Astronomy Test 3: Part 2

Origin of the Solar system

- A large amount of evidence supports this idea and explains major features of the solar system well : -Existence of two types of planets: - Terrestrial and Jovian: their sizes and the composition. Patterns of motion of the planets and large objects: - Orbit in same direction and plane, most rotating in the same direction Existence of smaller objects: - Asteroids and comets. -About 4.6 billion years ago, a cloud of interstellar gas and dust, few light years in size started contracting under its own gravity. • Contained mostly (98%) Primordial matter (hydrogen and helium) - and2%ofotherelementsintheformoficeand dust • These were formed in older stars and thrown into the interstellar space when they ended their lives in supernova explosions. - 0.2% Metals: iron, nickel, aluminum - 0.4% Rocks: silicon-based minerals - 1.4% Ices: hydrogen compounds like methane (CH4), ammonia (NH3), water (H2O) -Interstellar Gas clouds are mostly hydrogen and helium, primordial mix of elements: - about 92% hydrogen atoms and 8% helium atoms by number (or about 75% H and 25% He by mass), matter formed right after the Big bang. - There could be 1-2% heavier elements in gas clouds now. • Heavier elements were produced in stars and ejected into the interstellar space as a star blows off its outer layers during the final stages of its life.

• The nebular hypothesis :

- States that the solar system formed from the gravitational collapse of large interstellar cloud of gas and dust - Originally proposed by the philosopher Immanuel Kant and later by the mathematician Pierre Laplace in mid 18th century.

Earth's Atmosphere

-Earth's atmosphere can be divided into several distinct layers: -The Troposphere -The Stratosphere and Ozone Layer -The Mesosphere and Ionosphere

Formation of Terrestrial planets

-The friction between infalling gases heated the nebular disk. - Inner parts of the nebular disk got hotter than the outer parts. Hottest near the center where the density is highest and the protosun was formed. (no nuclear reactions yet, it is emitting energy from gravitational collapse) Higher temperature of the inner region melted and evaporated ice particles. • only dust particles made up of rocky material and metals remained solid. • As those dust particles collided and stuck together, larger bodies of rock and metal formed • Like a snowball thrown through a blizzard getting larger as it hits other snowflakes • They are called Planetesimals (small planets- ~1km in size) • Further coalescing of those planetesimals under gravity eventually formed planets • After protoplanets were formed, energy from the collapse of matter and accumulation of heat from radioactive decays melted them. • materials differentiated (separated according to their density, denser to the core, lighter to the surface) and planets took spherical shapes. • Terrestrial planets were formed from Rocky and metallic dust, - They made up only 0.6% of the material in the solar nebula, so the amount of material available to form inner planers was small. - Therefore terrestrial planets could not grow very large. • Their smaller size could not exert large enough gravitational pull on hydrogen and helium gas from the surrounding nebula. - Whatever gases they initially had, escaped due to higher temperatures. • Hence, the terrestrial planets (Mercury, Venus, Earth, and Mars) are dense small worlds composed mostly from 0.6% of heavier elements contained in the solar nebula.

The Stratosphere and Ozone Layer:

-where air flow is mostly horizontal, temperature rising with altitude.

Lunar Exploration

Moon is the only place in the universe people have visited beyond Earth: -First spacecraft to fly past Moon (Luna 1): January 1959 -First spacecraft to (crash) land on Moon (Luna 2): September 1959 -First pictures of far side of Moon (Luna 3): October 1959 -The United States is (so far) the only country to send people to the Moon: -First manned mission to Moon (Apollo 11): July 1969 -Last manned mission to Moon (Apollo 17): December 1972

asthenosphere

Sitting just below the lithosphere is a region of plastic consistency

Simeis 147: A Supernova Remnant,

Stellar debris cloud from a supernova occurred about 100,000 years ago. (now spans 150 light years).

lithosphere

The upper part of the mantle and the crust together

Jovian planets:

large, gaseous, less dense, farther from the Sun • Beyond the frost line temperatures were low. - Water and other hydrogen compounds (ices) were frozen and in a solid form that could coalesce and form larger objects. ⇒ in the outer solar nebula, planetesimals formed from ice flakes in addition to rocky and metal dust. • Since ices were more abundant (1.4%) the planetesimals grew to much larger sizes, becoming the cores of the four Jovian planets. • Large massive cores thus formed had strong enough gravitational pull to capture hydrogen and helium gases from the surroundings nebular disk, • This added gas made their gravity even stronger allowing them to capture even more gas and grow larger and larger in size. • Thus becoming the large, gaseous, low-density worlds rich in hydrogen and helium, with dense solid cores and thick atmospheres. • Some of the nebular material captured by the proto planet form a small nebular disk around them. - Fracturing of the disk into smaller rings and subsequent coalescing of material formed large moons orbiting them. • The protosun continued collapsing under gravity, further heating it up in the process. • When the core reaches 10 million K, hydrogen nuclei (protons) are moving fast enough to overcome the electrical repulsion between them and combine together. - Nuclear fusion of hydrogen begins, converting hydrogen in to helium and releasing energy. H + H+ H+ H →He • As the core temperature increased due to nuclear reactions so did the pressure of the gas pushing it outward, stopping further collapse under gravity. The protosun became a star. • A combined radiation pressure (photons) and the solar wind (outflowing matter from the Sun) blew away the leftover gases and dust, thus the formation of the solar system came to an end.

Differentiation

light material rise to the surface while dense material sink to the core

P-waves:

longitudinal waves that can propagate in both solids and liquids.

Terrestrial planets:

relatively small, rocky, dense, close to the Sun

S-waves:

transverse waves that can propagate in solids but not in liquids

The Troposphere:

where all weather phenomena takes place temperature decreasing with altitude.

Greenhouse Effect

• About 30% of sunlight is reflected back to the space, rest is absorbed by the Earth's surface, warming it. -Earth surface re-radiates as infrared thermal radiation (since earth is at a lower temperature ~300K compared to 6000K of Sun) -But atmosphere is not very transparent to this infrared radiation and absorb some causing further heating -Greenhouse gases like water, carbon dioxide and methane increases the absorption of infrared radiation which makes the atmosphere warmer, which in turn increase the overall surface temperature leading to an overall temperature rise.

Asteroids

• Between Mars and Jupiter there are thousands of rocky planetesimals from 1,000 km to a few meters across called Asteroids. - Those are thought to be debris of the formation of the solar system that could not merge to form a planets due to influence of Jupiter's gravity. • About 6 are larger than 300 km, most are smaller (< 10 km) and do not have enough mass to be spherical. • More than 300,000 asteroids have been identified and cataloged, over a million asteroids lager than 1km are estimated to be there and many millions smaller ones.

Kuiper Belt Objects/Comets

• Comets are also debris left over from the formation of the solar system. • They were located beyond the frost line, - so unlike for asteroids, in addition to metal and silicate dust, ice particles were also coalesce to form planetesimals • But away from the Sun, beyond Neptune in coldest regions of the nebula, the density was low that those icy/dusty planetesimals could not grow very large. - They ended up like loosely packed dirty snow balls, most few kilometers in size.

Global Warming

• Current "ideal" conditions are due to Earth's unique geology and biology, which keeps green house gases (CO2) and thus temperature at equilibrium. Human activity seems to be causing change faster than the natural CO2 cycle can correct for it. • Consequences of Global Warming: many potential problems. Sea level rise (melting Antarctic ice) , climate effects of ocean current change, etc., • key is global concerted understanding and action.

The Planet Earth

• Diameter: 12756 • Mass : 5.97x1024 • Density 5515 kg/m3 • Escape speed 11.2 km/s • Albedo 0.31 • Average distance from the Sun: 1.496x108 km • Atmosphere composition (dry): - 78% nitrogen - 21% oxygen - 0.93% Argon - 0.04% Carbon dioxide - Water vapor 0.001% - 5%

The Moon

• Diameter: 3474 km (0.27 of Earth) • Mass: 7.3× 1022 kg (0.012 of Earth) • Density: 3300 kg/m3 • Escape Speed: 2.4 km/s • Distance to earth: 384399 km -Moon has large dark flat areas, due to lava flow, called maria (early observers thought they were oceans) Moon also has many craters formed by meteorite impacts. - Few craters in maria, lava flows (3 billion years ago) had erased craters, there has not been much meteor bombardment since then. - Most lunar craters date to at least 3.9 billion years ago; much less bombardment since then -The iron-rich darker maria are a younger surface made from ancient lava flows and are 2 to 5 km below the average surface elevation. -The cratered highlands are an older surface several kilometers above the average elevation. -We see only one side of the moon, far side of the moon is observed by spacecrafts. Far side have had less volcanic activity, thus no major maria.

Earth's magnetic field and the magnetosphere

• Earth acts like a giant bar magnet, source of the Earth's magnetic field is believed to be the electric currents in of its molten metallic core, created by convection currents. • Sun emits charged particles, mostly electrons and protons, some of which reach the Earth • They do interact with the Earth's magnetic field to create the magnetosphere . • These charged particles are trapped in areas called the Van Allen belts, where they spiral (move) along the magnetic field lines.

