NASA’s Mars Reconnaissance Orbiter has been imaging the surface of Mars for over 10 years
now, with its powerful HiRise camera.
10 years of taking photos everyday means that its catalogue is huge, and we can pick out
pretty much any part of Mars that we want a closer look at.
Today, I thought it would be really interesting to investigate the sand dunes on Mars, and
see how they compare to the wonderful sand dunes we have on Earth.
You may have already seen imagery of the Sahara Desert with dunes hundreds of meters tall,
or the Namib desert with rolling sand dunes as far as the eye can see.
Did you know though that there are 5 different types of sand dunes, and that Mars has a type
of dune Earth doesn’t have?
And also, what are the biggest dunes on Mars?
The most common type of sand dune on Earth is the crescent, or barchan sand dune.
While not always as distinctive as these dunes in this image, they are plentiful and found
in most sand deserts.
They are defined by this kind of crescent shape with arms, and they are usually wider
than they are long like you can see here.
They form like this because in these places there is only one dominant wind direction.
They have a shallow slope on one side that faces the wind, and a slipface on the opposite
side.
They are a type of transverse dune, as they move gradually over time as the wind blows,
whipping sand from the front, over the top and down the slip face.
When many of them merge, they can create an almost wave of sand, smooth on the side facing
the wind, and with a slipface away from the wind, moving gradually over time with the
wind's direction.
They are also in plentiful supply on Mars!
All the ones I’m about to show you can be found at the base of various large craters.
Going to this old crater near Mars’ equator, Barchan dunes can be seen lumped together
somewhat here in this plain.
This happens when there is a plentiful amount of sand and the dunes merge together into
a big, lumpy transverse dune.
But individual dunes are really evident a bit higher up where the sand is contrasted
against the bedrock.
Here are some more, found in Arkhangelsky Crater.
When there is a smaller amount of sand, the dunes tend to stay separate, which looks really
peculiar from above.
These dunes do move, although very slowly.
Here’s a couple of examples of dunes on Mars moving slowly over the course of 2-3
years.
Scientists originally thought that sand dunes would stay pretty motionless on Mars as sand
dunes require wind to move them, and Mars only has 1% of the atmospheric pressure at
the surface compared to Earth, which can easily move sand.
We knew the wind on Mars could pick up tiny dust particles, but it would require extremely
strong winds to move sand.
This could be helped though by the fact that sand on Mars is not exactly the same as Earth’s
sand.
In my Opportunity video I mentioned that the sand particles Opportunity analysed were far
smaller than what we are used to on Earth, which may aid the movement of these dunes.
But what happens if the wind is bidirectional?
Going back to Earth, a beautiful type of dune which can be seen easily all the way from
the International Space Station is the longitudinal, or seif dune.
Seif is the Arabic word for sword, which describes these dunes nicely as they are narrow and
long.
These linear dunes can be hundreds of kilometres long, and are often found in parallel sets,
separated by gravel or rocky corridors.
These occur where wind directions can flip over, meaning that these ridges have a slip
face on either side of the dune, making them appear quite sharp.
Seif dunes also appear on Mars, and in my opinion, the most beautiful example of it
can be found here.
Again, found at the bottom of a crater, the dunes look remarkably similar to each other,
including this dusty band found on each one.
Although we are not totally sure how they form, one leading theory is that they are
the result of wind directions changing around barchan dunes.
In this image, some barchan dunes seem to be mid process of forming into seif dunes.
It’s a really pretty image that seemingly tells a story.
To our right here we have a mesa that’s being eroded by the wind, feeding
sand into the valley below.
Then you have these beautiful examples of barchan dunes, which show the step by step
process of becoming seif dunes.
Another spectacular example of this a bit further into the process can be found here.
The contrasts of the different materials in the image are stunning, and the dark sections
are the sand dunes merging together.
The third type of dune is the dome dune.
Typically, these are not very big on Earth, usually only a few feet high.
They have no slip face, they are smooth all over – hence the name.
They are quite rare, and are usually found in and around other types of dunes.
And yes, these are also found on Mars.
Here’s a field full of domes, a few of which can be seen starting to cross over into barchan
dunes.
And in this impressive expanse of dunes, and I do really mean an expanse as these dunes
continue on way beyond this image, you have wobbly lines of dunes with domes on the end
of them.
These domes seem to be much bigger here than they are on Earth, the domes here are about
tens of meters across.
This field is indicative of a changing wind direction, as the lines are nowhere near as
straight as in some of these previous images.
The fourth type of dune is a parabolic dune, which is the opposite of a barchan dune, meaning
the slip face is on the other side compared to a barchan dune.
These occur near coastal regions, and form the way they do because something, often vegetation,
anchors the arms of the dune in place.
Lack of vegetation on Mars means it is a bit tricky to find parabolic dunes.
The closest thing I found is this beautiful image, a dune wrapped around a large mound
in the middle.
The dune is clearly moving from left to right in this image, and you can even see the shadow
of the slipface on this side.
The existence of this dune is heavily influenced by the size and shape of this mound, funnelling
wind along its walls.
The fifth and last type of dune is the biggest type, the star dune.
This is a dune where wind direction changes frequently, meaning the dunes in these regions
have three or more arms and are often stationary, just building up over time.
This impressive image is the Sahara Desert as seen from the ISS, and it is full of star
dunes.
The scale of these dunes truly is impressive, zooming in you will realise that these dots
are people.
And of course, we have star dunes on Mars.
Some are small, a collection of dunes merged together.
But others are massive, like these found in Russel Crater, which reach far beyond this
image.
As this image is found nearer the poles, carbon dioxide ice can form on these dunes.
During the Martian spring, the carbon dioxide sublimates, which dislodges loose material,
leaving these streaks.
Moving to another image, again near Mars’s pole, we find one of the most beautiful images
of Mars I have ever seen.
This image was taken just as the sun was either rising or setting, meaning a lot of the image
is shadows cast by dunes.
I love this as it provides a sense of depth to the image.
These, again, are big star dunes, with their individual arms reaching out for kilometres.
There’s one last thing I want to talk about the dunes of Mars.
Have you ever been to the beach, and seen sand ripples like these?
These are actually a type of transverse dune, forming perpendicular to the direction of
the water waves as they flow over the sand, much like larger scale transverse dunes caused
by the wind.
Mars has these ripples too, but they are much, much bigger than what you would find on Earth,
a meter or two between ripples.
This size makes it difficult to see from space, but luckily we have another rover on hand,
Curiosity, seen here in this image, to take this remarkable photo of them.
Scientists didn’t know this type of dune existed, and it is a fitting reminder that
although Mars may appear quite familiar to us, it is still a world full of secrets and
unexplored facts, and there is still so much we can learn from it.
We’ve been comparing Mars to Earth today, and we can see that even though there are
similarities between the two, they are clearly not the same.
Some things happen on Mars that don’t on Earth, and vice versa.
So, planets are different, and you can see differences in solar systems and galaxies
just by looking at them!
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How could that be when we’ve just established that everything has differences?
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