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.
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%.
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 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.
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.