Thursday, December 31, 2009

Rotation: A New Twist

With all the bands and surface features of the biggest jovian planets, you’d think it would be relatively easy to calculate rotation rates “by eye.” Just look for a prominent surface feature and time how long it takes that feature to make one trip around.
Well, it’s not so easy. Because these planets lack solid surfaces, different features on the surface actually rotate at differing rates! This differential rotation is not dramatic in the case of Jupiter whose equatorial region rotates only slightly faster than regions at higher latitudes. East-west winds move at about 190 miles per hour (300 km/h) in Jupiter’s equatorial regions, and at a zippy 800 miles per hour (1,300 km/h) in the equatorial regions of Saturn. It turns out that the best way to clock the rotation rates of these planets is not to look at their atmospheres, but to measure something tied to the planets’ cores. The periods of fluctuation in the radio emission (which arise from the planets’ magnetic fields) are taken to be the “true” rotation rate.
While Neptune and Saturn are slightly tipped on their axes similar to the earth (30, 27, and 24 degrees, respectively), Jupiter’s axis is nearly perpendicular to the plane of its orbit; the planet tilts from the perpendicular a mere 3 degrees. The true oddball in this respect is Uranus, which tilts 98 degrees, in effect lying on its side. The result of this peculiarity is that Uranus has the most extreme seasons in the solar system. While one pole experiences continuous daylight for 42 Earth years at a stretch, the other is plunged into an equal period of darkness.
It’s interesting to note that if the earth were tipped on its axis like Uranus, a city like Atlanta would experience 70 days when the sun never rose, and 70 days when the sun never set. The North Pole would have 6 months of darkness, and 6 months of sunlight.
On the vernal and autumnal equinoxes, day and night in Atlanta would still each last 12 hours.

Views from the Voyagers and Galileo

During the 1970s and 1980s, two Voyager space probes gave us unprecedented images of the jovian planets. Voyager 1 visited Jupiter and Saturn, and Voyager 2 added Uranus and Neptune to the list.
The Voyager missions also revealed volcanic activity on Io, one of Jupiter’s moons. As
for Saturn, a new, previously unknown system of rings emerged: several thousand
ringlets. Ten additional moons were discovered orbiting Uranus, which also revealed
the presence of a stronger magnetic field than had been predicted. And the Neptune
flyby led to the discovery of three planetary rings as well as six previously unknown moons. The hitherto featureless blue face of the planet was now resolved into atmospheric bands, as well as giant cloud streaks. As a result of the Voyager 2 flyby, the magnetospheres of Neptune and Uranus were detected. As with the Van Allen belts around the earth, the magnetospheres of these planets trap charged particles (protons and electrons) from the solar wind.
If only its namesake could have lived to see it. Launched in 1989, Galileo reached Jupiter in 1995 and began a complex 23-month orbital tour of the planet and its almost 400 years after the Italian astronomer first gazed on its colored bands and moons. Among the most extraordinary of
Galileo’s discoveries is a new ring of dust that has a retrograde (backward) orbit around Jupiter. About 700,000 miles (1,120,000 km) in diameter, this doughnut-shaped ring moves in the opposite direction of the rotating planet and its moons. Why does it move in this fashion? No one yet knows.
The Cassini space probe passed Jupiter in early 2001 and sent back images from its many cameras.

Earthbound Views: Jupiter and Saturn

In contrast to Uranus and Neptune, Jupiter and Saturn make for easy viewing. On a good, dark night, even a quite modest telescope will reveal the planets’ belts. The use of colored filters can enhance bands in Jupiter’s atmosphere. Moreover, Jupiter rotates so fast (its day consumes a mere ten hours) that any details you see will perceptibly move across the planet’s face if you observe long enough. Its rapid rotation also makes the planet appear noticeably oblate (elongated). It is even possible to observe the near moons (like Io) emerging from behind Jupiter as they orbit. Although smaller and nearly twice as distant as Jupiter—and therefore appearing much smaller and dimmer than the larger planet—the sight of Saturn through a refractor of at least a 4-inch aperture or a reflector with at least a 6-inch aperture is thrilling. Expect to see the planetary disk and its belts and zones, as well as its celebrated rings (discussed later in this chapter). You may even catch a glimpse of the moons, including Titan, brightest and biggest of Saturn’s nine moons (which we will discuss in the next chapter). Titan’s atmospheric pressure is similar to Earth’s, although its composition and temperature are different. Titan is slightly larger in diameter than the planet Mercury.

Monday, November 30, 2009

Uranus and Neptune from Earth

It is possible for the amateur astronomer to see both Uranus and Neptune. In fact, if you know where to look, Uranus is visible, albeit very faintly, even to the naked eye, provided that the night is very dark, very clear, and you are far from sources of light pollution. To view Neptune, which is much fainter than Uranus, requires an advanced amateur telescope. You don’t have to spend years sweeping the skies to find these dim and distant worlds.
But what can you expect to see? Uranus will appear as a greenish disk, probably featureless—though it is not impossible, given a very good telescope and superb atmospheric conditions, to see atmospheric features and bright spots. It is even possible to see Titania and Oberon, the largest of the planet’s five moons. Even many advanced amateur astronomers have not seen Neptune. Blue in color, it is aptly named for the Roman god of the sea. If you locate the planet at all, it will be a featureless disk.

