One of my favorite websites is Astronomy Picture of the Day. I go there to marvel at the Purty Pictures, and there is a new one each day.
Recently this post contained and explained this image:
Taken by the Mars Express orbiter showing the north pole of Mars. Dig the swirly pattern formed by the frozen \(CO_{2}\) drifts, blown by the circumpolar winds on Mars. The swirling winds happen because the Coriolis force acts on moving air, causing it to deflect in a clockwise direction in the northern hemisphere and counter-clockwise in the southern hemisphere. The effect is strongest at the poles.
I know that Earth also has circumpolar winds. I wondered, why doesn’t Earth form these same swirlies in the snow at the poles? A quick web search told me that in fact this probably does happen on Earth too. So the question became, why have I never seen dramatic pictures from orbit showing this? That has multiple and more mundane answers, including:
- we don’t have many satellites in orbits appropriate for taking images of the poles
- the weather near the poles seldom provides an opportunity for clear images
- there isn’t enough ground showing through the white snow to give the nice contrast provided by the red Martian dust.
This page provides a couple more images of this phenomenon on Mars.
That page also has multiple images of what to me is one of the most remarkable things in planetary science; the hexagon formed by the clouds at the north pole of Saturn:
Why on EarthSaturn should a rotating vortex form a hexagon pattern? According to the Wikipedia entry on Saturn’s hexagon, similar shapes have been produced in labs in circular tanks of liquid where the liquid is rotating at different speeds at the center versus the edge. The difference in rotational speed forms vortices that arrange themselves into stable patterns that can form geometric shapes, but only if the liquid viscosity and the difference in rotational speeds are just right. It is interesting that the geometric shapes produced in this way are not always hexagons - shapes with different numbers of sides can also be produced.
Digging a little deeper, these polygonal shapes can also be formed in the eyes of hurricanes here on Earth. I was unable to find many really obvious, dramatic images to back this up, but these images from Hurricane Isabell in 2003 show at least a strong hint of a pentagonal shape to the eyewall:
These polygonal shapes formed by sub-vortices inside a bigger vortex are better understood than I initially thought - they seem to be due to phase locking of vortex Rossby waves. Sounds like something from a bad Star Trek Voyager episode, right? This video lays it all out. It is basically what I said above: sub-vortices form near an interface where there is a difference in rotational speed within a larger vortex, e.g. along the eyewall of a hurricane. These Rossby waves are the mechanism that moves these vortices around the eyewall, and when these Rossby waves phase lock such that an integer number of them fit around the circumference of the eyewall, it spaces out and stabilizes the vortices in a way that makes the circular eyewall appear polygonal. Computer simulations exist that show this happening.
