Types of Radiation from the Sun
The Sun gives off many kinds of radiation besides visible light and heat. These radiations include radio waves, ultraviolet rays and X-rays. Space probes that orbit the Sun make observations and take pictures in the different wavelengths of electromagnetic radiation.
The Sun’s chromosphere and corona are also emitters of radio waves. These were first recorded in 1942 during World War II by British radars as ‘radio noise’. Such radio emissions often originate from sunspots and produce what we call ‘solar storms’. These radio waves can be collected via radio telescopes on Earth. Observations of the Sun using radio waves provide information different to that obtained from visible wavelengths because the propagation of the two types of radiations are different. For example, coronal gas is transparent to visible light but is opaque to radio waves.
Ultraviolet rays are electromagnetic waves with a shorter wavelength than visible light. They are invisible to the human eye. The Sun gives off more ultraviolet radiation during times of increased solar activity. The Earth’s atmosphere absorbs much of this radiation. Scientists have divided the ultraviolet part of the spectrum into three regions: the near ultraviolet, the far ultraviolet, and the extreme ultraviolet. The three regions are distinguished by how energetic the ultraviolet radiation is, and by the “wavelength” of the ultraviolet light, which is related to energy. The near ultraviolet, abbreviated NUV, is closest to optical or visible light. The extreme ultraviolet, abbreviated EUV, is the ultraviolet light closest to X-rays, and is the most energetic of the three types. The far ultraviolet, abbreviated FUV, lies between the near and extreme ultraviolet regions.
X-rays are another form of solar radiation with a very short wavelength. The Sun’s X-rays can injure or destroy the tissue of living things. The Earth’s atmosphere shields human beings from most of this radiation. Hard X-rays are the highest energy X-rays, while the lower energy X-rays are referred to as soft X-rays. The distinction between hard and soft X-rays is not well defined.
X-rays do not penetrate the Earth’s atmosphere. Therefore they must be observed from a platform launched above most of our atmosphere (Fig. 3.10).
Fig. 3.10Types of radiations emitted by the Sun.
Solar Eclipses
A total solar eclipse is one of the most spectacular astronomical events seen from Earth.
Such an event occurs when the Moon passes directly between the Sun and Earth. An eclipse does not occur at every new moon because the Moon’s orbit often passes above or below the Sun instead of directly across it. During such an eclipse, the Moon’s shadow traces a curved path across the surface of the Earth. Any person standing in the path of the shadow will see the sky and landscape go dark as the Moon blocks out the sunlight (see Figs. 3.11 and 3.12).
Fig. 3.11How an eclipse of the Sun occurs.
Fig. 3.12The solar corona as seen during a total solar eclipse.
Solar eclipses can be total, partial, or annular, depending on how much Sun is covered by the Moon. Total eclipses occur when the Moon is exactly in line between the Earth and Sun and exactly covers the disc of the Sun. If the Moon is not exactly in line between the Earth and Sun only a partial eclipse occurs. An annular eclipse occurs when the Moon’s is far enough away from Earth its apparent size is smaller than the Sun’s. Hence a bright ring (annulus) remains visible, and the Sun’s corona cannot be seen.
There are as many as two total solar eclipses a year, and sometimes as many as five, but few people have a chance to see them. The paths along which eclipses can be seen are narrow, and totality can last only about seven and a half minutes at most.
Among the features of a total eclipse are the so-called “Bailey’s Beads”. These are seen just as the Moon’s black disc covers the last thin crescent of the Sun. Sunlight shining between the mountains at the Moon’s edge looks like sparkling beads. The Diamond Ring effect is a fleeting flash of light seen immediately preceding or following totality.
At the time of totality, an observer with a small telescope can see the Sun’s prominences as long flame-like tongues of incandescent gases around the edge of the Moon’s disc. Also during totality, the corona can be seen as a region of glowing gases stretching out from the blacked-out Sun. Care must be taken when observing the Sun, even during an eclipse—and advice should be sought to protect your eyes from damage.
