How’s it going to end? Climate evolution on Mars and Venus and its bearing on the very long term fate of the Earth’s climate system

The sun is getting brighter on the main sequence (1%/100Ma) Eventually the sun will grow into a bloated red giant reaching nearly to the orbit of Venus. Luminosity will increase 1000-fold. This will take 5 billion more years. The earth will be fried! Hertzsprung Russell Diagram of Stellar Evolution

A forward looking—billion year climate simulation The simulation predicts that increased silicate weathering in a wetter, warmer world will draw down atmospheric CO 2 forming carbonates. The stratosphere will become wet and water will be lost from the Earth by photo-dissociation and H 2 gas will escape to space. The Earth will become like Venus

Venus is the brightest object in the sky after the Sun and the Moon. The thick atmosphere strongly reflects sunlight preventing us from seeing the surface. Venus is slightly smaller than Earth. The internal structure of Venus is similar to Earth, with a metallic core, rocky mantle, and crust.

Venus has an atmospheric pressure of 92 bars (96.5% carbon dioxide). This is enough to heat the surface to 460°C from a runaway greenhouse effect.

How did Venus get its hot climate? Venus was more like the Earth very early in its history. It once had abundant surface water. Because Venus is closer to the Sun it receives more solar insolation and would have been warmer than the Earth. Increased warmth would have revved- up the hydrologic cycle, causing increased silicate weathering, drawing down atmospheric CO 2. High altitude water in the stratosphere would photo-dissociate in the presence of UV light and H 2 would be lost to space. Venus receives 1.9 times the Solar flux of the Earth

Evidence that water vapour was initially present, but subsequently lost, is the high 2 H/ 1 H ratio characterizing the small amounts of water vapour remaining on Venus (about 150 times higher than in the oceans on Earth). We would expect the lighter isotope of hydrogen ( 1 H) to escape from the Venusian atmosphere, preferentially to the heavier isotope ( 2 H) (also know as deuterium). Loss of water would reduce silicate weathering. Loss of water would cause the loss of the SO 4 sink for gaseous sulfur emissions (SO 2 ), giving rise to sulphuric acid clouds. Venus receives 1.9 times the Solar flux of the Earth How did Venus get its hot climate?

Earth The Earth has a similar mass to Venus and should have produced gases in approximately the same proportion as Venus. A high partial pressure of CO 2 on the young Earth could have led to significant greenhouse warming, initially. However, because this temperature is low enough for water to exist in the liquid state, it accumulated on the surface. CO 2 dissolved into the ocean, reacted with silicate rocks, and precipitated as carbonate.

Mars has a mass that is only 11% of Earth’s mass and it has a very thin atmosphere (~12% of Earth’s atmospheric pressure). The Martian atmosphere is mostly CO 2 (96.5%) It is so cold that CO 2 freezes at the N and S poles The solar flux is 43% of that reaching the Earth

South PoleNorth Pole Dry ice forms at C

The Martian interior is probably similar to that of Earth with a crust, mantle and core. Mars also displays some extreme topography (i.e. Candor Chasm).

Mars has the largest known volcano in the Solar System. Olympus Mons—25km high (>75,000ft high)! It may be as young as 400 million years. Surface image

Why did the Martian climate system fail? Since Mars is about half the size of the Earth, its internal heat engine cooled earlier, plate tectonics ceased, or never got going in the first place, allowing no tectonic mechanism like subduction-related metamorphism to return CO 2 (that is frozen in rocks and soils, or existing as carbonates) back to the atmosphere. Emissions of CO 2 from volcanoes will not build up in the atmosphere (if it is cold enough to freeze CO 2 on the surface), and the carbonate-silicate feedback will not operate. A permanent ice age is the result.

The habitable zone (HZ) Inner edge of HZ is 0.95 AU*, which is 1.11 times present solar luminosity. This is where water begins to be lost by photodissociation Outer edge is around 1.5 AU. However, in some climate models moving Earth outwards by 1.01 AU is enough to cause runaway global glaciations! * AU = astronomical unit = the distance between Sun and Earth

Good luck on the exam!