Our Solar System Montage
Our Solar System Montage. Credit: NASA/JPL

Astronomy, Guide to Space, Solar System

How Dense Are The Planets?

9 Aug , 2009 by

The eight planets of our Solar System vary widely, not only in terms of size, but also in terms of mass and density (i.e. its mass per unit of volume). For instance, the 4 inner planets – those that are closest to the Sun – are all terrestrial planets, meaning they are composed primarily of silicate rocks or metals and have a solid surface. On these planets, density varies the farther one ventures from the surface towards the core, but not considerably.

By contrast, the 4 outer planets are designated as gas giants (and/or ice giants) which are composed primarily of of hydrogen, helium, and water existing in various physical states. While these planets are greater in size and mass, their overall density is much lower. In addition, their density varies considerably between the outer and inner layers, ranging from a liquid state to materials so dense that they become rock-solid.

Density also plays a vital role in determining a planet’s surface gravity and is intrinsic to understanding how a planet formed. After the formation of the Sun at the center of our Solar System, the planets were formed from a protoplanetary disc. Whereas the terrestrial planets resulted from dust grains in the inner Solar System, planets in the outer Solar System accreted enough matter for their gravity to hold on to the nebula’s leftover gas.

The Solar System. Image Credit: NASA

The Solar System. Image Credit: NASA

The more gas they held onto, the larger they became. And the larger they became, the more gas they held onto, the greater their size became until they reached a critical point. Whereas the gas giants of Jupiter and Saturn grew exponentially, the ice giants (Uranus and Neptune), with only a few Earth masses of nebular gas, never reached that critical point. In all cases, density is measured as the number of grams per cubic cm (or g/cm³).

Density of Mercury:

Ad a terrestrial planet, Mercury is composed of metals and silicate material. Mercury’s mean density is the second-highest in the Solar System, which is estimated to be 5.427 g/cm3 – only slightly less than Earth’s density of 5.515 g/cm3.However, if the effects of gravitational compression – in which the effects of gravity reduce the size of an object and increases its density – then Mercury is in fact more dense than Earth, with an uncompressed density of 5.3 g/cm³ compared to Earth’s 4.4 g/cm³.

These estimates can be also used to infer details of its inner structure. Compared to Earth, Mercury is much smaller, which is why it inner regions are subject to less in the way of compression. Therefore, its high density is believed to be the result of a large, and iron-rich core. All told, metals like iron and nickel are believed to make up 70% of the planet’s mass (higher than any other planet), while silicate rock accounts for just 30%.

Internal structure of Mercury: 1. Crust: 100–300 km thick 2. Mantle: 600 km thick 3. Core: 1,800 km radius. Credit: MASA/JPL

Internal structure of Mercury: 1. Crust: 100–300 km thick 2. Mantle: 600 km thick 3. Core: 1,800 km radius. Credit: MASA/JPL

Several theories for this have been suggested, but the predominant one claims that Mercury had a thicker silicate crust earlier in its history. This crust was then largely blown off when a large planetesimal collided with the planet.Combined with its size and mass, Mercury has a surface gravity of 3.7 m/s2, which is the equivalent of 0.38 of Earth’s gravity (aka. 1 g).

Density of Venus:

The second planet from our Sun, as well as the second-closest terrestrial planet, Venus has a mean density of 5.243 g/cm3. Again, this is very close to Earth’s own density. And while much remains unknown about Venus’ geology and seismology, astronomers have an idea of Venus’ composition and structure based on comparative estimates of its size, mass and its density.

In short, it is believed that Venus’ makeup and internal structure are very similar to Earth’s, consisting of a core, a mantle, and a crust. Also like Earth, the interior is though to be composed of iron-rich minerals, while silicate minerals make up the mantle and crust. The slightly smaller size of Venus also means pressures are 24% lower in its deep interior than Earth’s.

The internal structure of Venus – the crust (outer layer), the mantle (middle layer) and the core (yellow inner layer). Credit: Public Domain

The internal structure of Venus – the crust (outer layer), the mantle (middle layer) and the core (yellow inner layer). Credit: Public Domain

Because Venus and Earth have been cooling at about the same rate, it is believed that Venus’ core must be at least partially liquid. However, the lack of a magnetosphere around Venus has led scientists to question this, with some claiming that the core must be uniform in temperature, while others insist it is fully cooled and solid. Some has gone so as far as to suggest that it has no core.

Density of Earth:

Earth has the highest density of any planet in the Solar System,

Density of Mars:

Density of Jupiter:

Density of Saturn:

Density of Uranus:

Density of Neptune:

Mercury is the second densest planet in our Solar System. It has a density of 5.427 grams per cubic centimeter. Mars’ density is not surprising because the planet is mostly comprised of heavy metals including iron.

As an inner planet, Venus is also relatively dense compared to some of the other planets. Venus is often called Earth’s twin, and scientists believe that it has a core and a rocky crust like Earth’s. Unfortunately, scientists have not been able to test this hypothesis yet. Venus’ mass is only approximately 81% of Earth’s mass, so it is not surprising that Venus is also less dense than our own planet. Venus has a density of 5.204  grams per cubic centimeter.

Earth is the densest planet in our Solar System. It has a density of 5.51 grams per cubic centimeter. This is only the average density of the planet. The core is much denser than the oceans for example. Actually, scientists believe that the massive amounts of iron in our planet’s core is what makes it so dense.

Mars is larger than Mercury, but it has a much lower density – only 3.94 grams per cubic centimeter. Because of Mars’ low density and small mass, the planet actually has less gravity than the smallest planet Mercury.

Jupiter is the largest and most massive planet in our Solar System, but it only has a density of 1.33 grams per cubic centimeter because it is made up almost entirely of gases, such as helium and hydrogen. Even if it does have a heavy core, that core is relatively small.

Saturn is the second largest planet, but it has the lowest density of any planet in our Solar System. Saturn only has a density of .687 gram per cubic centimeter; this is actually less dense than water, which has a density of 1 gram per cubic centimeter.

Uranus is the second least dense planet in our Solar System with a density of 1.27 grams per cubic centimeter. Like Saturn and Jupiter, Uranus is an ice giant, which means that it contains “ices” like ammonia, water, and methane.

Neptune is an ice giant that has a low density of 1.638 grams per cubic centimeter. Although it has a higher density than any of the other gas giants, Neptune’s density is only about one-third of Earth’s density.

Universe Today has articles on all the planets’ densities including density of Neptune and density of Mars.

If you are looking for more information, check out NASA’s Solar System exploration page, and here’s a link to NASA’s Solar System Simulator.

Astronomy Cast has episodes on all the planets including Mercury.

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By  -      
Matt Williams is the Curator of the Guide to Space for Universe Today, a regular contributor to HeroX, a science fiction author, and a Taekwon-Do instructor. He lives with his family on Vancouver Island in beautiful BC.

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