April 22, 2020

Australia on Earth as seen from space.
Credit: © Harvepino/Shutterstock

How are you going to improve the world today? On this date, 50 years ago, an estimated 20 million Americans, both young and old, took part in the very first Earth Day. The number of people who participate in Earth Day grows each year as the world becomes more connected via social media and internet access. This year, public Earth Day celebrations in many areas will be curtailed as a result of social distancing measures undertaken to stem the spread of the coronavirus disease COVID-19. However, billions of people around the globe will take time examine human impacts on the environment.

Perhaps the most serious issue facing the environment today is climate change due to global warming. Global warming is an observed increase in Earth’s average surface temperature, driven by human activities. Global warming drives changes to the climate that can damage ecosystems, drive species to the brink of extinction, and increase the frequency and severity of such disasters as wildfires. 

Earth Day began in the United States on April 22, 1970. In 1969, U.S. Senator Gaylord A. Nelson of Wisconsin suggested that a day of environmental education be held on college campuses, similar to the anti-Vietnam War demonstrations, called “teach-ins.” The following year, the lawyer and environmentalist Denis Hayes, then a recent graduate of Stanford University, led hundreds of students in planning and organizing the observance of the first Earth Day. 

While working around colleges’ academic schedules, Earth Day was also a tip-of-the-hat to another notable environmental action day, Arbor Day. Nebraska newspaper editor and politician Julius Sterling Morton began Arbor Day once he realized how much trees enrich and conserve moisture in soil. Through his efforts, April 10, 1872, was set aside as Nebraska’s first Arbor Day. The Nebraska Legislature later made Arbor Day a legal holiday and changed its date to April 22, Morton’s birthday. Today, however, National Arbor Day is celebrated on the last Friday of April.

The observance of the first Earth Day helped alert people to the dangers of pollution and stimulated a new environmental movement. That same year, Congress created the Environmental Protection Agency (EPA) to set and enforce pollution standards. Congress also passed the Clean Air Act of 1970, which limited the amount of air pollution that cars, utilities, and industries could release. Other new environmental laws soon followed.

February 3, 2017

One of the greatest questions in the formation of the solar system is in our own planetary back yard: how was the moon made? The current hypothesis (proposed explanation)—that the moon formed from chunks of Earth unloosed in a massive collision—has held sway among planetary scientists for over 30 years. But as more is learned about the moon, scientists are exploring other possibilities, and three scientists in particular—Raluca Rufu and Oded Aharonson of the Weizmann Institute of Science, Israel, and Hagai B. Perets of the Technion Israel Institute of Technology—are offering a different explanation. They published their new theory last month in the journal Nature Geoscience.

A new theory suggests that the moon may have formed from debris unloosed by many small impacts on Earth rather than one big one. Credit: Lunar and Planetary Institute

Since the early days of astronomy, people have speculated on how the moon was formed. In the 1800’s, astronomers used to think that the moon split from Earth—but in a very peculiar way. The accepted hypothesis of that era said that in the distant past Earth spun so rapidly that a portion of it tore away, forming the moon and leaving behind a basin that became the Pacific Ocean. Scientists now know that plate tectonics formed the Pacific Ocean over hundreds of millions of years, and that Earth lacks the rotational speed to create such a spectacular split. In recent years, engineers have developed powerful computers that allow geologists to take new and closer looks at rocks returned from the Apollo moon landings from 1969 to 1972.

This artist rendering depicts the “Big Whack” hypothesis of Earth colliding with a planetary body. The resulting dust and debris from Earth would then have created the moon. (Credit: NASA/JPL-Caltech)

Since the 1980’s, one hypothesis has stood up best to scrutiny: that the moon formed as a result of a massive collision known as the Giant Impact or the “Big Whack.” According to this idea, a Mars-sized object collided with Earth about 4.6 billion years ago. As a result of the impact, a huge cloud of vaporized rock shot off Earth’s surface and went into orbit around Earth. The cloud cooled and condensed into a ring of small, solid bodies, which then gathered together, forming the moon.

If the Big Whack is favored, why are Rufu, Aharonson, and Perets exploring alternative ideas? The Big Whack explains many of the orbital and rotational characteristics of both Earth and the moon, but the hypothesis must be tweaked to an uncomfortable degree to account for the remarkable similarity of Earth rocks to moon rocks. The giant impactor would have had to have struck Earth in an extremely precise way to produce a moon with the makeup shown by returned lunar samples.

