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Hurricanes part of forecast—for Saturn moon

May 22, 2013
Courtesy of NASA Jet Propulsion Laboratory
and World Science staff

Sat­urn’s moon Ti­tan might be in for some wild weath­er as it heads in­to its spring and sum­mer, if two new mod­els are cor­rect.

The re­search is part of a still-emerging sci­ence of weath­er fore­cast­ing on ce­les­tial bod­ies oth­er than Earth.

NASA sci­en­tists think that as the sea­sons change in Ti­tan’s north­ern hem­i­sphere, some of the first known waves could rip­ple across the moon’s seas of hy­dro­car­bons. Hur­ri­canes could al­so beg­in to swirl over these ar­eas. 

Li­ge­ia Ma­re, shown in here in da­ta ob­tained by NA­SA's Cas­si­ni space­craft, is the sec­ond larg­est known body of liq­uid on Sat­urn's moon Ti­tan. Sci­ent­ists say it is filled with liq­uid hy­dro­car­bons, such as eth­ane and meth­ane, and is one of the many seas and lakes that be­jew­el Ti­tan's north po­lar re­gion. Cas­si­ni has yet to ob­serve waves on Li­ge­ia Ma­re and will look again dur­ing its next en­coun­ter on May 23, 2013. (Im­age cred­it: NA­SA/JPL-Caltech/ASI/Cornell)


The mod­el pre­dict­ing waves tries to ex­plain da­ta from the moon ob­tained so far by NASA’s Cas­si­ni space­craft. Both mod­els help mis­sion team mem­bers plan when and where to look for un­usu­al at­mos­pher­ic dis­tur­bances as Ti­tan sum­mer ap­proaches.

“If you think be­ing a weath­er fore­cast­er on Earth is dif­fi­cult, it can be even more chal­leng­ing at Ti­tan,” said Scott Edg­ing­ton, Cas­si­ni’s dep­u­ty proj­ect sci­ent­ist at NASA’s Je­t Pro­pul­sion Lab­o­r­a­to­ry, Pas­a­de­na, Ca­lif. “We know there are weath­er pro­cesses si­m­i­lar to Earth’s at work on this strange world, but dif­fer­ences arise due to the pres­ence of un­fa­mil­iar liq­uids like meth­ane. We can’t wait for Cas­si­ni to tell us wheth­er our fore­casts are right as it con­tin­ues its tou­r through Ti­tan spring in­to the start of north­ern sum­mer.”

Hy­dro­car­bons, which in­clude meth­ane, are sub­stances that con­sist of hy­dro­gen and car­bon, and are the sim­plest or­gan­ic com­pounds.

Ti­tan’s north po­lar re­gion, which is be­jew­eled with sprawl­ing hydrocar­bon seas and lakes, was dark when Cas­si­ni first ar­rived at the Sat­urn sys­tem in 2004. But sun­light has been creep­ing up Ti­tan’s north­ern hem­i­sphere since Au­gust 2009, when the sun’s light crossed the equa­tor at equi­nox. Ti­tan’s sea­sons take about se­ven Earth years to change. By 2017, the end of Cas­si­ni’s mis­sion, Ti­tan will be ap­proach­ing north­ern sol­stice, the height of sum­mer.

Giv­en the wind-sculpted dunes Cas­si­ni has seen on Ti­tan, sci­en­tists were baf­fled about why they had­n’t yet seen wind-driven waves on the lakes and seas. A team led by Al­ex Hayes, a mem­ber of Cas­si­ni’s ra­dar team who is based at Cor­nell Uni­vers­ity, Ith­a­ca, N.Y., set out to look for how much wind it would take to gen­er­ate waves. 

Their new mod­el, just pub­lished in the jour­nal Ic­a­rus, is thought to im­prove up­on pre­vi­ous ones by ac­count­ing for sev­er­al fac­tors at once. These in­clude Ti­tan’s gra­vity; the vis­cos­ity, or syrupi­ness, of the liq­uid in the lakes; the rel­a­tive dense­ness, or com­pact­ness, of air and liq­uid; and the liq­uid’s “sur­face ten­sion.” That’s the prop­er­ty that lets drops of wa­ter, for in­stance, stay round­ish as they sit on a sur­face rath­er than just spread­ing out eve­ry­where.

“We now know that the wind speeds pre­dicted dur­ing the times Cas­si­ni has ob­served Ti­tan have been be­low the thresh­old nec­es­sary to gen­er­ate waves,” Hayes said. “What is ex­cit­ing, how­ev­er, is that the wind speeds pre­dicted dur­ing north­ern spring and sum­mer ap­proach those nec­es­sary to gen­er­ate wind waves in liq­uid eth­ane and/or meth­ane. It may soon be pos­si­ble to catch a wave in one of the so­lar sys­tem’s most ex­ot­ic loca­t­ions.”

The new mod­el found that winds of 1 to 2 mph (2 to 3 kilo­me­ters per hour) are needed to gen­er­ate waves on Ti­tan lakes, a speed that has not yet been reached dur­ing Ti­tan’s cur­rently calm pe­riod. But as Ti­tan’s north­ern hem­i­sphere ap­proaches spring and sum­mer, oth­er mod­els pre­dict the winds may in­crease to 2 mph (3 kilo­me­ters per hour) or faster. De­pend­ing on the com­po­si­tion of the lakes, winds of that speed could be enough to pro­duce waves 0.5 foot (0.15 me­ter) high.

