"Long before it's in the papers"
January 27, 2015


Findings called clear evidence of “dry ice” snowfall on Mars

Sept. 14, 2012
Courtesy of JPL
and World Science staff

“Dry ice,” or fro­zen car­bon dio­xide, drops to the ground in flakes as a sort of snow­fall at the south­ern pole of Mars, scien­tists re­port.

NASA’s Mars Re­con­nais­sance Or­biter da­ta have giv­en what sci­en­tists call the clear­est ev­i­dence yet of such events. This re­veals the only known ex­am­ple of car­bon-di­ox­ide snow fall­ing an­y­where in our so­lar sys­tem, re­search­ers say.

Ob­ser­va­tions by NA­SA's Mars Re­con­nais­sance Or­biter have de­tected what sci­en­tists say are carbon-dioxide snow clouds on Mars and ev­i­dence of carbon-dioxide snow fall­ing to the sur­face. De­posits of small par­t­i­cles of carbon-dioxide ice would be formed by snow­fall from carbon-dioxide clouds. This map shows the dis­tri­bu­tion of small-grain carbon-dioxide ice de­posits formed by snow­fall over the south po­lar cap of Mars. It is based on in­fra­red mea­sure­ments by the Mars Cli­mate Sound­er in­stru­ment on the Mars Re­con­nais­sance Or­biter. (Im­age cred­it: NA­SA/JPL-Caltech)

Fro­zen car­bon di­ox­ide, bet­ter known as dry ice, re­quires tem­per­a­tures of about mi­nus 193 de­grees Fahr­en­heit (mi­nus 125 Cel­sius), which is much colder than needed for freez­ing wa­ter. Carbon-di­ox­ide snow re­minds sci­en­tists that al­though some parts of Mars may look quite Earth-like, the Red Plan­et is very dif­fer­ent. The re­port is be­ing pub­lished in the Jour­nal of Geo­phys­i­cal Re­search.

“These are the first de­fin­i­tive de­tec­tions of car­bon-di­ox­ide snow clouds,” said the re­port’s lead au­thor, Paul Hayne of NASA’s Je­t Pro­pul­sion Lab­o­r­a­to­ry in Pas­a­de­na, Ca­lif. “We firmly es­tab­lish the clouds are com­posed of car­bon di­ox­ide—flakes of Mar­tian air—and they are thick enough to re­sult in snow­fall ac­cu­mula­t­ion at the sur­face.”

The snow­falls oc­curred from clouds around the Red Plan­et’s south pole in win­ter, the in­vest­i­gators said. The pres­ence of car­bon-di­ox­ide ice in Mars’ sea­son­al and re­sid­u­al south­ern po­lar caps has been known for dec­ades. Al­so, NASA’s Phoe­nix Lan­der mis­sion in 2008 ob­served fall­ing wa­ter-ice snow on north­ern Mars.

Hayne and six co-au­thors an­a­lyzed da­ta gained by look­ing at clouds straight over­head and side­ways with the Mars Cli­mate Sound­er, one of six in­stru­ments on the Mars Re­con­nais­sance Or­biter. This in­stru­ment records bright­ness in nine wave­bands, or color ranges, of vis­i­ble and in­fra­red light as a way to ex­am­ine par­t­i­cles and gas­es in the Mar­tian at­mos­phere. The anal­y­sis was con­ducted while Hayne was a post-doctoral fel­low at the Ca­lifornia In­sti­tute of Tech­nol­o­gy in Pas­a­de­na.

The da­ta pro­vide in­forma­t­ion about tem­per­a­tures, par­t­i­cle sizes and their con­centra­t­ions, he said. The anal­y­sis is based on da­ta from ob­serva­t­ions in the south po­lar re­gion dur­ing south­ern Mars win­ter in 2006-2007, iden­ti­fy­ing a tall car­bon-di­ox­ide cloud about 300 miles (500 kilo­me­ters) wide per­sist­ing over the pole and smaller, shorter-lived, lower-altitude car­bon di­ox­ide ice clouds at lat­i­tudes from 70 to 80 de­grees south.

“One line of ev­i­dence for snow is that the car­bon-di­ox­ide ice par­t­i­cles in the clouds are large enough to fall to the ground dur­ing the life­span of the clouds,” co-au­thor Da­vid Kass of the Je­t Pro­pul­sion Lab­o­r­a­to­ry said. 

