Title: The great solar system revision.   
Subject(s): PLANETS -- Exploration; ASTRONOMY -- History
Source: Astronomy, Aug98, Vol. 26 Issue 8, p40, 6p, 8c
Author(s): Hartmann, William K.
Abstract: Focuses on the revolution of planetary exploration.
Historical background of astronomy; How the astronomical findings of
scientists developed; How moon's discovery lead to better
understanding of the universe; Details on the discovery of the
planetary system.
AN: 781897
ISSN: 0091-6358
Note: Tucson-Pima Public Library subscribes to this magazine.
Database: MasterFILE Elite

THE GREAT SOLAR SYSTEM REVISION

The past 25 years have changed our view of the solar system forever.

The first 25 years of Astronomy magazine coincided with a unique
quarter-century in the history of humanity: In that period, our system
of planets went from unknown to known. Every field of astronomy made
astonishing progress, but planetary exploration experienced the most
profound revolution of all.

This is a strong statement, but a few moments of reflection help us
see how true it is. Other fields of astronomy exhibited a continuous
stream of progress throughout the whole century. Astronomers began to
understand stellar evolution, stellar and galactic distances, and the
Big Bang -- all in the first two-thirds of the century. The last 25
years have significantly added to those areas of knowledge, but
objects outside the solar system remained astronomical objects.

Within the solar system, however, many objects have been transformed
in less than a generation from purely astronomical objects to
geological places. Since 1973, we have become the first generation to
map the surfaces of planets and moons. We landed on Venus and Mars and
began to understand planetary atmospheres and rock mineralogies. We
also became the first to see asteroids and a comet close up, the first
to see the rings of Jupiter, Uranus, and Neptune, and the first to
examine Saturn's rings in detail. Ours will be the only generation in
history to make these discoveries for the first time.

Variety Is the Spice of Life 

When Astronomy was born in 1973, the Apollo moon landings had just
ended, earlier than originally planned. Humanity's space exploration
goals shifted to the planets.

How rudimentary our knowledge of planets, moons, asteroids, and comets
was at that time! We had no surface photos from any planets, and only
Venus and Mars had been imaged from close range. We were still
uncertain about surface conditions on Mars (some scientists thought we
might find primitive plant life there). Only Saturn had rings. Dozens
of small moons remained to be discovered. Pluto was the only known
body of its type.

Most planets were virtual blank slates. Visual observers published
sketches of vague shady markings on Mercury, but the sketches did not
agree very well. The best telescopes showed the four big moons of
Jupiter as fuzzy pinheads. The situation was even worse for the other
giant planet satellites. Except for Saturn's moon Titan, which was
known to have an atmosphere containing methane gas, the moons were
assumed to be uniformly bland, cratered iceballs. Because these moons
were cold and small, it was confidently felt that they would all be
geologically dead.

The situation began to improve in 1975 when the Soviet Venera 9 probe
returned the first surface photos from Venus. Data from Venera 9 and
subsequent Veneras showed that Venus's surface seemed to be covered by
basaltic volcanic lavas. Similarly, the two American Viking landers
returned the first surface photos of Mars in 1976. Instead of
primitive organisms, only sterile surface soil was detected at both
sites (although the possibility remains that microbial martians eke
out an existence underground). Chemical analysis indicated a generally
basaltic composition for the martian soils too, a result supported by
the Mars Pathfinder lander in 1997.

Comparing these results to Earth, scientists noted that although the
continents are granitic rock (more silicate-rich than basalts),
basaltic lavas are the main constituent of Earth's crust, especially
sea-floor crust. The spacecraft results thus led to a new appreciation
that basaltic volcanism is the leading internal shaper of planetary
surfaces in the inner solar system, paralleling the finding that
impact cratering is the leading external shaper.

