Most of the craters on the near side are named after famous figures in the history of science, such as Tycho, Copernicus and Kepler. Other craters bear the names of philosophers such as Plato and Archimedes. Features on the far side have modern names such as Apollo, Gagarin and Korolev, with a distinctly Russian bias, since the first images of the far side were obtained by Luna 3. The largest impact basin (crater) is the Aitken basin, 2500 km wide and 12 km deep, at the south pole on the far side. The Imbrium basin, about 1800 km wide and the Crisium basin, about 1100 km wide, are both found on the near side.
In contrast to the dark maria, the light-coloured highlands are elevated regions that make up about 84 % of the lunar surface. Some of the highlands are mountains or ridges that form the rims of large basins that were formed from material uplifted after impacts. One of the highest mountains on the Moon, the Apennines, forms part of the Imbrium basin (see Fig. 6.9).
Highland rocks are light anorthosites (feldspars) rich in calcium and aluminium. Many highland rocks brought back to Earth are impact breccias, which are composites of different rocks fused together as a result of meteorite impacts.
Radioactive dating of Moon rocks have shown the mare rocks to be between 3.1 and 3.8 million years old, while the highland rocks are between 4.0 and 4.3 billion years old.
In September 2009, India’s ISRO Chandrayaan-1 detected water-ice on the Moon and hydroxyl absorption lines in reflected sunlight. In March 2010, it was reported that the Mini-RF on board the Chandrayaan-1 probe had discovered more than 40 permanently darkened craters near the Moon’s north pole that are thought to contain an estimated 600 million metric tonnes of water-ice.
In November 2009, NASA reported that its LCROSS space probe had detected a significant amount of hydroxyl in the material thrown up from a south polar crater by an impactor; this may be attributed to water-bearing materials such as pure crystalline water-ice. The suite of LCROSS and LRO instruments determined as much as 20 % of the material kicked up by the LCROSS impact was volatiles, including methane, ammonia, hydrogen gas, carbon dioxide and carbon monoxide. The instruments also discovered relatively large amounts of light metals such as sodium, mercury and possibly even silver. The science team at NASA found the water-ice on the Moon is not uniformly distributed within the shadowed cold traps, but rather is in pockets, some of which may lie outside the shadowed regions.
Scientists believe water may have been delivered to the Moon over geological timescales by the regular bombardment of water-bearing comets, asteroids and meteoroids or continuously produced in situ by the hydrogen ions (protons) of the solar wind impacting oxygen-bearing minerals. The search for the presence of lunar water has attracted considerable attention and motivated several recent lunar missions, largely because of water’s usefulness in rendering long-term lunar habitation feasible.
Large concentrations of mass lurk under the lunar surface. These concentrations change the gravity field and can either pull a spacecraft in or push it off course. The GRAIL missions have now mapped where theses areas are, and scientists have a much better understanding of how they developed. In 2012, the GRAIL team confirmed the theory that the concentrations of mass (called Mascons) were caused by massive asteroid impacts billions of years ago. The researchers determined that ancient asteroid impacts excavated large craters on the Moon, causing surrounding lunar materials and rocks from the Moon’s mantle to melt and collapse inward. This melting caused the material to become denser and more concentrated.
In 2014, scientists studying GRAIL data reported that they have found evidence that the craggy outline of the Oceanus Procellarum region of the Moon is actually the result of the formation of ancient rift valleys. The rifts are buried beneath dark volcanic plains and are unlike anything found anywhere else on the Moon. Another theory arising from the data analysis suggests this region formed as a result of churning deep in the interior of the Moon that led to a high concentration of heat-producing radioactive elements in the crust and mantle of this region.
The Atmosphere of the Moon
The Moon has no real atmosphere and no liquid water so life could not exist for long. Atmospheres are held in place by gravity, and the Moon has so little gravitational pull that it is unable to hold any of the gases such as those that make up the Earth’s atmosphere.
