Lecture
23
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Lecture
23
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The
Nine Planets |
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It is rather naïve and exclusive to imagine that landscape development is something confined to the Earth only. Despite the differing chemical composition of the other planets within the solar system (and their satellites), the history of the solar system is so ancient that comparable environmental conditions of one sort or another have existed on a number of planetary bodies, at some point in time.
Earth-like planetary bodies are planets and large moons with a solid crust on which landforms can be developed and preserved. Examples are Mars and Venus, as well as certain moons of both Jupiter and Saturn. The present day conditions experienced on the surface of various planets is largely (but not solely) a function of the mean distance of the planet from the Sun. Mean distance from the centre of the solar system influences the amount of solar energy at the surface of the planets. This together with the rotational period and the Nature of the atmosphere largely controls the average and extreme temperatures experienced on a planetary surface.
Atmospheric pressure and temperature are crucial variables – these determine whether water can exist in a liquid state. The presence of liquid water determines whether physical and chemical weathering can occur, and the Nature of that weathering. Naturally the availability of liquid water will also determine the presence or absence of fluvial activity. Surface gravity will affect mass movement processes. The rate and therefore efficacy of process, changes with planetary seasons.
227.9 million km from the Sun (4th planet from the Sun)
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Known
as the Red Planet - rocks/soil have a reddish-pink hue (given the name Mars
by the Romans in honour of their God of War). Believed to have been formed 4-4.5 billion years ago (Earth = 4,600 million - just a baby by comparison...). Freshness of Martian volcanoes indicate that they were formed within the last billion years...! Atmosphere is predominantly CO2 (95 %) & Nitrogen (2.7%), some argon, 0.13% oxygen, 0.03% water and a tiny bit of Neon. 1/1000th as much water as on Earth. Mean surface temperature = -23°C (Earth = 15°C) Max. temp. = 20°C, Min. = -140°C Full range of topography over 19 miles (30km). 1.5 x the elevation range on Earth. Smooth, lower north. Heavily cratered, higher relief - south (3 miles higher on average) |
An early observation was of the ‘canals’ - straight lines on the surface of Mars. Surely these are not natural, but created by intelligent life? A further observation was of apparent colour changes on the surface of the planet, it was thought that seasonal changes in surface colour could be vegetation blooms. Mars seemed to offer the hope, or more often the fear, of extra-terrestrial life. The famous adaptation of H.G.Wells’ War of the Worlds, including Orson Welles’ now notorious ‘News Bulletin’, caused widespread panic and alarm in the USA in the 1930s.
After WWII the USA & FSU were dominant in exploration of the planet. The first close view of the planet was provided by Mariner fly-by of 1964. In 1969 Mariner 6 & 7 gathered more data from fly by activity. In 1971 - Mariner 9 arrived. It made observations of a large dust storm for several months - then provided spectacular images. During 1 year Mariner 9 orbiter imaged almost the entire surface at resolutions of 1-10km with selected areas at 100m. Later in the decade the Viking Mission to Mars (NASA) was composed of two spacecraft, Viking 1 and Viking 2.
More recently the Pathfinder mission made further exploration of the planet: for example by the Sojourner rover on 4th July 1997. Most recently success has been less evident. The Mars Observer mission of 1992 lost contact with Earth for no known reason. The Mars Global Surveyor is a cheaper programme designed to recover some of the losses of the Observer mission.The Mars Surveyor '98 program was comprised of two spacecraft launched separately, the Mars Climate Orbiter and the Mars Polar Lander. The Mars Climate Orbiter was destroyed when a navigation error caused it to miss its target altitude. Instead of entering Mars atmosphere at 80 to 90 km, the orbiter entered the Martian atmosphere at an altitude of 57 km, and was destroyed. Most recently the Mars Polar Lander was due to send back data from the planet’s surface. The last telemetry from Mars Polar Lander was sent just prior to atmospheric entry on 3 December 1999. No further signals have been received from the lander. (OOOPS, two of our spacecraft are missing...Houston control, we have a cock-up). Most recently the Mars Odyssey orbiter went into orbit around the planet in January 2002 and began transmitting data in March 2002. It will act as a relay satellite for planned rover missions in 2004.
In NASA speak - 'New lands create new opportunities' - but the absence of an inhabitableable natural environment and the fact that people can only return to Earth once every 26 months when the alignment of the planets allows, makes the prospect of exploration (let alone colonisation) daunting. A commitment to a manned launch is a commitment to three years in space...
Martian landscapes are POLYGENETIC - relict forms abound but more recent processes have locally obscured the key relationships for analysis of ancient terrains. This, of course, once more exemplifies the principle of equifinality... (again!)
The surface of
Mars is self-sterilising. UV radiation, the dryness of soil and oxidising Nature
of soil chemistry prevent living organisms in Martian soil. Biological impacts
upon the land surface are likely to be minimal.
(And, cue
Richard Burton...).
Despite the very small amount of water present, it can still condense out forming clouds that ride high in the atmosphere; fogs also form in valleys, as do thin layers of winter frost. Despite the apparent limitations upon landforming processes, there are many distinct landforms on the surface of Mars.
Impact craters from meteorites, comets and asteroids are very common on planets (including Earth, & satellites of Jupiter, Europa & Io). Craters on Mars are unusual. 'Splosh' craters – so called because of the Nature of the material disturbed on impact, are believed to form due to the presence of subsurface ice (permafrost), which either as gas or liquid, becomes incorporated into ejecta to form a debris flow. It is thought that active resurfacing processes have erased the record of the primordial episode of heavy bombardment. Cratering density is used as a means of dating events on Mars – it is thought that the Martian surface is extremely ancient. The uplands of Mars are saturated with craters.
