Transcript Evolving with our Neighbours

Module 13:
Planets
as Habitats
Activity 2:
Evolving with
our Neighbours
Summary:
In this Activity, we will
(a) compare the evolution of Mars, Venus & Earth,
and
(b) investigate the natural satellites of Mars.
Our Neighbours: Venus, Earth & Mars
We are now in a position to compare our models of the
evolution of the three outer terrestrial planets – Venus,
Earth and Mars – both their surface evolution and their
atmospheric evolution.
Surface Evolution
Let’s first have a look at the surface evolution and various
surface activities of these three planets and see if we can
understand the differences in terms of the different physical
properties of the planets.
Condensation
Earth

Accretion
Differentiation


Venus



Mars



We can
assume that
the three
planets
formed in a
similar
manner.
Condensation
Accretion
Earth


Venus


Mars


Differentiation



Cratering



The different
cratering
histories of
these three
terrestrials is
strongly
related to their
atmospheres.
Condensation
Accretion
Earth


Differentiation



Cratering



Basin Flooding



(Volcanism)
Venus


Mars


Basin flooding
on all 3 planets
involved lava
flows, and
involved liquid
water on Earth
and possibly
Mars too
(see previous
Activity).
Condensation
Accretion
Earth


Differentiation



Cratering



Basin Flooding



(Volcanism)
Plate tectonics

Venus


Mars


There is no
evidence for
Earth-type
plate tectonics
on Venus, and
only very
limited
indications of
tectonics on
Mars.
Condensation
Accretion
Earth


Venus


Mars


Differentiation



Cratering



Basin Flooding



(Vulcanism)
Plate tectonics

Weathering

(Slow decline)