Understanding Uranus and Neptune

Since ancient times, the inventory of the solar system was clear and seemingly complete: a sun and, in addition to Earth, five planets, Mercury, Venus, Mars, Jupiter, and Saturn. Then came along one of those scientific busybodies that the eighteenth century produced in abundance. Johann Daniel Titius, or Tietz (1729–1796), a Prussian born in what is now Poland, poked his curious nose into everything.
He was a physicist, biologist, and astronomer who taught at the University of Wittenberg.
It occurred to him, in 1766, that the spacing of the planetary orbits from the sun followed a fairly regular mathematical sequence. He doubled a sequence of numbers beginning with 0 and 3, like this: 0, 3, 6, 12, 24, 48, and so on.
He added 4 to each number in the sequence, then divided each result by 10. Of the first seven answers Titius derived—0.4, 0.7, 1.0, 1.6, 2.8, 5.2, 10.0—six very closely approximated the relative distances from the sun, expressed in astronomical units (remember, an A.U. is the mean distance between the earth and the sun), of the six known planets.
No one paid much attention to Titius’s mathematical curiosity until another Prussian astronomer, Johann Bode (1747–1826), popularized the sequence in 1772. Neat as it was, the sequence, which became known as the Titius-Bode Law or simply Bode’s Law, is now generally thought to be nothing more than numerology. For one thing, there is no planet at 2.8 A.U. This gap would be filled later by the discovery of the asteroid belt at this location. While the rule gives a number that is close to Uranus, it breaks down for the positions of Neptune and Pluto. Since those planets had yet to be discovered, no one saw it as a problem. But what about the numbers beyond 10.0 A.U.? Did the Titius-Bode law predict other, as yet unknown, planets?
The people of our planet did not have to wait long for an answer. On March 13, 1781, the great British astronomer William Herschel, tirelessly mapping the skies with his sister Caroline, took note of what he believed to be a comet in the region of a star called H Geminorum. On August 31 of the same year, a mathematician named Lexell pegged the orbit of this “comet” at 16 A.U.: precisely the next vacant slot the Titius-Bode Law had predicted.
Herschel, with the aid of a telescope, had discovered the first new planet since ancient times.
Once the planet had been found, a number of astronomers began plotting its orbit. But something was wrong. Repeatedly, over the next half century, the planet’s observed positions did not totally coincide with its mathematically predicted positions. By the early nineteenth century, a number of astronomers began speculating that the new planet’s apparent violation of Newton’s laws of motion had to be caused by the influence of some as yet undiscovered celestial body—that is, yet another planet. For the first time, Isaac Newton’s work was used to identify the irregularity in a planet’s orbit and to predict where another planet should be. All good scientific theories are able to make testable predictions, and here was a golden opportunity for Newton’s theory of gravity.
On July 3, 1841, John Couch Adams (1819–1892), a Cambridge University student, wrote in his diary:
“Formed a design in the beginning of this week of investigating, as soon as possible after taking my degree, the irregularities in the motion of Uranus … in order to find out whether they may be attributed to the action of an undiscovered planet beyond it ….”
True to his word, in 1845, he sent to James Challis, director of the Cambridge Observatory, his calculations on where the new planet, as yet undiscovered, could be found. Challis passed the information to another astronomer, George Airy, who didn’t get around to doing anything with the figures for a year. By that time, working with calculations supplied by another astronomer (a Frenchman named Jean Joseph Leverrier), Johann Galle, of the Berlin Observatory, found the planet that would be called Neptune. The date was September 23, 1846.

Gas Planets Statistics

The most immediately striking difference between the terrestrial and jovian worlds are in size and density. It is useful to recall our rough scale: If the earth is a golf ball 0.2 miles from the sun, then Jupiter is a basketball 1 mile away from the sun, and Pluto is a chickpea 8 miles away. At this scale, the sun’s diameter would be as big as the height of a typical ceiling (almost 10 feet). While the jovian planets dwarf the terrestrials, they are much less dense. Let’s sum up the jovians, compared to the earth:
We have given a gravitational force and a temperaturenat the surface of the jovian planets, but as we’ll see, they do not really have a surface in the sense that the terrestrial planets do. These numbers are the values for the outer radius of their swirling atmosphere. One surprise might be that the surface gravity of Saturn, Uranus, and Neptune is very close to what we have at the surface the earth.
Gravitational force depends on two factors, you’ll recall, mass and radius. Although these outer planets are much more massive, their radii are so large that the force of gravity at their “surfaces” is close to that of the smaller, less massive Earth. Of the jovians, Jupiter and Saturn have the most in common with one another. Both are huge, with their bulk mainly hydrogen and helium. If you recall from our description of the early, developing solar system, the outer solar system (farther from the sun) contained more water and organic materials, and the huge mass and cooler temperatures of the outer planets meant that they were able to gravitationally hold on to the hydrogen and helium in their atmospheres.
The terrestrials consist mostly of rocky and metallic materials, and the jovian planets primarily of lighter elements. The density of a planet is determined by dividing its mass by its volume. While the outer planets are clearly much more massive (which, one might think, would make them more dense), they are much larger in radius, and so encompass a far greater volume. For that reason, the outer planets have (on average) a much lower density than the inner planets, But what of Uranus and Neptune—distant, faint, and unknown to ancient astronomers? While they are both much larger than the earth, they are less than half the diameter of Jupiter and Saturn; in our scale model, they would be about the size of a cantaloupe.
Uranus and Neptune, though less massive than Saturn, are significantly more dense. Neptune is more dense than Jupiter as well, and Uranus approaches Jupiter in density. Take a look at the following “Astronomer’s Notebook” sidebar to understand
why this is so.
Consider Neptune. Remember, density is equal to the mass of an object divided by its volume. While the mass of Neptune is about 19 times smaller than that of Jupiter, its volume is 24 times smaller. Thus, we expect its density to be about 24⁄19 or 1.3 times greater.
While we cannot yet peer beneath the atmospheric surface of Uranus and Neptune, the higher densities of these two planets provide a valuable clue to what’s inside.
Reflecting their genesis, all of the jovian planets have thick atmospheres of hydrogen and helium covering a core slightly larger than Earth or Venus. The rocky cores of all four of the jovian planets are believed to have similar radii, on the order of 4,300 to 6,200 miles (7,000 to 10,000 km); but this core represents a much smaller fraction of the full radius of Jupiter and Saturn than do the cores of smaller Uranus and Neptune— thus the higher average density of the latter two planets.
The atmospheres of the jovians are ancient, probably little changed from what they were early in the creation of the solar system. With their strong gravitational fields and great mass, these planets have held onto their primordial atmospheric hydrogen and helium, whereas most of these elements long ago escaped from the less massive terrestrial planets, which have much weaker gravitational pull. But here’s where it gets really strange. On the earth, we have the sky (and atmosphere) above, and the solid ground below. In the case of the jovians, the gaseous atmosphere never really ends. It just becomes denser with depth, as layer upon layer of it presses down.
There is no “normal” solid surface to these planets! As the gases become more dense, they become liquid, which is presumably what lies at the core of the jovian worlds. When astronomers speak of the “rocky” cores of these planets, they are talking about chemical composition rather than physical state. Even on the earth, rock may be heated and pressed sufficiently to liquefy it (think of volcanic lava). Thus it is on the jovians: gas giants, whose atmospheres become increasingly dense, but never solid, surrounding a liquid core. In the case of Jupiter and Saturn, the pressures are so great that even the element hydrogen takes on a liquid metallic form.

