Mega-Mars

[Cray]

I was kicking around an alternate history for a roleplaying session and got to thinking about swelling up Mars a bit, like in Harry Turtledove's "A World of Difference."

The idea is that plus-sized Mars would still be red and lifeless (?), so it wouldn't alter human history. (Lots of religions depend on Mars being red.) However, it'd be larger (surface gravity ~0.8G) and have a thicker atmosphere (~0.7 bar.) If that atmosphere was 80% nitrogen and 20% carbon dioxide, what would that do for the Martian climate?

Are there any handy planet climate modelers to figure out a planet's average temperature based on pressure, CO2 levels, and insolation?

[radtech497]

I was kicking around an alternate history for a roleplaying session and got to thinking about swelling up Mars a bit, like in Harry Turtledove's "A World of Difference."

The idea is that plus-sized Mars would still be red and lifeless (?), so it wouldn't alter human history. (Lots of religions depend on Mars being red.) However, it'd be larger (surface gravity ~0.8G) and have a thicker atmosphere (~0.7 bar.) If that atmosphere was 80% nitrogen and 20% carbon dioxide, what would that do for the Martian climate?

Are there any handy planet climate modelers to figure out a planet's average temperature based on pressure, CO2 levels, and insolation?

Assuming the Martian albedo of 0.25 remains unchanged (it likely would increase, because of the increased size of the polar caps, though this would be offset somewhat by open bodies of liquid), a quick BOTE calculation gives a mean surface temperature of 276.587 K, or just above the freezing point of water.

Radtech497

[Cray]

Assuming the Martian albedo of 0.25 remains unchanged (it likely would increase, because of the increased size of the polar caps, though this would be offset somewhat by open bodies of liquid), a quick BOTE calculation gives a mean surface temperature of 276.587 K, or just above the freezing point of water.

Ooo...I didn't expect that much. This is good. Throw in some very saline water and you could have some open bodies of water around the equator.

Thank you!

[Cray]

So...if you trade out 10-15% of the nitrogen in an 80 nitrogen/20 CO2 atmosphere for methane, what's that do for temperatures on Mega-Mars? Methane's a much more potent greenhouse gas.

Actually, it seems like that much methane could make the planet quite a bit warmer and ruin my Eskimos-in-space alternate history.

[radtech497]

So...if you trade out 10-15% of the nitrogen in an 80 nitrogen/20 CO2 atmosphere for methane, what's that do for temperatures on Mega-Mars? Methane's a much more potent greenhouse gas.

Actually, it seems like that much methane could make the planet quite a bit warmer and ruin my Eskimos-in-space alternate history.

Again, assuming the planetary albedo remains the same, an atmospheric methane concentration of 10% raises the mean surface temperature to 520.048 K (much too hot for water, and hotter even than Venus). A 15% concentration warms the surface even more, to 545.066 K.

As you noted, methane is a potent greenhouse gas.

Radtech497

[Cray]

Alrighty, then. The Martians can make their fuel the hard way, from water and CO2. No freebies from the atmosphere.