In­side Mars

Focus-Science and Technology - - CONTENTS - WORDS: COLIN STU­ART

What will the In­Sight mis­sion dis­cover when it digs be­neath the sur­face of the Red Planet?

The Red Planet has been an ob­ject of in­trigue for cen­turies, and an ar­mada of or­biters and rovers have ex­plored Mars up close. Yet for all our ex­plo­ration ef­forts there re­mains a per­plex­ing mys­tery: just what is go­ing on deep within Mars?

Pre­vi­ous mis­sions to Mars have been bi­ased towards what’s hap­pen­ing on the sur­face. That’s no sur­prise given the menu of mar­vels on of­fer, such as sweep­ing sand dunes, soar­ing vol­ca­noes and scin­til­lat­ing blue sun­sets. We now know Mars’s outer fa­cade so well that we have a bet­ter map of the Mar­tian sur­face than we do of the ocean floor here on Earth. Yet it’s the deep­est lay­ers of a planet that re­ally make it tick, and rel­a­tively lit­tle is known about the Red Planet’s in­te­rior. Now, that’s all about to change thanks to a mis­sion that has been long in the plan­ning. “It was first pro­posed 25 years ago,” says Dr Suzanne Sm­rekar from NASA’s Jet Propul­sion Lab­o­ra­tory in Cal­i­for­nia. “We’re ex­cited to fi­nally be do­ing this.” Sm­rekar is the deputy prin­ci­pal in­ves­ti­ga­tor for the In­Sight mis­sion. It was launched in May aboard an At­las V-401 rocket from Van­den­berg Air Force Base in Cal­i­for­nia and is cur­rently en route to the Red Planet. Due to touch­down on Mars on 26 Novem­ber, close to NASA’s ex­ist­ing Cu­rios­ity rover, it will spend a min­i­mum of one Mar­tian year (nearly two Earth years) sur­vey­ing deep be­neath the fa­mously ruddy dirt.

The goal of In­Sight is to give Mars’s in­te­rior the plan­e­tary equiv­a­lent of a full body health check. It will take its ‘pulse’ by care­fully mon­i­tor­ing seis­mic ac­tiv­ity (oth­er­wise known as ‘Marsquakes’) and record its tem­per­a­ture by keep­ing track of the heat flow un­der the planet’s sur­face. That will help us un­der­stand how rocky plan­ets, such as Earth and Mars, formed in the first place. On Earth, most of these clues have been erased due to the ac­tion of our tec­tonic plates over bil­lions of years. While seis­mic ac­tiv­ity has been mea­sured on the Moon, thanks to in­stru­ments left by the Apollo as­tro­nauts, it’s a much smaller world and it formed in a dif­fer­ent way to the So­lar Sys­tem’s four rocky plan­ets. Mars could hold se­crets about how we came to be here in the first place, and In­Sight hopes to find them. “Mars is the per­fect place for us to learn about ter­res­trial planet for­ma­tion and evo­lu­tion,” says Sm­rekar.

TOUCH­DOWN ON MARS

The lan­der it­self is based on NASA’s Phoenix probe that touched down close to the Mar­tian north pole in May 2008. By closely fol­low­ing a pre­vi­ous de­sign, mis­sion sci­en­tists have kept costs down. Since In­Sight will ini­tially strike the Mar­tian at­mos­phere at over 10,000 kilo­me­tres per hour, the craft has an outer shell that will shield the sen­si­tive equip­ment from the heat gen­er­ated by fric­tion with Mars’s thin at­mos­phere. A para­chute will then de­ploy to lower In­Sight down through the bot­tom half of the Mar­tian at­mos­phere, then rock­ets will fire when it is 100 me­tres above the sur­face to gen­tly de­posit it onto the Red Planet. While Phoenix’s land­ing went smoothly, that’s no guar­an­tee of a hic­cup-free ride this time around – land­ing any­thing on Mars is a no­to­ri­ously tricky busi­ness. “A third of pre­vi­ous Mars mis­sions have been un­suc­cess­ful,” says Sm­rekar. Still, she’s con­fi­dent that their well-tested sys­tem has a 99 per cent chance of stick­ing the land­ing within the 130km-wide des­ig­nated touch­down zone in a flat plain known as the Ely­sium Plani­tia. Six­teen min­utes after land­ing, the Mar­tian dust will have set­tled back down, after which In­Sight’s so­lar

“Mars is the per­fect place for us to learn about ter­res­trial planet for­ma­tion and evo­lu­tion”

ar­rays will whirr into ac­tion to un­furl and charge its so­lar pan­els. Then the mis­sion be­gins in earnest.

