What Was It Like When Venus And Mars Became Uninhabitable Planets?
“But the changes that Mars endured were rapid and sweeping. Planets are born with a fixed amount of internal heat, which radiates away over their lifetime. A planet like Mars, with half the diameter of Earth, is born with only about 10-15% the amount of internal heat as our world, and will therefore see a greater percentage of it radiate away much faster than Earth will.
Approximately 3 billion years ago, the core of Mars became cool enough that it stopped producing that protective magnetic dynamo, and the solar wind began striking the Martian atmosphere. In short order, which is to say in just tens of millions of years, the atmosphere was knocked off into interplanetary space. As a result, the oceans were unable to remain in liquid form, and either froze beneath the surface or sublimated away.”
When our Solar System first formed, it wasn’t just Earth that looked promising for life, but also Venus and Mars. All three of these planets had large, liquid water oceans, substantial atmospheres, and the ingredients for complex biochemistry and even life. Over on Venus, its close proximity to the Sun and the large presence of atmospheric water vapor led to a runaway greenhouse effect, boiling the oceans after just ~200 million years. But Mars, despite being small and distant, maintained Earth-like conditions for 1.5 billion years. Considering that life arose on Earth after just one-sixth of that duration, perhaps Mars once had life, too?
“At some point on our planet, in the very early stages, the molecules that are abundant and precursors to life, under the right energy and chemical conditions, began to simultaneously metabolize energy, respond to the environment, grow, adapt, evolve, and reproduce. Even if it would be unrecognizable to us today, that marks the origin of life. In a radically unbroken string of biological success, our planet has been a living world ever since.
While Venus and Mars may have had similar chances, radical changes to Venus’ atmosphere rendered it a searing hothouse world after just 200-300 million years, while the death of the Martian magnetic field caused its atmosphere to be stripped away, rendering it solid and frozen. While asteroid strikes may send Earth-based life off-world, throughout the Solar System and galaxy, all the evidence suggests that we are where it started.
By 9.4 billion years after the Big Bang, Earth was teeming with life. We’ve never looked back.”
When most of us learned about the history of life on Earth in school, all we had were ideas and theories about how it might have gotten started. As you may have guessed, science progresses quickly, and we now have extraordinary evidence for an ancient universal common ancestor, for life existing just a few hundred million years (at the latest) after Earth’s formation, and for life beginning on Earth itself, not before. We even have candidate locations for where!