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Interview with Sean Solomon

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假如其他星球也有生命

普林斯顿大学

4.8（277 个评分） | 34K 名学生已注册

Are we alone? This course introduces core concepts in astronomy, biology, and planetary science that enable the student to speculate scientifically about this profound question and invent their own solar systems. All the features of this course are available for free. It does not offer a certificate upon completion.

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4.8（277 个评分）

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FZ

Mar 23, 2017

Really enjoyed the course. It gave a very comprehensive introduction to Astrobiology and I enjoyed being pushed to write a science fictional short story at the end.

PT

Nov 10, 2016

This is simply fantastic! Such a charismatic lecturer leading us so professionally through heavy subjects in a light manner! Looking forward to more courses!

从本节课中

Mercury and Venus

This lecture discusses some of the remarkable properties of the two innermost planets, Mercury and Venus. We discuss how Mercury, a very hot planet, can have ice at its poles. We describe its surprisingly strong magnetic field. We discuss the structure of Venus’s atmosphere and how a “runaway greenhouse effect” made its surface uninhabitable.

Interview with Sean Solomon27:28

教学方

David Spergel
Charles Young Professor of Astronomy on the Class of 1897 Foundation and Chair

脚本

Let me welcome Sean Solomon.

Sean is the leader of the Messenger Mission that has been exploring Mercury.

And Sean has graciously agreed to spend a little time with us and talk to us

about the mission, and some of the fascinating

things that they have discovered about the planet.

Being involved in a NASA mission is a big commitment.

It's many years of your life and why did you,

why does Mercury interest you?

What made you decide to become part of

the Messenger mission and help make it all happen?

You're right that being involved in a mission and leading a mission

particularly a mission that spends many years in space is a big commitment.

And I became interested in the planet Mercury when I was a young faculty member.

And Mariner Ten, that first spacecraft to visit Mercury flew by

three times in 1974 and 75. It was on the heels of

the earliest exploration of Mars.

After the Apollo exploration of the moon, and

Mercury was seen, for the first time at

close range, to seem rather like the moon

on the outside, a heavily cratered surface, ancient crust.

And yet Mercury had its own magnetic field.

It was made out of denser material

than any of the other planets. And even in the 70s, we knew that the next

step in exploring Mercury would be to send to send spacecraft to orbit that planet.

In the 1970s, nobody knew how to

do that with chemical propulsion, with conventional rocketry.

And it took another ten years before some mission

design experts at J.P.L., one in particular Qian

Xuesen discovered that you could send a spacecraft orbit Mercury if you

use multiple flybys of Venus and Mercury to fine tune the orbit with each flyby.

And put yourself in a position to

do an orbit insertion maneuver with a conventional rocket.

In the 1970s,

I wasn't involved in any space missions, and later in my career, I

would have the opportunities to participate

in missions to Mars and to Venus.

On top of a lot of work I had done on the Moon earlier in my career.

But for some reason, one reason or another the missions

that NASA chose to fly in the, in the 80s,

even when we knew how to send an orbiter to Mercury.

We're big, expensive, relatively infrequent missions to other targets.

And it wasn't until the mid 1990s, when I was much more senior in my career.

That NASA opened up the opportunities for competed missions in the discovery line.

And there was, it was just a coincidence of new

technologies that had been developed to make space craft function at high

temperatures to make space craft lightweight

and these mission opportunities that NASA opened

up to peer competition that permitted a variety of

ideas to flourish on Mercury missions once again more than 20 years after the

Mariner Ten series.

So I was at a stage in my career where I could afford the kind of

time investment that you referred to without jeopardizing

my opportunities for promotion as a faculty member.

And it had been something that had interested me

since I was fairly young in my scientific career.

So I took advantage of the opportunity to

propose, to assemble the team and NASA selected our mission.

What was

the most exciting moment for you in the mission so far?

Well there, probably

the most dramatic moments were those of highest risk.

