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Interview with Sean Solomon
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Imagining Other Earths
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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.
与讲师见面
David SpergelCharles Young Professor of Astronomy on the Class of 1897 Foundation and Chair
Department of Astrophysics
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.
