Astronomy unit 3

Where are the best candidates for habitability in outer solar system?

few large moons of Jovian planets

what order do spacecraft missions normally go in?

flyby missions > orbital missions > lander missions. - In each case we learn something that enables us to better design the follow-on investigations.

How is martian surface divided up?

3 regions that refer to 3 diff ages

tidal heating

arises from the effects of tidal forces

most common type of star?

main sequence

what did Viking 1 and viking 2 study?

mars

Does life seem plausible on the Moon or Mercury?

no, b/c no liquid water or any other liquid medium for life.

Could there be life on moons or other small bodies?

- A few large moons may contain liquid water or other liquids, potential candidates for life. - Smaller moons and other small bodies probably do not have any liquids present, many may have had liquid water in the distant past.

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

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

How long can life survive on Earth?

- At a minimum, earth could 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.

How does a planet's location affect its prospects for life?

- At any particular time, a star's habitable zone is the range of distance around it at which a planet could potentially have surface temperatures that would allow for abundant liquid water.

What is the leading hypothesis concerning how Mars lost its once-thick atmosphere? What role does Mars' size play in this hypothesis?

- Because Mars is smaller than the Earth, magnetic field would have been lost due to the cooling of its core. - The magnetic field helps protect atmosphere from the solar wind --> w/ no magnetic field, solar wind would have stripped away Mars' atmosphere. - Mars would also have had weaker gravity due size --> made it more difficult to hold onto an atmosphere.

What are the 3 Viking experiments?

- Carbon assimilation experiment: see if any of the CO2 or CO would become incorporated into soil - Gas exchange experiment: look for gases that might be released by the respiration of martian microbes: - labeled release: if life were present, expect the level of radioacitivty to rise as organisms consumed the nutrients and released radioactive gases - GCMS: Baked soils to release volatiles which can be measured by molecular weight

Based on our understanding of the chemistry of life, where should we expect such chemistry to be possible? What are the implications of this idea to the search for life beyond Earth?

- Chemical reactions that sustain life are enabled by conditions that tend to introduce disequilibrium into the reactions. - On Earth, we would expect these conditions to exist near the deep sea vents and at the interfaces between rock and water. - In our search for life elsewhere, we would start by looking at worlds that might be geologically active and where there might be liquid water.

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

- Earth 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.

How does the strength of sunlight vary with distance from the Sun? Discuss the implications for photosynthetic life.

- Energy from the Sun decreases with the square of the distance ("inverse square law"). - photosynthetic life would have to be larger, more efficient at collecting sunlight, have a slower metabolism compared to Earth life in order to exist. - This is the same reason that when we send robot spacecraft to the outer solar system, we need to use something other than solar arrays to supply power. We generally use RTGs, which are little nuclear reactors, to supply energy. The exception is the JUNO spacecraft which uses solar arrays event though it's in orbit around Jupiter.

Why does Mars seem such a good candidate for life?

- Evidence gathered by the Mars Exploration Rovers, Mars Curiosity, and the various orbiting satellites show Mars once had water on the surface and a thicker atmosphere - The temps were warmer in the past, as well. - Although there is no surface water at the present time, there is underground water ice and perhaps liquid water under the surface as well. - Conditions on Mars 3 - 4 billion years ago were far more Earth-like than they are today suggesting that life could have gotten started on Mars in a similar manner to the way life got started on the Earth

What is the potential significance of atmospheric methane to the search for life on Mars?

- For it to be detected now, something must be introducing methane into the atmosphere. - Possible sources of methane: comet or meteorite impacts, volcanism, and biology. - Calculations have shown that the number of impacts by comets or meteors needed to account for the methane would be detectable. - no evidence of current volcanism - plausible methane could have been created in past, has been locked up in reservoirs underground, Periodic releases from those reservoirs could account for the detection. - discriminate between biology and volcanism by measuring other gases that would be expected to be produced with methane in the two different processes, or by measuring the 12C/13C ratios.

What does water on mars mean for a warm and wet, possibly with rainfall, in its distant past.

- For water to have been liquid on the surface, temps must have been warmer and the atmosphere thicker in the past. - Mars could have had an early atmosphere created by volcanic outgassing and comet impacts. - GHE would have warmed the planet.

Is there evidence of life in martian meteorites?

- Four lines of evidence have been presented as suggesting the presence of past life in a martian meteorite, each also has a potential nonbiological explanation

How might future observations confirm an ocean beneath Europa's surface?

- Future observations could make the case more definitive by measuring the amplitudes of the surface bulges with a laser altimeter - The amount of stretch that Europa undergoes could discriminate between solid ice and liquid water under the surface. - Radar measurements could, in principle, establish the existence of subsurface water in the same manner that it was used to discover Lake Vostok in Antarctica, which is a body of liquid water under a thick sheet of ice.

Briefly summarize the Viking experiments and their results.

- GCMS: Baked soils to release volatiles which can be measured by molecular weight - Gas Exchange: Mars atmosphere removed and water plus nutrients added to soil sample. Any off-gassing was analyzed for changes due to life. - Labeled Release: Soil was exposed to nutrient solution with 14C and experiment analyzed for 14CO2 - Pyrolytic Release: Soil was introduced to light, 14CO and 14CO2 and subsequently heated to measure metabolized 14C.

Should we send humans to Mars?

- Human missions to Mars could probably answer scientific questions about life much more quickly than robotic missions - pose a risk of contamination.

possible evidence of past life discovered in studies of ALH84001

- Hydrocarbons (PAH's) are found which are the same as breakdown products of dead micro-organisms on Earth - Mineral phases consistent with by-products of bacterial activity - Tiny carbonate globules which may be microfossils of primitive bacteria, all within a few hundred-thousandths of an inch of each other. (Similar to products of nanobacteria on Earth). - Carbonates are consistent with the exposure of ALH84001 to water - Morphologically, the "fossils" resemble nanobacteria

Why does water seem the most likely liquid medium for life?

- It is abundant in the Milky Way - It also remains liquid over a greater range of temperature - water is polar, allows water to become less dense as it freezes providing advantages for life that lives in water. - b/c polar, good at dissolving other molecules - Biochemistry on Earth is dependent on these hydrogen bonds.

Describe the evidence suggesting that Europa has a liquid water ocean beneath its icy crust.

- It's possible to calculate the amount of tidal heating that results from Europa's orbit around Jupiter and the models show that there should be enough heat to keep water liquid beneath the icy crust - The surface of Europa is relatively young as determined by crater counts implying that there is some process that resurfaced the moon in the last few million years. - The characteristics of the surface, including what's described as "chaotic" terrain, suggest that liquid water wells up through surface cracks and freezes into ridges and blocks. - Europa has an induced magnetic field implying the presence of a conducting medium under the surface. Most likely this is a salty ocean. - HST spectroscopy has found evidence of water vapor over Europa's south pole. Taken together, these pieces of evidence imply that there is an ocean under the icy surface of Europa. - Future observations could make the case more definitive by measuring the amplitudes of the surface bulges with a laser altimeter. The amount of stretch that Europa undergoes could discriminate between solid ice and liquid water under the surface. Radar measurements could, in principle, establish the existence of subsurface water in the same manner that it was used to discover Lake Vostok in Antarctica, which is a body of liquid water under a thick sheet of ice.

What characteristics make some of the large moons of jovian planets seem like potential candidates for habitability?

- Jupiter and Saturn large moons formed where ices were present in the protostellar disk. - Evidence supports the conclusion that much of the ice is water ice. - At least some of the jovian moons have a source of energy due to tidal forces from Jupiter or Saturn --> provides a heating mechanism and possibility that some of the water is liquid under the moons' frozen surfaces. - We know that organics are prevalent in the solar system and in the case of Enceladus organics have been detected in the water plumes. - So, in the case of some of the large jovian moons, the most constraining requirement, liquid water, could very well be present. With a known source of energy and the likelihood of organic molecules, that makes them potential candidates for habitability.

Do the Viking experiment results constitute evidence of life?

- Labeled release reported an increase in the amount of radioactive CO2 - Gas Exchange experiment detected release of oxygen. Both of these results were consistent with the presence of life. - other experiments did not find evidence consistent with life. - GCMS found no evidence for organic molecules. Other results were inconsistent with life. - concluded that reactions that were observed were more easily explained by surface chemistry than biology. - As we discussed in class, with the knowledge that there were perchlorates in the soil at the Mars Phoenix location (and also now discovered at the Mars Curiosity location), an experiment was done that showed that organic molecules would have been destroyed by the Viking experiment protocol had there been perchlorates present in the soil at the Viking location. That would argue that the Viking experiments were inconclusive. Mars Curiosity has found perchlorates at its location in Gale Crater.

Does life need liquid water?

- Life almost certainly requires a liquid - water has 3 advantages over other liquids: a wider and higher range of temperatures in which it is liquid; ice floats; type of chemical bonding made possible by charge separation within water molecules. - can't completely rule out other liquids, such a liquid ammonia, methane, or ethane.

what is the role of disequilibrium in life?

- Life as we know it can exist only in places where natural conditions maintain a state of chemical disequilibrium. - disequilibrium can cause chemical reactions that may be used to create complex molecules, to string complex molecules together into complicated structures, or to support metabolism of living organisms.

What types of chemical reactions supply energy for life?

- Life on Earth gains energy from redox reactions in which one molecule gains electrons and another loses them - redox reaction used by terrestrial life includes those that occur near deep-sea vents and at underground rock/water interfaces - same types of reactions might be used by life on worlds that have similar conditions, such as Mars (underground rock/water interfaces) and Europa (deep-sea vents)

Could moons of Uranus or Neptune have life?

- Life seems less likely on such distant moons, still possible that some could have habitable zones beneath their surfaces - Neptune's moon Triton shows evidence of tidal heating and ice volcanism, suggesting it might have liquid beneath its surface. - Other moons seem like much longer shots, but the lesson of Enceladus tells us we should study them further before concluding that they lack liquids and chemistry that might sustain life

What are the prospects for life on jovian planets?

- Liquid water could exist at certain depths in the atmospheres of the jovian planets - strong vertical winds make life seem unlikely - Uranus and Neptune may have "oceans" of water and other liquids in their deep interiors, no way to search such depths for life.

Could other moons of Jupiter have life?

- Magnetic field measurements suggest Ganymede and Callisto could have subsurface oceans - Ganymede shows some evidence of water having gushed out onto parts of its surface - Ganymede and Callisto could conceivable offer conditions for life, although energy sources are even more limited on these moons than on Europa.

Is mars habitable?

- Mars almost certainly had a habitable surface during its wet periods more than 2-3 billion years ago. - surface or near-surface might still sometimes be habitable when its axis tilt is greater than its now, and the subsurface may still have habitable regions today.

How do Martian seasons differ from Earth seasons? Describe major seasonal changes that occur on Mars.

- Mars' rotation axis is tilted approximately the same amount as the Earth's so it has seasons. - last 2x as long as Earth seasons b/c Mars' orbital period is roughly twice as long. - Mars' orbit is more elliptical than Earth's, so the variable distance from the Sun is more of a factor on Mars. - In particular, in the southern hemisphere winter, Mars is farthest from the Sun and in southern hemisphere summer Mars is closest to the Sun. Since it's farther from the Sun in southern winter, Mars is also moving more slowly in its orbit (Kepler's Second Law) so winter lasts longer than summer. - All of this means that the seasons are more extreme in the southern hemisphere than in the northern hemisphere. - Seasonal changes result in the sublimation of the carbon dioxide in the polar caps in the summer and strong winds which cause significant dust storms.

What is the evidence for global warming?

- Measurements show that human activity is causing a substantial increase in the atmospheric concentration of CO2. - 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 to the human contribution to global warming, rather than to natural factors

How does the requirement of organic building blocks further constrain the prospects of habitability?

- Miller-Urey experiment suggest that at least some of the organic building blocks could be created in naturally occurring chemical reactions on early Earth. - Conclude the elements important to life as well as the basic organic building blocks are common - more complex organic molecules are less common but still perhaps common enough that we can be optimistic about the possibility of life elsewhere. - Based on studying Earth life, some sort of medium is necessary to support the reactions that are necessary for life. Therefore, it would make sense to constrain our search to look for worlds with evidence of either surface or sub-surface water.

What evidence tells us that water once flowed on Mars?

- Orbital images eroded craters, dry river channels, and floodplains all point to past water flows - supporting evidence found in chemical analysis of martian rocks. The era of lakes (or possibly oceans) seems to have ended at least 2-3 billion years ago, but some flooding may have occurred later. - Mars today still has water ice underground and in its polar caps, and could possibly have pockets of underground liquid water.

What are the necessary but not sufficient conditions for life?

- Organic molecules: complex organics exist in comets and meteorites, survival of molecules could depend on an atmosphere or getting underground - Water: Enables chemical reactions - Energy: Sunlight (starlight), chemical energy, thermal energy, Assuming life metabolizes, some source of energy is required

Summarize the evidence suggesting that Mars must have had water

- Photos from early orbiting spacecraft showed features that appeared to have been created by flowing water; e.g., river beds, dry lake beds, river deltas, etc. - There's photographic evidence of erosion of craters, some investigators attribute to rainfall. - Remote sensing experiments onboard orbiting spacecraft found evidence of hydrated materials, which is evidence of there having been liquid water - Mars Exploration Rovers and Curiosity have found minerals that are formed in water. - layered deposits that were most likely the result of standing water.

Briefly summarize the evidence, both real and imagined, that led to widespread belief in a Martian civilization by the end of the nineteenth century.

