Astrobiology in the Classroom

 

NASA – CERES Project –http://btc.montana.edu/ceres

Montana State University

Preliminary Edition

 

 

“Who Can Live Here? --

Life in Extreme Environments

 

 

 

 

 

 

 

Students explore the limits of life on Earth to extend their beliefs about life to include its possibility on other worlds. In this four-part activity, students first explore the environments of several mammals and birds to better understand how living things and their environments interact and depend on each other. This part is designed to illustrate that the kinds of animals found in different parts of the Earth are related to the climate and environment there. In the second part, students match bacterial types with their more extreme environments. Students discover that an environment's temperature, salinity, pH, and sources of carbon and energy are important for what can live there. Next, students are given readings on life in extreme environments that cover the latest scientific findings in this field. Students answer reflective-questions designed to probe their understanding of the limits that are placed on both the organism and the environment. With their new understanding of the limits for life on Earth, students are asked in the final part to explore environments on other planets and moons in our Solar System. Like true astrobiologists, they are challenged to imagine what type of organisms could live in these extreme environments.


Who Can Live Here?

 

Part I – Exploration

 

From our perspective as humans we typically think of ourselves as the dominant form of life on the planet.  From an overall perspective that includes all forms of life from tiny single-celled bacteria to enormous whales we must consider that we, as humans, are simply one of the players in the larger ecosystem at play on Earth.  In the activities that follow we are going to begin by considering familiar life forms in familiar settings from around the world and then move to studying very specific and unfamiliar life forms in very extreme living conditions that exist on Earth.  

 

Activity #1  Why do they live there?

                             

Consider the two lists shown below containing the names of different types of bears and birds. 

 

BEARS

BIRDS

Koala

Parrot

Grizzly

Penguin

Polar

Bald Eagle

Panda

Ostrich

 

The following questions are asked to help you begin to reason about different life forms in terms of their connection to the surroundings they inhabit. 

 

 

 

A.  Consider each of the bears and birds listed in the table above.  In general why do these animals exist only in specific regions on the Earth?   Are there specific features of their surroundings that strongly influence why they live at these particular locations?  If so, what are they?

 


B.  Complete the following tables by filling in the blank next to each characteristic with an approximate numerical value and/or a brief description.  Consider how the animal and its environment are interconnected to the survival of the animal.

 

Koala Bear

 

Characteristics:

 

Location

 

 

Temperature

 

 

Elevation

 

 

Amount of Water

(low, medium, high)

 

 

Source of Food

 

 

Mode of Transport

 

 

 

 

Grizzly Bear

 

Characteristics:

 

Location

 

 

Temperature

 

 

Elevation

 

 

Amount of Water

(low, medium, high)

 

 

Source of Food

 

 

Mode of Transport

 

 

 

 


 



Polar Bear

 

Characteristics:

 

Location

 

 

Temperature

 

 

Elevation

 

 

Amount of Water

(low, medium, high)

 

 

Source of Food

 

 

Mode of Transport

 

 

 

Panda Bear

 

Characteristics:

 

Location

 

 

Temperature

 

 

Elevation

 

 

Amount of Water

(low, medium, high)

 

 

Source of Food

 

 

Mode of Transport

 

 

 

Parrot

 

Characteristics:

 

Location

 

 

Temperature

 

 

Elevation

 

 

Amount of Water

(low, medium, high)

 

 

Source of Food

 

 

Mode of Transport

 

 

 

Penguin

 

Characteristics:

 

Location

 

 

Temperature

 

 

Elevation

 

 

Amount of Water

(low, medium, high)

 

 

Source of Food

 

 

Mode of Transport

 

 

 

Bald Eagle

 

Characteristics:

 

Location

 

 

Temperature

 

 

Elevation

 

 

Amount of Water

(low, medium, high)

 

 

Source of Food

 

 

Mode of Transport

 

 

 

Ostrich

 

Characteristics:

 

Location

 

 

Temperature

 

 

Elevation

 

 

Amount of Water

(low, medium, high)

 

 

Source of Food

 

 

Mode of Transport

 

 

 

  1. Which of these birds could live in the same environment as which of these bears?  Explain why these birds and bears could live together.

