Astronomy Final - Part 3

Why must there be a reservoir of atmospheric methane on Titan?

UV sunlight destroys methane quickly in the upper atmosphere. In order for there to continually be methane and other simple hydrocarbons in Titan's atmosphere there must be some way it is being continually replenished.

Why couldn't small stars have Earthlike planets?

"Red dwarfs" are faint and emit low energy photons so a planet would have to be very close to get the right energy for life. This closeness would force the planet into synchronous rotation making on side too hot for life and the other too cold. However, life could possible be in the places in perpetual twilight.

How did Saturn's rings form?

Close to Saturn the tidal distortion induced by Saturn is too great for a moon to be held together. The rings are made of these small pieces that could never stick together to form moons.

What is the criteria for a planet?

Discovery of KBOs larger than Pluto started the debate over what a planet is. To be a planet, a body has to meet the following criteria: 1. heliocentric orbit 2. large enough to be roughly spherical 3. have to have cleared out its orbit They then decided that objects like Pluto, Eris, and Ceres should be called "dwarf planets." These meet the first two criteria but not the third.

Why must there be reservoirs for comets?

Each time a comet passes close to the sun it loses some material with sublimation and eventually disappear. This means that comets from 4.5 billion years ago cannot be the same as those today. There must be reservoirs constantly replenishing the comets.

Where are the reservoirs for comets?

For short period comets (takes less than 200 years to complete a trip around the sun, orbits roughly coplanar with the planets) their origin is in the Kuiper Belt, the outer disc of the solar system beyond Neptune, where they start as icy bodies and are pulled into orbit. For long period comets (take more than 200 years, tilted orbit) their origin must be in a spherical cloud of icy bodies that surrounds the sun and planets (Oort Cloud).

How to gravity assists work?

Gravity assists make it possible for a spacecraft to reach the distant outer planets without using a lot of propellant. The spacecraft flies by planets, causing those planets to lose a tiny bit of energy and give it to the spacecraft. The spacecraft gains that energy, causing a great increase in speed.

What's up with the lunar cataclysm?

Jupiter was able to fling nearby plenetesimals far outward from the sun (may be how Oort Cloud formed) and also would have flung them inward towards us. While many probably plunged into the sun, others may have impaceted the Earth and Moon. Simulations of this show that this wouldn't have happened steadily, but rather would have happened in a brief spurt triggered when Saturn and Jupiter reached a 2:1 orbit-orbit resonance. This could explain the lunar cataclysm.

Why couldn't large stars have Earthlike planets?

Massive stars put off a lot of UV light which is lethal to life. These stars also burn out quickly, meaning there would be no time for life to evolve.

Why the the rings have gaps?

Particles in the rings collect in different zones because they were gravitationally attracted by the moons of Saturn and pulled into a new orbit. One of the largest gaps is called the Cassini Division.

What is the Transit method?

The Kepler mission used this method to find many exoplanets. In this method, you monitor the star's brightness to look for slight dips. From this, we can learn orbital period, semimajor axis, diameter, and atmosphere.

What did the Stardust mission do and find?

The Stardust mission went to the comet Wild 2 and collected interstellar dust particles. These provide a window into the distant past and helped to unravel mysteries about the creation and evolution of the solar system. It also found a fundamental chemical building block for life on a comet (glycine, an amino acid) showing that these chemicals were all over the solar system.

What are the centaurs?

The centaurs, such as Chiron, are Kuiper Belt Objects that are in the middle of the transition to a comet. This works as they begin to feel a tug from Neptune, elongating its orbit. Then, it feels a tug from Uranus and its orbit elongates ever more. Eventually it will either become a comet or be flung into the outer solar system.

What is the methane cycle on Titan and evidence to support it (Cassini)?

The methane cycle on Titan is just like the water cycle on Earth, with clouds, rain, and seas. Telescopes have observed clouds appearing and disappearing. Wet areas of the ground have also been seen, like what we would expect from rain. Titan also has a temperature that would allow methane to exist in solid, liquid, and gas forms, like we have for water on Earth.

