What does Mars smell like?


M ars is a good ex­am­ple of a planet we think has un­der­gone dras­tic cli­mate change in its early days. It is presently a cold desert, but in win­ter there are large weather sys­tems very sim­i­lar in scale to those we ex­pe­ri­ence in the UK. The in­trigu­ing ques­tion is: while Mars is cur­rently very cold and dry, could it have been warmer and wet­ter in the past? Cer­tainly, there’s ev­i­dence in the rocks on the sur­face that it was. So was it ever hab­it­able? Could it have even been – in the dis­tant past, four bil­lion years ago or so – a place where life could have started?

My role in­volves mon­i­tor­ing and un­der­stand­ing the cur­rent state of the at­mos­phere on Mars but also look­ing for tiny traces of gases that may re­veal some­thing about Mars’s past us­ing the Trace Gas Or­biter (TGO) to ‘sniff ’ them out.

The TGO was launched in March 2016 and ar­rived at the Red Planet in Novem­ber 2016. There were no ob­ser­va­tions un­til it got into its sci­ence or­bit in late spring of this year. We are now get­ting sci­ence data on a reg­u­lar ba­sis.

We know that the bulk of the at­mos­phere is car­bon diox­ide, ni­tro­gen and a few no­ble gases, but what we re­ally want to know is are there any other trace gases that don’t fit in with the equi­lib­rium gases that pos­si­bly give a hint of some­thing that’s hap­pen­ing on Mars to­day, or long, long ago?

A very use­ful heat haze

We are look­ing down to parts per bil­lion; it takes some very sen­si­tive spec­troscopy to find gases that are such tiny com­po­nents of the at­mos­phere. So to give us a bet­ter chance of find­ing them, the space­craft will also be look­ing as the Sun rises and sets through the Mar­tian at­mos­phere.

Why? Well, cur­rently TGO is about 400km above the sur­face, or­bit­ing every cou­ple of hours. In every or­bit it sees one sunrise and one sun­set. For a brief few min­utes as the Sun sets or rises, the TGO will, be­cause of its an­gle, be look­ing at it through a long stretch of Mars’s at­mos­phere; that gives you a large sam­ple for spec­troscopy and a greater chance of de­tect­ing even mi­nor gases in the at­mos­phere. This is a tech­nique known as limb sound­ing. The same in­stru­ments used for this will look straight down at the planet as well. This will give us an­other source of in­for­ma­tion be­cause the sur­face is rel­a­tively warm and gives off in­frared ra­di­a­tion that will be ab­sorbed and re-emit­ted by gases in the at­mos­phere as it rises up towards to the TGO.

Life lessons

Why are we do­ing this? One rea­son is so that we can test our mod­els of Earth’s cli­mate sys­tems. It might change our ideas about how our own at­mos­phere works or help us un­der­stand what’s im­por­tant about Earth’s cli­mate, and what causes cli­mates to change and be­come less hab­it­able. The sec­ond rea­son is learn­ing how Mars has evolved and ask­ing if Mars were ever a hab­it­able place, which opens up some huge ques­tions. Just be­cause life could evolve some­where, does it al­ways evolve? Or is life still in­cred­i­bly un­likely? We have only one ex­am­ple: Earth. Was that just an in­cred­i­ble fluke? Of course, you can’t ac­tu­ally smell Mars – if you took your hel­met off you’d die fairly quickly. But think­ing about what it might smell like is a way spec­u­lat­ing about what Mars’s at­mos­phere might be made up from. The main gases, in­clud­ing car­bon diox­ide and ni­tro­gen, don’t smell of any­thing. But we are try­ing to look for tiny trace amounts of out-of-bal­ance gases. We are not di­rectly smelling them, but look­ing at their spec­tra. Es­sen­tially, there would be a sort of rusty smell that you’d get from the dust of ox­i­dised rocks. The other smell might be a meth­ane-farty smell – to put it crudely – if there were bac­te­rial or mi­cro­bial life on Mars. Meth­ane is one of the trace gases we are look­ing for, but its pres­ence wouldn’t nec­es­sar­ily prove the pres­ence of life, now or ever. It could have come from a to­tally non-liv­ing process, like geo­ther­mal ac­tiv­ity.

The Trace Gas Or­biter will be us­ing sun­sets and sun­rises to have a re­ally good look for anoma­lous gasses in Mars’s at­mos­phere

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