Saturday, October 31, 2009

Understanding gas planets

If you’ve ever been outside late at night looking to the south, chances are you’ve already seen the largest planet in the solar system, Jupiter. You may have thought that it was just a bright star, and that is exactly what the ancients thought, except that they realized it moved in a way unlike the other stars. Imagine Galileo’s surprise, then, in 1610, when he pointed a telescope at the planet and saw its surface and four smaller bodies orbiting it. His discovery would cause a good deal of upheaval in the way humans viewed themselves in the universe, and Galileo himself would end up in trouble with the Church. All this because of that wandering star in the sky. All of the planets are found near an imaginary arc across the sky that we call the ecliptic. Long before astronomers knew that the terrestrial planets shared common features, they knew that two of the “wanderers” that they watched were different. While Mercury and Venus never strayed far from the sun, and Mars moved in a fairly rapid path across the sky, Jupiter and Saturn moved ponderously, majestically across the stellar ocean. In that motion, we had a clue that the outer planets—those farthest from the sun—were unique long before we had telescopes. Mercury, Venus, and Mars may seem inhospitable, forbidding, and downright deadly, but our sister terrestrial planets have more in common with the earth than the giants of the solar system’s farthest reaches. The jovian planets are truly other-worldly, many times larger and more massive than the earth, yet less dense: They are balls of gas that coalesced around a dense core, accompanied by multiple moons and even rings. In recent years, thanks to the Hubble Space Telescope and planetary probes such as the Voyagers and Galileo, the jovian planets and their moons have given up some of their mysteries.

Martian Moons

Mars and Earth are the only terrestrial planets with moons. As we have said, the earth’s moon is remarkably large, comparable in size to some of the moons of Jupiter. The moons of Mars, colorfully named Phobos (Fear) and Deimos (Panic), after the horses that drew the chariot of the Roman war god, were not discovered until 1877. They are rather unimpressive as moons go, resembling large asteroids. They are small and irregularly shaped (Phobos is 17.4 miles long 12.4 miles [28 km 20 km] wide, and Deimos is 10 miles 6.2 miles [16 km 10 km]). They are almost certainly asteroids that were gravitationally captured by the planet and fell into orbit around it.

Wednesday, September 30, 2009

Water in Mars

Clearly visible on images produced by Martian probes are runoff and outflow channels,
which are believed to be dry river beds, evidence that water once flowed as a liquid
on Mars. Geological evidence dates the Martian highlands to four billion years
ago, the time in which water was apparently sufficiently plentiful to cause widespread
flooding. Recent theories suggest that at the time, Mars had a thicker atmosphere that
allowed water to exist in a liquid state, even at its low surface temperatures.
The Mars Global Surveyor mission, which has established an orbit around the red
planet and is transmitting early data back to the earth, has found further geological
evidence for the presence of liquid and subsurface water. Such evidence has kept alive
hopes that life may have existed—or may even yet exist, perhaps on a microbial
level—on Mars.

Volcanoes, Craters, and a “Grand Canyon”

The Mariner series of planetary probes launched in the 1960s and 1970s revealed a startling difference between the southern and northern hemispheres of Mars. The southern hemisphere is far more cratered than the northern hemisphere, which is covered with wind-blown material as well as volcanic lava. There have even been recent proposals that the smooth northern hemisphere hides a frozen ocean.
Volcanoes and lava plains from ancient volcanic activity abound on Mars. Because the planet’s surface gravity is low (0.38 that of the earth), the volcanoes can rise to spectacular heights. Like Venus, Mars lacks a strong magnetic field, but, in contrast to Venus, it rotates rapidly; therefore, astronomers conclude that the core of Mars is nonmetallic, nonliquid, or both. Astronomers believe that the core of the smaller Mars has cooled and is likely solid, consisting largely of iron sulfide.
Unlike the earth, Mars failed to develop much tectonic activity (instability of the crust), probably because its smaller size meant that the outer layers of the planet cooled rapidly. Instead, volcanic activity was probably quite intense some 2 billion years ago.
Also impressive are Martian canyons, including Valles Marineris, the “Mariner
Valley,” which runs some 2,500 miles (4,025 km) along the Martian equator and is as
much as 75 miles (120 km) wide and, in some places, more than four miles (6.5 km)
deep. The Valles Marineris is not a canyon in the earthly sense, since it was not cut
by flowing water, but is a geological fault feature.

Why Mars Is Red

If we feel any disappointment at the loss of our cherished Martian canals, at least we can still enjoy the image of the “angry red planet.” Yet the source of the reddish hue is not the bloody spirit of the Roman god of war, but simple iron ore. The Martian surface contains large amounts of iron oxide, red and rusting. As Viking 1 and Mars Pathfinder images revealed, even the Martian sky takes on a rust-pink tinge during seasonal dust storms.
The dust is blown about by winds that kick up in the Martian summer. These winds play a prominent role on Mars, forming vast dunes and streaking craters. An especially large dune is found around the north polar cap.

Monday, August 31, 2009

The Martian Chronicles

Percival Lowell was born in Boston in 1855, son of one of New England’s wealthiest and most distinguished families. His early career was absorbed in literature (his sister Amy Lowell became a famous poet) and Far Eastern travel. He became a diplomat, serving as counselor and foreign secretary to the Korean Special Mission to the United States. But in the 1890s, he read a translation of an 1877 book by Giovanni Schiaparelli, the same Italian astronomer who had concluded that Mercury’s rotation was synchronized with its orbit. Reporting his observations of the surface of Mars, Schiaparelli mentioned having discovered canali.
The word, which means nothing more than “channels” in Italian, was mistranslated as “canals” in what Lowell read, and the budding astronomer, already charmed by exotic places, set off in quest of the most exotic of all: Mars—and whatever race of beings had excavated canals upon it.
Lowell dedicated his considerable family fortune to
the study of the planet Mars. He built a private observatory in Flagstaff, Arizona, and, after years of observation, published Mars and Its Canals in 1906. Noting that the canal network underwent seasonal changes, growing darker in the summer, Lowell theorized that technologically sophisticated beings had created the canals to transport crop irrigation water from the Martian polar ice caps. In 1924, astronomers searched for radio signals from the planet (using a technique that anticipated the current search for radio signals from the universe), but to no avail. Yet the idea of intelligent life on Mars was so ingrained in the public imagination that, on October 30, 1938, Orson Welles’s celebrated radio adaptation of H. G. Wells’s 1898 science fiction novel about an invasion from Mars, War of the Worlds, triggered national panic. A variety of space probes have now yielded very high resolution images of Mars, revealing the apparent canals as natural features, such as craters and canyons. While it is true that Mars undergoes seasonal changes, the ice caps consist of a combination of frozen carbon dioxide and water.

Mars: “That Looks Like New Mexico!”

Those of us who were glued to our television sets when NASA shared images of the Martian surface produced by the Mars Pathfinder probe were struck by the resemblance of the landscape to the earth. Even the vivid red coloring of the rocky soil seems familiar to anyone who has been to parts of Australia or even the state of Georgia—though the general landscape, apart from its color, more closely resembles desert New Mexico.
In contrast to Mercury and Venus, which are barely inclined on their axes (in fact, their axes are almost perpendicular to their orbital planes), Mars is inclined at an angle of 25.2 degrees—quite close to the earth’s inclination of 23.5 degrees. And that’s only one similarity. While Mercury and Venus move in ways very different from the earth, Mars moves through space in ways that should seem quite familiar to us. It rotates on its axis once in every 24.6 hours—a little more than an Earth day—and because it is inclined much as the earth is, it also experiences familiar seasonal cycles.
The peculiarities of Mercury and Venus make Mars look more similar to the earth than it really is. Generations have looked to the red planet as a kind of solar system brother, partly believing, partly wishing, partly fearing that life might be found there. But the fact is that life as it exists on Earth cannot exist on the other terrestrial planets.

