Water in the Solar System
Oceans of Liquid Water are Unique to Planet Water
The earth appears to be unique in our solar system in that it contains an enormous amount
of water (70% of its surface), and that water has existed in a form not too different
from its present state for billions of years.
What makes the earth different from the other planets?
There are two parts to this question:
- How did the earth acquire such a large amount of water in the first place?
- Once acquired, how was it retained?
The first question has to do with how the earth was formed and the second involves the
evolution of the earth and its atmosphere. The most recent theories of planet formation describe the process of planet formation as
having two steps.
First, gravitational collapse takes place forming small asteroid like bodies some as large
as 1/500 of the mass of the earth. The planetesimals begin to collide and form the larger
bodies of the planets. The rain of bodies on the surface of the earth generates large
amounts of heat, enough to cause the heavier elements, such as iron to migrate to the centre.
A second factor has to do with the fact that when a meteor hits anything, some of it sticks
and some is scattered back into space by the impact. The lower the density of the material,
the more likely it is to escape. In the early stages, the earth collects heavier stuff more
easily, leaving lighter stuff such as silicon and water still in orbit about the sun.
As the earth gets bigger, however, it more effectively traps the lighter material during
the latter stages of planet formation.
The formation of the earth probably took a few hundred million years to be completed.
That is to be compared with the time of about 3.5 billion years since the earth has
developed a solid crust. About the time the earth was formed, the sun became large enough
that the fusion reactions in the sun ignited.
This didn't happen smoothly, but likely in sputtering way for a while.
Each flaring up of the sun sent streams of particles sweeping out.
If the earth had an atmosphere at this time, it would have been blown off leaving the earth
as a rock with neither air nor water on its surface. In fact, after the sun stabilized,
the earth went through a process of releasing gases from its interior in a process called
Over a relatively short time, something like a 100 million years, enough material had been
released to form the oceans and to give the earth an atmosphere.
There was no free oxygen in the atmosphere at this time, but it was a collection of gases,
largely ammonia, methane and carbon dioxide, held to the earth by gravitational attraction.
Fortunately, early in its history, the temperature of the earth dropped below 212 degrees
Fahrenheit, and the water condensed into the oceans we know today.
In fact, the mass of water present in the oceans, now about 10(24) grams, is about the same
as the mass of water that was contained in the crust when the degassing process started.
We can estimate the rate at which water is being lost today by estimating the rate at which
water molecules in the atmosphere are dissociated into its constituent hydrogen and oxygen.
The hydrogen is light enough that it easily moves off into space.
The net effect of hydrogen loss decreases the amount of water vapor in the atmosphere. A good estimate is that 5x10(11) grams are lost this way each year.
his amounts to a volume of a cube about 100 yards on a side.
The total water lost to space since the beginning of the earth thus amounts to about
2x10(21) grams, about 0.2 percent of the water in the oceans.
This means that most of the water you see on the earth was the very same stuff that degassed
from the crust when the earth was only a few hundred million years old.
Fortunately, the water lost to space is replaced by the same geologic processes that formed
the oceans originally.
At the present time, about 70%of the surface of the earth is covered with water.
The present coastlines are where they are because some of the water is locked up in the
polar ice caps.
In terms of volume, the water on earth is distributed in the following way:
oceans => 1.35 x10(17) cubic meters (97.3%)
polar ice and glaciers => 29x10(15) cubic meters (2.1%)
underground aquifers (fresh) => 8.4x10(15) cubic meters (0.6 %)
lakes and rivers => 0.2x10(15) cubic meters (0.01%)
atmosphere (water vapor) => 0.013x10(15) cubic meters (0.001%)
biosphere => 0.0006x10(15) cubic meters (0.00004%)
If the water locked up in polar ice were to completely melt, the oceans would rise
about 73 metres above its present level.
Why is the water still here on the earth?
This is more difficult to answer. It has to do with the changing nature of the atmosphere
due to evolution of life, specifically algae. The algae produced free oxygen by
photosynthesis which destroyed ammonia and methane, so called greenhouse gases,
just as the sun's luminosity was increasing by about twenty five percent.
If that hadn't happened the oceans would have boiled away long ago.
In fact, we are the beneficiaries of an incredible balancing act which allowed just enough
heat to escape from the earth to keep the oceans from boiling, but not so much as to cause
the earth to freeze solid.
Water on Other Planets or Moons in the Solar System?
Water in its various forms pervades the solar system, from traces of water vapour on the
Sun itself to water ice in the likely composition of Pluto and the Kuiper Belt objects
beyond it. However, large amounts of liquid water are not clearly seen elsewhere in the solar system
at the surface.
But in recent years, NASA spacecraft have found evidence that liquid water may
persist below the dry surface of Mars and the icy surfaces of three large moons circling
Water on Mars?
In the 1970s, three Mars orbiters sent back images that revealed landscape shapes apparently
formed by flowing water in the distant past. NASA's Mariner 9, Viking 1 and Viking 2
spacecraft showed us Martian channels carved as if by rivers and out-wash plains scoured
as if by floods.
Geologists estimate that very heavy flows, equal to thousands of Mississippi Rivers,
would have been necessary to shape some of the surface features on Mars.
Yet Mars' atmosphere is too thin and cold for water to remain liquid at the surface.
