Thursday 7 May 2020

Is there life on Mars?

I doubt it, but what I do know is that it has an atmosphere:

Despite the high concentration of CO2 in the Martian atmosphere [95%], the greenhouse effect is relatively weak on Mars (about 5°C) because of the low concentration of water vapour and low atmospheric pressure. While water vapour in Earth's atmosphere has the largest contribution to greenhouse effect on modern Earth, it is present in only very low concentration in the Martian atmosphere.

So the CO2 makes little difference then? It's the water vapour all of a sudden? (Venus also has hardly any water vapour and is 95% CO2).

What makes the big difference is the low atmospheric pressure, but they only mention that in passing. On the surface of Mars this is only 0.61% of sea level atmospheric pressure on Earth (and on the surface of Venus it is 92 times as much).

The effective temperature of Mars, i.e. its surface temperature if it had no atmosphere, is 210K. It's actual average surface temperature is 215K. To get the Barometric Formula to balance and give you an average temperature of 210K for the whole atmosphere*, you have to put in a surface temperatures between 215 and 218K.

This is not far off the official figure and gives you a lapse rate of a bit less than -1K/km altitude. NASA say it is almost exactly -1K/km. And they've been there and measured it. Interestingly, the altitude/temperature chart in the Wiki article (first link) also shows a lapse rate of about -1K/km, even though the article itself says actual -2.5K/km and predicted -4.3K/km,

So the formula comes through for us yet again; all that matters is that a mole of CO2 has an atomic mass of 44, about one-and-a-half times the mass of a mole of the Earth's atmosphere (mix of N2 and O2 = 29). You don't need to know anything else about the gases' properties.

As an approximation, density and pressure are proportional, so Mars has about fourteen times as many CO2 molecules per unit volume as Earth does. Earth = 420 ppm; Mars = 1,000,000 x 0.61% x 95% = 5,800.

So this next bit is a real cop-out:

Moreover, under low atmospheric pressure, greenhouse gases cannot absorb infrared radiation effectively because the pressure-broadening effect is weak.

CO2 molecules are a lot closer to each other on Mars than on Earth!

* Earth's effective temperature is given as 255K, which is very close to the average temperature of the atmosphere, depending on how it is calculated. So let's assume that the the average temperature of a planet's atmosphere is equal to its effective temperature. The charts tell us that the surface temperature is higher than the average and the temperature at the top of the stratosphere is lower than the average.

15 comments:

Dinero said...

Hello again.

This is where you are going wrong. In your reasoning You start with a heat source, the surface, and you add a stratum that you know has a temperature graduation across its thickness. Then in the next step, to maintain the correct average and graduation, you make the surface, the heat source, hotter.

That is wrong. Thermodynamics does not work like that. Placing a conductive medium in contact with a heat source cannot make the heat source hotter.

Mark Wadsworth said...

Din, and that's where you are going wrong. The heat source is the sun.

Dinero said...

The barometric formula does not predict what the total heat in the atmosphere is, it only gives a vertical distribution for the heat present in the atmosphere.

Mark Wadsworth said...

Din, to repeat: the source of energy is the sun. That dictates average temp of atmosphere. The bf then helps you calculate vertical distribution. I have never said anything else.

Dinero said...

No that is wrong, the thermal characteristics of the atmosphere are also a factor of the average temperature of the atmosphere.

Mark Wadsworth said...

Din, how do you mean "that is wrong"? I am never sure what level of reality you are operating on.

You said "the bf... gives a vertical distribution for the heat present in the atmosphere", with which I completely agree, I have never said anything else.

Then you introduce this made up term "thermal characteristics". The average temperature is what it is and can only be dictated by outside heat source, i.e. the sun.

Dinero said...

The heat comes from the sun. The average temperature is partly a product of what is retained in the atmosphere, rather than leaving to space,because of the infra red properties of the component gases. And so the barometric formula is not a way of obtaining the surface temperature independently of the effect of infra red properties of the constituent gases, because that effect is in the average temperature of the atmosphere.

Mark Wadsworth said...

Din, you really haven't understood a word I wrote.

I assume that avg temp of atmosphere is equal to effective temp, and then adapted bf accordingly.

Mark Wadsworth said...

And as it happens, actual avg temp of earth atmosphere is equal to effective temp. Hardly surprising, as constituent gases have no impact (another of you flawed assumptions is that they make a difference).

Dinero said...

You used the effective temperature for the average temperature. But the temperature effect of The infra red properties of the gases is not removed from the formula by doing that because the effect of them is a factor in the pressure term. This is because pressure varies with temperature. And the pressure term you used is measured in an atmosphere with that effect in it.

Dinero said...

"avg temp of earth atmosphere is equal to effective temp "

Well that is the nature of a surface covered with an insulating layer. The previous surface temperature then moves to within the insulating
ting layer , the surface temperature goes up and the outside of the insulating layer is cold. The average across the insulating layer is the same as the previous surface temperature was.

Dinero said...

Insulating in the general term, is not the quite the correct term for this specific case, but the graduation of temp and average temp are consistent.

Mark Wadsworth said...

Din, why don't you read what I write and pick holes in that, instead of pretending I said something stupid and picking holes in that?

"You used the effective temperature for the average temperature."

Yes I did

"But the temperature effect of The infra red properties of the gases is not removed from the formula by doing that because the effect of them is a factor in the pressure term"

Nope. Effective temp means likely temperature of surface of a planet if it had no atmosphere at all.

Dinero said...

Are you seriouse?

You used the air pressure term when you, quote,

9 May 09.20
"adapted bf accordingly" .
end quote.

Mark Wadsworth said...

Din, especially for you, I have explained how to work out the likely lapse rate, based entirely on observations from earth's surface (known temperature and pressure).

It's a two step thing

1. Use Barry to find the altitude at which pressure = 0.5 atm.

2. Use 'energy equivalence' concept to find lapse rate at which Joules/m3 is constant up to that point.

You can then work out likely temperature 'half way up'.