Question

What evidence do we have that Jupiter and Saturn must be made of more than hydrogen and helium alone?

Short answer

Jupiter and Saturn's densities, spectroscopic evidence, and magnetic fields indicate components beyond hydrogen and helium.

Step-by-step solution

Identifying the Primary Composition

Most of the solar system's giant planets—Jupiter and Saturn—are primarily composed of hydrogen and helium, much like the Sun. This composition reflects the abundance of theses gases in the primordial solar nebula from which the planets formed.

Analyzing Density

The average density of these planets provides vital clues. Jupiter has an average density of about 1.33 g/cm³, and Saturn's density is even lower at 0.69 g/cm³, which is less than water. These densities are higher than what one would expect if these planets were made only of hydrogen and helium.

Understanding Contribution from Other Elements

The higher density suggests the presence of heavier elements and compounds beyond just hydrogen and helium. These could include rocky and icy materials, which are denser, affecting the planet's overall density.

Spectroscopic Evidence

Spectroscopic analysis of Jupiter and Saturn's atmospheres shows the presence of other elements and compounds like methane (CH₄), ammonia (NH₃), and water vapor (H₂O), indicating that their compositions are not limited to hydrogen and helium.

Exploring Magnetic Fields

Both Jupiter and Saturn possess strong magnetic fields, which suggest the presence of metallic hydrogen inside their interiors. The conduction of these fields requires something other than pure hydrogen and helium.

Key concepts

Density of Giant Planets

The density of giant planets like Jupiter and Saturn offers fascinating insights into their composition. Although both planets are primarily composed of hydrogen and helium, their densities reveal the presence of other elements. Jupiter's average density is about 1.33 g/cm³, and Saturn's is even lower, at 0.69 g/cm³—less dense than water.
These values might seem low, but they are higher than what would be expected if the planets were made purely of hydrogen and helium.
This higher density indicates that Jupiter and Saturn contain heavier elements. These elements could be rocky materials or ices, which are denser and thus increase the planets' total density.
Understanding these densities helps us conclude that giant planets are more complex in composition than they appear at first glance.

Spectroscopic Analysis

Spectroscopic analysis is a key tool in astronomy used to understand the composition of distant celestial bodies. By analyzing the light emitted or absorbed by these planets, scientists can identify various elements and compounds present in their atmospheres.
Jupiter and Saturn's atmospheres contain gases like methane (CH₄), ammonia (NH₃), and water vapor (H₂O).
These gases, detected through spectroscopy, provide critical evidence that these giant planets have a more complex chemical makeup than hydrogen and helium alone.

• Methane in the atmosphere indicates ongoing chemical reactions or processes.
• Ammonia can hint at deeper atmospheric dynamics and temperature conditions.
• Water vapor further suggests the presence of oxygen in some form, which could contribute to other atmospheric phenomena.

Spectroscopy allows us to peak into the atmospheric composition and understand a planet's formation and evolution.

Magnetic Fields of Gas Giants

The magnetic fields of Jupiter and Saturn offer additional clues about their interior structure. Both planets have strong magnetic fields, generated deep within their interiors.
For such magnetic fields to exist, it's believed there must be a fluid layer capable of conducting electricity.
In the case of Jupiter and Saturn, this layer is likely metallic hydrogen. Under immense pressure and heat, hydrogen can become metallic, enabling electric currents and thus generating magnetic fields.

• This transformation requires conditions not achievable by hydrogen and helium alone.
• The presence of metallic hydrogen suggests that these gas giants have cores or regions with compositions different from just gases.

These magnetic fields are not only vital for understanding the planets' interiors, but also for studying their interaction with solar winds and cosmic particles.