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7.1 Overview of our planetary system  (Page 3/12)

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Surface of mercury.

Image of the surface of Mercury taken from Mariner 10. Large craters, with many overlapping one upon the other, cover the surface of this 400 km wide scene.
The pockmarked face of the terrestrial world of Mercury is more typical of the inner planets than the watery surface of Earth. This black-and-white image, taken with the Mariner 10 spacecraft, shows a region more than 400 kilometers wide. (credit: modification of work by NASA/John Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington)

The next four planets (Jupiter through Neptune) are much larger and are composed primarily of lighter ices, liquids, and gases. We call these four the jovian planets (after “Jove,” another name for Jupiter in mythology) or giant planets —a name they richly deserve ( [link] ). More than 1400 Earths could fit inside Jupiter, for example. These planets do not have solid surfaces on which future explorers might land. They are more like vast, spherical oceans with much smaller, dense cores.

The four giant planets.

Diagram of the Four Giant Planets Shown to Scale. Arranged from left to right are Jupiter, Saturn, Uranus, and Neptune. Also shown to scale at lower center is the Earth.
This montage shows the four giant planets: Jupiter , Saturn , Uranus , and Neptune . Below them, Earth is shown to scale. (credit: modification of work by NASA, Solar System Exploration)

Near the outer edge of the system lies Pluto , which was the first of the distant icy worlds to be discovered beyond Neptune (Pluto was visited by a spacecraft, the NASA New Horizons mission, in 2015 [see [link] ]). [link] summarizes some of the main facts about the planets.

Pluto close-up.

Image of a portion of the surface of Pluto. In this photograph from New Horizons, the smooth, white Sputnik plains are seen covering most of the upper right of the image. Rugged, heavily cratered terrain covers the lower center and upper left.
This intriguing image from the New Horizons spacecraft, taken when it flew by the dwarf planet in July 2015, shows some of its complex surface features. The rounded white area is temporarily being called the Sputnik Plain, after humanity’s first spacecraft. (credit: modification of work by NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute)
The Planets
Name Distance from Sun
(AU) An AU (or astronomical unit) is the distance from Earth to the Sun. 		Revolution Period
(y)		 Diameter
(km)		 Mass
(10 23 kg)		 Density
(g/cm 3 ) We give densities in units where the density of water is 1 g/cm 3 . To get densities in units of kg/m 3 , multiply the given value by 1000.
Mercury 0.39 0.24 4,878 3.3 5.4
Venus 0.72 0.62 12,120 48.7 5.2
Earth 1.00 1.00 12,756 59.8 5.5
Mars 1.52 1.88 6,787 6.4 3.9
Jupiter 5.20 11.86 142,984 18,991 1.3
Saturn 9.54 29.46 120,536 5686 0.7
Uranus 19.18 84.07 51,118 866 1.3
Neptune 30.06 164.82 49,660 1030 1.6

Comparing densities

Let’s compare the densities of several members of the solar system. The density    of an object equals its mass divided by its volume. The volume ( V ) of a sphere (like a planet) is calculated using the equation

V = 4 3 π R 3

where π (the Greek letter pi) has a value of approximately 3.14. Although planets are not perfect spheres, this equation works well enough. The masses and diameters of the planets are given in [link] . For data on selected moons, see Appendix G . Let’s use Saturn’s moon Mimas as our example, with a mass of 4 × 10 19 kg and a diameter of approximately 400 km (radius, 200 km = 2 × 10 5 m).

Solution

The volume of Mimas is

4 3 × 3.14 × ( 2 × 10 5 m ) 3 = 3.3 × 10 16 m 3 .

Density is mass divided by volume:

4 × 10 19 kg 3.3 × 10 16 m 3 = 1.2 × 10 3 kg/m 3 .

Note that the density of water in these units is 1000 kg/m 3 , so Mimas must be made mainly of ice, not rock. (Note that the density of Mimas given in Appendix G is 1.2, but the units used there are different. In that table, we give density in units of
g/cm 3 , for which the density of water equals 1. Can you show, by converting units, that 1 g/cm 3 is the same as 1000 kg/m 3 ?)

Check your learning

Calculate the average density of our own planet, Earth. Show your work. How does it compare to the density of an ice moon like Mimas? See [link] for data.

Answer:

For a sphere,
density = mass ( 4 3 π R 3 ) kg/m 3 .
For Earth, then,
density = 6 × 10 24 kg 4.2 × 2.6 × 10 20 m 3 = 5.5 × 10 3 kg/m 3 .
This density is four to five times greater than Mimas’. In fact, Earth is the densest of the planets.

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Source:  OpenStax, Astronomy. OpenStax CNX. Apr 12, 2017 Download for free at http://cnx.org/content/col11992/1.13
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