Skip to main content
Astronomy

Return to Answer

Added references and calculations.
Source Link
pela
pela
  • 38.9k
  • 114
  • 145

Mars does have a greenhouse effect, only somewhat weaker than Earth's.

 

Mars' atmosphere is very dilute, with a with a surface pressure only 0.6% of Earth's. So even if 95% of it is CO2, that's not a lot. However, it is actually a higher absolute abundance of CO2 molecules than on Earth, which only has a CO2 abundance of 0.06%04% (by volume; e.g. NOAA, corresponding to roughly 0.06% by mass).

That isThe exact calculation depends on how the atmosphere decreases in density with height (the "scale height"), but for an order-of-magnitude calculation, we can use the abundancetotal masses of the atmospheres $M_\mathrm{atm,Earth} = 5.15\times10^{18}\,\mathrm{kg}$ (Trenberth & Smith 2004), and $M_\mathrm{atm,Mars} = 2.5\times10^{16}\,\mathrm{kg}$ (NASA). The total amount of CO2 in Mars' atmosphere, compared to Earth's, is then $$ \frac{M_\mathrm{CO_2,Mars}}{M_\mathrm{CO_2,Earth}} = \frac{95\%\times M_\mathrm{atm,Mars}}{0.06\%\times M_\mathrm{atm,Earth}} \simeq 7.9, $$ i.e. almost an order of magnitude higher. The fact that Mars is smaller than in Earth'sEarth means that this number is probably somewhat larger.

Roughly 10–20% of the radiation emitted by Mars' surface is absorbed in the atmosphere (Haberle 2015). One way to quantify the greenhouse effect is the difference between the effective temperature and the planet's average surface emission temperature. For Earth, the difference is 33 K, while for Mars it's a much lower 5 K (but must have been much higher in the past in order for liquid water to exist).

So why is the effect so much larger on Earth? Well, CO2 is not the only agent of the greenhouse effect. Other gases — e.g. methane, nitrous oxide, and ozone — add as well. But actually the largest contribution comes from water vapor. Exactly how much is a matter of heated debate, I think, but it's probably not controversial to say that at least 1/3 of the effect is due to water (RealClimate says 36–66%).

Water molecules are less reflecting than CO2, but constitutes 0.3% of Earth atmosphere. In contrast, only 0.03% of Mars' atmosphere is water vapor (e.g. Trokhimovskiy et al. 2015).

In summary, the smaller greenhouse effect, and the fact that Mars is roughly 1.5 times as far from the Sun as Earth, and hence receives less than half of the incident radiation, is the reason that Mars is so cold.

Mars does have a greenhouse effect, only somewhat weaker than Earth's.

Mars' atmosphere is very dilute, with a surface pressure only 0.6% of Earth's. So even if 95% of it is CO2, that's not a lot. However, it is actually a higher absolute abundance of CO2 molecules than on Earth, which only has a CO2 abundance of 0.06% (by mass).

That is, the abundance of CO2 in Mars' atmosphere is an order of magnitude higher than in Earth's.

Roughly 10–20% of the radiation emitted by Mars' surface is absorbed in the atmosphere (Haberle 2015). One way to quantify the greenhouse effect is the difference between the effective temperature and the planet's average surface emission temperature. For Earth, the difference is 33 K, while for Mars it's a much lower 5 K (but must have been much higher in the past in order for liquid water to exist).

So why is the effect so much larger on Earth? Well, CO2 is not the only agent of the greenhouse effect. Other gases — e.g. methane, nitrous oxide, and ozone — add as well. But actually the largest contribution comes from water vapor. Exactly how much is a matter of heated debate, I think, but it's probably not controversial to say that at least 1/3 of the effect is due to water (RealClimate says 36–66%).

Water molecules are less reflecting than CO2, but constitutes 0.3% of Earth atmosphere. In contrast, only 0.03% of Mars' atmosphere is water vapor (e.g. Trokhimovskiy et al. 2015).

In summary, the smaller greenhouse effect, and the fact that Mars is roughly 1.5 times as far from the Sun as Earth, and hence receives less than half of the incident radiation, is the reason that Mars is so cold.

