Density and Composition
Mercury is thought to be one of the densest planets in the Solar System. This high density suggests that an iron core that makes up 85 % of the planet by weight dominates Mercury. During 2007, a team of astronomers announced that they have evidence suggesting that some of the core is molten. Bouncing radio waves off the planet and analysing the return signals made the discovery. The latest information suggests the solid iron core is surrounded by a liquid outer layer of iron, followed by a layer of iron-sulfide and a thin, solid silicate mantle and crust (see Fig. 4.4). Little is known about Mercury’s crust, but it is thought to extend down less than 100 km. The crust seems to have cooled rapidly once formed, and is solid enough to preserve surface features.
Fig. 4.4The interior of Mercury as determined by Messenger data. The core contains mostly iron and is larger than previously thought (85 % radius).
The Messenger probe has found sodium, magnesium, calcium, oxygen, helium and even water molecules streaming away from the planet. These streams are not uniform, with sodium and calcium concentrated over the polar regions, and magnesium and helium more evenly distributed. Bursts of energetic electrons are regularly detected whenever the Messenger probe passes over the mid-northern latitudes of Mercury (the source of these electrons is unknown).
Mercury is about one third the size of Earth and a little more than one-third the gravity of Earth. A 75 kg person on Earth has a weight of 735 N, but on Mercury the same person would weigh 247 N.
The Surface
The surface of Mercury is heavily cratered and looks very similar to our Moon. Most of the craters are impact craters formed from bodies colliding with the surface. The distribution of craters on Mercury, the Moon and Mars are similar, suggesting the same family of objects was responsible for the impacts on each body. One of the largest impact features on Mercury is the Caloris Basin. This basin measures about 1300 km across and has been partially flooded with lava from volcanic activity. It was probably formed by a very large impact early in the history of the Solar System. As the basin floor settled under the weight of volcanic material, fractures and ridges formed (Figs. 4.5 and 4.6).
Fig. 4.5Cameras on the Messenger probe took this image of lava flooded craters and large areas of smooth volcanic plains on Mercury (Credit: NASA/JHU).
Fig. 4.6The southern hemisphere of Mercury as seen by cameras on the Messenger probe in April, 2013. This image highlights Bach crater (lower double ringed) and the bright rays of Han Kan crater (Credit: NASA/JHU).
In addition to the heavily cratered areas, Mercury also has large regions of smooth plains caused by ancient lava flows. Mariner 10 also revealed some large escarpments over the surface, some up to hundreds of kilometres in length and 3 km high. Some of these cut through the rings of craters and others seem to be formed by tectonic forces. The largest scarp or cliff observed to date is Discovery Rupes, which is about 500 km long and 3 km high. Such scarps are thought to be due to global compression and tectonic activity as Mercury cooled.
A reanalysis of data collected by Mariner 10, suggests that recent volcanism has occurred on Mercury. Many of Mercury’s craters are not covered by volcanic flows indicating that they were formed after volcanic flows ceased. Heavy bombardment of the planet ended about 3.8 million years ago.
Recent radar analysis has found that a number of craters near each pole have high radar reflectivity, suggesting the presence of ice. The interiors of these polar craters are permanently shaded from the Sun’s heat, making the preservation of ice possible. In November 2012, NASA announced that Messenger had discovered both water ice and organic compounds in some of the permanently shadowed craters on Mercury’s north pole.
Data from cameras on the Messenger probe have confirmed that broad lava plains fill most of the land between the planet’s abundant craters. The north pole is distinctly lower in elevation than elsewhere, and was probably inundated early in Mercurian history by a volcanic outpouring of almost unimaginable size. Although there is no evidence of recent or ongoing eruptions, Mercury is not geologically dead. Its surface abounds with clusters of small shallow pits, ranging in size from tens of metres to several kilometres across. These hollows appear to be associated with some unknown black material found scattered over the surface.