Earth and Mars
have largely
settled down to
steady
weathering, but
Venus appears
to be still
dominated by
its active
surface.
Tectonic activity is defined as “any crustal deformation
caused by motions of the surface”, e.g. stretching and
compression.
The tectonic history of these three terrestrial planets can be
understood in terms of internal heat loss, which in turn is
related to their size. All the planets were heated during their
formation (by the conversion of gravitational energy to thermal
heat energy) and slowly this heat is radiated away.
Plate tectonics is a form of global tectonic
activity, whereby the lithosphere consists of
individual “plates” that move across the
asthenosphere.
crust
lithosphere
asthenosphere
upper mantel
crust
upper mantel
lower mantel
core
Mars, being the smallest of the three planets, cooled the
quickest before wholesale plate tectonics had a chance
to take hold.
Venus and Earth are similar in size - and hence we might
expect similar cooling rates. But Venus’ runaway greenhouse
effect ensures that its surface remains pliable and it probably
never obtained a sufficiently rigid crust for Earth-type plate
tectonics to occur.
All three planets, however, contain evidence of some tectonic
activity - volcanism - which indicates at least early heat loss.
Volcanism not only changes the surface of planets but can
also have a strong effect on their atmospheres.
Mars, which has the highest volcanic
mountain in the Solar System, clearly
had an active past.
24km high
volcano
Mars’ Olympus Mons
Valles Marineris is a tectonically formed
canyon system which stretched one fifth
the way around Mars. It is over 3000 km
long, 600 km wide and up to
8 km deep.
Mars’ Valles Marineris
Venus’ Maat Mons
Venus has also had a very tectonically
active past. About 10% of Venus’
surface is covered by highlands which
are probably volcanic in origin, it has
over 1000 volcanic structures, and most
of its surface is covered in lava plains.
8km high volcano
Since the mass of Earth & Venus are very similar, the height of their
volcanoes are similar (Mauna Loa is ~9km above the seafloor). With
a surface gravity only about 40% that of the Earth’s, Mars has much
higher volcanoes.
It also has long linear mountain ridges and strain pattern that
extend over hundreds of kilometres, which occurred both
before and after volcanic episodes. It is not known whether
Venus is still active today, though variations in atmospheric
sulfur dioxide suggest volcanic outbursts.
Atmospheric Evolution
If we assume for a moment that Venus, Earth & Mars were
formed from essentially the same material, then we would
expect them all to have similar atmospheres.
But this is not the case. The compositions and masses of
the terrestrial atmospheres are all different, which indicates
that they have evolved since their formation. (Of course life
on Earth has greatly effected its atmosphere, but let’s ignore
that for now.)
The loss of a planetary atmosphere depends strongly on the
planet’s mass (and hence gravitation) and the atmospheric
temperature. Generally the less massive the planet, the
more easily it loses its atmosphere.
But that’s not the full story...
A particle can escape from an atmosphere if its kinetic energy*
(associated with the speed of the particle) is greater than the
gravitational binding energy* (associated with the mass of the
planet).
The hotter the planet, the faster the atmospheric gas
molecules will be moving and the more easily they can
escape the planet’s gravitation. The general rule of thumb is
that fast moving lighter particles will escape more readily than
slow moving heavy particles. This explains why the
atmospheres of Venus, Earth and Mars are devoid of light
gases like hydrogen and helium (which, we’ll see later, is
what the atmospheres of the giant planets are primarily
composed of).
But that’s not the full story either...
Tell me more...
Let’s have a closer look at the compositions of the terrestrial
atmospheres. Venus and Mars are mostly carbon dioxide
(presumably released from volcanic emissions), with little or
no water vapour.
Even if they did originally have water in their atmospheres,
the lack of an ozone layer meant that any water vapour (H2O)
would have been broken up in H and O atoms by the Sun’s
ultraviolet radiation over time. This process is called
photodissociation.
H2O
The lighter hydrogen atoms are then
“free” to escape the atmosphere,
UV
leaving the heavier oxygen atoms
behind. Oxygen is highly reactive
H
and would quickly combine with
H
other atoms and molecules.
O
The solar wind is a stream of energetic charged
particles (mainly protons and electrons) that stream
out of the Sun and permeate interplanetary space.
This means that all bodies in the Solar System are
constantly being bombarded, with those objects
closer to the Sun receiving a more intense flux of
particles than bodies further from the Sun.
Solar wind particles are very effective at removing O and H
atoms from planetary atmospheres (that have been
photodissociated from H2O molecules). When an energetic
solar wind particle collides with a hydrogen atom, it imparts
some of its energy, increasing the velocity of the H atom so
that it can then escape the planet’s gravitational pull. In this
way, the water would be lost forever.
If a planet has a magnetic field,
such as the Earth does, the solar
wind particles will flow around the
magnetosphere (which is actually
shaped by the solar wind).
Path of solar
wind particles
This means that the solar wind particles can not “energise” the
dissociated H ad O atoms and reduce the planet’s atmosphere.
The Earth’s magnetosphere was probably fundamental in
reducing the loss of water from the Earth’s surface.
Venus and Mars, are the other hand, probably lost their
magnetospheres billions of years ago and therefore the amount
of water lost would have increased dramatically.
The composition of Earth’s atmosphere probably started out
similar to that of Venus and Mars, but the presence of liquid water
(the oceans) removed most of the carbon dioxide and the