Influence of the Sun on Earth
The Sun has a steady output of charged particles and other matter that is collectively known as the solar wind. This wind streams through the Solar System at about 400 km/s. This wind interacts with the atmosphere of Earth and charged particles in particular get trapped in the Earth’s magnetic field (the magnetosphere). Our magnetic fields and atmosphere in some ways protect us from some solar radiation and cosmic rays from outer space, but they also let in some of the radiation. Outbursts of radiation from the Sun can have dramatic effects on Earth. When a burst of solar radiation strikes the Earth’s magnetic field the result can be geomagnetic storms that spark huge electric currents and distort the magnetosphere. This can adversely effect radio communication and navigational systems, and pump extra electricity into power lines (sometimes causing blackouts).
Researchers believe that changes in solar activity are having an indirect effect on Earth’s climate. Satellite measurements, for example, have detected a small change in the Sun’s total output during the course of each sunspot cycle. The ebb and flow of solar radiation can heat and cool the atmosphere of Earth enough to change its circulation patterns, which may have significant impacts on regional weather. Researchers have developed powerful computer models to simulate the impact of the Sun on our climate. One such effort, the Whole Atmosphere Community Climate Model (WACCM), helps researchers study interactions among different levels of the atmosphere, ranging from the surface of Earth to the upper atmosphere and the edge of space. The modelling work is combined with the analyses of data from observing instruments aboard satellites to track the impacts of solar radiation throughout the atmosphere.
Much of what we have learned has been realized in only the last few decades. Solar space missions such as NASA’s TRACE (Transition Region and Explorer Spacecraft) and the SOHO (Solar and Heliospheric Observatory) have provided answers to many questions regarding the effect of the Sun on Earth. But there is a lot of work still to be done and many new questions need answering.
The Sun’s Future
The Sun is about 4.5 billion years old and is not quite half way through its life cycle. Throughout the second half of its life the Sun is expected to increase gradually in size, luminosity and temperature. In about 5 billion years time the Sun will have expanded to about three times its present size. As the Sun uses up its hydrogen it will become more orange in colour. By this time, temperatures on Earth will be much hotter and all the water will have evaporated. As the Sun continues to expand to about 100 times its present size it will become a red giant engulfing Mercury and Venus. The Earth will be scorched to a cinder. As hydrogen is used up, the core of the Sun will slowly contract, forcing the Sun’s central temperature to increase. When the core temperature reaches 100 million degrees Celsius, helium fusion begins to generate carbon and oxygen. Temperature in the core continues to rise causing helium to fuse at an increasing rate. An explosion (the helium flash) will result and a third of the Sun will be blown away. Eventually the Sun will lose its outer layers and contract to become a white dwarf about the size of Earth.
Further Information
See the book “New Eyes on the Sun—a guide to satellite images and amateur observation” by John Wilkinson and published by Springer (2012).
www.solarsystem.nasa.gov (click on the Sun)
www.nineplanets.org (click on the Sun)
© Springer International Publishing Switzerland 2016
John WilkinsonThe Solar System in Close-UpAstronomers' Universe10.1007/978-3-319-27629-8_4
4. Mercury: The Iron Planet
John Wilkinson1
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(1)Castlemaine, Victoria, Australia
Highlights
NASA’s Messenger spacecraft is the first probe ever to orbit the planet Mercury. It began orbiting the innermost planet in the solar system in 2011 and has recorded more than 200,000 photos of Mercury.
Pictures taken by Messenger of the far side of Mercury show it is a shrinking, ageing planet.
Mercury is dominated by an iron core, which makes up 85 % of the planet by weight (much larger than previously thought).
In November 2012, Messenger discovered both water ice and organic compounds in some of the permanently shadowed craters on Mercury’s north pole.
Magnetometers on the Messenger probe found that the source of the magnetic field on Mercury is not dead centre in the planet’s interior.
