Henrik Hargitai
Published: 2015-08-15
Total Pages: 0
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The technique of the mapping of planetary surfaces and the methods used for the identification of various planetary landforms improved much in the last 400 years. Until the 20th century, telescopic observers could interpret planetary landforms solely based on their appearance, while today various data sets acquired by space probes can be used for a more detailed analysis on the composition and origin of the surface features. Before the Greeks, the Earth and the Heavens were indisputably of different origin and nature. It was a major philosophical breakthrough - first appeared as an a priori theory, later based on observations - that the Heavens (planetary bodies) and the Earth share common features: gravity, composition and solar distance may be different, but the nature of the physical processes shaping the landforms are essentially the same. It has been a long way since we have arrived from the first telescopic description of lunar craters to the identification of various geological formations on Mars or on minor planets. Relief features of the Moon have first been observed by Galileo Galilee, via his telescope. During the next centuries, a multitude of Lunar landforms have been identified. Theories based on observations have been connected together by a scientific paradigm which explained their origin in a logical and seemingly undisputable manner. Telescopes showed a Lunar surface full of circular landforms, called craters, a landscape with no parallel on Earth. But the individual landforms had a morphological equivalent, volcanoes, which naturally led to the conclusion that craters had been created by volcanic processes. Maria ("seas") served as natural basins for water bodies. Observations clearly showed that water and air are hardly found on the Moon, the lack of clouds indicated the lack of precipitation. But the flat surface of the maria (obviously composed of marine sediments) and the meandering valleys suggested the presence of liquid water and a higher atmospheric pressure in the past - during the age of active volcanism and degassing. There were no observable active volcanic processes but some craters (though to be volcanoes) have been observed as being active: flashes of light - interpreted as eruptions - have been reported by several observers. The presence of pyroclasts thrown out from the volcanic vents of craters provided an independent evidence: meteor showers and individual meteorites falling from the sky - originating from Lunar craters. The logical and interconnected set of explanations based on observations proved to be completely false by the second half of the 20th century. The new paradigm interpreted the very same features in a new context. The case of Mars was different. There were no telescopes capable of observing relief forms (no shadows on Mars are visible from the Earth, because Mars always shows a nearly full Mars phase), so only albedo features could be seen and used for interpretation. The lack of visible relief features were interpreted as a lack of considerable topography: an unnoticed distortion in the observational data. The hue and contrast of dark and bright, orange, grey and white spots have changed seasonally, the polar areas clearly showed a polar cap made of ice and snow, but clouds have not been observed. Since Mars is farther away from the Sun than the Earth, it was evident that temperature values are lower there. Scientists concluded that Mars is an ancient, arid world. Then contemporary geology taught the theory according to which waters on the Earth are going to infiltrate underground in time, making the surface dry - observations showed that this had already happened on Mars. The last surface reservoirs of water were the polar caps. Some observers reported seeing a global network of linear features, but other have only seen very few of such albedo markings. These features were interpreted as "canals," made by a civilization for irrigation, carrying water from the poles to all around the flat plains of Mars. What was observable from the Earth were the broad stripes of irrigated vegetation (like those along the Nile), the canals themselves were too narrow to be visible from here. All theories converged - supposing that the features seen by some, but not seen by others, were real. There was no chance for verification until spacecrafts have been developed which were able to make local observations. Instead of canals, the first pictures returned revealed a surface full of craters - a landform not expected by anyone. A paradigm shift was needed to explain the features of the "new" Mars. On the Moon, features were observable, but the interpretation was wrong. On Mars, only blurred albedo markings could be observed, along with sharp lines of imagination, which again were interpreted falsely. In the case of Venus, there was no data on surface features. Only its bright cloud top could be observed from the Earth. But this fact along with the planet's orbital parameters provided enough information for a popular view on its surface conditions: a hot world (inferred from its proximity to the Sun) and also a rainy one (from its complete cloud cover). The conclusion: Venus is a global jungle possibly with dinosaurs, like the hot and wet world of the then-discovered Mesozoic era. Our current knowledge originated from these early attempts of interpreting surface conditions and geological origin of landforms from a very little set of available data. Today we have a huge set of images and other physical data which makes it possible to create models on the inner structure and thermal history of planetary bodies. Combined data sets lead to better supported models on the formation of surface features. Today we believe that most models give reliable explanation for the origin of planetary landforms. New, higher resolution images reveal new sets of meso- and microscale landforms, while images from previously not imaged dwarf planets, satellites, asteroids and cometary nuclei show landforms never seen before. In the future exoplanets are expected to provide brand new types of relief features no predictable by our Earth-and Solar System bound imagination. There are so many different landforms on planetary surfaces that it is nearly impossible for anybody to overview all of them who does not work exactly with that certain feature type. The Encyclopedia helps with presenting the landforms in searchable, alphabetical order. The book contains more than a simple list of various features: it provides context and connections between them and point to their origin. For example sand dunes were found on Venus, Mars and Titan, fluvial valleys and shorelines are present on Mars and Titan, impact craters have many different types - all are presented and explained here. Beyond the texts, references, schematic figures, images and planetary maps accompany the description of landforms, providing a wide background for detailed analyses even for geomorphologists working in planetary science. This book is to help the reader to discover the great variety of planetary landforms.
