"You Had to Be There." Firsthand Observations Solve the Mysteries of Solar System

 

"You Had to Be There." Firsthand Observations Solve the Mysteries of Solar System

by Vicky L. Oldham, April 26, 2022

The Space Age evokes fond memories for many who grew up during its heyday.  Officially beginning in 1957, it started with the Soviet Union's launch of the world's first artificial satellite, Sputnik I.  Sputnik II blasted off with Laika, the dog, for a passenger a month later.  The surprising succession of these missions raised fears by the United States that it was falling behind the rest of the world in technological progress.  Thus began the space race between the United States and the Soviet Union (Office of the Historian, n.d.).  A flurry of tests and human-crewed missions culminated in what was, at the time, the biggest show on Earth—the Moon landing in 1968.  The event generated a huge sensation with unprecedented television viewership of 600 million worldwide.  The audience for space-related topics mushroomed with the younger generation's eagerness to follow every step of intrepid astronauts like Neil Armstrong.  Suddenly, droves of school-age kids wanted to either become an astronaut or an astronomer.

A little book written for young middle-school-age students was already circulating in elementary schools during the same period.  As an eight-year-old, I remember it as my hands-down favorite book, one I could barely put down.  It was titled Rusty's Spaceship and featured two children and their dog led by a friendly "ET-like" alien on a trip through the Solar System (Lampman, 1957).  Although framed by a fictional space voyage, it represented my first exposure to the science of astronomy as they encountered the Moon, other planets and their moons, meteors, and asteroids.  Each chapter's destination reflected astronomers' best understanding of the Solar System in 1957 while providing children with a virtual adventure.

In hindsight, a book like Rusty's Spaceship seems ahead of its time because it expressed that direct exploration of phenomena in space is the best way to ascertain its true nature, especially considering objects "in our backyard," the Solar System.  Although the book reveals a surprising state of knowledge well before the lunar landings and fly-bys of space probes like Voyager I and II, its information later proved inaccurate or incomplete.  Only after close observation by space probes could a wealth of new data provide the course correction needed to go to the next level of discovery.  We've learned that no matter how much we gain by instruments gazing from afar, there is no substitute for direct observation and sampling.

Figure 1. Rusty's Spaceship Book Cover

Note: Photo of the cover of Rusty's Spaceship, popular children's book about a ride through the Solar System, including astronomy facts at the start of the Space Age. Written by Evelyn Sibley Lampman. Published by Doubleday © 1957.

Following a close-up investigation, beliefs about objects in the Solar System could either be confirmed or amended.  One example from the time of Rusty's Spaceship was reflected in the view that Mercury always kept one face to the Sun.  Based on observations from ground telescopes, this assertion was widely accepted by astronomers for decades and unchallenged until the late 1960s (Prockter & Bedini, 2010).  More definitive answers to questions about Mercury only occurred after space probes like Mariner 10 flew by Mercury in the early 1970s.  Mariner's results were later updated by NASA's MESSENGER mission to Mercury (2004 to 2015), correcting previous data that erroneously included oxygen as a constituent of Mercury's atmosphere (Tillman, 2016).  Besides close-flying probes, nothing can match the advantage of sampling materials at the source.  The 440 kilograms of lunar rock samples brought back by the Apollo astronauts revealed more about the Moon "than all the other Moon studies combined" (Fraknoi, 2016, para. 4).  Comparing astronomers' knowledge in 1957 with information accumulated during the last decade, followed by the almost daily news about discoveries in the Solar System (in 2022), assures anyone interested in astronomy that the most remarkable discoveries lie just ahead.

Born from the Solar Nebula

The eight planets of our Solar System fall into two general categories: the terrestrial planets and the Jovian (or gas giant) planets.  But why did we end up with rocky planets and gas giants rather than eight homogenous spheres neatly arranged in evenly spaced circular orbits around the Sun?  The answer lies in the behavior, composition, and temperatures of the early solar nebula from which the planets formed.

The terrestrial planets, Mercury, Venus, Earth, and Mars, have solid surfaces and are composed mainly of silicate rocks with an inner iron or iron-nickel core.  Although Mercury lacks a significant atmosphere, Venus, Earth, and Mars have atmospheres containing varying amounts of oxidized compounds like carbon dioxide.  Besides shared traits, all the terrestrial planets display unique properties on their own.  Earth stands out from all others for its abundance of life, so far found nowhere else.

