In his hit song “Rocket Man”, musical legend Elton John aptly remarks that “Mars ain’t the kind of place to raise your kids / In fact it’s cold as hell.” The average surface temperature of Mars is 220 Kelvin, or about -64 degrees Fahrenheit. Now, I’m not sure about hell, but that’s definitely too cold to be raising any kids.
For centuries, us humans have wondered about our next-door neighbor in the solar system: the red planet of Mars. Mars’ semi-major axis is about 1.5 times that of Earth, and Mars’ radius is only slightly more than half the size of Earth’s radius. Mars is the last of the four inner planets and the five terrestrial worlds; however, Earth is ten times more massive than Mars.
Mars is a truly fascinating world; from humongous volcanoes to polar caps of dry ice, the planet is a wonder to behold. Mars is too cold for liquid water today, but we have found evidence to suggest that Mars was a wet planet at some point in its history. Even if water was still around, human beings could not survive on Mars without a spacesuit due to low air pressure, freezing temperatures, little breathable oxygen, and the lack of an atmospheric ozone layer.
Though no human has step foot on Mars (yet), we have “visited” our neighboring planet with help from exploration rovers. There have been four successful rovers; in fact, the Opportunity mission was only recently terminated after working for 15 years and covering a total distance of just over 25 miles. As of now, the Curiosity rover is still active, and NASA is planning to launch a new mission in 2020.
Though we do not currently have the means to see directly inside the Earth (or any other planet), we can use clues to make inferences about what may be lying beneath their surfaces. On Earth and the Moon, our most helpful data stems from the analysis of seismic waves, or vibrations that travel along the world’s surface and through its interior after earthquakes. For other terrestrial worlds, we can use other measurements like average density and gravity to determine the distribution of mass in the world’s interior. In this blog post, I will be discussing the three major layers that are present inside the terrestrial worlds: the core, the mantle, and the crust.
The Core: The innermost layer of the terrestrial worlds also has the highest density. It is primarily composed of metals, including iron and nickel. Mercury has a very large core of iron that comprises around 85 percent of its interior. The cores of Earth and Venus are made up of a solid, inner core and a molten outer core. Tectonic activity is also caused by heat in the world’s core.
The Mantle: Thick, rocky, moderate-density mantles surround the cores of the terrestrial planets. They are composed of mostly minerals that contain oxygen, silicon, and other elements. With the exception of Mercury, the mantle makes up a large portion of a terrestrial world’s volume; Earth’s mantle makes up 84 percent of the planet’s total volume.
The Crust: The terrestrial planets have thin crusts composed of low-density rock that make up their outermost layer. The Earth’s crust contains a great assortment of metamorphic, sedimentary, and igneous rocks; however, it makes up less than 1 percent of the planet’s total volume. The crusts of the terrestrial planets were formed through various igneous processes, and they frequently change due to erosion, sedimentation, volcanism, and cratering.
Over a single night, the planets behave much like the stars; they appear to rise in the east and set in the west. However, over the course of many nights, one will recognize that the movement of planets among the stars is quite intricate. The speeds and brightnesses of the planets fluctuate significantly, and while they typically travel eastward through the zodiac, they will periodically reverse course and move westward through the stellar background. This phenomenon is called apparent retrograde motion, and these periods can last anywhere from a few weeks to a few months.
For ancient astronomers who believed in a geocentric universe, this presented a problem. If planets supposedly moved in perfect circles around a stationary Earth, then what could be causing this peculiar backward motion? Greek astronomers like Ptolemy suggested that each planet traveled around Earth on a small circle, or epicycle, that simultaneously moved upon a larger circle, or deferent.
Apparent retrograde motion can be explained much more simply with a heliocentric universe. Each of the planets orbits the Sun at a different rate; Mercury and Venus have shorter orbital periods than Earth since they are closer to the Sun, but Mars and the gas giants take a longer time to complete their revolutions. As the Earth passes or is passed by another planet in its orbit, the other planet appears to move back and forth relative to the stars in the distance. We know today that the heliocentric theory is the right one, but it would take almost 2,000 years from the time it was first suggested by Greek astronomer Aristarchus in 260 B.C. to be widely accepted. Nevertheless, the complexities of planetary motion would spur much of the debate over our planet’s place in the cosmos.
What if I told you that in a couple thousand years from now, your Zodiac sign would no longer be your Zodiac sign? It may be devastating to devout followers of astrology, but the relative positions of the Zodiac constellations are changing very, very slowly, at least from our viewpoint. This is due to a process called precession, the continuous, gradual wobble that changes Earth’s axial orientation in space. The Earth really is like a spinning top – just an extremely slow one. In fact, this top only makes a complete spin every 26,000 years.
Because Earth protrudes at its equator, the planet is not quite a perfect sphere. The equator is also tilted with respect to the ecliptic plane, and as a result, the gravitational attractions of the Sun and the Moon attempt to draw the equatorial bulge into the ecliptic plane. In simpler words, gravity from the Sun and the Moon tries to pull the Earth into straightness. However, because Earth tends to keep rotating, gravity fails to straighten out the Earth and instead causes the axis to precess.
It is not likely that we will see any major changes during our lifetimes, but the night sky will look a lot different thousands of years from now. The North Celestial Pole is pointed toward the star Polaris now, but in 3,000 BC, the North Star was actually Thuban, a star in the constellation Draco. In roughly 12,000 years, Vega, a star in the constellation Lyra, will be the new North Star. Precession also alters the points in Earth’s orbit at which equinoxes and solstices occur; this means that in 13,000 years, the seasons on Earth will have switched times of year.
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