- Real evidence: fact that Mars has polar ice caps as does the Earth, Mars has a rotation axis tilted approximately as much as the Earth's, Mars rotates in a period of just over 24 hrs. - real observations of color changes that were season-dependent, which some imagined to be growing vegetation. - observed that the Martian polar caps shrink in the summer. That fact, coupled with the imagined presence of canals, led to assumption that intelligent Martians were transporting melting snow to the planet's equatorial regions to support agriculture.

Why do we think that some moons could harbor life?

- Some moons have substantial internal heat as a result of tidal heating, along w/ radioactive decay - Tidal heating explains the volcanic activity on Io and the heating thought to melt subsurface ice on Europa. - A few other moons may also have liquid water or other liquids, and thus would seem to meet the minimum requirements for life.

How do robotic spacecraft work?

- Spacecraft can be categorized as flyby, orbiter, lander/probe, or sample return missions. - robotic spacecraft carry their own propulsion, power, and communication systems, can operate under preprogrammed control, or with updated instructions from ground controllers.

How does Mars invade popular culture?

- Superficial similarities b/w mars and earth led to speculation about martian civilization - Astronomer Percival Lowell thought he saw canals built by and advanced society, canals do not really exist

What is synchronous rotation

- Synchronous rotation means that a moon rotates exactly once per orbit. - The result is that the moon keeps the same face pointed at the planet - Synchronous rotation is a natural consequence of the tidal force of the planet on the moon. - Over time, tidal friction on the moon causes its rotation rate to decrease. Once synchronous rotation is established, there is no more tidal friction.

why this evidence generates controversy

- Taken together, these pieces of evidence suggested biology to McKay et al. - However, the arguments are not conclusive and there are non-biological ways to account for some or all of the findings in the meteorite. - Proving there used to be life on Mars would be one of the most important discoveries of all time. Therefore, as Sagan said, "extraordinary claims require extraordinary evidence."

Is there any evidence of life on Mars?

- The Viking experiments produced results that some scientist think may be evidence of life, nonbiological explanations seem more likely. - Recent observations have detected methane in the atmosphere, which may be due to life or may simply be due to volcanism.

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 life 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.

What makes the recent discoveries about Enceladus so surprising? Could Enceladus be habitable? What lessons does Enceladus hold for our more general search for life in the universe?

- The most surprising discovery was that Enceladus had active ice volcanoes. - Measurements from Cassini proved that the geysers were water and they contained organic molecules. That observation coupled with the young surface with cracks and grooves suggest that there's liquid water under the surface. - Tidal heating would be able to melt the undersurface ice. The fact that Enceladus has a source of energy, liquid water, and organic molecules makes life there plausible, though not necessarily likely. The most important lesson for our search for life elsewhere is that the conditions that might be favorable for life could be more common than we initially thought.

Summary for and against ALH84001

- The summary of the arguments for and against looks something like this: - PAHs are found all over the place - Mixture of PAHs in ALH84001 is very different - The findings could be Earth contamination - You'd expect the contamination to be more concentrated near the surface - No cell cavity has ever been found - Higher resolution assessments are required - The "organisms" are smaller than Earth life - Nanobacteria are of similar size (but are they alive?) - Carbonates can be made non-biologically - Carbonates in ALH are always found in a form fitting pit - In the lab they form in cracks between two pyroxene surfaces • Magnetites can be formed without biology - They don't look like the ones in ALH; mixed composition carbonates don't make pure Fe-magnetites

How does Mars' axis tilt change affect the climate?

- The tilt of a planet's rotation axis with respect to the normal orbital plane determines the extent to which there would be seasons - Since Mars' rotation axis can change tilt rather dramatically, the result is episodes of dramatic climate change on Mars, including epochs where the polar caps would sublimate affecting the composition of the atmosphere. - Mars' orbit is more elliptical than most other planets and that contributes to the extremes in the seasons at a specific rotation axis tilt.

How and why does Mars' axis tilt change with time, and how do all these changes affect the climate?

- The tilt of rotation axis is affected by Jupiter's gravity --> results in the rotation axis changing tilt over time - Models suggest that the changes could be as great as 0-80 degrees. - Earth's rotation axis is affected less b/c Earth is farther from Jupiter, also the presence of the Moon has a stabilizing effect on Earth's rotation axis - Mars has no large moon and so its rotation axis changes orientation dramatically over periods of > 100,000 yrs

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. - 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.

What energy sources might be available to life on Europa? Overall, what can we say about the likelihood and abundance of life on Europa?

