The Universe Within: Discovering the Common History of Rocks, Planets, and People
Page 5
Our history has been shaped by water, our existence made possible by it, and our future likely defined by our relationship to it. Events far and wide have conspired to define our watery existence and with it, the fundamental structure of our bodies.
EVIL TWINS
Exhausted following a three-day meeting in California, I slumped on a sofa in a hotel lobby waiting for an airport shuttle bus. Seated across from me was an eminent colleague, his face partially hidden by the computer he had open on his lap. His facial expressions drew my attention. He was staring at his laptop, alternately laughing to himself and shaking his head in disbelief. I felt guilty watching him, so I tried staring at my bags to avert my gaze. Noticing my failed attempts to be discreet, he kindly beckoned me over to look at his computer. On the screen was a cliff face with a surface I had seen many times before. The way the layers crisscrossed is characteristic of rocks that formed in ancient dunes. I was familiar with this pattern, having seen it during fossil-hunting trips to Canada and Africa. I had even found fossils in this kind of rock. The rock beckoned; paleontologists dream of these kinds of geological exposures. But these photos were not from Earth. My colleague was part of the scientific team analyzing images returned from one of the Mars rovers, Spirit, and the computer images had just beamed back to Earth the day before.
In the 1988 movie Twins, the character played by Arnold Schwarzenegger is a virtual superman who goes looking for his long-lost brother. He ultimately finds his twin in a character played by Danny DeVito, a short, ungifted brother with a criminal past. They were born of the same mother, but an accident of fate left one brother with the gifts, the other with considerably less. Arnold’s character, after meeting his brother, learns much about himself. So too can we learn when we look to neighbors in the solar system—Venus, Mars, and Jupiter—for insights into our planet and even the makings of our bodies. We have Arnold in our past and Danny in our future.
Separated at birth? Sandstones from the American West (left) and Mars (right). (Illustration Credit 3.2)
For thousands of years, humans have looked to the sky for answers about life, time, and our place in the universe. Telescopes have enhanced our view, revealing moons on distant planets and canals on Mars. For the past forty years, we’ve sent hundreds of craft to the moon, asteroids, other planets and their moons, even deep space beyond the gravitational pull of our sun. Apollo 8 propelled the first humans beyond the gravitational pull of Earth to enter one dominated by another celestial body. Circling the moon on Christmas Eve 1968, William Anders captured the rise of Earth over the surface of the moon. About twenty-five years later, the unmanned Voyager spacecraft began to leave our solar system, departing from the gravitational pull of our sun to enter deep space. Engineers turned the cameras back to reveal Earth. What was for Voyager a single pixel in space, and for Apollo 8 a globe, was a blue oasis of water and air in a world unique among all known ones in the universe.
Even before Project Apollo, observations of Venus changed the way we see our place in the universe. The bright planet looks like a sphere, but next time you have the opportunity, scan it with binoculars or a telescope. What you will see is something that nearly got Galileo executed when he first interpreted it in 1610. Venus, like our moon, has phases that extend from crescent to full and back again. From these kinds of observations, Galileo was able to prove that the planets, including our own, rotate around the sun rather than Earth.
Being near the size of Earth, and relatively close to the sun, Venus has long been thought to be our closest planetary relative—so much so that the earliest interplanetary missions were sent there in the hopes of finding life. Some scientists even thought that when we landed on Venus, we would discover a tropical world, almost like that of Earth during the age of the dinosaurs.
The first hint that something strange was happening on Venus came in the 1930s, when observers looked at the planet with a new kind of telescope. Rather than measure the intensity of light, this telescope, at the Mount Wilson Observatory in California, deconstructed the light into its electromagnetic spectrum. The spectral pattern hinted that Venus’s atmosphere was composed of 99 percent carbon dioxide.
In 1962, Venus won the lottery as the first extraterrestrial planet to receive visitors from Earth when NASA planners launched the first spacecraft of the Mariner project. This mission was a huge undertaking; liftoffs are always dangerous, but in 1962 they were particularly so. Mariner 1’s liftoff went so awry that the destruct button had to be pushed to prevent a disaster for the towns of coastal Florida. The mission that followed, Mariner 2, only had the capability of carrying about forty pounds for all the scientific measurements it would perform. After a successful liftoff, Mariner 2’s trip to Venus took about three and a half months. The small number of instruments that the probe carried were to make some very large discoveries. They revealed that Venus has surface temperatures that were roiling hot, about 900 degrees Fahrenheit. Venus has surface pressures about ninety times those of Earth; you would need to go half a mile underwater to feel that kind of pressure. And the instruments confirmed that the atmosphere is almost entirely carbon dioxide. Mariner 2 found that our close planetary relative, our near twin in size, is most similar to hell.
How could such a dead ringer for Earth in so many ways be so different? Part of the answer would come from new kinds of probes.