The oth­er mod­el about hur­ri­canes, re­cently pub­lished in Ic­a­rus, pre­dicts that the warm­ing of the north­ern hem­i­sphere could al­so br­ing hur­ri­canes, al­so known as trop­i­cal cy­clones. Trop­i­cal cy­clones on Earth gain their en­er­gy from the build-up of heat from seawa­ter evapora­t­ion and min­ia­ture ver­sions have been seen over big lakes such as Lake Hu­ron. The new mod­eling work, led by Tet­suya Tokano of the Uni­vers­ity of Co­logne, Ger­ma­ny, shows that the same pro­cesses could be at work on Ti­tan, ex­cept that it is meth­ane rath­er than wa­ter that evap­o­rates from the seas. The most likely sea­son for these hur­ri­canes would be Ti­tan’s north­ern sum­mer sol­stice, when the sea sur­face gets warm­er and the flow of the air near the sur­face be­comes more tur­bu­lent. The hu­mid air would swirl coun­ter­clock­wise over one of the north­ern seas and boost the sur­face wind there to pos­sibly 45 mph (a­bout 70 kilo­me­ters per hour).

“For these hur­ri­canes to de­vel­op at Ti­tan, there needs to be the right mix of hy­dro­car­bons in these seas, and we still don’t know their ex­act com­po­si­tion,” Tokano said. “If we see hur­ri­canes, that would be one good in­di­ca­tor that there is enough meth­ane in these lakes to sup­port this kind of ac­ti­vity. So far, sci­en­tists haven’t yet been able to de­tect meth­ane di­rect­ly.”


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Saturn’s moon Titan might be in for some wild weather as it heads into its spring and summer, if two new models are correct. The research is part of a still-emerging science of weather forecasting on celestial bodies other than Earth. NASA scientists think that as the seasons change in Titan’s northern hemisphere, waves could ripple across the moon’s seas of hydrocarbons. Hurricanes could also begin to swirl over these areas. The model predicting waves tries to explain data from the moon obtained so far by NASA’s Cassini spacecraft. Both models help mission team members plan when and where to look for unusual atmospheric disturbances as Titan summer approaches. “If you think being a weather forecaster on Earth is difficult, it can be even more challenging at Titan,” said Scott Edgington, Cassini’s deputy project scientist at NASA’s Jet Propulsion Laboratory, Pasadena, Calif. “We know there are weather processes similar to Earth’s at work on this strange world, but differences arise due to the presence of unfamiliar liquids like methane. We can’t wait for Cassini to tell us whether our forecasts are right as it continues its tour through Titan spring into the start of northern summer.” Hydrocarbons, which include methane, are substances that consist of hydrogen and carbon, and are the simplest organic compounds. Titan’s north polar region, which is bejeweled with sprawling hydrocarbon seas and lakes, was dark when Cassini first arrived at the Saturn system in 2004. But sunlight has been creeping up Titan’s northern hemisphere since August 2009, when the sun’s light crossed the equatorial plane at equinox. Titan’s seasons take about seven Earth years to change. By 2017, the end of Cassini’s mission, Titan will be approaching northern solstice, the height of summer. Given the wind-sculpted dunes Cassini has seen on Titan, scientists were baffled about why they hadn’t yet seen wind-driven waves on the lakes and seas. A team led by Alex Hayes, a member of Cassini’s radar team who is based at Cornell University, Ithaca, N.Y., set out to look for how much wind would be required to generate waves. Their new model, just published in the journal Icarus, is thought to improve upon previous ones by accounting for several factors at once. These include Titan’s gravity; the viscosity, or syrupiness, of the liquid in the lakes; the relative denseness, or compactness, of air and liquid; and the liquid’s “surface tension.” That’s the property that lets drops of water, for instance, stay roundish as they sit on a surface rather than just spreading out everywhere. “We now know that the wind speeds predicted during the times Cassini has observed Titan have been below the threshold necessary to generate waves,” Hayes said. “What is exciting, however, is that the wind speeds predicted during northern spring and summer approach those necessary to generate wind waves in liquid ethane and/or methane. It may soon be possible to catch a wave in one of the solar system’s most exotic locations.” The new model found that winds of 1 to 2 mph (2 to 3 kilometers per hour) are needed to generate waves on Titan lakes, a speed that has not yet been reached during Titan’s currently calm period. But as Titan’s northern hemisphere approaches spring and summer, other models predict the winds may increase to 2 mph (3 kilometers per hour) or faster. Depending on the composition of the lakes, winds of that speed could be enough to produce waves 0.5 foot (0.15 meter) high. The other model about hurricanes, recently published in Icarus, predicts that the warming of the northern hemisphere could also bring hurricanes, also known as tropical cyclones. Tropical cyclones on Earth gain their energy from the build-up of heat from seawater evaporation and miniature versions have been seen over big lakes such as Lake Huron. The new modeling work, led by Tetsuya Tokano of the University of Cologne, Germany, shows that the same processes could be at work on Titan as well, except that it is methane rather than water that evaporates from the seas. The most likely season for these hurricanes would be Titan’s northern summer solstice, when the sea surface gets warmer and the flow of the air near the surface becomes more turbulent. The humid air would swirl in a counterclockwise direction over the surface of one of the northern seas and increase the surface wind over the seas to possibly 45 mph (about 70 kilometers per hour). “For these hurricanes to develop at Titan, there needs to be the right mix of hydrocarbons in these seas, and we still don’t know their exact composition,” Tokano said. “If we see hurricanes, that would be one good indicator that there is enough methane in these lakes to support this kind of activity. So far, scientists haven’t yet been able to detect methane directly.”