Section 2 of the course: Life On Earth and in the Solar System

Relevant text book chapters: 5 - 9

Chapter 5 and 6.

This chapters deals with the nature of life as we have made discoveries about it on earth. Naturally, a large section is devoted to Darwin's theory of biological evolution. There is also an important section in the book on the difference between creationism (and why it is not science) versus the theory of biological evolution (Sectrion 5.6). Read it!

The chapter starts off with a description of how to define "life". The key characteristics are:

order
reproduction
growth and development
energy utilization ("metabolism")
response to the environment
evolutionary adaptation

EVOLUTION

The key individual in the subject of biological evolution is of course Charles Darwin (1809-1892), who published what is perhaps the most famous book in the sciences every written, "Origin of the Species" in 1859. This was based on his voyage on the ship the Beagle, which visited mostly Latin America, and the observations Darwin made there of how species had adapted to the environment.

Key aspect: Central concept in Darwin's theory of evolution is

NATURAL SELECTION

What Darwin observed was that species adapt to their environment by gradually changing their characteristics, to enhance their chances of survival. This process takes places spontaneously, not by some particular controlled process, other than natural selection itself. (Contrast this with modern "selective breeding techniques" which make use of Darwin's observations that species can change in particular ways. In selective breeding, we help nature a hand by focusing on the traits that we want to pass on, in nature, natural selection preferentially picks the traits that enhance survival chances for the species).

So, the species that are best adpted to their environment survive and may even thrive, while those that don't, disappear. Not only do species adapt to the point where they survive (e.g. black crabs on the Hawaiian lava) but the biological changes can lead to entirely different species in the long run.

Natural selection does not always lead to improved  ("better") or "more advanced" species, although this is a natural consequence of evolution. Very low life forms, like the simplest bacteria, can also survive and thrive in the right conditions.

When a species goes extinct, it is a reflection of the inability of that species to adapt quickly enough to changing circumstances, or maybe even a recognition that over time the species has changed into a different one, which is still genetically closely tied to its origin, but biologically classified as a different species.
Three principles of natural selection:

1. There is heritable genetic variation
2. Parents over-proliferate
3. Successful offspring are the ones best adapted to the environment

Natural selection does not lead to a particular goal, it is not always leading to more advanced species, it cannot anticipate anything.

The issue of biological evolution is still hotly debated in some circles in the US, though it is not controversial in the scientific world and not an issue of controversy in most industrialized countries. The following objections have been raised, but are all shown to be wrong:

Five Major Misconceptions About Evolution

The rest of Chapter 5 discusses cells as the basic units of life, classifications of life on earth, metabolism (the chemistry of life), before getting to DNA and extreme life forms.

For this course, sections 5.1, 5.4, and 5.5 are most important, but do read the other two sections too. Metabolism is important in connection with what is required to create and sustain life, since it specifies internal and external conditions that need to exist for certain life forms to thrive. Water is one of the key ingredients. One can speculate about life forms in environments with no water, but it is clear that water is and has played a crucial role on earth for life to florish.

The discovery of the structure and behavior of DNA in cell divisions has provided key support to Darwin's theory of biological evolution: how DNA is replicated in cell division and how it changes when species of opposite sex create new life provides a key mechanism for how gradual changes in the living creatures can occur. It is known that tiny changes in DNA can produce very important changes in the characteristics of living beings. If the changes persist and continue to evolve in particular directions under the guiding principle of natural selection, in successive generations the species can develop new properties and eventually evolve into other species. Make sure you know what DNA is, and what the connection is between DNA and chromosomes, and genes.

Extremophiles are extreme life forms formed on earth that may provide us with clues as to under which conditions life can still exist on other planets. This is a section that discusses work at the forefront of "astrobiology", the new study of astronomy and biology, which tries to apply knowledge of life on earth to conditions elsewhere to see where life might originate and what its properties might be.

Here are a few interesting links.
Bit of information on extremophiles on Earth
The NASA astrobiology institute's web pages


If we combine Darwin's observations with our knowledge of the early earth and the old age of the earth, we arrive at a central question:

HOW DID LIFE ORIGINATE ON EARTH?

It is obvious, that if the Earth and Sun formed from a contracting gas cloud which heated up as it contracted, that the early Earth was very inhospitable to life: life MUST have formed later, or have been "brought there", perhaps in primitive form from organic molecules on comets. Also, the early atmosphere was certainly not an oxygen rich atmosphere.

The early development of life is poorly understood. We don't know how the first "living things" appeared; the formation from simple molecules into amino acids is not that difficult, apparently, given that we do find evidence for amino acids outside earth on meteorites and comets, but the transition from amino acids to the much more complex DNA, RNA, let alone cellular life, is not understood and has not been done in a laboratory.