 

 

 

 

 

 

  1. Which bear lives in an environment that would be the harshest for the parrot to live in?  Describe the key characteristics of the bear’s environment that would make life difficult for the parrot.

 

 

 

 

 

  1. When an environment presents characteristics that are difficult for a particular life form to exist in we often use the word extreme to label the environment.  Which of the birds’ environments would be extreme for the Grizzly bear?  Explain your reasoning for each choice that you make.

 

 

 

 

 

 

 

 

  1. Which animals live in environments that would be considered extreme for human beings?  Explain your reasoning with each choice that you make.

 

 

 

 

 

 

 

 

  1. Describe a set of conditions on Earth that make up an environment so extreme as to exclude the possibility for any life form to exist.

  2. As a class compare the environments that each group created in response to question G.  Create a class list of the characteristics that most strongly influence whether or not life can exist at a particular location on Earth.  Try to describe why these characteristics create an extreme environment for living organisms.  Record your class list below.

 

 

 

 

 

 

 

 

 

 

 

 


Part II – Concept Introduction

 

Most scientists agree that the Earth is approximately 4.5 billion years old.  By the time the Earth was approximately 1 billion years old microscopic organisms had found a way to live on the volatile young Earth.  However, it would take another 3 billion years before plants and animals would appear.  We see that humans, plants and animals have been around for only a very short time in comparison to the time that microscopic organisms has existed.  During the last three billion years these tiny life forms have gone through a tremendous evolution so as to adapt to the changing conditions on Earth.  They can be found living in almost any environment imaginable.  Of great interest to scientists is the unique way that these tiny organisms live in condition in which all other forms of life fail.  By better understanding how these life forms interact with their surroundings we hope to better understand how life could exist in the extreme environments found on other planets and moons in our solar system and beyond. 

 

Activity #2  Who Lives Where?

 

            In this activity you will investigate three hypothetical environments and the bacterial life forms that could exist on Earth.  For each we have provided a table with a partial list of characteristics that describe: (1) how the different environments support life and (2) the different needs of each bacteria in order to live within a particular environment.  It will be your task to examine the characteristics that are provided for each environment and bacteria and, then based on this information, you will need to complete each table by deciding which bacteria could live in which environment. 

 

            In the table below we have listed the characteristics for each environment and bacteria.  In the column next to each characteristic are the possible range of values that you will need to consider when matching each bacteria with its environment.

 

Characteristics

Range of Values

Temperature

0oC – 100oC

Salinity

Low, Medium or High
<5%          to         25%

pH level

1 –14

Energy Sources/Uses

 Sunlight (photons) or Chemical Potential

Carbon Sources

Organic (sugars/proteins/fats)

or Inorganic (CO2 or HCO3)

Oxygen provided by the environment or used by bacteria

Yes or No

 


A.  Complete (fill in) each of the tables for these hypothetical environments and bacteria by determining which of the bacteria could Text Box: Environment X	
Temperature	2oC
Salinity	Low
pH level	
Energy Source	Light-Photons
Carbon Source	
Oxygen Provided	Yes

live in which of the environments.

Text Box: Bacteria A	
Metabolism	Fermentation
Preferred Temperature 	20oC – 30oC
Preferred Salinity 	
Preferred 
pH level	7
Energy Source used	Chemical Potential
Carbon Source used	
Oxygen needed

 

 

 

 

 

 

 

 

 

Text Box: Environment Y	
Temperature	90oC
Salinity	Low
pH level	
Energy Source	
Carbon Source	Inorganic
Oxygen Provided	No

 

 

 

Text Box: Bacteria B	
Metabolism	Respiration
Preferred Temperature 	
Preferred Salinity 	
Preferred 
pH level	7
Energy Source used	
Carbon Source used	CO2
Oxygen needed

 

 

 

 

 

Text Box: Environment Z	
Temperature	25oC
Salinity	High
pH level	
Energy Source	
Carbon Source	Organic
Oxygen Provided	No

 

 

 

 

 

 

 

 

Text Box: Bacteria C	
Metabolism	Gluconeogenesis
Preferred Temperature 	
Preferred Salinity 	
Preferred 
pH level	3
Energy Source used	Chemical Potential
Carbon Source used	CO2
Oxygen needed

 

Note: Only one bacteria will be able to live in each environment.

