Welcome to Earth, third rock from the Sun. In this segment on Knowledge of the Spheres, we will continue our quest through the cosmos by taking a look at our very unique observational platform. Our planet is rare, it is distinct from other bodies in our solar system. Through this differentiation, we are able to look outside the atmosphere and observe the universe. In this post, it is my goal to astronomically and geologically surmise what forces give rise to our very privileged and unique position.
When we think about our planet, the first thing that comes to mind is life. Yes, complex intelligent life is very uncommon in the Universe. There are many factors that contribute to this such as the atmosphere, oceans, size, distance and size of our local star, protection contributed by Jupiter’s mass and distance, size and distance of our moon and location within our galactic home.
All of these factors we will explore. While all of these factors are very important, there is one crucial mechanism that paved the way for our existence in the Universe. It is the combination of all of these factors that gave rise to our very unique and privlidged observational platform. So, without further ado, I present Earth, third rock from the sun.
As we look at our planet from the perspective of another planetary body, we notice several things. Earth is a terrestrial planet, meaning it has a solid surface. It is rather small compared to other planetary bodies, especially the gas and ice giants (Jupiter, Saturn, Uranus, and Neptune). It resides within the inner solar system accompanied by other terrestrial (rocky) planets (Mercury, Venus, and Mars).
All of these factors allow us to look outside our atmosphere and observe the celestial realms in Earth inhabits. Cosmically speaking, Earth is a planetary gem-a rocky planet where liquid water exists by the oceanful. Positioned in a benign region of our galaxy where both comet and asteroid bombardment are tolerable. A moon possessing the right amount of solar mass, just large enough to stabilize axial tilt and create tides, but not so large that gravity pulls our planet apart. Without these seemingly mundane factors, life would not exist.
To fully understand the uniqueness of our observational platform, we need to surmise how these random factors integrate into the cohesive whole we call Earth. We will begin with the atmosphere.
Earth’s atmosphere is pivotal in permitting life. The atmosphere is a mix of nitrogen (78%), oxygen (21%), and other gases (1%) that encompass the planet. This envelope of gases is held in place around the planet by Earth’s gravity. High above the planet, the atmosphere becomes thinner until it reaches space.
The atmosphere is divided into five layers:
The troposphere starts at the Earth’s surface and extends 5 to 9 miles. This part of the atmosphere is the densest. All of our weather happens in this region.
The stratosphere starts just above the troposphere and extends to 31 miles high. Most commercial airlines cruise within this layer of the atmosphere. The air is thin and cold, reducing air resistance. The ozone layer, which absorbs and scatters solar ultraviolet radiation also resides in this layer.
The mesosphere starts just above the stratosphere and extends to roughly 53 miles high. Meteors burn up in this layer
The thermosphere starts just above the mesosphere and extends to 372 miles above the surface. Auroras occur in this layer. It is also the layer where satellites orbit the Earth.
The ionosphere is an abundant layer of electrons and ionized atoms and molecules. Stretching from about 30 miles to the edge of space at about 600 miles, overlapping into the mesosphere and thermosphere. This region is what makes radio communications possible.
This is the upper limit of our atmosphere. It extends from the top of the thermosphere up to 6,200 mi.
The atmosphere is an important factor in allowing Earth to be a habitable planetary body. The atmosphere is crucial in blocking some of the most dangerousaspectst of the light spectrum produced by the Sun. Ionized atoms in the atmosphere burn up meteors and other space debris, protecting the Earth from impacts.
Acting as a warm blanket on a cold night, the atmosphere traps heat keeping the planet warm. This envelope of warm air acts as a safeguard keeping water present on our planet in a liquid state. Without this warming effect, our planet would suffer the same fate as Mars. Contrary to popular belief, Mars has abundant water on the planet, enough to cover the entire planet at depths of 30 feet. However, due to the lack of an atmosphere, Mars is extremely cold thus, all water present on the planet is frozen.
Finally, our planet can retain the atmosphere essential for life thanks in part due to the perfect size of our moon. If the Earth were smaller, such as the size of the moon, the gravity needed to retain an atmosphere would be too weak. The moon does possess an atmosphere, but it is tenuous, and not sufficient to retain heat or protect against impacts. For this reason, our moon is heavily cratered and not habitable without specialized equipment. Thanks to our atmosphere, Earth can regulate heat from the sun and sustain human, aquatic and animal life.
Our Moon is a wonder of astronomy and cosmology. Above is the famous picture taken by the Apollo 16 crew that orbited the moon on April 20, 1972. I love this picture because it shows just how large our moon is and how close it orbits our planet. This is not just amazing; it is fundamental to our existence on Earth.
