Download Day 12 - Ch. 5 - Mercury and Venus

Mercury, seen from Earth through a moderate telescope.
Venus
Mars
Ch. 5: the Terrestrial Planets
We survey the four terrestrials first, then look
at details for Mercury, Venus, and Mars.
Planetary Radar allows us to determine the distance to Venus.
It also lets us determine the rotation of the planet.
Terrestrial Planets’ Spin Axis and Rotation Rate
Terrestrial Planets’ Spin
Mercury’s sidereal rotation is tidally locked to 2/3 of an orbit.
Venus rotates very slowly backwards compared to the other
Planets, so that it is rotating clockwise (others are CCW).
Mercury and Venus both have almost no axial tilt, Earth and
Mars both have similar axial tilts and rotation rates.
Mercury’s Rotation is tidally locked to the
orbital revolution period (the Mercury-year).
The solar day on Mercury is 176 days long,
which is two Mercury-years!
See following slides for details.
During the
Mercury-year the
planet rotates 1.5
times with respect
to the stars,
but only half of
a solar day.
For an observer at
the location of the
arrow,
noon to sunset
would take
44 Earth-days.
After 88 Earth-days
the observer would
experience
midnight (next slide).
During the
Mercury-year the
planet rotates 1.5
times with respect
to the stars,
but only half of
a solar day.
For the observer,
midnight to
sunrise would take
another
44 Earth-days.
Then after another
44 Earth-days the
observer would
experience noon
(day 176, next slide).
So it takes two
Mercury-years
for the planet to
rotate through
a solar day.
This solar day
lasts 176
Earth-days.
Mercury orbits
the sun in 88
Earth-days.
Mercury’s Rotation is tidally locked to the
orbital revolution period.
The solar day on Mercury is 176 days long,
which is two Mercury-years!
This tidal locking is different from that of the
Moon revolving around Earth because Mercury’s
orbit is elliptical and not nearly circular like the Moon’s.
Atmospheres and Temperatures
Mercury has no atmosphere, just like our moon. Because
there is no atmosphere to trap heat, the night side of Mercury
gets very cold (100 K or -280oF). The day side of Mercury
gets up to 700 K (or 800oF).
The atmosphere of Venus is made up of carbon dioxide, with
clouds of sulfuric acid. The atmosphere is some 90 times
denser than Earth’s. The Greenhouse effect causes the
surface temperature of Venus to be close to 730K (or 860oF)
day or night.
Mars has a very thin atmosphere (less than 1% of Earth’s) of
mainly carbon dioxide. The surface temperature is
approximately 50 K (50oC) lower than Earth’s.
Mercury, Up Close
Mercury, Very Close, from Mariner 10 photos
Mercury’s Surface
Mercury’s surface has
a large number of these
scarps or cliffs like giant
cracks in its surface.
Mercury never had plate
tectonics like the Earth.
When the crust of
Mercury cooled it shrank
causing the crust to crack.
Mercury’s Caloris Basin, due to a very large impact
Mercury Maps and images
•
•
•
•
USGS has a website with planet maps
http://astrogeology.usgs.gov/maps/planetary-maps-and-globes
These can provide tennis ball projects for kids
http://planetarynames.wr.usgs.gov/Page/mercuryQuadMap
• Newer results have resulted from the MESSENGER spacecraft.
• Launched in August 2004, the Mercury Surface, Space
Environment, Geochemistry and Ranging spacecraft, known as
MESSENGER, entered into orbit around Mercury in 2011. The
following photos are from this spacecraft.
MESSENGER insertion into a Mercury orbit
occurred in March 2011
Launched Aug. 3, 2004 from Kennedy Space Center
Earth flyby in August 2005
Venus flybys in October 2006 and June 2007
Mercury flybys in January 2008, October 2008
and September 2009
MESSENGER Fact Sheet from JPL is here
Either here or after the entire PowerPoint presentation,
you should look at a tribute movie from NASA which
shows some animations of the spacecraft orbiting
Mercury and has a wide variety of pictures of
terrain on the surface. See this webpage for the
movie and for some of the images:
http://photojournal.jpl.nasa.gov/catalog/PIA16893
Venus, Up Close
Because of Venus’s dense
cloud cover most of what
we know about Venus’s
surface and rotation comes
from using radar.
There have been only a few
spacecraft to land on Venus,
but each survived for only a
short time.
(This is an ultraviolet photo.)
Venus Radar Map taken by the Pioneer Venus spacecraft
Earth Radar Map, shown with scale similar to the Venus map
Venus map made by the Magellan spacecraft
In 1995 the Magellan
spacecraft was able
to make a much more
detailed radar map of
Venus.
Possibly active shield
volcanoes, craters,
and volcanic structures
called coronae were
seen by Magellan.
Venus’s Surface Features
including some
volcanic cones and “coronae”
A Venus Corona – a collapsed volcanic cone
Impact Craters on Venus – as seen with radar from above
Venus surface, taken with a camera on the Soviet “Venera”
probe (“Venera” just means “Venus” in Russian).
Atmospheres and Temperatures
Mercury has no atmosphere, just like our moon. Because
there is no atmosphere to trap heat, the night side of Mercury
gets very cold (100 K or -280oF). The day side of Mercury
gets up to 700 K (or 800oF).
The atmosphere of Venus is made up of carbon dioxide, with
clouds of sulfuric acid. The atmosphere is some 90 times
denser than Earth’s. The Greenhouse effect causes the
surface temperature of Venus to be close to 730K (or 860oF)
day or night. This is hot enough to melt lead or tin.
Mars has a very thin atmosphere (less than 1% of Earth’s) of
mainly carbon dioxide. The surface temperature is
approximately 50 K (50oC) lower than Earth’s.
Venus’s Atmosphere has an extreme Greenhouse effect
Terrestrial Planet Interiors
Mercury’s mantle is solid (not semi-molten like the Earth’s) and
its iron core may have a solid center. There should be a liquid
iron layer which causes the magnetism of Mercury.
Not much is known about the interior of Venus – but it is thought
to be like a young Earth. (so we don’t have a figure for it above)
Mars is mostly solid and no longer geologically active in any way.
Earth’s Magnetosphere is due to it’s magnetism.
The Earth’s magnetic field is formed because of the
Earth’s fairly rapid rotation and because the Earth
has a molten iron outer core which can flow and
can have electric currents in it.
There is so Lunar magnetic field so it has little iron.
Terrestrial Planet Magnetic Fields
Mercury has a very weak magnetic field, partly due to its slow
rotation, even though it may have liquid iron in the core.
Venus has no measurable magnetic field, probably due to its
very slow rotation rate.
Mars has a magnetic field weaker than Mercury, meaning
that its core is either not liquid or not metallic.
Theories of the origin of the Terrestrials
The “rocky” planets all formed in a warmer
environment near the Sun in the early
solar nebula. Current theories are based
on computer models, which show that
the inner planets formed from a band
of planetesimals that may have been
influenced by a Jupiter which was closer
to the Sun and then moved away. For
an article about this, if you are curious, see
http://www.skyandtelescope.com/astronomy-news/our-new-improved-solar-system/
For a video showing how the Late Heavy
Bombardment could have happened, see
http://www.skyandtelescope.com/sky-and-telescope-magazine/beyond-the-printed-page/video-evolution-of-the-solar-system/