Therefore, the Israeli team started from scratch. They reasoned that because impacts were common in the early solar system, Earth should have been hit with objects large enough to create moons many times, not just once. They ran hundreds of computer simulations and found that a series of smaller impacts over the course of millions of years could explain the compositional similarity of Earth and its moon. A smaller body (more the size of the dwarf planet Ceres) would slam into Earth, forming a disk of debris that would eventually come together to form a moonlet, or mini-moon. Later, another body would collide with Earth, creating a new debris disk and another moonlet. Eventually, these moonlets would merge with one another. To reach the size of the current moon, a number of such collisions and moonlet creations and mergings (their guess was roughly 20) would be needed.

The new study is intriguing, but it does not disqualify the Giant Impact Hypothesis just yet. Rufu and her colleagues admit that much more research needs to be done to confirm their findings. For instance, the group did not determine if some of the moonlets could have been sucked back into Earth or flung out into the solar system. This would increase the number of impacts needed to make our moon, making this explanation less likely than a precise Giant Impact.

August 31, 2016

On August 24, scientists from the European Southern Observatory (ESO) announced that they had discovered an extrasolar (beyond our solar system) planet, or exoplanet, that may harbor conditions favorable to life. This exoplanet, called Proxima b, orbits Proxima Centauri, the star closest to the sun. Astronomers have nicknamed Proxima b the “pale red dot,” a play on Earth’s appearance as a “pale blue dot” in a distant photo taken by the Voyager 1 space probe in 1990. Astronomers believe that Proxima Centauri, a red dwarf star, will cast its close-orbiting planet in a pale red glow. The search for a planet orbiting Proxima Centauri, which began just seven months ago, was dubbed the “Pale Red Dot” campaign.

This artist’s impression shows the red dwarf star Proxima Centauri, the closest star to our solar system, on the red horizon of Proxima b, a planet scientists think could support life. The double star Alpha Centauri AB appears to the upper right of Proxima Centauri. Credit: ESO/M. Kornmesser

Proxima Centauri is part of a three-star system called Alpha Centauri. Two of the stars, Centauri A and Centauri B, are roughly the size of the sun and orbit each other. Proxima Centauri is a much smaller red dwarf star and orbits the larger pair of stars every million years or so.

Proxima b (nicknamed the “pale red dot” by astronomers) and its orbit around Proxima Centauri are compared with the same region of our own solar system. Proxima Centauri is smaller and cooler than the sun, and Proxima b orbits much closer to its star than Mercury orbits the sun. A planet in the green habitable zone could possibly have liquid water, which could possibly support life. Credit: ESO/M. Kornmesser/G. Coleman

The newly discovered exoplanet, Proxima b, is at least 1.3 times the size of Earth. Its size indicates that it is probably a rocky planet, like Earth and Mars. It orbits Proxima every 11 Earth days (Mercury, the closest planet to the sun, completes a year every 88 Earth days). If Proxima Centauri were a star like our sun, the planet would be little more than a charred husk. But red dwarf stars are small and relatively cool. Consequently, it is possible that liquid water could exist on the exoplanet’s surface. Scientists call this region around a star, where temperatures are suitable for liquid water, its habitable zone. Liquid water is a necessary building block for life as we know it.

If red dwarf stars can harbor habitable worlds, then the odds that life exists elsewhere in the universe increase significantly. Red dwarfs make up about 70 percent of the stars in the universe. They also burn slowly and steadily, for up to 10 trillion years. In contrast, the sun has a stable lifespan of only 10 billion years. As a result, if favorable conditions exist, life could have countless chances to form over countless eons.

Astronomers still have a lot to learn about Proxima b and whether it can host life. It might lack an atmosphere or get bombarded by powerful X ray blasts from Proxima. The James Webb Space Telescope (JWST), to be launched in 2018, should answer some of these questions. It will be able to determine the exoplanet’s composition and whether it has an atmosphere. JWST will also gather more information on the planet’s size and makeup. Future space telescopes may even be powerful enough to see the “pale red dot” directly.

Most exoplanets are too far away to be explored by spacecraft from Earth. They orbit stars many light-years away. A light-year is the distance light travels in a year, or about 5.88 trillion miles (9.46 trillion kilometers). Nothing can travel faster than the speed of light. Proxima Centauri, however, is just 4.2 light-years away, tantalizingly close in astronomical terms. But the fastest spacecraft ever created have reached just tiny fractions of the speed of light. Using such technology, a traditional space probe would still take thousands of years to reach the system.

