TED技术:Anjali Tripathi: Why Earth may someday look like Mars

So when you look outat the stars at night,it's amazing what you can see.It's beautiful.But what's more amazingis what you can't see,because what we know nowis that around every staror almost every star,there's a planet,or probably a few.

So what this picture isn't showing youare all the planets that we know aboutout there in space.But when we think about planets,we tend to think of faraway thingsthat are very different from our own.But here we are on a planet,and there are so many thingsthat are amazing about Earththat we're searching far and wideto find things that are like that.And when we're searching,we're finding amazing things.But I want to tell youabout an amazing thing here on Earth.And that is that every minute,400 pounds of hydrogenand almost seven pounds of heliumescape from Earth into space.And this is gas that is going offand never coming back.So hydrogen, helium and many other thingsmake up what's knownas the Earth's atmosphere.The atmosphere is just these gasesthat form a thin blue linethat's seen here fromthe International Space Station,a photograph that some astronauts took.And this tenuous veneer around our planetis what allows life to flourish.It protects our planetfrom too many impacts,from meteorites and the like.And it's such an amazing phenomenonthat the fact that it's disappearingshould frighten you,at least a little bit.

So this process is something that I studyand it's called atmospheric escape.So atmospheric escapeis not specific to planet Earth.It's part of what it meansto be a planet, if you ask me,because planets, not just here on Earthbut throughout the universe,can undergo atmospheric escape.And the way it happens actually tells usabout planets themselves.Because when you thinkabout the solar system,you might think about this picture here.And you would say, well,there are eight planets, maybe nine.So for those of youwho are stressed by this picture,I will add somebody for you.


Courtesy of New Horizons,we're including Pluto.And the thing here is,for the purposes of this talkand atmospheric escape,Pluto is a planet in my mind,in the same way that planetsaround other stars that we can't seeare also planets.So fundamental characteristics of planetsinclude the fact that they are bodiesthat are bound together by gravity.So it's a lot of materialjust stuck togetherwith this attractive force.And these bodies are so bigand have so much gravity.That's why they're round.So when you look at all of these,including Pluto,they're round.

So you can see that gravityis really at play here.But another fundamentalcharacteristic about planetsis what you don't see here,and that's the star, the Sun,that all of the planetsin the solar system are orbiting around.And that's fundamentally drivingatmospheric escape.The reason that fundamentally starsdrive atmospheric escape from planetsis because stars offer planetsparticles and light and heatthat can cause the atmospheres to go away.So if you think of a hot-air balloon,or you look at this pictureof lanterns in Thailand at a festival,you can see that hot aircan propel gasses upward.And if you have enough energy and heating,which our Sun does,that gas, which is so lightand only bound by gravity,it can escape into space.And so this is what's actuallycausing atmospheric escapehere on Earth and also on other planets —that interplaybetween heating from the starand overcoming the forceof gravity on the planet.

So I've told you that it happensat the rate of 400 poundsa minute for hydrogenand almost seven pounds for helium.But what does that look like?Well, even in the '80s,we took pictures of the Earthin the ultravioletusing NASA's Dynamic Explorer spacecraft.So these two images of the Earthshow you what that glowof escaping hydrogen looks like,shown in red.And you can also see other featureslike oxygen and nitrogenin that white glimmerin the circle showing you the aurorasand also some wisps around the tropics.So these are picturesthat conclusively show usthat our atmosphere isn't justtightly bound to us here on Earthbut it's actuallyreaching out far into space,and at an alarming rate, I might add.

But the Earth is not alonein undergoing atmospheric escape.Mars, our nearest neighbor,is much smaller than Earth,so it has much less gravitywith which to hold on to its atmosphere.And so even though Mars has an atmosphere,we can see it's much thinnerthan the Earth's.Just look at the surface.You see craters indicatingthat it didn't have an atmospherethat could stop those impacts.Also, we see that it's the "red planet,"and atmospheric escape plays a rolein Mars being red.That's because we thinkMars used to have a wetter past,and when water had enough energy,it broke up into hydrogen and oxygen,and hydrogen being so light,it escaped into space,and the oxygen that was leftoxidized or rusted the ground,making that familiarrusty red color that we see.

