Astronomy chapters 8,9,10

Could life exist outside the habitable zone?

There could be many worlds that have underground or underwater life fueled by energy other than sunlight, in which case these worlds need not be (and are unlikely to be) in habitable zones. However, outside our solar system it would be difficult to detect life on such worlds, and it seems unlikely that complex, intelligent life could arise in such environments.

Martian surface gravity

is about 40% that on Earth, so everyone and everything would weigh about 40% of Earth weight. Astronauts could walk around easily even while wearing space suits with heavy backpacks. It would also be easy to adapt to patterns of day and night, since the mar- tian day is only about 40 minutes longer than an Earth day.

habitable zone

is the range of distances from the Sun within which we could in principle move our planet without fundamentally changing the characteristics that make it home to abundant life.

synchronous rotation

means that each moon completes exactly one rotation around its axis while it makes one orbit around the planet. Same face turned toward the planet

Tree general types of hydrated minerals

minerals containing water or hydroxide (OH), indicating that they formed in the presence of liquid water—have been found at numerous locations on Mars: clay minerals, hydrated sulfates, and hydrated silica, more commonly known as opal.

Ganymede

the largest moon in the solar system, has both old and young regions on its surface of water ice. The dark regions are heavily cratered and must be billions of years old, while the light regions are younger landscapes where tectonic faulting and eruptions of liquid water or slush have presumably erased ancient craters; the long grooves in the light regions were probably formed by tectonic stresses. Notice that the boundary between the two types of terrain can be quite sharp.

oxidation

the process of losing electrons in general, even when the electrons are lost to something besides oxygen. Thus, the overall process of making water from hydrogen and oxygen

Giovanni Schiaparelli

Claimed to have seen a network of 79 linear features that he called canali, by which he meant the Italian word for "channels." However, it was often translated incorrectly as "canals." Coming amid the excitement that followed the 1869 opening of the Suez Canal, Schiaparelli's discovery soon inspired visions of artificial waterways built by a martian civilization.

What is the evidence for global warming?

Measurements show that human activity is causing a substantial increase in the atmospheric concentration of CO2. The well-understood mechanism of the greenhouse effect suggests that this should lead to an increase in the global average temperature, and such an increase has indeed been observed over the past century. Climate models indicate that this temperature increase is due primarily to the human contribution to global warming, rather than to natural factors.

Other heat sources

if Iapetus and other moons could have incorporated enough short-lived radioactive material during their formation to explain the level of heating that they have apparently experienced.

three key environmental requirements for life:

(1) a source of elements and molecules from which to build living organisms, (2) a source of energy for metabolism and growth, and (3) a liquid medium for transporting the molecules of life.

Evidence for a liquid ocean

1. Calculations show that tidal heating can supply enough heat to keep most of Europa's ice melted beneath a solid ice crust. 2. The relatively small number of craters implies that the moon's surface is young, perhaps only a few tens of millions of years old, indicating that it has been recently repaved. 3. Various features on the surface (chaotic and flooded terrain) suggest that liquid water sometimes wells up from below. 4. Europa has a magnetic field that is likely caused by currents produced in something that conducts electricity—like a salty ocean—as Jupiter's magnetic field changes.

four types of evidence have been cited as pointing to the existence of biology on Mars:

1. The carbonate grains have a layered structure, with alternating layers of magnesium-rich, iron-rich, and calcium-rich carbonates. On Earth, this type of layering generally occurs only as a result of biological activity. 2. The carbonate grains contain complex organic molecules known as polycyclic aromatic hydrocarbons, or PAHs. These molecules can be produced by both biological and nonbiological processes, and they have indeed been found in many meteorites that are not from Mars. 3. Under a microscope, we see crystals of the mineral magnetite within the iron-rich layers of the carbonate grains. The sizes, shapes, and arrangements of these crystals appear to match those of magnetite grains that on Earth occur only when made by bacteria 4. Most intriguingly, highly magnified images of the carbonate grains reveal rod-shaped structures that look much like fossilized bacteria, except they are much smaller in size

Habitable zone conditions

1. The concept of a habitable zone is based on the range of distances at which worlds similar to Earth could exist. In other words, a habitable zone is a zone in which it is possible for a world to have abundant liquid water on its surface. 2. Simply being in a star's habitable zone is not sufficient to make a world habitable. The Moon presents an obvious case in point: As a companion to our planet, it is located at essentially the same distance from the Sun as Earth, but it is not habitable. 3. Habitable zones evolve with time. In particular, because stars like the Sun tend to brighten as they age, we expect a star's habitable zone to move outward over time.

The case for life on Europa

1. There is strong, indirect evidence that a liquid water ocean exists. 2. We expect the elements needed for life to be present in that ocean and on its floor. 3. There are possible energy sources to support life, but the total available energy is small compared to the energy available for life on Earth.

Alternative explanations for ALH84001

1. There may be nonbiological ways to get layered carbonate. For example, several pulses of hot water with different dissolved elements might have passed through the rock and laid down the different mineral layers. 2. Other meteorites prove that PAHs can be produced by chemical rather than biological processes, and their high abundance might also be explained by terrestrial contamination during the time the rock resided in Antarctica. 3. The resemblance between the magnetite crystals in the meteorite and those made by bacteria on Earth may be coincidental, and some scientists have proposed nonbiological ways in which the crystals and chains might have been formed. 4. The rod-shaped structures may look like bacteria, but they are about 100 times smaller than typical terrestrial bacteria. Indeed, they are so small (only 10 to 20 nanometers in width) that it is difficult to see how the complex molecules presumably needed for life (such as RNA- or DNA-like molecules) could fit inside them.

Origin of Titan's atmosphere

1. Titan's outer layer should have substantial amounts of methane and ammonia ice. These compounds can evaporate or sublimate into gas at lower temperatures than does water. Thus, if internal temperatures rose on Titan during differentiation, methane and ammonia ice might have turned to gas, bubbled out of the crust, and built up an atmosphere. 2. Titan's atmosphere is that comets and asteroids hitting a moon of Saturn are traveling at lower speeds than are those that fall onto the moons of Jupiter. When such bodies slam into an atmosphere, they can blast away much of the atmosphere. If Ganymede once had an atmosphere, it would have been more likely than Titan's atmosphere to have been blown away by impacts.

history of ALH84001

4.1 billion years ago Solidifies from molten rock in the southern highlands of Mars 4.0-4.1 billion years ago Affected by nearby impacts, but not launched into space 3.9 billion years ago Infiltrated by water, leading to the formation of carbonate grains within the rock 16 million years ago Blasted into space by an impact on Mars 13,000 years ago Falls to Earth in Antarctica December 27, 1984 Found by scientists October 1993 Recognized as a martian meteorite August 1996 Announcement that ALH84001 contains possible evidence of martian life

What factors influence surface habitability?

According to present understanding, a planet can have a habitable surface only if it is within its star's habitable zone, is large enough to retain internal heat and have plate tectonics, and has enough of an atmosphere for liquid water to be stable on its surface.

Redox reactions

An exchange or reshuffling of electric charge (which occurs through movement of electrons) between the reacting atoms or molecules.

How will the Sun's habitable zone change in the future?

As the Sun ages, its luminosity gradually increases. As a result, the habitable zone gradually moves outward with time.

How long can life survive on Earth?

At minimum, Earth should remain habitable for another several hundred million years. By about a billion years from now, a moist greenhouse effect could cause Earth's oceans to evaporate away, though natural feedback processes might prevent this from occurring so soon. In 3-4 billion years, the Sun will become bright enough that our planet will certainly be subject to a runaway greenhouse effect, ending surface habitability.

Possible Redox reactions for other worlds

Bacteria known as Thiobacillus ferrooxidans, which can thrive in highly acidic conditions such as in mine tailings, obtain energy by oxidizing iron: 2 Fe^+2 + 1/2 O2 + 2 H^+ --> 2 Fe^+3 + H2O

How Mars lost its water

Because Mars lacks an ultraviolet-absorbing stratosphere, atmospheric water molecules would have been easily broken apart by ultraviolet light from the Sun.

terraforming

But for the more distant future, some people wonder if we might be able to alter the martian environment in ways that would make it more hospitable to us. Making such changes goes by the name terraforming, because the changes would tend to make the planet more Earth-like.