In contrast to the rocky terrestrial planets of the inner Solar System, the Jovian planets, Jupiter, Saturn, Uranus, and Neptune, have no solid surfaces and possess rings.  They are massive compared to terrestrial planets and composed chiefly of hydrogen and helium over a small, solid metallic core.  In a chapter of Rusty's Spaceship, the crew visits Jupiter, and they encounter extreme temperatures, intense gravity, ammonia rivers, and a poisonous atmosphere.  Current information paints an even more daunting picture as Jupiter displays unfathomably severe storms and temperatures ranging from minus 260 F at the interior to 1,340 F in the upper atmosphere!  Although referred to as "gas giants," both Jupiter and Saturn are primarily composed of compressed, liquefied hydrogen and could more accurately be called "liquid planets" (Fraknoi et al., 2016a, para. 2).  Of all the Jovian planets' rings, Saturn's ring system made almost entirely of water ice is, by far, the most iconic as it extends into space by thousands of miles.

Earth and Saturn — Like Comparing Apples to Oranges?

It is hard to understand how planets like Earth and Saturn that are so different could form from the same nebular material.  It evokes the phrase "comparing apples to oranges," or trying to make sense of two things that appear entirely unrelated.  Yet apples and oranges are both types of fruit!  Furthermore, these planets' differences help confirm their origins by the solar nebula theory. 

Earth, the third planet from the Sun, named the "Blue Marble" in a photo taken by Apollo astronauts, ranks among the most beautiful planets in the Solar System, especially when seen from orbit.  Its composition of silicates over a molten mantle and solid iron-nickel core results in a natural dynamo that generates a magnetic field.  The magnetic field protects the planet from destructive solar energy.  Earth is located in the "Goldilocks" zone, describing a distance from the Sun that is not too warm and not too cold.  It possesses an atmosphere of nitrogen, oxygen, and other gases, liquid water, and is the only known habitable planet.

Viewed from Earth, Saturn, easily identified by its extensive ring system, is arguably the most spectacular Jovian planet.  However, its beauty belies its inhospitable, forbidding environment.  Its lovely golden color is due to fractions of toxic ammonia in its upper clouds, and it hosts raging storms with incredibly ferocious winds.  The sixth planet from the Sun, Saturn grew enormous by attracting large amounts of hydrogen and helium abundant in the early solar nebula.  Although its mass is 95 times that of Earth, its mean density is less than water (Fraknoi et al., 2016c).  If one could imagine an interplanetary water-filled bathtub large enough for Saturn, it would float.  The punchline?  It would leave a ring!  (Launch Pad Astronomy, 2015, 00:13:17).  Still, the idea that Saturn is less dense than water seems to defy the reality of its enormous mass and unique properties.

Orbiting Saturn from 2004 to 2017, NASA's Cassini spacecraft revealed much about Saturn that was previously unknown.  Since Saturn's axis is tilted the same as Earth's with respect to its orbit around the Sun, it has seasons and solstices, with each season lasting seven Earth years!  Cassini observed Saturn's 170,000-mile-wide rings in detail and found them incredibly thin, a mere 330 feet deep, but exceptionally complex (Buratti et al., n.d.).  It also found that Saturn's strong gravity heats its moons' interiors, exerting tidal forces with potentially profound implications (Kelly, 2016).  For example, fissures in the frigid shell of Saturn's moon Enceladus are believed to result from tidal activity.

Diverse Planetary Properties Support the Solar Nebula Theory

With expanded knowledge about the planets, moons, asteroids, and comets in our Solar System, support for the solar nebular theory has also increased.  Solar nebula theory posits that the Solar System grew from a spinning, condensing cloud of interstellar gas and dust from the remnants of a supernova explosion (Fraknoi et al., 2016b).  Small bodies of dust and gas in the nebula bonded together at first by electrostatic attraction and later by gravity.  The small bodies grew larger to become planetesimals, building their masses through accretion, a process in which gravitational forces attract and incorporate freely circulating material from the solar nebula.  Eventually, impacts and collisions combined elements to form roughly spherical protoplanets, all revolving in the same direction as the original spinning nebula. 

As the cloud condensed, it concentrated heat and energy at its center, forming a protostar.  Temperatures rose so high that the protostar ignited by thermonuclear fusion into a star, our Sun.  Planets' varied compositions, densities, atmospheres, and masses reflect this process.  Gasses evaporated from planets forming nearest the Sun, leaving rocky worlds sculpted by collisions with other space rocks that melted their surfaces, allowing lighter silicates to rise while heavier elements sunk to their cores.  Further from the Sun in the nebular disk, the Jovian planets attracted and retained the still-abundant hydrogen and helium circulating in the colder outer regions (CrashCourse, 2015a).  Consequently, the Jovian planets are far less dense than the terrestrial planets despite their enormous size and gravity.

