3.4.1 Why are Planets Round-ish?

Choose to either watch the below video or read the cited article to learn why spheres are so common in the universe and, in particular, why so many planetary bodies have an oblate spheroid shape. If you are feeling extra, you can do both!

Online Article: Tyson, Neil deGrasse. “On Being Round.” Natural History Magazine, March 1997, https://neildegrassetyson.com/essays/1997-03-on-being-round/.

3.4.2 Gravity and Topography

No planetary body has a surface that is completely smooth; they all have high points and low points. In other words, they exhibit topography—differences in the elevation of their land surface. Topography encompasses landforms such as mountains, hills, plains, valleys, canyons, craters and more. Interestingly, some planets can build taller mountains and dig deeper canyons than others. But why? Let’s talk about the answer in the context of mountains on the terrestrial planets.

The mountains on the terrestrial planets owe their origins to different processes. On the Moon and Mercury, the major mountains are ejecta thrown up by the large basin-forming impacts that took place billions of years ago. Most large mountains on Mars are volcanoes, produced by repeated eruptions of lava from the same vents. There are similar (but smaller) volcanoes on Earth and Venus. However, the highest mountains on Earth and Venus are the result of compression and uplift of the surface. On Earth, this crustal compression results from collisions of one continental plate with another.

It is interesting to compare the maximum heights of the volcanoes on Earth, Venus, and Mars (Figure 3.16). On Venus and Earth, the maximum elevation differences between these mountains and their surroundings are about 10 kilometers. Olympus Mons, in contrast, towers more than 20 kilometers above its surroundings and nearly 30 kilometers above the lowest elevation areas on Mars.

One reason Olympus Mons (Figure 14.20) is so much higher than its terrestrial counterparts is that the crustal plates on Earth never stop moving long enough to let a really large volcano grow. Instead, the moving plate creates a long row of volcanoes like the Hawaiian Islands. On Mars (and perhaps Venus) the crust remains stationary with respect to the underlying hot spot, and so a single volcano can continue to grow for hundreds of millions of years.

A second difference relates to the strength of gravity on the three planets. The surface gravity on Venus is nearly the same as that on Earth, but on Mars it is only about one third as great. In order for a mountain to survive, its internal strength must be great enough to support its weight against the force of gravity. Volcanic rocks have known strengths, and we can calculate that on Earth, 10 kilometers is about the limit. For instance, when new lava is added to the top of Mauna Loa in Hawaii, the mountain slumps downward under its own weight. The same height limit applies on Venus, where the force of gravity is the same as Earth’s. On Mars, however, with its lesser surface gravity, much greater elevation differences can be supported, which helps explain why Olympus Mons is more than twice as high as the tallest mountains of Venus or Earth.

By the way, the same kind of calculation that determines the limiting height of a mountain can be used to ascertain the largest body that can have an irregular shape. Gravity, if it can, pulls all objects into the most “efficient” shape (where all the outside points are equally distant from the center). All the planets and larger moons are nearly spherical, due to the force of their own gravity pulling them into a sphere. But the smaller the object, the greater the departure from spherical shape that the strength of its rocks can support. For silicate bodies, the limiting diameter is about 400 kilometers; larger objects will always be approximately spherical, while smaller ones can have almost any shape (as we see in photographs of asteroids).

Text Attributions

This text of this chapter is adapted from:

• Section 14.5 of OpenStax’s Astronomy 2e (2022) by Andrew Fraknoi, David Morrison, and Sidney Wolff. Licensed under CC BY 4.0. Access full book for free at this link.

Media Attributions

• Why Planets Are Round.” YouTube, uploaded by StarTalk, 19 Mar 2024, https://www.youtube.com/watch?v=8AwRB_wqjao.