The Earth: Just Right

In our march through the terrestrial planets, the next logical stop would be Earth. We have mentioned some of the unique aspects of our home planet in earlier chapters, and will mention more in the course of the book. In particular, we will look at the earth as a home to life when we discuss the search for life elsewhere in the Milky Way. But let’s take a brief moment to think of the earth as just another one of the terrestrial planets. The earth is almost the same size as Venus, and has a rotational period and inclination on its axis almost identical to Mars. How is it, then, that the earth is apparently the only one of these three planets to support life?
As in real estate, it comes down to three things: location, location, and location. The earth is far enough from the sun that it has not experienced the runaway greenhouse effect of Venus. It is close enough to the sun to maintain a surface temperature that allows for liquid water, and massive enough to hold onto its atmosphere. The molten rock in the mantle layer above its core keeps the crust of the earth in motion (called plate tectonics), and the rotation of this charged material has generated a magnetic field that absorbs and holds on to charged particles that escape from the sun in the solar wind.
These conditions have created an environment in which life has gotten a foothold and flourished. And life has acquired enough diversity that the occasional setback (like the asteroid that struck the earth some 65 million years ago) may change the course of evolution of life on the planet, but has not yet wiped it out. Our home planet is truly remarkable, and remarkably balanced. The more we learn about our terrestrial neighbors, the more we should appreciate the delicate balance that supports life on Earth.

Thursday, July 30, 2009

Venusian Atmosphere

Chemically, the atmosphere of Venus consists mostly of carbon dioxide (96.5 percent). The remainder is mostly nitrogen. These are organic gases, which might lead one to jump to the conclusion that life—some form of life—may exist on Venus. Indeed, during the 1930s, spectroscopic studies of Venus revealed the temperature of the planet’s upper atmosphere to be about 240 K—close to the earth’s surface temperature of 290 K. Some speculated that the environment of Venus might be a dense jungle.
In the 1950s, radio astronomy was used for the first time to penetrate the dense cloud layer that envelops Venus. It turned out that surface temperatures were not 240 K, but were closer to 600 K. Those temperatures are incompatible with any form of life we know. But the outlook got only worse. Spacecraft probes soon revealed that the dense atmosphere of Venus creates high surface pressure—the crushing equivalent of 90 Earth atmospheres—and that surface temperatures actually top 730 K. And what about those clouds?
On Earth, clouds are composed of water vapor. But Venus shows little sign of water.
Its clouds consist of sulfuric acid droplets.

The Sun Sets on Venus (in the East)

As we’ve seen, Mercury’s peculiar rotational pattern can be explained by its proximity to the sun. But no such gravitational explanation is available for the peculiar behavior of Venus. If at 59 days, Mercury rotates on its axis slowly, Venus is even more sluggish, consuming 243 Earth days to accomplish a single spin. What’s more, it spins backwards! That is, viewed from a perspective above the earth’s North Pole, all of the planets (terrestrial and jovian) spin counterclockwise—except for Venus, which spins clockwise.
Nobody knows why for sure, but we can guess that the rotational peculiarities of Venus were caused by some random event that occurred during the formation of the solar system—a collision or close encounter with another planetesimal, perhaps. A violent collision, like the one that formed the earth’s moon, might have started Venus on its slow backward spin.

Forecast for Venus: “Hot, Overcast, and Dense”

Venus’s thick atmosphere and its proximity to the sun are a cruel combination. The planet absorbs more of the sun’s energy (being closer to the sun than the earth) and because of its heavy cloud cover, is unable to radiate away much of the heat. Even before astronomers saw pictures of the planet’s surface, they knew that it would not be a welcoming place.
Until the advent of radar imaging aboard space probes such as Pioneer Venus (in the late 1970s) and Magellan (in the mid-1990s), the surface of Venus was a shrouded mystery. Optical photons bounce off the upper clouds of the planet, and all we can see with even the best optical telescopes is the planet’s swirling upper atmosphere. Modern radio imaging techniques (which involve bouncing radio signals off the surface) have revealed a Venusian surface of rolling plains punctuated by a pair of raised land masses that resemble the earth’s continents. Venus has no coastlines, all of it’s surface water having long ago evaporated in the ghastly heat. These land masses, called Ishtar Terra and Aphrodite Terra, are plateaus in a harsh waterless world.
The Venusian landscape sports some low mountains and volcanoes. Volcanic activity on the surface has produced calderas (volcanic craters) andcoronae, which are vast, rough, circular areas created by titanic volcanic upwellings of the mantle.
Venus is surely lifeless biologically, but geologically it is very active. Volcanic activity is ongoing, and many astronomers believe that the significant, but fluctuating, level of sulfur dioxide above the Venusian cloud cover is the result of volcanic eruptions. Probes sent to Venus thus far have not detected a magnetosphere; however, astronomers still believe that the planet has an iron-rich core. Scientists reason that the core of Venus might simply rotate too slowly to generate a detectable magnetic field.

Monday, June 29, 2009

“I Can’t Breathe in Mercury!”

Like the earth’s moon, Mercury possesses insufficient mass to hold—by gravitation—an atmosphere for very long. In the same way that mass attracting mass built up planetesimals, so the early planets built up atmospheres by hanging on to them with their gravitational pull. If an atmosphere was ever associated with Mercury, the heating of the sun and the planet’s small mass helped it to escape long ago. Without an atmosphere to speak of, the planet is vulnerable to bombardment by meteoroids, x-rays, and ultraviolet radiation, as well as extremes of heat and cold. In sunlight, the planet heats to 700 K. In darkness, with no atmosphere to retain heat, it cools to 100 K. Despite the absence of atmosphere, regions at the poles of Mercury may remain permanently in shadow, with temperatures as low as 125 K. These regions, and similar regions on the earth’s moon, may have retained some water ice.

