Climate Change in the Solar System

We are all familiar with the climate on Earth: the seasons, the range of surface temperatures that are just right for being a water world, the oxygen we breathe, the ozone layer that protects us from UV radiation. In short: habitable.

So what other bodies in the Solar System might be (or might have been) habitable, and why aren’t they today?

Mars probably comes to mind, and for good reason. Mars has the most similar climate to our own, with water ice caps at the poles, seasonal snow, and dust storms. This is because Mars has a similar axial tilt as the Earth, which creates similar seasonal temperature variations. However, the colder average temperatures and the thin atmosphere mean liquid water can only exist on the surface around midsummer and at the lowest elevations (where the atmospheric pressure is greatest). The thin atmosphere also means the surface is exposed to intense UV radiation. Mars may not be habitable today (for life on the surface), but climates change.

 

Mars

Hubble image of Mars engulfed in a global dust storm, with its polar caps peeking through. Image courtesy of NASA.

Several lines of evidence point to Mars being wet and warm early in its history. Water-carved channels, minerals formed by interaction with groundwater (like gypsum), river delta deposits, and what may be a shoreline all the way around the northern lowlands (which would have been a giant ocean) all point to lots of liquid water on the surface sometime in the distant past.

So why was Mars so much warmer and wetter than it is today, and why did it change? These are fundamental questions about climate change that have yet to be fully answered.

Early Mars likely had a thicker atmosphere, made of mostly CO2 like it is today, which would have warmed the surface through the greenhouse effect. One way to understand the climate early in Mars history is to study the oldest rocks and landforms. Another is to look at more recent climate changes, which are likely preserved in the polar ice caps.

Just as ice cores on Earth provide a record of annual changes in climate, the thick stacks of polar ice on Mars have internal layering that suggests they were built up one layer at a time, for millions if not billions of years. (Some of the research I do here at the Museum is directly related to the internal structure of these ice caps, which I mapped out using orbital radar data. I am currently working to understand smaller-scale features buried in the ice.)

So if one of our neighbors may have been habitable in the past, what about our nearest neighbor, Venus?

Venus is almost the same size as Earth, and only slightly closer to the Sun. However, its axis does not tilt relative to the Sun, so it has no seasons like Earth and Mars. We know less about ancient Venus than we do about Mars, because the surface of Venus is relatively young (~1 billion years old). However, we think the atmosphere is much older than the surface, made up of mostly CO2 (like Mars, and like early Earth). With 100 times the atmosphere of Earth, its runaway greenhouse effect long ago boiled all the water off the surface. Some of that water is bound to sulfur and makes up the sulfuric acid clouds that circle the planet, but much of it was broken down in the atmosphere and removed by the solar wind. Venus is dry and hot, despite its clouds reflecting 80% of the sunlight that arrives, since it very effectively traps the remaining 20%.

 

Venus

Clouds swirl around the south pole of Venus, imaged in UV by Venus Express. Image courtesy of the European Space Agency.

So was Venus ever more like Earth?

Being so similar to Earth, Venus likely formed from the same material. The key to their different climates today may be in part due to Earth having plate tectonics, which buries carbon-rich sedimentary rocks (taking CO2 out of the atmosphere). Venus instead keeps all of its CO2 in the atmosphere. The clues to climate change on Venus will probably be found in the composition of its atmosphere, with isotopic ratios of elements like carbon and hydrogen pointing the way to understanding when and why it became so hot and dry.

Only those three inner planets in our Solar System have atmospheres thick enough and persistent enough to have climates that change over time. However, one moon in our Solar System, more massive than the planet Mercury, has an atmosphere. In fact, Titan, a moon of Saturn, was once thought to be the largest moon in the Solar System precisely because its atmosphere is so thick (1.5 times the atmosphere of Earth).

 

Titan

Titan is the only moon in the Solar System with a thick atmosphere, imaged by Cassini. Image courtesy of NASA.

Titan is particularly interesting because its atmosphere is made up mostly of nitrogen, just like the Earth. The remainder is mostly methane, which breaks down easily in the atmosphere and has to be replenished every ~50 million years; this implies some unknown but ongoing process. Titan gets 100 times less sunlight than the Earth, so its surface is frigid, cold enough that water ice is as hard as rock. So while Titan is not currently habitable for life as we know it on Earth, it is the only other place in the Solar System with rain (made of methane and ethane). However, in another 5 billion year the Sun will become a red giant star, and Titan probably will be warm enough to have liquid water on its surface, making it habitable at last.

