Module 11: Venus - the Sulphurous Greenhouse Activity 2: Observing the Surface of Venus

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Module 11: Venus - the Sulphurous Greenhouse Activity 2: Observing the Surface of Venus Slide 2 Summary: In this Activity, we will investigate (a) exploring Venus by radar, (b) landing on Venus, and (c) the surface of Venus - cratering, volcanism, tectonic activity Slide 3 (a) Exploring Venus by Radar Venus thick covering of clouds rules out the use of optical astronomy. However radio waves travel through clouds, and so we can use the techniques of radar astronomy to send radar signals to Venus (either from Earth, or from an orbiting radar transmitting space probe) and analyse them on their return. Without radar astronomy, we would not even know how fast the surface of Venus rotates. Slide 4 The Arecibo 305 Metre Radio Telescope, formed out of a natural limestone sinkhole in Puerto Rico, can be used as a powerful radar transceiver* for planetary studies. * transmits & receives radar signals Slide 5 Radar astronomy is used to measure accurately the distance to Solar System objects by using the round-trip time for the signal (to accuracy of 1 microsecond) - and is also important for navigation on space missions. Radar astronomy can also be used to map surface features of Solar System targets. Using Doppler shift techniques, radar astronomy has been used to determine the rotation rates of Mercury and Venus. Slide 6 Radar imaging can directly resolve surface features on the Moon: (not to scale!) But Venus is much further away, and so only low resolution maps of Venus are achieved by radar imaging from Earth. Relatively high resolution radar mapping of the surface of Venus came about once radar-equipped space probes could be placed in orbit around Venus. Slide 7 Radar imaging of Venus transformed our view of our closest planetary neighbour: Slide 8 The first orbital radar mapping of Venus was carried out by Pioneer 12, the Pioneer Venus Orbiter. Pioneer 12 entered orbit around Venus in 1978, and made radar maps of 93% of the planets surface. Designed to carry out an 8 month mission, it in fact stayed in operation until October 1992! Slide 9 Radar mapping was also carried out by the USSR Venera 15 & 16 probes, launched in 1983, and then the US Magellan spacecraft, between 1990 and 1994, mapped the surface of Venus to a resolution of 100m. Magellan was the first space probe to be launched by the Shuttle program. Magellan Slide 10 This 3-kilometer resolution radar map is a composite of Magellan images compiled between 1990 and 1994. Gaps were filled in by the Earth-based Arecibo Radio Telescope. Slide 11 In this false-colour map, red represents mountains, while blue represents valleys. Ishtar Terra highlands featuring Maxwell Montes, the largest mountain on Venus Slide 12 On a radar map, bright regions correspond to terrain which is highly reflective to radar rough terrain, and dark regions correspond to terrain which does not reflect radar well smooth terrain. Slide 13 So radar maps of Venus can look rather strange, but then, so can radar maps of Earth: This image of Mt. Rainier, Washington USA, planet Earth, was produced by the Spaceborne Radar Laboratory which flew on the Space Shuttle Endeavour in 1994. As with all fields of astronomical imaging, it takes training and practice to interpret such images correctly. Slide 14 (b) Landing on Venus Crafts that land on Venus have to contend with the extreme temperature (hot enough to melt lead and zinc) and crushing pressure at its surface. The first spacecraft to land on Venus* was Soviet Venera 7, which arrived at the surface on 15 December 1970 and transmitted data for 23 minutes. This was followed by Venera 8, which landed on 22 July 1972. Neither of these spacecrafts, however, had cameras so no images were returned. * One could argue that Venera 3 was the first spacecraft to reach Venus impacting the surface (rather than landing) on 1 March 1966. The communications systems failed however and no data was transmitted. Slide 15 The first spacecraft to return images from Venus was Venera 9, which arrived at the surface on 22 October 1975 and operated for 53 minutes. It transmitted a single image: Three days later, Verena 10 arrived and also managed to transmit an image in the 65 minutes it remained operational. Slide 16 Images were returned from a total of four of the Soviet Venera spacecrafts (Venera 9, 10, 13 & 14) which landed on Venus between the 1970s and early 1980s, and managed between them to conduct experiments, collect data and take pictures for typically an hour before being put out of commission by the extreme conditions. They carried out radioactive testing on rocks at their landing sites, finding what appeared to be basaltic at some locations and granitic at others - both types of rock which are originally formed from lava. Venera 13 survived for 2h7mins and took 14 images on 1 March 1982 Lens cap! Slide 17 (c) The Surface of Venus The