Close Encounter with Mercury

If Mercury was difficult for a professional astronomer like Schiaparelli to observe, it is even more challenging for the amateur. It is never farther than 28 degrees from the sun (due to its small orbital radius) and always seen very low in the sky, either in the west just after sunset or in the east, just before sunrise. Because it is visible only close to the horizon, obstacles and atmospheric conditions (light pollution, smog, and turbulence) may often make it impossible to see. Like the moon (and, as we saw in Chapter 2, Venus), Mercury exhibits phases as different fractions of its face are seen to be illuminated by the sun. The best time to see Mercury is at its crescent phase, because it appears largest in the sky at this time. The reason for the variation in size with phase is that when the planet is on the near side of the sun (at a distance of approximately 0.6 A.U. from us), it is backlit and closer and thus appears large. When it is on the far side of the sun, it is fully illuminated (full), is 1.4 A.U. away, and appears smaller. To get a good look at Mercury, you need a telescope, preferably fitted with an eyepiece that offers about 150magnification. It is also possible to see Mercury in the daytime, but this can be dangerous. Because the planet is so close to the sun, there is a real danger that you might accidentally focus on the sun. Doing so for even a moment can permanently damage your eyesight! If you want to look for Mercury during the day, you should consult a good ephemerides guide (see Chapter 17 and Appendix E) and use a telescope fitted with setting circles (see your telescope’s instruction manual and Chapter 17) to locate the planet precisely. For added safety, always keep a solar filter on the telescope until you have precisely located the planet.
Better yet: Restrict your viewing of Mercury to just before sunrise or shortly after sunset.

Lashed to the Sun

In the days before space-based telescopes and probes, earthbound astronomers did the best they could to gauge the rotation of Mercury. The nineteenth-century astronomer Giovanni Schiaparelli observed the movement of what few, indistinct surface features he could discern and concluded that, unlike any other planet’s, Mercury’s rotation was synchronous with its orbit around the sun.
Synchronous orbit means that Mercury always keeps one face toward the sun, and the other away from it, much as the moon always presents the same face to the earth. Technology marches on. In 1965, by means of radar imaging, unavailable to Schiaparelli in the nineteenth century, astronomers discovered that Mercury’s rotation period was not 88 days, but only 59 days. This discovery implied that Mercury’s rotation was not precisely synchronous with its orbit, but that it rotated three times around its axis every two orbits of the sun.

Saturday, May 30, 2009

Mercury: The Moon’s Twin

In many ways, Mercury has more in common with the lifeless moon of our own planet than with the other terrestrial planets. Its face is scarred with ancient craters, the result of massive bombardment that occurred early in the solar system’s history. These craters remain untouched because Mercury has no water, erosion, or atmosphere to erase them. The closest planet to the sun—with an average distance of 960,000 miles (1,546,000 km)—
Mercury is difficult to observe from the earth, and can only be viewed near sunrise or sunset.
Its surface, revealed in detail for the first time in images transmitted by such unmanned probes as Mariner 10 (in the 1970s), is pocked with moonlike craters.
Mariner 10 also discovered a weak but detectable magnetic field around Mercury. As a result, astronomers concluded that the planet must have a core rich in molten iron. This contention is consistent with the planet’s position closest to the center of the solar system, where most of the preplanetary matter—the seeds that formed the planets—would have been metallic in composition

The Terrestrial Roster

The terrestrial planets are Mercury, Venus, Earth, and Mars. Except for Earth, all are named after Roman gods. Mercury, the winged-foot messenger of the gods, is an apt name for the planet closest to the sun; its sidereal period is a mere 88 Earth days, and its average orbital speed (30 miles per second or 48 km/s) is the fastest of all the planets. Mercury orbits the sun in less than a college semester, or about four times for each Earth orbit.
Venus, named for the Roman goddess of love and fertility, is (to observers on Earth) the brightest of the planets, and, even to the naked eye, quite beautiful to behold. Its atmosphere, we shall see, is not so loving. The planet is completely enveloped by carbon dioxide and thick clouds that consist mostly of sulfuric acid. The name of the bloody Roman war god, Mars, suits the orange-red face of our nearest planetary neighbor—the planet that has most intrigued observers and that seems, at first glance, the least alien of all our fellow travelers around the sun. Here are some more numbers, specifically for the terrestrial planets. Notice that the presence of an atmosphere (on Venus and Earth) causes there to be much less variation in surface temperature.
If you recall, when we discussed the formation of the solar system, we mentioned a few observational facts that “constrained” our models of formation. A few rules of planetary motions are immediately apparent. All four terrestrial planets orbit the sun in the same direction. All except Venus rotate on their axes in the same direction as they orbit the sun. The orbital paths of the inner four planets are nearly circular. And the planets all orbit the sun in roughly the same plane. But the solar system is a dynamic and real system, not a theoretical construct, and there are interesting exceptions to these rules. The exceptions can give us insight into the formation of the solar system.

Wednesday, April 29, 2009

April Showers (or the Lyrids)

Whenever a comet makes its nearest approach to the sun, some pieces break off from its nucleus. The larger fragments take up orbits near the parent comet, but some fall behind, so that the comet’s path is eventually filled with these tiny micrometeoroids. Periodically, the earth’s orbit intersects with a cluster of such micrometeoroids, resulting in a meteor shower as the fragments burn up in our upper atmosphere.
Meteor showers associated with certain comets occur with high regularity. They are known by the constellation from which their streaks appear to radiate. The following table lists the most common and prominent showers. The shower names are genitive forms of the constellation name; for example, the Perseid shower comes from the direction of the constellation Perseus, the Lyrids from Lyra. The dates listed are those of maximum expected activity, and you can judge the intensity of the shower by the estimated hourly count. The table also lists the parent comet, when known.
You can detect meteor showers on your FM radio or even on unused VHF television frequencies. But if it’s clear outside, we suggest that you take your radio outside, and as you listen for distant radio stations to pop up, look up at the skies and watch as well. It might be hard to believe that most of those streaks of light are following meteoroids no larger than a pea. But be thankful that they are!

Meteors, Meteoroids, and Meteorites

Meteors are commonly called shooting stars, although they have nothing to do with stars at all. A meteor is a streak of light in the sky resulting from the ionization of a narrow channel in the Earth’s upper atmosphere. The heat generated by friction with air molecules ionizes a pathway behind the piece of debris.
While smaller meteoroids (often called micrometeoroids) are typically the rocky fragments left over from a broken-up comet, the meteor phenomenon is very different from a comet. A meteor sighting is a momentary event. The meteor streaks across a part of the sky. As we have seen, a comet does not streak rapidly and may, in fact, be visible for many months because of its great distance from the earth. A meteor is an atmospheric event, whereas a comet is typically many A.U. distant from the earth.
Meteor is the term for the sight of the streak of light caused by a meteoroid—which is the term for the actual rocky object that enters the atmosphere. Most meteoroids are completely burned up in our atmosphere, but a few do get through to strike the earth. Any fragments recovered are called meteorites.
While most of the meteors we see are caused by small meteoroids associated with comet fragments (about the size of a pea), larger meteoroids, more than an inch or so, are probably asteroid fragments that have strayed from their orbit in the asteroid belt. Such fragments enter the earth’s atmosphere at supersonic speeds of several miles per second and often generate sonic booms. If you see a very bright meteor—the brightness of the planet Venus or even brighter—it is one of these so-called fireballs. It is estimated that about 100 tons of meteoric material fall on the earth each day.