For the time being, understanding the methane cycle on Titan (perhaps analogous to the water cycle on Earth) will help us understand climate change on Titan, and may give us insight into the behavior of climate on early Earth.

Titan, Venus, and Mars all have something to teach us about the possibilities for climate change and habitability on Earth. While nothing as dramatic as the changes experienced by Mars or Venus is likely to happen anytime soon on Earth, we do know that smaller changes in climate have had big effects on life, and vice versa.

When photosynthesis appeared on Earth ~2.5 billion years ago, it put oxygen into the atmosphere for the first time. When the “snowball Earth” episode ended ~500 million years ago, the warmer and friendlier climate produced macroscopic life for the first time. When extensive volcanism occurred ~250 million years ago, ~95% of life on Earth was wiped out. When the aftermath of a large impact cooled the climate ~65 million years ago, the dinosaurs died off. In the last million years, according to ice core records from Greenland and Antarctica,  recurring periods of warming and cooling (correlated with increasing and decreasing amounts of CO2 in the atmosphere) have caused repeated ice ages and interglacial periods; during the most recent interglacial period (from ~10,000 years ago to today), humanity has thrived.

 

Earth

The one climate in our Solar System that is "just right" for life, imaged by Apollo 17. Image courtesy of NASA.

Currently we are blessed with a friendly climate. What will help us best understand it? What more might we want to know about changes in other climates? What is the role of humanity in the future climate of Earth?

Michelle Selvans is a planetary geophysicist in the Center for Earth and Planetary Studies at the National Air and Space Museum.

The Pilot as Hero in the Aviation Film Genre

The relationship between film, history, and mass culture is especially intriguing when we examine the correspondences between the representation of pilot-heroes in film and public perceptions of aviation. These connections are applicable during the heyday of the aviation genre film—the interwar years and WWII. For many years, films in the aviation film genre have been overlooked by scholars because they were perceived to be formulaic and cliché-ridden. While this charge may be true, these films contain important elements that confront and resolve problems of the individual pilot versus the group, and of the individual and group versus the outside world. Moreover, the best of these films not only reflect but often define American notions of heroism, masculinity, responsibility, and nationalism.

The aviation films of Howard Hawks—The Dawn Patrol (1930), Ceiling Zero (1935), Only Angels Have Wings (1939), and Air Force (1943), seem to me to be the solidifying force in the definition of the aviation film genre and the genre’s significance in molding public perceptions of masculinity and  heroism, particularly during the 1930s and WWII. More than any other director of his era, Hawks was responsible for defining the aviation hero in popular film, and the generic elements in his aviation films helped to reinforce the popular image of the pilot-hero. The formula Hawks established for heroic behavior in his aviation genre films can be seen in other film genres like the Western and the combat film.

Dawn Patrol

A First National Pictures (Warner Brothers) still photo from "The Dawn Patrol" (1930), directed by Howard Hawks. "The Dawn Patrol" solidified the aviation film genre by focusing on a group of professional military pilots in WWI who face danger and death stoically.

Hawks’s The Dawn Patrol, made in 1930, set the standard for portraying pilots in a dramatic wartime situation. The Dawn Patrol blended the realism of William Wellman’s Wings (1927) and Howard Hughes’s Hell’s Angels (1930) with more fully developed generic elements. More than any other WWI air combat film, The Dawn Patrol redefined and reinterpreted the genre, by exploring the psychological pressures and stresses of wartime flying. These tensions are characterized by situations in which fighter pilots must come face to face with their impending death and the death of their comrades, and by the emotional difficulties faced by squadron commanders who must send young men to die.

The film chronicles the 59th Squadron of the Royal Flying Corps during 1915, when the RFC was suffering severe casualties at the hands of the German air force.  Squadron Commander Brand (Neil Hamilton) faces the difficult task of sending young and inexperienced fliers to fight the Germans because so many of his veteran pilots have been killed.  Flight Commander Courtney (Richard Barthelmess) accuses Brand of butchery, but finds himself in the same position when Brand is promoted and Courtney becomes squadron commander.  Nerves stretched to the breaking point by the pressures of command, Courtney finds that he must send the unseasoned younger brother of his best friend Scott (Douglas Fairbanks, Jr.), into battle.  Scott’s brother is killed, and Courtney’s relationship with Scott is damaged irreparably.  Courtney himself takes on a suicidal mission that Scott had volunteered for and is killed.  At the end of the film, Scott finds that now he must assume the duties of command, re-initiating the deadly cycle.