surface of Venus is fairly flat: 20% lowland plains, 70% rolling uplands, and 10% highlands. Slide 18 With the exception of the three highland regions, elevation differences are only 2 to 3km. Even the highlandlowland elevation differences are only about 12km, compared to 20km on Earth and 25km on Mars. * * Mt. Everest reaches slightly under 9km above sea level, and the Marianas trench descends 11km below sea level. Slide 19 Map of the terrain, false-coloured to emphasize altitude differences: Slide 20 the northern hemisphere is relatively mountainous, with uncratered upland plateaus the southern hemisphere is mainly rolling, cratered, lava plains Slide 21 Which parts of the surface of Venus would you expect to be the oldest - the plateaus in the north, or the lava plains in the south? Slide 22 The presence of cratering on the southern lowland plains gives us a clue. We expect significant cratering to indicate relatively old surfaces, perhaps dating from the era of intense bombardment in the early Solar System. The plateaus in the north are relatively uncratered, indicating that the surface has been reworked there, by volcanic flows or tectonic activity, in more recent times. Slide 23 So we conclude that the cratered southern lowlands represent a more ancient venusian surface than do the highlands. This is opposite to the situation here on Earth: here the lowlands, which are our ocean basins, are the newest part of the terrestrial surface. Slide 24 Venus is a rocky world, illuminated by orange sunlight: - picture sent back by Venera 13, which landed on Venus in 1982. Part of the Venera lander is visible here Slide 25 More cratering is evident on Venus than on Earth, but as we have noted before, other influences including plate tectonics and biological effects have probably acted to wipe out or obscure much of the terrestrial cratering record. Cratering on Venus Planetary scientists study cratering on Venus to learn more about the planet, but also to learn more about cratering on Earth. Both planets have similar accelerations due to gravity, and both have substantial atmospheres. Slide 26 This Magellan radar image of Lavinia Planitia is again false- coloured to emphasize differences in elevation, with red indicating high- and blue indicating low-lying ground. 3 large meteorite impact craters, diameters of 35 to 65 km, are surrounded by bright (rough) ejecta, and appear to be partially filled with dark (smooth) material, probably lava risen to the surface through fractures produced by the impacts. Slide 27 Ejecta from the original impact litter the surrounds of the craters, and come out as light, bright areas in a radar map because they are rough terrain. They are coded in the red-orange-yellow range, because they occur high up on the sides of the craters. Lava flows are predominantly smooth, so they appear dark in a radar map. They are coded blue, because they occur deep down on the crater floor. Slide 28 Compared to the Moon and Mercury, Venus has a much smaller number of craters, and in particular lacks very small craters. The latter is probably due to the very dense atmosphere of Venus. Think about why this might not favour very small craters, then click here to see if you agree with our here Slide 29 Volcanism on Venus From what we know about the surface of Venus, it is largely dominated by volcanism. Thousands of volcanoes dot the surface, which is also marked by lava flows of up to thousands of kilometers in length - responsible for wiping out much of the cratering record on Venus. Slide 30 Maat Mons, an 8-kilometer volcano, is shown in this three- dimensional perspective reconstruction of the surface of Venus, as seen from a viewpoint 560 kilometers north of Maat Mons at an elevation of 1.7 kilometers. Lava flows extend for hundreds of kilometers across the fractured plains shown in the foreground, to the base of Maat Mons. In order for us to more clearly see detail, NASA 3D reconstructions are usually exaggerated by a factor of 22.5 in height! Slide 31 Here is a similar image, this time of Gula Mons, a 3 km high volcano: Slide 32 but here is the same volcano, without the factor of 22.5 exaggeration in height! As you can see, exaggerating the vertical scale helps us see detail, especially as the terrain all appears to have similar colouring. However it can give us a false sense of how steep the terrain really is. Slide 33 Volcanoes on Venus are shield volcanoes, which form from particularly fluid lava and have extremely shallow slopes. Shield volcanoes are common on Earth too - the largest terrestrial volcano, Mauna Loa in Hawaii, is also a shield volcano. Slide 34 Below is a radar image of one fairly common type of venusian volcanic feature which showed up in the Magellan images. Known as a venusian tick, it represents a volcano about 32 kilometers wide at the summit with ridges and valleys radiating down its sides lending it an insect like appearance. Slide 35 Other volcanoes on Venus form as pancake-shaped domes up to 2