So probably the leading moment of excitement from

a technical point of view, or the second in excitement was the launch itself.

Mm-hm. >> Since launches

are not conventional scientific

experiments that one can control completely.

But by far the next leading contender, if not the most dramatic was orbit insertion.

Imagine that a spacecraft has been launched up

to Mercury to orbit it for the first time.

And in order to be in a position to do orbit insertion,

you've got to fly by Venus twice, after going around the Sun multiple times.

And then, fly by Mercury three times, making

a total of 15 revolutions around the Sun.

Spending six and a half years in the inner

solar system, before you have one chance to fire

your propulsion system at the right time for the

right duration to get into the orbit that you want.

So we had built up

expertise in doing the kinds of maneuvers that

are required but there is only one orbit insertion.

So that was perhaps the most tense event and

knowing once we were done that we were in orbit

and in the orbit that we had designed that

upon which all of the scientific plans had been based.

Was very satisfying and a

great relief.

>> What do you think was the biggest scientific surprise for you?

>> So, far the biggest surprise has been what Mercury is made out of.

We've known for more than 60 years that Mercury is,

has the highest uncompressed density of any of the planets.

Meaning if you took average material

of Mercury and you took a

standard volume and you weighed it on a scale, it would be heavier than

the same volume of material from any other planet.

And the only material that's common in the solar system

and in the Sun that has the requisite density is iron.

So the idea has been out there since Harold Dury

in the 1950s that Mercury has got a much higher

fraction of iron, probably mostly iron metal than any of

the other planets, including the other inner planets of which Mercury

is a member of a four-planet family.

And so theoreticians have been dealing with this question for half a century.

How do you make the inner planets so that one of them ends up much richer in iron

metal that the others? And all of the ideas pointed to some

peculiarities for the origin of Mercury that imparted

a different outcome to the final composition.

It might have been that because Mercury's the

planet, is the planet closest to the Sun,

that it formed out of material, the gas

and dust that was closest to the early sun.

And in terms of the solid materials

that were around, available to condense into planets was more metal rich.

Or it could be that and this

was another idea by some famous astrophysicists,

that Mercury formed as a planet a

little larger than its current size, maybe the size of Mars, with a composition

closer to that of Mars, and was bathed in a very hot nebula

of gas that was still around the Sun that vaporized the outer silicate shell.

Leaving a central iron core that had

been protected from that thermal environment.

And yet a third idea is that the final stages of the accretion

of the inner planets involves some

collisions of objects nearly the same size.

As each other, and so those outcomes were often catastrophic, but in some cases,

would have disrupted the target planet but left us some portion of it remaining.

And if that planet had a metal core surrounded

by a rock shell, then the material lost from the impact

might have been preferentially the rocky part, leaving an iron-rich planet.

In any case, all of those ideas made predictions for

the composition that we would find in the rocky shell.

And in particular, the predictions were that we would find a body that

had been depleted in the elements that are easily lost by high temperature processes.

They become gas, they vaporize and they can

be lost to space and to the immediate environment of the planet.

So the prediction was that Mercury would be poor in volatile elements.

Well we took a suite of, geochemical remote

sensing instruments with us on the Messenger mission.

Specifically to look for the kinds of chemical signatures

of the different ideas for how Mercury got assembled.

And somewhat to our surprise, we found that

the abundances of the elements thought to

be volatile, like the alkaline metals, like sulfur,

are high on Mercury, but are not depleted.

In fact, sulfur is ten times more abundant at

the surface of Mercury than it is at the Earth.

And, the ratio of the moderately volatile

element potassium to the not at all volatile element thorium.

Those are both radioactive elements that you

can measure with a gamma ray spectrometer.

And they both have somewhat similar geochemical tendencies in

the sense that they tend to be concentrated in the crust.

They tend to go into a molten phase when silicate rock partially melts.

The ratio of potassium to thorium on Mercury is about

the same as it is on the Earth and on Mars.

And is much higher than it is on the moon.

The moon.

