The Earth and Moon
The Earth and Moon
The Earth and Moon form a bound system - the Moon orbits the Earth
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The Moon is quite large
- just over ¼ of Earth's radius and just over 1% of Earth's
mass. - Note the Moon has lower density than the Earth.
The Earth-Moon system can
almost be considered a binary planet.
Surface gravity of the
Moon is less than that of the Earth:
By Fg =
GM1M2/r2 - if the radius is 4 times
smaller and the mass of the planet (M1) is 100 times
smaller, than the force is <
Fg(Moon)
= Fg(Earth)*[1/100]/[ (1/4*4) ] ~ 16/100 ~ 1/6 smaller.
The force of
gravity on the Moon's surface is therefore about 1/6th of what it is
on the surface of the Earth.
The Earth
Solar system started forming about 4567 million years ago, as evidenced by
radioisotope calibration of unprocessed meteorite granules (specifical CAI inclusions).
About 2 million years later, the proto-planetary material had condensed into
large solid bodies (at least 100+ km in radius), large enough to undergo
internal heating, water processing and internal chemical interactions,
as evidenced by radioisotope dating of chondrules in meteorites.
The terrestrial planets appear to have been substantially formed within 5-10 million
years of the meteorite parent body formation, Jupiter may have formed more quickly still.
The Earth was substantially formed, and impacted by a Mars sized proto-planet
about 30 million years after the beginning of the formation of the solar system.
The solid crust of the Earth was in place about 300 million years after formation.
The Earth's composition has segregated. It has a molten interior, and many of the heavier metals have sunk into the core. The inner core is probably solid, with and outer liquid core. The core has a radius of about 3500 km.
The centre of the core has a temperature of about 5000K
Surrounding the core is the mantle this is a rocky layer which is semi-fluid or plastic.
Above the mantle is the crust. The crust is the outermost 30km or so of rock.
The continents and ocean are on the surface of the crust. The crust is fractured into plates - the plates move relative to each other. This continental drift leads to earthquakes as adjacent plates rub against each other, and mountain uplift as plates run into each other. On long (100s of My) time scales, the topography of the continents changes. Where one plate slips under another, subduction leads to crust material being recycled down into the mantle.
At the plate boundaries and discrete hotspots, upwellings of molten rock from the mantle can burst out of the crust, forming volcanoes.
Rock and gases ejected from volcanoes continually brings fresh mantle material to the surface and atmosphere.
The crust is mostly silicates, the mantle is iron/magnesium compounds while the core is iron/nickel rich.
Ocean
About 70% of the Earth's surface is covered by water. The ocean has a mean depth of a couple of kilometers.
Atmosphere
The Earth's atmosphere is 80% nitrogen, 20% oxygen + trace gases like argon, carbon dioxide, water vapour and methane.
The atmosphere is out of chemical equilibrium with the surface.
This is because of life. Metabolism, driven by solar radiation, continually cycles carbon, nitrogen, oxygen from geological reservoirs through the ocean, atmosphere and biosphere.
The Earth is heated by the Sun (small amounts of heat also come from radioactive decay of metals in the core - mostly potassium, thorium and uranium).
The Earth must radiate away as much heat as it receives.
The Sun mostly radiates in the visible, the Earth's atmosphere is transparent to the visible light, some is reflected, some is absorbed by the surface.
The surface then radiates in the infra-red. The amount radiated must equal the amount absorbed or the heat energy of the Earth will increase, leading to higher temperature.
Applying the Stefan-Boltzmann law to the Earth, we predice that Earth's mean temperature should be below the freezing point of water. The actual mean temperature is about 298K, 15K above the freezing point of water.
The difference is due to the greenhouse effect. This was discovered by Arrhenius in the 19thC. Gases in the Earth's atmosphere, particularly water vapour and carbon dioxide, absorb the infra-red radiation emitted by the surface and block it from escaping to space. The trapped radiation drives the Earth's temperature up, increasing the energy emitted per second until equilibrium is reached again.
Greenhouse effect | ClimatePrediction.net - do your own climate models at home
The amount of carbon dioxide in the Earth's atmosphere is about 0.033%, this is up from about 0.028% in the middle ages. The increase is due to human activity, mostly because of fossil fuel burning but also partly because of changing land use. The increase in carbon dioxide leads to warming, on average - human activity drives climate change - how large this effect is, and how critical it is for civilization and life in general, is a matter of some controversy.