Probably the most dramatic changes in our perceptions of the solar
system came when the two Voyager probes reached Jupiter in 1979.
Instead of the expected indistinguishable dead iceballs, the Voyagers
astounded everyone by showing that the four large moons had distinct
personalities. Callisto had an old, cratered surface, as expected.
Ganymede also had old, cratered regions, but they were broken by
swaths of bright, fractured ice. Europa turned out to have a very
young crust of fractured ice, apparently overlying an ocean of liquid
water that could conceivably support life.

Io was the biggest surprise, with nearly a dozen actively erupting
volcanoes and a colorful surface of sulfur compounds. Just days before
Voyager 1 arrived and discovered Io's volcanoes, California
geophysicists Stanton Peale, Patrick Cassen, and Ray Reynolds
predicted in Science there might be volcanic activity on Io. Peale and
colleagues examined how tidal forces arising from the gravitational
influence of Jupiter and the nearby satellites would flex Io, heating
the moon's interior and leading to active volcanism. The power of
science lies in its predictive ability; no other system of human
thought, from classical philosophy to occult mysticism, could have
predicted this.

Even after the revelations at Jupiter, most scientists still thought
that dormant iceballs would prevail among the smaller and colder
satellites of the more distant planets. But Voyager photos from 1981
to 1989 revealed that several of Saturn's icy moons have swaths of
fractures, Uranus's moon Miranda has a surface completely twisted and
deformed by systems of fractures and faults, and, most surprising of
all, Neptune's moon Triton has volcanic vents that lift columns of
black smoke high into its thin atmosphere. These discoveries suggest
that tidal heating is even more important than had been thought. The
bottom line? There is much greater variety among the moons and planets
than we had anticipated.

Two Sides of the Same Coin 

In 1973, our understanding of asteroids and comets was, in some ways,
even more Neolithic than our understanding of planets and moons.
Asteroids and comets were treated as two unrelated phenomena. In
telescopes, asteroids looked like stars (hence the name aster-oid).
Comets looked fuzzy and gave off gas that must have come from ices.
Comets thus came to be studied by astronomers trained in gas
spectroscopy, and virtually nobody talked about the surface or
geologic properties of a solid comet itself. If you went to a
planetary science meeting, you would find asteroids discussed in one
session by scientists interested in rocks and minerals and comets
discussed in another session by scientists interested in gas spectra,
dust emission, and tail structure.

On closer inspection, asteroids and comets turned out to be two sides
of the same coin. The seeds of the new view were sown in the
mid-1970s, when researchers such as Tom McCord, Clark Chapman, and
David Tholen began to show that asteroids were dramatically divided
into groups based on color, reflectivity, and spectral properties. In
1977, Charles Kowal discovered a 500-mile-wide (300 km) "asteroid"
between Saturn and Uranus named Chiron. As with other outer solar
system asteroids, it was black. But in 1988, it turned into a comet,
brightening and sprouting gas and a cloud of dust. Reverse cases were
also observed, in which objects initially cataloged as comets had
become indistinguishable from asteroids.

Astronomers were also finding that comet nuclei were not the
dirt-speckled white icebergs that had been visualized, but black
objects whose colors matched those of asteroids in the outer solar
system. In 1986, a consortium of European countries flew the Giotto
probe close enough to Comet Halley to show its potato-shaped structure
and jets of gas streaming from discrete vents on the surface. Comets
at last were thought about as solid worldlets.

The seeming distinction between these classes broke down. Instead of
thinking of two distinct classes, it's more helpful to think of the
original ancient planetesimals that aggregated into the planets. The
ones closer to the sun were rocky, and contained metal, which in some
cases sank to the center to make iron-nickel cores. The ones farther
from the sun were dominated by lower temperature materials, ices, and
black, carbonaceous material. When these icy bodies pass close enough
to the sun, the ice sublimes into gas, creating a head and tail of gas
and dust. But when they are far from the sun and inactive, they look
merely like black asteroids.