The "Frost Line"

An interesting concept that explains the approximate region separating the terrestrial planets from the Jovian planets is the "frost line" (Launch Pad Astronomy, 2015, 00:08:14).  It roughly marks the distance from the Sun, where temperatures are low enough to prevent vaporization of the gas left over from the original solar nebula.  This region lies past Mars and the Asteroid Belt in our Solar System.  From the frost line to Jupiter's orbit and beyond, water, ice, hydrogen, helium, and other gases condense to become liquid or ice.  As a bonus, this region also preserves the refractory materials like silicates and metals that later contributed to the formation of Jovian moons.

Worlds in Collision: The Origins of Earth's Moon

About four billion years ago, terrestrial planets like Earth commonly experienced impacts from other large space rocks.  Although craters are a typical feature of terrestrial planets, Earth's geological activity has largely erased evidence of impacts.  However, a past, nearly catastrophic collision particularly stands out for its relevance to the origins of Earth's Moon.

Earth, of course, has a single lifeless Moon, but it is disproportionately large compared to its planet, with the greatest planet-to-moon ratio in the Solar System.  Its relative size reflects the chaos unleashed as the Solar System's young planets differentiated through collisions with objects as they orbited the Sun.  About 4.5 billion years ago, an object perhaps the size of Mars is believed to have crashed into Earth's side, exploding material outward into an orbiting ring that condensed to form the Moon (Fraknoi et al., 2016e).  The impact almost shattered the early Earth.

Figure 2. Celestial Body Slamming into a Planet.

Note: Artist concept of planetary impact by NASA/JPL-Caltech, 2009.  Wikimedia Commons. https://commons.wikimedia.org/wiki/File:Artist%27s_concept_of_collision_at_HD_172555.jpg. In the public domain.

Known as "the giant impact hypothesis" and hotly debated in the past, support for this theory of the Moon's origin relies on findings in the composition of Apollo Moon rocks (Howell, 2020).  Perhaps the best evidence lies on Earth, underground.  Intriguingly, there are two uncharacteristic "blobs" sitting beneath West Africa.  Qian Yuan, a Ph.D. student in geodynamics at Arizona State University, notes, "they are the largest thing in the Earth's mantle" and offers the exciting possibility they may be the unexplored remains of the impact object known as Theia (Voosen, 2021, para. 2).  How interesting it would be if samples of this object could one day be obtained, not just to help confirm the theory about the origin of the Moon but also to learn more about its material and other properties.

After it first formed, the Moon orbited the Earth just 15,000 miles away (Science Channel, 2015, 00:00:43).  Imagine how incomprehensibly massive it must have appeared!  Over billions of years, it came to orbit in the familiar position, approximately 240,000 miles away from the Earth.  Ames Research Center astrobiologist Chris McKay considers the possibility that gravitationally generated tides caused by our huge Moon are the key to the origin of life on our planet (00:01:43).  The process may have occurred when the Earth was covered by oceans powered by enormous, miles-long tides, much like tsunamis.  Seawater spilled over the land, filling warm, sunlit tide pools awash with organic chemicals.

Worlds Within Worlds: The Origins of Jovian Moons

When Rusty's Spaceship was published in 1957, Jovian planet moons consisted of just a few large enough to be observed using ground-based telescopes.  Eventually seen by NASA flyby missions, Jovian moons are now known to number in the dozens: Jupiter with 79, Saturn with 82, Uranus with 27, and Neptune with 14 (NASA Science, 2021).  The surprising realization about these moons is that many appear more like the cratered worlds of the inner terrestrial planets, with characteristics that include icy crusts, rocky compositions, plate tectonics, volcanism, an atmosphere, a magnetic field, and even liquid water beneath a frozen surface.  The fantastic variety of Jovian moons formed as each gas giant's gravity created its own "subnebula."  Orbiting material accreted into planet-like bodies from the still-circulating rocky debris and gas, mirroring the process of planet formation around a central star (Owen, n.d.).  Additionally, gravity exerted by Jovian planets on some orbiting moons produced tidal forces that generate significant heat in their interiors. 

