Monday, 25 November 2019

Why Study Planetary Atmospheres?

How does looking at weather on other worlds help inform our understanding of the Earth’s atmosphere?

Think of all the planets in our solar system as offering a continuum of different outcomes for atmospheric evolution, based primarily on the soup of species that were initially accreted by the protoplanet, the distance of the planet from the Sun today, and the dynamical properties of the planet (e.g., its size and rate of rotation, which determine the banded pattern of winds).  By comparing how atmospheric meteorology, chemistry, clouds and dynamics differ from one world to the next, we get a better understanding of the physical and chemical rulebook that governs what an atmosphere (or a climate) will actually be like.  Science is all about experimentation – tweaking one parameter and seeing how a system changes.  We can’t do that sort of mass experimentation on our home world (well, ahem, ignoring the massive amount of CO2 we’re pumping into our fragile atmosphere), but the planets of our solar system (and indeed, exoplanets) offer a ready-made experiment, out there for us to explore.  If we find that a theory of atmospheres based on Earth doesn’t work for Mars, or for Saturn, then we need to know why it doesn’t work – and that might mean tweaking the theory.

Now, that all might sound straightforward, and ultimately is the direction planetary atmospheric scientists might want to head.  But for now, we know that the conditions are so different from world to world that it’s hard to determine whether our universal theories are inaccurate, or whether the data are simply incomplete or misleading.  For example, Saturn has no solid boundaries (mountains, continents, valleys) to get in the way of perfect fluid dynamical flows, so it’s a very different regime from Earth.  Furthermore, its clouds and precipitation exist in a hydrogen rich atmosphere (the lightest gas in the universe), which is totally different to the Earth’s nitrogen-oxygen atmosphere.  So today, I’d say we’re at the stage (with missions like Cassini and Juno) of trying to properly measure and observe these unusual atmospheres.  The next step is to confirm that we can recreate them numerically as a simulation (in just the same way as a terrestrial weather and climate model works).

How can other planets give us a glimpse of our future?

A lot of what I said above also applies here – our planet exists of a continuum of planetary types, showing how worlds can end up in different states depending on small changes at the start of their evolution.  Take Venus, Earth, and Mars, for example:  these worlds might all have started off under conditions that weren’t so very different, but over the aeons those differences were amplified – Venus becoming the hellish world of high temperatures and corrosive atmosphere; Mars freezing to become a barren desert.  These are great examples of divergent evolution from shared origins.  Now, we humans have been doing an uncontrolled experiment with our own fragile atmosphere since the industrial revolution, vastly upsetting the balance of molecules in our atmosphere and leading to today’s climate emergency.  Venus stands as a stark warning of what happens to worlds with too much carbon dioxide.  Mercifully, we know that this is avoidable, if we only had the willpower to change our ways.