 

 

 

 

 

 

 

 

 

 

 

B.  State which bacteria (A, B, or C) you decided could live in which environment (X, Y, or Z.)

 

 

 

 

C.      How did you choose which environment bacteria A could live in?  How did you rule out the other environments?  What characteristics of the other environments made them too extreme for bacteria A?  What were the determining/limiting characteristics for the other bacteria and their corresponding environments?  Explain your reasoning.

 

 

 

 

 

 

 

 

D.  If the number of photons that arrive at environment Y were to decrease to nearly zero would the bacteria that you chose still be able to live in this environment?  Explain why, or why not.

 

 

 

 

 

 

 

E.   Would your answer to part D change if we were instead considering environment X or Z and the corresponding bacteria?  Explain your reasoning.

 

 

 

 

 

 

 

 

F.   Which of the bacteria use a carbon source that is organic and which of them use a source of inorganic carbon? 

 

 

 

 


To describe how bacteria interact with their environment it is useful to consider the different ways the bacteria use energy and produce or consume food.  To describe these different processes we use the following labels.

 

chemo

photo

and

autotroph

heterotroph

Uses Chemical Energy

Uses light or photon energy

 

Uses an inorganic carbon source

Uses an organic carbon source

 

By combining the label for how the bacteria uses energy (chemo or photo) with the label that describes the type of carbon source needed by the bacteria (autotroph and heterotroph) we can generate a label that describes the interaction between the bacteria and its environment.

 

G.  Label bacteria A, B, and C using the labels above.

 

      Bacteria A is a _____________ - ___________

                                   energy source                    carbon source

 

 

      Bacteria B is a _____________ - ___________

                                   energy source                    carbon source

 

 

      Bacteria C is a _____________ - ___________

                                   energy source                    carbon source

 

 

H.  Which of these bacteria live anaerobically and which live aerobically?  Explain how you know?

 

 

 

 

 

I.    Life forms that can live in extreme environments are often given special names.  For instance a “Hyperthermophile” can live at extremely high temperatures near the boiling temperature of water.  A “Psychrophile” can live at extremely cold temperatures near the freezing temperature of water, and a “Halophile” is able to live in conditions that have an extremely high concentration of salts. 

     

      Which of these hypothetical bacteria (A, B, or C) is a Hyperthermophile, a Psychrophile or a Halophile?


Activity #3  Extreme life styles.  What are the limits?

 

Read the following on-line written materials about life in extreme environments.

 

“Extremophiles” from Scientific American:

http://www.sciam.com/0497issue/0497marrs.html

This article provides a great deal of background information into the names and life styles of many extremophiles found on Earth.  Be sure to examine the link labeled Punishing Environments at the beginning of the article and the link labeled Images of Extremophiles at the end of the article.

 

“Life in Extreme Environments” from Encyclopedia Britannica:

http://www.britannica.com/bcom/eb/article/1/0,5716,109621+8+106478,00.html

Only read up to the section titled Behavior and sensory capabilities.  This is an excellent reference for details on the limits that different organisms can live in on Earth.

 

“Chapter #17: Microbial Diversity in Archaea” from Brock –Biology of Microorganisms by Madigan, Martiko and Parker:

http://cw.prenhall.com/bookbind/pubbooks/brock/chapter17/deluxe.html

This excerpt provides a brief synopsis of Chapter #17 on the ways different archaea exist in extreme environments.

                     

Answer the following questions based on your readings from these three sources.

 

A.  What are the three primary branches of the tree of life?

 

 

B.  In which branch(es) of the tree of life do we find plants and animals?

 

 

C.  In which branch(es) do we find single celled organisms that lack a nucleus?

 

 

 

D.  At how high of a temperature does life become to extreme for eukarya? 

 

 

 

E.   What is the name of the organism thought to live at the greatest temperature?  At what temperature does it live?  Where does it Live?

 

 

 

 

 

 

F.   What is thought to happen at temperatures above 150o C that prevents all life forms from existing above this temperature?