But why are some Jovian moons so crucial to further study and exploration?  In addition to contributing to our understanding of solar nebula theory, several moons offer provocative clues to conditions for life.  Moons with heated interiors like Europa (Jupiter's moon) and Enceladus (Saturn's moon) are now known to contain liquid water far below their frozen crusts. 

Figure 3. Enceladus.

Note: Enceladus with 1/4 removed in order to see the interior. Image by NASA, 2018. Wikimedia Commons. https://commons.wikimedia.org/wiki/File:Enceladus%27s_interior.png. In the public domain.

Worlds Under Water: Europa and Enceladus

For decades, Jupiter's moon Europa has been a target of interest because it is believed to have a warm, saltwater ocean under 18 miles of ice.  Previously, scientists assumed a probe would need to drill through all that ice to reach the liquid water, but recent studies suggest there are pools and lakes only a mile or so below Europa's surface (Kluger, 2022).  Occasionally, water may breach its crust where organic chemicals on the surface could combine with saltwater, providing conditions for life.  This process echoes recent theories about how life began on Earth when its vast oceans interfaced with surface chemicals after the Moon formed, generating the first organic molecules in tidepools.  A NASA mission planned for 2024 will explore Europa close up and in detail using ice-penetrating radar.  Interestingly, recent modeling of Europa's icy surface results from studies of ice patterns on Greenland!

Enceladus, a moon of Saturn, generates intense interest because it also hides an interior saltwater ocean beneath an icy crust.  Astoundingly, Enceladus is mentioned in Rusty's Spaceship (p. 188) as the fictitious rocket and its crew pass it on the way to Saturn.  If only they had known more about Enceladus, it could have been the highlight of their trip.

Enceladus is a small, icy world, just 310 miles in diameter.  Despite its tiny size, about the size of the state of Colorado, it has the most reflective surface of any object in the Solar System (CrashCourse, 2015b).  The Cassini mission to Saturn (launched in 1997) first began to explore Saturn's largest moon Titan.  Then it turned to focus on Enceladus, where, as Cassini project scientist Linda Spilker exclaims, "discoveries have changed the direction of planetary science" (NASA Science, 2018, para. 7).  What possible findings could inspire such a drastic statement?  Perhaps, the tantalizing possibility that Enceladus' interior conceals a living ocean.

 Underneath Enceladus' frozen white surface is a liquid saltwater sea warmed by deep hydrothermal vents, features previously known to exist only on Earth (NASA, 2020).  Volcanic geysers spew liquid saltwater filled with organic chemicals hundreds of miles out into space (DCODE by Discovery, 2018).  Complex organic molecules from Enceladus' geysers also become incorporated into Saturn's E-ring.  Cassini detected the chemical signature of the ring material and determined it could only result from hot, hydrothermal vents.  Made of water-ice crystals, the E-ring must be "continually replenished by a source at Enceladus" (Fraknoi et al., 2016d, para. 11).  Scientists look to a future mission following Cassini to further investigate conditions for life inside Enceladus.

The Story Continues

While astronomers and astrobiologists hope that future space missions pursue direct observations by sending a probe to drill beneath the surface of Europa or sampling Enceladus' geysers at the source, there's no telling what we'll learn in the interim.  Data from decommissioned space telescopes and probes still require years of analysis, even as the Hubble Space Telescope continues to operate past its prime, revealing new, untold wonders as it peers deep inside distant galaxies.  The recently deployed James Webb Space Telescope is about to demonstrate its capabilities in analyzing atmospheres of exoplanets, searching for signs of life.  It promises to exponentially compound our knowledge by the advantage of its advanced design and cutting-edge infrared instruments to observe space in ways we've yet to imagine. 

The realization that planets and other bodies have formed from the same solar nebula is reinforced by the multiple ways we've studied its diverse objects.  In addition to the variety of planets, moons, asteroids, and comets, we may ask: what if conditions for the evolution of life result from the same nebular processes?  The answer to the biggest question, whether there is life in our immediate Solar System, could lie closer than we think.  With ongoing improvements in technology enabling close-up observations and direct sampling of distant worlds, our understanding of the Solar System and its potential for life beyond Earth grows exponentially. 

With everything we've learned, it is impressive to reflect on the information astronomers got right way back in 1957.  Still, there's no substitute for firsthand observation.  The phrase "you had to be there" emphasizes that nothing informs us better than a visit to another world, from walking on the Moon to direct sampling of materials by space probes.  It may be time for an updated version of Rusty's Spaceship, filled with current astronomy facts, inviting a whole new generation to explore space to continue the grand adventure that beckons us "out there."

 

 

 

 

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