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...write a short description of the kinds of features you can see on Europa's surface, noting for example relative brightness and characteristic shapes or textures. Concentrate on a simple description of appearance; we do not expect you to explain the origin or precise nature of what you can see. (b) Try to deduce the relative ages of the features you have described. Answer...

...writing Using the values L = 1038 W and c = 3 × 108 m s−1, we get Q = 1022 kg s−1. Converting this into solar masses per year using 1M⊙ ≈ 2 × 1030 kg and 1 year ≈ 3 × 107 s, we get Q ≈ 0.2M⊙ per year. Is there a large enough supply of matter for a fraction of a solar mass to be accreted every year? Most astronomers think that the answer is yes, and that...

...writing (2009), 20 p/litre less than that on ultra-low sulphur petrol. A recent review has shown that biofuel production is a high-cost process overall; only the production of bioethanol from sugar cane was considered to be cost-effective. If, however, the price of crude oil rises then biofuels may start to become more competitive, although this would depend on...

...writing). AIDS-related deaths fell from 2 million in 2008 to 1.8 million in 2010 due to the expansion of access to effective antiretroviral therapy. One outcome of this success is that HIV prevalence (the number of people living with HIV infection) is steadily increasing – to 34 million in 2010. Just over 2 million of those people were children under 15 years infected...