>> But then the moon's depleted in volatiles.

>> The moon is depleted in volatiles.

Depleted in water, depleted in potassium, depleted in other alkaline metals.

And that has always been one of the arguments that the

Moon also was born by some process that involved very

high temperatures, very high energies.

Possibly the the formation of a satellite out of a debris

thrown into Earth orbit by a giant impact late in the accretion of the Earth.

And so the signatures, the high temperature processes

predicted to be there for Mercury for decades by

all the ideas that people had come up with

to explain the high density, they weren't borne out.

And so, the biggest surprise that we are wrestling with is that

Mercury has to end up as a small, rocky planet that has

got much more iron metal than any other terrestrial planet but is not

depleted in the volatile elements, like potassium and sodium and sulfur.

Compared to the bigger planets that have other kinds of volatiles as well.

And so it's causing us to rethink

how all of the inner planets were assembled how material might have

varied with not only composition but oxidation state in the inner solar system.

And how material must have been mixed as, over

different solar distances as the planets were being assembled.

So the fun part of finding

a surprise like that is it does drive all of the ideas for how the planets got

formed back to the starting blocks to come

up with explanations that can match the new observations.

>> Yeah, I mean, this is I think what makes

missions like this, you know, just, astronomical discovery so exciting.

Is you go you're surprised, right?

>> Right.

>> Our theoretical ideas are challenged.

And we have to come back and then identify what are the new

sets of observations, and we'll want to do to test these new ideas.

>> One of the nice things had been that

the different ideas that had, even the ones that

had predicted that Mercury would be depleted in volatiles made

different predictions for other chemical elements that we would see.

And that drove us to

put a lot of emphasis on geochemical remote sensing, so

that we can distinguish the ideas that were out there.

We ended up throwing all of the ideas out and we're starting over again.

But even though we were surprised we had the right tools

to make the measurements that have led us to where we are.

So we're still following the scientific method in

the sense that had some hypothesis, we put on

instruments that would test those hypotheses and distinguish between them.

And we found something that was different from all of the ideas.

>> Another to me fascinating thing was the observations of Mercury's pole.

>> Right.

>> You know, that I have always had this image

in my mind of Mercury being an incredibly hot place.

>> It is so the idea.

>> [LAUGH] It is a hot place.

On the day side of mercury we've known for almost 50 years now that Mercury

is in this very unusual Uu, resonant state between its day and its year.

And the so called Sidereal Day there are three

of those for every two revolutions of Mercury around the Sun, precisely in

that ratio of three to two.

And that resonant state means that almost every part of Mercury

sees day and night, cause prior to 50 years ago we

thought was in synchronous rotation like Earth's moon with one side

facing the Sun at all times, and one side in permanent darkness.

And thinking still that a lot of extra-solar planets

near the stars have that kind of resonance state.

But Mercury is the only object we know so far that's

in the so-called three-two resonance, where

three days precisely equals two years.

And that state stabilizes the position of the spin axis on Mercury.

So it's nearly in a constant location relative to Mercury's orbital plane.

And that means that at, because the

spin axis is almost perpendicular to the orbital plane

that not every part of Mercury's surface sees the Sun.

In particular, at the poles impact craters, which are bowl shaped

depressions surrounded by a ring of mountains are an in permanent shadow.

And so it's been known for more than 30 years that Mercury was

capable of storing water ice and other frozen volatiles in these deep freezes.

And then about 22 years ago, Earth-based radar produced the

first maps of Mercury, and showed that both polar regions

have radar bright areas, that also affect the polarization of

the radar signals, in a way that is best matched elsewhere

in the solar system by water ice.

So we had this hypothesis that goes back to

the radar results, to test with Messenger as well.

Are the areas where the polar deposits

are seen in radar, in fact in permanent shadow?

Are the temperatures in those regions cold enough to

preserve water ice over the history of the solar system?

And is

the polar deposit material in fact water ice?

Can we do some kind of chemical analysis that will verify that?

