AST101H1 Lecture Notes - Tidal Acceleration, Tidal Heating, Orbital Speed
DepartmentAstronomy & Astrophysics
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Jupiter and Saturn are made mostly of hydrogen and helium, while Uranus and Neptune are primarily made of
hydrogen compounds such as water (H2O), methane (CH4), and ammonia (NH3).
There is no obvious trend in the densities of the jovian planets. For example, Jupiter is the nearest and most massive
of the four planets, but it is the second-lowest in density. The lack of a clear trend tells us that we need to look deeper
at the nature of these planets in order to understand their densities.
Recall that planets are made from four major categories of ingredients, shown in the table below. In general, the
densities of the ingredients in the table increase from top to bottom; metals are the most dense, and hydrogen and
helium gas are the least dense. Consider how this trend in the densities of the different materials would be expected to
affect the densities of the jovian planets.
Which of the following statements best explains why the densities of Uranus and Neptune are higher than those of
Jupiter and Saturn?
o They have a higher proportion of hydrogen compounds and rock.
o At similar temperature and pressure, hydrogen compounds and rock are much higher in density than
hydrogen and helium. The internal conditions in the jovian planets are similar enough so that composition
has a major effect on average density. Uranus and Neptune are higher in density than Jupiter and Saturn
because they are made primarily of higher-density hydrogen compounds and rock.
Which of the following best explains why Jupiter’s density is higher than Saturn's?
o Jupiter is more massive than Saturn.
o Because Jupiter and Saturn have nearly identical composition, Jupiter’s higher density indicates that its
interior is more compressed than Saturn’s. This greater compression is due to gravity, which is stronger for
Jupiter because of its greater mass.
Based on the leading scientific theory of solar system formation, which of the following statements best explains why
Uranus and Neptune have a significantly different composition and higher density than Jupiter and Saturn?
o Jupiter and Saturn captured more gas from the solar nebula than Uranus and Neptune.
o Accretion is thought to have occurred more rapidly in the denser regions of the solar nebula that were closer
to the Sun. Therefore, although all four jovian planets captured hydrogen and helium gas from the solar
nebula around similar-mass planetesimals (made of hydrogen compounds and rock), Jupiter and Saturn had
more time to capture this gas. As a result, Jupiter and Saturn accreted so much hydrogen and helium gas that
these ingredients ended up dominating their composition. In contrast, Uranus and Neptune were left with
compositions dominated by hydrogen compounds and rock, which also led to their higher densities
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