by Tom Schlatter
Q: Why are there no clouds on Mars, a perpetual overcast on Venus, and a mixture of clear and cloudy skies on Earth? Has there ever been a time when Earth was like the other two planets? If so, could it happen again?
A: Earth has an abundance of water in the oceans and water vapor in the atmosphere. It so happens that atmospheric temperatures support water in all three phases: vapor, liquid, and solid. The hydrologic cycle causes water to pass from earth to sky and back again with a cycling time of roughly 9 to 10 days. Any volume of air containing water vapor can be lifted and cooled to the point where some of the vapor begins to condense into a cloud. Similarly, any volume of cloudy air can be forced downward and warmed to the point where the cloud evaporates. Traveling weather systems cause upward motion, which builds clouds, and downward motion, which destroys them. Thus, in most parts of the world, there is alternation between clear and cloudy periods.
On Mars, the average surface pressure is about 7 millibars (mb), compared with a standard sea-level pressure of 1013 mb on Earth. Mars is also much colder than Earth. Even at low latitudes during summer, the surface temperature seldom reaches the freezing point. When the vapor pressure at the surface of a liquid becomes equal to the total atmospheric pressure, the liquid boils. At sea level on Earth, water must be heated to 100°C before it will boil, because that's when the vapor pressure over liquid water becomes 1013 mb. Most people know that as altitude increases, the boiling point lowers, which accounts for the fact that foods must be cooked longer in Denver, the Mile-High City, where the boiling point is slightly less than 95°C, than in Washington, D.C., at sea level. On Mars, if liquid water ever did suddenly appear at the surface, it would quickly boil away. Why? Because, even at 3°C, the vapor pressure over liquid water is 7.6 mb, which is about the same as the total surface pressure on Mars. Since the temperature of the Martian atmosphere decreases with altitude, and even surface temperatures very seldom reach freezing, it is safe to say that water droplet clouds are virtually impossible on Mars. Ice-crystal clouds (cirrus clouds) have been observed on Mars. The vapor is thought to come from the polar caps, when water ice sublimates (turns directly into vapor) during the polar summer. Though it is incorrect to say that there are no clouds on Mars, ice-crystal clouds, the only kind found there, are not common.
Venus has a very dense and thick atmosphere. The surface pressure is more than 90 times that at sea level on Earth, equivalent to the pressure one would experience over 900 meters beneath the surface of the ocean. The surface of Venus is hotter than any cooking oven, about 460°C. From the surface to 40 kilometers in altitude, any liquid water would quickly boil away. There is very little water vapor in the Venusian atmosphere, about 20 parts per million by volume. What little water vapor there is participates in photochemical reactions high in the atmosphere to form sulfuric acid clouds at 50–70 km altitude, near the top of the troposphere. These clouds constantly wreathe the entire planet, reflecting about 75 percent of the incident sunlight and making Venus the brightest planet.
Rather than discuss whether the Earth's atmosphere ever resembled that of Mars or Venus, it is probably easier to consider whether our atmosphere could become like theirs in the future.
The key to habitability on Earth has been its equable temperature range for many millions of years. Earth is not so close to the sun that liquid water cannot exist at the surface, as on Venus, nor so distant as to be in a deep freeze, as on Mars.
On a time scale of millions of years, the interplay between liquid water and atmospheric CO2 has acted as a kind of thermostat. The weathering of silicate-based rock removes CO2 from the atmosphere. Volcanic activity adds CO2 to the atmosphere. These two processes are in approximate balance, and they are connected in the following way. Refer to the figure, supplied by James Kasting at Pennsylvania State University. CO2 dissolved in raindrops is slightly acidic. This breaks down silicate rocks (calcium silicate and magnesium silicate) during precipitation. The dissolved materials (calcium and magnesium ions, bicarbonate, and silica in solution) are carried by rivers to the oceans. In the ocean, further chemical reactions result in magnesium carbonate and calcium carbonate, the latter often being incorporated into the shells of small organisms living near the ocean surface. Eventually the carbonates and the silica settle to the bottom, where they are incorporated in sediment, layer by layer, over many thousands of years. These minerals have names that are familiar to many: calcium carbonate is the major ingredient in limestone, magnesium carbonate is called magnesite, and the silica takes the form of quartz.
Tectonic processes take over from here: the slow motion of the earth's crust eventually moves the sedimentary rock to subduction zones, where it sinks deep into the Earth's mantle and is subjected to intense heat and pressure. The calcium and magnesium carbonates react with the silica to form calcium and magnesium silicate. A byproduct of these reactions is CO2. The CO2 finds its way back into the atmosphere directly through volcanic eruptions or indirectly through hydrothermal vents on the ocean floor. Note that plate tectonics is responsible for 1) subduction and metamorphosis of carbonate sedimentary rock, which creates CO2, 2) the volcanism that releases CO2 into the atmosphere, and 3) the mountain building that brings silicate rocks to the surface and makes them susceptible to weathering, thus completing the cycle. Also note that liquid water is a necessary component of this cycle.
