Planets

Lecturer: NASA astronaut Jessica Watkins, PhD, geologist.
Focus Area: Mass wasting at the Rio Grande del Norte National Monument, New Mexico.

Robert Goddard: Considered one of the fathers of rocketry, he launched a liquid oxygen-gasoline rocket on March 16, 1926, in Auburn, Massachusetts.
Hubble Space Telescope: On November 2, 1995, it captured a notable photograph of the Eagle Nebula, highlighting dark pillar-like structures made of cool hydrogen gas and dust that serve as nurseries for new stars.

Nebular Hypothesis: Proposed by Immanuel Kant in 1755, it describes how a nebula, which is a diffuse, slowly-rotating sphere of gas, begins to contract due to gravitational forces.
- The universe's origin is described by the Big Bang theory, which occurred approximately 13.7 billion years ago as a cosmic explosion.
Formation Processes:
- Gravity pulls material toward the center of the nebula, initiating the formation of a proto-Sun.
- A disk of gas and dust, termed the solar nebula, flattens and spins more rapidly around the proto-Sun.

In the spinning disk, gas and dust accumulate into kilometer-sized chunks known as planetesimals.
- Rocky inner planets are formed from these planetesimals, while the outer gas giants primarily accumulate from gaseous materials.

Rocky Inner Planets: Mercury, Venus, Earth, Mars.
Gas Giants: Jupiter, Saturn, Uranus, Neptune.
- The inner planets are smaller and rockier, whereas the outer planets are larger, gaseous, and often possess rocky cores.
Pluto: Composed of methane, water, and rock, classified as a dwarf planet.

During the latter stages of Earth's accretion, an object approximately the size of Mars collided with Earth.
This immense impact resulted in a significant amount of debris being ejected from both the impacting body and Earth into space.
The impact altered Earth's rotational speed and tilted its orbital plane by 23 degrees.
Approximately 4.2 minutes post-impact, Earth began re-forming as a largely molten mass.
The Moon coalesced from the debris generated by the impact approximately 8.4 minutes after the event.
The age of Moon rocks collected during the Apollo missions supports this impact hypothesis, indicating they are 4.47 billion years old.

Standard Model of Moon Formation: Proposed the giant impact model, documented by Nakajima & Stevenson in 2015.

**Gravitational Differentiation: **
- This process led to the sinking of dense materials, resulting in a stratified structure of the Earth.
- Timeline of Differentiation: Approximately 4.4 billion years ago, this process defined the core and mantle of the Earth.

A mixture of ice, water, and other volatiles (lighter elements) contributed from planetesimals helped form Earth's surface.
Substantial volcanic activity released these materials to the surface, creating an atmosphere and oceans.
Some volatiles originated from volatile-rich bodies such as comets.

The planets differ significantly in size and mass due to their varying distances from the sun, leading to distinct surface characteristics.
Despite these differences, all four rocky planets share properties, such as differentiated structures with an iron-nickel core, silicate mantle, and outer crust.

Moon Terrain Types:
- Lunar Highlands: Characterized by numerous craters.
- Lunar Lowlands (Maria): Fewer craters due to being younger geological formations.
The differences in the density of craters on surfaces can be tied to their ages: newer surfaces display fewer craters, while older terrains show more due to cumulative impacts over time.
Smaller bodies impact more frequently than larger ones.
Processes of crater counting help establish a relative timeline, wherein older surfaces have larger craters that intersect newer craters, indicating a history of geologic activity.

Calibration Methodology:
- Developing a geologic time scale through crater counting and matching it with absolute ages derived from lunar rocks to analyze the geological history and activity on celestial bodies.

Venus: Displays tectonic activity, with convection currents reshaping the surface and preventing a thick crust
Mercury: Exhibits a heavily cratered surface akin to that of the Moon; notable features like Maat Mons (a volcanic mountain) at 3 km height and 500 km diameter.
Mars: Houses Vallis Marineris, a substantial canyon extending 4000 km long and up to 4 km deep, making it the longest canyon in the solar system. It also has volcanic features such as Maat Mons.

Evidence of water presence on Mars surfaces through specific channel networks suggesting liquid water was involved in their formation.
Sedimentary Structures: The Becquerel Crater exhibits sedimentary strata with distinct regularity indicating historical water activity.
The exploration rovers contributed significantly to our understanding of Martian geology, particularly in sedimentary rock formations containing sulfate minerals indicative of aqueous conditions.
Impact Crater Records: Although Earth has experienced recycling of its crust through tectonics, leaving few large craters, remaining evidence can be found predominantly on continental landforms.