emergence of life on Earth supplied oxygen to the atmosphere.
As we have seen, Venus’ atmosphere has reached crushing
pressures due to a runaway greenhouse effect, whereas Mars’
atmosphere is now so thin that walking on the surface of Mars
without a space suit would make you lose consciousness within
20 seconds.
Presumably volcanic emissions on Mars released a once-thicker
primeval atmosphere, including water vapour. However as Mars
is small, the internal heating due to differentiation would have
been less and would have mostly escaped relatively quickly. That
in turn implies less volcanism, and so less outgassing (release
of volcanic gases into the atmosphere).
(b) The Natural Satellites of Mars
While exploring Mars, Mariner 9 and later spacecrafts
studied the two natural satellites of Mars - Phobos and
Diemos - close-up.
Earth’s natural satellite, the Moon, is so large that the
Earth and Moon could almost be thought of as a double
planet system. Compared to the Moon, Phobos and
Diemos are very minor chunks of rock indeed!
Phobos (which means “fear”):
Phobos is highly irregular in shape,
and so its size is difficult to specify.
The dimensions are usually quoted
as 27 x 21.6 x 18.8 km.
Phobos, orbiting lower than any
other natural satellite in the Solar
System, has a sidereal period of
only 7.7 Earth hours. It is just
6000 km from the surface of Mars
(which is about 1% of the
Moon-Earth distance).
Phobos and Diemos both orbit
Mars in the same direction as Mars
rotates. Both undergo
synchronous rotation - that is, they
are tidally locked into keeping the
same face towards Mars at all
times.
Phobos, however, orbits faster than
Mars rotates, so as seen from the
Martian surface, Phobos moves
across the sky retrograde (west to
east), and so low that it cannot be
seen from some locations on Mars.
As it orbits so close to Mars,
Phobos undergoes tidal stresses
which are gradually affecting the
period and radius of its orbit, in
a similar fashion to the tidal effects
on our Moon.
Unlike our Moon (which is gradually
orbiting further and further from the
Earth), calculations show that the
effect on Phobos is to make it
gradually spiral in towards its parent
planet. In approximately 50 million years
from now it will either have smashed into the
Martian surface, or have broken up to form
a ring around Mars!
As you can see, the colour of Phobos in these images is
different - it depends on the filters
used to take the image.
Phobos and Diemos are actually
extremely dark grey: Phobos has an
albedo of only ~ 0.02.
Data from the Mars Global Surveyor
indicates that Phobos is covered with
a layer of fine dust about a meter thick,
similar to the regolith on our Moon.
Both moons resemble highly-cratered asteroids, and may well
be captured asteroids as the asteroid belt lies between the
orbits of Mars and Jupiter (and some orbit outside that range).
If so, they are likely to be composed of rock and ice,
and the Soviet spacecraft Phobos 2
detected some material outgassing
from its surface - probably water.
However, while it is not difficult to model
scenarios where stray asteroids
come close to Mars in their orbits, it is
harder to model their capture by Mars as that would involve their being slowed down somehow.
An alternative theory pictures them as fragments of a
former Martian natural satellite, but this does not easily
explain their similarity to asteroids.
Prominent on Phobos are Stickney, a huge crater,
plus parallel linear grooves which appear to originate
at Stickney and end at a featureless region on the other side
of the satellite.
The impact crater Stickney
is so large that the impact
must have come close to
destroying Phobos: the
grooves may be deep
cracks produced by the
impact, and ending in a
region on the other side of
the satellite like the “jumbled
terrain” region on the Moon and Mercury’s “weird terrain”.
Close-up images taken by
the Mars Global Surveyor,
along with temperature data,
indicate the surface of
Phobos is covered in a fine
powder at least one metre
thick from eons of impacts.
The day side of Phobos can
be a “warm” -4 degrees,
while the night side is a
freezing -110 degrees.
-4o C
-110o C
Diemos (“panic”):
Diemos, the smaller of Mars’
moons, is also cratered and
highly irregular in shape, with
dimensions 15 x 12.2 x 11km.
While it looks smoother than
Phobos, it has a thicker layer
of dust which covers small
features, and may well bear
the scars of large impacts
under the dust.
Diemos orbits Mars 2.5 times further away than does
Phobos, completing one orbit in 30 hours.
Compared to our
Moon’s orbit around
the Earth, however,
they are very close
to Mars indeed.
Even Diemos’
orbital radius is
only about
one sixteenth
the size of the
orbital radius of
the Moon.
In the next Activity we will look beyond Mars at the
Asteroid Belt.
Image Credits
Venus, Earth & Mars:
http://photojournal.jpl.nasa.gov/
Mars’ volcanoe Olympus Mons
http://mars.jpl.nasa.gov/
Venus’ Maat Mon volcano
http://nssdc.gsfc.nasa.gov/photo_gallery/photogallery-venus.html
Phobos
http://nssdc.gsfc.nasa.gov/photo_gallery/caption/vik_phobos_caption.html
http://www.anu.edu.au/Physics/nineplanets/thumb/phobos.gif
http://www.anu.edu.au/Physics/nineplanets/moons/Phobos.jpg
http://www.anu.edu.au/Physics/nineplanets/thumb/show9.jpg
Diemos
http://www.anu.edu.au/Physics/nineplanets/deimos.html
http://nssdc.gsfc.nasa.gov/image/planetary/mars/deimos.jpg
Now return to the Module 13 home page, and
read more about the evolution of Venus, Earth
and Mars in the Textbook Readings.
Hit the Esc key (escape)
to return to the Module 13 Home Page
Escape Velocity
A gas particle of mass m can escape from a planet with mass M
and radius R if the gas particle’s kinetic energy is greater than
the gravitational binding energy of the planet.
That is:
where G is the gravitational constant
given by 6.67 x 10-11 N m2/kg2
We can re-arrange this expression to solve for the velocity that
the gas particle must be travelling at in order to escape the
gravitational pull of the planet:
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