How cold does it feel on Mars?

[tony873004]

I know that Mars is rarely over freezing, and often nearly 200 below zero F. But how cold would it feel to an astronaut on Mars? The air is so thin, that it has almost no ability to suck the heat out of you. Water that is 33 degrees F feels much colder than air that is 33 degrees F because it is more dense. Likewise, the air on Mars is so thin compared to Earth's air that I would imagine it would lose most of its ability to draw the heat out of you. Mark Twain once said the coldest winter he ever spent was a summer in San Francisco (where I live). But in San Francisco, it never gets lower than 55 F in the summer. Why does San Francisco at 55 degrees feel colder than Lake Tahoe when it's 10 degrees? It's because San Francisco has a lot of moisture in the air, and a lot of wind. Its air just sucks the heat out of you quickly. But because Mars' atmosphere is so thin, it has no ability to suck the heat out of you. You will only lose the heat that your body radiates. I'm guessing that an astronaut would feel comfortable in just a heavy sweater under his spacesuit. Is it possible that if Mark Twain visited Mars he might still think of San Francisco as colder ?

[Mikeydude750]

I still believe it would be a bit colder than San Francisco. One thing's for sure, however...you wouldn't have to worry about wind chill for the most part(atmosphere being too thin; even though the winds can reach a very high speed...the atmosphere is less than 1% the thickness, therefore it wouldn't seem like a lot).

On an unrelated note...today it almost seems as cold as Mars over here(-15 real, -30 windchill)

[granthutchison]

I'm guessing that an astronaut would feel comfortable in just a heavy sweater under his spacesuit.

The problem would be more one of losing heat than of staying warm. As you say, the heat loss from convection is negligible (because of the low heat capacity of the Martian atmosphere). The major conduction loss would be through the soles of your feet, to the frigid Martian soil, and you'd need to have insulated boots to minimize that in order to stop your feet freezing. Evaporation of sweat won't happen through a conventional spacesuit. So that leaves radiation cooling, which for a human-sized black body at human body temperature amounts to just 100W - neatly equivalent to the human basal metabolic rate. So even if you were just to stand still on Mars in a spacesuit, you'd be producing marginally more heat than you were losing (because of the work being done by the postural muscles that hold you upright) - as soon as you started doing any work at all, you'd begin to cook.
Which is why astronauts working in vacuum wear those water-cooled long-johns connected to a high-surface-area radiator in their backpacks - so they can dump excess heat. No sweater required.

Grant

[tony873004]

Thanks, Grant. I knew you'd have detailed explanation. That's funny. An astronaut standing on the surface of a planet whose temperature is 200 below zero would have to worry more about cooling off than keeping warm. That's pretty much what I thought although I figured they'd need at least a sweater.

[Mikeydude750]

So what you're saying is that humans would only need air tanks, and not heaters?

That would make things a little easier for any humans who go to Mars...only bring a suit with a self-contained air supply instead of a heating unit.

[granthutchison]

You'd need a pressure suit like the ones used on the Moon, with insulated boots so that you didn't freeze your feet.
You really only lose heat efficiently if you're in contact with a large mass of something cold, and the Martian atmosphere is too thin and dry to do the job well - it would only take a small amount of your body heat to warm the air around your suit.

Grant