Sunrise on Mars. Image via NASA.
It's almost June, which means the Northern hemisphere is gearing up for a summer filled with beach days, beer gardens, and the annual schlepping of AC units. Like all of our planet's seasons, summer was born billions of years ago, when a massive object collided with the early Earth and permanently tilted it to one side.
Currently, the axial tilt or, if you want to get fancy, the “obliquity of the ecliptic” is around 23.4 degrees. The North Pole is angled towards the Sun, ready to soak up some warm stellar rays, while the Southern hemisphere prepares for chillier weather.
The word “summer” naturally has a earthly connotation to us humans, but our planet is far from the only world in the solar system that experiences a warm season. In fact, Mercury is the only planet with no axial tilt and thus no discernible summer. It does go through minor seasonal changes, such as variations in sodium levels, but it's nothing like the dramatic summer season we greet on Earth. While the Venusian summer is somewhat more distinguishable than that of Mercury's, Venus has an axial tilt of only 2.7 degrees, so it doesn't have much seasonal variation either.
Mars, on the other hand, experiences by far the most dramatic seasons in the solar system. Though it has a similar obliquity to Earth (it's currently at 25 degrees), it does not share our planet's almost perfectly circular orbit. Over the course of the 684-day Martian year, the planet's distance from the sun fluctuates from 1.36 astronomical units to 1.64. No other planet in the solar system has a higher orbital eccentricity than Mars (unless you are a purist who still counts Pluto).
The upshot is that Martian summers vary drastically depending on the hemisphere. In the northern hemisphere, the summers are long and cool because the planet is orbiting slowly at the aphelion, or the farthest point in its orbit around the Sun.
In the southern Martian hemisphere, summers are short and hot because the planet is moving quickly around the perihelion, or its closest point around the Sun. The upshot is that, counterintuitively, the Martian winter solstice occurs during these hotter, wilder months. The intense differences in temperature of the planet during the winters can kick up massive dust storms as the planet flies closely by our star.
A dust storm on Mars. Image via NASA/JPL-Caltech/Univ. of Arizona.
Another bizarre characteristic of Martian seasons is the huge variation in atmospheric pressure. When the planet is nearing its perihelion around the winter solstice, the northern pole cap is facing away from the Sun. During this cold, short winter, large amounts of carbon dioxide gas are absorbed by the expanding northern pole cap.
But when summer rolls around and the southern pole cap is facing away from the Sun, it absorbs much less CO2 than its northern counterpart. Accordingly, the atmospheric pressure on Mars is a full 25 percent higher at its summer solstice than at the winter solstice. And we think the thickening humidity of Earth's summers are bad! We'd all be flattened if Earth summers were as cavalier with pressure increases as Martian summers are.
Gas giants technically have summers as well, but the only one that shows real seasonal variation is Uranus. With an insane axial tilt of 82 degrees, the planet has been completely bowled over to one side. The Sun never sets during Uranian summers, which last about 42 years. Accordingly, the Sun never rises during Uranian winters, plunging the Northern hemisphere into 42 years of darkness. Spring and fall on the planet are particularly stormy, as surfaces either warm up or cool down for the first time in decades.
Point being: summers on Earth certainly have their downsides, including stink, heatstroke, and criminally high utility bills. No doubt climate change will ratchet all that up a notch over the coming centuries. But there's nothing like looking at the messed up summers of other planets to make you realize that we really have a good thing going here on our own. As much as sunburn and sweat marks are a drag, at least we don't have to wait half a lifetime for a season change.
"Damn, Summer" is a semi-regular series exploring the science behind summer's various miseries and pleasures.