Firstly, Mars has a mean distance from the Sun of 1.524 AU, so by the inverse square law the energy it gets from the Sun is about 40% of what the Earth gets.

But the main reason that Mars is so cold is that its atmosphere is very thin compared to Earth's. From Wikipedia Atmosphere of Mars:

The atmosphere of Mars is much thinner than Earth's. The surface pressure is only about 610 pascals (0.088 psi) which is less than 1% of the Earth's value.

In comparison, the mean surface pressure on Earth is 101,300 pascals. So the atmosphere of Mars is barely more than a vacuum compared to Earth's.

So even though the Martian atmosphere is over 95% carbon dioxide, there simply isn't enough of it to trap much heat.

Although carbon dioxide is a greenhouse gas, the predominant greenhouse gas on Earth is actually water vapour. However, water is usually cycled in and out of the atmosphere very quickly in response to temperature and pressure changes. Carbon dioxide is a problem because it stays in the atmosphere for a long time, and its presence shifts the equilibrium temperature upwards from that of the plain water cycle.

Firstly, Mars has a mean distance from the Sun of 1.524 AU, so by the inverse square law the energy it gets from the Sun is about 40% of what the Earth gets.

But the main reason that Mars is so cold is that its atmosphere is very thin compared to Earth's (as well as very dry, see below). From Wikipedia Atmosphere of Mars:

The atmosphere of Mars is much thinner than Earth's. The surface pressure is only about 610 pascals (0.088 psi) which is less than 1% of the Earth's value.

In comparison, the mean surface pressure on Earth is 101,300 pascals. So the atmosphere of Mars is barely more than a vacuum compared to Earth's.

So even though the Martian atmosphere is over 95% carbon dioxide, there simply isn't enough of it to trap much heat.

Although carbon dioxide is a greenhouse gas, the predominant greenhouse gas on Earth is actually water vapour. However, water is usually cycled in and out of the atmosphere very quickly in response to temperature and pressure changes. Carbon dioxide is a problem because it stays in the atmosphere for a long time, and its presence shifts the equilibrium temperature upwards from that of the plain water cycle.

Firstly, Mars has a mean distance from the Sun of 1.524 AU, so by the inverse square law the energy it gets from the Sun is about 40% of what the Earth gets.

But the main reason that Mars is so cold is that its atmosphere is very thin compared to Earth's. From Wikipedia Atmosphere of Mars:

The atmosphere of Mars is much thinner than Earth's. The surface pressure is only about 610 pascals (0.088 psi) which is less than 1% of the Earth's value.

In comparison, the mean surface pressure on Earth is 101,300 pascals. So the atmosphere of Mars is barely more than a vacuum compared to Earth's.

So even though the Martian atmosphere is over 95% carbon dioxide, there simply isn't enough of it to trap much heat.

By the way, although carbon dioxide is a greenhouse gas, the predominant greenhouse gas on Earth is actually water vapour. However, water is usually cycled in and out of the atmosphere very quickly in response to temperature and pressure changes. Carbon dioxide is a problem because it stays in the atmosphere for a long time, and its presence shifts the equilibrium temperature upwards from that of the plain water cycle.

Firstly, Mars has a mean distance from the Sun of 1.524 AU, so by the inverse square law the energy it gets from the Sun is about 40% of what the Earth gets.

But the main reason that Mars is so cold is that its atmosphere is very thin compared to Earth's. From Wikipedia Atmosphere of Mars:

The atmosphere of Mars is much thinner than Earth's. The surface pressure is only about 610 pascals (0.088 psi) which is less than 1% of the Earth's value.

In comparison, the mean surface pressure on Earth is 101,300 pascals. So the atmosphere of Mars is barely more than a vacuum compared to Earth's.

So even though the Martian atmosphere is over 95% carbon dioxide, there simply isn't enough of it to trap much heat.

Although carbon dioxide is a greenhouse gas, the predominant greenhouse gas on Earth is actually water vapour. However, water is usually cycled in and out of the atmosphere very quickly in response to temperature and pressure changes. Carbon dioxide is a problem because it stays in the atmosphere for a long time, and its presence shifts the equilibrium temperature upwards from that of the plain water cycle.