It is said that planets are continuously moving away from the sun. At one time Mars was just the right distance to support life at least temperature wise? Could Venus be far enough from the sun eventually to cool down?

Related: Are planets moving away from the sun?

Is Earth losing orbit?

  • $\begingroup$ If you read and understand the other answers, they already contain all you want to know about this. $\endgroup$ – AtmosphericPrisonEscape Jan 22 '18 at 18:24
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    $\begingroup$ That Mars was once habitable is not related to any change in it's orbital radius. $\endgroup$ – Rob Jeffries Jan 22 '18 at 19:17

It's actually the opposite. Venus, Mars and I'll include the Earth as well, are likely getting more solar energy per square meter, not less, over time, even as they slowly move away from the sun.

1.5 cm per year, from the Rob Jeffries' answer to the 2nd question you linked is too slow to make much difference. That's 1 km every 66,000 years and about 70,000 km over the age of the solar-system at that rate. Now, planets may have moved away at different rates, especially when the solar-system was young and orbits were more chaotic and when Jupiter was thought to be doing it's dance in and back out, so it's not precisely known how close Mars was to the sun, say, 3.7 to 4.1 billion years ago, but we do know that the Sun was a little smaller and not as hot at that time, perhaps about 25-30% less luminous overall, though it was also more active with solar storms. But Mars probably got less solar energy 4.1 billion years ago than it gets now, and it will get more solar energy a billion or two billion years in the future, even though it will probably be a little further from the sun.

The primary factor is how much of an atmosphere it had. Planets can't have liquid water oceans without a reasonably thick atmosphere and Mars is believed to have had an atmosphere long ago. How close it was to the sun matters too, but the thickness and composition of the atmosphere matters more. Heat from the planets interior also matters and all the planets had more internal heat after formation, in the 3.7 to 4.1 billion years ago range.

Its also not 100% certain that Mars ever had relatively permanent liquid oceans and what we'd consider moderate temperatures. Mars' geology suggests it had floods, but flooding could have been caused by local or temporary heating from volcanic activity or large meteor impacts. It might also have been driven by a a more extreme axis leading to greater seasonal variation with glacial melt every summer (687 days in a year so summer lasted longer), and perhaps re-freezing every winter.

The Mars Ocean Hypothesis is a good hypothesis, don't get me wrong, but it's worth pointing out that its not a certainty. It's possible that the oceans were ice covered on top but liquid underneath from heat coming from inside the planet. Nobody knows for certain how temperate Mars was 3.7 to 4.1 billion years ago.


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