there is a small planet so near the Sun
that it is scarcely visited in the
bright glare of our star like an elusive
ghost it is best observed from Earth in
the Twilight hours when the Sun is gone
from the sky
the ancient Romans gave it its name so
little was known about the planet but
scientists could only guess at its
origin and evolution like the planet
Mars it could have had violent volcanoes
deep canyons scarred by violent winds
more like our own moon its surface might
be pitted with craters a record of
ancient events written on a planet's
surface the best photographs of mercury
added little to our knowledge but when
man learned to travel in space the
electronic cameras of a spacecraft
called Mariner 10 revealed the planet in
startling detail
in June of 1975 scientists from five
nations assembled at the California
Institute of Technology to exchange the
information they had gained from the
exploration of the planet Mercury by the
spacecraft Mariner 10 included in the
group for scientists representing many
techniques of exploration including the
detection of various forms of radiation
a study of gases in a planet's
atmosphere and the powerful scientific
tool photography of the surface of a
planet
welcome to Pasadena we are convened the
photographic exploration of mercury was
led by dr. Bruce Murray of the
California Institute of Technology the
objective I have is to present a
relatively simple interpretation of
history of mercury and to draw some
conclusions about the other terrestrial
planets the work that dr. Murray and his
colleagues are engrossed in goes beyond
the exploration of a single planet
mercury they are seeking insights into
the history of our entire inner solar
system the search for new information
began with the launch of Mary Murphy
these photographs appear were taken to
test the cameras that were to photograph
both venus and mercury for the first
time after leaving the gravitational
field of Earth Mariner 10 began its long
voyage to mercury the spacecraft's
flight path was carefully chosen to skim
past the planet orbiting between Earth
and Mercury cloud-covered Venus the
Mariner 10 cameras at close range
were to show us a startling new Venus a
Venus we couldn't see from Earth Venus
has been a real frustration to
photograph from the ground because in
the visible it's perfectly featureless
there is occasionally a little bit of
indentation along the Terminator and
occasionally one can see a little
structure around the limb but basically
appears from the earth to be obscured by
an incredible haze and the ultraviolet
however that's the wavelengths beyond
the blue that the eye cannot see but
photographic film can respond to very
faint markings have been observed and it
was those faint markings in the
ultraviolet that persuaded us to make a
maximum effort to photograph Venus in
the ultraviolet up close
the Mariner 10 cameras photographed
Venus in high-resolution sections
each photograph covering only a small
portion of the planet the frames were
then pieced together into mosaics to
give a detailed picture of the entire
planet the results of the Mariner 10
photography in the ultraviolet are
really good in fact much much better
than we had hoped because as you can see
looking at first a picture of Venus in
the visible that was taken by Mariner 10
and then when we switched to one that
was taken in the ultraviolet and has
been enhanced for the computer to bring
up the details there's a tremendous
pattern of organization of markings
let's take a look at what's in these
pictures the most prominent thing that
one can see is a very bright south polar
cap probably a great vortex almost like
a huge cyclone and there are dark
streaks that come into this as though
they're winding up like taffy so the
whole series of mosaics is like a series
of snapshots of Venus in time and from
those mosaics it was possible to create
a time-lapse movie of the rotation of
the Venus atmosphere during the period
of time mirror ten flew by and flew away
from the front so the pattern of
rotation of features on Venus and the
upper atmosphere way up high by our
standards is remarkable in that the
atmosphere on Venus rotates faster than
the surface in the case of the earth the
atmosphere tends to drag behind a little
bit as one might imagine case of Venus
the atmosphere rotates all the way
around the planet once every four days
the surface rotates once every 243 days
so the atmosphere is rotating almost 60
times as fast at the top anyway as a
planet itself
the gravitational field of Venus was
used to bend the flight path of Mariner
10 inwards towards mercury then the
gravity of the Sun pulled Mariner
towards its first encounter with the
planet
in March of 1974 Mariner 10 flew past
mercury during the next year it would
perform two more flyby Mariner 10
returned almost 3,000 pictures of the
