ASSIGNMENT 2:

THE EVOLUTION OF SURFACE WATER ON EARTH AND MARS

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MELTING

FREEZING

SUBLIMATIONDEPOSITION

VAPORIZATION

CONDENSATION

Recall from Assignment 1 the impact of temperature and

pressure on water state.

Earth’s surface temperature and pressure accommodates all three

states of water, while Mars’ balances just shy of the triple

point.

Though the Mars we know today may seem like a dry and barren wasteland, we have reason to believe this may not

have always been the case.

Many features on the surface of the Red Planet have provided us with significant

evidence that Mars may have once been much wetter.

It may have even been a second Earth.

The evidence for ancient liquid water on Mars is overwhelming.

The entire planet is rich with geological features resembling rivers, lakes and oceans.

But just as evidence points to ancient liquid water on the Martian surface, so does it point to its long absence from the geologic record.

This removal was by no means coincidental. This lesson will review several reasons behind the loss of Martian water.

In order to understand the fate of water on the Red Planet, we must first trace

back to its beginning.

Forming from the same gas and dust which built our Sun, Mars settled into place as the fourth inner planet nearly 4.6 billion years ago.

It formed at the edge of an invisible boundary known as the “Goldilocks Zone” – a circumstellar range in which planetary surface temperatures were appropriate for the existence of liquid water.

Such a close proximity would render Mars’ birth very similar to Earth’s.

It is widely accepted that water inventories rose on Earth and Mars via interior degassing and cometary impacts.

Water vapor – along with other gases such as methane, ammonia and carbon dioxide – formed a primordial atmosphere as they were outgassed from molten rock in the mantle.

Approximately 4.1-3.8 billion years ago, after the planets had accreted to their present-day sizes, a period known as the Late Heavy Bombardment brought the final large impacts, and the water and volatiles they carried, to their partially molten surfaces.

Eventually, the bombardment slowed, giving the inner planets time to cool and proceed to the next step of planetary formation.

Water stayed in the atmosphere until the planet’s surface cooled below boiling point. At this point, it condensed and fell

to the ground as rain.

The runoff collected in low-lying areas with water which had leached out of

hydrous minerals in the ground.

Over time, these pools grew to the size of lakes and oceans, rendering young

Earth and Mars near twins in their infancy.

But Mars did not retain its water for long….

Perhaps the greatest protector of liquid water on Earth is its magnetosphere, which is a region of magnet shielding

surrounding our planet caused by conduction of electric currents in its

molten iron outer core.

We can apply a basic understanding of physics to explain why Mars lost its

water so long ago.

A magnetosphere protects its parent body from harmful cosmic rays and solar

winds which can strip lighter atmospheric elements and molecules off

into space.

The Earth has a magnetic field in part because of the heat transfer between its rotating molten core and the relatively

cooler mantle layer on top of it.

This temperature difference helps create an electric dynamo, which keeps the

magnetic field stable over time. The rock record suggests that Earth’s has been stable for at least 3.4 billion years.

We can see evidence of a magnetic field on Mars via its rock record.

If you look at a projection of Mars’ remnant magnetic field, it can give you

insight to its relative age.

As you can notice from this map, evidence of Mars’ magnetic field

appears least prevalently in the northern hemisphere and in the south in the

Hellas region.

We can relatively date regions on Mars by counting the number of impact

craters. By that logic, all locations with little evidence of a remnant magnetic

field are younger than the more heavily cratered localities that do exhibit the

stronger signature.

Looking at the volcanic region known as Tharsis, we estimate that volcanic

activity has clearly occurred within the past 1 billion years. So from that

knowledge, we can deduce that Mars has been without a magnetic field for at

least that long.

Next we look at the great impact regions, Utopia and Hellas craters. We

date these large impacts back to the end of the Heavy Late Bombardment

3.8 billion years ago.

Not only do they give us evidence that Mars has been without a magnetic field

for at least 3.8 billion years, but they also provide us insight and to why Mars

may have lost it.

Mars’ diameter is only about half of that of Earth’s. Consequently, its

smaller surface area to volume ratio meant that it cooled off faster than our

home planet.

Mars’ mantle is also half as thick as Earth’s. By cooling off faster, Mars lost its interior heat, which drove its own

geodynamo.

This on its own would have resulted in the decline of the Martian magnetic

field. But it is believed that an additional factor hastened the process.

The Giant Impact Theory states that the loss of Mars’ magnetosphere was expedited by four especially large

impact craters, before being completely snuffed out by a fifth – believed to be

Utopia.

This final impact at Utopia, whose crater is over 2000 miles wide, injected

so much heat into the mantle that it drastically reduced the temperature difference driving Mars’ geodynamo.

Once the mantle became too hot, it was unable to cool the core as efficiently –

as a result, Mars’ dynamo lost the effectiveness of its conductivity, and the

magnetic field shut down.

Without a magnetic field, Mars was exposed to a full brunt of solar wind,

which stripped its atmosphere of all but its heaviest molecules.

Left behind with a tenuous atmosphere 1/100 the density of Earth’s, Mars was prone to lose its surface liquid water to

space.

Nearly 98% comprised of carbon dioxide – which was the one of few

remaining molecules heavy enough to not be lost to space, Mars did not have

the atmospheric pressure to sustain liquid water on the surface.

Without a magnetic field, Mars was exposed to a full brunt of solar wind,

which stripped its atmosphere of all but its heaviest molecules.

Any liquid water ocean it may have harbored was soon boiled off into

space.

So what is the current state of Martian water?

Well, it is not completely lost from the planet. Water still exists, trapped in the

poles and underground as ice,

the depth of which is variable upon the

latitude and obliquity of the planet.

Water vapor still exists in the

atmosphere, as well; however, it is being lost to space at the rate of 100 tons a

day.

But not all hope is lost in the quest to find liquid water on Mars.

It is possible that conditions may be favorable for liquid water to exist

underground in some places.

Hypothetically speaking, pockets of water underground could be kept warm enough by insulating blankets of porous

materials, such as sand.

At present, Mars’ heat flow is 4 times less than when hydrological features formed, so theoretically, you would

need 4 times the thickness (~400 ft) of porous sediments for liquid water

insulation.

This means that we may find Martian aquifers beneath dunes.

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END OF ASSIGNMENT 2