- Tidal heating and radioactive decay would likely heat the rocky interior surface, potentially melting the rock. - This could create a situation similar to the deep ocean vents on the Earth where there could be something like undersea volcanoes. It is conceivable, that there could be some sort of molecules that life would use for energy produced near the surface by interactions between the surface ice and UV photons from the Sun and high energy particles from Jupiter's radiation belts. - The book suggests that radioactive decay of potassium could produce hydrogen and oxygen that might facilitate chemical reactions for life. - In general, the energy available for life would be small on Europa compared to Earth, which doesn't say life couldn't exist but argues that it would not likely be abundant.

Briefly describe the cause of the tides on Earth, why they lead to tidal friction, and how tidal friction affects Earth's rotation and the orbit of the Moon.

- Tides caused by difference between the force of the Moon's gravity at different places on the Earth due to the different distances of those places to the Moon. -The point on the Earth closest to the Moon experiences a greater gravitational force than does the center of the Earth or the point on the Earth farthest from the Moon. - As a result, a bulge of ocean water is established along the line between the center of the Earth and the Moon. - b/c the Earth rotates faster than the Moon orbits the Earth, friction pulls the ocean bulge slightly ahead of the Earth-Moon line. -The displacement of the bulge tugs on the Moon and the result is that the Moon's orbital distance increases. - The Moon is moving away from the Earth at a few centimeters per year. The friction causes the Earth's rotation rate to decrease. So, the rotational energy that the Earth loses goes into increasing the orbital energy of the Moon.

could titan have life?

- Titan has a thick atmosphere, lakes of liquid methane and ethane, and numerous other surface features reminiscent of Earth - cold temperatures would slow chemical reactions, metabolism difficult and decreasing the chances for life. - possible titan sometimes has surface or near-surface pockets of liquid water and subsurface ocean of a cold ammonia/water mixture. Some energy sources for life might also be available.

Why was Titan chosen for such intense study by the Cassini-Huygens mission? Why is it surprising to find methane in Titan's atmosphere?

- Titan is the only moon in the solar system with a significant atmosphere. That atmosphere has some interesting constituents and properties. - Understanding the atmospheric chemistry was an important motive for the Cassini-Huygens mission. - Methane is unstable in the atmosphere and would be broken down by UV photons. The abundance of methane still present suggests that there must be a source. - A plausible source would be lakes of liquid methane on the surface that slowly evaporate.

Could life exist on Venus?

- Venus = too hot for liquid water on or under its surface, life seem unlikely. - life might be possible high in Venus's atmosphere, where clouds contain droplets of water.

What factors influence surface habitability?

- 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 - has enough of an atmosphere for liquid water to be stable on its surface.

Why do we expect the elements of life to be widely available on other worlds?

- all of the elements heavier than hydrogen and helium are made in stars. - Elements that are the most important to Earth life, (C, O, N) are among the most abundant of the heavy elements. - We know from studying supernovae that these elements are reintroduced into the interstellar medium, and new generations of stars and planets are made from material that's enriched in these important elements - From studying comets and meteorites we have evidence that organic building blocks, including amino acids, were created naturally in space and deposited on planets in the solar system or were incorporated in the planets when they formed.

what are a few other potential liquids for life.

- ammonia, alcohol, methane - Other potential solvents are not polarized in the way the water molecules are. - not as abundant as water

Features of Uranus and Neptune

- atmosphere colder than Jupiter and Saturn bc distance from sun - cloud where liquid water can form, vertical winds unlikely for life - outer cores of water, methane, and ammonia are potential zones of habitability

Why was Mars warmer and wetter in the past?

- atmosphere must once had been much thicker with stronger greenhouse effect, - do not know whether this made Mars warm and Wet for an extended period of time or only intermittently

Why isn't liquid water stable at the Martian surface today? What happens to water ice that melts on Mars?

- b/c temps so cold and atmosphere is so thin - water would not be a liquid on the surface of Mars - Water would normally be ice on the surface and any water ice that melts would quickly sublime. - scientists argue that subsurface water is released sometimes because there's photographic evidence of places where something resembling ice has recently appeared - This is observed on the side of a hill or wall of a crater. In this case, water would have probably both frozen and evaporated as it was released onto the surface. This idea has confirmed by the detection of hydrated salts associated with the features that appear to be water flows.

What is the restraint of chemical reactions?

- can only occur if the potential reactants are brought into contact w/ each other - ongoing reactions can occur only where materials are being continually mixed

Overall, what "litmus test" seems appropriate for constraining our search for habitable worlds, and why?

