快猫短视频

Over the past 25 years, spacecraft have visited all the planets of the
Solar System except for tiny Pluto. Even more stunning than the images we have
of these worlds is what we now know of their science

ONLY 30 years ago, astronomers knew little about the other planets, the
Earth鈥檚 neighbours in space. So far as they were concerned, nine planets
orbited the Sun; the four closest to the Sun were small, the next four much
larger, and Pluto 鈥� at the edge of the Solar System 鈥� another small planet.

Even with today鈥檚 telescopes, astronomers on the Earth can tell relatively
little about the other planets. For a start, the other planets are a very long
way off; even the nearest, Venus, never comes closer than 38 million
kilometres. When the light from one of the planets penetrates our atmosphere,
moving currents of air distort our view.

In the late 1950s, engineers developed rockets powerful enough to take
spacecraft well beyond the Earth and Moon, and off to the other planets. Space
probes from Earth have now flown past all the planets except Pluto, and have
also photographed a comet at close quarters. Some craft have landed on Mars
and Venus.

Even before the advent of space probes, astronomers had deduced some idea
of what the planets were made of. They could see through their telescopes how
big a planet was. They could work out its mass 鈥� from the pull of its gravity
on moons orbiting it, or, less accurately, from its pull on other planets.
From such information, the astronomers could calculate its density.

The densities of Mercury, Venus and Mars are very similar to the Earth鈥檚,
so they too must be made largely of rock (possibly with a core of iron).
Astronomers named these planets the 鈥渞ocky dwarfs鈥�.

The next four planets are obviously different; they are all very much
larger than the Earth, and their densities are much lower 鈥� not much more than
that of water. Astronomers realised that the low density meant that these
planets consist mainly of gases or liquids; so they called them the 鈥済as
驳颈补苍迟蝉鈥�.

At the edge of the Solar System is tiny Pluto. Because no spacecraft has yet
visited it, we still know very little about it, but astronomers believe that
it is a small solid planet, made of a mixture of rock and ice.

Patterns of weather

Out of this world

AS we all know, predicting the weather is a thoroughly difficult business.
This is because the Earth鈥檚 atmosphere behaves in a complicated way, even
though the basic processes that drive the weather are simple. The Sun heats
the equator more than the poles, so the air at the equator rises, moves north
or south, and comes down at the poles, only to travel back to the equator at a
lower level. The Earth鈥檚 rapid rotation twists these wind flows around, so
there are also east-west and west-east winds at different latitudes.

By studying other planets, and in particular Venus and Mars, meteorologists
have been able to test their theories, and improve them. Venus, for example,
rotates slowly 鈥� once in 243 days 鈥� but winds at the top of the atmosphere
move extremely fast, up to 300 kilometres per hour.

On Mars, regions of high and low pressure (anticyclones and depressions)
exist that are similar to those that we all experience from time to time on
Earth. But on Mars they only occur in one hemisphere at a time, in the half of
the planet that is then experiencing winter. (The seasons there last almost
twice as long as they do on Earth.)

The weather on the gas giants is surprisingly similar to ours;
surprisingly, because our weather occurs only in a thin layer of gas over a
solid surface, whereas the gas giants consist of fluid (gas or liquid)
throughout. Meteorologists believe that these giants have weather similar to
that on Earth because the same processes operate on them. The Sun鈥檚 heat (and
heat from within, in the case of Jupiter, Saturn and Neptune) causes small
swirling eddies of gas to rise. These eddies give their energy to winds that
blow in various directions at different latitudes.

What has surprised scientists is that this process is much more efficient
on the gas giants. In our atmosphere, eddies convert one-thousandth of the
total energy in the atmosphere into high-speed winds, known as jet streams.
Jupiter鈥檚 eddies turn one-tenth of the energy in the atmosphere into winds.
Neptune seems to be even more efficient.

Magnetic fields abound

Age old faces

THE gas giants can tell us a lot about how the planets create magnetic
fields. Of the four dwarf planets, the Earth is the only one with a strong
magnetic field; this it generates by electric currents deep within its
molten iron-nickel core (see Inside Science No 26). Both Jupiter and Saturn
have magnetic fields stronger than the Earth鈥檚. These rapidly spinning giant
worlds consist largely of hydrogen, but it is under such pressure that it
behaves rather like a liquid metal, and conducts electricity. So, although
these planets have hydrogen 鈥渕etal鈥� rather than iron at their cores, they seem
to confirm the basic theory of the Earth鈥檚 magnetism.