Callisto

Callisto is heavily cratered, indicating an old surface that nonetheless may hide a deeply buried ocean. The inset shows how a dark powder appears to cover low-lying areas of the surface

Could other moons of Saturn have life?

Cassini photos of Enceladus show a moon with some very young surface regions. The bluish "tiger stripes" (which are not really blue in color, because the image was made with ultraviolet and infrared as well as visible light) show regions near the moon's south pole that are measurably warmer than the surrounding terrain, and close-up examination suggests that they are cracks or grooves through which material can well up from below. These regions appear to be covered by "fresh" ice—ice that has solidified on the surface within no more than the past few thousand years, and possibly within just the past few decades or less. Moreover, images taken by Cassini as it looked at Enceladus backlit by the Sun show that fountains of ice particles and water vapor are spraying out from the tiger stripe regions. This is clear evidence of icy volcanism on Enceladus.

What are the potential consequences of global warming?

Continued global warming could raise the average worldwide temperature by 3°C to 5°C during this century. Regional climate changes will be greater, and we can expect increased polar melting and a rise in sea level. Additional heat should increase ocean evaporation, which may lead to more numerous and more intense storms. Many other serious effects could also occur, but precise consequences are difficult to predict.

Titan

Discovered by Dutch scientist Christiaan Huygens in 1656.

How Mars lost its Carbon Dioxide

Early in its history, Mars probably had molten, convecting metals in its core, much like Earth today. The combination of this convection with Mars's rotation should have produced a magnetic field and a protective magnetosphere. The magnetic field would have weakened as the small planet cooled and core convection ceased, leaving the atmosphere vulnerable to solar wind particles. These solar wind particles could have stripped gas out of the martian atmosphere and into space.

Could Venus have once been habitable, and could life still exist there?

Early in its history, when the Sun was some 30% dimmer than it is today, Venus may have been within the Sun's habitable zone and hence could have had rain, oceans, and perhaps life. If so, it is conceivable that life could still survive among liquid droplets in high-altitude clouds.

Valles Marineris

East of Tharsis and just south of the equator is the long, deep system of valleys on mars. Valles Marineris may be one of the best places to look for fossil evidence of past martian life.

Why is Venus so hot?

Venus's distance from the Sun ultimately led to a runaway greenhouse effect: Venus became too hot to develop (or keep) liquid oceans like those on Earth. Without oceans to dissolve outgassed carbon dioxide and lock it away in carbonate rocks, all of Venus's carbon dioxide remained in its atmosphere, creating its intense greenhouse effect.

Could other moons of Jupiter have life?

Europa is the most likely of the Galilean moons to be habitable. However, both Ganymede and Callisto are also composed of significant amounts of water ice.

Does Europa have an ocean?

Europa seems to consist of a central metallic (probably iron) core, overlaid with a thick mantle of silicate rock and an 80-170-kilometer-thick outer skin of water or water ice. From the gravity measurements alone, the water layer could contain just about any combination of solid ice, liquid water, or slush (partially melted ice), because all of these have about the same density As a result, over the past couple of decades scientists have debated which form of water is most likely.

Most habitable moon

Europa, is kept warm inside by a mechanism quite different from that which warms Earth's interior. Tidal heating, the result of orbital resonances that occur among three large moons of Jupiter, can provide a continuous source of heat for billions of years. Because orbital resonances can arise quite naturally, tidal heating may be common among moons of jovian planets throughout the universe.

Disequilibrium

From a chemical energy standpoint, the basic requirement for life is a situation in which chemicals naturally exist in a state that is "unbalanced," which we describe as a state of disequilibrium. The idea is similar to that of a scale on which you place objects on both sides to see which is heavier. If the two sides weigh the same, the scale is balanced. But if you then add a little extra to one side, the scale quickly tips because it is no longer balanced. In a similar way, if there is disequilibrium among chemicals, they will start to react just the way the scale starts to tip. With a scale, the movement quickly stops, because a scale can move only so far. But with chemicals, the reactions can continue as long as the disequilibrium remains.