 

 

 

 

 

G.  List the different species that scientist have found living in the extremely cold Antarctic sea-ice. 

 

 

 

H.  How do organisms live without freezing in extremely cold environments?

 

 

 

 

I.    At what temperature do Polaromonas vaculota grow best?  At what temperature does life begin to become to warm for Polaromonas vaculota?

 

 

 

 

J.   How do halophiles adjust their structure to cope with life in extremely salty conditions?

 

 

 

 

K.  What range of values in pH does an acidophile prefer?  What about an Alkaliphile?

 

 

 

L.   Do acidophiles have high acidity in their cells?  Explain why or why not?

 

 

 

 

 


M. Create a list of the most extreme conditions that life has been found to exist in here on Earth.  Include information about extreme temperatures, pH, elevation limits, light levels, radiation exposure, size, and oxygen availability.  If possible try to list an example organism that lives at each extreme.


N.  What is unique about the cell walls of archaea?

 

 

O.  What would a halophile do to adapt to a change in the salinity of the solution in which it was living?

 

 

P.   What happens to the concentration of hydrogen ions (protons) as pH is lowered?

 

 

 

Q.  How would the synthesis of ATP change, if at all, if you lowered the pH of the solution in which a photosynthetic halo-bacterium was living?

 

 

R.  Which extremophiles use inorganic carbon in anaerobic respiration to produce organic carbon and the by product CH4?

 

 

S.   What is the electron acceptor utilized by all hyperthermophiles in metabolism?

 

 

T.  Is the electron acceptor from question S an energy source or a carbon source?  Is it oxidized or reduced?

 

 

 

 

Part II – Concept Application

 

Activity #4  Who Can Live Here?

 

Obtain a handout from your teacher that describes an extreme environment found in our solar system.  Read the description and answer the following questions. 

 

A.  Clearly we do not have all the information that we need to fully understand the environment described in the handout.  What question would you want to answer about this environment if you could send a single lander or orbiter to the planet or moon and perform just one test?  Explain your reasoning.

 

 

 

 

 

 

B.  Could any of the bacteria that we have studied survive on the moon or planet that you have read about?  If so, state the type of bacteria that could survive and list the energy source and carbon source that the bacteria would use.  If not, explain why none of these bacteria could survive.

 

 

 

 

 

C.  What changes in the atmosphere or surface would most strongly increase the chance for life to exist on the planet or moon that your read about?

 

 

 

 

 

 

 

 

D.   Design a hypothetical life form that could live in the environment that you read about.  Describe in detail how this life form would interact with its environment.  What would it use as an energy source or a carbon source?  Would it be an aerobic or an anaerobic life form?

 

 

 

 

 

 

 

 

 


E.   Recently many Jupiter-sized planets have been discovered orbiting other stars within the galaxy.  Some of these planets are close enough to their companion stars that the planet’s average temperature could be high enough for the presents of liquid water.  However, since these planets are gas giants it is unlikely that life would have developed on these planets?  Why then are these discoveries so important to our search for life?


 Europa (moon of Jupiter)

 

Discovered by: Galileo Galilei, 1610

Distance from the Sun: 780,000,000 km

Distance from Jupiter: 671,000 km

Radius: 1570 km        

Mass: 4.8x1022 kg

Density: 3010 kg/m3

Surface Composition: Water Ice

Major atmospheric constituent: Oxygen

 

Europa is the smallest of Jupiter's four planet-sized moons, yet it is only slightly smaller than Earth's Moon.  Europa is somewhat similar in bulk composition to the other terrestrial planets (primarily composed of silicate rock).  Recent data from Galileo indicate that Europa has a layered internal structure perhaps with a small metallic core. However, Europa's surface is not at all like anything in the inner solar system.  Its surface is exceedingly smooth with few features more than a few hundred meters high. 

 

There are very few craters on Europa; only three craters larger than 5 km in diameter have been found.  From the observations of water ice absorption bands, and due to the near absence of impact craters we have inferred that the surface is ice rich and also very young and active, perhaps only 30 million years old.  The precise age of Europa's surface is unknown.  Voyager mapped only a fraction of the surface at high resolution.  The images of Europa's surface strongly resemble images of sea ice on Earth.  Scientists have postulated that a water-ice shell covers Europa and is more than 150 kilometers thick.  It is possible that beneath Europa's surface ice there is a layer of liquid water, perhaps as much as 50 km deep, kept liquid by tidally generated heat due to the pull of Jupiter and its other moons.  If so, it would be the only place known in the solar system besides Earth where liquid water exists in significant quantities.

