Reading Practice Test 22

The Sun: Our Nearest Star

The Sun is our nearest star and it dominates our sky from a distance of 'only' 150 million kilometres. Even though it appears to be the same size as the full Moon, it is over 400,000 times brighter, and dictates when we have night and day here on Earth. The Sun is the largest body in the Solar System and it is also the most massive, containing 99.9 per cent of the total mass of all the planets, moons, dwarf planets, asteroids and comets combined. This concentration of mass, and the accompanying gravitational force, is why the Sun sits at the very centre of the Solar System, pulling all the other bodies in orbit around it. We are entirely dependent on the Sun for the habitability of our planet, as it provides us with the energy in the form of heat and light that we require to survive. But it also brings many potential hazards, from the continual flow of hazardous radiation that always lurks just beyond Earth's atmosphere, to the sporadic and violent space weather that threatens much of our society's infrastructure.

Given that the Sun has a volume that is over a million times that of the Earth, yet contains only 330,000 times the mass, we can immediately deduce that its average density is far lower than that of a terrestrial planet. Indeed, the average density is about the same as that of water, and less than a quarter of the density of the Earth. The Sun is made mainly of the lightest elements, hydrogen (the Sun's fuel) and helium, in a gaseous form.

The source of the Sun's energy remained a mystery until Einstein's 1905 special theory of relativity highlighted the promise of efficient nuclear fusion. For nuclear fusion to occur, matter needs to be under conditions of tremendous pressure and of extreme heat, so that the electric repulsion can be overcome, and the nuclei get close enough to smash into each other. It was the English astronomer Sir Arthur Eddington who realised in the 1920's that the physical conditions within the core of the Sun were extreme enough to permit the necessary nuclear reactions. The Sun converts 600,000 million kilograms of hydrogen to helium every second to sustain its phenomenal energy output.

The Sun's core is approximately 15,000,000 degrees Celsius and is the site of the nuclear fusion. The energy from the core travels outwards through the radiation zone by the transfer of the energy from one molecule to another. Heated gases move the energy from the radiation zone through to the convection zone, where the gases start to cool and this causes them to sink back down to the radiation zone. Outside the convection zone is the photosphere, which is approximately 500 kilometres thick and is the surface layer of the sun. Beyond, there is a thin layer of gas that surrounds the photosphere called the chromosphere. Finally, the corona is another layer of gas that extends a long way outside of the Sun.

Observations of more evolved objects around us in the galaxy lead to our understanding of the eventual fate of the Sun. The Sun has sufficient hydrogen at the right temperature and density to continue creating helium for a further six billion years. Then, the supply of fuel, and all possibility of future nuclear reactions, will eventually be exhausted. By this point, the Sun will appear very different from how it does today. It will have become a red giant; a much cooler, redder and far more bloated version of itself, with an atmosphere puffed so large as to swallow up the planets Mercury and Venus and make conditions pretty uncomfortable on Earth. Eventually, the outer envelope of the red giant will be lost, expanding away to form a planetary nebula. The remaining hot core of the star will be left exposed as a white dwarf, which will slowly cool and fade over billions of years, until finally fading into a cold, dark and dense ball of compressed matter.

From time to time, there are eruptions of matter from the Sun. The magnetic energy in an exceptionally powerful sun flare can heat and speed up a huge cloud of charged particles to form a coronal mass ejection. The cloud produced by such an eruption escapes away out into interplanetary space, but can cause concern if directed towards Earth. When a coronal mass ejection reaches the Earth, it rattles the Earth's magnetic field to generate what is known as a 'geomagnetic storm'. The occurrence of the flare gives us advance notice of this event and that it will arrive between 15 hours and a couple of days later, depending on how fast it's moving, and how clear the passage between Sun and Earth is. The major effect for humans of a coronal mass ejection is on our satellites, which can be seriously damaged. Power cuts on Earth can also take place.

Although we may now understand the basics of the Sun, we remain unable to reliably predict everything about it. There is much still to understand and learn about it, and it seems the more intensely it is studied, the more questions there are to answer!

The Exploration of Mars

A  In 1877, Giovanni Schiaparelli, an Italian astronomer, made drawings and maps of the Martian surface that suggested strange features. The images from telescopes at this time were not as sharp as today. Schiaparelli said he could see a network of lines or canali. In 1894, an American astronomer, Percival Lowell, made a series of observations of Mars from his own observatory at Flagstaff, Arizona, USA. Lowell was convinced a great network of canals had been dug to irrigate crops for the Martian race! He suggested that each canal had fertile vegetation on either side, making them noticeable from Earth. Drawings and globes he made show a network of canals and oases all over the planet.