In the 1960s, while NASA was gunning for the moon, the Russians were developing machines to land on Venus. Getting a mission to actually visit and record data on Venus is a tricky business. These machines need to be lightweight so that they can be put aloft, but therein lies a huge trade-off: the immense pressures of the planet’s atmosphere give precious little time to make measurements before the probe is crushed like a beer can at a football game. Not surprisingly, the list of early missions reads like a list of calamities. Venera 1 lost contact en route. Venera 2 lost contact upon arrival. Venera 3 crash-landed on the planet. Venera 4 entered the atmosphere, sent a few signals, then was lost. But persistence paid off. Venera 9, launched fourteen years after Venera 1, landed on Venus and sent back the first grainy black-and-white photographs. Subsequent missions landed and were able to analyze soil samples and the environment. What did they find? Venus has thunder and lightning. Venus has lava rocks much like those of Earth. Venus may be a hot, high-pressure, and carbon-dioxide-rich world, but it is strangely similar to our own planet.
Then came NASA’s Pioneer mission, launched in 1978. This probe was a miniature science lab in space, carrying equipment that could measure the composition of the clouds and the chemical constituents of the atmosphere, among other things. When Pioneer entered a Venusian cloud, some of the sulfuric acid inside touched one of the devices. The device could then look at the atoms inside, particularly the different kinds of hydrogen. The ratio of different atoms of hydrogen in a sample of gas is influenced by the presence of liquid water. In the measurement of these atoms came a surprise. Venus is dry as a bone today, but at some point in the very distant past it had oceans.
Venus and Earth were born twins, but our fates have been completely different thus far—Venus lost its water, while our planet kept it. Venus’s relatively close position to the sun defines a world where liquid water cannot be maintained. The loss of water may be behind many of the differences between the two planets. On Earth, water facilitates the removal of carbon dioxide from the atmosphere through a long chain of chemical interactions with rocks. These reactions are not possible on Venus. Having no liquid water, Venus is like a closed container being pumped with gas; with volcanoes spewing carbon, and no way of removing it, pressure just builds over time. As a result, the planet gets hotter and hotter. Venus is in a runaway greenhouse, set up by its loss of water.
Our neighbor on the opposite side of our planet’s orbit from the sun, Mars, tells a different story. On Mars, we have yet to find active volcanoes spewing gases, lava floes, or a moving crust. Canyons and canals carry the signature of having been sculpted by flowing wate
r. Dormant volcanoes dot the surface. If there was liquid water, then there needed to be temperatures in the ranges we experience here on Earth. The surfaces that reveal extensive flowing water are scarred by time, often pockmarked by small and large impacts that may have happened billions of years ago. Recent probes reveal seasonal flowing water on Mars today, but these flows are a far cry from those that created the deep canyons that exist on the planet. Mars’s vigorous and aquatic past is largely frozen in time.
Many of the differences between Venus and Mars derive from the heat balance of the planets. Venus lost its water because, being close to the sun, its water evaporated and set off a runaway chain reaction of ever-increasing heat. Since Mars is relatively far from the sun, it likely did not receive enough heat to sustain liquid water. Mars’s relatively small size also contributed to its loss of heat. All else being equal, small entities have more surface area for their size than do big ones. For example, children have relatively more surface than adults. More surface area means more loss of heat, so children start to shiver in a cold pool quicker than adults do. Planets are no different. Mars’s shivers led it to lose its heat and much of its geological activity.
In the planet business, being in the right place, at the right size, with the right materials, at the right time is everything. We live on a planet that is habitable because it formed at just the right distance from the sun, in a gravitational balance with its neighbors, with just the right amount of material to support a world with liquid water, recycling crust, and an atmosphere. What can we thank for being the lucky recipients of such an inheritance?
BY JOVE
For the Romans, Jupiter was the god who oversaw oaths and laws and thereby defined the social balance that maintains a just society. The planet Jupiter plays an analogous role in the physical and biological worlds.
Jupiter has two and a half times the mass of the rest of the planets in our solar system combined. Its mass is over three hundred times that of Earth; more than eleven hundred Earths could fit inside Jupiter. This colossus exerts, through its gravitational pull, an enormous effect on its neighbors. It sucks asteroids and comets into its field. As for the planets, Jupiter competes with the sun to pull them into its orbital plane. This cosmic tug-of-war defines our own orbit and has guided much of our history.
Over 4.6 billion years ago, as dust swirled the star that would become our sun, clumps of debris formed, as Swedenborg, Kant, and Laplace envisioned. Jupiter was the equivalent of the two-ton gorilla in the crowd: being the most massive planet in the solar system, it had a profound impact on its neighbors. The gravitational attraction that produced the planets made them, in effect, compete and interact with one another. Imagine the tugs felt on the forming Earth everywhere from the sun, from other planets, and from the center of attraction inside the young planet itself. A huge planet, such as Jupiter, with a proportionally large gravitational field around it, influenced how much material was available for Earth to form and where it would lie relative to the sun.