The LADEE spacecraft orbited close to the surface of the Moon during the first few months of 2014 and found a veil of micron-size dust particles continuously encases the Moon. The rain of meteorite matter that hits the surface kicks up these particles. The spacecraft picked up helium, neon, and argon in the Moon’s tenuous, transient atmosphere, and it detected atoms of magnesium, aluminium, titanium, and oxygen—the remnants of rocky mineral blasted upwards from the lunar surface. LADEE smacked into the farside at 5700 km/h on 17 April 2014.
Temperature
There is a range of temperatures on the surface of the Moon, because of its lack of atmosphere. At night temperatures can fall to −184 °C, while on parts of the Moon facing the Sun temperatures can reach 130 °C. At the poles, temperatures are constantly low, about −96 °C. Some polar regions are in permanent shadow.
Because the Moon rotates once on its axis every 27.3 days, night and day at any point on the Moon last about 14 Earth days. On the side of the Moon that always faces Earth, ‘phases of the Earth’ would be observed. Part of the long period of night would be ‘Earth lit’, just like we have ‘Moon lit’ nights on Earth.
Magnetic Field
The Moon has no global magnetic field, but some rocks brought back by the Apollo astronauts exhibit permanent magnetisms. This suggests that there may have been a global magnetic field early in the Moon’s history.
With no atmosphere and no magnetic field, the Moon’s surface is directly exposed to the solar wind. Since the Moon’s early days many charged particles from the solar wind would have become embedded in the Moon’s regolith (surface material). Samples of regolith returned by the Apollo astronauts confirmed the presence of these charged particles.
Further Information
http://nssdc.gsfc.nasa.gov (click on the Moon)
www.moon.nasa.gov
For specific information about the Moon and its features see the book: “The Moon in Close Up”, written by John Wilkinson and published by Springer (2010).
© Springer International Publishing Switzerland 2016
John WilkinsonThe Solar System in Close-UpAstronomers' Universe10.1007/978-3-319-27629-8_7
7. Mars: The Red Planet
John Wilkinson1
(1)Castlemaine, Victoria, Australia
Highlights
The two rovers Spirit and Opportunity have found that water once existed on the Martian surface in its past.
The Martian surface contains many large basins formed when large meteors or asteroids have hit the surface.
The soil on Mars is mostly iron-rich clay that contains iron, silicon and sulfur; and it is slightly magnetic.
The Curiosity probe that landed in Gale crater has found evidence that rivers and streams once flowed over the crater floor.
Small amounts of water vapour in Mar’s atmosphere can form fogs and clouds.
The two moons of Mars are probably captured asteroids.
Mars is regarded as one of Earth’s neighbours in space. Many people have considered Mars to be the most likely planet, apart from Earth, to contain life. Mars is the fourth planet from the Sun, orbiting at an average distance of 228 million km. This distance is about one and a half times the distance Earth is from the Sun. Radio signals take between 2.5 min and 20 min to travel one way between Earth and Mars, depending on where Mars is in its orbit in relation to Earth. At times Mars is the third brightest planet we see in our night sky, after Venus and Jupiter. It has a diameter of 6794 km, about half Earth’s diameter, making it the seventh largest planet. Mars is often referred to as ‘the red planet’ because it appears red from Earth. This colour is due to the large amounts of red dust that cover its
surface. The planet is thought to have formed about 4.5 billion years ago, at the same time as the other planets in the solar system. Because it is relatively close to the Sun, Mars must have been hot and in a molten state before it cooled to become a solid planet (see Fig. 7.1).
Fig. 7.1A view of Mars as seen by Mars Global Surveyor. Notice the three volcanoes on the right hand side and the “Valles Marineris” canyon across the middle of the image (Credit: NASA).
Early Views About Mars
Mars is named after the ancient Roman god of war. Both the ancient Greeks and Romans associated Mars with war because its colour resembles that of blood. The Greeks called the planet Ares. The two moons of Mars, Phobos (fear) and Deimos (panic) are named for the sons of the Greek god of war. The month of March, derives its name from Mars.
