Some asteroids have hit the Earth’s surface and left craters. There is a 1-km-wide crater in northern Arizona that was made by an asteroid impact some 50,000 years ago. This asteroid was made of iron and nickel and was about 30 m wide. As recently as 1908, what is thought to have been a 100,000 tonne asteroid exploded in the Earth’s atmosphere at an altitude of about 10 km with the force of a 20–30 megaton nuclear bomb. The explosion occurred over Siberia and the blast from the explosion flattened forests and burned an area about 80 km across. Many scientists believe an asteroid impact about 65 million years ago was responsible for the extinction of the dinosaurs on Earth. The asteroid created a huge circular depression called the Chicxulub Basin centred in Mexico’s Yucatan Peninsula. The diameter of the basin is about 180 km.
Size and Composition
Asteroids vary greatly in size. The largest asteroid, Ceres, is 950 km in diameter and contains about one third the total mass of all the asteroids. When it was discovered in 1801, Ceres was thought to be another planet because of its size. The discovery of other bodies in the same region made it an asteroid. Ceres was classified by the IAU in 2006 as a ‘dwarf planet’ because although it had a near spherical shape, it was too small to have cleared out the smaller chunks of matter in its orbital path (Figs. 8.6 and 8.7).
Fig. 8.6An image of Ceres as seen by the Dawn probe on 19th February 2015 from a distance of 46,000 km. It shows a surface covered with small craters. Inside one crater are two bright spots thought to be due to water ice, volcanic action or salts reflecting sunlight. However, infrared images reveal the spots have different thermal properties (Credit: NASA).
Fig. 8.7Ceres’s northern hemisphere as seen by the Dawn probe on 15 April 2015 from a distance of 22,000 km (Credit: NASA/Dawn).
The second largest asteroid is Vesta. A recent analysis of Vesta’s shape and gravity field using data gathered by the Dawn spacecraft has shown that Vesta is not spherical enough to be in hydrostatic equilibrium and is therefore not a dwarf planet. Vesta has several impact craters and a large concavity and protrusion near its south pole caused by impacts. The asteroid’s crust is thicker than previously thought (about 80 km), and it lacks olivine on its surface a major component of planetary mantles). Vesta is the only asteroid that has an earth-like internal structure (core, mantle and crust). The asteroid also has sinuous gullies in some crater walls and a series of concentric troughs that are some of the longest chasms in the solar system. These troughs are thought to be caused by shock waves after collisions (see Table 8.3 and Fig. 8.8).Table 8.3Details about Vesta (the second largest asteroid)
Distance from Sun
353,260,000 km (2.50 AU)
Diameter
525 km in diameter
Mass
2.59 × 1020 kg
Density
3.456 g/cm3 or 3456 kg/m3
Orbital eccentricity
0.088
Period of revolution
1325 Earth days or 3.63 Earth years
Rotation period
5.34 h
Orbital velocity
19.3 km/s
Axial tilt
29°
Average temperature
−188 °C to −20 °C
Atmosphere
None
Strength of gravity
0.25 N/kg
Fig. 8.8Image of Vesta as seen by the Dawn spacecraft. The towering mountain at the bottom of the image (south pole) is more than twice the height of Mount Everest. The set of three craters known as the “snowman” can be seen at the top left (Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA).
A detailed geological map has been made of Vesta using data from the Dawn probe. The map shows Vesta is geologically divided into three provinces linked to the formation of its three largest craters—the Veneneia basin, the Rheasilvia basin and the Marcia crater. The canyons around Vesta’s equator are thought to be due to stress from the Rheasilvia impact (Fig. 8.9).
Fig. 8.9South polar scarp on the asteroid Vesta, as seen by the Dawn spacecraft (Credit: NASA).
Scientists have found out about the composition of asteroids by analysing the light reflected from their surface and from analysing meteorite fragments. There are two main types of asteroids based on their composition. One group that dominates the outer part of the belt, is rich in carbon with a composition has not changed much since the solar system formed. The second group is located in the inner part of belt, and is rich in minerals such as iron and nickel.
Scientists believe Ceres and Vesta have followed a different evolutionary path. Vesta’s origins seemed to have been hot and violent because it has basaltic flows on its surface. Vesta also formed earlier than Ceres and is a very dry body. By studying these contrasts and comparing these two asteroids, scientists hope to develop a better understanding of the transition from the rocky inner regions of the solar system to the icy outer regions.