Instruments on the Messenger probe have also shown that very little iron exists on Mercury’s surface. Instead, the rocks are infused with lots of magnesium, a chemical signature unique among the terrestrial worlds. There are also high abundances of sulfur, potassium, and sodium. These volatile elements vaporise at relatively low temperatures and their presence has caused scientists to rethink their ideas on how the planet formed.
In February 2013, NASA published the most detailed and accurate 3D map of Mercury to date, assembled from thousands of images taken by Messenger.
Mercury’s Atmosphere
Mercury has a very thin atmosphere. In 1974 the Mariner 10 space probe detected traces of oxygen, sodium, helium, potassium and hydrogen vapours. Earth based telescopes have detected gaseous sodium, potassium and calcium. In 2008, the Messenger spacecraft discovered magnesium and water vapour in the atmosphere of Mercury. The hydrogen and helium may have originated from the solar wind while the sodium may have come from surface rocks bombarded by the wind or meteorites. Astronomers have observed clouds of sodium vapour occasionally rising from the surface of Mercury. There are striking differences in the amounts of calcium, magnesium and sodium when the planet was closer to and further from the Sun.
The atmospheric pressure is only a million-billionth that of Earth’s—as low as many vacuums created in Earth laboratories. Mercury has very little atmosphere because its gravity is too weak to retain any significant gas particles, and it is so hot that gases quickly escape into outer space. The quantities of these gases have been poorly determined and vary depending on the position of Mercury in its orbit. The atmospheric gases are much denser on the cold night-side of Mercury than on the hot dayside.
Temperature and Seasons
Because of its closeness to the Sun, surface temperature variations are extreme on Mercury—more than on any other planet. You could roast during the day at 430 °C, and freeze during night at −180 °C. The high day temperature would be hot enough to melt the metals zinc and tin. One day on Mercury is equal to 58.65 Earth days, so it takes a long time to warm up from the cool of night and it takes a long time to cool down from the heat of day.
Mercury is not the hottest planet on average. The temperature on Venus is slightly hotter than on Mercury but is more stable because of thick clouds on that planet.
On Earth, the seasons change in a regular way due because the rotational axis is inclined (at 23.5° from the perpendicular to its orbital plane). As a result, each hemisphere on Earth receives more direct sunlight during one part of the orbit than the other. Mercury’s axis is very near perpendicular to its orbital plane (2° from perpendicular), so no seasonal changes occur. Some craters near Mercury’s poles never receive any sunlight and are permanently cold (Figs. 4.7 and 4.8).
Fig. 4.7The International Astronomical Union (IAU) has named an impact crater on the planet Mercury after John Lennon, the British pop music sensation who helped make The Beatles the most popular group of their generation. Lennon is one of ten newly named craters on the planet, joining 114 other craters named since NASA’s Messenger spacecraft’s first Mercury flyby in January 2008 (Credit: NASA/Johns Hopkins Applied Physics Lab/Carnegie Institution).
Fig. 4.8An image taken by cameras on the Messenger probe of Rembrandt impact crater on Mercury. The surface contains impact craters, wrinkle ridges and plains covered with lava (Credit: NASA/JHU).
Magnetic Field
The magnetic field of a planet is generated by electric currents flowing in its molten metal core as the planet rotates. Because Mercury rotates slowly (59 times slower than Earth) it was not expected to have a magnetic field. In 1974 Mariner 10 did detect a very weak magnetic field but the Messenger probe was
able to measure it more accurately. The magnetic field tends to be stronger at the equator than at other areas of Mercury but is only 1 % the intensity of Earth’s magnetic field. Magnetometers on the Messenger found that the source of the magnetic field is not dead centre in the planet’s interior; rather it is offset toward the north pole by 480 km, about 20 % of Mercury’s radius. This northward offset leaves the planet’s southern hemisphere more vulnerable to bombardment by space radiation. The offset also suggests that the dynamo responsible for the magnetic field originates not in the planet’s heart but closer to its core-mantle boundary.
