ASTR Midterm II

Recall from the lecture on light and heat that some gas molecules absorb light in narrow bands called absorption bands. Gas molecules can absorb in a narrow range of wavelengths, where the range depends on the complexity of the molecule (how many ways it can internally vibrate). Diatomic molecules that vibrate in a simple way absorb at specific wavelengths (narrow band).

Greenhouse gasses (H20, CO2, CH4) trap heat of the atmosphere by absorbing infrared light and re-radiating this light in all directions: i.e., they slow the rate of heat escape from the atmosphere, raising surface temperatures. Without the greenhouse effect, our planet would be as cold as the Moon.

What are the four steps of the Carbon Cycle?

outgassing, dissolution in rainwater, precipitation in oceans, subduction

The effect of clouds is poorly understood. Clouds seem to absorb or reflect light (depending on elevation).

For reasons that aren't totally understood, our planet's climate (long-term conditions, not “weather”) goes between hot and cold (icy) conditions. During ice ages, the advance and retreat of ice sheets depends on changes in rotation axis tilt, which affects how much solar energy the poles are receiving (this also determines the seasons on a yearly basis).

Earth’s long term habitability is sustained by creating an environment where large amounts of liquid water can be stable on the surface of the Earth. This is due to three phenomena, all of which are largely a consequence of processes that have cooled the Earth over time. What are they?

- A magnetic field that protects the atmosphere
- Volcanism that releases gas from the interior
- Plate tectonics (via the “carbon cycle”)

How does our atmosphere stay warm?

Earth's atmosphere keeps much of the Sun's energy from escaping into space. This process, called the greenhouse effect, keeps the planet warm enough for life to exist. The atmosphere allows about half of the Sun's heat energy (50%) to reach Earth's surface.

How has Earth's climate changed over time?

Over the past million years, Earth’s globally averaged surface temperature has risen and fallen by about 5˚C in ice-age cycles, roughly every 100,000 years or so (Figure 2.1a). In the coldest period of the last ice age, about 20,000 years ago, sea level was at least 120 metres lower than today because more water was locked up on land in polar ice sheets. The last 8,000 years, which includes most recorded human history, have been relatively stable at the warmer end of this temperature range. This stability enabled agriculture, permanent settlements and population growth.

How does light interact with matter?

- Absorbed photons
- Transmitted photons
- Reflected photons
- Emitted photons
→ So, we can figure out what stuff is made of from a distance just by measuring its spectrum!

How do greenhouse gas molecules emit/absorb infrared light?

More complex molecules absorb more wavelengths. Greenhouse gas molecules absorb the longer wavelengths of outgoing infrared radiation from Earth's surface.

Where does radiative energy from sunlight go?

Greenhouse effect: solar radiation passes through the earth’s atmosphere; most radiation is absorbed by the surface, warming the planet; the light that is
A significant portion of the Sun's ultraviolet (high-energy, shortwave) radiation is absorbed by ozone (O3) in the upper atmosphere (the stratosphere). Solar radiation that is reflected back into space by Earth's surface or atmosphere does not add heat to the Earth system. Absorbed radiation is transformed into heat.

How do we maintain a constant energy level?

The earth-atmosphere energy balance is achieved as the energy received from the Sun balances the energy lost by the Earth back into space. In this way, the Earth maintains a stable average temperature and therefore a stable climate.

What does “climate” mean?

“Climate” refers to long-term conditions, especially temperature at surface

Where does CO2 come from?

planetesimals from outer regions containing ices or rocks chemically bound with H2O, common gasses

Why doesn't CO2 simply build up in the atmosphere?

mostly trapped in carbonated rocks (such as limestone)

What is the carbon cycle?

Greenhouses gasses maintain the Earth’s temperature by trapping heat the of the atmosphere by absorbing infrared light and re-radiating it. Doing this slows the rate that heat escapes from the atmosphere, which raises Earth’s surface temperature. Greenhouse gasses are also a part of the carbon cycle, which regulates Earth’s surface temperature by decreasing temperatures by increasing evaporation and rainfall and removing CO2 when temperature rise and rising temperatures by allowing CO2 to increase when temperature decrease.