Asteroids become darker and redder with age due to space weathering. However evidence suggests most of the color change occurs rapidly, in the first hundred thousands years, limiting the usefulness of spectral measurement for determining the age of asteroids. Asteroids also contain traces of amino acids and other organic compounds, and some scientists speculate that asteroid impacts may have seeded the early Earth with the chemicals necessary to initiate life, or may have even brought life itself to Earth (Table 8.4).Table 8.4The ten largest Asteroids
Number
Name
Diameter (km)
Year discovered
Discovered by
1
Ceres
950
1801
G. Piazzi
4
Vesta
550
1807
H. Olbers
2
Pallas
540
1802
H. Olbers
10
Hygeia
443
1849
De Gasparis
704
Interamnia
338
1910
V. Cerulli
511
Davida
335
1903
R. Dugan
65
Cybele
311
–
–
52
Europa
291
1858
Goldschmidt
451
Patientia
281
–
–
31
Euphrosyne
270
–
–
The Surface
Scientists have classified asteroids according to the amount of light they reflect. The dark stony kind reflects less than 5 % of the sunlight that falls on them. The brighter, light-coloured kind reflects about 20 % of incident light. Rarer third kinds resemble iron meteorites and may be the shattered core of older asteroids.
Photographs of most asteroids show they are covered with craters and dust, made by impact with other smaller rock-like bodies. The surfaces of asteroids are generally rock-like, with a layer of soil-like material.
On 12 February 2001, the NEAR probe landed on the asteroid Eros. Eros is irregular in shape and about 33 km long by 13 km wide. The first images of Eros showed it has an ancient surface covered with craters, grooves, house-sized boulders and other complex features. NEAR’s gamma-ray spectrometer was able to analyse material in the surface to a depth of about 10 cm, detecting the elements iron, potassium, silicon and oxygen. The density of Eros is about 2.4 g/cm3, about the same density as the Earth’s crust. Now turned off, the NEAR probe could remain preserved in its present location, the vicinity of the huge, saddle-shaped feature called Himeros, for millions of years. As the asteroid orbits the Sun, the spacecraft’s solar panels will be repeatedly turned toward the Sun, offering the possibility of reawakening the probe.
In 2012, NASA released the preliminary results of Dawn’s s
tudy of Vesta. Vesta is thought to consist of a metallic iron–nickel core 214–226 km in diameter, an overlying rocky olivine mantle, with a surface crust. Data from Dawn suggested the dark spots and streaks on Vesta’s surface were likely deposited by ancient asteroid impacts. The dark material contains hydrated minerals and might be carbon-rich. Gullies on the surface of Vesta are thought to have been eroded by transiently flowing liquid water. Numerous fragments of Vesta were ejected by collisions 1 and 2 billion years ago that left two enormous craters occupying much of Vesta’s southern hemisphere. A recent analysis of Vesta’s shape and gravity field using data gathered by the Dawn has shown that Vesta is currently not in hydrostatic equilibrium.
Observations of Ceres, the largest known asteroid, by NASA’s Hubble Space Telescope, have revealed that the object may contain pure water beneath its surface. Scientists estimate that if Ceres were composed of 25 % water, it may have more water than all the fresh water on Earth. However, unlike the water on Earth, water on Ceres would be in the form of water ice and be located in the mantle. Today, Ceres’ surface is too hot for ice to be stable anywhere except possibly, at the poles. If there ever was ice exposed at the surface, it has sublimed away. The shape of this asteroid is almost spherical, suggesting it may have an interior with a rocky inner core, and watery mantle with a thin, dusty outer crust.
Ultraviolet observations by spacecraft have revealed the existence of hydroxide water vapour near the north pole of Ceres. On 22 January 2014, ESA scientists using the far-infrared abilities of the Herschel Space Observatory reported the detection of water vapour on Ceres. Plumes of water vapour have been detected shooting up periodically from Ceres when portions of its icy surface warm slightly (such as when it is closer to the Sun).
The Atmosphere
The asteroids are much too small to retain an atmosphere. Any gases would be lost to space because of the very low gravitational attraction. Ceres may have a tenuous atmosphere containing water vapour.
Temperature
The maximum surface temperature of Ceres is –38 °C. This is relatively warm compared to the average surface temperature of a typical asteroid of –100 °C. This low temperature is largely because of the large distance between the Sun and the asteroid belt. The temperature on Vesta varies from –188 °C (dark side) to –18 °C (sunlit side). There are no seasons on the asteroids, although they do undergo day and night, dependent on which side is facing the Sun.
Magnetic Field
The NEAR probe that landed on the asteroid Eros also contained a magnetometer to measure any magnetic field. This instrument found no surface magnetic field exists.
It is unlikely that any asteroid would contain a magnetic field large enough to be detected.
Further Information
http://nssdc.gsfc.nasa.gov/planetary/
www.space.com/51-asteroids/
http://dawn.jpl.nasa.gov
© Springer International Publishing Switzerland 2016
John WilkinsonThe Solar System in Close-UpAstronomers' Universe10.1007/978-3-319-27629-8_9
9. Jupiter: The Gas Giant
John Wilkinson1
(1)Castlemaine, Victoria, Australia
Highlights
Jupiter has a system of four thin rings composed of rocks and dust particles surrounding its atmosphere in an equatorial plane.
New Horizon’s is the fastest spacecraft ever (80,000 km/h) to travel between Earth and Jupiter taking only 13 months.
NASA’s Juno spacecraft, launched in August 2011, will be the first solar-powered spacecraft to orbit Jupiter.