A-Hunting We Will Go

Visitations by major comets, such as Comet Hyakutake in 1966 and Hale-Bopp in 1997, are newsworthy events. Turn on the television or read a newspaper, and you’ll be told where to look and when. But most comets don’t make the front pages. For the latest comet news, check out the NASA comet home page at Sky and Telescope magazine also publishes comet information Of course, you don’t have to limit yourself to looking for comets whose presence or approach is already known. You can head out with your trusty telescope and hunt for new ones.
Comet hunting can be done with or without a telescope, but a good telescope greatly increases your chances of finding a new comet. Remember that telescopes catch more light than our eyes, and most comets are discovered as a tiny, wispy smudge. The coma will not appear much different from a star, but you should see a gradual, not sharply defined, tail attached to it. The tail may be a short, broad wedge or a long ion streamer.
The following tips will increase your chances of finding a comet:
  • Set up your telescope in a rural area, away from city lights. Choose a moonless night so that the skies are as dark as possible. You will be looking for a faint object.
  • According to David H. Levy, just before dawn, two days before or five days after the new moon, is an ideal time to search.
  • Comets can be seen in any part of the sky, but they are brightest when they approach within 90 degrees of the sun. You might concentrate on this part of the sky. That is, at sunset you could look from directly overhead to the western horizon.
  • Gradually and methodically sweep the sky with your telescope. Stake out perhaps 40 degrees of sky and sweep in one direction (either from east to west or west to east).
  • Remember one thing. Discovering a comet requires you to see something unusual or different in the sky. For this reason, you would do well to spend time becoming familiar with the sky, the constellations, and your telescope, so that you will be better able to recognize when something is not quite right.

Monday, March 30, 2009

“Mommy, Where Do Comets Come From?”

The solar system has two cometary reservoirs, both named after the Dutch astronomers who discovered them. The nearer reservoir is called the Kuiper Belt. The short-period comets, those whose orbital period is less than 200 years, are believed to come from this region, which extends from the orbit of Pluto out to several 100 A.U. Comets from this region orbit peacefully unless some gravitational influence sends one into an eccentric orbit that takes it outside of the belt. Long-period comets, it is believed, originate in the Oort Cloud, a vast area (some 50,000–100,000 A.U. in radius) surrounding the solar system and consisting of comets orbiting in various planes. Oort comets are distributed in a spherical cloud instead of a disk.
The Oort Cloud is at such a great distance from the sun, that it extends about 1⁄3 of the distance to the nearest star. We don’t see the vast majority of these comets, because their orbital paths, though still bound by the sun’s gravitational pull, never approach the perimeter of the solar system. However, it is believed that the gravitational field of a passing star from time to time deflects a comet out of its orbit within the Oort Cloud, sending it on a path to the inner solar system, perhaps sealing our fate.
Once a short-period or long-period comet is kicked out of its Kuiper Belt or Oort Cloud home, it assumes its eccentric orbit indefinitely. That is, it can’t go home again. A comet will, each time it passes close to the sun, lose a bit of its mass as it is boiled away. A typical comet loses about 1⁄100 of its mass each time it passes the sun, and so, after 100 passages, will typically fragment and continue to orbit or coalesce with the sun as a collection of debris. As the earth passes through these orbital paths, we experience meteor showers.

A Tale of Two Tails

Most comets actually have two tails. The dust tail is usually broader and more diffuse than the ion tail, which is more linear. The ion tail is made up of ionized atoms—that is, atoms that have lost one or more electrons and that, therefore, are now electrically charged. Both the dust tail and the ion tail point away from the sun. But the dust tail is usually seen to have a curved shape that trails the direction of motion of the comet. Careful telescopic or binocular observations of nearby comets can reveal both of these tails.
What we cannot see optically is the vast hydrogen envelope that surrounds the coma and the tail. It is invisible to optical observations.
Common sense tells us that the tail would stream behind the fast-moving nucleus of the comet. This is not the case, however. The ion tail (far from the sun) or tails (the dust tail appears as the comet gets close to the sun) point away from the sun, regardless of the direction of the comet’s travel. Indeed, as the comet rounds the sun and begins to leave the sun’s proximity, the tail actually leads the nucleus and coma. This is because the tail is “blown” like a wind sock by the solar wind, an invisible stream of matter and radiation that continually escapes from the sun. It was by observing the behavior of comet tails that astronomers discovered the existence of the solar wind.

Anatomy of a Comet

The word comet derives from the Greek word kome, meaning “hair.” The name describes the blurry, diaphanous appearance of a comet’s long tail. But the tail is only part of the anatomy of a comet, and it is not even a permanent part, forming only as the comet nears the sun. For most of the comet’s orbit, only its main, solid body—its nucleus—exists. It is a relatively small (a few miles in diameter) mass of irregular shape made up of ice and something like soot, consisting of the same hydrocarbons and silicates that we find in asteroids.
The orbit of the typical comet is extremely eccentric (elongated), so that most comets (called longperiod comets) travel even beyond Pluto and may take millions of years to complete a single orbit.
So-called “short-period” comets don’t venture beyond Pluto and, therefore, have much shorter orbital periods.
As a comet approaches the sun, the dust on its surface becomes hotter, and the ice below the crusty surface of the nucleus sublimates—immediately changes to a gas without first becoming liquid. The gas leaves the comet, carrying with it some of the dust. The gas molecules absorb solar radiation, then reradiate it at another wavelength while the dust acts to scatter the sunlight. The effect of this is the creation of a coma, a spherical envelope of gas and dust (perhaps 60,000 miles across) surrounding the nucleus and a long tail consisting of gases and more dust particles.

Friday, February 27, 2009

Impact? The Earth-Crossing Asteroids

Most of the asteroids in the asteroid belt remain there, but some have highly eccentric orbits that take them out of the asteroid belt and across the orbital path of the earth (as well as the paths of other terrestrial planets). Nearly 100 of these so-called Apollo asteroids have been identified so far, and a number of astronomers advise funding efforts to identify and track even more, because the potential for a collision with Earth is real. With advance warning, scientists believe, missiles with thermonuclear warheads could be exploded near an incoming asteroid, sufficiently altering its course to make it avoid the earth, or shattering it into a large number of smaller asteroids. You’re local movie theater or video store is a good source to study Hollywood’s take on these nightmare scenarios, but they are a very real threat. Project NEAT (Near Earth Asteroid Tracking) is funded by NASA. For more information see
It is believed that a few asteroids of more than a halfmile diameter might collide with the earth in the course of a million years. Such impacts would be disastrous, each the equivalent of the detonation of several hydrogen bombs. Not only would a great crater, some eight miles across, be formed, but an Earth-enveloping dust cloud would darken the skies. It is thought that the great extinction of dinosaurs 65 million years ago was due to such an impact. Were the impact to occur in the ocean, tidal waves and massive flooding would result.
Earth impacts of smaller objects are not uncommon, but on June 30, 1908, a larger object—apparently the icy nucleus of a very small comet—fell in the sparsely inhabited Tunguska region of Siberia. The falling object outshone the sun, and its explosive impact was felt at a distance of more than six hundred miles. A very wide area of forest was obliterated—quite literally flattened. Pictures from the time show miles of forest with trees stripped and lying on their sides, pointing away from the impact site.