- can't rule out inorganic molecules for building cells nor other solvents than water, although there are several lines of evidence that say organic molecules and water are the most likely - The most constraining requirement need for liquid water, seems to be less common on other worlds than organics or a source of energy.

evidence of life based on the martian meteorite ALH84001

- carbonate grains have a layered structure, w/ alternating layers of magnesium, iron, and calcium rich carbonates - carbonate grains contain complex organic molecules known as PAHs - under a microscope, we see crystals of mineral magnetite w/in iron-rich layers of carbonate grains

Where can we expect to find building blocks of life?

- chemical elements needed for life should be present on almost any world - smaller number of worlds contain more complex organic molecules that serve as building blocks for life. - building blocks are present in asteroids and comets suggest that we'll find them in many places

What is mars like today?

- cold and dry - atmospheric pressure so low that water is unstable. - Martian weather is driven largely by seasonal changes that cause CO2 alternately condense and sublime at the poles, creating winds that sometimes generate huge dust storms.

Where could the methane on mars come from?

- comet impacts: unlikely b/c rare events - volcanic activity - life

Where do organic molecules came from? Combo of three sources:

- deep-sea vents - chemical reactions in atmosphere - brought to earth from space

Features of Large Jovian moons

- different than terrestrial planets: born w/ more ice - moons have ice geology - Io and Europa: ongoing source of internal heat, in principle melt subsurface ice into liquid water - Europa possible subsurface ocean of liquid water

What would photosynthetic life be like on a world farther away?

- either much larger than life on Earth (larger surface area for collecting light), - more efficient at collecting solar energy - or slower in its metabolism and reproduction.

Why is a liquid medium important for life?

- enables the chemical reactions that support life - Organic molecules dissolve so that they are available to react - chemicals need to be transported into and out of cells - plays a role in metabolic reactions.

What are the advantages and disadvantages of flybys, orbiters, landers or probes, and sample return missions

- flyby missions: less expensive, time available for taking data is limited, can study multiple targets more cheaply than can orbiters - Orbiters: study targets for longer durations, more expensive b/c have to carry propellant for entering orbit at the destination - Landers: give most detailed investigations of planets, expensive - Sample return missions:analysis can be done in a terrestrial lab using techniques and equipment that wouldn't be practical to put on a spacecraft. Sample return missions such as Stardust are relatively inexpensive, although a Mars sample return mission would be a significant cost.

Could Europa have life?

- it's probable that Europa has both a liquid water and elements necessary for life, possible energy forces for life are much more limited than on Earth. - Volcanic vents on the ocean floor could provide some energy, perhaps enough for life to have arisen but probably not enough to support life in great abundance - A few other energy sources may contribute additional energy, but overall we would expect any life on Europa to be simple and small.

explain how the larger jovian moons tend to differ in general from the smaller ones. How does the formation process of the moons explain this difference?

- larger moons: orbit in the same direction as the planet rotates, orbit planes are generally in the equatorial plane of the planet. - Small moons: more likely to have orbits that are highly elliptical, and/or inclined with the plane of the planet's equator. Some even have retrograde orbits. often irregular in shape. - These characteristics taken together imply that the larger moons co-accreted with the planet while the smaller moons are more likely to have been captured asteroids or comets.

features of mercury

- may have water chemically bound in surface rock, unlikely water takes liquid form - contain water ice in craters near poles

What are the general characteristics of the jovian moons?

- moons of jovian planets range greatly in size: a few kilometers across to somewhat larger than mercury. - tend to have ice mixed in with their rock: water ice for all the jovian moons, plus ammonia, methane, and other ices for moons of the more distant jovian planets - Nearly all are the synchronous rotation, keeping one side perpetually turned toward their host planet.

Why did mars change?

- must have occurred b/c loss of atmospheric gas --> weakened the greenhouse effect - Some gas was probably blasted away by impacts - more probably was stripped away by the solar wind as Mars cooled and lost its magnetic field and protective magnetosphere. - Water was probably lost b/c UV light could break apart water molecules in the atmosphere, the lightweight hydrogen then escaped to space

Features of mars

- once had flowing water, w/ thicker and warmer atmosphere - mars lost atmospheric gas -> surface pressure too low for liquid water - retains enough internal heat, liquid water might be possible underground

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

- optimistic assumptions: boundary extends from a distance of about .84 AU to 1.7 AU - conservative assumptions: boundary extends from about .95 AU to 1.4 AU.

first requirement of life:

- presence of most or all of the elements used by life - Probably met by almost any world, all chemical elements besides H and He were produced by stars - Although the heavy elements are rare compared to H and He, they are found just about everywhere

crater evidence of water that once flowed on Mars

- rainfall may have eroded craters, stronger evidence argues lakes in the bottom of craters - 3 types of hydrated minerals: clay, hydrated sulfates, and hydrated silica, indicate they were formed the presence of liquid water

What are the potential consequences of global warming?

