Mercury
An
Unusual
Planet
Astronomy 1-1
Lecture 08-1
Vital Statistics
Mean Distance from Sun
Mean Orbital Speed
Sidereal Period
Rotation Period
Inclination of Axis
Inclination of Orbit
Mass
Mean Density
Albedo
Astronomy 1-1
5.79 x 107 km
0.387 AU
47.9 km/sec
87.969 days
58.646 days
0 degrees
7 degrees
3.3 X 1423 kg
0.0558 ME
5.42 g/cm3
0.06
Lecture 08-2
Expeditions
Mariner 10
Launched November 1973
Gravity assist from Venus
3 Encounters
Mapped only ~45% of surface
Detected a magnetic field
Surprise! as Mercury rotates
far too slowly for something
called the dynamo effect
Astronomy 1-1
Lecture 08-3
Expeditions
Messenger
Launched Aug. 2004
Gravity Assists
Earth 2005
Venus Oct. 2006 and June
2007
First flyby of Mercury Jan.
2008
Two more – Oct. 2008 and
Sept. 2009
Then elliptical orbit about
Mercury
Astronomy 1-1
Will map remainder of planet
Study
Mercury’s high density
Its geological history
Nature of its magnetic field,
Structure of its core,
Ice at its poles?,
Tenuous atmosphere comes from where
Lecture 08-4
Planet of Extremes
Extremes in Temperature
Day
Night
~ 400o C
~ -200o C
Highly eccentric orbit – 0.2056
Closest Approach (perihelion) 46 x 106 km
Furthest Distance (aphelion) 69 x 106 km
Astronomy 1-1
Lecture 08-5
Orbit
Highly eccentric orbit
The Aphelion (furthest distance from Sun) is
1.5 times the Perihelion (distance of closest
approach to Sun)
Orbital plane is highly tilted at 7 degrees
Astronomy 1-1
Lecture 08-6
Orbital Precession
The point of Mercury’s perihelion
slowly shifts as a function of time
Not all of the shift could be
accounted for by standard
gravitational interactions
Mercury is in deep gravity well
of Sun
Additional shift was
accounted for by Einstein’s
Theory of General Relativity
Astronomy 1-1
Lecture 08-7
3:2 Spin-Orbit Coupling
Mercury initially thought to be in synchronous rotation about
the Sun
Temperature measurements of dark side inconsistent with
this picture
Doppler measurements showed that
Mercury was in fact rotating with a
rotation period of about 59 days
When the sidereal period is
compared with the rotation period it
is noticed that the ratio is 3:2
Astronomy 1-1
Lecture 08-8
3:2 Spin-Orbit Coupling
3-2 ratio is due to tidal interactions of the Sun with
Mercury's non-spherical shape and its highly eccentric
orbit
Equivalent solar day to be 176 days
They would fry for about 88 days
Freeze for about 88 days
The explorers, at times, would also notice the Sun in
retrograde motion
When Mercury is at closest approach its orbital velocity
would be faster than the surface speed.
Astronomy 1-1
Lecture 08-9
Basic Mercury Information
Most information comes from the
Mariner-10 flyby in 1974-75
Earth-like magnetic field, though
very weak
Not understood as to why this field
exists
Very tenuous atmosphere
Atomic helium and hydrogen – derived from solar wind
Atomic oxygen, sodium, and potassium – probably from soil, or
impacting meteorites
Gases tend to accumulate on night side and dissipate in morning
sunlight
Ices seen in deep depressions at the pole regions
Astronomy 1-1
Lecture 08-10
Surface Features - Craters
Bombarded in its early history by
meteorites
So Mercury is a cratered planet
Not as heavily cratered as the
Moon and craters are different
Very little overlapping of craters
Cratered areas separated by plains
Why is Mercury so different from
the Moon?
Astronomy 1-1
Lecture 08-11
Creating a Crater
Astronomy 1-1
Lecture 08-12
Craters on Mercury
Different then on the Moon
Mercury’s gravity is twice that on the Moon
Higher gravity keeps material from being
ejected as far 65%
Complex forms and structures characteristic of
large craters occur in smaller craters
Astronomy 1-1
Lecture 08-13
Caloris Basin
Roughly 1400 km across
Interior is occupied by smooth
plains
Concentric ridges
Radial fractures
Outside of basin there is there
are two basin rims and ejecta
material
Smooth plain are between basin
rims
Astronomy 1-1
Lecture 08-14
Antipodal to Caloris
180o to Caloris is a region a
weirdly contorted terrain
Though to be caused by the
focusing of seismic waves
from the collision that caused
the Caloris Basin
Astronomy 1-1
Lecture 08-15
Antipodal to Caloris
“Weird terrain” is thought to result from
focusing of seismic waves
Astronomy 1-1
Lecture 08-16
Surface Features - Plains
Relatively flat or smoothly undulating surfaces
Three ways planets are resurfaced
•Raising temperature – reducing strength of
crust and its ability to retain high relief
•Flow of material to toward lower elevations
under influence of gravity
•Fragments of material are deposited from above
How the plains on Mercury were formed is unknown
Astronomy 1-1
Lecture 08-17
Surface Features - Scarps
Scarps are steep, cliff-like slopes that
separate large areas lying at different surface
levels – 100m Æ 3km high
Generally run North-South
As Mercury cooled, the inner core regions
contracted
Overlaying material fell inwards
Planet has shrank ~ 2km in diameter and it is
felt that it has not finished shrinking
Astronomy 1-1
Lecture 08-18
Surface Composition
Range of surface brightness on Mercury is less than that of
the Moon
Color differences less pronounced
These suggest that surface lacks iron and titanium silicates
Astronomy 1-1
Lecture 08-19
Mercury's Structure
This can occur if the core region was iron rich and that
the core region represented a large fraction of the
planet's volume. The core region of Earth is - 16% of
the volume, while Mercury's core region is about 41%.
The larger percentage for the core region can be
understood by realizing Mercury's position. Because of
the high temperatures, all the easily meltable minerals
were vaporized off the planet
Astronomy 1-1
Lecture 08-20
Mercury's Structure
Core, nucleus, region occupies
about 42% of Mercury’s volume
and is about 1800km in radius
Surrounding core is the mantle
which is about 600km thick
On top of the mantle is the crust
which is about 100 – 200km thick
Astronomy 1-1
Lecture 08-21
Why Moon – Mercury Differences
Mercury is slightly larger than the Moon.
It had more energy to radiate away.
Mercury, therefore took longer to cool down.
The crust remained thin longer and could
easily be broken.
This would allow the subsurface magma greater
opportunity to break through and flow.
Astronomy 1-1
Lecture 08-22
Astronomy 1-1
Lecture 08-23