Extraterrestrial Climate Change

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Extraterrestrial Climate Change

Introduction

Venus, Earth, and Mars are the second, third, and fourth planets in the solar system counting outward from the sun. Of all the planets in the solar system, Mars and Venus are most similar to Earth. Venus is almost exactly the same size and has a thick, cloudy atmosphere. Mars, although having a mass only about one-tenth that of Earth, has an atmosphere, clouds, frosts, and dust storms, with polar caps of frozen carbon dioxide and water and large amounts of subsurface permafrost. In fact, since both planets have atmospheres, they have climates, and since they have climates, they can experience climate change. Study of Martian and Venusian climate in the 1960s and 1970s helped spark scientific thought about climate change on Earth, and the validity of mathematical principles used to model climate change on Earth is sometimes tested today against observations of those planets.

Historical Background and Scientific Foundations

Since the nineteenth century, many writers, including some astronomers, had fantasized that Mars might have a climate that was extremely similar to Earth's, with intelligent beings building canals across its surface to channel water from the ice caps to the supposedly thirsty tropical zones. Gradually such ideas were corrected in the first half of the twentieth century when better telescopes were built.

In the 1940s, long before it was yet possible to launch robot space probes to the planets, scientists were speculating that data about atmospheres elsewhere in the solar system might improve the understanding of Earth's atmosphere. In 1948, the head of the U.S. Weather Bureau, Harry Wexler (1911–1962), suggested the formation of a “Project on Planetary Atmospheres.” However, the idea was premature, since earthbound telescopes could not tell enough about what was happening on Venus and Mars to yield interesting comparisons between their atmospheres and Earth's.

By the mid-1950s, scientists knew from telescopic observations that the Martian atmosphere was almost entirely carbon dioxide (CO₂). They also knew that the atmosphere was very thin compared to Earth's, with a surface pressure of less than one one-hundredth of the pressure at terrestrial sea level. Mariner 4, which in 1965 became the first space probe to reach Mars, radioed back pictures of craters that looked more like the surface of the moon than of an Earth-like world.

In the 1970s, 1990s, and 2000s, many probes successfully orbited and landed on Mars, making on-site measurements of its weather and climate. Proof of the prior existence of large amounts of liquid water on the surface of Mars was discovered by NASA's twin Mars Exploration Rovers, which landed on the planet in late 2003 and early 2004. Since liquid water does not appear on the Martian surface today, Mars had clearly undergone massive climate change at some time in its deep past, several billion years ago.

Meanwhile, knowledge of Venus also was growing. Early fantasies had concluded that it might be an Earth-like jungle world covered by a perpetual deck of clouds, but this turned out to be very far from the truth. From telescopic observations, scientists already knew in the 1940s that Venus had an atmosphere consisting largely of CO₂. In the late 1950s, scientists learned that the surface of Venus was astonishingly hot—about 860°F (460°C)—by bouncing radio waves off the planet's surface. The Venusian surface temperature is hotter than any household oven and hot enough to melt lead (which melts at 621°F or 327°C). Soviet and American space probes visited the planet in the 1960s and 1970s, verifying this fact. In addition, the U.S. Magellan probe orbited Venus from 1990 to 1994, making a detailed radar map of its surface and gathering data on the Venusian atmosphere as well. The European Space Agency's

Venus Express probe, devoted mostly to observing the Venusian atmosphere, began orbiting Venus in April 2006 and was still functioning as of late 2007.

Scientific knowledge of the two planets' climates is now extensive. Their past history of climate change, or possible ongoing climate changes, is a subject that continues to be investigated. Scientists presently believe that both planets once had climates that were more similar to Earth's, but underwent radical climate changes that made one into a burning desert and the other into a frozen desert.

Mars

Of all the bodies and moons in the solar system, Mars has the climate that is least unlike Earth's. Its day, by coincidence, is almost exactly the length of a terrestrial day (24 hours). The tilt of the Martian axis with respect to the plane of the ecliptic (the imaginary flat plane in which all the orbits of the major planets lie, like rings on a disk), is 25°, similar to Earth's tilt of about 23°. The northern and southern hemispheres experience alternating winter and summer seasons, as do those of Earth, with increased precipitation in the winter hemisphere and increased evaporation in the summer hemisphere.

But the dissimilarities are also great. Mars's year is 1.8 times longer than Earth's; its atmosphere is about one-hundredth as thick; and its surface is bone-dry compared even to the driest Earthly conditions. Snow and rain never fall on its surface, although frosts form and clouds sometimes form. It is also far colder, with minimum temperatures of -225°F (-143°C) and an average surface temperature of -69°F (-56°C). Mars's orbit is not as neatly centered on the sun as Earth's, and its orbital shifts over thousands of years produce cyclic climate changes.