Mercury is the planet in the solar system nearest to the Sun, with an average orbital radius of 58 million km. It is also the smallest planet of the inner solar system with a diameter of only 4880 km, making it about the size of our Moon. Mercury travels fast, taking just 88 days to orbit the Sun, but it takes a slow 58.65 days to rotate once on its axis. Of all the terrestrial planets, Mercury’s orbit is the most elliptical. Its elliptical orbit and slow rotation gives it large variations in surface temperatures. During the day temperatures can reach a blistering 430 °C, while at night they can drop to a freezing −180 °C. No other planet experiences such a wide range of temperatures.
Mercury is thought to have formed at the same time as the other planets in the solar system about 4.5 billion years ago. Because it is so close to the Sun, Mercury must have been very hot and in a molten state before it cooled to become a solid planet. As Mercury cooled, it also began to contract. Since it formed, the surface of Mercury has been churned up by many meteorite impacts (Fig. 4.1).
Fig. 4.1Mercury as seen by the Messenger probe during its 2008 flyby of the planet. The sprawling Caloris basin (upper right) is one of the solar system’s largest impact basins. Created during the early history of the solar system by the impact of a large asteroid-sized body, the basin spans about 1500 km and is seen in yellowish hues in this enhanced colour mosaic. Orange splotches around the basin’s perimeter are now thought to be volcanic vents, new evidence that Mercury’s smooth plains are indeed lava flows (Credit: NASA).
Early Views About Mercury
Mercury has been known since the time of the Sumerians (3rd millennium BC). The planet was given two names by the ancient Greeks: Apollo for its apparition as a morning star and Hermes as an evening star. Greek astronomers knew, however, that the two names referred to the same body. To the Greek’s, Hermes was the messenger of the Gods. In Roman mythology Mercury was the god of commerce, travel and thievery. The planet probably received this name because it moves so quickly across the sky.
Giovanni Schiaparelli using a simple telescope made the first map of Mercury in the 1880s. The map only showed areas of dark and light. A more detailed map was produced by Eugenios Antoniadi between 1924 and 1933, but has since been proved inaccurate. Both these astronomers believed Mercury to rotate once on its axis in 88 Earth days, with one hemisphere permanently facing the Sun. This meant that Mercury’s day was the same length as its year. However, radar measurements carried out in the early 1960s showed that the true axial rotation period was 58.6 days. Thus it is now known that Mercury rotates three times during two orbits of the Sun. The result of this is that the same hemisphere is pointed towards Earth every time the planet is best placed for observation. This effect also means that the Mercurian day (sunrise to sunset) is 176 Earth-days long, or two Mercurian years.
Antoniadi also believed that Mercury had an atmosphere because he thought he could see clouds above its surface. We now know that Mercury’s atmosphere is far too tenuous to support clouds. The lack of clouds is also due to the fact that Mercury’s escape velocity is only 4.3 km/s, so any gas particles would be moving too quickly to be restrained by Mercury’s gravity (Table 4.1).Table 4.1Details of Mercury
Distance from Sun
57,910,000 km (0.38 AU)
Diameter
4880 km
Mass
3.3 × 1023 kg (0.055 Earth’s mass)
Density
5.43 g/cm3 or 5430 kg/m3
Orbital eccentricity
0.206
Period of revolution (length of year)
88 Earth days or 0.241 Earth years
Rotation period
58.65 Earth days
Orbital velocity
172,400 km/h
Tilt of axis
2°
Day temperature
430 °C
Night temperature
−180 °C
Number of Moons
0
Atmosphere
Practically none (some oxygen, sodium, helium)
Strength of gravity
3.3 N/kg at surface
Probing Mercury
Mercury is the least explored of our solar system’s inner planets. To date the planet has been visited by only two spacecraft—Mariner 10 and Messenger. Mariner 10 flew past Venus on 5 February 1974, in order to get a gravity assist to Mercury. It flew by the planet three times between March 1974 and March 1975. Mariner 10 was also the first spacecraft to have an imaging system, and the encounter produced over 10,000 pictures that covered 57 % of the planet. Mercury is too close to the Sun to be mapped by the Hubble Space Telescope.
