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The inner planets of our solar system, Mercury, Venus, Earth, and Mars, are all similar in some ways. They are rocky planets with a firm surface like Earth’s, and they are much smaller than the gas giants Jupiter and Saturn. But they differ in many ways too, in terms of their size, mass, and atmospheres. To understand the conditions on each planet, you need to know about their size and how they developed. Here’s what I’ve learned about the formation of Mars over my career, including interviewing scientists and engineers working on current Mars missions.
One reason that Mars is such a focus of study, with rovers and landers being sent there and many people interested in whether humans could live there long-term, is that it is the planet in our solar system which is most similar to Earth. But a key difference between the planets is their size, with Mars being about half the size of Earth. This affects everything from the gravity to the weather there.
We know that Mars formed around 4.5 billion years ago. We’re still learning about how planets formed in the early solar system, but Mars is a great place to learn about this as it has some extremely ancient rocks on its surface (via Digital Trends). Unlike Earth, which is tectonically active and so recycles rocks over billions of years, Mars doesn’t have tectonic activity. That means that its oldest rocks stay on its surface where they are available for study. The Perseverance rover, which is currently exploring Mars, is planning to visit some of the oldest known rocks near to the Jezero crater, which are thought to be 4 billion years old. Studying these can help us learn more about the formation of both Mars and the other rocky planets.
We do know that planets form from clouds of dust and gas which swirl around stars like our sun (via NASA). Before there were planets, there was a swirl of dust and gas which was thicker in some places than others. Gravity acted on this swirl to pull it into clumps and form the basis of planets, called planetesimals. A planetesimal attracted more material because of its gravity, growing to eventually become a planet. The size of the planet would depend on how much material was available and how lumpy the original cloud of dust and gas was.
One of the main effects of Mars being smaller than Earth is that its gravity is different. Gravity there is around one third the strength of gravity on Earth, because there is less mass attracting things to its surface. This can be useful for exploration in some ways – for example, the Ingenuity helicopter is able to fly there in part because the lower gravity makes it easier to get off the surface (via Digital Trends). But it causes problems in other ways, like making the atmosphere very thin.