What are the examples of greenhouse gasses?

H20, CO2, CH4

Does the Carbon Cycle ever fail?

Not really, however: The Earth's climate sometimes succumbs to extremes called “Ice-house” or “Hot-house” climates. We're in an “Ice house” now (a.k.a. “Ice age”: large amounts of ice at poles): it's lasted 2.5 million years. Ice ages are marked by advance & retreat of ice sheets;when this happens depends on Earth's tilt.

What is the effect of Earth's tilt? What causes the seasons?

The tilt of the Earth is what causes seasons to occur. These are the seasons in relation to the Northern Hemisphere. The tilt also produces effects such as the Midnight Sun, where the Sun never sets during some summer nights in very high-latitude regions.

What is corpuscular radiation?

fast-moving particles of matter (i.e., having mass) with some electric charge; is especially harmful to biological tissue if exposed in large, sustained doses

Radioactive decay refers to the change of one isotope of an element to another. An element is defined by the number of protons in the nucleus (known as the atomic number), and isotopes of an element have different numbers of neutrons in the nucleus. Some isotopes are unstable and “decay” via beta decay or alpha decay.

The half life of a decay process is the time interval over which any given atom has a 50% chance of decaying. Said another way, about 50% of the atoms originally present will decay in that time. Knowing the half-life (from lab experiments) and knowing the original amount of an unstable isotope in a mineral grain, and assuming only the “parent” isotope was present, we can use this principle to estimate the time since the mineral formed (usually at high temperatures, when the rock was last molten).

Layered sedimentary rocks are made up of grains from many different source rocks. Those source rocks were broken down by weathering and erosion and then transported to a place where the grains settled and collected. Over time,they hardened to form rocks.

Layered sedimentary rocks have been found also on Mars, such as by the Curiosity Rover in Gale Crater.

These rock layers preserve a history of events, with the oldest layers at the bottom of the stack and the youngest at the top. But because sedimentary rocks are made of grains from many different source rocks, age estimates vary widely.

Unfortunately, it is mainly igneous rocks contain mineral grains that, at the time of formation, only contained parent isotopes, and whose ages can be estimated directly using radiometric techniques.

How do we use radioactivity to estimate ages?

After the death of the organism, the amount of radiocarbon gradually decreases as it reverts to nitrogen-14 by radioactive decay. By measuring the amount of radioactivity remaining in organic materials, the amount of carbon-14 in the materials can be calculated and the time of death can be determined.

What are the three main sources of corpuscular radiation?

1. radioactive rocks in the Earth’s interior
2. cosmic radiation from other stars and galaxies
3. the Sun and its solar wind
→ The Earth's magnetic field and atmosphere largely shield us from corpuscular radiation emanating from outside the Earth's environment.

Name the two types of radioactive decays, as explained in class. What are the resultant particles in both types of decays?

The two types of decay are alpha decay and beta decay. The resulting particle of alpha decay is an alpha particle, consisting of two protons and two neutrons which are emitted from the nucleus. The remaining element of alpha decay has two less neutrons and two less protons, meaning its mass number is decreased by 4. The resulting particle of beta decay is an emitted electron. The remaining element of beta decay has an additional proton, but the atomic mass remains the same.

Since the planets and moons are made of objects that are asteroid- and comet-like (i.e., “planetesimals”), do they have the same composition and structure as asteroids and comets?

The larger planetesimals are differentiated bodies consisting of an iron-rich core, a rocky mantle, and an impacted, brecciated regolith.Comets are icy bodies in the solar system that have wide orbits. These orbits can take anywhere from a few years to hundreds of thousands of years to complete. A planet, on the other hand, is a large object that orbits around a star or a stellar remnant.

