Mars could have been as cold in its past as it is now

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Sky at Night Magazine - - BULLETIN -

S ome of the clear­est signs that an­cient Mars once sported huge amounts of flow­ing wa­ter on its sur­face are pro­vided by net­works of rivers. These val­ley net­works have long-since dried up, but their dis­tinc­tive forms carved into the land­scape are both abun­dant and wide­spread across the south­ern up­lands of Mars. They tell us that dur­ing the pe­riod in the planet’s his­tory when they formed – the late Noachian, which was around 4.1 bil­lion to 3.7 bil­lion years ago – the Mar­tian en­vi­ron­ment was very dif­fer­ent from what we find to­day.

But what ex­actly was the na­ture of the early Mar­tian cli­mate? Was it once very Earth-like, with con­di­tions warm enough for a gen­uine wa­ter cy­cle of rain­fall and evap­o­ra­tion, re­sult­ing in long-term lakes and rivers? Or was an­cient Mars the same very cold, icy, planet we know to­day, with av­er­age tem­per­a­tures well be­low the freez­ing point, the val­ley net­works hav­ing been formed by short-lived melt­ing episodes trig­gered by a vol­canic erup­tion or as­teroid im­pact?

Has Mars ever been much warmer and wet­ter, or has it al­ways been in­hos­pitably cold, bleak and dry? The an­swer to this ques­tion is not only of in­ter­est to plan­e­tary cli­ma­tol­o­gists, but is also im­por­tant for de­ter­min­ing the chances of Mars ever hav­ing de­vel­oped life of its own.

The pre­served re­mains of the val­ley net­works might also be able to pro­vide in­sights into the an­cient Mar­tian cli­mate. In re­cent years plan­e­tary sci­en­tists have used or­biter im­agery and sur­face mod­els to es­ti­mate the vol­ume of liq­uid wa­ter that would have been nec­es­sary to carve out the val­ley net­works, and over what timescales. The prob­lem is, these cal­cu­la­tions vary quite widely.

One re­search group es­ti­mated that erod­ing all of the val­ley net­works would have re­quired a vol­ume of wa­ter equiv­a­lent to sub­merg­ing the en­tire Mar­tian globe to an av­er­age depth of al­most 5km. If true, this would in­di­cate that the flow­ing wa­ter must have been re­cy­cled many times down the val­leys by evap­o­ra­tion and rain­fall, and thus that an­cient Mars was a great deal warmer. On the other hand, dif­fer­ent stud­ies cal­cu­lated a re­quired vol­ume of wa­ter equiv­a­lent to only 3-100m glob­ally, which could eas­ily be ex­plained by tran­sient river ac­tiv­ity in an oth­er­wise very cold and icy cli­mate.

These ear­lier es­ti­mates were based on cer­tain as­sump­tions and lim­ited data sets and vary greatly from each other. So, to try to bring some clar­ity to the is­sue, Eliott Rosen­berg and his col­leagues at the Depart­ment of Earth, En­vi­ron­men­tal and Plan­e­tary Sciences, Brown Univer­sity, have used more com­plete data for the vol­umes of the val­ley net­works and an im­proved method for es­ti­mat­ing the liq­uid flow that would have been re­quired. They find that the min­i­mum amount of liq­uid wa­ter needed to carve out the val­ley net­works is around 640m glob­ally.

This lat­est cal­cu­la­tion sits in the mid­dle of pre­vi­ous es­ti­mates. And it also seems to be con­sis­tent with ei­ther a warm, rainy an­cient Mars, or a cold and icy cli­mate. In­trigu­ingly, the lo­ca­tions of many of the val­ley net­works and lakes are where cli­mate mod­els pre­dict ice lay­ers would form in a cold sce­nario, and so they could be ex­plained by melt­wa­ter run­ning off sur­face ice. Per­haps an­cient Mars was not as hab­it­able as we might have thought.

LEWIS DARTNELL was read­ing… The vol­ume of wa­ter re­quired to carve the Mar­tian val­ley net­works: Im­proved con­straints us­ing up­dated meth­ods by Eliott N Rosen­berg, et al. Read it on­line at: www.sci­encedi­rect. com/science/ar­ti­cle/pii/S001910351­7305900

A NEW STUDY FINDS THAT THE MIN­I­MUM AMOUNT OF LIUID NEEDED TO A CARVE OUT MARS'S VAL­LEY NET­WORK IS AROUND 640M GLOB­ALLY

An aerial image – taken by the Mars Re­con­nais­sance Or­biter – of Mars’s Uzboi Val­lis, be­lieved to have been formed by run­ning wa­ter

LEWIS DARTNELL is an as­tro­bi­ol­ogy re­searcher at the Univer­sity of West­min­ster and the au­thor of The Knowl­edge: How to Re­build our World from Scratch (www.the-knowl­edge.org)

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