Transcript Mercury & its Evolution

Module 10: Mercury Planet of Extremes
Activity 2:
Mercury & its Evolution
Summary:
In this Activity, we will investigate
(a) the surface of Mercury
- cratering, lava flows, rupes
(b) ice on Mercury?
(c) formation models for the Moon and Mercury.
(a) The Surface of Mercury
Mercury’s 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.
• Cratering on Mercury
Like the Moon, Mercury’s
surface is largely covered
with craters of all sizes,
however the amount of
cratering is somewhat
less than that on the Moon.
The largest basin on Mercury is
called the Caloris Basin,
seen here on the edge
of the terminator (the
division between day
and night on the planet).
The Caloris Basin is approx.
1300km in diameter, caused by
an impact which threw ejecta
600 - 800km across the planet.
The Caloris Basin is surrounded
by rings of mountains which are
up to 3km high,
and partly filled with lava flows.
These finely structured
hills are believed to be
ripples from the
seismic waves
due to the Caloris
impact.
They cover an area
which is approximately
100km by 100km.
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 Moon’s surface opposite its largest impact craters.
• Lava Flows
Mercury has intercrater plains - broad plains, probably
lava flows, which separate groups of craters.
Collisions with planetesimals over 3.8 billion years ago
probably weakened the crust, allowing lava to well up from
the mantle and flood low-lying areas.
In particular, the Caloris impact may have single-handedly
been the cause of many of these lava flows due to its
weakening of Mercury’s crust.
• 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.
This is the Santa
Maria Rupes.
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:
Moon
Core - small,
possibly partly
molten
Mercury
Core - large,
probably solid
We saw that Mercury is 60% denser than the
Moon. This is probably due to its large core being made
of iron.
We also saw that small Solar System bodies like the
Moon and Mercury would have cooled relatively quickly
after they differentiated.
Metals like iron expand or contract noticeably as the
temperature changes - which is why railway tracks
sometimes buckle in extreme heat.
As Mercury cooled, the iron core would have contracted,
and the radius of the whole planet would have shrunk
correspondingly. 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 Earth’s crust is not brittle - it is kept relatively plastic
by the heat flow from the interior, and any scarps which
did form in this fashion would have been wiped out long
ago by plate tectonics.
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 to
if your computer is equipped with a sound card and
speakers.)
Click on these images
in turn to see movies
about Mercury and its
evolution!
Condensation
Accretion
Earth


Differentiation

Cratering

Basin Flooding

(Volcanism)
Plate tectonics

Weathering

(Slow decline)
Moon
Mercury
We will look
at models for
the earliest
stages of the
evolution of
the Moon
and Mercury
soon.
Condensation
Accretion
Earth


Differentiation

Cratering

Basin Flooding

(Volcanism)
Plate tectonics

Weathering

(Slow decline)
Moon

Mercury

Condensation
Accretion
Earth


Differentiation



Cratering



Basin Flooding

(Volcanism)
Plate tectonics

Weathering

(Slow decline)
Moon
Mercury
Condensation
Accretion
Earth


Differentiation



Cratering



Basin Flooding



(Volcanism)
Plate tectonics

Weathering

(Slow decline)
Moon
Mercury
Condensation
Accretion
Earth


Differentiation



Cratering



Basin Flooding



(Volcanism)
Plate tectonics

Weathering

(Slow decline)
Moon
Mercury
There is no
evidence for
plate
tectonics on
either
Mercury or
the Moon, ...
Condensation
Accretion
Earth


Differentiation



Cratering



Basin Flooding



(Volcanism)
Plate tectonics

Weathering

(Slow decline)
Moon
Mercury
… and the
lack of
atmospheres
or liquid
water on the
Moon or
Mercury
means that
weathering is
essentially
absent.
We will go on to look at models for the formation
of Mercury and the Moon soon, but first let’s look
at another (surprising!) similarity between the two
bodies:
(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 Mercury’s
polar regions would have temperatures of only about -170°C.
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
… 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 Mercury’s poles:
Mariner 10 only imaged about half of Mercury’s surface.
With important issues like the presence of water ice
still 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 as
200 km above Mercury’s
surface over a total
period of one Earth
year.
For Messenger Mission updates, visit:
http://messenger.jhuapl.edu/
(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.
In the Activity Planetary Evolution we saw a model
for 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 similar
composition 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.
• 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:
(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 Earth’s crust
and mantle.
Click here to see an animation illustrating the
Fission Theory
However although the Moon’s composition does bear
some similarities to that of the Earth’s mantle and crust,
significant differences exist.
For example, the Moon
(1) has fewer volatile elements
(2) has the wrong nickel to iron and magnesium 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!
(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
(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
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 be
captured by the Earth without being tidally ripped apart
unless 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.
(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
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.
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!
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. onetenth 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 planetesimals
larger than the Moon were loose in the inner solar system,
as well as many more smaller surviving planetesimals.
If so, the early Solar System’s 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.
• 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.
Early in Mercury’s 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’ illustrates
a supercomputer simulation of the formation of
present-day Mercury by collisional stripping.
In the next Module, we will investigate our nearest
planetary neighbour, Venus.
Image Credits
NASA:
Mariner 10 mosaic of one hemisphere of Mercury
http://nssdc.gsfc.nasa.gov/image/planetary/mercury/mercuryglobe1.jpg
Mosaic of the Bach area of Mercury
http://nssdc.gsfc.nasa.gov/image/planetary/mercury/bach.jpg
Mosaic of the Caloris Basin and surrounding area
http://nssdc.gsfc.nasa.gov/image/planetary/mercury/caloris.jpg
Hills of Mercury
http://learn.jpl.nasa.gov/projectspacef/ME_03.jpg
Antoniadi Ridge
http://learn.jpl.nasa.gov/projectspacef/ME_08.jpg
Large Faults on Mercury (Santa Maria Rupes)
http://learn.jpl.nasa.gov/projectspacef/ME_07.jpg
Image Credits
NASA: Mercury
http://pds.jpl.nasa.gov/planets/welcome/thumb/merglobe.gif
Earth
http://pds.jpl.nasa.gov/planets/welcome/earth.htm
Three-filter color image of the Moon (Galileo)
http://nssdc.gsfc.nasa.gov/image/planetary/moon/gal_moon_color.jpg
Mercury evolution animations (Space Movie Archive)
http://graffiti.u-bordeaux.fr/MAPBX/roussel/anim-e.html
Goldstone/VLA radar maps of Mercury
http://wireless.jpl.nasa.gov/RADAR/mercvla.html
Mercury Orbiter
http://umbra.nascom.nasa.gov/SEC/secr/missions/meo.html
Now return to the Module home page, and read
more about Mercury and its Evolution in the
Textbook Readings.
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