Convection transports heat through the liquid outer core and the solid (but hot and therefore slightly malleable) mantle. Conduction dominates heat flow through the cool, brittle outer layer of the planet (the lithosphere). When magma reaches the surface of the planet and pressure is released, dissolved volatile gases escape. This process, called outgassing, generated the atmosphere.

Convection in the liquid outer core generates a magnetic field that protects our atmosphere and the surface from the solar wind: a stream of charged particles moving at high speeds.

Convection in the mantle drives (1) ______, which (2) _______

1. plate tectonics
2. regulates the surface temperature over long periods of time

What are the stages of plate tectonics?

1) rigid crystal plates drift toward, alongside, and away from each other
2) at mid-ocean ridges, new crust is generated that cools and therefore becomes more dense (volume shrinks)
3) oceanic crust sinks into the mantle at subduction zones

Since subducting crust contains a lot of water and carbon (volatile elements), its melting temperature is lower. The sinking crust melts, releasing magma that forms volcanoes at the surface. Slipping that occurs as one plates slides beneath another is one cause of earthquakes. This explains why we find volcanoes and earthquakes along subduction zones.

What are the kinds of heat transport?

1. Convection – hot material expands and rises. Cool material contracts and falls. It transports heat through the liquid outer core to the rock in lower mantle. Rock near top layer of mantle loses heat to the surface by conduction and cools.
2. Conduction dominates heat flow through the cool, brittle outer layer of the planet (the lithosphere – crust and upper mantle).

What is outgassing?

When magma reaches the surface of the planet and pressure is released, dissolved volatile gasses escape. This process, called outgassing, generated the atmosphere.

What are the two types of crust? What happens when they collide?

The two types of crust are oceanic crust and continental crust. When oceanic crust and continental crust collide, the oceanic crust will always subduct under the continental crust because it contains a lot of water and carbon and therefore its melting temperature is lower. This process of subduction as the plates move past one another can cause earthquakes. When the oceanic crust melts, it releases magma that forms volcanoes at the Earth’s surface.

Why do continents drift? What process and what “force” is driving the motion?

Gravity causes denser (colder, heavier) materials to sink into the Earth's interior.
The movement of lithospheric plates referred to as continental drift, is believed to be caused by the radioactive decay of elements in the core and mantle that produces heat. The heat in turn creates convection currents in the mantle which "drive" the plates along their path of movement.

How does Earth's slow cooling affect its habitat?

1. Volcanism (advective cooling) creates the atmosphere via outgassing: gas escapes from magma when pressure released.
2. The magnetic field (partly generated by core convection, partly by rotation) protects our atmosphere from the solar wind.
3. Plate tectonics regulates the climate.

Apparent brightness =

#photons/time/area, and decreases with 1/R2, where R = distance from light source

Absolute brightness =

total photons emitted by source

Cooling rate is proportional to ____.

proportional to A/V = 3/r, where r = radius of world

Mercury and the Moon are airless worlds, heavily pocked with craters and rotate slowly (with extreme day/night temperature differences owing to (1) absence of atmosphere that can spread heat via convection and (2) advection, and (2) very slow rotation).

large iron core = ?
small iron core = ?

large iron core = high density (mars)
small iron core = low density (moon)

The Moon has large basins flooded with lavas (3-4 billion yr ago) and a more ancient highland. More craters / area → older surface. The surface is hostile: no atmosphere (no protection from cosmic rays, solar wind, hard radiation like UV, or meteoroid impacts).

Apollo brought back moon rocks that helped us understand the history of the solar system and the origin of the Moon (in a giant impact).

What are the conditions for life? Where are we likely to find these?

- Energy
- Carbon
- liquid water
- various other elements

What is the nature of dead, airless worlds?

Mercury and the Moon are airless worlds, heavily pocked with craters and rotate slowly (with extreme day/night temperature differences owing to (1) absence of atmosphere that can spread heat via convection and advection, and (2) very slow rotation).

What properties of a planet and its location are important for making a planet habitable?