Rocks and Hard Places

Asteroids are composed of stony as well as metallic materials—mostly iron—and are basically tiny planets without atmospheres. Some asteroids have a good deal of carbon in their composition as well. These, called carbonaceous chondrites, are thought to represent the very first materials that came together to form the objects of the solar system. Carbonaceous chondrites are truly the solar system’s fossils, having avoided change for billions upon billions of years.
Earlier astronomers surmised that asteroids were fragments resulting from various meteoric collisions. While some of the smaller meteoroids were likely produced this way, the major asteroids probably came into being at the time of the formation of the solar system as a whole. Theoretical studies show that no planet could have formed at the radius of the asteroid belt (at about 3 A.U. from the sun). The region between Mars and Jupiter is dominated by the gravitational influence of the giant planet Jupiter. These forces stirred up the potential planet-forming material, causing it to collide and break up instead of coming together to create a planet-sized object. The smaller asteroids come in a wide variety of shapes, ranging from nearly spherical, to slab-like, to highly irregular.
During 1993–1994, the Galileo probe passed through the asteroid belt on its way to Jupiter and took pictures of an asteroid orbited by its own miniature moon. Potato-shaped, the asteroid was named Ida, and is about 35 miles (56 km) long, orbited at a distance of roughly 60 miles (97 km) by a rock less than 1 mile in diameter. This little moon is the smallest known natural satellite in the solar system.

The Asteroid Belt

Astronomers have noted and cataloged more than 6,000 asteroids with regular orbits, most of them concentrated in the asteroid belt, between the orbits of Mars and Jupiter. So far, every asteroid that has been noted orbits in the same direction as the earth and other planets—except one, whose orbit is retrograde (backward, or contrary to the direction of the planets). Although the asteroids move in the same direction—and pretty much on the same plane—as the planets, the shape of their orbits is different. Many asteroid orbits are more eccentric (the ellipse is more exaggerated and oblong) than those of the planets.
In early 2001, an asteroid-exploring probe orbited and finally landed on the surface of Eros. As it (slowly) crashed to the asteroid’s surface, it sent back tantalizing close-up images of the surface.

Friday, January 30, 2009

Serving Up the Cosmic Leftovers

What’s the oldest stuff in your refrigerator (aside from that rubbery celery you bought but never ate)? Leftovers! The same is true in the solar system. The fragmentary leftovers of the formation of the sun and planets are some of the oldest objects in the solar system. For a long time, few scientists paid much attention to this debris or knew much about it. More recently, however, astronomers have come to realize that many significant clues to the origin and early evolution of the solar system are to be found not in the planets, but in the smaller bodies, the planetary moons and solar system debris. For the most part, the planets are very active places. Atmospheres have produced erosion, and internal geological activity has erased ancient surfaces. On the earth, weather, water, and tectonic motion have long since “recycled” the earth’s original surface.
So studying the planets reveals relatively little about the origins of the solar system. However, on moons and asteroids, atmospheres are sparse or nonexistent, and geological activity is minimal or absent. The result? Many of these bodies have changed little since the solar system was born. They are, in effect, cosmic leftovers.

The Inner and Outer Circles

Astronomers think of the planets as falling into two broad categories—with one planet left over. The four planets (including the earth) closest to the sun are termed the terrestrial planets. The four farthest from the sun (not counting Pluto) are the jovian planets. And Pluto, usually the farthest out of all, is in an unnamed class by itself. Its location is jovian, while its size and composition put it more in a class with the moons of the jovian planets. Some astronomers prefer to think of it as the largest Kuiper Belt object rather than the smallest (and hardest-to-categorize) planet.

Snapshot of the Terrestrial Planets
Mercury, Venus, Mars, and Earth are called the terrestrial planets because they all possess certain Earth-like (terrestrial) properties. These include proximity to the sun (within 1.5 A.U), relatively closely spaced orbits, relatively small masses, relatively small radii, and high density (rocky and solid-surfaced). Compared to the larger, more distant jovian planets, the terrestrials rotate more slowly, possess weak magnetic fields, lack rings, and have few moons or none. In fact, within the terrestrial “club” the earth’s large moon is unique. The moon is only slightly smaller than the planet Mercury and larger than Jupiter’s moon Europa! As we have seen, the moon’s large size is one clue to its origin.

Snapshot of the Jovian Planets
The jovians are far from the sun and travel in widely spaced orbits. They are massive planets with large radii, yet they are of low density with predominantly gaseous makeup and no solid surface. In contrast to the terrestrial planets, they rotate faster, possess strong magnetic fields, have rings, and are orbited by many large moons.
The outermost jovian planet is Neptune.

Planetary Report Card

Let’s make a survey of the planets. Here’s what we’ll be measuring and comparing in the table that follows:
  • Semi-major axis of orbit. You’ll recall from Chapter 4 that the planets orbit the sun not in perfectly circular paths, but elliptical ones. The semi-major axis of an ellipse is the distance from the center of the ellipse to its farthest point. This distance does not exactly correspond to the distance from the sun to the farthest point of a planet’s orbit, since the sun is not at the center of the ellipse, but at one of the ellipse’s two foci. We will express this number in A.U.
  • Sidereal period. The time it takes a planet to complete one orbit around the sun, usually expressed in Earth years.
  • Mass. The quantity of matter a planet contains. The mass of the earth is 5.977 1024 kg. We will assign the earth’s mass the value of 1.0 and compare the masses of the other planets to it.
  • Radius. At the equator, the radius of the earth is slightly less than 6,400 km (3,963 miles). We will assign the radius of the earth a value of 1.0 and compare the radii of other planets to it.
  • Number of known moons. Self-explanatory—an ever-changing number for the outer planets.
  • Average density. This value is expressed in kilograms of mass per cubic meter. The substance of the inner planets is dense and tightly packed; in the outer planets, the densities are typically lower.