H. G. Wells

H. G. Wells's The War of the Worlds, published in 1898. Public belief in Martians became so widespread that, decades later, a radio broadcast of The War of the Worlds created a famous panic as many people thought an invasion was actually under way

Energy to support ongoing life on Europa

High-energy particles accelerated and trapped in Jupiter's magnetic field, as well as ultraviolet light from the Sun, regularly slam into the surface ice. These particles and photons hit the surface with enough energy to break up molecules in the ice, leading to the production of small quantities of other molecules such as hydrogen peroxide molecular oxygen and hydrogen (which quickly escapes); this process explains why Europa has an extremely thin atmosphere (not noticeable to the eye, but detected by instruments). These molecules can facilitate energy-producing reactions, and they should be mixed into the uppermost portion of the europan surface by the frequent churning caused by small meteorites.

William Herschel

In 1784, William Herschel claimed that Mars possessed an atmosphere and that consequently, "its inhabitants probably enjoy a situation in many respects similar to our own."

Mariner 4

In 1965, NASA's Mariner 4 spacecraft flew to within 6000 miles of the martian surface, transmitting a few dozen television-quality images of the landscape below. Mars's surface was littered with craters, not canals, and measurements of the atmospheric pressure and temperature made from the spacecraft indicated a cold, dry planet seemingly incapable of supporting life.

Phoenix lander

In 2008, confirmed presence of water on mars (frozen in martian soil)

Reduction

In accepting electrons, the electrical charge of the oxygen is reduced (because electrons are negatively charged); hence the first three letters in redox refer to this process of reduction of electrical charge.

Tidal heating

Io's tidal heating arises from a combination of two factors: (1) Its proximity to Jupiter means it experiences a strong tidal force from the massive planet; and (2) Io has a slightly elliptical orbit, which causes the strength and direction of the tidal force to change slightly as Io moves through each orbit.

Opportunity rover

Landed in the Meridiani Plains, where orbital spacecraft had detected the presence of hematite, an iron-rich mineral that often forms in water but that can also form through volcanic processes. Further study by Opportunity indicates that the water must have been fairly shallow and either acidic or salty, suggesting that the rover landed at a site that was once a pond or shallow lake.

Percival Lowell

Lowell mapped close to 200 canals that he claimed to see on Mars, publishing his first book about them in 1895. Because he assumed Mars's polar caps were similar to Earth's and made of water ice, he imagined that the canals were built to carry water from the poles to thirsty cities nearer the equator. From there it was a short leap to imagine the Martians as an old civilization on a dying planet.

Green house effect on Mars

Mars almost certainly had a much stronger greenhouse effect in the past. Calculations suggest that martian volcanoes should have outgassed enough carbon dioxide to make the atmosphere about 400 times as dense as it is today (and enough water to fill oceans tens to hundreds of meters deep).

Color of the Martian sky

Martian winds and dust storms also leave Mars with perpetually dusty air, which helps explain the colors of the martian sky. Without suspended dust, the air on Mars is so thin that the sky would be essentially black even in daytime. Instead, light scat- tered by the suspended dust tends to give the sky a yellow-brown color.

ALH84001

Meteorite found in Allen Hills, Antarctica in 1984, thought to be from Mars. Surface contained suspected nanobacteria.

Mars tilt

Models suggest that the martian axis tilt varies over time from 0° to as much as 80°, which means that the current 25° is significantly smaller than the average. These changes in tilt would have dramatic effects on the climate

jovian moons

Moons that orbit Jovian planets

Viking 1

On July 20, 1976, Viking 1 lander touched down on the Chryse Planitia, a sprawling, rock- strewn plain about 1300 kilometers north of the martian equator.

