I have to say, I learn something new about space pretty much every day. A couple of weeks ago I was covering a lecture for an introductory level astronomy class here at UNH. The lecture topic was the formation of the solar system with a highlight on the details of the inner terrestrial planets. So, in preparation for the lecture (the night before), I looked through the textbook: The Cosmic Perspective. And while reading in there about Venus, my mind was blown. What exactly blew my mind? That Venus’s fate could easily have been Earth’s.
Venus is almost the same size as Earth (95% in radius and 81.5% in mass) and made up of pretty much the same stuff. The similarity in size and density between Venus and Earth leads scientists to believe that they share a similar internal structure: a core, mantle, and crust. Both have significant volcanic activity, although Earth has active plate tectonics and we don’t see evidence of that on Venus. The big difference between the two planets comes in the stuff above the planet’s surface: the atmosphere. Earth’s atmosphere is made up primarily of nitrogen (78%) and oxygen (21%), with trace amounts (~1% or less) of argon, carbon dioxide, water vapor, and other things. On Venus it’s an entirely different story. The Venusian atmosphere is almost entirely made up of carbon dioxide (97%), a very efficient greenhouse gas.
Now you’ve probably heard of greenhouse gases and their role in the greenhouse effect which is playing a role in concerns about global warming. A quick review of how the greenhouse effect works: a greenhouse gas, is a gas (most commonly a hydrocarbon) that allows visible light to pass through it but traps heat from infrared radiation. Okay, so who cares? Well the atmosphere is full of small amounts of naturally occurring greenhouse gases (such as carbon dioxide, methane, and water vapor) which allow the visible light from the Sun to pass through our atmosphere and reach the surface. That’s awesome, because if those gases were opaque to visible light– meaning they did not allow them to pass through– then all the plants on Earth would die and we’d all be sickly pale. It’s this next part that makes greenhouse gases significant. The Earth is warm, astronomically speaking compared to the cold vacuum of space, and emits its own radiation (or light) in the infrared. Back in the mid-19th century, scientists realized that anything that’s warm emits radiation– called blackbody radiation— most of it at wavelengths that we can’t see. The relation between the spectrum of light that an object radiates and its temperature is governed by Planck’s Law. A practical example are night-vision goggles which pick up on the thermal infrared radiation that’s given off by the warmth of a human’s body. Our Sun (10,340º F) is much, much hotter than Earth (~40º F) or a human (~98º F), so it emits a blackbody spectrum which peaks in the visible, whereas humans and the Earth emit mostly in the infrared. It’s the shifting of this peak which gives stars their different colors.
So Earth (and Venus because of its similar size) radiates most of its light in the infrared, which greenhouse gases do not allow to pass through the atmosphere. So instead of that newly radiated heat (IR light) being transmitted out through the atmosphere and into space, it gets trapped and increases the temperature for us on the surface. Now, normally for us on Earth that’s fine because our planet does a great job of regulating the surface temperature through a process known as the carbon dioxide (CO2) cycle. The CO2 cycle helps Earth to self-regulate its temperature. If it’s too hot, the excess carbon dioxide in the atmosphere dissolves in the water in the ocean, then settles and is stored in rocks of the ocean floor. If it’s too cold, then the carbon dioxide which returns to the interior of the planet (via subduction caused by plate tectonics) is released back into the atmosphere by volcanoes. So Earth can do a really good job at keeping track of its own thermostat, which has helped Earth maintain a stable surface temperature even though the temperature of the Sun has changed significantly over time. The problem with the greenhouse effect on Earth is that if we add too many new greenhouse gases (specifically carbon dioxide) that Earth can’t/isn’t ready to handle, then we can mess up the delicate balance that Earth is working so hard to maintain for us.
So what, Venus has more carbon dioxide in its atmosphere, why does that matter to Earth? Well, because it could very easily have happened to us. The only reason that Venus is a horrible, deadly world, the hottest in the solar system, with temperatures over 700º F is because it’s closer to the Sun than Earth is. The average distance of Venus’s from the Sun (called the semi-major axis) is ~72% of that of Earth’s; that means it’s roughly 66,928,200 miles away from the Sun (that’s a mere 26,027,600 miles– or 109 times the distance to the Moon– closer than Earth is). Astronomically speaking, that’s a very small distance. If primordial Earth’s orbit was altered enough to move it that roughly 26 million miles closer to the Sun– by, say an asteroid impact similar to the one that we believe created the Moon— then Earth could have ended up the same as Venus. If Earth were to move that mere 30% closer to the Sun, then the liquid water oceans which we have on Earth that dissolve the excess carbon dioxide, removing it from our atmosphere, would have evaporated. Water vapor is actually one of the best greenhouse gases, so the evaporation of the water vapor into the air combined with the failure to remove carbon dioxide from the atmosphere, would result in a runaway heating effect that the planet would have no way to stop.
So it’s by a very small distance, a distance that’s only 30 times larger than the diameter of the Sun, that Earth escaped being a hellish, fiery deathtrap and became the one oasis of life that we know of in the universe.
Manager, UNH Observatory
Modified version of original blog post from The Sky’s the Limit.