Evolving with our Neighbours

1
Module 13 Planets as Habitats
Activity 2 Evolving with our Neighbours
2
Summary
In this Activity, we will (a) compare the
evolution of Mars, Venus Earth, and (b)
investigate the natural satellites of Mars.
3
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.
4
Surface Evolution
Lets 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.
5
We can assume that the three planets formed in a
similar manner.

• Earth Venus Mars
• Condensation ? ? ?

Accretion ? ? ?
Differentiation ? ? ?
6

• Earth Venus Mars
• Condensation ? ? ?
• Accretion ? ? ?
• Differentiation ? ? ?
• Cratering ? ? ?

The different cratering histories of these three
terrestrials is strongly related to their
atmospheres.
7

• Earth Venus Mars
• Condensation ? ? ?
• Accretion ? ? ?
• Differentiation ? ? ?
• Cratering ? ? ?
• Basin Flooding ? ? ?
• (Volcanism)

Basin flooding on all 3 planets involved lava
flows, and involved liquid water on Earth and
possibly Mars too(see previous Activity).
8

• Earth Venus Mars
• Condensation ? ? ?
• Accretion ? ? ?
• Differentiation ? ? ?
• Cratering ? ? ?
• Basin Flooding ? ? ?
• (Volcanism)
• Plate tectonics ?

There is no evidence forEarth-type plate
tectonics on Venus, and only very limited
indications of tectonics on Mars.
9

• Earth Venus Mars
• Condensation ? ? ?
• Accretion ? ? ?
• 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.
10
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.
11
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.
12
Mars, which has the highest volcanic mountain in
the Solar System, clearly had an active past.
13
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.
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 Earths, 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.
14
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 lets ignore that for now.) The
loss of a planetary atmosphere depends strongly
on the planets mass (and hence gravitation) and
the atmospheric temperature. Generally the less
massive the planet, the more easily it loses its
atmosphere. But thats not the full story...
15
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 planets 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,
well see later, is what the atmospheres of the
giant planets are primarily composed of). But
thats not the full story either...
Tell me more...
16
Lets 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 Suns
ultraviolet radiation over time. This process is
called photodissociation.
The lighter hydrogen atoms are then free to
escape the atmosphere, leaving the heavier oxygen
atoms behind. Oxygen is highly reactive and
would quickly combine with other atoms and
molecules.
17
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 planets gravitational pull. In this way, the
water would be lost forever.
18
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 planets atmosphere. The Earths
magnetosphere was probably fundamental in
reducing the loss of water from the Earths
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.
19
The composition of Earths 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).
20
(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.

Earths 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!
21
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).
22
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.
23
As it orbits so close to Mars, Phobos undergoes
tidal stresseswhich are gradually affecting
theperiod and radius of its orbit, ina similar
fashion to the tidal effects on our Moon.
Unlike our Moon (which is graduallyorbiting
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!
24
As you can see, the colour of Phobos in these
images isdifferent - it depends on the filters
used to take the image.
Phobos and Diemos are actuallyextremely dark
grey Phobos has analbedo 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.
25
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
modelscenarios 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.
26
Prominent on Phobos are Stickney, a huge crater,
plus parallel linear grooves which appear to
originateat Stickney and end at a featureless
region on the other side of the satellite.
The impact crater Stickneyis so large that the
impactmust have come close todestroying Phobos
the grooves may be deepcracks produced by the
impact, and ending in a region on the other
side ofthe satellite like the jumbled terrain
region on the Moon and Mercurys weird terrain.
27
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.
28
Diemos (panic)
Diemos, the smaller of Mars moons, is also
cratered andhighly irregular in shape,
withdimensions 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.
29
Diemos orbits Mars 2.5 times further away than
does Phobos, completing one orbit in 30 hours.
Compared to our Moons orbit aroundthe Earth,
however,they are very closeto Mars indeed. Even
Diemos orbital radius is only about one
sixteenththe size of theorbital radius ofthe
Moon.
30

• In the next Activity we will look beyond Mars at
  the Asteroid Belt.

31
Image Credits
Venus, Earth Mars http//photojournal.jpl.nasa.
gov/ Mars volcanoe Olympus Monshttp//mars.jpl.n
asa.gov/ Venus Maat Mon volcanohttp//nssdc.gsfc
.nasa.gov/photo_gallery/photogallery-venus.html Ph
oboshttp//nssdc.gsfc.nasa.gov/photo_gallery/capt
ion/vik_phobos_caption.html http//www.anu.edu.au
/Physics/nineplanets/thumb/phobos.gif http//www.a
nu.edu.au/Physics/nineplanets/moons/Phobos.jpg htt
p//www.anu.edu.au/Physics/nineplanets/thumb/show9
.jpg Diemos http//www.anu.edu.au/Physics/nineplan
ets/deimos.html http//nssdc.gsfc.nasa.gov/image/p
lanetary/mars/deimos.jpg
32

• 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
33
(No Transcript)
34
Escape Velocity
A gas particle of mass m can escape from a planet
with mass Mand radius R if the gas particles
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
35
Return to Activity