B  The idea that there was intelligent life on Mars gained strength in the late 19th century. In 1898, H.G. Wells wrote a science fiction classic, The War of the Worlds about an invading force of Martians who try to conquer Earth. They use highly advanced technology (advanced for 1898) to crush human resistance in their path. In 1917, Edgar Rice Burroughs wrote the first in a series of 11 novels about Mars. Strange beings and rampaging Martian monsters gripped the public's imagination. A radio broadcast by Orson Welles on Halloween night in 1938 of The War of the Worlds caused widespread panic across America. People ran into the streets in their pyjamas-millions believed the dramatic reports of a Martian invasion.

C  Probes are very important to our understanding of other planets. Much of our recent knowledge comes from these robotic missions into space. The first images sent back from Mars came from Mariner 4 in July 1965. They showed a cratered and barren landscape, more like the surface of our moon than Earth. In 1969, Mariners 6 and 7 were launched and took 200 photographs of Mars's southern hemisphere and pole on fly-by missions. But these showed little more information. In 1971, Mariner 9's mission was to orbit the planet every 12 hours. In 1975, The USA sent two Viking probes to the planet, each with a lander and an orbiter. The Landers had sampler arms to scoop up Maritain rocks and did experiments to try and find signs of life. Although no life was found, they sent back the first colour pictures of the planet's surface and atmosphere from pivoting cameras.

D  The Martian meteorite found in Earth aroused doubts to the above analysis. ALH84001 meteorite was discovered in December 1984 in Antarctica, by members of the ANSMET project; The sample was ejected from Mars about 17 million years ago and spent 11,000 years in or on the Antarctic ice sheets. Composition analysis by NASA revealed a kind of magnetite that on Earth, is only found in association with certain microorganisms. Some structures resembling the mineralized casts of terrestrial bacteria and their appendages fibrils of by-products occur in the rims of carbonate globules and pre-terrestrial aqueous alteration regions. The size and shape of the objects are consistent with Earthly fossilized nanobacteria, but the existence of nanobacteria itself is still controversial.

E  In 1965, the Mariner 4 probe discovered that Mars had no global magnetic field that would protect the planet from potentially life-threatening cosmic radiation and solar radiation; observations made in the late 1990s by the Mars Global Surveyor confirmed this discovery. Scientists speculate that the lack of magnetic shielding helped the solar wind blow away much of Mars's atmosphere over the course of several billion years. After mapping cosmic radiation levels at various depths on Mars, researchers have concluded that any life within the first several meters of the planet's surface would be killed by lethal doses of cosmic radiation. In 2007, it was calculated that DNA and RNA damage by cosmic radiation would limit life on Mars to depths greater than 7.5 metres below the planet's surface. Therefore, the best potential locations for discovering life on Mars may be at subsurface environments that have not been studied yet. The disappearance of the magnetic field may be played a significant role in the process of Martian climate change. According to the valuation of the scientists, the climate of Mars gradually transits from warm and wet to cold and dry after magnetic field vanished.

F  NASA's recent missions have focused on another question: whether Mars held lakes or oceans of liquid water on its surface in the ancient past. Scientists have found hematite, a mineral that forms in the presence of water. Thus, the mission of the Mars Exploration Rovers of 2004 was not to look for present or past life, but for evidence of liquid water on the surface of Mars in the planet's ancient past. Liquid water, necessary for Earth life and for metabolism as generally conducted by species on Earth, cannot exist on the surface of Mars under its present low atmospheric pressure and temperature, except at the lowest shaded elevations for short periods and liquid water does not appear at the surface itself. In March 2004, NASA announced that its rover Opportunity had discovered evidence that Mars was, in the ancient past, a wet planet. This had raised hopes that evidence of past life might be found on the planet today. ESA confirmed that the Mars Express orbiter had directly detected huge reserves of water ice at Mars' south pole in January 2004.

G  Researchers from the Center of Astrobiology (Spain) and the Catholic University of the North in Chile have found an 'oasis' of microorganisms two meters below the surface of the Atacama Desert, SOLID, a detector for signs of life which could be used in environments similar to subsoil on Mars. "We have named it a 'microbial oasis' because we found microorganisms developing in a habitat that was rich in rock salt and other highly hygroscopic compounds that absorb water" explained Victor Parro, a researcher from the Center of Astrobiology in Spain. "If there are similar microbes on Mars or remains in similar conditions to the ones we have found in the Atacama, we could detect them with instruments like SOLID" Parro highlighted.

H  Even more intriguing, however, is the alternative scenario by Spanish scientists: If those samples could be found to that use DNA, as Earthly life does, as their genetic code. It is extremely unlikely that such a highly specialised, complex molecule like DNA could have evolved separately on the two planets, indicating that there must be a common origin for Martian and Earthly life. Life-based on DNA first appeared on Mars and then spread to Earth, where it then evolved into the myriad forms of plants and creatures that exist today. If this was found to be the case, we would have to face the logical conclusion: we are all Martian. If not, we would continue to search the life of signs.