The Earth-Moon relationship that our planet possesses is indeed rare. Let’s take a closer look at our moon and see why it is so rare.
Without the moon, there would be no moonbeams, no month, no lunacy, no Apollo program, less poetry, and a world where every night is dark and gloomy. Although there are dozens of moons in the solar system, the familiar ghostly white moon that illuminates our night sky is highly unusual and its presence has played an important role in life on Earth.
What is most fascinating is the likelihood that an Earth-sized planet should have a moon in the first place. The fact that our planet has a moon and then a moon as large as ours is indeed one in a billion. Due to the gravitational pull of our planet, our moon should be much smaller than our celestial companion.
Of the many moons that orbit within the regions of our solar system, nearly all of them orbit the large gas and ice giants. Most of the warm, Earth-like planets that orbit close to the Sun are nearly devoid of moons. The only other terrestrial planet that has moons is Mars, orbited by Phobos and Deimos.
These moons are small and irregularly shaped. Although a spectacular sight to see two moons in the blue Martian sunset, these moons do not aid Mars as our moon aids Earth. Some of the solar system’s moons are huge. Jupiter’s Ganymede is nearly as large as Mars. Saturn’s Titan is so large that it has an atmosphere, although much denser than Earth’s and much colder.
Our moon is a freak of cosmology because of its large size in comparison to the parent planet. Our moon is nearly a third the size of Earth and is in some ways more of a twin rather than a subordinate. The only other case and point are Pluto and its companion Charon. I will discuss Pluto and the Kuiper Belt Objects in a later post.
The Moon’s Role in Life on Earth:
The moon plays two pivotal roles in affecting life on earth, tides, and stabilizing Earth’s rotational axis. Both these effects are due to the combined gravitational effects of the Sun and Moon. The pull of these two bodies produces bulges in the ocean, toward and away from the Sun and Moon. The alternating effects of these two celestial bodies cause the ocean to bulge respectively to the near and far sides of our planet.
As Earth spins under the bulges, the sea rises and falls, moving the water of the oceans. This motion oxidizes the oceans, allowing life to flourish under the waves. When the moon is in line with the Sun every 2 weeks (full moon and new moon), the tidal ranges are at a maximum (high tide). When the Sun and Moon are 90 degrees apart in Earth’s sky (quarter moon overhead at sunrise and sunset), the tidal change is minimized (low tide). Through these motions, the moon provides a way to keep the oceans from becoming stagnate and harboring harmful bacteria. As waves ebb and flow, the ocean renews itself, allowing oxygen to permeate, breathing life into the ocean.
Secondly, the moon affects the angular tilt of the axis relative to the plane of Earth’s orbit, commonly referred to as Obliquity. This means that the moon causes seasonal changes. Without the moon, the tilt and angle of our planet would wander in response to the gravitational pulls from the Sun and Jupiter. The monthly motion of our moon prevents any tendencies for the tilt of the axis to change.
If the Moon were smaller or more distant, or if Jupiter were large or closer, or if Earth were closer or farther from the Sun; the moon’s stabilizing influence would be less effective. Without a large moon, Earth’s rotational axis might vary by as much as 90 degrees. Mercury provides an example of what can happen to a planet whose rotational axis is nearly perfectly perpendicular to the plane of its orbit.
Mercury is the closest planet to the Sun. Most of its surface is hellishly hot. However, radar images from Earth show that the poles of the planet are covered in ice. Despite Mercury’s proximity to the Sun, viewed from the poles, the Sun is always on the horizon. In contrast to Mercury’s lack of axial tilt, the planet Uranus has a 90-degree tilt; one pole has exposed the sunlight for half a year, while the other experiences cryogenic darkness for a year.
The situation on Earth is very peculiar. Most terrestrial planets in our solar system have experienced large-scale and chaotic behaviors due to the obliquity of axial tilt. The presence of the Moon has a stabilizing effect on Earth, allowing life to thrive. Without this effect, Earth could very easily be another Mercury or Uranus. We owe our present climate stability to an exceptional celestial companion, the Moon.
Our Moon is not only rare but a freak of cosmology. The moon is one of the primary reasons that life on Earth exists.
I will take a break here. I’m sure that many of you are getting tired of reading or have other important things to attend to. I will be back soon to conclude our discussion on why our observational platform is rare. I will discuss the impact of Jupiter, the Milky Way, and my reflection of why Earth is so rare.
If you have any astronomy-related questions, feel free to post in the comments or drop me a line at firstname.lastname@example.org
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As always, thanks for reading.