So it's fine to look at pictures of Marsand say that atmospheric escapeprobably happened,but NASA has a probe that's currentlyat Mars called the MAVEN satellite,and its actual jobis to study atmospheric escape.It's the Mars Atmosphereand Volatile Evolution spacecraft.And results from it have alreadyshown pictures very similarto what you've seen here on Earth.We've long known that Marswas losing its atmosphere,but we have some stunning pictures.Here, for example,you can see in the red circleis the size of Mars,and in blue you can see the hydrogenescaping away from the planet.So it's reaching out more than 10 timesthe size of the planet,far enough away that it'sno longer bound to that planet.It's escaping off into space.And this helps us confirm ideas,like why Mars is red,from that lost hydrogen.But hydrogen isn'tthe only gas that's lost.I mentioned helium on Earthand some oxygen and nitrogen,and from MAVEN we can also lookat the oxygen being lost from Mars.And you can seethat because oxygen is heavier,it can't get as far as the hydrogen,but it's still escapingaway from the planet.You don't see it all confinedinto that red circle.

So the fact that we not only seeatmospheric escape on our own planetbut we can study it elsewhereand send spacecraftallows us to learnabout the past of planetsbut also about planets in generaland Earth's future.So one way we actuallycan learn about the futureis by planets so far awaythat we can't see.And I should just note though,before I go on to that,I'm not going to show youphotos like this of Pluto,which might be disappointing,but that's because we don't have them yet.But the New Horizons missionis currently studying atmospheric escapebeing lost from the planet.So stay tuned and look out for that.But the planetsthat I did want to talk aboutare known as transiting exoplanets.

So any planet orbiting a starthat's not our Sunis called an exoplanet,or extrasolar planet.And these planets that we call transitinghave the special featurethat if you lookat that star in the middle,you'll see that actually it's blinking.And the reason that it's blinkingis because there are planetsthat are going past it all the time,and it's that special orientationwhere the planets are blockingthe light from the starthat allows us to see that light blinking.And by surveying the starsin the night skyfor this blinking motion,we are able to find planets.This is how we've now been ableto detect over 5,000 planetsin our own Milky Way,and we know there aremany more out there, like I mentioned.

So when we look at the lightfrom these stars,what we see, like I said,is not the planet itself,but you actually seea dimming of the lightthat we can record in time.So the light drops as the planetdecreases in front of the star,and that's that blinkingthat you saw before.So not only do we detect the planetsbut we can look at this lightin different wavelengths.So I mentioned looking at the Earthand Mars in ultraviolet light.If we look at transiting exoplanetswith the Hubble Space Telescope,we find that in the ultraviolet,you see much bigger blinking,much less light from the star,when the planet is passing in front.And we think this is because you havean extended atmosphere of hydrogenall around the planetthat's making it look puffierand thus blockingmore of the light that you see.

So using this technique,we've actually been able to discovera few transiting exoplanetsthat are undergoing atmospheric escape.And these planetscan be called hot Jupiters,for some of the ones we've found.And that's becausethey're gas planets like Jupiter,but they're so close to their star,about a hundred times closer than Jupiter.And because there's all thislightweight gas that's ready to escape,and all this heating from the star,you have completely catastrophic ratesof atmospheric escape.So unlike our 400 pounds per minuteof hydrogen being lost on Earth,for these planets,you're losing 1.3 billionpounds of hydrogen every minute.

So you might think, well,does this make the planet cease to exist?And this is a questionthat people wonderedwhen they looked at our solar system,because planetscloser to the Sun are rocky,and planets further awayare bigger and more gaseous.Could you have startedwith something like Jupiterthat was actually close to the Sun,and get rid of all the gas in it?We now think that if you startwith something like a hot Jupiter,you actually can't end upwith Mercury or the Earth.But if you started with something smaller,it's possible that enough gaswould have gotten awaythat it would havesignificantly impacted itand left you with something very differentthan what you started with.

So all of this sounds sort of general,and we might think about the solar system,but what does this have to dowith us here on Earth?Well, in the far future,the Sun is going to get brighter.And as that happens,the heating that we find from the Sunis going to become very intense.In the same way that you seegas streaming off from a hot Jupiter,gas is going to stream off from the Earth.And so what we can look forward to,or at least prepare for,is the fact that in the far future,the Earth is going to look more like Mars.Our hydrogen, from waterthat is broken down,is going to escapeinto space more rapidly,and we're going to be leftwith this dry, reddish planet.

So don't fear, it's notfor a few billion years,so there's some time to prepare.


But I wanted youto be aware of what's going on,not just in the future,but atmospheric escapeis happening as we speak.So there's a lot of amazing sciencethat you hear about happening in spaceand planets that are far away,and we are studying these planetsto learn about these worlds.But as we learn about Marsor exoplanets like hot Jupiters,we find things like atmospheric escapethat tell us a lot moreabout our planet here on Earth.

So consider that the next timeyou think that space is far away.

Thank you.


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