Although Mars is blanketed with a greenhouse gas, CO₂(with traces of another, methane), it is cold because

it is farther from the sun and its atmosphere is so much thinner than Earth's. Once, several billion years ago, Mars had a much thicker, warmer, and moister atmosphere. Oceans briefly washed its surface. The gases of its atmosphere were supplied by large volcanoes. However, because of its low gravity, once the volcanoes became extinct, the atmosphere began to leak away to space. The water froze and Mars became the planet we see it today.

Venus

The climate of Venus is dominated by its thick atmosphere, which has a surface pressure 92 times that of Earth and is composed mostly of CO₂. This CO₂, along with deep cloud layers of sulfur-compound particles, creates an intense greenhouse effect. In 1940, German-American astronomer Rupert Wildt (1905–1976) performed calculations showing that Venus's carbon-dioxide atmosphere must create a hot surface, about 260°F (126°C).

In 1960, American astronomer Carl Sagan (1934– 1996) studied the problem and concluded that Venus was at least as hot as Wildt had calculated. He also speculated that Venus had once had a surface ocean like Earth and Mars, but had suffered a runaway greenhouse effect, where CO₂ caused heating, which evaporated the water of the oceans, which enhanced the greenhouse effect. Although Sagan was mistaken about water in the atmosphere of Venus—it is now known that its clouds are particles of sulfur compounds, not water particles like Earthly clouds—his idea that Venus once had an Earth-like climate, early in its history, is now commonly accepted.

In 1979, astrophysicist Michael Hart (1932–) proposed that Earth is luckily located in a very narrow orbital zone between too much energy from the sun and too little. If so, it might be easily tipped over into a Venus-like or Mars-like condition by human-caused global climate change. Scientists have since confirmed that Earth's climate is not as precarious as Hart thought. It is not plausible that Earth will come to resemble either of its neighbors. Rather, Earth is vulnerable to human-caused climate change in a narrower range—enough to cause problems, perhaps severe ones, but far from the sort of change that would threaten to sterilize the planet.

Recent research has suggested that the greenhouse effect on Venus is very intense. When massive volcanoes spew large quantities of greenhouse gases into the Venusian atmosphere, these additional gases may make the greenhouse effect even hotter for millions or hundreds of millions of years. This extra heat can then work its way down into the planet's crust from the atmosphere, causing stresses that deform and wrinkle the crust. These wrinkles have been mapped from orbit by spacecraft using radar to see through the thick haze layer that completely enshrouds the planet.

Impacts and Issues

Efforts to understand the climate on Mars and Venus aid efforts to understand Earth's climate. David Grinspoon, a scientist with the Denver Museum of Nature and Science and a member of the Venus Express team, has urged scientists to test their mathematical models of Earth's climate by applying them to the simpler systems of Venus and Mars. According to Grinspoon in 2007, the planets's systems contain invaluable information for the study of Earth's climate.

In the early 2000s, it was reported that Mars had begun undergoing global warming. Some skeptics or doubters of the reality of anthropogenic (human-caused) climate change claimed that there must be some common, non-human cause for simultaneous global warming on Earth and Mars—perhaps increased heat from the sun. However, measurements made by spacecraft orbiting at Mars show that slightly less, not more, solar energy is reaching Mars. The real reason for Mars's recent warming is its shifting orbit around the sun, which causes repetitive or cyclic changes of climate called Milankovitch cycles. Earth also experiences Milankovitch cycles, but these well-understood changes are not the cause of recent global climate change on Earth.

Primary Source Connection

Although climate change is often thought of only in the context of Earth, climate change can occur on other planets, often with severe impacts. On Venus, climate change influences, and is influenced by, the planet's volcanic and tectonic processes. This article from the journal Science describes the process by which large volcanic eruptions cause climate change by releasing greenhouse gases, primarily water vapor (H₂O) and sulfur dioxide (SO₂), into the atmosphere. The atmospheric warming caused by these volcanic events then increases surface temperatures to such a degree that it ultimately leads to tectonic deformations.

CLIMATE CHANGE AS A REGULATOR OF TECTONICS ON VENUS

Tectonics, volcanism, and climate on Venus may be strongly coupled. Large excursions in surface temperature predicted to follow a global or near-global volcanic event diffuse into the interior and introduce thermal stresses of a magnitude sufficient to influence widespread tectonic deformation. This sequence of events accounts for the timing and many of the characteristics of deformation in the ridged plains of Venus, the most widely preserved volcanic terrain on the planet .

Venus has had a volcanic and tectonic history differing in important respects from that of Earth. The distribution and states of preservation of impact craters indicate that much of the surface is indistinguishable in age from a mean value of 300 to 700 million years (My). Venus lacks evidence for global plate tectonics, but widespread volcanic plains record globally coherent episodes of deformation that appear to have occurred over short intervals of geological history. These characteristics have been difficult to reconcile with interior dynamical models for the thermal and mechanical evolution of the planet.

