Presentation: the runaway planets

In this time-lapse sequence, each frame represents one martian day (a "sol") for the NASA Mars Exploration Rover Opportunity, starting from its landing on Mars. What are some details that you notice appearing in some of these frames? (Video link: "3 4 OpportunityHazCam.")

In this presentation we will complete our discussion on terrestrial planets with Venus and Mars...

...explicitly comparing their similarities and differences with each other and with Earth (as discussed in a previous presentation).

Venus and Mars can be considered the "runaway" planets, as their greenhouse gas cycles have each gone awry in different ways for different reasons, which will also be covered in more detail on the subsequent in-class activity. Let us hope that our planet's greenhouse gas cycle does not fail us as well...

First, factors that affect the amount of greenhouse effect. Note that mass appears in this list twice. It's that important.

If a planet is massive, it will retain heat for a longer time (due to the "turkey/cornish hen effect"), and be more geologically active, spewing out more greenhouse gases into its atmosphere.

In contrast, a low-mass planet will lose heat quickly, and be less geologically active for a shorter time, spewing out less greenhouse gases into its atmosphere.

Mass is also important to retain an atmosphere. A massive planet will have stronger gravity, such that it hold onto its atmosphere of greenhouse gases, which would otherwise leak out into outer space. (Video link: "Office Time Lapse.")

In contrast, a low-mass planet will not have a lot of gravity, such that it will quickly lose the small amount of atmosphere that it outgassed.

Given that a planet has an appreciable amount of greenhouse gases in its atmosphere, the distance from the sun to the planet then determines how much warming there will be from the greenhouse effect. A planet near the sun will be able to absorb more visible light energy from the sun, and trap this energy as infrared, maintaining warm temperatures.

In contrast, a planet far from the sun will not absorb a lot of visible light energy from the sun, such that it will not trap much of this energy as infrared, and cooler temperatures result.

Let's summarize these three greenhouse factors. The left-to-right arrow denotes a cause-and-effect, and matching up-and-down arrows on both sides of the causal arrows represent a correlated effect: a large or small mass planet will have a lot or a little outgassing, while a large or small mass planet will have gravity to retain much or very little of its atmosphere. Note the opposite down-and-up arrow and up-and-down arrow for distance and temperature, which represents a inverse effect: a planet very close or very far from the sun will have high or low temperatures as result of the greenhouse effect.

We will later apply these greenhouse factors to Venus and Mars, after looking at their surface features.

Second, surface features on Venus and Mars.

These are cylindrical maps of Venus and Mars, if you unwrapped their exteriors and laid them flat by unrolling them. Note that Venus' surface is obscured by clouds, while Mars' surface is obscured by dust storms, sparse clouds, and polar ice caps.

Whenever visible light is obscured, longer electromagnetic radiation wavelengths will penetrate, so the surfaces of Venus and Mars have been radar-mapped by bouncing radio waves. Darker regions represent smoother areas, and brighter regions represent rougher areas on these radar maps, tinted with the natural colors of Venus' and Mars' surfaces. Which part of which planet is geologically oldest? Newest? What overall similarities or differences do you observe for Venus and Mars, compared to each other and with Earth?

A unique feature found on Venus is its coronae, which at first glance seem like conventional Earth-like volcanoes that have spewed out lava. However, most of Venus's surface has been completely covered over with lava not very long ago, as revealed by the density of impact craters. Peculiarly many of these volcanoes have collapsed and flattened out, as in the foreground, giving them their "crown-like" appearance.

The prevailing theory for the relatively recent, nearly complete volcanic resurfacing of Venus can be illustrated in this clip from The Simpsons ("Tennis the Menace," Twentieth Century Fox, 2001), where the gophers have been paved over with a tennis court. If the pavement is thin and flexible enough, then the gophers will eventually push their way out, once they get angry enough. Recall that Venus has nearly the same mass as Earth, and will have retained nearly as much heat, which is represented by the gophers. The magma below is trying to push out through the crust, but being thin and flexible, the crust does not easily allow this, such that the hot magma is stored until finally bursting out simultaneously everywhere on Venus, paving over its impact craters. Later as the heated magma subsides, the crust collapses, forming the flattened coronae. How many of these suppressed-widespread eruption cycles Venus has gone through in its history, and whether Venus still has enough internal heat stored up for another eruption cycle are open questions. Maybe the gophers under Venus' crust are sleeping, saving up for another push through...or maybe they're now too weak, or dead...