Instead of melting, warmed water ice on Mars turns directly into vapor, the way
carbon-dioxide "dry" ice does on Earth. To account for the signs of copious water flows in the past, scientists at first suggested
that long ago Mars had a thicker atmosphere than it does now. allowing for liquid water on
Other scientists suggested that it was liquid carbon dioxide rather than water that
have formed these features. A debris flow dominated by carbon dioxide would flow faster and farther than a water-based
flow. Also, carbon dioxide is more volatile than water at lower temperatures, and the cold
temperatures found on Mars would mean that less carbon dioxide- based magma would be
required to produce the observed erosion than magma containing mainly water.
There is now very clear evidence of erosion in many places on Mars including large floods
and small river systems. There must have been some sort of fluid on the surface. Liquid water
is the obvious fluid but other possibilities exist.
Recent Mars data suggests that large lakes or even oceans were once present on Mars.
The images of layered terrain taken by Mars Global Surveyor and the mineralology results
from MER Opportunity clearly suggest lakes of oceans. These data suggest wet episodes that
occurred only briefly and very long ago; the age of the erosion channels is estimated at
about nearly 4 billion years.
Images from Mars Express released in early 2005 show what appears to be a frozen sea that was
liquid very recently (maybe 5 million years ago). This still needs to be confirmed.
Perhaps early in its history, Mars was much more like Earth.
As with Earth almost all of its carbon dioxide was used up to form carbonate rocks. But lacking
the Earth's plate tectonics, Mars is unable to recycle any of this carbon dioxide back into its
atmosphere and so cannot sustain a significant greenhouse effect. The surface of Mars is therefore
much colder than the Earth would be at that distance from the Sun. Mars has a very thin atmosphere
composed mostly of the tiny amount of remaining carbon dioxide (95.3%) plus nitrogen (2.7%),
argon (1.6%) and traces of oxygen (0.15%) and water (0.03%). The average pressure on the surface
of Mars is only about 7 millibars (less than 1% of Earth's), but it varies greatly with altitude
from almost 9 millibars in the deepest basins to about 1 millibar at the top of Olympus Mons.
But it is thick enough to support very strong winds and vast dust storms that on occasion engulf
the entire planet for months. Mars' thin atmosphere produces a greenhouse effect but it is only
enough to raise the surface temperature by 5 degrees (K); much less than what we see on Venus and
Further evidence of water on Mars
Early telescopic observations revealed that Mars has permanent ice caps at both poles; they're
visible even with a small telescope. We now know that they're composed of water ice and solid
carbon dioxide ("dry ice"). The ice caps exhibit a layered structure with alternating layers of
ice with varying concentrations of dark dust. In the northern summer the carbon dioxide completely
sublimes, leaving a residual layer of water ice.
ESA's Mars Express has shown that a similar layer of water ice exists below the southern cap as well.
The mechanism responsible for the layering is unknown but may be due to climatic changes related to
long-term changes in the inclination of Mars' equator to the plane of its orbit. There may also be
water ice hidden below the surface at lower latitudes. The seasonal changes in the extent of the
polar caps change the global atmospheric pressure by about 25% (as measured at the Viking lander sites).
Plenty of frozen water may persists in permafrost layers underground, near the surface
at the poles, and also buried at lower latitudes. If some underground areas are warm, they might
even hold liquid water in the pores between grains of rock.
The discovery of signs of liquid water near the surface of Mars in the past and perhaps underground
suggests that Mars has the precursors for life: carbon, certain minerals, liquid water and energy.
The question remains, however, as to whether the presence of all of those ingredients - most
importantly water, under similar conditions would lead to life on Mars or any other planet. The question
whether life existed on Mars or is perhaps still there remains to be answered.
Water on Europa?
Liquid water may also be present on Europa, one of Jupiter's four major moons.
Europa is covered by a thick layer of ice. But the gravity of giant Jupiter exerts tidal
tugging that warms Europa's insides, possibly enough to keep a layer of water melted
under its frozen surface.
Water clues appeared in pictures taken by NASA's Galileo spacecraft in 1996 as it orbited
Jupiter. The pictures supported earlier theories about a hidden Europan ocean.
On some parts of Europa's surface, for example, blocks of ice appear to have broken apart
and rearranged themselves as if by floating, like Arctic ice floes, on a fluid underlayer.
Europa's fractured surface shows signs of liquid water, ice or slush.
Galileo's magnetometer instrument has sent home the strongest indication that a layer of
saltwater remains melted under Europa's crust today. As Europa moves through different
parts of Jupiter's strong magnetic field, its own weaker magnetic field changes direction,
indicating that the moon has a layer of electrically conducting material.
Since ice would not conduct electricity well enough, saltwater is the best candidate.
Similar magnetic evidence from Galileo indicates that two of Jupiter's other large moons,
Ganymede and Callisto, may have liquid saltwater layers, too.
The Moon and Comets
Earth's own Moon divulged no trace of water to NASA astronauts who explored six landing
sites more than 25 years ago. But all those sites were far from the poles.
In the 1990s, the Clementine and Lunar Prospector robotic spacecraft each found indications
that the Moon may hold supplies of water ice in permanently shaded areas near its poles.
Like the water ice on Mars, those supplies could become useful for future exploration.
Water is not only a vital resource in itself, but it can also be split into oxygen and
hydrogen for breathing and for rocket fuel for return trips or journeys beyond the Moon.
Comets may possibly be a water supply to the planets. Comets are largely ice, and they
have been colliding into the Earth, the Moon and the rest of the solar system for billions
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