“An­other comes from ob­serva­t­ions when the in­stru­ment is point­ed to­ward the ho­ri­zon, in­stead of down at the sur­face. The in­fra­red spec­tra sig­na­ture of the clouds,” that is, the de­tailed break­down of the char­ac­ter­is­tics of its in­fra­red light, “viewed from this an­gle is clearly car­bon-di­ox­ide ice par­t­i­cles and they ex­tend to the sur­face. By ob­serving this way, the Mars Cli­mate Sound­er is able to dis­tin­guish the par­t­i­cles in the at­mos­phere from the dry ice on the sur­face.”

Mars’ south po­lar re­sid­u­al ice cap is believed to be the only place on the Red Plan­et where fro­zen car­bon di­ox­ide per­sists on the sur­face year-round. Just how the car­bon di­ox­ide from Mars’ at­mos­phere gets de­posited has been in ques­tion. It is un­clear wheth­er it oc­curs as snow or by freez­ing out at ground lev­el as frost. The new re­sults suggest snow­fall is es­pe­cially vig­or­ous on top of the year-round snow area, or “re­sid­u­al cap.”

“The find­ing of snow­fall could mean that the type of de­po­si­tion—snow or frost—is some­how linked to the year-to-year pre­serva­t­ion of the re­sid­u­al cap,” Hayne said.

* * *

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NASA’s Mars Reconnaissance Orbiter data have given scientists what they call the clearest evidence yet of carbon-dioxide snowfalls on Mars. This reveals the only known example of carbon-dioxide snow falling anywhere in our solar system, researchers say. Frozen carbon dioxide, better known as “dry ice,” requires temperatures of about minus 193 degrees Fahrenheit (minus 125 Celsius), which is much colder than needed for freezing water. Carbon-dioxide snow reminds scientists that although some parts of Mars may look quite Earth-like, the Red Planet is very different. The report is being published in the Journal of Geophysical Research. “These are the first definitive detections of carbon-dioxide snow clouds,” said the report’s lead author, Paul Hayne of NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “We firmly establish the clouds are composed of carbon dioxide—flakes of Martian air—and they are thick enough to result in snowfall accumulation at the surface.” The snowfalls occurred from clouds around the Red Planet’s south pole in winter. The presence of carbon-dioxide ice in Mars’ seasonal and residual southern polar caps has been known for decades. Also, NASA’s Phoenix Lander mission in 2008 observed falling water-ice snow on northern Mars. Hayne and six co-authors analyzed data gained by looking at clouds straight overhead and sideways with the Mars Climate Sounder, one of six instruments on the Mars Reconnaissance Orbiter. This instrument records brightness in nine wavebands of visible and infrared light as a way to examine particles and gases in the Martian atmosphere. The analysis was conducted while Hayne was a post-doctoral fellow at the California Institute of Technology in Pasadena. The data provide information about temperatures, particle sizes and their concentrations, he said. The analysis is based on data from observations in the south polar region during southern Mars winter in 2006-2007, identifying a tall carbon-dioxide cloud about 300 miles (500 kilometers) wide persisting over the pole and smaller, shorter-lived, lower-altitude carbon dioxide ice clouds at latitudes from 70 to 80 degrees south. “One line of evidence for snow is that the carbon-dioxide ice particles in the clouds are large enough to fall to the ground during the lifespan of the clouds,” co-author David Kass of the Jet Propulsion Laboratory said. “Another comes from observations when the instrument is pointed toward the horizon, instead of down at the surface. The infrared spectra signature of the clouds,” that is, the detailed breakdown of the characteristics of its infrared light, “viewed from this angle is clearly carbon-dioxide ice particles and they extend to the surface. By observing this way, the Mars Climate Sounder is able to distinguish the particles in the atmosphere from the dry ice on the surface.” Mars’ south polar residual ice cap is the only place on the Red Planet where frozen carbon dioxide persists on the surface year-round. Just how the carbon dioxide from Mars’ atmosphere gets deposited has been in question. It is unclear whether it occurs as snow or by freezing out at ground level as frost. These results show snowfall is especially vigorous on top of the residual cap. “The finding of snowfall could mean that the type of deposition—snow or frost—is somehow linked to the year-to-year preservation of the residual cap,” Hayne said.