Module 10: Mercury Planet of Extremes

1
Module 10 Mercury -Planet of Extremes
Activity 2 Mercury its Evolution
2
Summary
In this Activity, we will investigate
(a) the surface of Mercury - cratering,
lava flows, rupes (b) ice on Mercury? (c) format
ion models for the Moon and Mercury.
3
(a) The Surface of Mercury

• Mercurys surface bears a strong resemblance to
  that of our Moon, with a couple of noticeable
  differences
  
• - no dark maria, and the presence of curved
  cliffs called scarps.

4
Cratering on Mercury
Like the Moon, Mercuryssurface is largely
coveredwith craters of all sizes,
however the amount of cratering is somewhat
less than that on the Moon.
5

• The largest basin on Mercury is called the
  Caloris Basin,

seen here on the edgeof the terminator
(thedivision between dayand night on the
planet).
6
The Caloris Basin is approx.1300km in diameter,
caused by an impact which threw ejecta600 -
800km across the planet.

7
The Caloris Basin is surroundedby rings of
mountains which are up to 3km high,
and partly filled with lava flows.
8
These finely structuredhills are believed to be
ripples from the seismic wavesdue to the Caloris
impact.
They cover an area which is approximately 100km
by 100km.
9
The resulting seismic shock waves would have
focussed on the other side of the planet, in a
region of weird terrain similar to the jumbled
terrain on the Moons surface opposite its
largest impact craters.
10
Lava Flows

• Mercury has intercrater plains - broad plains,
  probably lava flows, which separate groups of
  craters.

Collisions with planetesimals over 3.8 billion
years agoprobably weakened the crust, allowing
lava to well up fromthe mantle and flood
low-lying areas.
In particular, the Caloris impact may have
single-handedlybeen the cause of many of these
lava flows due to itsweakening of Mercurys
crust.
11
Scarps

• Unlike our Moon, Mercury has great curved cliffs
  called scarps (or rupes) which are up to 3km high
  and 100s of km long.

This is a 450 kilometer cliff called the
Antoniadi Ridge. It cuts through a large 80
kilometer crater.
12

• This is the Santa Maria Rupes.

13

• Scarps are common on Mercury, but not on our Moon.

To see how they formed, we need to look within
the core. Recall that Mercury has a relatively
large core
14
We saw that Mercury is 60 denser than the Moon.
This is probably due to its large core being
madeof iron.

We also saw that small Solar System bodies like
the Moon and Mercury would have cooled
relatively quicklyafter they differentiated.
Metals like iron expand or contract noticeably as
thetemperature changes - which is why railway
trackssometimes buckle in extreme heat.
15
As Mercury cooled, the iron core would have
contracted,and the radius of the whole planet
would have shrunkcorrespondingly. As this
happened relatively quickly,it is reasonable to
expect that the brittle crust would have formed
fault lines as it buckled - lines we see today as
scarps.
Why are there no shrinkage scarps on the Moon and
Earth?
This is because the Moon only has a small core,
and the Earths crust is not brittle - it is
kept relatively plasticby the heat flow from the
interior, and any scarps whichdid form in this
fashion would have been wiped out long ago by
plate tectonics.
16
The following NASA movie clips show animations of
the currently accepted model for the evolution of
Mercury The first shows the formation of Mercury
out of the Solar Nebula, its early bombardment,
lava flows, differentiation and further
cratering.
The second shows the evolution of its surface
after its crust formed, including the formation
of scarps.
Click on the pictures to start the movie clips.
(The movie clips have sound tracks which you can
listen toif your computer is equipped with a
sound card and speakers.)
17
Click on these imagesin turn to see moviesabout
Mercury and its evolution!
18
We will look at models for the earliest stages of
the evolution of the Moon and Mercury soon.

• Earth Moon Mercury
• Condensation ?
• Accretion ?
• Differentiation ?
• Cratering ?
• Basin Flooding ?
• (Volcanism)
• Plate tectonics ?
• Weathering ?
• (Slow decline)

19

• Earth Moon Mercury
• Condensation ?
• Accretion ?
• Differentiation ? ? ?
• Cratering ?
• Basin Flooding ?
• (Volcanism)
• Plate tectonics ?
• Weathering ?
• (Slow decline)

20

• Earth Moon Mercury
• Condensation ?
• Accretion ?
• Differentiation ? ? ?
• Cratering ? ? ?
• Basin Flooding ?
• (Volcanism)
• Plate tectonics ?
• Weathering ?
• (Slow decline)

21

• Earth Moon Mercury
• Condensation ?
• Accretion ?
• Differentiation ? ? ?
• Cratering ? ? ?
• Basin Flooding ? ? ?
• (Volcanism)
• Plate tectonics ?
• Weathering ?
• (Slow decline)

22

• Earth Moon Mercury
• Condensation ?
• Accretion ?
• Differentiation ? ? ?
• Cratering ? ? ?
• Basin Flooding ? ? ?
• (Volcanism)
• Plate tectonics ?
• Weathering ?
• (Slow decline)

There is no evidence for plate tectonics on
either Mercury or the Moon, ...