 

Europa's most striking surface features are the series of dark streaks or cracks that crisscross the entire globe.  The larger of these streaks or cracks are roughly 20 km across with diffuse outer edges and a central band of lighter material.  These features indicate that the surface ice sheets of Europa are tectonically active.  The latest theory for their origin is that they are produced by a series of volcanic eruptions or geysers.  It is believed that these cracks are locations of eruptive sites from which liquid water has intermittently flowed out onto the surface and then frozen and thus erased the traces of impact craters.  The Jupiter moon called Io has highly active volcanic systems that are driven by the pull of Jupiter.  Similarly, although much less intense, heating may exist in the subsurface of Europa, accounting for the resurfacing processes on the planetary surface.

 

One the most compelling insights to emerge from 15 years of research on submarine volcanic-hydrothermal systems on Earth is the idea that volcanoes in the presence of liquid water can sustain. Whether or not life can originate in these hydrothermal systems is controversial, but the evidence is unequivocal regarding the linkages between volcanic processes as we know them and abundant carbon-based life forms on and below the seafloor in the vicinity of active spreading centers. 

 

Recent observations with the Hubble Space Telescope reveal that Europa has a very thin atmosphere (1e-11 bar) composed primarily of oxygen.  Of the 61 moons in the solar system only four others (Io, Ganymede, Titan and Triton) are known to have atmospheres.  Unlike the oxygen in Earth's atmosphere, Europa's is almost certainly not of biologic origin.  It is most likely generated by sunlight and the subsequent splitting of water into hydrogen and oxygen. The hydrogen escapes leaving the oxygen.

 

 


Io (moon of Jupiter)

 

Discovered by: Galileo Galilei, 1610

Distance from the Sun: 780,000,000 km

Distance from Jupiter: 422,000 km

Radius: 1815 km (Earth’s moon: 1740 km)

Mass: 9 x 1022kg

Density: 3550 kg/m3

Major atmospheric constituent: Sulfur dioxide

Surface constituents: sulfur, silicon, sodium

 

Looking like a giant pizza covered with melted cheese and splotches of tomato and ripe olives, Io is the most volcanically active body in the solar system. Volcanic plumes rise 300 kilometers (190 miles) above the surface.

 

The energy for all this activity derives from the gravitational forces between Io, Europa, Ganymede and Jupiter.  These gravitational forces cause Io's surface to bulge up and down (or in and out) by as much as 100 meters (330 feet)!  This tidal pumping generates a tremendous amount of heat within Io, keeping much of its subsurface crust in liquid form.  Thus, the surface of Io is constantly renewing itself, filling in any impact craters with molten lava lakes and spreading smooth new floodplains of liquid rock.

 

In contrast to most of the moons in the outer solar system, Io is thought to be somewhat similar in bulk composition to the terrestrial planets, primarily composed of molten silicate rock.  Recent data from Galileo indicates that Io has a core of iron (perhaps mixed with iron sulfide) with a radius of at least 900 km. 

 

The material erupting from Io's vents appears to be some form of sulfur or sulfur dioxide.  The volcanic eruptions change rapidly.  In just four months between the arrivals of Voyager 1 and Voyager 2 some of them stopped and others started up.  The deposits surrounding the vents also changed visibly.

 

Io has an amazing variety of terrains: calderas up to several kilometers deep, lakes of molten sulfur, mountains which are apparently NOT volcanoes extensive flows hundreds of kilometers long of some low viscosity fluid (perhaps some form of sulfur), and volcanic vents.  Sulfur and its compounds take on a wide range of colors which are responsible for Io's diverse appearance.

 

Analysis of the Voyager images led scientists to believe that the lava flows on Io's surface were composed mostly of various compounds of molten sulfur.  However, subsequent ground-based infrared studies indicate that they are too hot for liquid sulfur.  One current idea is that Io's lavas are molten silicate rock.  Recent HST observations indicate that the material may be rich in sodium.  Or there may be a variety of different materials in different locations.