The Messenger spacecraft also found that Mercury’s magnetic field is responsible for several magnetic ‘tornadoes’—twisted bundles of magnetic fields connecting the planetary field to interplanetary space—these are around 800 km wide or a third the total radius of the planet.
Further Information
www.nasa.gov/mission_pages/messenger/main/index.html
www.space.com/36-mercury-the-suns-closest-planetary-neighbor.html
http://solarsystem.nasa.gov (and click on Mercury)
© Springer International Publishing Switzerland 2016
John WilkinsonThe Solar System in Close-UpAstronomers' Universe10.1007/978-3-319-27629-8_5
5. Venus: A Hot, Toxic Planet
John Wilkinson1
(1)Castlemaine, Victoria, Australia
Highlights
Venus was volcanically active as recently as 2.5 million years ago.
Venus is a much more inhospitable world than Earth, with surface temperatures topping 450 °C and a super-dense atmosphere composed of toxic gases.
Venus has an unusual super-rotating upper atmosphere, which flies around the planet once every 4 days.
A huge double atmospheric vortex exists at the south pole of Venus.
Flashes of lightning regularly occur in the sulfuric acid clouds of Venus.
In July 2014 the Venus Express probe used aerobraking maneuvers to lower its orbit to within 250 km of Venus’s north pole (just above the top of the atmosphere). It found conditions to be more variable than previously thought.
Venus is the second planet from the Sun, orbiting on average at a distance of 108 million km from the Sun. It is the sixth largest planet with a diameter of 12,104 km. Venus is sometimes regarded as Earth’s sister planet since it is similar in size and mass to Earth. Venus orbits the Sun between Mercury and Earth but is twice as far from the Sun as Mercury. It comes closer to Earth than any other planet in the solar system.
One of the strange things about Venus is that it spins on its axis in the opposite direction to that of the other terrestrial planets—it seems to be upside down with its north and south poles reversed. The unusual state is thought to be due to a massive impact early in the planet’s life.
Venus 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 close to the Sun, Venus must have been hot and in a molten state before it cooled to become a solid planet. Out of all the planets, Venus still has the hottest surface temperature, even though it is not the closest planet to the Sun. This is mainly due to its dense atmosphere, which traps heat and pushes down on the surface with a pressure 92 times that experienced on Earth. In fact temperatures are hot enough to melt the metals tin, zinc and lead.
The surface of Venus is completely covered by thick clouds and this makes it impossible to see the surface from Earth or from space without special radar imaging techniques.
Venus has no natural satellites (Moons) (Fig. 5.1).
Fig. 5.1The planet Venus is completely covered by dense clouds. In order to see the surface, special radar imaging techniques need to be used. This image was taken by the Messenger probe during a recent flyby of the planet (Credit: NASA).
Early Views About Venus
Venus is the brightest planet as seen from Earth. At certain times of the year it can be seen in the evening sky just after sunset; at other times of the year it appears to rise in the east just before sunrise.
Venus was well known to the ancient Greeks and Romans because of its brightness in the night sky, but the Greeks believed Venus to be two different objects: Phosphorus as the morning star and Hesperos as the evening star.
To the ancient Romans, Venus was the goddess of love and beauty (Venus is the only planet named after a goddess). The brightness of Venus as seen from Earth is due to its covering of dense clouds which reflect over three-quarters of the sunlight received by the planet. These clouds completely hide the surface of the planet from view (Table 5.1).Table 5.1Details of Venus
Distance from Sun
108,200,000 km (0.72 AU)
Diameter
12,104 km
Mass
4.87 × 1024 kg (0.82 Earth’s mass)
Density
5.25 g/cm3 or 5250 kg/m3
Orbital eccentricity
0.007
Period of revolution
224.7 Earth days
Rotation period
243 Earth days
Length of year
0.615 Earth years
Orbital velocity
126,108 km/h
Tilt of axis
177.3°
Day temperature
480 °C
Night temperature
470 °C
Number of Moons
0
Atmosphere
Carbon dioxide
Strength of gravity
8.1 N/kg at surface
Probing Venus
People have long thought Venus being close to Earth and similar in size and mass, would have conditions suitable for life. However, early space probes sent to Venus disproved this theory. The probes found Venus to have a hostile-to-life or hell-like environment. The thick atmosphere, high surface temperature and high pressure hampered early exploration by spacecraft, and many probes were unsuccessful. Both the USSR and USA have sent more probes to Venus than any other planet, mainly because of its closeness to Earth.
