This question comes from a local fifth-grader, courtesy of the Science Outreach Program’s (SOP) question box. Teams from the Science Outreach Program make weekly visits to elementary school classrooms in the Portland area to encourage scientific learning in younger students—most importantly, by making it fun. One of the ways they do this is by having a question box, where any student can submit a question they have about the world, and the Science Outreach team will explore the answer with them. Now, this spirit of curiosity has taken on a new form in the Quest. If you have any questions, please contact scienceoutreach@reed.edu.
To answer this question, let’s look not just to our own planet’s atmosphere, but also to that of our neighbor: Mars, the Red Planet. Mars’s atmosphere is over 100 times thinner than Earth’s, and 95% percent of it is carbon dioxide. The remaining 5% is made up of nitrogen, argon, and trace gases such as carbon monoxide and methane. In comparison, Earth’s atmosphere is composed of 78.1% nitrogen, 20.9% oxygen, and 1% trace gases. Already, there are two clear differences between the two atmospheres: their chemical composition and their relative thickness. But what if this wasn’t always the case? What if, long ago, Mars’s atmosphere was more similar to what Earth’s atmosphere looks like today?
Based on results from NASA’s Mars Atmosphere and Volatile Evolution (MAVEN) and Curiosity missions, among others, scientists believe this could be true. By measuring the different isotopes of oxygen present in the Martian atmosphere and comparing them to the isotopes of oxygen present in Earth’s atmosphere, it is possible to calculate how much oxygen the Martian atmosphere once contained. Once, Mars’s atmosphere was oxygen-rich, and over billions of years it has transformed to the cold and inhospitable atmosphere we know today. Over this time, the rivers Mars once had froze and disappeared, dust storms began to rage across the planet, and any possibility for life was extinguished. According to NASA, if you were to stand on Mars’s equator at noon, “it would feel like spring at your feet (75° Fahrenheit or 24°Celsius) and winter at your head (32° Fahrenheit or 0° Celsius).” Weird, right? Would the same thing happen to Earth if our atmosphere became weak like Mars’s?
The short answer is yes. On Earth under current conditions, water must be heated to 212° Fahrenheit (100° Celsius) to boil at sea level, but at the summit of Mount Everest, water only has to be heated to 160° Fahrenheit (71° Celsius) to boil. The reason for this phenomenon is atmospheric pressure, the push exerted on Earth’s surface by the layers of our atmosphere. So, without Earth’s atmosphere, all water would evaporate up into space, even the oceans, which is what scientists theorize occurred on Mars. Similarly, there would be no clouds and no rain. We would also lose our blue sky, because the presence of nitrogen and oxygen in the atmosphere are what allow blue wavelengths of light to scatter more than any other color. In their absence, the sky would be dark all the time, which is how it is on Mars. With no clouds and no hazy atmosphere, it would be perfect for stargazing—but, unfortunately, there would likely be no species left on Earth to do so. Earth’s atmosphere acts as a shield to protect us from harmful cosmic and solar radiation, while at the same time trapping heat that makes the planet hospitable for life. The gases that trap heat are called greenhouse gases, and while humanity is currently pumping the atmosphere too full of these, life as we know it wouldn’t exist without them. Earth would be colder than Mars, at around -4° Fahrenheit or -20 ° Celsius.
And, of course, there is the problem of breathing. Life on Earth did not always depend on oxygen, but it certainly does today. If the atmosphere no longer contained enough oxygen, all oxygen-dependent species would die out—that is, if they hadn’t already been eliminated by the boiling of all liquid and the constant bombardment of radiation. Much like Mars, Earth would become a lifeless planet, save for a few microscopic organisms that might survive, like tardigrades, which are able to endure extreme temperatures and high amounts of radiation. It remains to be seen if similar microorganisms exist on Mars; no alien life has been discovered on the Red Planet yet, despite what popular science fiction might tell you.
All in all, if Earth’s atmosphere were to become significantly weaker, the results would be depressing. Luckily, we don’t have to worry about such a sudden, drastic change—the atmospheric degradation of Mars took billions of years. You won’t wake up one day to Reed Lake boiling up into an apocalyptically dark sky.