What are the rings made of?

The rings consist of thousands of small bands which differ in size and composition. There are also spokes on the rings, dark markings in reflected light and bright streaks when lit from the rear. These are extremely small dust particles held together by electrostatic forces.

Why is the study of Titan important?

The study of Titan is important for many ways. The Methane cycle on Titan is an extreme version of our water cycle (there is vastly less evaporation but the moon can hold a lot of liquid) and shows what could happen here is global warming gets out of control on Earth. While there is no life possible on Titan, the organic content may be similar to early Earth so we can use it to understand something about the origin of life on Earth.

What are the two types of tails in comets?

There is the ion tail. This type is straight and has emission lines of numerous ionized molecules such as carbon monoxide, nitrogen, and water. There is also the dust tail. These are broad, diffuse, and curved. They are rich in microscopic dust particles that reflect sun. These are the tails that are usually visible from Earth.

What are Hot Jupiters?

These are large, gaseous planets that orbit absurdly close to their parent star. These made us rethink planetary formation. Instead of giant gas planets forming far away so there is ice to gather atmosphere and stay there, we theorize that they do form this way but then migrate inward due to gravity with other planets.

What is Gravitational Lensing?

This is a method we can use to find exoplanets. For this we look at the starlight of a distant star. When it grazes the top of an exoplanet we will see a difference in its light. Then, if we graph brightness v. time, there will be a jump to signal an exoplanet.

What role did planetary migration play in our own solar system?

This process happened early in our own solar system. Gravity between the four large outer planets and the many ice/rock planetesimals caused Saturn, Uranus, and Neptune to migrate outward and Jupiter to migrate inward.

How does the Doppler method work?

This was the predominate method to find exoplanets until recent years. In this method you look for tiny changing Doppler shifts in the absorption lines present in the star's spectrum. This implies that the star is moving slightly back and forth as a planet orbits it and pulls on it. Graphing the Dopple shift as a function of time allows you to figure out the planet's semimajor axis and the flattening of its orbit. We can also estimate the planet's mass. However, this is biased toward large planets close to their parent star.

What is the atmosphere of Titan like?

Titan's atmosphere is 60% thicker than our own. It is mainly nitrogen and the rest is mostly argon. There is also trace amounts of ethane, propane, and carbon monoxide which sunlight converts to a photochemical smog. This reddish haze hides the surface from our gaze so we have to use microwave radar and reflected infrared to see surface.

What does Titan's landscape look like (found by Cassini)?

Titan's landscape is surprisingly Earthlike. It has vast eareas of rugges hills, dark seas of sand, and sand dunes as high as those on Earth. There is also evidence of old streambeds, such as rounded pebbles and sand like we see around rivers. There are many lakes and seas around the poles. These lakes are made up of liquid hydrocarbons and their surfaces have changed over time.

How would we check for life on a planet?

To look for life we could use reflectance spectroscopy on the faint reflected starlight on the planet to look for absorption bands of particular gases in the atmosphere. We would look for gases like oxygen, ozone, and methane. If we found oxygen and methane in the atmosphere we would know that something would have to be continually supplying this as these to gases quickly react with each other to produce carbon dioxide and water unless something is constantly producing it.

Why did we think that Pluto was a planet?

When Pluto was first discovered it was thought to be roughly the size of Earth; however, now we know that it is only 1/6 the size of the moon. Pluto also has a moon, Charon, which we thought showed it was a planet. However, we now know that many KBOs have satellites now.

What is the structure of a comet?

When comets are far from the sun they are only the nucleus, which is extremely small and basically a dirty chunk of ice. When the comet reaches the sun the icy surface becomes too warm and sublimates into gas, this is the coma of the comet which can reach great sizes. Engulfing the coma there is the hydrogen envelope which stretches across millions of km of space. Finally, there is the tail, which is pronounced when the comet is is close to the sun and are directed away from the sun by solar wind.