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.

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.

Our Neighbours: Venus, Earth & Mars

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.

Surface Evolution

Earth Venus Mars

Condensation

Accretion Differentiation

We can assume that the three planets formed in a similar manner.

Earth Venus Mars

Condensation

Accretion

Differentiation

Cratering

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

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).

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.

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.

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.

Tectonic activity is defined as “any crustal deformation caused by motions of the surface”, e.g. stretching and compression.

Plate tectonics is a form of global tectonic activity, whereby the lithosphere consists of individual “plates” that move across the asthenosphere.

core

lower mantel

upper mantel

crust

upper mantel

asthenospherelithosphere

crust

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’ Olympus Mons

24km high volcano

Mars, which has the highest volcanic mountain in the Solar System, clearly had an active past.

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 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.

Venus’ Maat Mons

8km high volcano

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.

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.

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.

Atmospheric EvolutionIf 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.

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.

UV

H2O

H

H

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.

Path of solar wind particles

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).

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.

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).

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.

While exploring Mars, Mariner 9 and later spacecrafts studied the two natural satellites of Mars - Phobos and Diemos - close-up.

(b) The Natural Satellites of Mars

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 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!

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.

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.

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.

Prominent on Phobos are Stickney, a huge crater,

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 Mercury’s “weird terrain”.

plus parallel linear grooves which appear to originateat Stickney and end at a featureless region on the other side of the satellite.

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 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.

Diemos orbits Mars 2.5 times further away than does Phobos, completing one orbit in 30 hours.

Compared to our Moon’s 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.

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

Venus, Earth & Mars:

http://photojournal.jpl.nasa.gov/

Mars’ volcanoe Olympus Monshttp://mars.jpl.nasa.gov/

Venus’ Maat Mon volcanohttp://nssdc.gsfc.nasa.gov/photo_gallery/photogallery-venus.html

Phoboshttp://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

Image Credits

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 Mand radius R if the gas particle’s kinetic energy is greater than the gravitational binding energy of the planet.

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:

where G is the gravitational constant given by 6.67 x 10-11 N m2/kg2

That is:

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