Most relate to the conditions required to make water stable, as a liquid, at or just below the surface.

What is brightness? What is the difference between absolute and apparent brightness?

Brightness is a measure of the amount of light reflected by a material. Absolute brightness is the total number of photons (of any wavelength) emitted by the source per unit time, in all directions. Apparent brightness is number of photons per unit time, per unit area (e.g., hitting your 'detector')

How does radiative energy vary with distance from the Sun?

Radiation from the Sun is lessened by the inverse square law as it reaches further and further away from the Sun. So the further away that a planet is from the Sun then the less radiation it receives.

Life on Earth has survived many mass extinctions in the past 600 million years since the advent of animal life. Possible explanations include supernovae (high energy radiation and ionization of the atmosphere), volcanism (dust/ash blocks sunlight; changes in atmospheric composition), snowball earth (deep freeze), or large impact.

When rocks tumble through atmosphere at supersonic speed, they compress air in front, heating it to thousands of Kelvins so that it glows brightly, and melts and ablates the object.

Objects that are large enough (diameter > 50 m or so) reach the surface at supersonic speeds, forming an impact crater. In these events, the ground and projectile are compressed and thereby heated (material is melted or vaporized or damaged). The rebound from this compression excavates the crater. The shock wave from impact is a supersonic compression wave: a sound wave that breaks and heats (sometimes vaporizes, melts) the material it passes through.

Large craters collapse & flatten to make complex craters (smaller ones are “simple”). The transition diameter decreases with increasing planetary gravity.

We find evidence of large impacts in the geological record. Large events put a lot of dust in the atmosphere, blocking sunlight, frustrating photosynthesis.

Giant impacts have left their mark on most worlds in our solar system.

What are possible causes of mass extinctions?

- Supernovae: high energy radiation and ionization of the atmosphere
- Volcanism: dust or ash blocks sunlight, which changes in atmospheric composition
- Snowball earth: earth undergoes deep freeze, such as ice ages
- Large impact from an impactor

How do we study impacts?

- Laboratory experiments
- Computer simulations (requires equations of state, the laws of motion, and powerful computers).
- Large-scale explosions
- Field studies of craters
- Watch as they happen in nature
- Spacecraft observations: orbiters, landers, and rovers

What happens during an impact?

If the asteroid impacts a land mass, in addition to a huge shock wave, massive amounts of particles would be released into the atmosphere, causing serious environmental damage on a global scale and mass extinctions. As we know, the impact of large asteroids will have devastating effects no matter where they land.

What are the stages of a simple crater formation?

- Contact/compression stage: the projectile and the target are compressed
- End contact/compression stage: the shock wave from impact causes a supersonic compression wave that breaks and heats the material it passes through
- Evacuation stage: the rebound from compression excavates the crater via melting or evaporation
- End evacuation stage: “rebound” & excavation; the crater walls extend as the cavity deeps
- Modification stage: crater walls slump
- Final crater

What are the three components of an impactor as it enters the earths’ atmosphere?

- Turbulent wake: the trail behind the impactor showing its path through the atmosphere
- Supersonic bullet: the rock itself, which is heated and melted by the compressed air created by its speed. If the rock is less than 50m in diameter, the heat will ablate it; if it is large enough, it will reach surface and cause an impact crater
- Bow shock: the boundary that the solar wind of the atmosphere forms against the compressed air in front of an impactor

How has life been affected by impacts?

- Moon formation
- Sterilizing impacts?
- Atmospheric erosion
- Mass extinctions
- Tabula rasa effect (post mass extinction)
- Migration
- Influx of raw materials for life: complex organic molecules, water

How have other planets been affected by impacts?

- Venus: very slow, retrograde rotation
- Mercury: Very massive for its size; Made of 60% iron, twice the solar nebula abundances; implies a core that's 75% of the planet's radius.
- Mars: Lopsided! Northern hemisphere is lower than southern hemisphere: is it an impact basin?
- Uranus: Huge axial tilt, of ~97 degrees.
- Moon: Lopsided! It's shape and the Moon itself probably formed via giant impact.