The largest volcanoes in the solar system are found on Mars. OLYMPUS MONS rises to a height of 26km above the surrounding plain. (cf Everest = 8872m - 8.9km). . The caldera complex is 80km in diameter. . The characteristics of the volcano are comparable to Hawaii - but there is a vast size difference. . The reason for the sheer size of the martian volcanoes is due to the lower surface gravity, therefore the reduced isostatic effects means that they do not load the crust beneathto the extent that dense accumulations of rock on Earth do. The volcanoes are all shield type with massive lava flows – for example ALBA PETERA is 1500 km across. Other important volcanoes are VALLES MARINERIS, which is fault-like; and the THARSIS BULGE...
Atmosphere and surface conditions on Mars are closer to those on Earth than any other body in the Solar system. There are, however, marked differences in atmospheric pressure, atmospheric composition and surface temperatures.
There are observed similarities in weathering processes on the surface of mars. It is thought that the abundance of fine-grained material results from salt weathering. Frost weathering is significant as water and CO2 ice have been identified on the surface. The closest terrestrial analogues for certain martian landscapes that we may have are found in the cold Dry Valleys of Antarctica. Here a seasonal cycle of frost deposition is noted, as on the surface of Mars.
The abundance of high escarpments on Mars gives scope for mass movements, the scale of which exceeds anything on Earth. In the Valles Marineris area there are 25 huge landslides ranging from 40-7500km² - one is recessed 30km into an escarpment wall. The landforms could be due to a) Dry avalanching of unconsolidated material, or b) Sapping - evaporation of ground water or permafrost ice.
The lack of surface water, the absence of vegetation to stabilise surfaces, high winds and an abundance of surface sediment means that aeolian processes are very powerful. An enormous sand desert surrounds the northern polar region; it is characterised by wind-laid dust and ice deposits. The equatorial zone is also noted for considerable deposition and erosion. Massive dust storms can sometimes shroud the whole planet for up to 4 months - wind speeds in the storms are in excess of 500km/h. (Earth maximum = 160km/h). Dust rises up to 70km above the Martian surface.
The power of martian storms is derived from the lower atmospheric pressure of mars compared to that of Earth. The lower pressure results in greater threshold drag velocities; thus air bound grains can achieve 20 x velocity of similar sized particles on Earth: consequently there is high erosion potential. So powerful are the forces that ‘Kamikaz. Grains’ have been observed, so-called due their destruction upon impact. Many of the dunes (especially in the north circumpolar dune fields) and forms are similar to desert landforms on Earth.
Water in liquid state is known not to exist in significant quantities on the present day Martian surface – it may occur at shallow depths in the equatorial zone. Freeze-thaw and frost creep may have been widespread in the past when mean temperatures were higher and surface water was more abundant. Patterned ground phenomena - (including ice-wedge polygons on the northern plains that are 5-10km in diameter, with cracks 100s of m across) - imply a contraction mechanism. If the cause is not periglacial, then cooling it is possible that the patterned ground is a result of cooling lava.
The present water content of mars is unknown, there is abundant evidence for high ice content during the past - chaotic terrain in equatorial zone - large scale ground collapse. Clear latitudinal segregation. More widespread melting could be caused by future climate change - further permafrost degradation.
The frost caps/ice caps of Mars expand to cover up to 30% of the planet's surface area during the Martian winter but retreat to small residual ice caps covering only 1% in summer. Annual shrinkage and expansion has been watched for 200 years...
Northern ice cap consists of water ice, the Southern ice cap is constituted of water & CO2 and doesn't melt completely. The ice appears reddish due to dust incorporated into cap. There is no real evidence for glacial erosion on Mars – and the existence of flowing ice is not known for sure. Glacial processes have been proposed for the formation of the ‘glacial outflow channels’ because their morphology is similar to terrestrially eroded glacial valleys.
There are 3 main types of channel:
FRETTED CHANNELS: These have smooth, wide floors; steep walls and mark the transition between the cratered uplands and the plains
RUNOFF CHANNELS: these are connected to form dendritic networks - similar to terrestrial fluvial systems. They are found throughout the cratered terrain. Possible modes of formation include their being due to surface runoff processes, or springs from permafrost. Their form resembles river cut valleys on Earth - Mars could have been moist at some stage.
OUTFLOW CHANNELS: These are found mainly in the young surfaces in the Northern lowlands. They are enormous features whose origin is keenly debated. They are 10s of km wide and 100s of km long. Massive scour marks characterise the valley floors. There is argument as to what agent was involved - water, ice, wind, debris flows, liquified crustal material or lava. Catastrophic floods could even be a cause of formation cf Channelled Scablands - although the channels are much larger.(Gravity and pressure are important.)
a) Volcanic eruptions beneath glaciers led to rapid melting and catastrophic release of water - this requires existence of ice outside the polar regions.
b) Catastrophic release of artesian water under high pressure.
c) Lucchitta et al. (1981) – they argue that ice stream flow remnants compare to the size of ice flow streams in Antarctica. The ice is in sheets, rather than the valleys.
One key question remains if the flood theory is to be proven: Why are there no obvious deposits at ends of channels? A key issue concerns the fact that Mars is dry, yet these features are best explained by running water of some sort. Whatever has happened suggests major changes in hydrological history of Mars!
©Fiona Tweed: 3/00
Arvidson,
R., Guinness, E. & Lee, S. 1979. 'Differential aeolian redistribution
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374, 595-596.
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Various authors (1997) 'Mars Pathfinder' Science, 278 (5344), 1734-1773 (Special issue, focus on Mars with 3D images etc.)
Acknowledgement:
images of Mars pinched from NASA. I hope they don't mind. They're not used for commercial gain, and if you want to see lots of brilliant images, visit their web sites (I have provided some links...)
Aims: