Mars is Earth like in some ways, but in other ways it's very different with its global dust storms every two years, its thick sheets of dry ice at its ice caps in winter. Any fresh water is close to its very low boiling point in the near vacuum. And the eccentric orbit also has a big impact on its seasons. So how do its seasons work exactly and what effect does this have on its climate?

I was asked this on Quora, and I can't find a page anywhere that explains the Mars seasons really clearly in one place. Lots of information about bits and pieces of it, but hard to find the whole picture. So here we go:

First, Mars has a year that lasts for two Earth years. So each season is roughly twice as long as the corresponding Earth season.

It's axial tilt, by coincidence, is about the same as Earth's. This is very much a coincidence. Our Earth keeps the same tilt for billions of years, stabilized by the Moon. But Mars's tilt keeps changing. Sometimes it tilts so far that it has equatorial ice sheets.

Mars is tilted like the picture at top left at present. If it was like the one at top right, with a big ice sheet at the equator, and warm ice free poles, it would be very unlike Earth. The bottom one shows it tilted almost vertically with the ice sheets much more extensive than they are now. See Changes in Tilt of Mars' Axis

Anyway right now its:

Mars axial tilt: 23.5°

Earth's axial tilt: 23.4°

It's one of those strange coincidences, like the Moon seeming the same size in our sky as the Sun. Mars has almost the same axial tilt, and it also almost the same day length as Earth. Even though its axial tilt keeps varying - and the Earth and Mars days also keep changing too. It's just a coincidence.

Anyway though it's day and axial tilt is similar to Earth, its orbit is very different. The eccentricity of Mars' orbit keeps changing a lot, unlike Earth's, which is always more or less circular. Mars' orbit is sometimes more circular than Earth's and sometimes very eccentric.

As it turns out, Mars has a rather eccentric orbit at present.

The result is that it is close to the sun for its Northern winter, and a long way from the sun for its southern winter (northern summer).

So the northern winter is much warmer its southern winter. It's also shorter because by Kepler's second law of equal areas swept out in equal times - when closer to the sun, it orbits the sun more quickly.

For the same reasons, the southern winters are much longer and colder.

Also, it just so happens that Mars is lumpier at its southern pole, while its northern pole is much lower elevation - it's the dried up bed of a huge ocean. Actually we don't know why it is that Mars has this big difference between the two hemispheres.

This is the puzzle of the Martian dichotomy. In this map, blue and green is low, orange and red is high. The region up to 60 degrees away from each pole is low around the North pole and high around the south pole.

In between there's a more complex region, with that big deep blue crater there Hellas basin, site of an ancient big impact, and the long gash to the left is Valles Marineres, which is where early Mars started to tear apart, the very beginning of continental drift but it never went any further and it never divided up into continental plates as happened on Earth..

It's not yet known why there's this difference in elevation. It could be one truly huge impact in the early solar system when the planets were still forming - around when our Earth's Moon formed, or many impacts, or some tectonic process - a bit like the plate tectonics on Earth. Martian dichotomy

Anyway so the result of all this is that the northern ice cap is both lower, and warmer, and the winter shorter And it doesn't vary so much in temperature, nevertheless, it shrinks and loses most of its ice in the northern summer. And in winter it is cold enough to have dry ice there.

The southern ice cap is much colder in winter, with thick layers of dry ice. You'd think it would be warmer in summer but because it is so much higher in elevation, then it's actually pretty cold in summer too, especially the polar regions. The ice doesn't melt, but its dry ice evaporates.

Actually the dry ice in the southern hemisphere produces the probably spectacular Martian Geysers when it gets hot enough. This is an artist impression, and no rover has ever been able to see this phenomenon. There are ideas to make a "geyser hopper" to study them in the future.

What happens is that the dry ice is semi-transparent. It lets the sunlight through and this then heats up a layer below the surface rather than the surface itself, through what is called the "solid state greenhouse effect".

This happens every Southern spring. As it warms up, the dry ice underneath is not under nearly enough pressure to form a liquid, but turns directly into gas. This leads to explosions of dry ice, so to the Martian geysers.

That's not the end of the story. After the geyser is done, you end up with these dark patches, which are particularly noticeable in a crater called Richardson crater. And they aren't static, they move, they extend fingers down the slopes, the so called "flow like features".

Flow-like features in Dunes on Richardson Crater, Mars. They form around the dark dune spots, in the debris of the hypothesized Martian Geysers. The dark material at the end of the flows moves at between 0.1 and 1.4 m/day in late spring / summer on Mars.

And as you see, these dark streaks spread out from the dots.

They grow at a rate of up to 1.4 meters per Martian sol, so pretty fast for Mars.

Flow-like-features detail - on Dunes in Richardson Crater, Mars. - detail. This flow moves approximately 39 meters in 26 days between the last two frames in the sequence

All the models for these features, to date, involve some form of water.

Indeed the leading idea is that they form in a similar way to the Martian geysers - through the solid state greenhouse effect. This is later in the year. Though the surface is still very cold, far too cold for ice to melt - yet if the ice is clear enough, then the ice will melt below the surface, because the ice layers above trap the infrared.

Ice of course turns into a liquid, not a gas, so it will just stay there, not explode out of the ice like the geysers. And ice is very insulating. So the ice will remain liquid overnight. So over a period of time a layer of liquid centimeters thick can form. Because it's trapped by the ice, it can be stable on Mars even though there is a near vacuum above the ice.

Of all the many suggestions for liquid water on Mars, this is the only one I know of that could potentially consist of ordinary reasonably fresh water - and what's more at zero degrees c or even warmer. It was like Wow, when I found out about this in Nilton Renno's survey paper of habitats that could exist on Mars. I don't know why it doesn't get much more publicity.

It's just as surprising as the warm seasonal flows, which we'll come to in a minute, indeed even more so, these happen in the bitterly cold southern sub polar region, the region around the area of permanent ice, about the most unlikely place you can think of for liquid water on Mars, if you don't take a look at the models and ideas behind it first.

There's another idea for how these fingers could form involving thin interfacial liquid water on ice / rock boundaries, which would merge and pool together to provide enough liquid to flow.

It all depends on whether Mars has clear ice. It's common on Earth. Here it is usually blue in colour like this.

If ice as clear as this forms in the Martian south pole region in Richardson's crater, or anywhere else for that matter at similar temperatures, then the models predict that we could have liquid water form there. Liquid water forms in similar conditions in Antarctica, about half a meter below the surface, where the ice is reasonably clear.

That then would explain the streaks. The water picks up salt and dirt and then flows out onto the surface from under the ice. Because it is so much more salty by then, it can stay liquid for long enough to start flowing down the slope.

So - water is very very rare on Mars. The conditions are such that ice turns directly into water vapour. If it does form liquid then it is close to boiling point and will dry quickly like clothes drying on your line on a sunny day.

Still, there are places where water could form.

Other places include on salt / ice interfaces, where the water can form, little mm or so scale droplets, but as Nilton Renno says, just a droplet of water is a "swimming pool for bacteria".

Or salt can take up water from the atmosphere as Phoenix first suggested. Then Curiosity discovered indirectly, that this can happen even in equatorial regions.

This subsurface liquid layer is thought to be sometimes warm enough for life in the dunes where Curiosity is exploring, but then too salty, and sometimes not so salty but then too cold. So it might not be habitable but Nilton Renno has said he thinks there is a possibility of habitable conditions even where Curiosity is, if in some way life exploits it, with biofilms or other ways of creating its own microclimate.

By the way there are flow like features in the Northern hemisphere too, but they form at much colder temperatures (rather paradoxically) and the explanations of these may or may not involve liquid water.

Seasonal processes in the Northern polar dunes with Flow Like Features. Time differences between the images are 22 days and 12 days. The final picture shows a long feature that formed new between the two images, and its length is 60 meters so it grew at a rate of at least 5 meters per day.

Much better known are the warm seasonal flows

Warm Season Flows on Slope in Newton Crater (animated)

These form in much warmer conditions, sun facing slopes, and they occur right down into the valles Marineres in the equatorial regions.

They were recently shown to include hydrated salts. So almost certainly formed by water in some sense. They are dark lines - which grow in spring, spread out in summer and fade in winter. However, they are not damp patches. Water would not be stable enough.

It's frustrating, a bit, the only camera we have able to photograph then well, comes close to the sunny side of Mars twice a day, on opposite sides of Mars - but its orbit is such that it always takes these photographs at around mid afternoon. That's the very worst time for looking for water. We'd like to photograph them close up in the early morning. Sadly, we can't do that, not unless we send another satellite to Mars able to to take similar photographs in the early morning. It's possible that liquid water could be detected there directly, on occasion, if only we could photograph them early in the morning.

Anyway apart from that, you get the frosts, many mornings in the Equatorial regions.

Ice on Mars Utopia Planitia. These frosts formed every morning for about 100 days a year at the Viking location.

Scientists believe dust particles in the atmosphere pick up bits of solid water. That combination is not heavy enough to settle to the ground. But carbon dioxide, which makes up 95 percent of the Martian atmosphere, freezes and adheres to the particles and they become heavy enough to sink. Even in equatorial regions it gets cold enough at night for this to happen for 100 days a year

Warmed by the Sun, the surface evaporates the carbon dioxide and returns it to the atmosphere, leaving behind the water and dust as this icy frost, which then soon evaporates.

Water is not stable in the equatorial region, even as ice, on the surface. And the atmosphere has very little water vapour. Nevertheless, at night the air cools down hugely - often it gets below the temperatures of dry ice at night even in the equatorial regions. And that's when the ice frosts form, as mixtures of ice and dry ice.

Then as the atmosphere warms up in the morning - it has hundred percent humidity because it is so cold,. even though there is hardly any water vapour there. So the frosts can remain until well into daylight.

Gilbert Levin has wondered if it is possible that life could somehow exploit this ice. If not, well it could exploit the 100% humidity directly. A team of biologists in DLR (German aerospace) have tested  various lichens and cyanobacteria, and some, from places like Antarctica, very dry and cold places, are able to survive in Mars conditions and even metabolize and photosynthesize using only the night time humidity.

Now apart from that, there is one more thing that makes the Mars climate unusual - the often global dust storms.

They happen once every two years. They can form in hours, cover the planet in days, and then last for weeks before they dissipate.

You get strong winds, hundreds of miles an hour, and the rather charming dust devils.

Though these are not nearly as harmful as you would think. The dust is so fine it's about as fine as cigarette ash. The winds, though they are so fast, are winds in a near vacuum, and at their most powerful, they could just about manage to move an autumn leaf on Earth.

But Mars is covered in this very fine dust, fine as cigarette smoke. So great clouds of it get lifted up in these global dust storms.

The dust storms always happen in the Southern summer - remember that's the season on Mars when it is closest to the sun, and its much warmer than the northern summer.

They often start in Hellas basin.

And within a short time they may cover the entire planet like this.

Space Today Online - What We Know About Mars - Dust Storms

They block out 99% of the Mars sunlight when they are at their thickest.

Here is a photo showing progression of a dust storm as seen by Opportunity.

In the middle of this dust storm, less than 1% of the light that reaches the top of the Mars atmosphere made its way to the ground where Opportunity photographed it.

This gif animation by Emily Lakdawalla shows how the sun faded during the dust storm as viewed from Opportunity. There's a big gap at the end where the sun was too dark to do these images. See her post from 2007: Dust storm update: rovers still OK

Many dark streaks form seasonally on Mars. Most of these are thought to be due to dry ice and wind effects. This image shows an example, probably the result of avalanche slides and not thought to have anything to do with water:

Slope Streaks in Acheron Fossae on Mars - these streaks are thought to be possibly due to avalanches of dark sand flowing down the slope

They look a bit like the warm seasonal flows, but are easily distinguished by experts.

Mars also has sand dunes that move, much like they do on Earth. Indeed they move at about the same speed they do on Earth, which was rather surprising with the thinner air and lower gravity.

Advancing Dune in Nili Patera, Mars. Images taken nearly three years apart by the HiRISE camera on Mars Reconnaissance Orbiter.

This discovery shows that entire dunes as thick as 200 feet (61 meters) are moving as coherent units across the Martian landscape. The sand dunes move with about the same flux (volume per time) as dunes in Antarctica. This was unexpected because of the thin air and the winds which are weaker than Earth winds. It may be due to "saltation" - ballistic movement of sand grains which travel further in the weaker Mars gravity.

Advancing Dune in Nili Patera, Mars

So - there are some similarities with Earth, but many things that are unique to Mars.

The biggest difference is that flowing water or pools are completely impossible on Mars at present, except briefly as flash floods or rapidly freezing over lakes after an asteroid or comet impact or a volcanic eruptions.

Though at times, as its axis tilts, it may be possible for liquid water to flow and be a permanent feature of its landscape.

The dry gullies on Mars were first thought by many scientists to be formed by activity of water. Nowadays, it is thought that recent gullies are formed by dry ice processes.

 Above, gullies on dunes in Russel Crater (54.3°S, 12.9°E) are partially covered by CO2 ice. Below, sinous gullies in a crater in Newton Basin (41°S, 202°E). Image credits: NASA/JPL/University of Arizona - see Gullies on Mars sculpted by dry ice rather than liquid water?

This hypothesis for recent gullies was confirmed, reasonably conclusively, when new sections of gullies were seen to form at temperatures far too low for water activity. So the only likely explanation is dry ice.

There's another phenomenon caused by dry ice too, on Mars, another thing we don't have at all on Earth

Dry Ice "Snowboards" on Mars - and Grooves on Mars may be result of blocks of dry ice sliding down slopes

These grooves are thought to be caused by "boulders" of dry ice rolling down the slopes. Because they are made of dry ice, they evaporate into the atmosphere when they reach the bottom, not even leaving a damp patch to freeze over.

Making Dry Ice Sleds on Mars | NASA JPL Space Science HD Video

But many of the older dry ice gullies, they now think, result from the action of water.

This hypothesis strong support in January 2015. They may well have been formed by floods of melt water associated with melting of glaciers that form when the Mars axis tilts beyond 30 degrees. This could have happened within the last two million years (between 400,000 and two million years ago).

Sharp-featured recent gullies (blue arrows) and older degraded gullies (gold) in the same location on Mars. The older ones are thougth to have occured at times in the past when the region was warmer due to changes in the Mars axial tilt - so CO2 would have been sparse, and they are also associated with ice rich deposits.  

This suggests that many of the older dry gullies are water formed and associated with cyclical climate change within the last two million years. For more details: Gully patterns document Martian climate cycles

Then further back in the early solar system, Mars probably had seas. They came and disappeared several times perhaps - this is an artist impression of the most recent one. It shows the low lying northern hemisphere from above the north pole.

Artist's impression of an ancient northern ocean on Mars. Credit: ESA

At those times its atmosphere must have been thicker and it must have been warmer. Depending on its orbit, it's possible that the ocean was frozen over much of the time and only melted every two years in the northern summer.

There's a lot of debate about how much liquid water there was and to what extent it was covered in ice. If it had liquid water year round, it must have had potent greenhouse gases in the atmosphere, perhaps methane, because CO2 isn't nearly warm enough by itself to do this.

Back then it might have been quite habitable for algae, lichens, perhaps sea weed even if it evolved higher lifeforms. Then after that there was a later period of flash floods and many lakes in crater floors, for instance Gale Crater which Curiosity is now exploring contains abundant evidence that it had flowing water and was also filled with water over much of its early history. Curiosity also found out that the water was not too acid or alkaline or too salty for life but was just perfect for Earth-type life.

Mars as it is now seems an unlikely place for life to evolve. But early Mars seems a place where life could have got off to a start rather easily.

Or else perhaps Mars was seeded by meteorites from Earth or even Mars seeded Earth with life. Mars was habitable first in the early solar system as it was first to form, and also our Earth got a sterilizing impact when the Moon formed at a time that Mars was probably very habitable.

If so, and if the life evolved to the stage where it became reasonably hardy, and especially if it got to the stage where it developed photosynthesis - it might still be there, and there are now many habitats known where it just possibly it might still survive in some form. Though most likely it would be microbes, at most lichens, because in similar places on Earth, despite the biodiversity of Earth life, there is not much life. Indeed quite often you get ecosystems consisting of a single species on Earth.

Of the Earth lifeforms, one of the most likely to live on Mars is Chroococcidiopsis - MicrobeWiki

This is one of the most hardy of all lifeforms on Earth. It might have been one of the main contributors of oxygen to our atmosphere when oxygen first formed. Over those billions of years it's developed an incredible number of metabolic pathways.

It can survive in almost any habitat. Give it salt water and it's just fine, yummy, "I know how to deal with that". Give it fresh water and it is just as at home. Hot springs, or nitrate caves - no problem. Driest deserts, coldest places like Antarctica. Even put it in a near vacuum Mars atmosphere and zap it with ultraviolet light, and it turns out it can handle that too, it's one of the microbes able to photosynthesize and metabolize in semishade, unprotected on the Mars surface using the 100% night time humidity. And it is one of the most radioresistant microbes, able to repair any DNA damage in real time and withstand high levels of cosmic radiation and solar storms. Rather puzzlingly since it never encounters those extreme conditions on Earth, but it may be a byproduct of its dessication resistance.

Yet it's also right at home in somewhere nice and warm - you find it on beaches, and in household water supplies and so forth in tropical places like Sri Lanka.

It's one tough cookie :).

Of all our Earth microbes, well there are several candidates that could be shared with Mars. But this must be one of the top ones. It seems likely it could survive a meteorite transfer to Mars - and then once there, it's one of the most likely to find a habitat easily, and because it is so widespread on Earth, it's got a decent chance of finding its way into debris from a giant impact on Earth.

Yet, it plays very nicely. It's a primary producer and doesn't need anything else except CO2, light, water and trace elements - it often forms single species ecosystems in deserts. Yet it's not going to eat Mars microbes or make them extinct, except possibly through competition. It might even be food for them, if they are compatible with it biologically. Of course it must have evolved a lot on Mars since it got there, if it is there.

None of this is proved. Nobody has yet discovered any microbe common between Earth and Mars. It could be that they have no lifeforms in common. 'But if they do, I wouldn't be surprised if this was one of them.

Or, perhaps Mars has some early form of life that hasn't yet evolved as far as Earth life, maybe RNA only life. If so - well I wonder also if perhaps Chroococcidiopsis would play nicely enough so that they could co-exist. Perhaps Mars could have both the toughest of microbes imaginable, along with some very fragile early life form, maybe even not based on DNA if it evolved separately on Mars?

So as with the seasons, life there also could be a mixture of the familiar -with a species that is related to Earth life - and unfamiliar with some form of Mars life, not related at all to Earth life.

That's just science fiction type speculation, but it maybe helps to give an idea of some of the range of possibility for what we could find there.

It's going to be a long time before we know for sure. For that reason I think we absolutely must have as top priority to preserve the Mars life and not make the mistake we have made so often on Earth of introducing foreign species without knowing what we are doing.

Nearly all the habitats I've described here are new ideas suggested or discovered only in the last decade or so. It's a rapidly evolving field and much of what I've just said may be way out of date again by the 2020s.

SEE ALSO

Here are some of my Science20 articles on related topics:

And many other answers here and articles on Science20

This originated as my answer to How do seasons on Mars compare and contrast with seasons on Earth?

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