Physics 101 - Astronomy - Spring 2019
Class notes for day 11, Feb. 21, 2019
We watched the movie "Earth is Born" on Tuesday, and your paper on that is due next Tuesday. You might be interested in an update about the Earth's magnetic poles. The north magnetic pole of the Earth has been moving around since it was first located in the 1600's, and an interesting map of this motion is at https://en.wikipedia.org/wiki/File:Magnetic_North_Pole_Positions.svg
Ch. 9: Cratered Worlds: the Moon and Mercury
To start talking about the Moon, we recall that Earth is a layered object.
The Inner Core is made of solid iron, and is surrounded by an Outer Core of
liquid iron, which is the source of the Earth's magnetism. The Mantle is a dense
rocky material outside of the iron core, and this is about 80% of the Earth's
total volume. Floating on the mantle is a relatively thin Crust of less dense
rocks. The Hydrosphere is the oceans which fill the lower areas of the crust.
Our Atmosphere has several layers, and then above that is a Magnetosphere which
has very little gas and ions, but is influenced by the magnetic field of the
Earth.
The Moon, in contrast, does not have a hydrosphere, atmosphere, or
magnetosphere.
Contrast the near side and the far side of the Moon. The near side has more of
the Maria than the far side. Both sides have Highlands which are older
structures and have lots of Craters. The far side also has a deep Impact basin
from a very large impact early in its history.
Craters and ejecta are described in a slide in the PowerPoint.
Tides in the Earth's oceans are due to the gravitational pull of the Moon, which
is stronger on one side of the Earth close to the moon. This pulls the water in
the oceans around and we get high tides twice a day. See the slides for a
picture of the cause of tides. Solar and Lunar effects on Tides depend on the
relative orientations of the Earth, Moon, and Sun. "Spring tides" occur when the
Sun, Moon, and Earth are along a line, and "Neap tides" occur at first and third
quarter.
The tidal bulge is actually offset somewhat from being directly underneath the
Moon. This produces a torque, a kind of twisting force on the Earth. The tides
cause the Earth to slow down and the Moon’s orbit to increase in radius (4 cm
per century). Also, the Moon’s rotation is synchronous with its orbit as a
result of tidal effects early in its history when the interior was still liquid.
The Moon’s formation is believed to be due to a collision of a massive
(Mars-sized) object with the early Earth. Computer simulations show that this
could leave a moon-sized satellite in the correct orbit. After a few million
years, the Moon was formed from the fragments that had been knocked off the
Earth by the collision. The Moon was uniformly covered by craters during
the early bombardment period, but then as the interior slowly heated up from
radioactive decay of elements in its core, the Maria were formed from large lava
flows. Presently, another 3 billion years of bombardment has covered the maria
in more craters to give the Moon its present appearance.
Some of the key facts about the rotation of the terrestrial planets are:
Mercury’s rotation is tidally locked to 2/3 of an orbit.
Venus rotates very slowly backwards (clockwise as seen from above the north
pole) compared to other planets (this is also called retrograde rotation).
Mercury and Venus both have almost no axial tilt, Earth and Mars both have
similar axial tilts (which gives them seasons) and rotation rates.
The atmosphere of a planet has a big influence on the temperature.
Mercury has no atmosphere, just like our moon. Because there is no atmosphere to
trap heat, the night side of Mercury gets very cold (100K or -280oF). The day
side of Mercury gets up to 700K (about 800oF). This is the biggest variation of temperatures
(from day to night) of any planet in the Solar System.
Mercury’s surface has a large number of scarps (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. There are lots of craters just like the Moon, but none of the Maria that are seen on the Moon. Some craters have double rings around them, but otherwise are similar to those on the Moon. A spacecraft that went to Mercury in the past decade was the MESSENGER mission. No spacecraft has landed on Mercury.
Recall that we can learn something about the core of the Earth because we can have sensitive equipment (seismographs) which record the waves that travel through the Earth from distant earthquakes. These are refracted as they traverse the boundaries between the core and mantle, etc. and we can work backwards to deduce the internal structure of the Earth. We have not put similar types of equipment on other planets, so we have no direct evidence of the structure of their interiors.
Planetary interiors have not been studied directly, but we can make
estimates based on density and magnetic measurements.
Mercury’s mantle is solid
(not semi-molten like the Earth’s) and its iron core also may be solid as well.
Magnetic fields give some clues about the interiors.
Mercury has a very weak magnetic field, probably due to its large iron core that
solidified into a large magnet.
The Earth’s magnetic field is formed because of the Earth’s rotation and because
the Earth has a molten core.
There is no Lunar magnetic field!
So, among the terrestrial planets, only Earth has a significant magnetic field, which helps shield our atmosphere from the effects of the solar wind.
Links to extra material about Mercury (for your interest, much more than you need to know):
http://www.nasa.gov/mission_pages/messenger/main/index.html
Various movies about Mercury and the MESSENGER mission:
http://messenger.jhuapl.edu/Explore/Videos.html