 

Some of the hottest spots on Io may reach temperatures as high as 2000 K (1723 oC) though the average is much lower, about 130 K (-143 oC).  The hot spots are the principal mechanism by which Io loses its heat.

 

Unlike the other Galilean satellites, Io has little or no water.  This is probably because Jupiter was hot enough early in the evolution of the solar system to drive off the volatile elements in the vicinity of Io.  Sulfur dioxide is the primary constituent of a thin atmosphere on Io.


Mars

 

Distance from the Sun: 227,900,000 km

Radius: 3,397 km

Mass: 6.42 x 1026 kg

Density: 3940 kg/m3

Mean Temperature at Solid Surface: 186 to 268 K (-86oC to -5 oC)

Major Atmospheric Constituents: carbon dioxide, nitrogen, and argon

 

Mars has inspired our imagination over the centuries, and has been the focus of intense scientific interest for many years.  Mars has shown itself to be the most Earth-like of all the planets; as it has polar ice caps that grow and recede with the change of seasons, and markings that appear to be similar to water channels on Earth.

 

Mars is a small rocky planet that has experienced volcanic eruptions, numerous impact events, and displays many atmospheric changes.  Areas of layered soils near the Martian poles suggest that the planet's climate has changed more than once, perhaps caused by a regular change in the planet's orbit.  Martian tectonics-the formation and change of a planet's crust--differs from Earth's.  Where Earth tectonics involve sliding plates that grind against each other or spread apart in the seafloors, Martian tectonics seem to be vertical, with hot lava pushing upwards through the crust to the surface. Periodically, great dust storms occur that engulf the entire planet. The effects of the storms are dramatic, including dunes, wind streaks, and wind-carved features.

 

One of the early mysteries pondered by scientists was why Mars does not have oceans like Earth.  Mars has an atmosphere that is now too thin and its temperature too cold to allow liquid water.  Mars certainly had surface water and groundwater once.  This liquid water shaped the valley networks in the highlands and the huge flood channels that cut from the highlands to the northern lowlands.  Scientists are not certain of exactly how much water was present.  Estimates range from the equivalent of an ocean 10 meters deep covering the entire surface to the equivalent of a layer kilometers deep. However much water there was, it is not now on the surface, except for a bit in the polar ice caps.  One question that has been raised is where did the water go?  It could be underground in pools of groundwater, either small or huge depending on how much water Mars started with. Or it could have escaped to space and been lost completely (the hydrogen from water can escape easily through Mars's low gravity and small magnetic field).

 

We don't know if there is or was life on Mars.  There are currently no clear signs of any life on the inhospitable surface of Mars.  We do know however that the climate of Mars was once quite different than today.  We could image the past where Mars had a thicker atmosphere, flowing water, volcanoes, lava flows, open lakes, and perhaps even an ocean.  These conditions could have supported live similar to that which develops in hot springs here on Earth.

 

In a recent study Scientists found a huge deposit of the mineral hematite.  This discovery has led to speculation that there was water on Mars long enough for life to form.  At a recent meeting of the American Geophysical Union, Arizona State University Prof. Phil Christensen suggested that the hematite deposit "is really the first evidence we have that hot water was around long enough for a geological period of time so that potentially life could have had an opportunity to form."  Hematite is an iron oxide mineral that forms by a variety of ways that often involve water. The coarse-grained hematite spotted on Mars also occurs on Earth around volcanic regions such as Yellowstone National Park. It is evidence that a large-scale hydrothermal system may have operated beneath the Martian surface, said the scientists working on the Mars Global Surveyor Mission.  "If you want to find out about possible life on Mars, the deposit is a good place to start," Christensen said. "You've got water, you've got heat, and you've got energy.


Titan (moon of Saturn)

 

Discovered by: Christiaan Huygens, 1655

Distance from Saturn: 1,220,000 km

Radius: 2,580 km

Mass: 1.35 x 1023kg

Mean density: 1880 kg/m3

Mean temperature at solid surface: 94 K (-178°C)

Atmospheric pressure: 1.5 bars

Major atmospheric constituents: nitrogen, methane

 

Saturn's moon Titan was long thought to be the largest satellite in the solar system, however, recent observations have shown that Titan's has a very thick, opaque atmosphere which hides its solid surface.  Due to this extensive atmosphere the surface of Titan cannot be seen at all with visible light however some surface details are visible in the infrared.