23

• Earth Moon Mercury
• Condensation ?
• Accretion ?
• Differentiation ? ? ?
• Cratering ? ? ?
• Basin Flooding ? ? ?
• (Volcanism)
• Plate tectonics ?
• Weathering ?
• (Slow decline)

and the lack ofatmospheres or liquidwater on
the Moon or Mercury means that weathering is
essentially absent.
24
We will go on to look at models for the
formationof Mercury and the Moon soon, but first
lets lookat another (surprising!) similarity
between the twobodies
25
(b) Ice on Mercury?

• As we saw in the Activity The Moon and its
  Evolution, there is recent evidence which
  suggests that there is water ice present in deep,
  permanently shaded craters near the lunar poles.

Although the Moon and Mercury share many
similarities, one might not expect that a planet
as close to the Sun as Mercury is could also
contain locations cold enough to sustain water
ice.
However deep, permanently-shadowed craters at
Mercurys polar regions would have temperatures
of only about -170C.
26

• Radar mapping of Mercury using the Very Large
  Array, a radioastronomy interferometer made of
  27 dishes and located in New Mexico, USA, as a
  receiver ...

inner dishes of the VLA
27

• has found bright reflective regions on images
  of the side of Mercury not imaged by Mariner 10,
  consistent with the presence of water ice near
  Mercurys poles

28
(No Transcript)
29
Mariner 10 only imaged about half of Mercurys
surface. With important issues like the
presence of water icestill not resolved, another
mission to Mercury is planned- the Mercury
Orbiter (later renamed MESSENGER MErcury
Surface, Space ENvironment, GEochemistry, and
Ranging).
MESSENGER was launced on the 3rd of August, 2004.
It will travel for nearly 5 years (using gravity
assists from two Venus and two Mercury flybys)
before being placed in an 8 hour polar orbit,
coming as close as200 km above Mercurys
surface over a totalperiod of one Earth year.
For Messenger Mission updates, visithttp//messe
nger.jhuapl.edu/
30
(c) Formation Models for the Moon and Mercury

• The Moon and Mercury have much in common, as well
  as some important differences, and comparisons
  between the two should help us come to some
  conclusions about how they were formed.

31

• In the Activity Planetary Evolution we saw a
  modelfor the formation of terrestrial planets in
  the Solar System, involving gradual accretion of
  solar nebula debris, differentiation, cratering
  and so on.

This model works quite well in predicting overall
similarcomposition for the interior of the
Earth, Venus and Mars,but not so for Mercury and
the Moon.
In particular, the Moon contains too small a
percentage of metals, and Mercury contains too
large a percentage of metals.
32
Formation Models for the Moon

• Analysis of Apollo rock samples has shown that
  the Moon is similar to the Earth in many ways,
  but is significantly different in its lack of
  metals and volatile compounds.

These Apollo findings have been a considerable
challenge to astronomers trying to form theories
of how the Moon formed. We will briefly review
the most popular theories
33
(i) The Fission Theory

• This theory claims that the Moon was once part of
  a young, rapidly-spinning Earth. Tidal forces
  due to the Sun made it break into two parts, with
  the Moon forming mainly from material thrown off
  from the Earths crust and mantle.

Click here to see an animation illustrating the
Fission Theory
34

• However although the Moons composition does bear
  some similarities to that of the Earths mantle
  and crust, significant differences exist.

For example, the Moon (1) has fewer volatile elem
ents (2) has the wrong nickel to iron and magnesi
um to silicon ratios, and
(3) has twice as much aluminium and calcium as
does the Earth.
Also, it is not clear why the Earth would have
been spinning so fast to start off with, and,
more importantly, where all that excess angular
momentum has gone to!
35
(ii) The Binary Accretion (or double planet)
Theory

• This theory (also sometimes called the Sister
  Theory) claims that the Earth and Moon formed
  together as a double planet system from the same
  part of the Solar Nebula.

Again, this theory has a fundamental difficulty
in explaining why, if the Earth and Moon formed
from the same material, they now have important
differences in chemical composition and chemistry.
Click here to see an animation illustrating the
Binary Accretion Theory
36
(iii) The Capture Theory

• This theory claims that the Moon formed elsewhere
  in the Solar System - for example, a little
  inside the orbit of Mercury where the local
  condensation temperatures would have given it
  approximately the right composition - and that a
  gravitational interaction with Mercury would have
  boosted it out of its original orbit and
  brought it close enough to Earth to be
  gravitationally captured.

Click here to see an animation illustrating the
Capture Theory
37
This theory, once very popular, has fallen out of
favour because

• (1) it emphasises the differences between the
  chemical composition of the Moon and Earth, but
  fails to explain many significant similarities
  
• (2) the Moon would have been travelling too fast
  to becaptured by the Earth without being tidally
  ripped apartunless another large (unknown)
  nearby object was involved,
• (3) if it was captured in this way, one would
  expect it to have taken up a highly eccentric
  orbit, and
  
• (4) it relies on a sequence of unlikely events.

