Astronomy Exam 3

RNA World

A hypothetical stage in the early history of life on Earth, proposing that RNA (ribonucleic acid) served as the primary molecule for both storing genetic information and catalyzing biochemical reactions, long before DNA and proteins took on these roles. This theory suggests RNA had the dual capabilities of modern DNA (information storage) and proteins (enzymatic function).

Extremophiles

Organisms, primarily microorganisms, that thrive and survive in environments considered extreme or hostile to most life forms. These environments can include extremely high or low temperatures, high salinity, high pressure, high radiation, or very acidic/alkaline conditions. Examples include thermophiles, psychrophiles, halophiles, and acidophiles.

Characteristics of Extremophiles (regarding early life)

Extremophiles are thought to resemble Earth's early life forms. They have adapted to environments like hot water and derive energy from chemical reactions (chemosynthesis) rather than photosynthesis, which suggests early life might have done the same.

Hypothesized Origin of Life on Earth

Based on DNA evidence pointing to extremophiles as the closest living relatives to early life, it is hypothesized that life may have developed near deep-sea vents and hot springs. These environments offered protection from harmful UV radiation and provided ample chemical energy for metabolism.

Necessities for Life (as we know it)

The fundamental requirements for life include:

1. A nutrient source

2. Energy (e.g., sunlight, chemical reactions, internal heat)

3. Liquid water (often considered the hardest to find on other planets, though other liquids might also support life)

Star System Constraints for Habitable Planets

For a star system to potentially host habitable planets, it generally needs to meet these constraints:

1. Old enough: Sufficient time for evolution to occur (rules out high-mass stars, which have shorter lifespans).

2. Stable orbits: Planets must have stable orbits, which might rule out many binary or multiple star systems.

3. Habitable zone: Existence of a region where a planet of the right size could maintain liquid water on its surface. It's important to note that surface life is not the only possibility for habitability.

Habitable Zone

The region around a star where conditions, primarily temperature, are suitable for liquid water to exist on a planet's surface. Generally, the more massive the star, the larger its habitable zone, and thus the higher the probability of finding a planet within this zone.

Caveats of the 'Habitable Zone' Concept

Simply being within a star's habitable zone does not guarantee suitability for life. Factors such as:

• The planet's atmosphere and geological activity (e.g., Venus or Earth's Moon, which are in the zone but not habitable).

• Changes in the star's luminosity over time, altering the zone's boundaries.

• The possibility of liquid water existing outside the surface habitable zone (e.g., subsurface oceans on icy moons).

• More 'active' stars (e.g., red dwarfs with frequent flares) can still pose challenges for life.

Key Ingredients for Planetary Habitability

Beyond merely being in the habitable zone, a planet needs several key ingredients for long-term habitability:

• Habitable zone: The correct distance from its star to maintain liquid water.

• Volcanism: Necessary for creating and replenishing atmospheres and oceans.

• Plate tectonics: Facilitates the carbon-dioxide cycle, which helps regulate planetary temperature.

• Planetary magnetic field: Protects the atmosphere from erosion by the stellar wind.

Detecting Extraterrestrial Life

One primary method for detecting life on other planets is to search for Earth-like atmospheres, looking for biosignatures or chemical imbalances that would indicate biological activity. Observatories like the James Webb Space Telescope (JWST) can analyze exoplanet atmospheres.

Fermi's Paradox

The apparent contradiction between the high probability of extraterrestrial civilizations existing (given the vast number of stars and planets) and the lack of observational evidence to support their existence. If civilizations are common, why haven't we detected them or been visited?

Carl Sagan's Principle on Evidence

Carl Sagan famously stated, "Extraordinary claims, require extraordinary evidence." This principle applies to the search for extraterrestrial intelligence, emphasizing the need for robust proof before accepting claims of alien contact.

SETI (Search for Extraterrestrial Intelligence)

SETI is a collective term for scientific efforts that aim to detect deliberate communications from extraterrestrial civilizations. These experiments typically involve scanning the cosmos for radio or optical signals that could be artificial in origin.

