Ice crystals on Mars
Clarence Bennett and I are the discoverers of the shape of dry ice snowflakes grown under Martian conditions.
Read my story about the discovery of Martian Snowflakes.
There is ice on Mars. The planet has polar ice caps which can be seen in telescopes from earth. The ice in the polar caps of mars is both water ice and also carbon dioxide ice.
There is probably underground ice as well. Images of mars show pedestal craters which are craters surrounded by what look like mud flows. These craters are thought to be created when a meteorite hits a region containing substantial subsurface ice. there are also polygonal patterns on the surface of mars that look like polygons found on earth in regions with permafrost.
Left, Rock polygons on Mars. Right, Polygonal fractures in antarctica. Click either picture to enlarge. Left image Malin Space Science Systems, JPL, NASA.
The Mars Express spacecraft which went into orbit of Mars in 2003 contains ground penetrating radar that will look beneath the surface of Mars to find underground ice.
Sometimes Martian ices appear on the surface of Mars away from the polar caps.
The Viking2 lander awoke one morning to find that it was surrounded by a white landscape. The ice surrounding it had formed when the overnight temperature dropped to 140 K, (-200 °F) the temperature of carbon dioxide ice crystallization, but it didn't melt away at higher temperatures. Scientists assumed that the white frost contained water ice that formed on carbon dioxide ice seed crystals..
Heavy white frost on the ground around the Viking lander. Click to enlarge.
There are clouds on Mars. One region of clouds on Mars that is visible from earth astronomers was named "Nix Olympica", the snows of Olympus. When spacecraft revealed that the clouds formed over a giant volcano it was named Olympus Mons.
We can also see Martian clouds forming waves over the raised rims of craters.
Left. Wave clouds form over a crater on Mars, the crater rim is white with ice deposits. Right Wave clouds also form on earth. Click either image to enlarge.
We also saw clouds at dawn from the pathfinder lander. These are probably high thin ice crystal clouds.
Bluish ice crystal clouds at dawn viewed from the pathfinder lander.
On earth clouds are made of water drops and water ice crystals. On Mars clouds are made of water ice and carbon dioxide ice (dry ice.)
Clouds can also be seen over the dusty atmosphere at the limb of Mars.
Clouds over the dusty atmosphere at the limb of Mars.
Thinking about the ice crystal clouds of Mars I wondered what the size of the halos around the sun would be? To find out I needed to know the shape of martian snowflakes made by water ice and carbon dioxide ice. I found that many people had done research on the shape of water ice crystals, but I couldn't find information on carbon dioxide ice crystal shapes formed under Martian conditions. So I decided to make a Mars chamber and make my own Martian dry ice snowstorm within it. Then I would find out for myself what the shape of a Martian carbon dioxide ice crystal might be. I enlisted the aid of Clarence Bennett. (Clarence is a member of PIRA, physics instructional resource agents.)
Snowflakes on Earth
Below is a photograph of an ice crystal growing on a spiderweb on earth.
This is an ice crystal growing on a spider thread. It is six armed, and is how we envision snowflakes on earth.
"Snowflake" Bentley photographed thousands of snowflakes in Vermont. His photos show that snowflakes can take on many different shapes. Here is one page from his book which is still in-print and available.
Photographs of snowflakes by Bentley. Click to enlarge.
(Note Bentley never copyrighted his photographs so the are avaiable for non-commercial use. Many of his snowflake images will be found on the NOAA photograph website.)
Notice in particular the flat plate snowflake without arms.
Flat plate and six-sided prism snowflakes photographed by Bentley.
He also photographed pencil shaped snowflakes.
(Unsharpened pencils without erasers are 6 sided prisms just like some snowflakes.)
Images of hexagonal prism snowflakes (with internal air bubbles.) Photo from a book by Walter Tape.
I have a book by Walter Tape in which he shows images of snowflakes he collected in Antarctica at the same time he was observing ice crystal halos around the sun.
In an ice cave within an Antarctic glacier I photographed this large hexagonal ice crystal. It is an interesting size and shape it is a hollow hexagonal prism with flat wings sprouting from each vertex of the prism.