The next two planets, Uranus and Neptune, both produce a magnetic field,
but not in their cores. On Neptune, the magnetic field is generated within a
region four-fifths of the way from its centre to the surface. In both giants,
the magnetic poles are not near the planet鈥檚 real poles (the ends of its axis
of rotation), but lie half way to the equator. Astronomers think that here the
region of electrically conducting fluid lies above a large, insulating core.
This idea ties in with theories that Uranus and Neptune have cores made of
rock, surrounded by a thick layer of water. Water that contains impurities is
a good conductor of electricity, so on these planets the magnetic field
originates in a deep ocean of warm water.

Since the birth of the Solar System, the Earth has altered because
geological processes have changed its surface. Our Moon and Mercury are two
worlds where there has never been much geological activity. Their surfaces are
good museums of the way that all the rocky planets must have looked soon after
their birth. Both the Moon and Mercury are pocked with craters; giant pits
blasted out by the impact of solid bodies falling in from space.

The manned Apollo flights to the Moon brought back a third of a tonne of
Moon-rocks. When scientists had measured the ages of these rocks, they were
able to say that most of the Moon鈥檚 surface has remained unchanged for almost
4 billion years, so it is indeed a museum of the early days of the Solar
System.

Just as important, the Apollo results showed that scientists could tell the
age of a region of the Moon from the number of craters on it. A once pock-
marked area may have disappeared beneath lava flows, and show only few craters
as there are now very few solid bodies whizzing around in space to crash into
it. By counting craters, astronomers can tell, roughly at least, the ages of
portions of the other planets.

The first spacecraft to Mars disappointed astronomers who had hoped to find
a planet similar to the Earth, perhaps one that sustained life. The pictures
from Mariner 4 in 1965 and Mariners 6 and 7 in 1969 showed a planet covered in
craters almost as thoroughly as the Moon.

We now know that we were unlucky with our first close-up views of Mars. In
1971, Mariner 9 went into orbit around the planet, and found that previous
spacecraft had shown only half the picture 鈥� quite literally. Whereas half of
Mars consists of a heavily cratered surface, the rest is entirely different.
Here, we find vast volcanoes, and canyons 3000 metres deep, both larger than
anything on the Earth, and signs that water once flowed on the now-frozen
surface. These results 鈥� combined with those of the Viking 1 and 2 in 1976 and
the Soviet Phobos-2 in 1989 鈥� show that 鈥済eological activity鈥� has been
happening on Mars, and quite recently in terms of the age of the Solar System.
Flows of lava, and erosion by running water have erased the older craters on
more than half of the surface of Mars.

The largest volcano, Olympus Mons (Mount Olympus), is 25 kilometres high 鈥�
nearly three times as high as our own Mount Everest 鈥� and it has very few
craters caused by impacts from space. This scarcity of craters means that
Olympus Mons is young in geological terms, and eruptions would have occurred
until about 100 million years ago. In fact, this volcano may merely be
鈥渄ormant鈥�, and erupt again in the future.

Geology of a twin

Plateaux and plains

ASTRONOMERS and geologists hoped that Venus would provide some clues to an
important question: Would any planet that is similar in size to the Earth
necessarily have the same kind of geology?

The problem is that Venus is covered by cloud. In recent years, however,
astronomers have built up a picture of the surface below the cloud, using
radar. With this technique, they send radio waves through the clouds, and
reconstruct an image of the surface below from the way in which the waves are
reflected by the rocks.

Venus has two large, high plateaux and several volcanoes. The rest of the
planet is covered by low-lying plains. The most recent radar results show very
few impact craters on these areas, indicating that they are young; no more
than a few hundred million years old. In fact, lava probably seeps out onto
these plains even now.

Although both the Earth and Venus have volcanoes and lava plains, the
geology of the two worlds differs in one important respect. The Earth鈥檚
surface is broken up into a couple of dozen plates, which move around the
Earth鈥檚 surface, in a process called plate tectonics (see Inside Science No
6). Where two plates collide earthquakes happen, or a mountain chain (such as
the Himalayas), even a line of volcanoes, may form. But so far, images of
Venus have not indicated the same kind of plate tectonics. Recent radar
pictures show that Venus鈥檚 surface is probably splitting around the equator,
and the plates spreading north and south are crumpling into folds as they
move.

Strange activities

Freezing eruptions

ONE great surprise from the Voyager missions to the giant planets was that
most of their moons showed signs of geological activity having erased the old
cratered surface.

Although these moons look rather like our Moon, or like parts of Mars,
there is an important difference. At more than 700 million kilometres from the
Sun, water is frozen solid into ice 鈥� and, at the distance of Uranus and
Neptune, ammonia and methane are frozen too. So their moons are made up of a
mixture of rock, ice and perhaps other frozen material. The fact that these
worlds consist of different materials from the inner planets is very useful.
It shows what kind of general geological processes can occur, whatever the
world is made of; and the differences that depend on its particular makeup.