The Drake Equation (N)

Proposed by Frank Drake in 1961, this equation is used to estimate the number of intelligent, communicating civilizations in the Milky Way galaxy (N). It serves as a framework for discussion rather than a precise scientific calculation due to the highly uncertain nature of its factors.

Factors of the Drake Equation

The Drake Equation consists of several factors, most of which are currently unknown or highly speculative:

• n_{\text{stars}}: The number of stars born per year in the galaxy.

• f_{\text{planets}}: The fraction of those stars that have planets.

• n_{\text{life zone}}: The average number of planets within a star's habitable zone that could support life.

• f_{\text{life}}: The fraction of suitable planets on which life actually appears.

• f_{\text{intelligent}}: The fraction of planets with life where intelligence develops.

• f_{\text{communicate}}: The fraction of intelligent civilizations that develop technology to communicate across space.

• L_{\text{survive}}: The average length of time such a communicating civilization can survive.

History and Challenges of SETI

SETI efforts began around the 1960s. A significant project received 10-year funding in 1990 but was cancelled by Congress in 1993, later resurrected as "Project Phoenix" with private funding. SETI aims to survey thousands of sun-like stars for intelligent signals, but faces the immense challenge of searching for a "needle in a multi-dimensional haystack" across vast ranges of locations, frequencies, and signal variations.

The 'Dark Forest' Theory (Fermi's Paradox Solution)

This theory suggests that the universe is a 'dark forest' where intelligent civilizations are like hunters lurking silently. Given the finite resources and exponential advancement of technology, any detectable intelligent species could eventually become a threat to significantly more advanced species. Therefore, the safest strategy is to remain silent and destroy any civilization that reveals itself, explaining why we haven't heard from anyone.

Solution to Fermi's Paradox: 'We Are Alone'

This solution posits that life, intelligence, or advanced civilization is much rarer than we might expect. The Rare Earth Hypothesis supports this by listing unique conditions for Earth's habitability and the development of complex life:

• A planetary system being a dangerous place.

• Earth's plate tectonics system being unique.

• The Moon's stabilizing influence being unique.

• Life's genesis itself being a rare event.

• Technological progress not being inevitable.

• Intelligence at the human level being rare.

• Language being unique to humans. Humans also possess comparatively large brains, suggesting our level of intelligence might be statistically improbable.

Solution to Fermi's Paradox: 'Civilizations Are Common, but Interstellar Travel Is Not'

This solution suggests that intelligent civilizations either exist but choose not to travel, or cannot travel, due to various reasons:

• Interstellar travel is exceedingly difficult: The immense distances and energy requirements make it impractical. Current spacecraft travel at < 1/10,000c (speed of light), taking ~100,000 years to reach the nearest stars. Accelerating a cruise ship to $1\%$ the speed of light would require energy equivalent to all energy used in the United States in 4.5 years.

• The desire to explore is rare: Most civilizations might lack the drive or resources for vast interstellar exploration.

• The Great Filter: Civilizations destroy themselves or face insurmountable challenges (e.g., resource depletion, war, catastrophic impacts) before achieving interstellar travel or widespread communication. This implicitly limits the L_{\text{survive}} factor in the Drake Equation to a relatively short lifespan (e.g., ~10,000 years, compared to stellar lifetimes of 10^9 years).

The Great Filter

A hypothesized barrier or succession of barriers that makes the emergence of intelligent, communicating life extremely improbable. It could be any evolutionary step or natural phenomenon that prevents life from progressing to an advanced, space-faring stage. The crucial question is whether humanity has already passed the Great Filter (meaning life beyond us is rare) or if it lies ahead of us (meaning our destruction is highly probable).

Solution to Fermi's Paradox: 'There IS a Galactic Civilization'

This solution suggests that intelligent, technologically advanced civilizations do exist in our galaxy, but we haven't detected them for one of three reasons:

• They are hiding themselves from us: This aligns with the 'Dark Forest' theory, where active communication is avoided for safety.

• We can't interpret the signals they're sending: Their communication methods or frequencies might be beyond our current detection capabilities or understanding.