Another NASA spacecraft, called Messenger, was launched on a mission to Mercury on 2 August 2004. Messenger stands for ‘MErcury Surface, Space ENvironment, GEochemistry and Ranging’. This probes 7-year journey included 15 trips around the Sun, one Earth fly-by, two Venus fly-bys and three Mercury fly-bys (January 2008, October 2008, September 2009) before it entered orbit around Mercury in March 2011. The fly-bys helped focus the science mission before the spacecraft entered orbit. With a package of seven scientific instruments Messenger has been able to determine Mercury’s composition, map its surface and magnetic field, measure the properties of its core, explore the mysterious polar deposits to learn whether ice lurks in permanently shadowed regions, and characterise Mercury’s tenuous atmosphere and Earth-like magnetosphere.
Pictures taken by Messenger in January 2008 show the far side of Mercury contains wrinkles of a shrinking, ageing planet. There are scars from volcanic eruptions and craters with a series of troughs radiating from them (Figs. 4.2 and 4.3).
Fig. 4.2Messenger’s looping polar orbit around Mercury ranges in altitude from just 200 km to about 15,000 km. The spacecraft uses the close-ins for observations and beams back its data to Earth when it is far out.
Fig. 4.3The Messenger space probe took this photo of the far side of Mercury in January 2008. The previously unseen features suggest the planet is old and wrinkly (Credit: JHU/NASA).
Messenger’s primary mission finished on 17 March 2012, having collected over 100,000 images. In November 2013, NASA managers decided to extend the mission through to March 2015.
Messengers mission finally ended on 30 April 2015 when it crashed into the surface of Mercury creating a new crater. The mission exceeded all expectations.
A joint European-Japanese mission, to Mercury is due for launch on 15 August 2015. The spacecraft, called BepiColombo, will arrive at Mercury in January 2022 for a 1-year nominal mission with a possible 1-year extension. BepiColombo will use the gravity of the Earth, Venus and Mercury in combination with solar-electric propulsion to journey to Mercury. When approaching Mercury, the spacecraft will use the planet’s gravity plus conventional rocket engines to insert itself into a polar orbit (Table 4.2).Table 4.2Significant space probes sent to Mercury
Name of probe
Country of origin
Date launched
Notes
Mariner 10
USA
1973
Now in solar orbit
Messenger
USA
2004
Orbiting Mercury 2011–2015
Posi
tion and Orbit
Mercury has the most eccentric orbit of all the planets. At closest approach (perihelion) it is only 46 million km from the Sun but its furthest distance (aphelion) is 70 million km. At perihelion Mercury travels around the Sun at a very slow rate.
Observation of Mercury is difficult because of its close proximity to the Sun. The best time to view the planet is twice a year when it appears above the horizon at its greatest distance from the Sun. At these times, Mercury can be seen just before sunrise or just after sunset as an orange object.
The axis of rotation of Mercury is almost vertical. This means the plane of its equator coincides with the plane of its orbit.
Mercury has the shortest year of any planet, taking only 88 days to orbit the Sun. It has no known satellites (moons). Because it is closer to the Sun than Earth, Mercury is seen to go through phases just like our Moon. Mercury’s size appears to vary according to its phases because of its changing distance from Earth. When Mercury first appears in the evening sky, it is coming around the far side of its orbit toward us, and through a telescope appears as a full crescent.
At rare intervals, observers from Earth can see Mercury pass in front of the Sun. Such a passing is called a transit. Transits occurred on 7 May 2003 and 8 November 2006. A transit should be viewed by projecting the Sun’s image from a telescope, onto a white screen. A planet would appear as a black dot slowly moving across the Sun’s image. Care should always be taken when viewing the Sun—never look directly at the Sun with a telescope.
The Solar System in Close-Up Page 7