In the 1990s, spacecraft passed close enough to three asteroids to
photograph them. As expected, they turned out to be cratered,
potato-shaped objects. One out of the three, Ida, has a tiny satellite
of its own, confirming earlier speculations that some asteroids might
have satellites. The 1997 NEAR flyby of Mathilde showed an asteroid
with a surprisingly low density, indicating that many asteroids'
internal structures were highly fractured by collisions.

Also in the 1990s, Jane Luu, David Jewitt, Anita Cochran, and others
mapped out two new populations in the outer solar system. Chiron
turned out to be just the first of a number of bodies that cross among
the orbits of the giant planets. They were named Centaurs, after the
half human, half horse beasts of Greek mythology. Beyond the Centaurs,
a larger reservoir of objects exists in the general realm of Pluto and
beyond. These comprise the Kuiper belt, named after Dutch-American
astronomer Gerard Kuiper, who speculated about its possible existence
a generation ago. There is a pattern here: Just as Earth-approaching
asteroids are a small group that "leak" into the inner solar system
from the asteroid belt, the Centaurs are a small group that leak into
the giant planet region from the Kuiper belt.

When Pluto was discovered in 1930 there was little hesitation in
celebrating it as a new planet. Today, however, Pluto seems more
properly classified as merely the biggest known object in the Kuiper
belt. Pluto is smaller than our own moon, its orbit is more eccentric
and inclined than any other planetary orbit, and its path crosses
Neptune's. Some have argued that Pluto's relatively large satellite
Charon elevates it to planet status, yet now we know that some
asteroids have satellites. In the new view, Pluto is to the Kuiper
belt as Ceres is to the asteroid belt (Ceres being about twice as
large as the next biggest asteroid).

Chaos and Catastrophe 

The discovery of these different populations of small objects has in
turn led to the growing awareness that planetary rotational and
orbital properties involve a mixture between many small collisions
(which produce regularity, circular orbits, and uniform properties),
and rare, large-scale collisions or near-misses (which produce
irregularity, non-circular orbits, and distinctive properties).

In the 1960s, Soviet researchers, especially Victor Safronov, were at
the forefront of showing how planetesimals collided and aggregated
into planets. Safronov suggested that the aggregation of countless
small bodies would tend to give planets fairly uniform properties,
such as west-to-east spin directions, but that a few large collisions
had left a few planetesimals with distinctive "personalities."
Safronov suggested, for example, that a large impact had given Uranus
its peculiar sideways axial tilt.

In 1974-5, Donald R. Davis and I extended Safronov's ideas by
suggesting that the moon formed when a giant impact blasted mantle
material out of Earth. This would explain the resemblance between
chemical properties of the moon and Earth's mantle. We showed that
bodies big enough to do the job could have grown in Earth's zone as
Earth grew. Still, this scenario was considered too catastrophic to
satisfy most scientists in the 1970s, who felt that planet-shaping
processes had to be slow and gradual. The giant impact theory of lunar
origin was not accepted as the leading explanation until 1984.

Also in the early 1980s, compelling evidence emerged that the
dinosaurs were wiped out by the impact of a six-mile-wide (10 km)
asteroid 65 million years ago. Further studies suggested that other
changes in the biological and geological records were also linked to
impacts. The idea of random cosmic impact disasters as a major agent
of biological change was a radical departure from classic Darwinism,
in which biological change was forced by mutation and competition
within Earth's ecosystem. Philosophically, then, this quarter-century
has seen a shift from gradualism to a view that while planetary
evolution is generally gradual, it is also affected by catastrophic
cosmic accidents.

The realization of the significant role of random impacts has also led
to a wide interest in "chaos theory." The orbits of small bodies, like
Chiron, can evolve chaotically. This means that dynamicists can't
calculate their orbits forward or backward for long stretches of time,
because these bodies experience close approaches with planets. In
these encounters, tiny shifts in position can lead to whopping changes
millions of years later. The long-term future of the solar system is
not as predictable as we thought it was 25 years ago.