Pangea - the Supercontinent

• In early 20th century, Alfred Wegener, a German geologist noticed that the coasts of western Africa and eastern South America looked like the edges of interlocking pieces. • Wegener hypothesized that there was a gigantic supercontinent 200 million years ago, which he named Pangaea (All-earth). - It broke up and drifted atop a liquid core to leading to oceans, continents, and other land masses exist today. • Continental drift idea is now replaced by plate tectonic mechanism, - otherwise the idea of supercontinent and is breakup 225 million years ago is well supported by geological and fossil evidence.

Moon's Interior

• Like Earth, the Moon has a crust, a mantle, and a core, a liquid outer core surrounded by a solid inner core. • Crust is much thicker than that of earth (smaller size ⇒ faster cooling) • Moon's density is relatively low, and it has no magnetic field, so cannot have sizable iron/nickel core • Moon has no atmosphere, its gravity is too weak and temperatures are too high to retain an atmosphere.

Interior of the Earth

• Most of our knowledge about the interior of the Earth has comes from the study seismic waves -Pressure waves generated by natural events like earthquakes, volcanos or artificially by explosions and mechanical devices -P-waves -S-waves The Earth seems to have: • A Solid inner core surrounded by a liquid outer core, made mostly of iron and nickel. The diameter of the core is estimated to be ~7000 km. • A Crust, only a few tens of kilometers thick made of rock (granite and basalt) • The region between the core and the crust is called the mantle, made of silicates . - lithosphere. - asthenosphere.• • • -Differentiation -Decay of radioactive elements heats up the core -The Earth (or other planets) did not have this interior structure when it was formed. -Earth was probably molten when formed and due heat from bombardment by space debris and radioactivity.

Forming the planetary disk

• Most of the gas collapsed to the center forming the proto-Sun. • As the gas cloud contracts, its rotational speed increased due to the conservation of angular momentum -The rotation of the disk prevented further collapse of the disk stabilizing at a diameter of about 200 AU (0.003 ly) The rotating ball collapsed into a thin disk with most of the mass concentrated near the center forming a protostar (protosun) The disk (proto planetary disk) evolved into the planetary system.

Planetary Moons of the Solar system

• Seven large planetary satellites are comparable in size to the planet Mercury. - Most of them are with Jovian planets. • The remaining satellites of the solar system are much smaller (less than 250 km).

Formation of the Moon

• The leading moon formation theory is that some 4.5 billion years ago a planetary body (size of Mars) collided with the young Earth. • That collision ejected a large amount of planetary material into space, which accreted to create our Moon. • It is widely accepted due to the similarity in composition of the Moon and Earth's crust. Another piece of supportive evidence is the absence of a liquid metallic core

Plate Tectonics

• The lithosphere, typically 50-100 km thick is broken into large plates called tectonic plates. These plates sit on the asthenosphere. • The asthenosphere is kept plastic (deformable, can flow) largely through heat generated by radioactive decay. • Very slow convection currents flow in this plastic layer, and these currents provide horizontal forces on plates of the lithosphere, and slowly moving them (like a giant conveyor belt) • This movement of tectonic plates (few centimeters a year) drifts continents with them and cause other major geological activities like earthquakes and volcanos. • At tectonic plate boundaries, plates press or grind against each other, rise or subduct and develop different inter-plate stresses.

The Mesosphere and Ionosphere

• The original earth atmosphere which contained nebular gases similar to atmosphere of Jovian planets was lost to space at early stages of forming the Earth. • Current atmosphere was formed from compounds outgassed from the crust (volcanos) or have come from the impacts of comets and other planetesimals rich in volatile materials.

Aurora lights

• These charged particles are trapped in the magnetic field of the earth and taken to polar regions along the magnetic field lines, where they interact with atmosphere and produce auroras. - The Aurora Borealis (northern lights) or Aurora Australis (southern lights) are caused by charged particles from the Sun exciting oxygen and nitrogen atoms in theatmosphere.

The Solar System

• the Sun (99% of the mass in the solar system) • 8 major planets and their satellites, • asteroids, comets, and trans-Neptunian objects. • Most of the planets have slightly elliptical (nearly circular) orbits. • All planets orbit the Sun in almost the same plane and in the same direction.

Later Evolution of the Solar System

•Planetesimals did not merged into planets left as debris in the early solar system. - Some of the planetesimals debris collided with planets forming craters on them - The vast majority of the impacts occurred in the first few hundred million years. - Such caters can be seen on Moons, and planets which do not have thick atmospheres and much geological activity. - Erosion and geological processes on Earth have erased most of the craters.