Saturn鈥檚 moon Enceladus, for example, has smooth plains that probably
consist of ice that flowed from its interior as water 鈥渓ava鈥� when part of the
moon鈥檚 interior melted. The surface of Europa, a moon circling Jupiter, is
covered with a smooth, brilliant white plain. This is probably the surface of
an ocean of water that once covered the moon, but froze solid to make it look
like a billiard ball.

Triton, the largest moon of Neptune, also has few impact craters,
indicating that it, too, melted completely at one time. But its surface is not
smooth. Triton is covered by polygonal-shaped rings, long low ridges and
frozen lakes of ice. When the molten Triton cooled down, its surface did not
simply freeze 鈥� as did Europa鈥檚. It seems to have experienced some complicated
鈥済eology鈥� of a type that scientists do not yet understand.

Even stranger is the surface of Uranus鈥檚 moon Miranda. Here we find large
V-shaped features and oval rings of ridges and troughs. So far, no one is sure
how they formed. Some scientists think this moon once broke up into half-a-
dozen pieces, which then reassembled. But it is more likely that the strange
markings are regions where part of the moon鈥檚 interior softened and churned up
the surface.

As Voyager 1 passed Jupiter鈥檚 moon Io in 1979, its cameras revealed
umbrella-shaped plumes of gas rising 300 kilometres into space. Ten years
later, Voyager 2 found a smoke-like eruption of dark material from Triton,
Neptune鈥檚 largest moon.

Although people often refer to these outbursts as 鈥渧olcanoes鈥�, they are
more similar to geysers on the Earth. The difference is that a volcano spews
out the actual hot material that rises from inside the Earth 鈥� molten lava, or
magma. In a geyser, the magma is trapped below ground, and its heat boils
water in the rocks above to make the water erupt.

Io has molten rocky magma below the surface, and it heats up sulphur or
sulphur dioxide which then erupts through the surface as a plume of vapour.

Triton is much farther from the Sun, and the temperatures are much lower.
Indeed, at -236掳C 鈥� only 37 degrees above absolute zero 鈥� the surface of
Triton is the coldest place we know in the Solar System. Here the 鈥渉ot鈥�
material inside is ordinary ice, trapped deep within Triton. It heats up
pockets of frozen nitrogen, which burst through the surface as nitrogen gas,
carrying small specks of black, probably organic, material from below and
spreading them into dark streaks seen by Voyager鈥檚 cameras. (Our experience
from close-ups of Halley鈥檚 Comet tells us that 鈥渂lack material鈥� in the Solar
System is usually associated with carbon.)

A long family story

Birth of the planets

GEOLOGICAL processes such as erosion and plate tectonics have destroyed or
hidden the earliest rocks on the Earth, so we cannot tell directly how our
planet was born. However, astronomers can now piece together the early history
of the Earth and the other planets from their studies of the other worlds 鈥�
including the rocks brought back from the Moon and meteorites that fall to the
Earth from space.

About 4.5 thousand million years ago, a cloud of gas and dust in space
collapsed under its own gravity, to form a dense blob at the centre, which
became the Sun, and a swirling disc of matter farther out. The dust and gas in
this disc would eventually become the planets that we know today.

The solid 鈥渄ust鈥� in the disc consisted of particles of ice and rock, only a
millionth of a metre across. In the regions closest to the centre, the heat of
the nascent Sun boiled away most of the icy particles, to leave mainly grains
of rock. That is why the planets near the Sun are rocky. Towards the edge of
the Solar System, where it was colder, the icy particles survived, and we find
the planets Uranus and Neptune made up of a mixture of rock and ice (now
melted). Jupiter and Saturn began in the same way, but became so massive that
their gravity was able to capture some of the hydrogen and helium in the disc,
so they now consist mainly of these gases.

Astronomers think the way that the planets came together was much the same,
regardless of whether the original particles were icy or rocky. In the dense
disc, these 鈥渄ust鈥� particles merged to form 鈥減lanetesimals鈥�, a few kilometres
across. How the particles accumulated into planetesimals, we do not yet know;
they may simply have stuck together when they collided, or possibly the
gravitational effect of all the particles on one another became unstable, and
the matter in the disc broke up into clumps that then coagulated.