- raise the average worldwide temperature by 3 degreec C to 5 degrees C during this century. - Regional climate changes will be greater, 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.

What are the major geological features of Mars?

- regions that are densely cratered and must be very old - other regions with fewer craters must be much younger - Giant volcanoes dot certain regions of Mars, see evidence of past tectonics, probably created Valles Marineris

Could other moons of Saturn have life?

- relatively small moon Enceladus probably has a subsurface liquid that drives the fountains of ice and water vapor observed to be emerging from the moon - possible that Enceladus has zones of habitability - no direct evidence for similar possibilities on other moons of Saturn, the case of Enceladus tells us not to rule them out too quickly.

What are the environmental requirements for habitability?

- requires a source of molecules to build living cells - source of energy for metabolism - liquid medium for transporting chemicals - possibility of liquid water is the main requirement we search for in looking for habitable worlds.

similarities b/w moon and mercury

- smaller than earth, lost most of internal heat, no ongoing volcanism, no significant tectonic activity - no volcanoes = no outgassing - small size = weak gravity

Does Europa have an ocean?

- surface shows numerous features suggesting that liquid or slush from below has gushed through and repaved the surface - magnetic field makes sense only if we assume Europa has a salty ocean - observational data, combined w/ the known tidal heating of Europa, make it likely that the moon has a subsurface ocean, which may contain twice as much water as the oceans of Earth.

features of mars

- temp below freezing, atmospheric pressure <1% - lack of O2 - no liquid water on surface bc low atmospheric pressure - liquid water would either freeze to ice, or sublimate - surface shaped by liquid water

features on mars caused by season changes

- temps @ winter pole drop so low that CO2 condenses into dry ice at polar caps - frozen CO2 at summer pole sublimates into CO2 gas, atmospheric pressure increases at summer pole and decreases at winter pole - stone wind due to cycling of CO2 cause dust storms - cause dust devils: tornadoes from ground to sky

What is the litmus test?

- test for habitability - a world can be habitable only if it has a liquid medium

features of venus

- thick atmosphere, lots of CO2, very hot - extreme GHE - high pressure - covered in thick clouds

Features of jovian small bodies

- unlikely habitable - too small to have leftover internal heat - orbit too far from sun

features of our moon

- very little water in any form - water ice found at the bottoms of polar craters

Why do the Moon and Mercury seem unlikely to be habitable? Does evidence for ice in lunar craters affect the answer?

- water ice has been found on both the Moon and Mercury. - ice is found in the permanently shadowed craters near the poles. - Despite the fact that there's water ice (and organics) present, the conditions aren't right for the water to be liquid (too hot, no atmosphere) and, therefore, life being able to exist is highly improbable.

Where should we look for organic molecules/building blocks of life?

- worlds that have an atmosphere or a surface or subsurface liquid medium like water or both

What is the martian sky like?

-winds and dust storms cause perpetual dusty air - w/o suspended dust sky would be black b/c air is so thin - sky is a yellow-brown color - diff hues occur depending on amount of dust

- energy available in sunlight decreases with the square distance from the sun - how much sunlight would a leaf on a world 2x as far from the sun as earth receive?

1/4 as much energy as the same leaf on Earth

Why does solar system formation mean elements of life are common on other worlds?

As long as condensation and accretion can occur, we expect the resulting worlds to contain the elements needed for life

Cassini huygens

Cassini carried a probe called Huygens that parachuted to a soft landing on Titan's surface. Results from Huygens and observations from Cassini revolutionized understanding of Titan

How and when will Earth become uninhabitable? Why? Could humans still survive? Explain.

Due to the outward migration of the inner boundary of the habitable zone, the Earth will no longer be in the habitable zone in about a billion years. The Earth will get warmer and a "moist greenhouse effect" will occur and the Earth will lose its water to evaporation and eventually to space. Humans could possibly survive by leaving Earth for another planet. Mars would eventually be in the habitable zone and terraforming might make it actually habitable for humans.

Where can we expect to find energy for life?

Energy for life can come from sunlight or chemical reactions. - Sunlight weakens with distance, unlikely to be sufficient to sustain life at large distances. - Chemical energy is probably available in more places, likely on any world with a substantial atmosphere or a liquid medium that can mix and support chemical reactions.

Why was mars warmer and wetter in past?

GHE

significance of methane on mars?

If methane detection is real, mars must have an active source of it

4 Galilean moons

Io: very active volcanism, Europa: subsurface ocean Ganymede: biggest Callisto: covered w/ craters

Features of Jupiter and Saturn

J: temp below freezing in cloud tops, temp rises w/ depth - several cloud layers, presence of liquid water - atmosphere makes life unlikely: strong vertical winds

What are the Jovian planets?