A single crystal of water ice inside an Antarctic glacier. Paul Doherty image.
Scientists have discovered that the shape of snowflakes depends on the temperature and supersaturation of the air mass in which they form.
A graph showing how the shape of snowflakes depends on the temperature and supersaturation at which they form.
Extrapolating to cold, low water vapor densities found on Mars we expect to find water ice snowflakes shaped like hexagonal plates and prisms. These shapes will produce halos around the sun with a radius of 22.5 degrees. See the image below.
Martian Carbon Dioxide Ice Crystal Shapes
Clarence Bennett and I built a chamber at Oakland University in Rochester Michigan where we could grow carbon dioxide ice crystals under Martian conditions. We attached a vacuum pump to reduce the pressure of the pure CO2 gas in the chamber to less than 6 millibars. We also added plumbing to carry liquid nitrogen into the chamber to cool it to mars temperatures. Under Mars atmospheric pressure, carbon dioxide ice forms at 140 K, or -130 °C or -200°F.
Two views of the Mars chamber in operation. Click to enlarge.
Making a Mars chamber was easy in comparison with making it snow inside the chamber. After months of work we finally produced a carbon dioxide ice snowstorm in the Mars chamber and managed to photograph the resulting snow crystals.
Thin tendrils of snow fall near the right side of the central shadow band in the mars chamber. Click to enlarge.
Close up photographs of the crystals showed that they had square faces and triangular faces.
Carbon dioxide ice crystals formed under Martian conditions, they are cuboctahedrons and twinned crystals. Click to enlarge.
We knew that carbon dioxide ice can crystallize with cubic symmetry, this meant that it could form cubes, octahedrons, or cuboctahedrons, i.e. cubes with the corners cut off. We had discovered that under Martian conditions carbon dioxide ice formed cuboctahedrons.
Cuboctahedron illustration by Ron Hipschman. Click to Enlarge.
Demo. Show a model cuboctahedron made from plastic.
I bought a platic cube 2" (5 cm) on a side and cut off the corners making a plastic model of a Martian Snowflake.
I also found an iron pyrite crystal that had formed as a cuboctahedron.
Iron pyrite also makes crystals with cubic symmetry. While Iron Pyrite, or fools gold, often makes cubes, sometimes it crystallizes as cuboctahedrons. Show an iron pyrite cuboctahedron.
Demo. Make dry ice snow with a fire extinguisher or a tank of CO2 gas.
Demo. Show dry ice.
Ice crystal Halos
Ice crystals make halos on earth. On earth the main halo is at 22 degrees radius around the sun or moon, on mars it will be at the same angle for water ice crystals.
I calculated the angle of the ice crystal halos on Mars made by carbon dioxide ice cuboctahedrons and found that they would form at 28 degrees from the sun. If you stretch out your arm to its full length in front of you then spread out your fingers and thumb the angle between the tip of your thumb and the tip of your little finger will be about 20 degrees, just about the angle between the sun and the halo on earth. The mars halo will be about 25% larger in radius than this handspan.
A halo around the sun on earth. Click to enlarge.
Steve Wall of JPL searched the Viking lander images for traces of Martian ice crystal halos to see if my prediction was correct, he found no halos. Later, Pathfinder looked for ice crystal halos without finding any. So now I wait for the results from the Spirit Rover. Will it see ice crystal halos? Will they be at 22.5 degrees or at 28 degrees? or at some other angle? I'm waiting.
Freezing by Boiling, water on Mars will boil at 0°C until it freezes. This exhibit uses a vacuum chamber to reproduce Mars surface conditions.
I began my lectures with the most recent images from Mars.
Here is the full panorama of the landing site photographed by the Spirit Rover. Click to enlarge.
The temperatures expected at the rover now, in blue, and in 100 days.
hese are the daily temperatures experienced by the rover now in summer shown in blue. After 100 days shown in yellow.
Return to Mars
Scientific Explorations with Paul Doherty
25 Dec 2003