 

Titan's atmosphere has a surface pressure that is more than 1.5 bar (50% higher than Earth's). It is composed primarily of molecular nitrogen (as is Earth's) with no more than 6% argon and a few percent methane. Interestingly, there are also trace amounts of at least a dozen other organic compounds (i.e. ethane, hydrogen cyanide, carbon dioxide) and water. The organics are formed as methane, which dominates Titan's upper atmosphere, and is destroyed by sunlight.

 

There are probably two layers of clouds at about 200 and 300 km above the surface. Complex chemicals in small quantities are responsible for the orange color as seen from space.

 

Observations have revealed that Titan is about half water ice and half rocky material. It is probably differentiated into several layers with a 3400 km rocky center surrounded by several layers composed of different crystal forms of ice.  There is some speculation that its interior may still be hot.  At the surface, Titan's temperature is about 94 K (-178°C).  At this temperature water ice does not sublimate and thus there is little water vapor in the atmosphere.

 

It seems likely that the ethane clouds would produce a rain of liquid ethane onto the surface perhaps producing an "ocean" of ethane (or an ethane/methane mixture) up to 1000 meters deep. Recent ground-based radar observations have cast this into doubt, however.

 

Recent observations with the HST show remarkable near infrared views of Titan's surface. Voyager's camera couldn't see through Titan's atmosphere but in the near infrared the haze becomes more transparent, and HST's pictures suggest that a huge bright "continent" exists on the hemisphere of Titan that faces forward in its orbit. These Hubble results don't prove that liquid "seas" exist, however; only that Titan has large bright and dark regions on its surface. The landing site for the Huygens probe has been chosen in part by examining these images. It will be just "offshore" of the largest "continent" at 18.1 degrees North, 208.7 degrees longitude.

 


 

Triton (moon of the planet Neptune)

 

Distance from Neptune: 354,760 km

Radius: 1352 km

Mass: 2.140x1022 kg

Density: 2066 kg/m3

Mean temperature at solid surface: 38 K (- 235 oC)

 

Triton is the largest moon of Neptune, with a diameter of 2,700 kilometers (1,680 miles).   It was discovered by William Lassell, a British astronomer, in 1846 scarcely a month after Neptune was discovered.   Triton is colder than any other measured object in the Solar System with a surface temperature of -235° C.   It has an extremely thin atmosphere.   The atmospheric pressure at Triton's surface is about 14 microbars, 1/70,000th the surface pressure on Earth.   It is speculated that nitrogen ice particles might form thin clouds a few kilometers above the surface.  

 

Triton is the only large satellite in the solar system to circle a planet in a retrograde direction -- in a direction opposite to the rotation of the planet.  Triton contains more rock in its interior than the icy satellites of Saturn and Uranus.   Its relatively high density and retrograde orbit have led some scientists to suggest that Triton may have been captured by Neptune as it traveled through space several billion years ago.  If that is the case, tidal heating could have melted Triton in its originally eccentric orbit, and the satellite might even have been liquid for as long as one billion years after its capture by Neptune.   Triton is scarred by enormous cracks.  Voyager 2 images showed active geyser-like eruptions spewing nitrogen gas and dark dust particles several kilometers into the atmosphere. 

 

Triton is one of only three objects in the Solar System known to have a nitrogen-dominated atmosphere (the others are Earth and Saturn's giant moon, Titan).   Triton has the coldest surface known anywhere in the Solar System (38 K); it is so cold that most of Triton's nitrogen is condensed as frost, making it the only satellite in the Solar System known to have a surface made mainly of nitrogen ice.   The pinkish deposits that cover a vast portion of the south polar cap is believed to contain methane ice, which would have reacted under sunlight to form pink or red compounds.  The dark streaks overlying this pink ice are believed to be an icy and perhaps carbonaceous dust deposited from huge geyser-like plumes, some of which were found to be active during the Voyager 2 flyby.  There is a bluish-green band that extends all the way around Triton near the equator; and may consist of relatively fresh nitrogen frost deposits.