A private initiative called Breakthrough Starshot, however, has proposed launching a tiny spacecraft to the system that would take as little as 20 years to get there. The probe would be only a gram or two (the equivalent of less than an ounce on Earth) and be propelled by large Earth-based lasers to 20 percent of the speed of light. There are many technical hurdles to be overcome, however, and the group does not anticipate launching a probe for at least another 20 years, but there is hope that the planet could be explored close-up before 2100. Soon, we may get to say hello to our closest potential neighbor. Will it say hi back?

June 21, 2016

When the North Pole has its greatest slant toward the sun, left, summer begins in the Northern Hemisphere and winter begins in the Southern Hemisphere. Credit: WORLD BOOK map

Have you noticed that the days have been getting longer and longer (or the reverse, if you live in the Southern Hemisphere)? The increasing difference between day and night reached its peak yesterday at 22:34 Coordinated Universal Time. This moment is called the June solstice.

A solstice is one of the two moments each year when the sun is at either its northernmost or southernmost position in Earth’s sky. Earth’s axis of rotation is tilted at an angle of 23 degrees 27 minutes in relation to the plane of Earth’s orbit around the sun. As Earth orbits the sun, the angle of tilt points toward the same direction in space. Thus, the direction of this angle in relation to the sun changes throughout the year, causing seasons on Earth.

Solstices mark the beginnings of astronomical summer and winter. In the Northern Hemisphere, the June solstice marks the longest day of the year. In the Southern Hemisphere, yesterday was the shortest day of the year and signaled the arrival of astronomical winter. Now, the day and night will get closer to the same length until the next equinox in September. During an equinox, the tilt of Earth’s axis lies such that neither hemisphere is angled toward the sun. This means that each part of Earth receives the same amount of daylight, so day and night are roughly the same length.

Cultures around the world, but especially those in higher latitudes in the Northern Hemisphere, celebrate the June Solstice. For example, the people of Sweden celebrate Midsummer’s Eve and Midsummer’s Day around the time of the June Solstice. Solstices were also important to ancient people. Thousands of years ago, people living in what is now southwestern England built Stonehenge, a circle of large carved stones. Several features of Stonehenge align with the sun on solstices, suggesting that they were important to its builders.

October 3, 2014

Up to half of the water in Earth’s oceans may be older than the sun and the rest of the solar system, including Earth itself, according to a new study. The findings indicate that Earth and other bodies in the solar system “inherited” their water—in the form of water ice—from interstellar space. The findings also suggest that if interstellar water ice survived the formation of the solar system, other planetary systems in our Milky Way Galaxy may also have had access to the single most important ingredient necessary for life as we know it.

The solar system is actually awash in water. Oceans make up about 70 percent of Earth’s surface. But water ice also exists in comets and on Jupiter’s moons Europa and Ganymede and on Saturn’s moon Titan. Mars has vast amounts of water ice at its polar regions, and the planet Mercury and Earth’s moon also have water ice within craters that are never fully exposed to the sun. Where did all this water and water ice came from? Scientists already knew that the clouds of gas and dust from which stars form contain water in the form of ice. But they weren’t sure whether water ice could survive that violent processes that occurred when the sun was born. Perhaps the radiation given off by the new sun vaporized most or all of the water ice or broke the water molecules into atoms of hydrogen and oxygen. In that case, the water in the solar system must have reformed in some way.

Stars are born from clouds of gas and dust that contain water in the form of ice.  (NASA)

To determine how much of the solar system’s water or water ice is “original,” the scientists created a computer model that mimicked conditions in the early solar system. In particular, they wanted to know whether the processes that led to the formation of the solar system could account for the amount of deuterium in Earth’s oceans and in comets and meteorites. Deuterium, an isotope (form) of hydrogen, is a key part of a kind of water called heavy water. The nucleus of an ordinary hydrogen atom consists of a single negatively charged particle called a proton. The nucleus of a deuterium atom consists of a proton and a single electrically neutron particle called a neutron. Deuterium forms only under certain circustances, in extremely cold environments, such as interstellar space, for example.

When the scientists ran the model, they discovered that radiation given off by the newly formed sun could not account for the deuterium in the solar system. In fact, they calculated that up to 50 percent of the water in Earth’s oceans comes from water ice that formed in interstellar space before the solar system even existed. As much as 70 percent of water in comets may have survived the transition from insterstellar space to the solar system.