Ely­sium Plani­tia was cho­sen due to it tick­ing so many boxes. “First there were the en­gi­neer­ing con­straints,” says Sm­rekar. The land­ing site had to be less than two kilo­me­tres above Mar­tian ‘sea level’ so that the probe trav­elled through enough at­mos­phere to slow it down. Equally they wanted a wide, open space free of large rocks and other po­ten­tial ob­sta­cles. It also had to be close to the Mar­tian equa­tor so that the probe can bathe in enough sun­light to stay pow­ered for at least a Mar­tian year (687 Earth days) – the min­i­mum in­tended du­ra­tion of the mis­sion, Sm­rekar says. She does, how­ever, cau­tion against ex­pect­ing the stun­ning pic­tures of sweep­ing Mar­tian land­scapes that we’ve be­come ac­cus­tomed to with mis­sions like Spirit, Op­por­tu­nity and Cu­rios­ity be­cause In­Sight’s mis­sion sci­en­tists care about what’s un­der the sur­face of the planet, not what’s on it. “It’s a flat, bor­ing site,” Sm­rekar says. “There were only a cou­ple of sites that met all these con­straints.”

TEST­ING, TEST­ING

One of the key in­stru­ments on board is a seis­mome­ter (known as SEIS) for mea­sur­ing tremors from deep within the bow­els of Mars. For Dr Neil Bowles, from the Uni­ver­sity of Ox­ford, it’s the most ex­cit­ing part of the mis­sion. The thing he’s look­ing for­ward to most, he says, is “the first un­equiv­o­cal de­tec­tion of a Marsquake”. One of the Vik­ing lan­ders of the 1970s 2

2 did carry a seis­mome­ter, but, cru­cially, it wasn’t put in di­rect con­tact with the Mar­tian sur­face. Ac­cord­ing to Bowles, that meant any tremors were felt through the legs of the lan­der and were there­fore hard to mea­sure ac­cu­rately. It was bet­ter suited for sens­ing the fear­some Mar­tian winds, but not for pick­ing up un­der­ground vi­bra­tions. This time In­Sight’s seis­mome­ter will be placed in di­rect con­tact with the ground and cov­ered with a ther­mal shield to pro­tect it both from the wind and wildly swing­ing Mar­tian tem­per­a­tures. It is so sen­si­tive that it can pick up vi­bra­tions smaller in size than the width of a hy­dro­gen atom. That’s more than enough to also de­tect the tell­tale thump of me­te­orite im­pacts on the Mar­tian ter­rain.

SEIS has al­ready been fired up on the jour­ney to Mars to test its sen­si­tive in­stru­ments in the harsh en­vi­ron­ment of space, but the first full seis­mic data will start to trickle back to Earth in early 2019, a cou­ple of months after the probe touches down. “What­ever we find will be in­ter­est­ing and ex­cit­ing straight away, as so lit­tle is known about what’s go­ing on down there,” says Bowles.

It’s been over 130 years since sim­i­lar mea­sure­ments were made of earthquakes here on Earth, and our knowl­edge about our planet’s in­nards has been trans­form­ing ever since. “From the way vi­bra­tions are re­flected and re­fracted in­side the Earth, we’ve learned about its in­ner struc­ture,” Bowles says. With­out ever trav­el­ling down in­side our planet, we know that it has a solid iron in­ner core en­cased in a liq­uid outer core, which in turns sits be­neath the man­tle and then fi­nally the crust. In­Sight may bring the same level of knowl­edge about Mars, and in do­ing so shed light on one of Mars’s great­est mys­ter­ies: what hap­pened to its mag­netic field.

FIELD WORK

Earth’s mag­netic field is gen­er­ated by the move­ment of ma­te­rial in the liq­uid outer core as the planet spins. There’s weighty ev­i­dence that Mars once had a global mag­netic field too, but now all that re­mains are scant patches of lo­calised mag­netism scat­tered here and there. It could be that, as Mars is a smaller planet, there wasn’t enough ma­te­rial crush­ing down on the core to keep it liq­uid. If it so­lid­i­fied, there would no longer be move­ment, so the mag­netic field would have switched off. Mag­netic fields pro­tect plan­ets from the so­lar wind, which is a stream of charged par­ti­cles blow­ing out from the Sun. Once

Mars had no mag­netic field, the so­lar wind was able to peck away at the planet’s at­mos­phere.