Mars has been known since prehistoric times and many ancient astronomers have studied its motion in the night sky. The Danish astronomer Tycho Brahe (1546–1601) made decades of observations of Mar’s motion. Tycho’s work was continued by Johannes Kepler (1571–1630) who used the observations to develop the first two of his three laws of planetary motion. These included the conclusion that planets orbit in an elliptical path, with the Sun at one focus.
Another early astronomer to study Mars was the Italian-French Giovanni Domenico Cassini who, in 1666, made the first reasonably accurate measurements of Mars’s axial rotation period, which he found to be 37.5 min longer than that of the Earth. Cassini was also the first to observe the Martian polar ice caps. The first observations of the surface markings were made in 1659 by Christiaan Huygens who drew the dark triangular feature we now know as the large plateau Sytris Major. Huygens also estimated the length of the Martian day to be about 24 h.
In 1777, William Herschel measured the tilt of the axis of Mars and deduced it must have seasons like Earth because it underwent regular changes in its polar ice caps.
Wilhelm Beer and Johann von Madler made the first detailed maps of the Martian surface features during the 1830s.
In 1877 the Italian astronomer Giovanni Schiaparelli reported seeing ‘canali’ (channels) on the surface of Mars. American Percival Lowell reported seeing similar features in the early 1900s. Lowell thought Mars was a desert world and that inhabitants of Mars used the features to carry water from the ice caps to equatorial regions. Exploration of Mars by space probes and high-resolution telescopes has since disproved the existence of such features (Table 7.1).Table 7.1Details of Mars
Distance from Sun
227,940,000 km (1.52 AU)
Diameter
6794 km
Mass
6.42 × 1023 kg (0.107 Earth’s mass)
Density
3.95 g/cm3 or 3950 kg/m3
Orbital eccentricity
0.093
Period of revolution
687 Earth days
Rotation period
1.029 Earth days
Length of year
1.881 Earth years
Orbital velocity
86,868 km/h
Tilt of axis
25.2°
Average temperature
−60 °C
Number of Moons
2
Atmosphere
Carbon dioxide
Strength of gravity
3.6 N/kg at surface
Probing Mars
People on Earth have observed Mars through telescopes based on Earth and in space. Early space probes carried telescopes and cameras to observe Mars as they flew past it. Later probes went into orbit around Mars and collected much more data. More recently, probes have successfully landed on the Martian surface, but to date, no human has yet set foot on Mars.
The first probe to fly by Mars was Mars 1, launched by the USSR on 1 November 1962, failed to return data. Between 1965 and 1969 the USA’s Mariner 4, 6 and 7 probes passed by Mars and took many photographs of the desert-like surface. The thin atmosphere was confirmed to be composed of carbon dioxide, and a weak magnetic field was detected.
The Mars 2 space probe reached Mars in 1971 and released a lander that crashed into the Martian surface when its rockets failed to slow it down. No data was returned but it was the first human-made object to be placed on Mars. The Mars 3 probe arrived at Mars on 2 December 1971 and a lander was successfully placed on the surface, however it returned video data for only 20 s.
The first US spacecraft to enter an orbit around Mars was Mariner 9 on 3 November 1971. At the time of its arrival a huge dust storm was in progress on Mars and many experiments had to be delayed until the storm had finished. The probe sent back the first high-resolution images of the moons, Phobos and Deimos. Mariner 9 took over 7000 images of Mars and showed ancient volcanoes and river-like features exist on the surface.
In 1976 each of the Viking 1 and 2 probes placed landers on the surface of Mars. Both landers sent back a great deal of information about surface features and atmospheric conditions as well as conducting experiments to search for micro-organisms. No conclusive evidence of life was found.
The Mars Global Surveyor launched by the USA on 7 November 1996, consisted of an orbiter and robotic lander. The probe was inserted into a low altitude, nearly polar orbit on 12 September 1997 and it now circles Mars once every 2 h. The mission has studied the entire Martian surface, atmosphere and interior, and has returned more data about the red planet than all other missions combined.