Why are there so few craters on our planet compared to others? Where should we expect to find the most craters?

Earth is equipped with three processes that eat up craters relatively quickly: erosion, tectonics, and volcanism. These forces leave only the largest scars from meteorites or asteroids — unlike, say, the moon, which can't gobble up craters. Meteorite craters are more common on the Moon and Mars and on other planets and natural satellites than on Earth, because most meteorites either burn up in Earth's atmosphere before reaching its surface or erosion soon obscures the impact site.

Our modern definition of life in some ways resembles 17th century definitions of water: life is often in terms of phenomenal properties, a subset of which may be necessary and sufficient.

It's possible “life vs. non-life” is a false dichotomy: life may occur along a spectrum between, e.g., crystalline minerals and complex animals.

We might figure out what life is really using a top-down or bottom-up approach (cataloging traits of universal, essential properties, or trying to synthesize new life in the lab from primitive Earth conditions).

Life can be categorized by its sources of energy and raw material. Alien microbes and earliest Earth life were most chemoautotrophs.

What are evidence of early life on Earth?

stromatolites and microfossils

Major events: the rise of photosynthesis was key for the build-up of oxygen (for respirers like animals) & ozone. The Cambrian explosion was the origin of complex multi-celled life (w/skeletons & mobility!)

What is life?

"A self-sustained chemical system that can undergo Darwinian evolution". - Gerald Joyce
→ Our modern definition of life in some ways resembles 17th century definitions of water: life is often in terms of phenomenal properties, a subset of which may be necessary and sufficient.

What are the requirements for terrestrial life?

- Sources of energy
- Sources of raw materials (carbon): where does it come from?
- Medium for bringing these together (a solvent)

How do we classify life according to strategies for finding raw materials and sources of energy (resource usage)?

- Photoautotroph: utilizes raw material CO2 for carbon and sunlight for energy
- Chemoautotroph: utilizes raw material CO2 for carbon and inorganic matter such as iron, sulfer, and ammonia for energy
- Photoheterotroph: utilizes raw material organic compounds for carbon and sunlight for energy
- Chemoheterotroph: utilizes raw material organic compounds for both carbon and energy

Why does oxygen build up in the atmosphere? (And what are the consequences?)

The process of photosynthesis, which converts CO2 and water into oxygen and sugar, causes the buildup of oxygen in the atmosphere. One of the earliest organisms to utilize this process and is therefore responsible for the origin of our oxygen-rich atmosphere is Cyanobacteria, which is aquatic bacteria that originated 3.0 to 2.5 billion years ago.

Why is free oxygen vital to life on Earth and the nature of our world?

Most living things need oxygen to survive. Oxygen helps organisms grow, reproduce, and turn food into energy. Humans get the oxygen they need by breathing through their nose and mouth into their lungs. Oxygen gives our cells the ability to break down food in order to get the energy we need to survive.

What were Darwin’s observations about natural selection?

1. Overproduction of offspring
2. All species exhibit variation
3. Offspring resemble their parents
4. Some traits are more advantageous than others

What are the “great inventions of life”?

RNA/DNA
Photosynthesis
Sex
Mobility
Skeletons
Sight
Warm-bloodedness
Consciousness

Why is carbon so important?

A compound found mainly in living things is known as an organic compound. Organic compounds make up the cells and other structures of organisms and carry out life processes. Carbon is the main element in organic compounds, so carbon is essential to life on Earth. Without carbon, life as we know it could not exist. The reason is carbon’s ability to form stable bonds with many elements, including itself. This property allows carbon to form a huge variety of very large and complex molecules.

What is the standard sequence of planetary robotic missions (in order of increasing complexity and cost)?

flybys, orbiters, probes, landers, rovers, sample return

The primary cost of missions is rocket fuel. Once a rocket is burned, objects tend to remain in motion (no air resistance in space), so further fuel is only needed for course corrections and for slowing down (and landing).