• One day we'll meet them: This is an optimistic view that contact is merely a matter of time.

Lunar Libration

A phenomenon where the Moon appears to rock back and forth, allowing about 59% of its surface to be visible from Earth.

Tides

Gravitational effects caused by the differential pull of the Moon and the Sun on Earth’s oceans, leading to the bulging of oceans.

Spring Tides

Tides that occur when the Earth, Moon, and Sun are aligned, resulting in higher high tides and lower low tides.

Neap Tides

Tides that occur when the Moon and Sun are at right angles relative to Earth, resulting in lower high tides and higher low tides.

Impact Cratering

A process where craters are formed on a planetary surface due to impacts by asteroids or comets, most common in the early solar system.

Maria

Smooth, dark areas on the Moon formed by ancient volcanic activity, originally thought to be seas.

Highlands

Brighter, heavily cratered regions of the Moon that are older than the Maria and contain more rough terrain.

Rays

Splash patterns from materials ejected during a meteor impact, visible primarily under high illumination conditions.

Tidal Friction

The effect of the Moon's gravity slowing Earth's rotation and causing it to gradually move away from the Earth.

Apollo Missions

NASA missions that successfully landed humans on the Moon, notably Apollo 11, which was the first crewed mission to land on the Moon.

Lunar Exploration

The investigation and study of the Moon's surface and environment through various missions and technologies.

Eccentric Orbit

An orbit that is not circular, causing variations in the distance from the moon to the Earth, which influences its gravitational effects.

Volcanism

Eruption of molten rock onto a planet's surface.

Tectonics

Disruption of a planet's surface by internal stresses.

Impact Cratering

Impacts by asteroids or comets that create craters on a planet's surface.

Erosion

Surface changes made by wind, water, or ice.

Mars' Orbital Eccentricity

0.093, indicating the shape of Mars' orbit around the Sun.

Valles Marineris

A system of valleys on Mars thought to originate from tectonics, significantly larger than the Grand Canyon.

Olympus Mons

The tallest volcano in the solar system, located on Mars.

Curiosity Rover

A Mars rover tasked with exploring and conducting experiments on the Martian surface.

Mars 2020 Mission

Launched on July 30, 2020, aiming to seek signs of ancient habitable conditions and collect rock samples.

Martian Rocks

Rocks found by Mars rovers that appear to have formed in the presence of water.

Mercury

The closest planet to the Sun, with a diameter of 4878 km and a mass of 0.055 times that of Earth.

Caloris Basin

The largest impact crater on Mercury, characterized by stretch marks from surface rebounding.

Tidal forces

The gravitational effects from the Sun causing Mercury to rotate on its axis three times for every two orbits.

MESSENGER

NASA's mission launched in 2004 to study Mercury, marking the first mission to use Earth and Venus flybys for orbit insertion.

Orbital eccentricity

A measure of how much the orbit of a planet deviates from a perfect circle, with Mercury having an eccentricity of 0.206.

Volcanism

The geological process related to the eruption of molten rock from a planet's interior, evident on Venus.

Ishtar Terra

One of the two major continents on Venus, situated in the northern highland region.

Erosion

The process through which materials are worn away and transported, influencing planetary surfaces, like the Grand Canyon on Earth.

Lithosphere

The outer layer of a planet, which may be thicker or more rigid on Venus, affecting tectonic activity.

Differentiation

The process by which denser materials sink towards a planet's core, while lighter materials rise to form the crust.

Atmosphere

A layer of gas that surrounds a planet.

Molecular Nitrogen (N2)

The most abundant gas in Earth's atmosphere, making up about 78.08%.

Oxygen (O2)

The second most abundant gas in Earth's atmosphere, comprising about 20.95%.

Atmospheric Pressure

The force exerted by the weight of air above a certain point, commonly measured in bars.

Greenhouse Effect

The process by which certain gases trap heat in a planet's atmosphere, leading to warmer surface temperatures.

Albedo

The fraction of incoming sunlight that is reflected by a surface; a low albedo means more sunlight absorption.

