Eye opener

Focus-Science and Technology - - Contents - PHOTO: NASA

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NORTH­ERN HEMI­SPHERE, MARS

We’re dream­ing of a white Christ­mas… on Mars. These sand dunes in the planet’s north­ern hemi­sphere have been dusted with an un­usual kind of snow, formed not from wa­ter but from car­bon diox­ide (CO ). Bet­ter known as dry ice, it ap­pears dur­ing the Mar­tian win­ter when tem­per­a­tures drop and at­mo­spheric CO freezes, form­ing ice on the planet’s sur­face or fall­ing from the clouds as snow.

As the Sun reap­pears in spring, the pris­tine cov­er­ing be­gins to crack, re­leas­ing gaseous car­bon diox­ide that car­ries dark sand up from the ground be­low. The re­sult is these beau­ti­ful pat­terns, cap­tured last May by the High Res­o­lu­tion Imag­ing Science Ex­per­i­ment (HiRISE) cam­era on NASA’s Mars Re­con­nais­sance Or­biter.

“Frozen CO is com­mon on Mars,” ex­plains Dr Candice Hansen, se­nior sci­en­tist at the Plan­e­tary Science In­sti­tute in Tuc­son, Ari­zona. “A sea­sonal po­lar cap made of dry ice forms each year at the north and south poles, and we even get patchy de­posits close to the equa­tor.”

We won’t be get­ting any here on Earth, though. CO re­quires tem­per­a­tures of -78.5°C to freeze, so keep your fin­gers crossed for nor­mal snow in­stead.

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