What was it like when Venus and Mars both died? | by Ethan Siegel | Starts With A Bang! | Feb, 2024

by TexasDigitalMagazine.com


While Mars is known as a frozen, red planet today, it has all the evidence we could ask for of a watery past, lasting for approximately the first 1.5 billion years of the Solar System. Could it have been Earth-like, even to the point of having had life on it, for the first third of our Solar System’s history? (Credit: NASA’s Goddard Space Flight Center/Big Think)

In the early stages of our Solar System, there were three life-friendly planets: Venus, Earth, and Mars. Only Earth thrived. Here’s why.

If you could travel back in time to the early stages of the Solar System, some 4.5 billion years ago, you wouldn’t simply find the one life-friendly world you’d expect in the form of planet Earth. Instead, there would have been three worlds with similarly life-friendly conditions: Venus, Earth, and Mars. In terms of the physical conditions they possessed, all three of them looked very similar from a planetary perspective, as they all had:

  • substantial surface gravity,
  • copious amounts of volcanic activity,
  • and atmospheres similar to Earth’s in thickness and pressure.

They all possessed volcanoes, watery oceans, and complex interactions between the surface, oceans, plus clouds and hazes, enabling these worlds to retain significant amounts of the heat they absorbed from the Sun.

Moreover, at these very early stages, even the compositions of their atmosphere were similar, as they were all rich in molecules like hydrogen, ammonia, methane, nitrogen, and water vapor. For a time, conditions were favorable for life to potentially arise on all three worlds, and indeed it may have arisen on all three at some point in the distant past. However, on all but one of these worlds, it didn’t last. Venus experienced a runaway greenhouse effect, boiling its oceans away and rendering it inferno-like after only a few hundred million years. Mars lasted far longer before becoming inhospitable: perhaps as much as 1.5 billion years. These are the stories of our how planetary neighbors met their respective demises.

Rather than only the two Martian moons we see today, Phobos and Deimos, a collision followed by a circumplanetary disk may have given rise to three moons of Mars, where only two survive today. The idea is that Mars’s once-innermost moon was destroyed and fell back onto Mars long ago. This hypothetical transient moon of Mars, proposed in a 2016 paper, is now the leading idea in the formation of Mars’s moons, and helps explain the enormous differences in topography between Mars’s northern and southern hemispheres. (Credit: LabEx UnivEarthS | Université de Paris Diderot)

It’s remarkable that worlds that, today, are so different from one another might have had such similar histories in their early stages. It wasn’t just Earth, but also Mars, that likely experienced a catastrophic early collisions, with Earth’s creating our Moon and Mars’s creating three moons, the largest of which…


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