IELTS Academic Reading Practice Test 19

A The Moon is widely regarded as by far the largest natural satellite relative to its planet in the solar system and is said to have shaped Earth's evolution in ways only recently understood. Its mass is roughly 1 percent of Earth's; its distance averages 384,400 kilometres, growing by approximately 3.8 centimetres each year as orbital dynamics transfer momentum from Earth's rotation. The diversity of its surface — heavily cratered ancient highlands alongside smoother volcanic plains called maria — is widely said to encode more than four billion years of solar system history. Tidal interaction between the Earth and Moon is widely said to have gradually slowed Earth's rotation as well, lengthening the terrestrial day from perhaps six hours in the very early solar system to its present twenty-four.

B The dominant hypothesis for the Moon's origin is regarded as the Giant Impact theory, first proposed in 1975 by William Hartmann and Donald Davis. The theory holds that a Mars-sized body, sometimes called Theia, collided with the proto-Earth roughly 4.5 billion years ago. The collision is widely said to have vaporised a substantial portion of both bodies; the debris cooled in orbit around the surviving Earth and coalesced into the Moon. The theory is, in many researchers' view, supported by the similar composition of lunar and Earth mantle rocks and by the relative deficiency of volatile elements in lunar samples, which suggests the formation of a hot, vapour-rich precursor. Alternative hypotheses, including gravitational capture of an already-formed satellite or co-formation alongside the Earth, are widely said to have receded from serious consideration because they cannot account for the same compositional pattern.

C Apollo lunar samples, returned to Earth between 1969 and 1972, have, in researchers' view, been the dominant source of detailed compositional information about the Moon. The 380-kilogram total of Apollo samples is supplemented by smaller collections returned by Soviet Luna missions and, most recently, by the Chinese Chang'e-5 and Chang'e-6 missions of 2020 and 2024. Together these samples are widely said to have established a detailed chronology of the Moon's volcanic and impact history, including the so-called Late Heavy Bombardment of approximately 4 to 3.8 billion years ago, during which the inner solar system was struck by numerous large bodies. The samples returned by the most recent Chinese missions are widely said to have prompted important refinements to the lunar timeline, particularly for the period of late volcanism that remained poorly constrained by Apollo-era material alone.

D The discovery of water on the Moon has, in many researchers' view, fundamentally changed thinking about future lunar exploration. Spectroscopic measurements from the LCROSS impact probe in 2009 and subsequent observations from the SOFIA airborne observatory confirmed the presence of water in permanently shadowed craters near the lunar poles. The total amount is widely said to be uncertain but probably substantial, possibly hundreds of millions of tonnes in the lunar polar regions. Practical access to this water would, according to many space-policy analysts, dramatically reduce the cost of long-term human presence on the Moon, since launching water from Earth is enormously expensive. Water can, in principle, also be split into hydrogen and oxygen, allowing it to serve as both drinking supply and rocket propellant if appropriate extraction infrastructure were established.

E The current wave of lunar exploration is widely described as the most active since Apollo. NASA's Artemis programme aims to return humans to the Moon, with a particular focus on the south polar region near suspected water deposits. China's Chang'e programme has, in researchers' view, progressed steadily, including a sample-return mission to the lunar far side in 2024. India's Chandrayaan-3 successfully landed near the south pole in 2023. Several private companies have begun delivering scientific payloads to the lunar surface under contracts with NASA and other agencies, with mixed success in the first attempts. The combined cadence of national and commercial missions is widely said to make the next two decades the most active period of lunar exploration in history, surpassing the Apollo era in number of missions if not yet in dramatic events.

A Geothermal energy — heat from inside the Earth tapped for human use — is widely said to have been employed in some form for at least two thousand years, with Romans heating bathhouses with hot springs and Japanese and Icelandic communities using hot water for cooking, bathing, and snow-melt. The modern geothermal-electricity industry began in 1904 at Larderello in Tuscany, where Italian engineer Piero Ginori Conti coupled a steam-powered generator to natural geothermal steam to power a few light bulbs. The same site has, in researchers' view, remained a working geothermal power station ever since, making it widely regarded as the world's oldest continuously operating example. The Tuscan example is widely said to have inspired similar early installations in Iceland, New Zealand, and the western United States during the first half of the twentieth century, although serious global expansion is agreed to have begun only after the oil shocks of the 1970s.

B The Earth's interior heat generally has two principal sources: residual heat from the planet's formation about 4.5 billion years ago, and radioactive decay of naturally-occurring isotopes including uranium, thorium, and potassium in the crust and mantle. The temperature gradient — the rate at which temperature rises with depth — averages about 25 degrees Celsius per kilometre but varies considerably between regions. Volcanic and tectonically active regions including Iceland, New Zealand, Indonesia, the Andes, parts of Japan, and the western United States typically have temperature gradients five to ten times higher, widely said to make them attractive for geothermal exploitation. Such regions are widely described as the natural geographic frontier of the industry, although enhanced systems are intended to expand the range of practical sites well beyond the obvious volcanic settings.

C Conventional geothermal power requires three conditions: heat at accessible depth, fluid to carry the heat to the surface, and rock with sufficient permeability for that fluid to flow. Where all three are naturally present, drilling produces steam or hot water that can be used directly in steam turbines, or to heat a secondary working fluid in a binary-cycle plant. Total installed geothermal electricity capacity worldwide is widely said to stand at approximately 17 gigawatts as of 2024, with major capacity in the United States, the Philippines, Indonesia, Turkey, New Zealand, and Italy. The figure is widely said to represent only a small share of total electricity generation worldwide but a much larger share in the countries that have made consistent investment over decades.

