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How the seasons shift on Mercury, Venus and Mars


Chilly winds, orange leaves, sweltering heat and flowers in full bloom – they can indicate the changing of the seasons on Earth. But what about our neighbours in the solar system? Phil Ritchie takes us through the rocky planets.


Seasons on Earth and other planets arise from our tilted axis.
NASA / NOAA / GSFC / Suomi NPP / VIIRS / Norman Kuring

As summer approaches in the southern hemisphere and the weather cools in the north, be thankful that our planet’s tilt of 23.5 ° means the seasons keep ticking along – keeping us not too hot and not too cold (most of the time, at least).

Like Earth, each planet in the solar system technically has four seasons. But they vary wildly from planet to planet – from the near-constant baking heat of Venus to swings of more than 500 °C on Mercury.

Seasons are based on the orientation of a planet’s poles. A planet with no tilt and a perfectly circular orbit around the sun will cop sunlight along the equator at all times.

But on Earth, it’s summer above the equator and winter below when the north pole points at the sun.

Throw in characteristics such as atmosphere, orbit, distance from the sun and how many hours are in each day, and the intensity of any given season changes too.

Earth is also lucky enough to have a near-circular orbit, which keeps the overall climate steady and our seasons pretty much comparable between years. For a planet with an elliptical orbit, changes in distance from the sun can cause massive seasonal fluxes in temperature.

With these elements in mind, let’s take a look at the seasonal quirks of our planetary neighbours – starting closest to the sun and working our way out.

Even though it cops massive temperature swings, Mercury's vertical axis means it doesn't have seasons.
NASA / Johns Hopkins University Applied Physics Laboratory / Carnegie Institution of Washington

Mercury

As the closest planet to the sun, Mercury’s climate varies significantly throughout its year, which equates to 88 Earth days.

Mercury is also the only planet in our solar system without any tilt, so technically lacks seasons. But what it does have is a highly elliptical orbit, creating a version of summer and winter.

When Mercury is pulled closest to the sun, it can 427 °C during the day. But when it’s tugged further out – reaching almost twice as far as its closest point – the surface can plunges to -173 °C.

Complementing this is Mercury’s incredibly thin atmosphere, which allows any heat from the day side to ooze into space before it can be shared with the night side.

Night on Venus, snapped by the Japanese robotic Akatsuki spacecraft. The vertical orange stripe between night and day is so wide because light is diffused by Venus' thick atmosphere.
ISAS / JAXA

Venus

Venus has a similar size and density to Earth. But it's blanketed in an atmosphere of carbon dioxide so thick that nights are as hot as days. The temperature across the surface stays around 470 °C.

Venus is tilted too, but at a minuscule 2.6 °. The angle is too small to evoke any obvious seasonal change, but then again the atmosphere would spread the heat evenly if it did.

Bizarrely, the planet spins in the opposite direction to all other planets in the solar system (aside from Uranus – we'll come to that later). Planetary scientists think that it did once spin the same way as Earth, but something – perhaps a collision – flipped it 180 ° around its axis. So technically, its axial tilt is 177.4 °.

A composite image of the last days of the northern autumn/southern spring on Mars, captured by the Mars Global Surveyor Mars Orbiter Camera.
NASA / JPL / Malin Space Science Systems

Mars

Mars is prone to the most diverse seasons in the solar system, thanks to it 25.2 ° tilt.

Over the course of the 687-day Martian year, the planet’s elliptic orbit exposes it to the sun’s energy at varying proximities and intensities. This means seasons last different lengths of time in each hemisphere.

In the northern hemisphere, spring is the longest season, lasting seven months. Summer and autumn both last for around six months. Winter is just four months long.

On the surface, temperatures can range from -125 °C in winter to 20 °C in summer, falling as much as 100 °C overnight as its weak atmosphere releases daytime heat.

The polar ice cap can completely disappear in Martian summers too, only to grow back as ice when winter comes around.


Tomorrow, we'll take a walk through the gas giants.

Contrib philipritchie.jpg?ixlib=rails 2.1
Phil Ritchie is a Melbourne-based journalist.
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