Computer simulations of the origin of the solar system suggest that Jupiter formed before Earth. Competition with Jupiter for debris meant that the position of Jupiter defined the shape of the rest of the solar system. If Jupiter formed closer to the sun, it would have led to fewer but larger rocky planets in the interior of the solar system. If Jupiter formed farther from the sun, there would likely have been a larger number of smaller planets. Our planet’s mass and its distance from the sun—the benevolent conditions that have supported liquid water and life—are due in no small part to the influence of Jupiter.
Our dependence on Jupiter lies in every part of our being, from the presence of liquid water on the planet to the size, shape, and workings of our bodies. The formation of Jupiter defined the size of Earth and, in so doing, the pull of gravity on all things on its surface. A simple thought experiment reveals the web of interconnections. If Jupiter had formed closer to the sun, then Earth would have been larger and heavier and the pull of gravity experienced by Earth’s denizens greater. Even in the unlikely event that such a strange Earth managed to hold liquid water, life on the planet would be very different. As every engineer knows, if you want to make a beam resistant to bending with the same material, just make it relatively wider. All else being equal, a heavier Earth means fat and wide bodies to cope with the greater tug of gravity. Conversely, a smaller Earth would have meant less tug of gravity on evolving bodies; hence, their proportions would need to be longer and lighter. The mass of Earth defines the gravity we experience and, in so doing, controls virtually every aspect of our lives, from our size and shape to how we move about, feed, and interact with the planet.
From the imbalance of matter over antimatter in the moments after the big bang, and the formation of Jupiter that defined our livable planet, to the way a single sperm out of millions fertilized the egg that defined our genes, it is easy to celebrate the lottery each of us has won to be here on a habitable planet. But it is a virtual certainty that within the next billion years the sun will run through its hydrogen fuel, expand, and become superhot. In the process, Earth will almost certainly lose its water. The subsequent loss of water will cause a runaway greenhouse effect, superheating the surface of our planet. Earth will become like Venus. The next planet with liquid water, and the conditions for life, will likely lie farther from the sun. Perhaps it will be a more distant body in our solar system that currently has ice—one of the moons of Jupiter such as Europa, or Enceladus, a moon of Saturn. Our good fortune, the perfection of circumstances that have defined our existence, is just a moment in time.
Alternate fates? Seeing the effect of Jupiter on the shape of our bodies is almost like looking into fun-house mirrors. We would have had more elongated bodies and lived on a smaller planet if Jupiter formed farther from the sun (left), and been short and squat if it formed closer in (right).
CHAPTER FOUR
ABOUT TIME
A trip in a time machine to Earth of 4.5 billion years ago would not only be eerie; it would be perilous. With an atmosphere lacking free oxygen and raining acid, you’d need a space suit far beyond the technology of modern science to survive. Impact after impact of rock and ice from space made the surface sometimes roil at thousands of degrees Fahrenheit. With this heat, there were no oceans: liquid water may have formed a few different times, only to evaporate away. As a break from this desolation, you might hope for beautiful moonlit nights. Forget about it. There was no moon.
Artifacts of the transformation of this primordial world into our modern one are strewn across different bodies of the solar system. Six missions landed on the moon and returned samples to Earth. Carrying mini geological kits, astronauts collected rocks from craters, highlands, and lowlands of the lunar surface. The specimens are today stored in liquid nitrogen in repositories in Houston and San Antonio. A number of small moon fragments have been given as gifts to foreign dignitaries, while others grace public exhibits. The bulk of the rocks, about 850 pounds in all, remain to be studied. The few samples that have made it to labs tell important stories of the origin of our world.
One of the biggest lessons from moon rocks is how normal many of them are. In terms of mineral content and structure, moon rocks are more similar to those on Earth than others in the solar system. One similarity is particularly telling. Oxygen atoms can exist in different forms, defined by the number of neutrons in the nucleus. By measuring the neutron-heavy and neutron-light versions of oxygen in any rock, a very informative ratio can be calculated. Each body in the solar system carries a unique chemical signature written in the proportion of different versions of oxygen in their rocks. The reason is that the oxygen content inside a planet’s rocks is sensitive to its distance from the sun when it formed. The oxygen composition of moon rocks, though, is virtually identical to those of Earth. This means that the moon and Earth formed at the same distance from the sun—perhaps in the same orbit.
With all of these similarities, there remains one very significant differenc
e between moon rocks and those of Earth. Moon rocks almost entirely lack one class of elements, the so-called volatiles. These elements—nitrogen, sulfur, and hydrogen—share one important geological fact: they tend to vaporize when things get hot (hence the name volatiles). Some great event in the distant past must have baked the moon rocks, releasing their volatiles.