surface of mercury these pictures reveal
craters and other features which were
formed very early in the history of
mercury and may refer to events that
affected the entire inner solar system
Mars Earth Moon and Venus as well as
mercury the pictures of mercury revealed
craters and many other features and to
the geologist these features and these
pictures are like the pages of a history
book and the entry book for mercury may
go back almost to the formation of the
solar system itself
five million years ago a vast cloud of
gas and dust floated through the galaxy
we call the Milky Way the ghostly
remains of a great star that had died in
a gigantic explosion the gas and dust is
twisted and shaped by magnetic forces
electrical currents and the subtle pull
of gravity as it swirls around a
thickening core the huge cloud gradually
flattens into a disk dust and gas
gravitates slowly inwards and Eddie's
begin to form in the cloud
particles of matter clustering two solid
bodies
the larger bodies continue to grow
sweeping up particles and dust as they
orbit the condensing core of the disk
and begin to heat under the increasing
gravitational pressures in the central
core of the disk incredible heat is
being generated hotter and hotter until
it reaches critical temperatures and
then billions of years ago a nuclear
reaction occurred and our Sun was
ignited
it's intense radiation repels the
surrounding gas and dust the increasing
heat of the inner planets has formed
them into molten spheres mercury like
the other inner planets is continuously
bombarded by debris that form craters on
its surface in the final stages of its
formation mercury flowed from internal
heat hot lava was forced to a surface
being torn by collisions with masses of
rock that were shaping the planet the
same heat that triggered the lava flows
also melted rock and metals in the
interior of Mercury heavy iron
concentrated at Mercury's center to form
a dense core which was overlain by a
thin shell of lighter material brought
to the surface by lava flows again the
surface was gouged and pitted by a great
series of impacts leaving huge craters
on the surface the Mariner 10
photographs have revealed these early
craters to scientists for the first time
one of these craters was formed by a
massive rock perhaps a small planet that
crashed into mercury in the Mariner 10
photographs half of this huge crater was
hidden on the night side of the planet
today this huge crater is called Caloris
the impact spewed millions of tons of
debris across the surface of mercury
creating a ring of mountain ranges over
a mile high the floor of this basement
was split by great surface cracks wide
and deep as the surface continued to
form deep inside mercury it's heavy iron
core began to contract the surface
buckled cracked and great sections
hundreds of miles wide were split open
leaving immense mile-high walls of rock
called scarps again there was a great
flow of lava covering many of the large
craters and leaving smooth flat plains
this set the stage for the final episode
in the history of mercury a period of
light cratering of the Great Plains
in this time mercury died the internal
heat that triggered many of the events
in the planet's history turned off and
for three and a half billion years
mercury has remained as we see it today
before Mariner 10 there were a multitude
of possible Mercury's now there's only
one the mercury of craters and basins
and planes this illustrates how
photography can be an exploratory tool
it displays features which were perhaps
never even anticipated and and sometimes
could not even be imagined before the
experiment was flown in a sense it
provides answers to questions that were
not even asked this was true at Mars
when Mariner 4 returned pictures of what
appeared to be a a moon like surface it
had been expected that Mars might be
very much like the earth the folded
mountain ranges and many other
earth-like features instead it looked
like the moon and that was a big shock
and it was even bigger shock when
Mariner 9 in 1971 visited Mars again and
found huge volcanoes and even some
peculiar sinuous channels that looked
like they might have been formed by
flowing water at some point in Mars
history now Viking has transported man's
vision to the actual surface the fantasy
of science fiction has been replaced by
the strange lonely reality of the
Martian landscape when Mariner 10 flew
by Venus it discovered an unexpectedly
well-organized set of cloud motions and
the unexpected was certainly encountered
in case of mercury which turned out to
exhibit not only moonlight features on
its surface but a sequence of events
there that seemed to be very similar to
those of the moon and that's
extraordinary
because mercury is very different inside
compared to the moon and that means it