It is hoped that In­Sight can use the way vi­bra­tions travel through Mars to de­ter­mine whether any of its core is still liq­uid. Un­der­stand­ing the link be­tween the core and Mars’s mag­netic field could prove cru­cial if we’re to reg­u­larly send as­tro­nauts to Mars and pro­tect them from the harsh ra­di­a­tion gen­er­ated by the Sun and the rest of the stars in the Milky Way. In­Sight might also re­veal sub-sur­face reser­voirs of water, kept as a liq­uid by heat ris­ing up from the core. It might be the only place on Mars where its an­cient water has been able to sur­vive in liq­uid form. If life got started on Mars in its more tem­per­ate past, it may just still be cling­ing on in these sub­ter­ranean seas. The stakes are high.

HOT TOPIC

If SEIS is the equiv­a­lent of a doc­tor’s stetho­scope, lis­ten­ing for the planet’s beat­ing heart, the Heat Flow and Phys­i­cal Prop­er­ties Probe (known as HP3) is like putting a ther­mome­ter un­der Mars’s tongue. Sm­rekar de­scribes it as “a self-ham­mer­ing nail”. The whole ex­per­i­ment weighs only three kilo­grams and will send back just over 40 megabytes of data over the du­ra­tion of the mis­sion – around the same as a low-qual­ity YouTube video. Like a mole, it will bur­row down into the Mar­tian soil to a depth of five me­tres – far deeper than any Mars probe be­fore it. Ac­cord­ing to Sm­rekar, that’s deep enough to get away from any sur­face tem­per­a­ture vari­a­tions due to day and night or Mars’s swing­ing sea­sons. Every 50 cen­time­tres, the probe will let out a heat pulse and mea­sure how that pulse dis­si­pates through the Mar­tian sub-sur­face. The quicker it fades, the bet­ter the sur­round­ing ma­te­rial is at con­duct­ing heat – a sure­fire way to know what it is made of.

HP3 is also on the hunt for ev­i­dence of heat gen­er­ated by ra­dioac­tive de­cay. El­e­ments such as ura­nium, tho­rium and potas­sium spon­ta­neously break down over long pe­ri­ods into lighter el­e­ments, re­leas­ing en­ergy along the way. It is thought Mars and Earth formed in a sim­i­lar fash­ion from “plan­e­tary build­ing blocks crash­ing to­gether and melt­ing,” says Sm­rekar. If the two plan­ets formed from the same ma­te­rial, then we should ex­pect a sim­i­lar heat sig­na­ture as those ma­te­ri­als un­dergo ra­dioac­tive de­cay. “In­Sight will tell us if the heat com­ing out of Mars is con­sis­tent with that pic­ture,” says Sm­rekar.

That leaves the Ro­ta­tion and In­te­rior Struc­ture Ex­per­i­ment (RISE), which mea­sures the med­i­cal equiv­a­lent of the planet’s re­flexes. As Mars or­bits the Sun, it wob­bles on its axis much like the Earth. Ex­actly how it wob­bles de­pends on what’s go­ing on in the cen­tre of Mars. You can test this for your­self by com­par­ing how a raw egg spins com­pared to a hard-boiled one. A par­tially liq­uid Mar­tian core would lead to a dif­fer­ent wob­ble com­pared with a solid core. So mea­sure­ments from RISE will com­ple­ment those from SEIS in or­der to shed light on the enigma of why Mars’s mag­netic field shut off. The in­stru­ment will ac­cu­rately track In­Sight’s po­si­tion in space every day of the mis­sion by send­ing a sig­nal from Earth and hav­ing it re­flected back home by RISE. Changes in Mars’s po­si­tion and speed will leave tell­tale shifts in the fre­quency of the sig­nal, just as the pitch of an am­bu­lance siren changes as it hur­tles past you.

If all goes to plan, and these ex­per­i­ments all work as in­tended, we’ll fi­nally get cru­cial in­for­ma­tion about the most un­ex­plored part of the most ex­plored planet in the So­lar Sys­tem.

ABOVE: In a clean room, the so­lar ar­ray on In­Sight is de­ployed for test­ingRIGHT: As­sem­bling the SEIS in­stru­ment for mea­sur­ing Marsquakes

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