Pathfinder, Sojourner

On july 4, 1997, both these rovers landed on mars. More than a half-dozen other robotic spacecraft have reached Mars successfully since Pathfinder. By combining data from these missions with past data, we are beginning to put together a realistic portrait of the past and present habitability of Mars.

The Cassini-Huygens mission

Our closest views of Titan came from the Huygens probe. Released from the Cassini "mother ship" on Christmas Day, 2004, the probe spent 21 days coasting toward Titan. On January 14, 2005, the probe entered Titan's smoggy atmosphere. A series of parachutes was deployed to ease Huygens through its descent. As the descent proceeded, the probe radioed back information about the composition and temperature of Titan's atmosphere, and also snapped hundreds of photos of the landscape below.

Energy for the origin of life on Europa

Our models of Europa's interior suggest that it should have a rocky ocean floor, and tidal heating and the decay of radioactive elements may provide enough energy to melt pockets of interior rock that could erupt into the ocean. Europa might even have large undersea volcanoes. In that case, it certainly seems plausible to imagine an independent origin of life on Europa.

the gas chromatograph/mass spectrometer experiment

That measured the abundance of organic molecules in the martian soil. This experiment started by heat- ing the soil to temperatures as high as 500°C (930°F)—high enough to kill any organisms, break apart any organic molecules they contained, and release the molecules as gases. The gases were passed through a gas chromatograph, a device that separates different gases as they pass through it, and then analyzed with a mass spectrometer. The results showed no measurable level of organic molecules in the martian soil.

Redox Reactions on Earth

The basic process of aerobic respiration in animals involves combining a sugar acquired by eating, such as glucose with oxygen acquired by breathing. The reaction makes carbon dioxide and water, and releases energy in the process: C6H12O6 + 6 O2 --> 6 CO2 + 6 H2O + energy

The possibility of life on Titan

The ultraviolet light that hits Titan's atmosphere produces a wide range of organic molecules (the main contributors to the observed smog). Over billions of years, some of these compounds should have accumulated as a deep layer of organic sediment on the surface—these sediments may be the material in the dunes. Occasional impacts by comets or asteroids would provide enough heat locally to melt any water ice and create pockets of warm water that might persist for a thousand years or so. While it is not clear that life could form in such a short time period, some interesting chemistry would certainly occur. Moreover, if Titan really has icy volcanism, it might hint at the existence of liquid, subsurface aquifers of a water/ammonia mixture; in that case, it's conceivable that there could be volcanic "hot springs" where temperatures might rise slightly above 0°C.

Galilean moons

The name given to Jupiter's four largest moons, Io, Europa, Callisto & Ganymede. They were discovered independently by Galileo Galilei and Simon Marius.

Former water evidence on Mars

The first evidence of past water came from photos taken by Mariner 9 and the Viking orbiters, some of which showed features that look much like dry riverbeds on Earth seen from above.

Martian winds

The strong winds associated with the cycling of carbon dioxide gas can initiate huge dust storms, particularly when the more extreme sum- mer approaches in the southern hemisphere. At times, the martian surface becomes almost completely obscured by airborne dust.

Could moons of Uranus or Neptune have life?

Triton orbits Neptune "backward," suggesting that it was somehow captured from what was once an independent orbit around the Sun. The question of how such a capture might have occurred is quite interesting, but here we are more concerned with the issue of potential habitability. Triton was photographed close-up only once, by Voyager 2 in 1989. Its icy surface is an enigmatic mix of terrain that in some places is smooth and in others is crinkled into patterns resembling the skin of a cantaloupe. There are few impact craters. The crater count on Triton today leads scientists to estimate that its last resurfacing must have been no more than 10-100 million years ago. Clearly, Triton has had internal heat, possibly left over from tidal heating that would have occurred as it was being captured into its present orbit, and probably with an additional contribution from radioactive decay.

Viking 2

Two months later, Viking 2 landed on the other side of the planet. Meanwhile, two Viking orbiters began studying the planet from above.

Where are the boundaries of the Sun's habitable zone today?

