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The climate of Mars

Heat and matter · level 8, lesson 6 of 6

A question to start

Mars and Earth are tilted by almost the same angle, so both have seasons. But on Mars, about an eighth of the air freezes out every winter, and the biggest dust storms come in southern summer. What makes Mars's seasons so different?

The idea

Three things shape Mars's climate.

1. Tilt and orbit. Tilt gives both planets seasons. But Mars's orbit is far more oval, so its distance from the Sun, and the sunlight it gets, change much more through its year.

Seasons Mars Earth
Tilt (obliquity) 25.19° 23.44°
Orbit eccentricity 0.0935 0.0167
Sunlight, closest point against farthest about 46% more about 7% more
Length of the year 687 Earth days 365 days

Mars is closest to the Sun during southern summer, so southern summers are short and warm, and southern winters long and cold. The extra sunlight near the closest point helps lift dust, and the biggest dust storms usually start then.

2. The carbon dioxide cycle. In each polar winter, carbon dioxide frosts out of the air onto the cap: 3 to 4 trillion tonnes, 12 to 16% of all the air. In spring it sublimes back. The whole planet's air pressure rises and falls with it.

3. Thin, dry air. With little air and almost no water vapour, the ground heats and cools fast, and the daily swing is huge.

closest:southern summerfarthest:southern winterSun

The orbit is drawn seen at a slant. The Sun sits a little off-centre, towards the closest point.

Worked examples

1. The total air. Mars's surface area is \(4\pi R^2 = 4\pi \times (3.39\times10^6)^2 = 1.44\times10^{14}\) m². With 171 kg over each m², the whole atmosphere is \(2.5\times10^{16}\) kg, 25 trillion tonnes. 3 to 4 trillion tonnes is 12 to 16% of that, as NASA says.

2. Sunlight through the year. From the energy topic: 714 W/m² at the closest point, 490 at the farthest. \(714/490 = 1.46\).

3. A season. A Mars year is 687 Earth days, so a season lasts about 172 days on average, but the seasons near the closest point are shorter, because Mars moves faster there.

Try it

  1. Why do both Mars and Earth have seasons?
  2. Why are Mars's southern summers shorter and warmer than its northern summers?
  3. Why does Mars's air pressure change through the year?
  4. Use the figures above to find the mass of air that freezes out, as a share of the whole, if 3.5 trillion tonnes freeze.
Answers
  1. Their axes are tilted, so each hemisphere leans towards the Sun for part of the year.
  2. Southern summer happens near the closest point of the orbit, where Mars gets more sunlight and moves faster.
  3. Carbon dioxide freezes onto the winter pole and sublimes back in spring.
  4. \(3.5/25 = 14\%\).

Common mistake

"Seasons come from the distance to the Sun." On Earth, seasons come from the tilt; the distance barely changes. On Mars, the tilt still makes the seasons, but the oval orbit makes one hemisphere's seasons more extreme than the other's.

Where it leads