Mars does have a greenhouse effect, only somewhat weaker than Earth's.

 

Mars' atmosphere is very dilute, with a with a surface pressure only 0.6% of Earth's. So even if 95% of it is CO2, that's not a lot. However, it is actually a higher absolute abundance of CO2 molecules than on Earth, which only has a CO2 abundance of 0.04% (by volume; e.g. NOAA, corresponding to roughly 0.06% by mass).

The exact calculation depends on how the atmosphere decreases in density with height (the "scale height"), but for an order-of-magnitude calculation, we can use the total masses of the atmospheres $M_\mathrm{atm,Earth} = 5.15\times10^{18}\,\mathrm{kg}$ (Trenberth & Smith 2004), and $M_\mathrm{atm,Mars} = 2.5\times10^{16}\,\mathrm{kg}$ (NASA). The total amount of CO2 in Mars' atmosphere, compared to Earth's, is then $$ \frac{M_\mathrm{CO_2,Mars}}{M_\mathrm{CO_2,Earth}} = \frac{95\%\times M_\mathrm{atm,Mars}}{0.06\%\times M_\mathrm{atm,Earth}} \simeq 7.9, $$ i.e. almost an order of magnitude higher. The fact that Mars is smaller than Earth means that this number is probably somewhat larger.

Roughly 10–20% of the radiation emitted by Mars' surface is absorbed in the atmosphere (Haberle 2015). One way to quantify the greenhouse effect is the difference between the effective temperature and the planet's average surface emission temperature. For Earth, the difference is 33 K, while for Mars it's a much lower 5 K (but must have been much higher in the past in order for liquid water to exist).

So why is the effect so much larger on Earth? Well, CO2 is not the only agent of the greenhouse effect. Other gases — e.g. methane, nitrous oxide, and ozone — add as well. But actually the largest contribution comes from water vapor. Exactly how much is a matter of heated debate, I think, but it's probably not controversial to say that at least 1/3 of the effect is due to water (RealClimate says 36–66%).

Water molecules are less reflecting than CO2, but constitutes 0.3% of Earth atmosphere. In contrast, only 0.03% of Mars' atmosphere is water vapor (e.g. Trokhimovskiy et al. 2015).

In summary, the smaller greenhouse effect, and the fact that Mars is roughly 1.5 times as far from the Sun as Earth, and hence receives less than half of the incident radiation, is the reason that Mars is so cold.

Source Link
pela
pela
  • 38.9k
  • 114
  • 145

Mars does have a greenhouse effect, only somewhat weaker than Earth's.

Mars' atmosphere is very dilute, with a surface pressure only 0.6% of Earth's. So even if 95% of it is CO2, that's not a lot. However, it is actually a higher absolute abundance of CO2 molecules than on Earth, which only has a CO2 abundance of 0.06% (by mass).

That is, the abundance of CO2 in Mars' atmosphere is an order of magnitude higher than in Earth's.

Roughly 10–20% of the radiation emitted by Mars' surface is absorbed in the atmosphere (Haberle 2015). One way to quantify the greenhouse effect is the difference between the effective temperature and the planet's average surface emission temperature. For Earth, the difference is 33 K, while for Mars it's a much lower 5 K (but must have been much higher in the past in order for liquid water to exist).

So why is the effect so much larger on Earth? Well, CO2 is not the only agent of the greenhouse effect. Other gases — e.g. methane, nitrous oxide, and ozone — add as well. But actually the largest contribution comes from water vapor. Exactly how much is a matter of heated debate, I think, but it's probably not controversial to say that at least 1/3 of the effect is due to water (RealClimate says 36–66%).

Water molecules are less reflecting than CO2, but constitutes 0.3% of Earth atmosphere. In contrast, only 0.03% of Mars' atmosphere is water vapor (e.g. Trokhimovskiy et al. 2015).

In summary, the smaller greenhouse effect, and the fact that Mars is roughly 1.5 times as far from the Sun as Earth, and hence receives less than half of the incident radiation, is the reason that Mars is so cold.