As anyone who’s flown on an airplane can attest to, in Earth’s atmosphere, turbulence is everywhere. On sunny days, pockets of hot air churn up through the sky. Mountains act like rocks in rivers of air, leaving swirling eddies downwind. And high above, air currents shed vortices as they race around the Earth.
We accept this behavior as normal, albeit annoying when trying to sip weak drinks at 35,000 feet. Turbulence is a general property of gases and liquids that occurs when they are strongly stirred, and it is expected to be present in all planetary atmospheres: in the case of Earth and Mars, at scales greater than about 1 millimeter and 1 centimeter, respectively. Below this, the atmospheres are like molasses, so smaller whirls don’t exist.
Until now, scientists have concentrated on understanding the ways that solar energy stirs terrestrial and Martian atmospheres, focusing on structures that cover thousands of kilometers or more in size; they have neglected the scales from this size down to the dissipation scale. Theorists have proposed that over these scales the atmospheres should follow statistical (turbulent) laws that are not sensitive to the details of the stirring mechanism. Until now, we have been able to quantify these laws only on Earth. But to fully verify the the roles that these scales play, we need a new point of comparison. Now a new study from Chen et al. reports that the properties of turbulence in Mars’s atmosphere are remarkably similar to Earth’s.
Decades of observations and simulations have given scientists a good look at the dynamics of the Martian atmosphere on a global scale, including cells of circulation from equator to pole that resemble Earth’s. This behavior is driven by global factors like the warmth of the Sun and the planet’s rotation, and they can be predicted from physical laws. But unlike the classical laws of fluids, turbulent laws describe the chaos, the randomness, of strongly stirred fluids. Patterns here are stochastic: They can be scrutinized statistically but not precisely predicted. Therefore, the dynamics of the atmosphere on more graspable scales—for example, scales on Earth the size of mountains, buildings, trees, and other things that influence turbulence—can only be dealt with statistically. In currents of air, nobody can reliably predict whether a particular eddy will form, but we can calculate quantities like the spectrum of sizes, how often eddies are likely to form, and how far apart they are likely to be.
To that end, the authors have produced the most detailed statistical analysis yet of Mars’s atmosphere. They used a public data set called the Mars Analysis Correction Data Assimilation (MACDA), which includes 5 years of measurements from NASA’s Mars Global Surveyor orbiter and also uses a model of global atmospheric circulation to fill in gaps.
The authors find that statistically, the spectrum of how parameters like temperature, pressure, and wind speed behave on Mars is remarkably similar to Earth’s. In fact, mathematically, the exponents of the power laws that govern their behavior are virtually identical.
The finding suggests something profound about the underlying physics: Certain patterns likely emerge in all planetary atmospheres. And even though physics still can’t predict the exact state of a turbulent atmosphere, the classical laws that attempt to define the statistics of those patterns do hold.
In this way, the authors note, the study tells us as much about Earth as it does about Mars: It demonstrates that the laws that spill our in-flight drinks across our tray tables aren’t unique but hold across Mars and Venus, the other terrestrial planets with robust atmospheres. (Journal of Geophysical Research: Atmospheres, doi:10.1002/2016JD025211, 2016)
—Mark Zastrow, Freelance Writer