Mars Pathfinder (USA) arrived at Mars on 4 July 1997. It used an innovative method of directly entering the atmosphere, assisted by a parachute to slow its descent and a giant system of airbags to cushion the impact. The landing site was an ancient, rocky, flood plain in Mars’ northern hemisphere known as Ares Vallis. A six-wheel robotic rover, named Sojourner, rolled onto the Martian surface on 6 July. Mars Pathfinder returned 2.6 billion bits of data, including more than 16,000 images from the lander and 550 images from the rover, as well as more than 15 chemical analyses of rocks and extensive data on winds and other weather factors.
In April 2001, the United States launched the Mars Odyssey space probe. The probe carried instruments to analyse the chemical composition of the surface and rocks just below the surface. The mission also looked for the presence of water ice on Mars and looked for radiation hazards in the space surrounding Mars. Mars Odyssey went into orbit around Mars in October 2001.
In January 2004, NASA landed two robotic rovers (Spirit and Opportunity) on Mars, as part of the Mars Exploration Rover project. The rovers originally had 90-day missions that called for them to search for signs of past water activity on the red planet. Both rovers far outlasted their planned mission lengths. After becoming bogged in soft soil, Spirit was declared dead in 2011, but Opportunity has continued roving. Both rovers made big discoveries that have fundamentally reshaped scientists’ understanding of Mars and its environmental history. One of the key discoveries was that water once existed on the Martian surface (Fig. 7.2).
Fig. 7.2The Mars exploration Rover named Opportunity has been exploring Mars since landing inside Eagle Crater on 25 January 2004. In its first decade of driving on Mars, opportunity covered over 38 km (Credit: NASA).
Mars Express was Europe’s first mission to another planet (launched in 2003, reaching Mars in 2005). It provided subsurface measurements with the first radar instrument ever flown to Mars, and discovered underground water-ice deposits. It sent back mineralogical evidence for the presence of liquid water throughout Martian history and measured the density of the planet’s crust. The orbiter’s unique orbit also has allowed it to make up-close studies of Phobos, the larger of Mars’ two moons. The mission has been extended several times.
NASA’s Mars Reconnaissance Orbiter (MRO), arrived at the red planet in March 2006 and spent half a year gradually adjusting the shape of its orbit. During 2007, the probe orbited the planet once every 24 h and returned data to Earth at a rate faster than any previous mission. The orbiter is examining Mars in unprecedented detail including water and mineral distribution, featur
es and future landing sites. So far the probe has discovered channels in a fossil delta, troughs in sand dunes and evidence that liquid or gas has flowed through cracks in underground rocks. In December 2013 the orbiter imaged Curiosity and its tracks in Gale crater.
A US space probe named Phoenix was successfully launched from Cape Canaveral, Florida, on 4 August 2007. The probe took 9 months to reach Mars. A robotic arm on the lander dug for clues to past and present life in the polar region. Instruments checked the soil for water and carbon-based chemicals (considered essential for life). The lander completed its mission in August 2008, and made a last brief communication with Earth on 2 November as available solar power dropped with the Martian winter. The mission was declared concluded on 10 November 2008 after engineers were unable to re-contact the craft.
NASA launched Mars Science Laboratory (MSL) on 26 November 2011. The overall objectives include investigating Mars’ habitability, studying its climate and geology, and collecting data for a manned mission to Mars. The mission successfully landed Curiosity, a Mars rover, in Gale Crater on 6 August 2012. Curiosity is about twice as long and five times as heavy as the Spirit and Opportunity rovers, and carries over ten times the mass of scientific instruments (see Figs. 7.3 and 7.4).
Fig. 7.3The Curiosity Rover on the surface of Mars (Credit: NASA).
Fig. 7.4The Mars Rover Curiosity used its Mast Camera on 7 August 2014, to record this view of sedimentation in an ancient river bed (Credit: NASA/JPL-Caltech/MSSS).
The Solar System in Close-Up Page 12