So we specifically took instruments that could address those questions.

So the very first thing we were able

to accomplish was to image the poles repeatedly over

and course of one solar day on Mercury, which

because of this funny resonance is two Mercury years.

And because of the stability of the spin axis it

doesn't change its tilt relative to the orbital plane.

Once you measure completely one solar day you've measured

a billion years worth of sunlight on Mercury.

So in six months, we had enough data to say which areas were

in permanent shadow and which were not, and how much sunlight each other

part of the surface received.

And, indeed, all of the radar bright

deposits were in areas of permanent shadows.

So that part was confirmed.

We took along an instrument called a neutron spectrometer because

one of the best absorbers of neutrons is the hydrogen atom.

And so this has been a tool of choice for looking at water ice on other planets.

There, the neutrons are

around, because every planet, particularly one

without an atmosphere like Mercury,

not much of atmosphere like Mercury or the moon, is bombarded by cosmic rays.

And cosmic rays, through a variety of interactions

with near-surface material produce neutrons as a product.

So the question is the flux of

neutrons around the surface uniform, or are there

regions that are rich in hydrogen, which of course is part of H2O or water ice.

And so if the poles are regions where neutrons are absorbed,

that is a smoking gun for a hydrogen enhancement at the poles.

Plausibly water ice. And so it took a lot of work,

because we're in a challenging orbit, we're

not in a circular orbit for thermal reasons.

We're in a highly eccentric orbit that

has a very different altitude at different latitudes.

But over the course of about a year and a half, we built up

enough data to determine the signal of the neutrons over one of the poles.

And indeed, the neutron flux was reduced.

And it was reduced by a different amount, and different energy

bands for the neutrons, in such a way that the polar deposits had to consist

of thick deposits of hydrogen rich material consistent with water ice.

But most, in most of those locations the water ice must have been buried

by something that was hydrogen poor that was maybe ten or 20 centimeters thick.

We needed that to explain over

the full energy range of the neutrons that we recorded.

And then we had one more key experiment that did two things for us.

It was a laser altimeter.

The laser altimeter we flew in order to

determine the topography of Mercury, the shape of Mercury.

And indeed the topography of the poles was

important, but I'll come to that in a second.

The laser is also a reflectometer,

because at one wavelength precisely tuned by the laser,

in our case a near infrared wavelength a little

over a thousand nanometers we could determine the reflectance of the surface

by the amount of reflected power from the laser beam.

And everywhere, almost, everywhere we looked at

the polar deposits, the deposits were dark.

Mercury itself is a pretty dark planet. It's darker than the moon by about 15%.

And, it's darker than the lunar near side by 15% which

has some very dark lunar maria in addition to the bright uplands.

And Mercury's darker than that average lunar near side.

But these polar deposits were darker still.

By at least a factor of two. Darker than the rest of Mercury.

So, it couldn't just be Mercury's soil. There had to be some material

that was different from the average for Mercury.

And then, in a few locations at the highest latitudes,

near the north pole, the polar deposits were very bright.

Three to four times brighter than the average for mercury.

And here's where the topography came in.

Because we had investigators on our team who could use the topography, and the

models for sunlight, to build very precise models for what the

temperature was as the surface at any one time during a solar day.

And what the temperature was at any depth in the near surface.

A few centimeters, a few tens of centimeters below the surface.

And they could address the question of how the

maximum temperature at the surface, or say 20 centimeters down,

compared with the temperature at which ice would

no longer stay frozen over geologically long periods of time.

And they showed that in most of the areas of the radar

bright polar deposits, water ice would not be stable right at the surface.

But it would be stable about 20 centimeters, 30 centimeters down.

But in a few parts of the planet, the surface temperatures were so

cold, even throughout a solar day, that the ice could be stable indefinitely.

Those were the areas where the laser

reflectance was very high, higher than average.

In the areas where the water ice was not predicted to be stable

were the areas that the laser reflectance showed dark material.

