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Module 11: Venus the Sulphurous Greenhouse Activity 2: Observing the Surface of Venus 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 (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. 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 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. Using Doppler shift techniques, radar astronomy has been used to determine the rotation rates of Mercury and Venus. Radar astronomy can also be used to map surface features of Solar System targets. 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. Radar imaging of Venus transformed our view of our closest planetary neighbour: 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 planet’s surface. Designed to carry out an 8 month mission, it in fact stayed in operation until October 1992! 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 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. Ishtar Terra highlands featuring Maxwell Montes, the largest mountain on Venus In this false-colour map, red represents mountains, while blue represents valleys. 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. 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. (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. 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, Venera 10 arrived and also managed to transmit an image in the 65 minutes it remained operational. 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. Lens cap! Venera 13 survived for 2h7mins and took 14 images on 1 March 1982 (c) The Surface of Venus The surface of Venus is fairly flat: 20% lowland plains, 70% rolling uplands, and 10% highlands. With the exception of the three highland regions, elevation differences are only 2 to 3km. Even the highland–lowland 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. Map of the terrain, false-coloured to emphasize altitude differences: the northern hemisphere is relatively mountainous, with uncratered upland plateaus the southern hemisphere is mainly rolling, cratered, lava plains 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? 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. 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. 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 • Cratering on Venus 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. 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. This Magellan radar image of Lavinia Planitia is again falsecoloured 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. 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. 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 explanation. • 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. Maat Mons, an 8-kilometer volcano, is shown in this threedimensional 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! Here is a similar image, this time of Gula Mons, a 3 km high volcano: … 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. 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. 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. Other volcanoes on Venus form as pancake-shaped domes up to 25 km across and 2 km high, presumably as the product of very thick sticky lava. These volcanoes have no counterpart on Earth. How Venus manages to produce both lava which is free-flowing enough to extend thousands of kilometres, plus lava viscous enough to form the pancake-shaped domes that we have just seen, is an unresolved issue! Sometimes the lava has not erupted through the surface at all, but instead has formed large circular or oval bulges hundreds of kilometers across, called coronae. The limited radar images we have of Venus cannot confirm whether any volcanic activity continues. However there is some indirect evidence - for example, of lightning strikes possibly concentrated over volcanic regions, and fluctuations in the level of sulphur dioxide in the atmosphere - which suggest that volcanic activity is still happening on Venus. Perhaps future missions to Venus will manage to resolve the issue by imaging a venusian volcano in the act of erupting! • Tectonic Activity on Venus While no large-scale plate tectonics of the scale of that on Earth appears to be happening on Venus, the lava flows indicate convective flows in its mantle, as on Earth. Limited tectonic activity of a sort does seem to occur, creating patterns of ridges and rift valleys in the lowlands, and much of the highland mountain regions appear to have been produced by compression of the crust. One theory suggests that periodically sections of the hot flexible crust (kept that way by the thick atmosphere) may sink into the mantle and new crust may be formed from lava flows and volcanism - a periodic resurfacing of large sections of the planet. Gula Mons Sif Mons 700 km long rift valley (Computer reconstruction, with vertical scale again exaggerated) We’ll delay modelling the evolution of Venus so that we can compare it directly to that of Mars in a later Activity. In the meantime, when you have finished this Activity, use the CD-ROM which accompanies the Universe textbook to view simulated fly-pasts of venusian volcanoes, rift valleys and craters - in the Animations & Videos section. In the same section there is also a video showing views of the Arecibo telescope. In the next Activity we will go on to look at the Earth’s other neighbour, Mars. Image Credits Arecibo http://www.naic.edu/about/photos/aoviews/tescop.jpg Venera 13 surface view of Venus http://nssdc.gsfc.nasa.gov/image/planetary/venus/venera13-left.jpg NASA: Pioneer 12 http://loenardo.jpl.nasa.gov/msl/QuickLooks/pioneer12QL.html NASA: Venus globe http://nssdc.gsfc.nasa.gov/image/planetary/venus/venusglobe.jpg NASA: Earth globe http://pds.jpl.nasa.gov/planets/welcome/earth.htm NASA: Just passing by http://antwrp.gsfc.nasa.gov/apod/ap980501.html NASA: Impact Craters http://pds.jpl.nasa.gov/planets/gif/ven/dsnall.gif Mauna Loa, Hawaii,US Geological Survey, J.D. Griggs http://wwwhvo.wr.usgs.gov/maunaloa/4303062_M.jpg Image Credits NASA: Ovda Regio http://pds.jpl.nasa.gov/planets/gif/ven/ovdan.gif Topographic Map of Venus from Pioneer Venus (Mercator Projection) http://nssdc.gsfc.nasa.gov/image/planetary/venus/pvo_topo_mercator.jpg Beneath Venus' Clouds http://antwrp.gsfc.nasa.gov/apod/ap960924.html Complex Craters, Venus http://cass.jsc.nasa.gov/images/simp/simp_S05TN.gif Computer-generated surface view of Maat Mons http://nssdc.gsfc.nasa.gov/image/planetary/venus/mgn_maat_mons.jpg Computer generated surface view of Gula Mons. http://nssdc.gsfc.nasa.gov/image/planetary/venus/mgn_gula_mons.jpg Gula Mons (to realistic scale) http://nssdc.gsfc.nasa.gov/image/planetary/venus/mgn_gula_mons_squish.jpg Image Credits A Venusian Tick, http://antwrp.gsfc.nasa.gov/apod/ap950823.html Computer generated surface view of Alpha Regio http://nssdc.gsfc.nasa.gov/image/planetary/venus/mgn_alpha_regio.jpg Computer generated surface view of a corona, tentatively named Idem-Kuva. http://nssdc.gsfc.nasa.gov/image/planetary/venus/mgn_corona_idem-kuva.jpg Now return to the Module 11 home page, and read more about the surface of the Venus in the Textbook Readings. Hit the Esc key (escape) to return to the Module 11 Home Page Atmospheres and Craters As we will see when we investigate meteorites on Earth in a later Activity, Solar System debris striking a planetary atmosphere largely “burns up” before it hits the surface. As the atmosphere of Venus is very dense compared to that of the Moon and Mercury or even Earth, the thick atmosphere probably tends to destroy small debris before it can reach the surface, leading to a lack of very small craters. Also, even somewhat larger projectiles probably tend to break up into smaller fragments when they hit the venusian atmosphere, and those fragments in turn are likely to burn up. Click here to return to the Activity!