Moving on to a unique feature of Mars, where there are fewer volcanoes, but they are much larger than on Venus, and even larger than on Earth. This shield volcano in particular is approximately the size of Missouri, and for vertical scale the cliffs on the left are nearly a mile high. This represents a long, steady eruption of lava to build up something this size, as well as paving over some of the surrounding landscape as well.

The prevailing theory for Mars' shield volcanoes is illustrated here with a rather unappetizing pot of gravy. Notice that the gravy skin is very thick and inflexible, but there are certain locations where escaping steam has punctured through, which would allow gravy to slowly seep up through the skin. Recall that Mars is a distant third behind Venus and Earth in terms of mass, so Mars would not have retained much internal heat for very long. While Mars was geologically active, magma below did manage to find its way out through certain locations in the thick, inflexible crust, and slowly but steadily built up massive shield volcanoes at these locations.

When you think Venus' geological features and history, think about those gophers, which may still be alive under its thin, flexible crust; and for Mars' geological features and history, think about that thick, gooey gravy, which leaked out for a time through certain locations through thick, inflexible crust.

Third, the runaway atmospheres of Venus and Mars.

From your textbook reading, you should be familiar with conditions on Venus--a hot, thick atmosphere made up caustic acids, and many times more dense than Earth's atmosphere. This makes a visit to Venus problematic, but Russian probes have managed to land and record data, before their batteries failed due to the high temperatures, and/or high-pressure acids leaking into their electronics.

Let's look at the greenhouse factors on Venus. It has nearly as much mass as Earth, so Venus would have outgassed nearly as much greenhouse gases into its atmosphere, and would have also retained nearly as much of that atmosphere as Earth. With Venus being closer to the sun than Earth, conditions would then start out being much warmer than on Earth. Recall that Earth controls the amount of greenhouse gases in its atmosphere by soaking up carbon dioxide in its oceans. Venus is a "runaway greenhouse" because its greenhouse cycle was broken in a compounded way. As its atmosphere became too warm, its oceans evaporated, which not only reintroduced carbon dioxide back into the atmosphere, but also reintroduced all that water vapor back into the atmosphere--which is also a strong greenhouse gas. So the means of controlling greenhouse gases on Venus eventually contributed to a stronger greenhouse effect on Venus, making its atmosphere inhospitably hot and dense today.

You should also be familiar with conditions on Mars from your textbook reading--a cold, sparse atmosphere. Mars has been more successfully explored with rovers, provided that their batteries are kept warm and maintained with photovoltaic cells receiving enough sunlight.

Consider the greenhouse factors on Mars. It is much smaller than Earth, so Mars, would not have outgassed much greenhouse gases as Earth, and would not have retained much of that atmosphere as well. With Mars being farther from the sun than Earth, conditions would then start out being much cooler than on Earth. Recall that Earth controls the amount of greenhouse gases in its atmosphere by soaking up carbon dioxide in its oceans. Mars is a "runaway refrigerator" because its greenhouse cycle was broken in a compounded way. Its oceans were around to soak up carbon dioxide from the atmosphere, but as Mars got cooler, water vapor started to freeze out, and the remaining carbon dioxide in the atmosphere also froze, forming dry ice. So the means of controlling greenhouse gases on Mars eventually contributed to a weaker (or no) greenhouse effect on Mars, making its atmosphere inhospitably cold and sparse today.

For the subsequent in-class activity you should be able to fill in the remaining two pages on the histories and greenhouse cycles for Venus and Mars, after completing the greenhouse cycle for Earth on the first page.