Jupiter, Saturn, Uranus, Neptune

orbital evidence of water that once flowed on Mars

Mariner 9 and Viking found features that look like dry riverbeds, channels were almost certainly carved by running water

Could life exist on Mars?

Mars almost certainly had habitable conditions in the distant past and might still have habitable regions underground.

How do we expect the habitable zones of brighter stars to compare to that of the Sun?

More luminous stars would have larger habitable zones than does the Sun. The inner and outer boundaries would be farther from the star than those same boundaries for our solar system. If a more luminous star has planets, the larger habitable zone would make it statistically more likely that one or more of the planets would be in it.

On the main sequence what type of stars are most rare to common?

O > G > M

Spectra of stars

O, B, A, F, G, K, M

Most occurring natural elements of life on earth

O, C, H, and N make up 96% of the mass of living organisms

Why does the Sun gradually brighten, and how does this brightening affect the location of the habitable zone over time? What do we mean by the continuously habitable zone?

Over time, the core of the Sun becomes less dense because nuclear fusion is converting four hydrogen nuclei into one helium nucleus. As a result, the number of particles is decreasing causing the pressure to decrease. That alters the balance between the pressure in the core due to fusion and the force of gravity due to the outer layers of the Sun. As a result, gravity will compresses the core which heats it up which causes the nuclear reactions to occur more rapidly releasing more energy. Consequently, the boundaries of the Sun's habitable zone will change over time. As the Sun brightens, the boundaries of the habitable zone will move farther away. One consequence is that a planet might be in the HZ at one point in time and then not be at a later time as the boundaries of the HZ change. The continuously habitable zone refers to the distances from the Sun that have been part of the habitable zone for the last 4 billion years (according to the book).

Rover mineral evidence of water that once flowed on Mars

Spirit and Opportunity have confirmed and extended the mineral evidence for past water

why is synchronous rotation so common among the jovian moons?

Synchronous rotation is common among the jovian moons because the planet sizes are so much larger than the moons', which allows synchronous rotation to be established relatively quickly.

What is the habitable zone, and how is the idea useful? Is a planet in the habitable zone necessarily habitable? Explain.

The habitable zone is the range of distances around a star where a planet would be warm enough to have liquid water on the surface. The concept serves as an initial filter in evaluating planets found around other stars in terms of the likelihood that they might be homes to life of some kind. However, a planet in the habitable zone is not necessarily habitable. There are a number of factors that plausibly affect the likelihood of life existing on a planet. The Moon is in the habitable zone of the Sun, but it's not habitable. Clearly the presence of an atmosphere makes a difference in terms of habitability. Part of what makes the Earth's distance from the Sun be in the habitable zone is the amount of greenhouse effect at work in our atmosphere. Were it not for that, the temps on the Earth wouldn't be above freezing.

Why is mars a prime candidate for past life?

evidence of past water

Is there any evidence of life on Mars?

claims fall into three categories: - results of viking landers, evidence of methane in martian atmosphere, based on studied of martian meteorites found on Earth

Summarize the three major environmental requirements for life. Overall, what "litmus test" seems appropriate for constraining our search for habitable worlds, and why?

environmental requirements for life are: - a source of organic molecules - a source of energy to support metabolism - a liquid medium to support chemical reactions.

Roche limit

point where tidal forces become too large--they exceed the self-gravity holding a moon or rock together, i.e. the min. distance to which a large satellite can approach its primary body w/o being torn apart by tidal forces

Same return mission

required spacecraft to return to Earth carrying a sample of the world is has studied

results of viking experiments

ruled to be a chemical process vs biological

runaway GHE vs moist GHE

runaway GHE: positive feedback between surface temperature and atmospheric opacity increases the strength of the greenhouse effect on a planet until its oceans boil away moist GHE: the upper atmosphere would become moist with water, at least until all the water was lost—might cause Earth to lose its oceans

lander or probe

spacecraft designed to land on planet's surface or probe a planet's atmosphere by flying though it. Some landers carry rovers to explore wider regions

flyby

spacecraft goes past a world just once and then continues on its way

Orbiter

spacecraft orbits the the world it's studying, allows long-term observation

Tidal friction

tidal forces stretch Earth itself, process creates some friction - as moon gets farther away, earth's rotation slows down, moon gets farther from earth

Where does earth get it's seasons?

tilt of planet's axis

Why are martian volcanoes so large?

weaker gravity makes it easier for tall structures to be built up, lack of tectonic plates on mars means mantle plumes remain stationary