June 9, 2014

Material from the object whose impact with Earth some 4.5 billion years ago led to the formation of the moon has been found in lunar rocks, according to a team of German scientists. For several decades, the most widely accepted theory about the formation of the moon has focused on a cataclysmic collision between Earth and a Mars-sized object called Theia. According to this theory–known as the Giant Impact or the “Big Whack”–the crash caused Theia as well as some of Earth’s crust to melt and combine. During the impact, some of this melted rock vaporized, shot off Earth’s surface, and went into orbit around Earth. Over time, the cloud of vaporized rock cooled and condensed into a ring of small, solid bodies, which then gathered together to form the moon. However, no evidence of Theia had ever been found in moon rocks.

The rocks analyzed by the German researchers were brought back to Earth by NASA Apollo missions from 1969 to 1972. Earlier studies of the rocks indicated that the isotopes (forms) of oxygen in them were the same as those in Earth rocks. In other words, the moon had formed from Earth rock. This puzzled scientists because computer simulations of the Big Whack suggested that up to 70 percent of the moon should be material from Theia.

A tremendous collision between Earth and a smaller object led to the birth of the moon. Debris from the impact went into orbit in a ring around Earth, then gathered together to form the moon. (© William K. Hartmann)

For their research, the German scientists used a more advanced technique to analyze the rocks and so were able to measure smaller amounts of the isotopes. The new study indicated small but important differences in the isotopes between the two kinds of rocks. The scientists suggest that these differences can be explained by an alien origin for the lunar rocks. About 40 percent of the moon may be material from Theia, the scientists said. They noted that additional research comparing isotopes of other chemical elements could strengthen their conclusions. Because the tests are so difficult and the difference in the isotope levels are so small, however, other scientists questioned the results.

Additional World Book articles:

• Space exploration (Lunar probes)
• How the Moon Was Born (a Special Report)

July 23, 2013

The people waving at Saturn knew they would be too far away to be seen, but that didn’t stop tens of thousands of Earthlings from posing for a rare portrait of our home planet and Saturn taken by the Cassini space probe. In the photograph–a mosaic of images released yesterday–Earth appears as a pale blue dot hanging in the darkness of space between several of Saturn’s outer rings. NASA planned the “Wave at Saturn” event to coincide with a rare opportunity for Cassini to photograph Saturn and its entire ring system in the same frame as it was backlit by the sun on July 19. Scientists hope the new photographs will reveal additional details about Saturn’s thinner rings and allow them to study how the rings have changed since 2006, the last time Cassini was able to capture all of the Saturn system at once. The new images, taken 898 million miles (1.44 billion kilometers) from Earth, also show the moon as a bright white dot.

Earth appears as a pale blue dot in an image of Saturn and its glorious rings taken by the Cassini space probe. (NASA/JPL-Caltech/Space Science Institute)

The 2006 images and the new Cassini photos mark only the second and third times, respectively, that Earth has been photographed from the outer solar system. The first image was taken by the Voyager 1 spacecraft in 1990, from about 3.7 billion miles (6 billion kilometers) away. NASA engineers ordered the spacecraft to take the photograph at the request of American astronomer Carl Sagan.

Additional World Book articles:

• Close Encounters with Saturn (a special report)
• Exploring the Suburban Solar System (a special report)

March 22, 2013

Scientists at the 44th Lunar and Planertary Science Conference (LPSC) held this week announced that the object that hit Earth 65 million years ago, leading to massive extinctions of species across the planet, may have been smaller than previously believed. Scientists have long thought that the object that impacted Earth was a huge, relatively slow-moving asteroid. This belief was based in part upon the stratum (layer) of Earth discovered by the American physicist Luis Alvarez and and his son, geologist Walter Alvarez. This layer, called the Cretaceous-Paleogene Boundary (at one time called the Cretaceous-Tertiary, or K-T boundary), contains a large amount of the chemical element iridium (Ir). The element is very rare in Earth’s crust but is common in such space bodies as asteroids and meteorites.

Instead of a large asteroid, as shown below, some scientists now believe a much smaller comet, like the one shown above, led to the extinction event that occurred at the end of the Cretaceous Period. (Lick Observatory)

(NASA/JPL-Caltech)

The iridium layer was not the only evidence that led scientists to think a large asteroid had struck Earth. There is also evidence of the impact. The Chicxulub crater in Mexico’s Yucatan Peninsula, is more than 112 miles (180 kilometers) in diameter and dates to the same geologic time period as the iridium layer; most scientists think the object that caused this crater is the object that spread a layer of iridium over the Earth and led to the extinction of the dinosaurs and many other animals and plants. Based on the level of iridium deposited and the size of the crater, scientists thought that only a large asteroid could have caused such an event.