Troposphere

The lowest layer of Earth's atmosphere where temperature decreases with altitude and convection is important.

Stratosphere

The layer above the troposphere where temperature increases with altitude due to UV light absorption.

Mesosphere

The layer above the stratosphere where temperature decreases with altitude, and the top is the coldest part of the atmosphere.

Thermosphere

The layer at about 100 kilometers altitude where temperature increases with altitude due to solar radiation.

Exosphere

The highest layer of the atmosphere where individual gas atoms can escape into space.

Rayleigh Scattering

The scattering of light by particles much smaller than the wavelength of light, responsible for the blue color of the sky.

Ionization

The process of removing an electron from an atom or molecule, which can occur in the presence of X-rays and UV light.

Dissociation

The destruction of a molecule, often due to energy absorption or exposure to certain types of light.

Scattering

The change in direction of photons as they collide with particles in the atmosphere.

Absorption

The process by which matter takes up photons' energy and converts it into other forms.

Weather

The short-term atmospheric conditions in a specific area, including temperature, humidity, and precipitation.

Climate

The long-term average of weather patterns in a particular region.

Weather

The ever-varying combination of wind, clouds, temperature, and pressure.

Climate

The long-term average of weather.

Coriolis Effect

The deflection of moving objects caused by Earth's rotation.

Angular Momentum

The quantity of rotational motion an object has, conserved in the Coriolis effect.

Convection

The process of carrying water vapor higher to cooler regions where it condenses into droplets.

Albedo

The reflectivity of a surface; higher albedo tends to cool a planet.

Greenhouse Gases

Gases that trap heat in a planet's atmosphere, affecting temperature and climate.

Thermal Escape

The process by which atmospheric gases can escape into space.

Ecliptic

The apparent path of the Sun across the sky, used for understanding seasons and axis tilt.

Runaway Greenhouse Effect

A scenario where increased temperatures cause more water vapor, leading to further warming.

Hydrosphere

The combined mass of water found on, under, and over the surface of a planet.

Solar Brightening

The gradual increase in the Sun's brightness over time, warming the planets due to increased sunlight.

Albedo

A measure of reflectivity; higher reflectivity cools a planet, while lower reflectivity leads to warming.

Axis Tilt

The angle of a planet's rotational axis which affects the severity of seasons and polar temperatures.

Outgassing

The primary source of atmospheric gas for terrestrial planets, involving the release of gases from the interior of a planet.

Thermal Escape

The process in which gas molecules gain enough velocity to escape a planet's gravitational pull.

Carbon Cycle

The process through which carbon compounds are exchanged among the Earth's biosphere, geosphere, hydrosphere, and atmosphere.

Stratosphere

The layer of the Earth's atmosphere above the troposphere; contains ozone that absorbs ultraviolet light.

Importance of the Moon

The moon stabilizes Earth’s axial tilt, which helps in maintaining a stable climate.

Jovian Planets

The group of gas giant planets in our solar system, including Jupiter, Saturn, Uranus, and Neptune.

Density

A measure of mass per unit volume, often expressed in g/cm³.

AU (Astronomical Unit)

A unit of measurement equal to the average distance from the Earth to the Sun, approximately 93 million miles or 150 million kilometers.

Magnetosphere

The region around a planet dominated by its magnetic field, protecting it from solar wind.

Auroras

Natural light displays in the sky, typically seen in polar regions, caused by the interaction of solar wind with a planet's magnetic field.

Hydrogen Compounds

Chemical compounds that contain hydrogen, often in gas forms such as water (H2O), methane (CH4), and ammonia (NH3).

Great Red Spot

A giant storm on Jupiter that has been raging for at least 350 years, characterized by its reddish color and high-pressure systems.

Brown Dwarf

A type of celestial object that is not massive enough to initiate hydrogen fusion in its core, often more massive than a planet but less than a star.

Oblateness

A measure of how much a planet is flattened at its poles and bulging at its equator, often due to rapid rotation.

Phase Changes

Transitions between different states of matter that occur due to changes in temperature or pressure, such as gas to liquid.