How much more mass does Mars need to have in order to still be habitable today?

Question

In an alternate timeline where Mars is the second Earth-like planet in our Solar System, the drive for colonizing it would perhaps have been much stronger. There would've been more resources spent exploring it and studying its countless species completety unrelated to life on Earth. The first manned mission might even have been launched right before the dawn of the 21st century! But for that to happen, there needs to be some prerequisites, prerequisites that strech all the way back to 4.5 billion years ago, back when the planet was formed.

Mars resides in the outer reaches of the habitable zone around the Sun. If it had an atmosphere like Earth, with the same greenhouse effect like Earth, it would've been just another colder, icier version of our planet, with a more turbulent global climate cycle (due to it not having a large moon to moderate its axial wobbling). The reason why it's arid and barren today, is because it lost its magnetic field, allowing the solar wind to blow away most of its atmosphere. Atmospheric loss was rapid because Mars only has 10.7% the mass of the Earth. The low mass might also be why the magnetic field decayed so early. Without sufficient atmospheric pressure, liquid water would boil away, split into hydrogen and oxygen, with the oxygen getting absorbed into minerals on the ground and the hydrogen blown off into space by the solar wind.

But what if, Mars formed with more mass, especially with enough iron to retain its magnetic field until today, or at least enough mass to hold onto a thicker atmosphere till today? How much more mass does it need from the start in order to be another Earth-like planet today, ready for human exploration?

Answer 1

It's not only a matter of mass...

We know that Venus, with 0.815 Earth masses, has been capable of retaining an atmosphere, despite being closer to the Sun than Earth, and thus experiencing a more intense solar wind.

However, its magnetic field is way weaker

The lack of an intrinsic magnetic field at Venus was surprising, given that it is similar to Earth in size and was expected also to contain a dynamo at its core. A dynamo requires three things: a conducting liquid, rotation, and convection. The core is thought to be electrically conductive and, although its rotation is often thought to be too slow, simulations show it is adequate to produce a dynamo. This implies that the dynamo is missing because of a lack of convection in Venus's core. On Earth, convection occurs in the liquid outer layer of the core because the bottom of the liquid layer is much higher in temperature than the top. On Venus, a global resurfacing event may have shut down plate tectonics and led to a reduced heat flux through the crust. This insulating effect would cause the mantle temperature to increase, thereby reducing the heat flux out of the core. As a result, no internal geodynamo is available to drive a magnetic field. Instead, the heat from the core is reheating the crust.

But, despite the lack of a magnetic field, the atmosphere is still there.

To answer your question then, based on the amazing statistics based on two data points, the mass should be at least 0.815 Earth masses.

Answer 2

Nobody Knows

For one thing, mass alone is not a sufficient determiner of habitability. Composition also matters, and the minimum mass may be wildly different for different compositions (even if we only want to consider planets with habitable surfaces, as opposed to subsurface oceans). This is kind of extreme, and may or may be actually realistic, but in Still River, for example, Hal Clement describes the planet Enigma-88, which is only habitable because it randomly started out with such a huge load of outgas-able volatiles that it is still replenishing a high-pressure surface atmosphere from internal supplies even after a billion years of losing atmosphere like a comet! (And in fact, we have found exoplanets that are billions of years old and appear to have cometary tails; granted, they are all gas giants, but given enough time to keep losing atmosphere, they eventually won't be....)

If you look at the Wikipedia page for Planetary Habitability, the references for planetary mass ranges are all over the place. Some people put it at 0.3 Earth masses--which is actually smaller than Mars! (I guess it just needed a different composition to maintain its magnetic field? Or a moon to stir its interior with tides?) Others conclude that Earth itself is actually right on the edge of habitability--so a habitable equivalent of Mars would actually need to be at least as big as Earth itself. This is the approach taken by Harry Turtledove for his alt-Mars (named "Minerva") in the novel A World of Difference.

So... who knows? We will need to see a lot more planets in a lot more detail before anyone can say for sure.

Answer 3

Actually I recently ansered this question about inceasing the gravity of Mars:

What are the effects of Increasing the gravity on Mars?

One of many questions about the minimum mass of habitable planets I have answered.

Here is a link to a question which I answer in much more detail, too much detail for me to want to repeat here:

Would 25% less gravity produce dramatic differences in animal morphology?