. to understand the Martian meteorology of today, we must travel back in time to a Mars 3 billion years distant. that ancient Mars has left us clues to its nature. With Mariner 9s first global reconnaissance of the red Planet (see Chap. 3), cratered terrain surrendered before vast flood plains, dendritic riv-erbeds, and spreading deltas of fossilized waterways. Many of these drainage features formed early in Martian history, in the first of three geologic epochs. the Noachian epoch marked the formation of the most ancient Martian surfaces visible today. It was a nightmarish time of molten rock, downpours of stone and metal asteroids, and violent volcanoes. Whatever primitive atmosphere Mars harbored, left over from the Solar System formation, was either stripped away by massive impacts or leaked away into space under Marss low gravity, just a third that of earths.

    a new atmosphere arose, belched from the interior through the throats of volcanoes. toward the end of this period, the tharsis bulge, the largest vol-canic construct of any planet, rose from the cratered Martian plains. this planetary beer belly would play an important role in future meteorology. Late in the Noachian, waters roared across Martian lowlands in extensive flash flooding, engraving the plains with a calligraphy of riverbeds and can-yons. Seas may have circumscribed the Martian north; remnants of rivers seemed to flow into the area from the southern highlands. If this was the case, the weather was dramatically different, according to Lockheed/Martins Ben Clark, whose involvement in Mars spans a period beginning with the Viking landers. Part of the mystery is: Could the Martian atmosphere have been thicker at one time? the current thinking is that it was, but that once Mars lost its magnetic field, that allowed the solar wind to erode away a major fraction of the atmosphere. the issue that researchers like Clark are puzzling over is that the carbon dioxide should have been locked into the rocks by now. Its a little bit of a dilemma in the sense that we have CO

    2 in

    the atmosphere, and we know it can react with the rocks to make carbonates. Why, then, didnt the carbon dioxide disappear altogether? On earth, we have biology recycling the CO

    2 for us. the plants take it up and the animals

    eat the plants and convert it right back to CO2. It just goes in a circle. On

    Mars you dont have that.Volcanoes are responsible for CO

    2 in the atmospheres of earth and

    Venus, and Mars does exhibit a record of volcanism, with the largest volca-noes in the Solar System rising high into the Martian sky. In the tharsis region, Olympus Mons is more than twice as high as Mount everest, with three nearby volcanoes in close competition for altitude records. On the opposite side of the globe lies another volcanic region, the elysium prov-ince. Other volcanic features in the south are more ancient, with heavily eroded mountains, weathered lava flows, and fields of cinder cones. the volcanism on Mars appears to be extensive, but as Clark points out, Its

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    really a lot less than weve had on earth, at least ten times less and perhaps a hundred times less. Still, if you heat rocks hot enough in volcanoes the CO


    will come back out of them. that could be part of the answer.If volcanism explains the presence of Martian CO

    2, the planets atmo-

    sphere may have been pumped up substantially in the next epoch, the Hesperian. about 3.51.8 billion years ago, extensive lava plains spread across the face of the red Planet. During this period, Mars was a volcanic overachiever. eruptions during this period likely built the massive volcanoes sitting atop the tharsis province. the great bombardment of meteors, com-ets, and asteroids tailed off at the beginning of the Hesperian, so that most craters from this era are heavily eroded.

    Beginning about 1.8 billion years ago, the amazonian epoch continued to be marked by lava flows. Of the rare impact craters that survive from this quiet period on its relatively recent surfaces, many are well preserved com-pared to those of the Hesperian or Noachian. But volcanoes eventually died out. No active volcanism has been observed on Mars in modern times, although the search continues. the amazonian epoch stretches into the present. the environment of modern Mars is, in many ways, the closest to earth of any other world. Compared to Venus or the outer planets, Martian temperatures are welcoming, ranging near the equator from 128f below zero at night to a high reaching the melting point of water. Marss length of day is nearly identical to earths (24 h, 39 min),1 and its axial tilt, which gov-erns seasons, is also currently quite similar, coming in at 25.2 (compared to earths 23.5).

    rain has not fallen on the red Planet for perhaps billions of years. today, frosts of carbon dioxide dust the ruddy rocks. But during periods of volcanic activity, and perhaps in between, standing bodies of water were probably

    Three views of Martian terrain of different ages. Left to right: Maunder Crater, Noachian; Hesperian lava flows; and Amazonian-age features in Amazonis Planitia (ESA/DLR/FU Berlin, G. Neukum; NASA/JPL/ASU; NASA/JPL Viking 1 photo)

    1. this is the length of its solar day, the time it takes for the Sun to return to the same apparent spot in the sky. the sidereal day, the time it takes for the planet to make one rotation compared to the background stars, is 24 h, 37 min, 22 s.

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    stable enough to trigger an active water cycle in the distant past. Some of those bodies of liquid may have been large enough to qualify as oceans. flat lowlands encircle the northern polar region of Mars. few craters exist in this, the most geologically young region of the planet. there is only one other place in the Solar System where we see the same kind of flat profile in topography: earths ocean basins. the terrain to the south of these lowlands is more ancient, rugged, and heavily cratered in places.

    the two-faced nature of the red Planet is called planetary dichotomy. at the boundary between ancient cratered highlands and smooth northern plains, fossilized river valleys drain toward the lowlands in a web of branching val-leys, deltas, and oxbows. Bench-like features sim-ilar to earthly beachfront property rim the plains. Many researchers feel these features confirm a time when vast oceans ringed the north pole, with rainstorms feeding rivers, lakes, and a great expanse called Oceanus Borealis (the candidate northern ocean). at least two possible major shorelines of differing ages have been identified, each a thousand miles long, roughly the distance from Seattle to Los angeles.

    It is clear that Martian meteorology has changed dramatically over the lifetime of the planet. No liquid water can remain on the Martian surface today; it instantly boils into vapor in the thin air. But past epochs may have seen enough pressure and water vapor to result in precipita-tion. Some river valleys are clearly the product

    Volcanoes in different stages of erosion. Left to right: Ceraunius, with moderate cratering (Viking image JPL); well preserved vent in the Tempe region imaged by Mars Global Surveyor (JPL/Malin Space Science Systems); false-color image of Apollonius, a heavily eroded volcano (ESA/DLR/FU/Berlin, Neukum)

    Parallel ridges may mark an ancient shoreline of a great northern ocean (JPL)

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    of flash floods that may have been triggered by the melting of subsurface ice from volcanic eruptions, subsurface ice melt, or the impacts of meteoroids. But others open out in gentle fans, implying long-term precipitation. Some valleys appear to be glaciated. glaciers also require stable climates with long periods of snowfall. Debate continues to rage over just how much water was there in the past, how long that water lasted, and what happened to it.

    Orbital studies by Mars global Surveyor, first reported by Malin Space Science Systems, have uncovered evidence of active surface water even today. gullies on crater walls changed from one imaging session to another, showing shifts in drainage patterns and brightening of surfaces, implying that ice has condensed as groundwater escapes from the craters subsurface. a gully on the wall of an unnamed crater in terra Sirenum, at 36.6S, 161.8W, was initially imaged by the spacecraft in December 2001. It showed a group of gullies dusted by light-toned material. a second image was radioed to earth in april 2005 and again in august, confirming that new bright material had been deposited. the MgS images show that a material flowed down through a gully channel, then spread out near the base of the flow.

    Capping it all Off