Owing to misunderstandings & wishful thinking (canals!), Mars was guessedto be host to an extinct alien civilization. The first flyby missions found a tenuous atmosphere and cratered surface. After the dust settled in 1971, Mariner 9 orbiter found giant volcanoes and ancient river beds.

Mars probably had something akin to hotspot volcanism. Without plate tectonics and with low gravity, volcanoes build up to great heights.

How do we study planets using spacecraft?

- Flybys
Cheap
Limited measurements and time
- Orbiters
Long term monitoring; mapping
Expensive
- Atmospheric probes landers & rovers
Detailed observations; “Ground truthing” of remote observations; Long term monitoring of local conditions
Even more expensive;Limited sampling

Without spacecraft, how do we learn about other worlds?

Spectroscopy
​telescopic images
meteorite analysis
stellar occultation

What have we learned about Mars' ancient past?

The movement of water impacts the top layer of soil on the planet, leaving water shapes on the planetary surface that are visible today:
1. Glaciers, which leave massive ice flows
2. Drainage network channels that appear similar to the branches of a tree
3. Meandering rivers similar to those that form on earth in level flood plains

How is volcanism different on Earth vs. Mars?

Because the lower gravity of Mars generates less buoyancy forces on magma rising through the crust, the magma chambers that feed volcanoes on Mars are thought to be deeper and much larger than those on Earth. If a magma body on Mars is to reach close enough to the surface to erupt before solidifying, it must be big.

How does axis tilt affect climate & seasons on Mars?

On Mars, changes in axis tilt influence the atmospheric pressure.
- High tilt: thicker atmosphere
- Low tilt: thinner atmosphere

Name the two types of radioactive decays, as explained in class. What are the resultant particles in both types of decays?

The two types of decay are alpha decay and beta decay. The resulting particle of alpha decay is an alpha particle, consisting of two protons and two neutrons which are emitted from the nucleus. The remaining element of alpha decay has two less neutrons and two less protons, meaning its mass number is decreased by 4. The resulting particle of beta decay is an emitted electron. The remaining element of beta decay has an additional proton, but the atomic mass remains the same.

What is a sedimentary rock?

Sedimentary rocks are a layered rock composed of grains from various different source rocks, which were broken down by weathering and/or erosion, transported, collected and settled, and finally hardened to form rocks. Because of this process, sedimentary rocks can be estimated by examining their layers, with the oldest layers at the bottom and the youngest at the top.

Explain how the atmosphere is generated through the process of outgassing?

1) convection transports heat through the liquid outer core and the malleable, solid mantle of the Earth
2) conduction creates heat flow through the lithosphere
3) magma created reaches the surface, releasing pressure and allowing dissolved volatile gasses to escape

What is an isotope?

Isotopes are two or more types of atoms that have the same atomic number but different numbers of neutrons in the nucleus.

What are the genetic classifications of organisms?

Bacteria
Archaea
Eukarya

Earth's (1) _____ protects its atmosphere from (2) ______

(1) magnetic field
(2) solar winds

What components are necessary for life?

1. liquid water (dictated by goldilocks zone; can't be too cold to freeze or too hot to evaporate)
2. carbon
3. atmosphere
4. energy

Why do we study these moons (Io, Europa, Titan) over other moons?

- geologically active (Io)
- possibility of liquid water (Europa)
- thick atmospheres with lots of carbon (the atmosphere in europa’s neighborhood is changing)
- protective magnetic fields (salt content of underground water on Europa creates magnetic field)

What are the four Jovian moons?

Io, Europa, Ganymede, and Callisto

What is Titan?

Saturn's largest moon
Titan is only moon with an atmosphere; it has liquid ethane, methane that forms streams and lakes; but no liquid water on surface, and low temps

How does this pattern of varying force change the shape of an object?

The particles closest to the moon have a greater pull towards the moon, stretching the planet into an oblong, elliptical shape.