Professor Jason Moore, a paleoecologist at Dartmouth College, presented findings at the 2013 conference showing that the geochemistry of the Chicxulub crater shows lower iridium levels than originally reported and lower levels than he believes could be created by a body as large as an asteroid. Dr. Moore believes that a body that could leave as little iridium and leave such a huge crater could not be a large asteroid. Moore believes the body that created the crater in Mexico was a small, fast-moving comet.

Comet or asteroid, the effect on the dinosaurs was devastating. Except, of course, for a group evolved from feathered dinosaurs that still does very well on Earth–the vertebrate class Aves, or birds.

Additional World Book articles:

• Cretaceous
• Dinosaur
• Extinction
• Geology (Back In Time)

July 13, 2012

For nearly 100 years, scientists have known that the Andromeda Galaxy, our nearest galactic neighbor, was heading our way. But they didn’t know whether an encounter would be a glancing blow or a head-on collision or if Andromeda would miss our Milky Way altogether. Now, thanks to measurements made using the Hubble Space Telescope, scientists with the Space Telescope Science Institute in Baltimore have calculated that Andromeda, which is hurtling toward us at a speed of about 250,000 miles (400,000 kilometers) per hour, will plow directly into the Milky Way in about 4 billion years. The encounter will produce some dramatic changes, the scientists said. But the destruction of the sun and the solar system will not be one of them.

The Milky Way, our "home" galaxy, is a spiral galaxy. Astronomers believe that after neighboring galaxy Andromeda--also a spiral galaxy--smashes into the Milky Way, the two will form an elliptical galaxy. (Artwork © Jon Lomberg and the National Air and Space Museum)

As Andromeda gets closer, it will fill more and more of our night sky. Eventually, the two spiral galaxies will begin to merge. A blaze of new stars will appear in the sky as clouds of dust and gas are compressed by the gravitational forces tearing at the galaxies. Stars within galaxies are so far apart that the sun or planets will not collide with other space bodies. But scientists think the solar system will be flung into a different part of the Milky Way, probably even farther from the galactic core than it is today. Over the next 2 billion years, the two spiral galaxies will combine to form an elliptical galaxy that some scientists are calling “Milkomeda.”

Earth and the sun will probably not be around to witness the final product, however. Scientists have estimated that about 5 billion years from now, the sun will have used up its hydrogen fuel. Eventually, it will expand enormously, probably nearly to the current orbit of Mercury, and swallow Earth.

Additional World Book articles:

•  Galaxy
• The Formation of Galaxies and Other Structures  (a Special Report)

Jan. 9, 2012

The mystery of how a highly unusual material found in a rock from a Russian mountain could have been created naturally on Earth may have been answered: It didn’t. A new study by scientists from Princeton University suggests that the material, called a quasicrystal, came from outer space in a meteorite. Moreover, the meteorite may be older than Earth itself. The finding indicates that materials that could never form naturally on Earth can form in space–which has scientists wondering what other materials are out there.

In a crystal, atoms are arranged in an orderly manner, with a regularly repeating pattern. The atoms in a quasicrystal also have an orderly arrangement, but they do not have a regularly repeating pattern. Scientists once thought that quasicrystals were an impossibility. In fact, when Daniel Shechtman, an Israeli engineer, reported his discovery of quasicrystals in 1982, the scientific community dismissed his findings completely. (In 2011, Shechtman won the Nobel Prize in chemistry for his discovery.) Several years after his discovery, other scientists made more quasicrystals in various arrangements. Then in 2009, scientists reported finding a naturally occurring quasicrystal in a rock from Russia’s Koryak Mountains.

Now Princeton scientists have reported that the Koryak quasicrystal appears not to have formed naturally on Earth after all. The rock apparently is the remains of a meteorite that formed 4.5 billion years ago, even before Earth had taken shape in the early solar system. In the rock sample, the scientists found a tiny grain of a mineral called stishovite, which occurs only at the kind of high pressure achieved in meteorite impacts and collisions. The quasicrystal was encased in the stishovite.

Both quasicrystals and crystals, such as these salt crystals, contain atoms that are arranged in an orderly pattern. But the atoms in quasicrystals do not follow the kind of repeating pattern found in crystals. (c) Charles Falco, Science Source from Photo Researchers

Many quasicrystals have practical uses. Certain quasicrystals can be particularly strong and hard. They are mixed with such metals as aluminum and steel and other alloys (mixture of metals) to increase a metal’s strength and hardness. Quasicrystals may also exhibit nonstick abilities, such as those found in the nonstick coatings on cookware.

Additional World Book articles:

• Mineral
• Symmetry