DK Eyewitness - Astronomy

Wood
Looking at volume
These blocks—wood, aluminum, and iron—all have
the same volume, that is they occupy the same amount
of space. Despite being the same size, however, these
materials do not have the same mass and density, nor do
they weigh the same. This is also true of the planets.
For example, Mercury, though small, has a higher
density than that of some of the larger planets.
Wood
Aluminum
Iron
Measuring planets
Whereas we can weigh and
measure objects on Earth, we
have to assess the space a planet
occupies (volume), how much
matter it contains (mass), and its
density by looking at its behavior,
by analyzing its gravitational pull
on nearby objects, and by using
data gained by space probes
(pp.34–35). Density is the mass for
every unit of volume of an object
(mass divided by the volume).
Measuring mass
Mass is how much matter an object contains.
A beam balance can be used to find the mass of a
Mercury
material. Here a piece of wood and a piece of iron of
identical proportions and volume are placed on the
balance. The iron has the greater mass. By dividing the
Orbital period
mass (measured in grams) of the wood and the iron
The tidal force of Earth has locked
by their volume (measured in cubic centimeters), their
its Moon into rotating so that one
relative densities can be calculated.
side of the Moon always faces Earth
(p.40). This means the rotational period
of the Moon equals its monthly period of
revolution around Earth. Since the orbit of
Mercury is elongated, like an oval, it is locked
into a rotational period where the planet
spins 1½ times during each orbit of the Sun.
This means that its year (how long it takes to
orbit the Sun) is 88 Earth days, while its day
(the time it takes to rotate—sunrise to
sunrise) is 58.6 Earth days.
Iron
Saturn
Comparing density
Mercury has great mass for its size. Even
though it is only slightly larger than Earth’s
Moon, its mass is four times that of the Moon.
This means its density must be nearly as
high as Earth’s, most likely due to a very high
quantity of iron. Astronomers believe that
Mercury must have a massive iron core that
takes up nearly three-fourths of its radius to
achieve such great mass—a fact backed up
by Mariner 10’s evidence of a magnetic field.
When the densities of Mercury and Saturn,
the huge gas giant (pp.52–53), are compared,
Saturn would float and Mercury, whose
density is seven times as great, would sink.
Moon Earth Mercury
Crater
Facts about mercury
Mosaic of
separate
photographs
Rocky crust
Iron and
nickel core
Rocky mantle
• Sidereal period 88 Earth days
• Temperature at surface –292°F to 800°F (−180°C to 430°C)
• Rotational period 58.6 Earth days
• Mean distance from the Sun 36 million miles/
57.9 million km
• Volume (Earth = 1) 0.056 • Mass (Earth = 1) 0.055
• Density (water = 1) 5.43
• Equatorial diameter 4,879 km/3,030 miles
• Number of satellites 0