All these issues lead to questions about the origin and long term
history of planetary systems. Did other planetary systems form like
ours? How common are circular, stable orbits where a planet can stay
in a liquid-water zone all year? How common are planetary systems
where liquid water exists and where life can form and survive? How
common are Earthlike planets throughout the universe?

The Universe Beyond 

In the 1990s planetary science has made the giant leap out of the
solar system with the discoveries of planet-sized bodies orbiting
other stars. Our planetary system is not a unique accident in the
universe. In the next 25 years, we will see increasing emphasis on
comparisons between our own solar system and other planetary systems.
In fact, a new question has replaced the old one: Is it possible that
our system has had a unique history that produced circular, stable
planetary orbits?

Theorists had confidently predicted that planetary systems would
display circular orbits with Jupiter-sized giants at large distances.
Yet it seems clear that other systems don't always follow these rules.
Many of the newly-discovered planets have elliptical orbits and most
of these Jupiter-sized planets lie very close to their host stars.
These findings have led to reexamination of orbit evolution. A new
idea, discussed by dynamicist Stu Weidenschilling and others, is that
two, three, or more Jupiter-sized bodies may have aggregated in the
same region. Instead of the largest one outpacing the growth of the
others and sweeping them up, the second and third largest might have
become very big in some cases and then experienced close encounters
that might have thrown them into elliptical orbits. In such an
encounter, one giant planet might have been left at, say five AU, a
second would have been thrown into an orbit very near the star, and a
third planet would be ejected from its system altogether.

These questions remind us that planetary science, perhaps more than
any other field of astronomy, is really about us and our own role in
the universe. The question of microbial life on Mars or in the seas of
Europa, the rate of species-threatening impacts on planets, and the
quest for planetary systems around other stars: All these topics
reflect back to questions about whether we are unique in the universe.

If you were born on an island and saw other islands on the horizon,
you would yearn to go there and find out how they compared to your
own. In the first 25 years of Astronomy, the first voyages have been
taken to the nearby islands, and the blank maps of other worlds have
begun to be filled in.

PHOTO (COLOR): Scientists were stunned by Voyager 1's 1979 discovery
of frenetic volcanic activity on Jupiter's large moon Io.

PHOTO (COLOR): In 1976, a dream became reality when NASA's twin Viking
landers returned pictures from the ruddy, sandy plains of Mars.

PHOTO (COLOR): Neptune's satellite Triton has active geyser-like vents
that shoot columns of smoke into its thin air -- giving rise to the
black streaks seen in this Voyager 2 image.

PHOTO (COLOR): In the 1970s and 80s, four Soviet Venera landers
photographed the surface of Venus for about an hour each before
succumbing to the hellish atmospheric conditions.

PHOTO (COLOR): Uranus's small moon Miranda is wracked with fractures
and ...

Uranus's small moon Miranda is wracked with fractures and faults,
possibly a result of a giant impact that tore the moon asunder.

PHOTO (COLOR): En route to Jupiter, Galileo discovered that the
35-mile-long ...

En route to Jupiter, Galileo discovered that the 35-mile-long asteroid
Ida has a one-mile-wide satellite, later named Dactyl.

PHOTO (COLOR): Giotto imaged potato-shaped Comet Halley in 1986
--humanity's first ...

Giotto imaged potato-shaped Comet Halley in 1986 --humanity's first
close-up look at a comet nucleus.

PHOTO (COLOR): The discovery of 1992 QB[sub 1] opened the door to a
new frontier beyond Pluto, the Kuiper belt.

~~~~~~~~

by William K. Hartmann

Planetary scientist William K. Hartmann recently published the novel
Mars Underground and was awarded the first Carl Sagan Medal by the
American Astronomical Society, for communication of planetary science
to the public. His articles and artwork have appeared in Astronomy
almost since its inception.
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Source: Astronomy, Aug98, Vol. 26 Issue 8, p40, 6p, 8c.
Item Number: 781897
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