During the 1960s the USSR launched a series of Venera spacecraft on missions to Venus. Venera 1 was the first space probe to flyby Venus in 1961, but communications with Venera 2 and 3 failed just before arrival. The first successful probe to enter the Venusian atmosphere was Venera 4 on 18 October 1967. Although this craft was crushed during descent, it sent back useful data on the planet’s atmosphere including its chemical composition, pressure, and temperature. In 1969, both Venera 5 and 6 returned data indicating an atmosphere of 93–97 % carbon dioxide. Venera 6 returned data down to within 26 km of the surface before being crushed by the pressure. Venera 7 achieved the first successful landing of a spacecraft on any planet on 15 December 1970. The probe used an external cooling device to allow it to send back 23 min of data. Venera 9 included an orbiter and a lander—the lander arrived on the Venusian surface on 22 November 1975 and transmitted the first black and white images of the planets surface. Venera 13 survived for 2 h and 7 min on the Venusian surface. It took colour images and analysed a soil sample. The first colour panoramic views of the surface were sent back by Venera 14 in November 1981. This probe also conducted soil analysis using an X-ray fluorescence spectrometer. Venera 15 and 16 were the first spacecraft to obtain radar images of the surface from orbit. The images were used to produce a map of the northern hemisphere from the pole to 30° north latitude. During 1985 Vega 1 and 2 flew by Venus on their way for a flyby of comet Halley. Vega 1 dropped off a Venera-style lander and a balloon to investigate the cloud layers. The lander from Vega 1 failed, but Vega 2’s lander was able to collect soil samples. Both Vega 1 and 2 are now in solar orbit.
The USA through NASA was also active in sending probes to Venus. The first USA probe to make a flyby of Venus was Mariner 2 on 14 December 1962. Mariner 2 passed Venus at a distance of 34,800 km and scanned its surface with infrared and microwave radiometers, showing the surface temperature to be about 425 °C. Mariner 5 co
nfirmed the temperatures in 1967 as it passed within 3900 km of the planet. It also studied the magnetic field and found an atmosphere containing 85–97 % carbon dioxide. Mariner 10 flew past Venus on 5 February 1974 for a gravity assist to the planet Mercury. It recorded circulation in the atmosphere of Venus and showed the temperature of the cloud tops to be −23 °C.
In 1978 the USA launched two Pioneer Venus probes to orbit Venus. Pioneer Venus 1 (also known as Pioneer 12) operated continuously from 1978 until 8 October 1992, when contact was lost and it burnt up in the Venusian atmosphere. The orbiter was the first probe to use radar imaging in mapping the planet’s surface. Pioneer Venus 2 (also known as Pioneer 13) carried four atmospheric probes that were released on 9 December 1978. The four probes descended by parachute and collected data on the atmospheric layers before burning up in the atmosphere. One of the sub-probes landed intact and sent back data for over an hour.
The Galileo spacecraft flew past Venus on its way to Jupiter in February 1990. The USA also sent the Magellan spacecraft into orbit around Venus in August 1990. This probe was launched from the space shuttle Atlantis in May 1989 and took 15 months to reach Venus. Its main mission was to produce a high-resolution map of Venus using synthetic aperture radar, which can see through clouds. The spacecraft mapped 99 % of the planet’s surface using a polar orbit. In 1994, the craft was directed into the atmosphere where it burned up (Fig. 5.2).
The Solar System in Close-Up Page 8