Hells Angels

A dramatic but perhaps doctored United Artists still photo from Hell’s Angels (1930), directed by Howard Hughes. Originally shot as a silent, the film was remade as a talkie. Hughes went to great lengths to film the aerial combat footage, and he designed many of the dogfighting scenes himself.

The paradigm below outlines characteristics that are the most integral elements of the WW I air combat film genre as defined by The Dawn Patrol: 1. Characters:  the men of The Dawn Patrol are a closely-knit set of professionals who perform a difficult task and who adhere to their own carefully defined code of conduct.  2.  Values:  In The Dawn Patrol, the male characters unite into a fraternal organization with a code that stresses stoical adherence to professionalism, honor, and responsibility to oneself and the group. 3.  Actions:  Although very much part of a group, the men of The Dawn Patrol are involved in individualistic combat with chivalric overtones set against a backdrop of impending doom.  4.  Iconography:  Many of the iconographic elements of the air combat film originated in WingsThe Dawn Patrol, however, contains visual elements that have become trademarks of the genre.  Pilots are costumed in leather coats and helmets, and outfitted with goggles, scarves, and winged insignia.  Aircraft are both authentic WWI-vintage aircraft and 1930s era aircraft made up to look like WWI airplanes (e.g. in The Dawn Patrol, Travel Air 2000/4000s and Standard J-1s). Flight maneuvers, filmed by aerial cinematographers, include spectacular crashes, complete with smoke and flames.  Sound effects are commonplace and consist of aircraft engines, machine guns, and often a dramatic musical score.

Also, because the characterizations are so heavily masculine (a reflection of the male-dominated world of aviation in the interwar years), women in these films tend to be either non-existent or not essential. Aviation is thought of as a man’s work; women need not apply.

Only Angels Have Wings poster

A Columbia Pictures poster from Only Angels Have Wings, directed by Howard Hawks in 1939. The film, which stars Cary Grant and Jean Arthur, deals with a group of unsung pilots who fly mail and dangerous cargo in the fictional town of “Baranca,” set in the Andes Mountains.

The cultural impact of the narrative patterns of the genre on the audience was to mythologize the aviator, romanticizing and dramatizing his persona somewhat out of proportion to actuality, and reinforcing the notion that the pilot-hero could resolve the most severe cultural conflicts concerning the fear of technology and the loss of democratic ideals and values in the twentieth century. Another reason The Dawn Patrol struck a chord with audiences may have to do with the time in which it was made—the first year of the Great Depression. Financial hardship was on everyone’s mind and themes of wartime courage and group solidarity could easily be adapted to times of economic despair. The Dawn Patrol could thus be seen as celebrating values of teamwork, of “sticking together,” of community and sharing, contrary to the values of acquisitive individualism inherent in modern industrial capitalism.

The conventions of the WWI air combat film genre, as represented by The Dawn Patrol, eventually evolved into those common in the aviation genre films of the 1930s, the WWII air combat film, and beyond.  Eventually, however, the genre began to stagnate.  The Western and the combat film genres underwent significant changes during the Cold War, and especially after the Vietnam War, often questioning—and sometimes abandoning—the traditional values they had championed earlier.  The air combat genre, for the most part, maintained its traditional attitudes and values.

Has the aviation genre film become irrelevant? Perhaps. The notion of heroism in the early twenty-first century is much more complex than at any time in American history. If Hollywood were perhaps willing to renegotiate the aviation genre film’s formula to conform to a postmodernist mode of discourse, might it not be revivable?  Hollywood appears unwilling or unable to deviate from the winning formula of the past, and there is no guarantee that drastic changes in the formula would be widely accepted and successful at the box office.

Nevertheless, the formula prevalent in the aviation genre film, which extols without irony, the manly virtues of pilot-heroes, is simply outdated in this post-heroic age. Piloting an airplane may be looked upon as a dangerous but not particularly heroic activity. This is perhaps because aviation has become so all-pervasive and familiar that no one thinks much about it. The exoticism of aviation, so prevalent during the interwar years and extending into the 1960s, is a thing of the past. If the mystery and romance of aviation exist at all, they do so in terms of nostalgia for a bygone era.

Dominick A. Pisano is a curator in the Aeronautics Division of the National Air and Space Museum.