Caption: Diagram of carbonate-silicate cycle. Left: Weathering of calcium silicate rocks removes CO2 from the atmosphere. Center: Calcium carbonate accumulates on the ocean bottom through sedimentation. Right: Through metamorphism, calcium silicate rock forms in the earth's mantle under high heat and pressure. A byproduct is CO2, which returns to the atmosphere in volcanic eruptions.
Now, about the thermostat. Suppose an increase in volcanism injected more CO2 into the atmosphere than weathering could remove. CO2 is the most important greenhouse gas in Earth's atmosphere. Put more of it in the atmosphere, and the planet warms. A warmer atmosphere accommodates more water vapor, and increases evaporation from the ocean surface. The entire hydrological cycle accelerates, with more precipitation and more weathering. But, as noted earlier, more weathering accelerates the removal of CO2 from the atmosphere, and so it begins to cool.
Suppose volcanism were less than normal for many thousands of years. Weathering would deplete CO2 from the atmosphere faster than volcanism resupplied it, thus decreasing the greenhouse effect and cooling the planet. Perhaps glaciers and ice sheets would advance, increasing the earth's albedo, thereby further cooling the planet. The hydrologic cycle would slow, precipitation would decrease, and weathering of rocks would slow down. This, is turn, could foster an increase in atmospheric CO2 despite reduced volcanism.
These two processes describe a negative feedback loop, whereby the system acts to maintain its temperature when one component or the other departs from equilibrium.
Evidence of water on Mars in the distant past is compelling. A June 2009 news item described an ancient lake bed with layers of sediment deposited along the former shoreline. Erosional features abound on Mars. Where did all the water go? Huge volcanoes once existed on Mars. Now thought to be extinct, they were probably the source of CO2 that now comprises more than 95 percent of its atmosphere. Except for trace amounts of water vapor in the atmosphere (0.03 percent), all water is locked up as ice in the polar regions or as liquid or ice beneath the surface at lower latitudes.
Unlike Earth, Mars has no magnetic field and no ozone layer. The former protects Earth's lower atmosphere from the bombardment of energetic particles in the solar wind, and the latter screens out energetic radiation (at very short wavelengths) from the sun. Either can dissociate (chemically break apart) water molecules. In the Martian atmosphere, H2O readily dissociates, freeing hydrogen.
The gravitational force on Mars is only 0.37 of that on Earth, and so the escape velocity, the vertical speed that particles must acquire to escape from the atmosphere to space, is much less for Mars, 5.02 kilometers per second (km s-1), than for Earth (11.2 km s-1). There are many ways for particles to attain the escape velocity, but suffice it to say that the lightest particles, hydrogen in particular, attain the highest speeds and are therefore most likely to escape. It is nearly certain that most H2O on Mars was destroyed through dissociation by energetic radiation or particles from the sun and subsequent leakage of hydrogen into space. Oxygen, considerably heavier than hydrogen, could not so easily escape. There was probably much more oxygen in the early Martian atmosphere than there is today (only 0.13 percent), but much of that may now be locked in the iron oxides that color the Martian surface.
The atmosphere of Venus is about 96.5 percent CO2, 3.5 percent nitrogen, and trace gases, including sulfur dioxide (SO2) and water vapor. The excessive CO2 arose from extensive volcanic activity, which may or may not continue today. The greenhouse effect of CO2 is responsible for the extreme temperatures in the lower atmosphere, precluding any liquid water at the surface. Without liquid water, there is no weathering of silicate rocks, assuming such rocks even exist, and consequently no effective means for CO2 removal from the atmosphere. Any water vapor that escapes chemical reactions that form sulfuric acid clouds can diffuse to the high atmosphere, where photodissociation occurs, and hydrogen, thus released, can escape to space. The loss of water on Venus seems an irreversible process.
As long as liquid oceans remain on Earth, our atmosphere should remain stable. Perhaps in a billion years or so, when the sun's radiant energy will have increased by 10 percent, the atmosphere warms significantly, and water vapor (itself a potent greenhouse gas) markedly increases, more vapor will diffuse to altitudes where photodissociation can accelerate the loss of hydrogen on our own planet. When it becomes hot enough that the oceans boil, the demise of our atmosphere will come more quickly, because the weathering of silicate-based rock and the removal of atmospheric CO2 on geologic time scales will cease.
Weatherwise Contributing Editor THOMAS SCHLATTER is a retired meteorologist and volunteer at NOAA's Earth System Research Laboratory in Boulder, Colorado. Submit queries to the author Attn: Weatherwise; Taylor & Francis LLC; 325 Chestnut Street, Suite 800; Philadelphia, PA 19106; or by e-mail to email@example.com.