must have had a different history of
heating and modification the surface
from the inside and furthermore it lives
in a very different part of the solar
system much closer to the Sun and
therefore it must have experienced a
for exterior history as well and yet a
picture show that we're Creek has a
surface history very similar to that of
the moon and to me that's extraordinary
a comparison of the craters on the moon
Mercury and Mars shows that these inner
planets were subjected to cratering at
the same time before Mariner 10 it was
believed that the source of cratering of
the inner solar system was the asteroid
belt orbiting beyond Mars because of the
difference in distance from the belt it
was assumed that no two planets would be
cratered the same those planets farther
from the asteroid belt like mercury
would have fewer craters but a careful
count of the craters on mercury and a
comparison with those on the Moon and
Mars showed them all to be roughly equal
if the crater count is equal then the
source could not be the asteroid belt
but must have come from elsewhere in the
solar system
one strong possibility is the planet
Jupiter with a gravitational attraction
second only to the Sun it is possible
that on two occasions Jupiter literally
hurled millions of tons of rock inwards
towards the Sun to impact the planets of
the inner solar system including mercury
this cratered record on Mercury was read
three times because of a fortunate
coincidence Mercury's orbit around the
Sun is 88 Earth days Mariner tens orbit
was twice as long or a hundred
seventy-six days these synchronous
orbits allowed Mariner 10 to fly past
mercury a second and third time
the path of the first flyby was on the
dark side of mercury the second
encounter sent the spacecraft past the
light side of the planet specifically
for photography after another six-month
journey around the Sun Mariner 10 made
its third and final encounter to look
again at a major discovery made during
the first flyby the presence of a
magnetic field around mercury similar to
earth a bow shock wave blocks the solar
wind from the Sun and deflects it around
the planet creating an environment
similar to the magnetic field around
Earth this discovery was a surprise
because mercury slow rotation about once
every 57 Earth days had led scientists
to believe that the planet could not
generate a magnetic field it is believed
that Earth's magnetic field is generated
by an interaction between the planets
faster rotation and its molten core
mariner tens discovery may change
theories on how these fields are formed
and has given scientists their first
opportunity to compare two magnetic
fields in the inner solar system there
were other measurements made at Mercury
by Mariner 10 infrared showed that
mercury surface ranges in temperature
from 700 degrees Fahrenheit to 300
degrees below zero the widest
temperature range of any planet the
ultraviolet experiment revealed that
mercury has a very thin atmosphere of
helium sensors measured the invisible
cosmic rays which flood our solar system
at tremendous speeds penetrate any
surface and are unaffected by
gravitational forces their source is
unknown exploration is an adaptive
process each new piece of information
adds the value of others that were
accumulated earlier we had the results
of the Apollo program on the moon to
help us understand the timescales and a
some extent the processes we were seeing
when we looked at Mars than Mariner 9
now that Mariner 10 has looked at
mercury we can begin to compare it with
the Moon and Mars and begin to realize
that there is a common solar system
history which has been recorded on these
planets whose early services have not
been erased by erosion and other
atmospheric processes that same common
history affected the earth too and so by
looking at these services reading those
records we're in fact looking back into
the Earth's history into a heretofore
unexplored domain of time our own
history on the earth now we can begin to
compare and contrast to look for
similarities and differences and try to
recognize our family relationships among
the terrestrial planets are we cousins
or brothers or all of us bizarre
strangers it happened inhabit the same
portion of the solar system that's the
task of comparative planetology
that's where we go from here
in future planetary flights Venus will
be explored once again
the probes of one pioneer spacecraft
will penetrate its thick atmosphere
while a second pioneer orbits the planet
- Mariner class spacecraft will explore
the planet Jupiter and then fly upward
to reach Saturn one of the Mariners may
be sent on to the planet Uranus over a
billion and a half miles from the Sun
and someday in our future we will send
intelligent machines robots to explore
the hostile surface of Mars and perhaps
even the ancient moons of Jupiter and
Saturn