Using the most optimistic assumptions, the boundary currently extends from a distance of about 0.84 AU to 1.7 AU. Under more conservative assumptions, the boundary extends from about 0.95 AU to 1.4 AU

The labeled release experiment

also mixed martian soil with organic nutrients. The nutrients were tagged with radioactive carbon-14 and sulfur-35 so that, if they were consumed by martian microbes, by-products of metabolism and respiration would be released as gases and be detectable by virtue of their radioactivity. If life were present, we would expect the level of radioactivity to rise as the organisms consumed the nutrients and released the radioactive gases, and then level off as the nutrients were used up. This is precisely what happened.

The opaline minerals

are particularly significant, for two reasons. First, they are thought to form in hot springs or hydrothermal environments—and recall that such environments may have been important to the origin of life on Earth. Second, some of the regions in which they are found appear to have formed as much as a billion years later than the thick, ancient clay deposits

Lowell's Agathodaemon

coincides with the location of the huge canyon network now known as Valles Marineris on mars.

METHANE ON MARS

could mean there is life there because it should be destroyed by UV light but it persists, meaning it comes from the surface, maybe from a life that releases it? (its most likely from low-level volcanic activity)

tidal friction

created by tidal forces stretching earth itself. earth;s rotation pulls its tidal bulges slightly ahead of the earth-moon line

Martian Seasons

differ from Earth seasons in two important ways. First, because the martian year is nearly twice as long as an Earth year, each season lasts nearly twice as long on Mars. Second, while Earth's nearly circular orbit means that tilt is the only significant factor in our seasons, Mars's seasons are also affected by the ellipticity of its orbit.

the carbon assimilation experiment

mixed a sample of martian soil with carbon dioxide (CO2) and carbon monoxide (CO) gas brought from Earth. In some runs of the experiment, the soil was also mixed with water. The aim was to see if any of the carbon dioxide or carbon monoxide would become incorporated into the soil, as would be the case if living organisms were using either gas as a source of carbon in their metabolism. Experiment found that the carbon-14 was incorporated into the soil, a result that seemed to suggest that metabolism was occurring. When the soil was heated the same thing occurred, which means it was chemical rather than biological.

The gas exchange experiment

mixed martian soil with a "broth" containing organic nutrients from Earth. The experiment looked for gases that might be released by the respiration of martian microbes, including hydrogen (H2), nitrogen (N2), oxygen (O2), methane (CH4), and carbon dioxide (CO2). Again, the initial results were promising for life: As soon as the soil was exposed to the nutrients, oxygen was released into the chamber. However, further analysis again suggested chemical rather than biological reactions.

equilibrium

represents a balance between the reacting atoms and molecules and the product atoms and molecules. For example, molecular hydrogen and oxygen can react together to make water.

Dust Devils

swirling winds that you may have seen over desert sands or dry dirt on Earth. Dust devils look much like miniature tornadoes, but they rise up from the ground rather than coming down from the sky.

Mars surface water

the water would almost immediately either freeze or evaporate away (or some combination of both).

Could Titan have life?

titan has an atmosphere even thicker than Earth's, but it's largely composed of nitrogen, argon,ethane, and methane. The huygens probe showed that fluid hydrocarbons have coursed over Titan's landscape, but the bitterly cold temperatures on this moon would greatly slow chemical reactions, making metabolism difficult and decreasing the chances for life. However it is possible that Titan sometimes has surface or near- surface pockets of liquid water, and a subsurface ocean of acold ammonia/ water mixture. Some energy sources for life might also be available.

Spirit rover

turned up crucial mineral evidence, most notably its discovery of both opaline minerals and hydrated sulfates in a region of Gusev crater nicknamed "Home Plate". Combined with other geological clues found in the region, the minerals make a strong case for the idea that Home Plate was once the site of a volcanically heated thermal hot spring.

Alfred Russel Wallace

used physical arguments to suggest that Mars must be too cold for liquid water to flow. He also pointed out a major flaw in Lowell's interpretation of the canals: Lowell's canals followed straight-line paths for hundreds or thousands of miles, but real canals would be built to follow natural contours of topography