D Enhanced geothermal systems are said to extend the resource base by engineering reservoirs where one of the natural conditions is missing. The most common approach injects water under pressure into hot but impermeable rock, creating fractures through which water can subsequently flow and be heated. Pilot plants in France, Australia, and the United States have, in researchers' view, demonstrated the principle, though commercial deployment has been hampered by induced-seismicity concerns and economic challenges. New start-up companies, including Fervo Energy, are widely said to have integrated drilling techniques developed for the oil and gas industry to substantially reduce costs in recent demonstration projects. Some analysts are widely said to expect enhanced systems to play a substantial role in firm low-carbon electricity by the late 2030s if early demonstrations continue to deliver promising results.

E Direct-use applications — heating buildings and supplying hot water without first generating electricity — generally account for substantially more geothermal energy than electricity generation. Iceland heats virtually all of its homes from geothermal sources, and integrated geothermal district heating supplies large fractions of energy demand in Reykjavik and several other cities. Greenhouses, fish farms, and industrial processes including drying and dyeing also typically draw on geothermal heat in many countries. Ground-source heat pumps, which exploit the modest temperature difference between underground and surface conditions for residential heating, are widely said to have spread rapidly over the past two decades.

F The environmental profile of geothermal is, in researchers' view, generally favourable. Lifecycle carbon emissions are less than 5 percent of those of fossil-fuel generation. Land use per gigawatt-hour is widely regarded as among the lowest of any energy source. Water consumption is modest. Specific concerns include the release of dissolved gases and trace elements from geothermal fluids, which must be reinjected rather than vented, and the induced-seismicity associated with enhanced systems. Overall, geothermal expansion is widely seen as a complementary rather than competing element of broader renewable energy strategies.

A Among the most striking outcomes of evolution are widely regarded as the social insects — ants, termites, and some bees and wasps — whose colonies reportedly operate as collective entities in ways that recall the function of single organisms. A typical ant or honeybee colony contains thousands to millions of individuals, the majority of whom are functionally sterile workers who help raise the offspring of a single reproductive queen. The astonishing fact that natural selection produced such self-sacrificing systems was, in researchers' view, a challenge to Darwinian theory for nearly a century and is widely said to remain an active subject of biological investigation. Darwin himself, in The Origin of Species, is widely said to have described the existence of sterile worker castes as one special difficulty for natural selection, requiring an explanation that operated above the level of the individual.

B The puzzle was, according to most evolutionary biologists, substantially resolved by William Hamilton's 1964 development of inclusive fitness theory. Hamilton showed mathematically that altruistic behaviour favouring close relatives can be favoured by natural selection, because the helpers share genes with the offspring they help to raise. The peculiar genetic system of bees, ants, and wasps — known as haplodiploidy, in which females develop from fertilised eggs but males develop from unfertilised eggs — means that sisters share, on average, three-quarters of their genes rather than the one-half that ordinary diploid siblings share. This is widely said to raise the genetic stakes of helping siblings rather than reproducing oneself. Hamilton's mathematical formulation is widely treated as one of the most influential pieces of twentieth-century evolutionary theorising.

C Termite social structure reportedly has a different genetic basis. Termites are diploid like most animals, and both males and females generally contribute equally to reproductive caste-mate relatedness. The puzzle of why termite workers nevertheless become functionally sterile helpers is therefore widely said to be less obvious than for the haplodiploid groups, and is now attributed primarily to the long life of reproductive pairs in protected nests, monogamous mating, and the ecological circumstances in which termite ancestors first specialised. Comparative analysis of termite, ant, and bee eusociality is widely treated as one of the cleanest examples of evolutionary convergence on a complex life history.

D The complexity of social insect colonies has, in researchers' view, prompted comparison to multicellular organisms. A honeybee colony of perhaps 50,000 workers, a single queen, and a few hundred male drones makes coordinated decisions, defends itself, maintains internal temperature, and reproduces itself by colony division. Some researchers have proposed the term superorganism for such collective entities. Critics of the term reportedly note that colony members can reproduce more easily than cells of a body, and that the boundary between colony and environment is, in their view, much less sharp than the boundary of a body.

E Communication within colonies is widely regarded as highly developed. Honeybees famously use the waggle dance, decoded by Karl von Frisch in the 1940s, to communicate the direction and distance of food sources relative to the sun. Ants typically use a complex repertoire of chemical signals known as pheromones — laid as trails for nestmates, used as alarm signals during disturbance, and used to mark identity between colonies. Some ant species are said to use chemical signals identifiable across multiple senses: visual, olfactory, and even acoustic cues produced by stridulation against body segments.

F The ecological influence of social insects is, in researchers' view, substantial. Ants and termites together are estimated to make up between 15 and 25 percent of all terrestrial animal biomass, varying by region. They are widely said to be major movers of soil, generally comparable in effect to earthworms in many ecosystems. They are major predators on other arthropods and major prey for many vertebrates. Their pollination services, particularly those of bees, are widely regarded as central to both natural ecosystems and human agriculture, with crop dependence on insect pollination estimated at one-third of global food volume. Without social-insect activity, large parts of the temperate-forest soil profile would, in many ecologists' view, look very different.

G Threats to social insects have, in many entomologists' view, grown in prominence. Honeybees have suffered the much-publicised colony collapse disorder; many native solitary bees and bumblebees have suffered measurable population declines; some specialised ant species are widely said to be threatened by habitat loss and invasive competitors. The conservation implications are widely regarded as significant: the loss of social insects affects ecosystems far out of proportion to their species numbers, given their roles in pollination, predation, and soil processing.