X-ray telescopes and the Hubble Space Telescope regularly detect auroras on Jupiter that are thousands of times more powerful than those on Earth. Auroras also occur on Jupiter’s largest moon, Ganymede.
The moon Io is the most volcanically active body in the solar system. It has nine giant erupting volcanoes on its surface and up to 200 smaller volcanoes.
Jupiter’s moon Europa contains water and molecular oxygen making it a likely candidate for life.
The planets Mercury, Venus, Earth and Mars are regarded as the inner planets of the solar system because they orbit close to the Sun. In contrast, the orbits of the four large planets—Jupiter, Saturn, Uranus and Neptune—are widely spaced at great distances from the Sun. The four inner planets are composed mainly of rock and metal, with surface features such as mountains, craters, canyons, and volcanoes. The outer planets on the other hand, rotate much faster and consist of vast, swirling gas clouds.
The gas planets do not have solid surfaces; their gaseous material simply gets denser with depth. The diameter of such planets is given for levels corresponding to a pressure of one atmosphere. What we see when looking at these planets is the tops of clouds high in their atmosphere.
Jupiter is the first of the gas giants and the fifth planet from the Sun. This planet is the largest in the solar system and it travels around the Sun once every 11.86 years at an average distance of 780 million km. Jupiter is so large that over 1300 Earths could be packed into its volume. It is also twice as massive as all the other planets combined (318 times the mass of Earth). Jupiter also contains about 71 % of all the material in the solar system, excluding the Sun.
Jupiter formed from the same swirling mass of gas and dust as the Sun and other planets. But unlike the inner planets, Jupiter was far enough away from the Sun to retain its envelope of lighter gases, mainly hydrogen and helium. The outer layer of Jupiter forms a gaseous shell almost 20,000 km thick.
Astronomers have studied Jupiter for many years as it is the fourth brightest object in the sky (after the Sun, the Moon and Venus). Ancient observers knew the planet and its movement across the night sky had been accurately plotted against the background of stars for centuries. Because Jupiter takes about 12 years to orbit the Sun, it spends about a year in each constellation of the zodiac as seen from Earth. To the unaided eye, Jupiter appears as a brilliant white star-like object in the night sky.
Early Views About Jupiter
To the ancient Romans, Jupiter was the king of the gods, the ruler of Olympus and the patron of the Roman state. The planet was also associated with Marduk, the most important figure in Mesopotamian cosmology and the patron god of the city-state of Babylon. According to the story, Marduk fought with Tiamat, the goddess of chaos, and her 11 monsters. Marduk defeated them one by one and split Tiamat’s body in two, thus dividing heaven from Earth. Marduk came to symbolize the rule of heavenly order over the universe. The wandering star Jupiter was placed in charge of the night sky.
With the invention of the telescope in 1608, the planet Jupiter could be studied in more detail from Earth. In 1610, Galileo observed Jupiter through his telescope and discovered its four largest moons, Io, Europa, Ganymede and Callisto. They are named after the mythical lovers and companions of the Greek god Zeus. These moons became known as the Galilean moons after Galileo.
The motion of these moons around Jupiter provided evidence to support Copernicus’s heliocentric theory of the motions of the planets. Galileo was arrested because of his support for the Copernican theory and he was imprisoned for the rest of his life.
In 1665 the French-Italian astronomer, Giovanni Cassini was the first person to see the Great Red Spot on Jupiter. Twenty-five years later he observed that the speeds of Jupiter’s clouds vary with latitude. Nearer the poles, the rotation period of Jupiter’s atmosphere is more than 5 min longer than at the equator (Table 9.1).Table 9.1Details of Jupiter
Distance from Sun
778,330,000 km (5.20 AU)
Diameter
142,984 km
Mass
1.90 × 1027 kg (318 times Earth’s mass)
Density
1.33 g/cm3 or 1330 kg/m3
Orbital eccentricity
0.048
Period of revolution
4329 Earth days or 11.86 Earth years
Rotation period
9 h 50 min
Orbital velocity
47,016 km/h
Tilt of axis
3.12°
Average temperature
−153 °C
Number of Moons
At least 63
Atmosphere
Hydrogen, helium
Strength of gravity
24.6 N/kg at surface
In January 2016 researchers at the British Museum announced that analysis of a newly found ancient tablet (dated 350–50 bc) has revealed that Babylonian astronomers had calculated the movements of the planet Jupiter using an early form of geometric calculus some 1400 years before the technique was used by the Europeans.
This means that these ancient astronomers had not only figured out how to predict Jupiter’s paths more than 1000 years before the first telescopes existed, but they were using mathematical techniques that would form the foundations of modern calculus as we now know it. This tablet thus provides the key to understanding how the Babylonians used a trapezoid shape to predict Jupiter’s position, which was integral to their beliefs about the weather, the price of goods, and the fluctuating river levels throughout the year.
The Solar System in Close-Up Page 15