The theory becomes firmer after the planetesimals formed. Astronomers have
calculated by computer how planetesimals would collide with one another,
sometimes crashing at such speed that they break one another up, sometimes
meeting at a lower speed so that gravity binds them together. Any clump of
planetesimals that grows to be bigger than the others will have a stronger
gravity, and so will be better at holding onto other bodies that it collides
with. As a result, the largest clumps of planetesimals tend to gather up the
others, and become the cores of the planets. These calculations show that in
the inner part of the Solar System, the planets formed from planetesimals in
about 200 million years; the outer planets, which formed in regions where
there was less matter about, took rather longer.

The final stages of forming the planets probably involved high-speed
collisions between quite large bodies, and with very dramatic effects.
快猫短视频s claim that some calculations suggest Mercury originally formed
farther out than Mars, but a tremendous collision drove it in towards the Sun,
stripping it of its outer layers of rocks.

The composition of lunar rocks suggests that the Moon was born from a
similar collision, now nicknamed the Big Splash. When the Earth had reached
almost its present size, another body about the size of Mars crashed into it.
The 鈥渟plash鈥� threw out a spray of molten rock, that condensed into a ring of
particles surrounding the Earth. These rocky particles then came together to
make up the Moon.

After the basic nine planets had formed, with their attendant satellites,
they quickly swept up most of the planetesimals. The impact of these smaller
bodies left the craters we see on all the old surfaces that have survived in
the Solar System. Some planetesimals still exist. The asteroid belt, beyond
Mars, consists of rocky planetesimals that never accumulated into a planet
because Jupiter鈥檚 gravity kept shuffling their orbits.

Beyond the planets there is a region where we find large numbers of icy
planetesimals, each a few kilometres across. These become comets when their
paths are altered so that they come nearer the Sun. Eventually, solar heat
causes them to evaporate, which gives them long gaseous tails.

Now that astronomers have made their first reconnaissance of the major
planets and the large moons, they are turning their attention to the smaller worlds that are
fragments left over from the earliest days of the Solar System: the asteroids
and comets.

Late in 1989, astronomers obtained their first view of the surface of an
asteroid, 1989PB, by bouncing radar waves off its surface. They found it is
shaped like a peanut.

The spacecraft Galileo, now on its way to Jupiter, will take the first
close-up pictures of two asteroids, Gaspra in 1991, and Ida in 1993.
Astronomers are also planning a spacecraft to take a long look at a comet; the
first comet missions went to Halley鈥檚 Comet in 1986, but they gained only a
fleeting glimpse because they passed it at 240 000 kilometres an hour. A
detailed study of the surviving planetesimals should fill in the missing
details about how the major planets 鈥� including the Earth 鈥� came into being.

Running rings around the planets

EVEN a small telescope reveals that Saturn has a set of rings. Astronomers
deduced that Uranus also has rings when, in 1977, the planet moved in front of
a star and the star鈥檚 light was cut off briefly just before and just after the
planet itself hid it. Similar observations of Neptune passing in front of
stars led some astronomers to believe that it too had rings.

The Voyager spacecraft confirmed that there are rings around Uranus and
Neptune, and discovered rings around Jupiter as well. They also showed the
rings of Saturn in great detail.

Astronomers were surprised to find that the rings of Uranus and Neptune are
very narrow. They are only a few kilometres thick, even though they orbit the
planets at a distance of around 50 000 kilometres. (On a scale model, the
rings, given a radius of about one metre, would be as thin as strands of
thread.) The Voyagers found that even the broad rings of Saturn are made up of
thousands of narrow 鈥渞inglets鈥�, nested one inside the other.

These rings are composed of millions of chunks of ice, ranging in size from
a few millimetres up to several metres, and each of them orbits the planet as
if it were a tiny moon. The gravitational forces of small moons that orbit the
planet just inside and just outside each ring seem to 鈥渟hepherd鈥� chunks of
ice, keeping them in narrow rings.

Most important to astronomers, the chunks in these rings must be moving in
a similar way to the pieces of rock that made up the Solar System in its
earliest days. The rings of the planets, therefore, can help astronomers to
understand how the planets came together from smaller pieces of rock and ice.

Further reading

The Cambridge Photographic Atlas of the Planets, by Geoffrey Briggs and
Fredric Taylor (Cambridge University Press, 1986, 拢10.95) has
photographs and some detailed maps of the planets out to Uranus, while The New
Solar System, edited by J. Kelly Beatty, Brian O鈥橪eary and Andrew Chaikin
(Cambridge University Press, 1983, 拢6.95), goes into the science in more
depth. The Planets, by Heather Couper and Nigel Henbest (Pan 1986,
拢6.95) provides a colourful introduction to the Solar System; it
includes a loose-leafed update on spacecraft observations of Uranus and
Halley鈥檚 Comet in 1986.