38
(iv) The Large-Impact Theory

• Developed in the 1980s in response to the Apollo
  data, this theory claims that a large
  planetesimal (perhaps as large as Mars) impacted
  the Earth, largely merging with the Earth but
  throwing off a disk of debris in the process.
  The debris, mainly composed of iron-deficient
  mantle and crust material, would have gradually
  aggregated together to form the Moon.

Click here to see an animation illustrating the
Impact Theory
39
This theory has several advantages

• (1) The ejected material would have been low in
  metal content as it originated in the crusts and
  mantles of the two colliding bodies. It would
  initially have been very hot, and volatiles would
  have evaporated off, explaining the relative lack
  of volatile compounds on the Moon.
• (2) To eject sufficient material to form the
  Moon, the collision had to occur at a steep angle
  - not head-on. Such a collision would have
  spun-up the resulting Earth - Moon system,
  explaining its high angular momentum.
• (3) As the Moon is composed of crust and mantle
  material, this explains the many similarities
  between the Moon and Earths chemical
  compositions.

40

• There do not appear to be any fundamental
  problems with the Large-Impact Theory, and
  supercomputer simulations of such a collision
  support it.
• However it has to stand up to testing with data
  from future lunar missions it is always easier
  to form a theory after the experimental data is
  gathered and analysed!

41

• One of the reasons astronomers were initially
  reluctant to support such a theory was because it
  relied on an extremely violent collision in the
  early Solar System - the size calculated for the
  impacting body, approx. one-tenth the mass of
  Earth, is almost the largest impact that the
  Earth could have suffered without being totally
  broken apart.

However now astronomers can use the power of
supercomputers to simulate events like these.
Some simulations suggest that as many as 100
planetesimalslarger than the Moon were loose in
the inner solar system, as well as many more
smaller surviving planetesimals.
42

• If so, the early Solar Systems history would
  have been marked by many collisions, and near
  encounters, and the cratering history of the
  terrestrial planets contains a number of the
  scars of giant impacts to support this.

43
Formation Models for Mercury

• Superficially, the accretion model which we have
  used to describe the formation of the terrestrial
  planets should fit Mercury. However as we have
  seen, Mercury has a much higher percentage of
  metal as compared to rocky constituents than
  would be expected from this model.

It is tempting to suggest that Mercury and the
Moon might have one more thing in common
- the effects of giant impacts in the early
Solar System.
44

• Early in Mercurys history, a giant impact might
  have stripped off much of its lower-density rocky
  crust and mantle material.

The remaining denser core material could have
attracted some (but not most) of the debris to
reform a thin mantle and crust, leaving Mercury
rich in metals but short on rocky mantle
material.
Until further missions visit Mercury, we can only
speculate.
Figure 10.16 in the textbook Universe
illustratesa supercomputer simulation of the
formation of present-day Mercury by collisional
stripping.
45

• In the next Module, we will investigate our
  nearest planetary neighbour, Venus.

46
Image Credits
NASA Mariner 10 mosaic of one hemisphere of
Mercury http//nssdc.gsfc.nasa.gov/image/planetar
y/mercury/mercuryglobe1.jpg Mosaic of the Bach ar
ea of Mercury http//nssdc.gsfc.nasa.gov/image/pl
anetary/mercury/bach.jpg Mosaic of the Caloris Ba
sin and surrounding area http//nssdc.gsfc.nasa.
gov/image/planetary/mercury/caloris.jpg
Hills of Mercury http//learn.jpl.nasa.gov/projec
tspacef/ME_03.jpg Antoniadi Ridge http//learn.j
pl.nasa.gov/projectspacef/ME_08.jpg
Large Faults on Mercury (Santa Maria Rupes)
http//learn.jpl.nasa.gov/projectspacef/ME_07.jpg
47
Image Credits
NASA Mercury http//pds.jpl.nasa.gov/planets/wel
come/thumb/merglobe.gif Earth http//pds.jpl.nas
a.gov/planets/welcome/earth.htm
Three-filter color image of the Moon
(Galileo)http//nssdc.gsfc.nasa.gov/image/planeta
ry/moon/gal_moon_color.jpg Mercury evolution anim
ations (Space Movie Archive) http//graffiti.u-bo
rdeaux.fr/MAPBX/roussel/anim-e.html
Goldstone/VLA radar maps of Mercury
http//wireless.jpl.nasa.gov/RADAR/mercvla.html
Mercury Orbiterhttp//umbra.nascom.nasa.gov/SEC/s
ecr/missions/meo.html
48

• Now return to the Module home page, and read more
  about Mercury and its Evolution in the Textbook
  Readings.

Hit the Esc key (escape) to return to the Module
10 Home Page
49
(No Transcript)