So that raised the question of what's the dark material?

The bright material looks to be water ice. It has the right reflectance.

It shows up in the areas that are cold enough throughout the

Mercury day to always be in the stability field of solid water ice.

The dark material had to be volatile, because it

was present only in permanently shadowed regions that were cold.

But the temperatures at which that material was

stable extended to higher temperatures than water ice.

But it was extremely darker than the darkest planet, Mercury.

And as

dark as some of the darkest objects in the outer solar system.

And so our group has proposed we need to find ways

to test this idea, that the really dark material is organic rich

compounds of the sort, that are common in the

outer solar system, that are very dark, spectrally, somewhat red.

That coat the surfaces of small objects in solar orbit

asteroids and some of the small moons of the giant satellites.

Some of the so called Trojans that are captured by the giant satellites.

And that is solid only in the cold

outer reaches of the solar system, but that

is common material in comets and volatile rich asteroids.

And so, the scenario that we envision is one in

which the water ice on Mercury is delivered from outside.

By the

the encounter of Mercury with comets and some volatile rich asteroids

some of which impact the surface, some of which then vaporize.

And then some small fraction, a few percent of

the volatile material, bounces around in Mercury's gravitational field,

finds its way into the colder, polar cold traps,

and is trapped, is frozen in place.

And one particularly large comet might

have been responsible for everything we see.

Both the water ice and the volatile the the the carbon rich volatile could have

been delivered by the same object, a dirty

snowball to use Whipple's term for comets.

And then over time, the water ice would have retreated from the surface

in the areas that were a bit warmer.

Leaving the dark material behind, and whereas

the water ice would have stayed stable

at the surface in the coldest areas, matching what we see with the reflectance.

So this combination of measurements told us many things.

It told us that the cold trapping idea for water ice worked.

In terms of permanent shadow, in terms of thermal models.

It told

us that the neutron spectrometer results were entirely consistent with the

hypothesis that the radar deposit radar bright deposits were dominantly water ice.

But that there was a dark component, and that dark

component was also volatile and had properties best matched by organics.

So here you have a planet nearest the Sun. The daytime surface

temperature under, say, at the equator

is 450 degrees centigrade 800 degrees Fahrenheit.

Where no volatiles would have any stability whatsoever even over

the course of just seconds on those kinds of temperature scales.

But this planet, that has such

high temperatures nonetheless has these freezers,

some natural freezers in the polar cold traps.

These impact craters at the north and south

pole remain sufficiently cold that they can, they can

trap water ice and other ices and keep them

around for hundreds of millions or billions of years

despite being on the object closer to the Sun than

any other solid object in the solar system.

>> Given that the bombardment was so much higher in the past, it seems

plausible that ice has been there for four and a half billion years.

>> There's certainly a plausibility argument for a

contribution to ice from four and a half billion years.

But the the argument that the ice we're

seeing is more recent, is really based on its cleanliness.

Anything that was around for four and a half billion years has had many

opportunities to be remixed with the Mercury

soil, through multiple impacts large and small.

Because Mercury's constantly being bombarded by dust particles, by very small

micrometeorites that work the soil, that

mix it horizontally and especially vertically.

And so it's much more likely that the ice we're seeing, which has a

remarkably pure signature to the radar, which

is penetrating tens of centimeters into the surface.

and, and the the, the radar characteristics

are matched by those of icy satellites.

And the polar deposits of Mars, which are much

purer ices than one would expect for something that's sitting around

on Mercury's surface for even a few billion years.

So there have been some at least

back of the envelope calculations, that the ice deposits on

Mercury might be as young as a few tens of

millions of years old, in order not to be mixed

with soil, and look more like dirty ice than it does.

>> Huh, it's fascinating. Thank you so much.

This was terrific.

I know I've learned a lot. >> My pleasure.

>> And I hope the class did as well.

>> Well, good luck on your march out into

the solar system and to other planetary systems as well.

>> Thanks. >> Okay.

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