Now try to answer the following questions, to remind you of some of the things you have learned and test your understanding of them.
To the nearest order of magnitude, how much greater is the estimated rate of heat loss per square metre of surface on Io than on Earth?
The end of Section 1.9 quotes Io's heat loss as 2.5 watts per square metre and the Earth's as 0.08 watts per square metre. Dividing these two numbers we get 2.5/0.08 = 250/8 = 31.25.
This is closer to 10 than to 100, so Io's rate of heat loss per square metre is approximately one order of magnitude greater than the Earth's.
From the list, which is the correct reason why Io is so volcanically active?
Io orbits within a strong, magnetically confined, belt of radiation.
Io is tidally heated.
Io contains an anomalously high concentration of radioactive elements.
A lot of primordial heat is still leaking away on Io.
Io does indeed orbit within a zone of magnetically confined radiation; but this is not the origin of the heat that powers Io's volcanoes. The correct answer is 2, Io is tidally heated (Section 1.9).
Apart from the fact that they orbit Jupiter rather than the Sun, do (a) Io and (b) Europa lack any properties that would otherwise allow them to be regarded as terrestrial planets?
Both are believed to have a dense, iron-rich core surrounded by a rocky mantle, which is the key characteristic of terrestrial planets. In terms of size and density they are both similar to the Moon, which is often regarded as a terrestrial planet. The composition of Io's volcanic surface is likely to be sufficiently different from its mantle to be regarded as a distinct crust, which is another feature of terrestrial planets. Europa has about 100 km of ice (or ice + water) hiding its rocky part; this thickness is only a small fraction of the global radius, and in this respect Europa is transitional between a 'normal' terrestrial planet and a large icy body, such as Ganymede, in which the ice is much thicker.
From the list, which of the following is not likely to happen to molecules of water on Europa or Ganymede?
Radiation can break them down into oxygen and hydrogen.
Radiation can turn them into tholins.
They can react with chemicals in rock.
They can become components in cryovolcanic fluids.
1, 3 and 4 are all believed likely to happen, although 3 is described in Chapter 9 of Teach Yourself Planets only for Europa. 2 is impossible, because tholins are tarry substances and therefore require carbon (Section 3.4), which is not present in a molecule of water (H2O). [Comment: The tholins that may contribute to radiation-darkening of Ganymede's ice could originate from the action of radiation on carbon dioxide ice.]
Look at the image in Figure 15. This is part of a map of one of the galilean satellites derived from a mixture of Voyager and Galileo images and which shows some IAU-approved names. This extract covers 80° of longitude and nearly 60° of latitude.
(a) What are the features with one-word names, such as Dendera, Antum and Maa?
(b) What is the meaning of 'Sulcus' in two-word names such as Umma Sulcus and Nippur Sulcus?
(c) Which of the galilean satellites is this map from, and how can you tell?
(a) These features are rather too small for you to make out their nature on this figure, but the only kinds of feature that are given names without a descriptor term are (impact) craters and eruptive centres. There are no signs of eruption here, and in fact these are impact craters.
(b) 'Sulcus' is a descriptor term meaning parallel (or nearly parallel) ridges and furrows.
(c) The map shows belts of pale terrain cutting through dark terrain. This is a characteristic feature of Ganymede, and the identification is backed up by the abundance of impact craters (few of them actually named) on both terrain types. The sulci are belts of pale terrain that cut through dark terrain, and 'nearly parallel ridges and furrows' is a fair description of the visual appearance of Ganymede's pale terrain. Nippur Sulcus, near the northeast (top right) corner of the map, is in fact the belt of pale terrain that runs diagonally across Figure 9.17. That figure also shows Akitu Sulcus, the tapering belt of pale terrain that extends westwards from Nippur Sulcus.
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