At the top of the atmosphere, ultra-violet radiation from the Sun interacts with oxygen producing ozone, the resulting, very thin, layer of ozone absorbs most of the ultraviolet radiation. UV radiation is energetic enough to damage biological systems, and current life is adapted to low levels of UV.
Magnetosphere
The molten core of the Earth sustains a large scale magnetic field, this surrounds the Earth. The poles of the current magnetic field happen to be near the rotation poles of the Earth.
The magnetic field varies because of internal variations in the currents that generate it. The magnetic poles move, and at long intervals the magnetic field may reduce to much lower values, and may flip direction.
The magnetosphere shields the Earth from a lot of cosmic rays, and traps low energy cosmic rays and much of the solar wind in intense radiation belts above the Earth's atmosphere.
Aurora form near the poles when the trapped particles spiral down the magnetic field lines and hit the upper atmosphere.
In orbit
It is possible for us to get into orbit about the Earth.
To do this, we have to give an object - like a satellite or spaceship - enough energy to go into a circular orbit about the Earth. This requires us to lift the object above the atmosphere and then give the satellite enough velocity so that its trajectory "misses" the Earth as it falls back - the speed necessary to get into a circular orbit is about 7 km/sec - that is about 25 times the speed of sound.
To escape the Earth completely, a spaceship needs more energy.
There is a critical speed - the escape velocity which gives a spaceship enough kinetic energy to overcome the pull of Earth's gravity and escape (to go into a Solar orbit). The Earth's escape velocity is about 11 km/sec.
Vescape = SQRT(2)*Vorbit SQRT( G M/r )
We can launch things into orbit on a near routine basis.
Launch vehicle development and engineering is one of the more exciting and active areas at the moment.
Space craft engineering is a major field of activity that pushed technology to the bleeding edge of what can be done with modern materials.
Launch vehicle development and engineering is one of the more exciting and active areas at the moment.
The Moon is mostly rock. It has a small iron enriched, off-center, core and a deep mantle.
The Moon has no magnetic field and no atmosphere.
The Moon's surface is heavily cratered with Maria (singular Mare) - lava seas - in between the terrae - the lunar highlands.
In places there are peculiar wrinkles on the Moon, these are most likely cracks from the largest impacts that made the craters. Most of the craters we see are impact craters created by asteroids and comets hitting the Moon.
Since the Moon is geologically (selenologically!) inactive, and has no atmosphere and hence no weathering, the impact record is well preserved on the Moon.
At the lunar poles are craters in permanent shadow, these may form cold traps for water vapour from comet impacts, and there is evidence for water ice in these craters from the Clementine and Lunar Prospector missions.
Lunar formation - more formation details | after formation
People have been to the Moon - the Apollo missions landed 12 people on the Moon between 1969 and 1972 - no one has been back since, leading to the infamous question "If we can land people on the Moon, why can't we land people on the Moon?"
Tides - Earth-Moon interaction
The Moon interacts with the Earth through their mutual gravitational attraction.
A key aspect of the interaction are the tides.
The tidal force on an extended object, size d, a distance r from a planet, depends on d/r3 - that is to say the tidal force falls off like the distance cubed, but is bigger for bigger object. The tidal force is also proportional to mass.
The tides are due to the gradient of the gravitational force.
Tidals forces lead to tidal locking - this is what causes the Moon to be in synchronous rotation with the Earth. The Earth and Moon are squished by the tidal force they exert on each other, and the resulting bulge tries to line up as they rotate about each other.
For the Moon, the bulge has locked into pointing at the Earth.
In contrast, the Earth is rotating too quickly to have reached synchronous rotation - however, the Earth's rotation is slowing down due to the Moon's tidal pull, and the Moon is moving away from the Earth at the same time. Slowly!
Since the Earth rotates, the tidal bulge of the Earth doesn't stay pointed in a fixed direction. So the Earth flexes. The most important part of this motion happens in the oceans - they rise and fall in response to the Moon's pull, causing the tides.
There are two tides per day, because the tidal bulge has a front and a back!
So we get two low tides and two high tides per day.
The Sun also contributes to the tides from its gravitational pull on the Earth
When the Sun and Moon line up their tidal pull reinforces and we get spring tides - particularly high and low tides.
When the Sun and Moon are at right angles, their tidal pull partially cancels and we get neap tides - relatively modest high and low tides.
The spring/neap tides repeat every lunar month.
So we get two low tides and two high tides per day.