Wednesday, January 14, 2009

Some Points of Interest

The orbits of the planets lie nearly in the same plane, except for Mercury and Pluto, which deviate from this plane by 7 degrees and 17 degrees, respectively. Between the orbit of Mars and Jupiter is a concentration of asteroids known as the asteroid belt. Most of the solar system’s asteroids are here. The orbits of the planets are not equally spaced, tending (very roughly) to double between adjacent orbits as we move away from the sun.
To say that the distances between planets and
the sun are very great is an understatement.
Interplanetary distances are so great that it becomes awkward to speak in terms of miles or kilometers.
For that reason, astronomers have agreed on something called an astronomical unit A.U.), which is the average distance between the earth and the sun—that is, 149,603,500 kilometers or 92,754,170 miles.
Let’s use these units to gauge the size of the solar system. From the sun to the average distance of the outermost planet, Pluto, is 40 A.U.
(3,710,166,800 miles, or almost 6 billion km). At just about a million times the radius of the earth, that’s quite a distance. Think of it this way: If the earth were a golf ball, Pluto would be a chickpea about 8 miles away, Jupiter would be a basketball about 1 mile away, and the sun would go floor-to ceiling in a 10-foot room and be less than a quarter- mile away. However, compared to, say, the distance from the earth to the nearest star (after the sun), even Pluto is a near neighbor. Forty A.U. is less than 1⁄1000 of a light-year, the distance light travels in one year: almost 6 trillion miles. Alpha Centauri, the nearest star system to our sun, is about 4.3 light-years from us (more than 25 trillion miles). On our golf ball scale, Alpha Centauri would be about 55,000 miles away.

Let’s Take a Stroll

Our solar system is centered on a single star, the sun. We have recently come to appreciate that about 50 percent of all stars form in binary systems (containing two stars), so our sun is a bit lonely as stars go. In orbit around the sun are nine planets (in order of distance from the sun): Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. Around some of these planets orbit moons—more than 70, at latest count. By the 1990s, astronomers had observed more than 6,000 large asteroids, of which approximately 5,000 have been assigned catalog numbers.
(Such an assignment is made as soon as accurate orbital data is recorded.) Most asteroids are rather small; it is estimated that there are 1 million with diameters greater than 1 km (or about 3⁄5 of a mile). Some, perhaps 250, have diameters of at least 62 miles (100 km). About 30 have diameters of more than 124 miles (200 km). All of these planets and asteroids are the debris from the formation of the sun. They coalesced through the mutual attraction of gravity.
In addition, the solar system contains a great many comets and billions of smaller, rock-size meteoroids.

Solar System Family

A snapshot freezes an instant in time. When we think about our solar system, we usually assume that it has always been much as it is now, and always will be. But what we know of the solar system (4,000 years of accumulated knowledge) is only a mere snapshot in comparison to its 4.6 billion–year age. It took humankind millennia to reach the conclusion that our planet is part of a solar system, one of many planets spinning on its axis orbiting the sun. These were centuries of wrestling with the earth-centered planetary system first of Aristotle, then of Ptolemy, trying to make the expected planetary orbits coincide with actual observation. This knowledge arose in some sense as a side product of the real initial goals: to be able to predict the motion of the planets and stars for the purpose of creating calendars and (in some cases) as a means of fortune-telling. However, even the earliest astronomers (of whom we know) wanted to do more than predict the planets’ motions. They wanted to know what was “really” going on. When Copernicus, Galileo, Tycho Brahe, and Kepler finally succeeded in doing this quite well in the sixteenth and seventeenth centuries, it was a momentous time for astronomy and human understanding.
Understanding how the planets move is important, of course, but our understanding of the solar system hardly ends with that. In the last few decades of the twentieth century and now into the twenty-first, astronomers have learned more about the solar system than in all the 400 years since planetary motions were pretty well nailed down. As this chapter will show, the planetary neighborhood is a very interesting place, and our own world, the earth, is unique among the planets as a home to life.

Ashes to Ashes, Dust to Dust

In the chapters of this book’s final section, we will consider questions of time and eternity as they relate to the universe. But as to the solar system, we know that it was born about 4.6 billion years ago, and that it will die when its source of energy (the sun) dies of old age.
Just as the specifics of the formation of the solar system depended on the formation of the sun, so its death will be intimately related to the future of our parent star. The evolution of the sun will presumably follow the same path of other stars of its size and mass, which means that the sun will eventually consume the store of hydrogen fuel at its core. As this core fuel wanes, the sun will start to burn fuel in its outer layers, grow brighter, and its outer shell will expand. It will become a red giant, with its outer layers extending perhaps as far as the orbit of Venus. When the sun puffs up into a red giant, Mercury will slow in its orbit, and probably fall into the sun. Venus and the earth will certainly be transformed, their atmospheres (and, in the case of Earth, also water) being driven away by the intense heat of the swelling sun. Venus and Earth will return to their infant state, dry and lifeless.
Some recent models of solar evolution predict that the sun will slowly grow to this state sooner, giving us only another billion or so years before the earth becomes uninhabitable. But don’t fret. All of this is another one to five billion years away. The sun is in its midlife now, and, we hope, will avoid any crisis. The sun will then eject its outer layers (to become a planetary nebula), leaving behind a burned-out star called a white dwarf. A white dwarf does not have sufficient mass to continue fusing elements. It will slowly cool, radiating its internal heat into space, and eventually become a black dwarf—a strange object composed mostly of oxygen and carbon, the size of a planet with the mass of a star. Let’s hope humanity has pushed on by then!

Do the Pieces Fit?

By combining the nebular and condensation theories, we have arrived at an explanation that appears to address the major constraints that we listed at the beginning of the chapter. Does that mean that this theory is “right”? Perhaps.
But like any model, it is subject to future observations that might cause us to reject or revise it. Let’s revisit some of the constraints that we outlined.
  • A rotating cloud of gas, collapsing gravitationally, can account for the “counterclockwise” (as seen from the North Pole) orbit of the planets, rotation of the sun, and rotational orientation of their moons. What we are seeing in all of these is the direction of rotation of that original solar nebula.
  • The rocky nature of the inner solar system, and the gaseous nature of the outer solar system follow directly from the temperature of these regions as the dust grains were formed. Only the heaviest materials (metals) survived intact close to the sun, whereas more fragile molecules (like water) survived in the outer reaches.
  • The planets are all found close to the ecliptic because, as the solar nebula contracted gravitationally, it naturally flattened. This flattened disk was where the planets most likely formed.
  • The existence and location of asteroids, comets, and other debris is a natural byproduct of the accretion and early gravitational interaction process. Yet, as expressed here, the condensation theory does not account for absolutely everything we observe in the solar system.

For the theory allows for an element of randomness, primarily in the form of close encounters and collisions among the planetesimals and protoplanets, which probably influenced certain variations we see in the orbital motions and orientation of some of the planets. As we saw in the last chapter, it is very likely that our own moon is the remnant of a catastrophic collision between the earth and a planetesimal that was the size of Mars. That collision also likely explains the anomalous tip of the earth’s rotational axis, and thus the seasons that grace our planet.