It has recently been recognized that global or near-global volcanic events, such as those called upon to account for the history of crater preservation and plains emplacement, can have a significant influence on the climate of Venus. In particular, the injection into the atmosphere from erupting lavas of such volatile species as H₂O and SO₂ can lead to large excursions of surface temperature over time scales ranging from millions to hundreds of millions of years. Here, we explore the implications of these temperature excursions for the state of stress in the Venus interior. In particular, we examine possible coupling between the temporal variations in climate and the nearly global tectonic deformation that would have followed the largest distinct episode of widespread volcanism known to have occurred on the planet.

WORDS TO KNOW

ANTHROPOGENIC: Made by people or resulting from human activities. Usually used in the context of emissions that are produced as a result of human activities.

METHANE: A compound of one hydrogen atom combined with four hydrogen atoms, formula CH₄. It is the simplest hydrocarbon compound. Methane is a burnable gas that is found as a fossil fuel (in natural gas) and is given off by rotting excrement.

MILANKOVITCH CYCLES: Regularly repeating variations in Earth's climate caused by shifts in its orbit around the sun and its orientation (i.e., tilt) with respect to the sun. Named after Serbian scientist Milutin Milankovitch (1879–1958), though he was not the first to propose such cycles.

The most abundant geological terrain on the surface of Venus consists of ridged plains, volcanic plains material deformed by wrinkle ridges subsequent to emplacement. Ridged plains are concentrated in areas of low elevation and make up 60 to 65% of the present surface. Because they are overlain by younger plains units in some areas, the ridged plains must have occupied a still larger fraction of the surface area of Venus at the time of their emplacement. On the basis of stratigraphic relations, impact crater densities, and the small fraction of impact craters embayed by ridged plains lavas, these plains appear to have been emplaced over a narrow interval of geological time, at most a few percent of the average crater retention age for the surface or a few tens of millions of years. Widespread formation of wrinkle ridges evidently occurred shortly after ridged plains emplacement, on the grounds that few impact craters on these plains have been deformed by the wrinkle ridges. An interval no more than 100 My between ridged plains emplacement and the formation of most wrinkle ridges is implied.

Wrinkle ridges, by analogy with similar structures on the other terrestrial planets, are inferred to be the products of horizontal shortening of the lithosphere, the mechanically strong outer layer of the planet. The consistent orientation of most wrinkle ridges over areas thousands of kilometers in extent and the strong tendency for the ridges to encircle broad topographic and geoid highs implies that much of ridge formation was a response to variations in the lithospheric stress field over comparable spatial scales. Although models for gravitational stresses that arise from long-wavelength variations in topography and lithospheric density can match the distribution of many wrinkle ridges, such models do not account for a limited time interval for most ridge formation. Some researchers have suggested that wrinkle ridge formation was but one of a series of widespread episodes in the geological history of Venus during which a single tec-tonic style dominated deformation on a global scale, although others have questioned the basis for global synchroneity of regional deformational episodes.

The volume of lavas that formed the ridged plains can be estimated from the exposed surface area and estimates of typical thicknesses of such plains material. The exposed surface area of ridged plains is about 3 × 10⁸ km². On the basis of crater embayment relations, statistics on volcanic shield burial, and relations between wrinkle ridge width or spacing and plains thickness, the fraction of ridged plains material exceeding 500 m in thickness has been estimated to be 20 to 40%. Given that the greatest thicknesses of plains material may be as large as 2 to 4 km if such plains bury surfaces as rough as the oldest terrain on the planet, that additional plains units may have been nearly contemporaneous with the ridged plains, and that the surface area of ridged plains after emplacement was greater than the presently observed area, the total volume of lavas that erupted to form the ridged plains was probably at least 1 to 2 × 10⁸ km³. This volume exceeds by as much as an order of magnitude that of even the largest of the major igneous provinces on Earth. The emplacement of ridged plains on Venus represents the largest distinct volcanic episode in the preserved geological history of Venus, as measured either by total volume or by volcanic flux.

The widespread emplacement of the ridged plains should have released large quantities of water and sulfur gases into the Venus atmosphere. These gases affect both the atmospheric greenhouse and the albedo and opacity of the global cloud cover. Perturbations to the atmospheric SO₂ content are modulated by surface-atmosphere chemical reactions, which are limited by kinetics and diffusion into the uppermost crust. Photo dissociation and upper atmospheric hydrogen loss affect the atmospheric H₂O inventory. Climate evolution models incorporating all of these processes predict significant excursions of surface temperature after a large volcanic event…

There is reason to expect a strong coupling between the evolution of climate on Venus, the history of large volcanic eruptions, and the state of stress and large-scale deformation of the surface. Climate-induced changes in the stress field act on a planetary scale, so synchroneity of widespread deformational events is to be expected if this mechanism plays an important role in tectonics. Such planet-scale coherence of tectonic episodes distinguishes the geological history of Venus from that of Earth.

Sean C. Solomon, et al