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June 17, 2015

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Ancient Mars mostly icy, not wet, study claims

June 16, 2015
Courtesy of Harvard University
and World Science staff

Mars may well have had plen­ti­ful wa­ter bil­lions of years ago—but it was mostly ice, not liq­uid, a study claims.

Re­search­ers say a cold and icy plan­et bil­lions of years ago bet­ter ex­plains wa­ter drain­age and ero­sion fea­tures seen on the Red Plan­et to­day.

A conceptual rendition of the competing warm and cold scenarios for early Mars. (Courtesy of R. Wordsworth)


For dec­ades, re­search­ers have de­bat­ed the cli­mate his­to­ry of Mars and how the plan­et’s early cli­mate led to the many wa­ter-carved chan­nels seen to­day. 

The idea that 3 to 4 bil­lion years ago Mars was warm, wet and Earth-like with a north­ern sea—con­di­tions that could have led to life—is gen­er­ally more pop­u­lar than that of a frig­id, icy plan­et where wa­ter is locked in ice most of the time and life would be hard put to evolve. 

In the stu­dy, re­search­er Rob­in Words­worth of Har­vard Uni­vers­ity and col­leagues used a com­put­er sim­ula­t­ion of at­mos­pher­ic cir­cula­t­ion to com­pare a wa­ter cy­cle on Mars un­der dif­fer­ent sce­nar­i­os. The mod­el exam­ined an era 3 to 4 bil­lion years ago, dur­ing what’s called the late No­a­chi­an and early Hes­pe­ri­an pe­ri­ods. 

One sce­nar­i­o looked at Mars as a warm and wet plan­et with an av­er­age glob­al tem­per­a­ture of 10 de­grees C (50 de­grees F) and the oth­er as a cold and icy world with an av­er­age glob­al tem­per­a­ture of mi­nus 48 de­grees C (mi­nus 54 de­grees F).

The stu­dy’s au­thors found that the cold sce­nar­i­o was more likely, based on what’s known of the his­to­ry of the Sun and the tilt of Mars’ ax­is at the time. The cold mod­el al­so did a bet­ter job ex­plain­ing the wa­ter ero­sion fea­tures that have been left be­hind on the Mar­tian sur­face, they said, and which have puz­zled and in­trigued sci­en­tists since they were first dis­cov­ered by the Vi­king or­biters in the 1970s.

A pa­per on the re­sults has been ac­cept­ed for pub­lica­t­ion in the Amer­i­can Geo­phys­i­cal Un­ion’s Jour­nal of Geo­phys­i­cal Re­search – Plan­ets.

The colder sce­nar­i­o was more straight­for­ward to mod­el, Words­worth ex­plained, be­cause Mars only gets 43 per­cent of the so­lar en­er­gy of Earth, and early Mars was lit by a young­er Sun be­lieved to have been 25 per­cent dim­mer than it is to­day.

An ex­treme tilt of the Mar­tian ax­is would have point­ed the plan­et’s poles at the Sun and driv­en po­lar ice to the equa­tor, where wa­ter drain­age and ero­sion fea­tures are seen to­day, the sci­en­tists said. 

More im­por­tant­ly, they added, un­der a thicker at­mos­phere, equa­torial high­land re­gions get colder and north­ern low­land re­gions get warm­er. This is so-called “i­cy high­lands ef­fect” ac­counts for snow-covered moun­tain peaks on Earth to­day. De­spite some warm­ing fac­tors—in­cluding a thicker at­mos­phere filled with cli­mate-warm­ing car­bon diox­ide—Mars still would have been quite cold, Words­worth said.


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Mars may well have had plentiful water billions of years ago—but it was mostly ice, not liquid, a study claims. Researchers say a cold and icy planet billions of years ago better explains water drainage and erosion features seen on the Red Planet today. For decades, researchers have debated the climate history of Mars and how the planet’s early climate led to the many water-carved channels seen today. The idea that 3 to 4 billion years ago Mars was warm, wet and Earth-like with a northern sea—conditions that could have led to life—is generally more popular than that of a frigid, icy planet where water is locked in ice most of the time and life would be hard put to evolve. in the study, Researcher Robin Wordsworth of Harvard University and colleagues used a computer simulation of atmospheric circulation to compare a water cycle on Mars under different scenarios 3 to 4 billion years ago, during what’s called the late Noachian and early Hesperian periods. One scenario looked at Mars as a warm and wet planet with an average global temperature of 10 degrees Celsius (50 degrees Fahrenheit) and the other as a cold and icy world with an average global temperature of minus 48 degrees Celsius (minus 54 degrees Fahrenheit). The study’s authors found that the cold scenario was more likely to have occurred than the warm scenario, based on what is known about the history of the Sun and the tilt of Mars’ axis 3 to 4 billion years ago. The cold model also did a better job explaining the water erosion features that have been left behind on the Martian surface, they said, and which have puzzled and intrigued scientists since they were first discovered by the Viking orbiters in the 1970s. A paper on the results has been accepted for publication in the American Geophysical Union’s Journal of Geophysical Research – Planets. The colder scenario was more straightforward to model, Wordsworth explained, because Mars only gets 43 percent of the solar energy of Earth, and early Mars was lit by a younger Sun believed to have been 25 percent dimmer than it is today. An extreme tilt of the Martian axis would have pointed the planet’s poles at the Sun and driven polar ice to the equator, where water drainage and erosion features are seen today, the scientists said. More importantly, they added, under a thicker atmosphere, equatorial highland regions get colder and northern lowland regions get warmer. This is so-called “icy highlands effect” accounts for snow-covered mountain peaks on Earth today. Despite a number of warming factors—including a thicker atmosphere filled with climate-warming carbon dioxide—Mars still would have been quite cold, Wordsworth said.