Northern Hemi­sphere, Mars

BBC Earth (Asia) - - Contents -

Stun­ning snaps from across the fields of his­tory, na­ture or science

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 (CO2). Bet­ter known as dry ice, it ap­pears dur­ing the Mar­tian win­ter when tem­per­a­tures drop and at­mo­spheric CO2 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 CO2 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. CO2 re­quires tem­per­a­tures of -78.5°C to freeze, so keep your fingers crossed for nor­mal snow in­stead.


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