We know roughly when (within the first billion years of the Earth's history), but it took until the last billion years (out of 4.5) for life to take on more complex forms that left most of the fossil record. A big challenge is that old rocks on Earth to look for evidence are very rare, due to the constantly changing surface or Earth in the plate-tectonic process.

There is suggestive evidence that rocks called stromatolites were actually deposited by microbes , that may have produced early photo-synthesis 3.5 billion years ago.  Early fossilized cells may have existed shortly after that.

Some important results:

- Advanced life, as defined by multi-cellular organisms and far more complex plant and animal life only developed in the last billion years of the Earth's history. At least, we have evidence in the fossil record that it existed this long ago; creatures that have no skeletons or other dense supporting structures do not leave much of a fossil record.

- While development of early life is a difficult subject to find conclusive evidence for, the fossil record of the last 750 million years is very strong and provides conclusive evidence for biological evolution. The real "missing link" may not be the ancestor of humans from apes, but the earliest life forms on Earth. We should keep in mind that this is not necessarily a weakness of the theory of how life may have started, but as much a problem that the evidence has been erased due to the active geological and atmospheric processes (weather!) on Earth.

- Even today life can survive in extreme conditions on Earth, from the most frigid environments in Antarctica to hot vents deep inside the Earth's crust where no oxygen is present. This provides hope that life may exist elsewhere in extreme environments, even if only in primitive form. These life forms are called "extremophiles" (Book section 5.5)

- We are very biased in our view of life on Earth. We mostly think of plants and animals, but the actual "tree of life" is broken up into three broad classes (Bacteria, Archaea, Eukarya) and all animal and plant life only forms two branches of the Eukarya category. As the book states: "The true diversity of life on Earth is found almost entirely within the microscopic realm. Biochemically and genetically, we humans (and all other animals) are much more closely related to mushrooms than most microbes are related to one another." (section 5.2). Also, microbes on earth contain about 5000 times more mass than all humans combined.


Experiments in the 20th century (e.g. Miller and Urey) tried to mimic conditions on early earth and create primitive life forms, but this has not (yet) succeeded. The real "missing link" in our understanding of life and evolution is the creation of that first life form. However, this does not mean it is impossible that it happened. It just means that we don't understand it yet. Some day we may. An equally challenging problem is to be able to understand how "easy" or "frequent" this spontaneous formation of life may happen. This is a critical question to which we have no answer now and it determines very much how prevalent life elsewhere in the universe might be.

Modern evidence in strong support of biological evolution is provided by our understanding at the molecular level of the basic building blocks of life, the role of proteins, amino acids, the structure of DNA, the reproduction of DNA in cell division and forming of new life. This shows that indeed all life forms on Earth stem from a "Common Ancestor", that we have a very high overlap in genetic material with other life forms, in particular the ones people are believed to have originated from, and that evolution does indeed occur as a result of RANDOM MUTATIONS of the genetic material guided by Natural Selection which winds up bringing great order to the randomness of the mutations.

Evolution is a consequence of random mutation which, when coupled with natural selection, leads to the creation of new species and determines which species thrive and which become extinct.

Note that while the process of mutations, now understood to be responsible for evolution of species may be random, the influence of "natural selection" imposes a strong constraint on this random process so that the existence of complicated species in itself is not an argument against evolution. Today we still have very primitive life forms, not only advanced ones. Evolution reflects survival of species that are well adapted to their current environment, be it advanced or primitive ones.

The understanding of the basic building blocks of life have helped shape our understanding of biological evolution: all life forms are based on the same set of common building blocks (amino acids), and the entire "model plan" for a species is passed on through the DNA to its offspring.

Set of terms you should familiarize yourself with:

Biological evolution, natural selection, mutations, DNA, amino acid, double helix, gene, genome, chromosome, mutation.

When looking at the evolution of life, get an idea for the basic time scales:

A good way to realize the incredibly fast pace of evolution over the last billion years, and the short time people have been around is through the cosmic calendar by Carl Sagan





You may be interested in: Artifical Intelligence and Artificial Life. See section 6.6 in book.


Chapter 7

Searching for Life in Solar System

Some important issues:

Environment requirements for life: what are suitable conditions?

a. Chemical elements for life (especially oxygen, carbon, hydrogen, nitrogen). Not so big an issue, since chemical elements are found everywhere (although perhaps not in usable form).

b. To have the elements available as possible suitable complex molecules from which to build life, they need to be shielded somehow, e.g. the Earth's atmosphere is quite important to shield molecules from UV light of Sun and cosmic rays from space. Likewise, water in liquid form would be very useful...

c. Energy source to fuel metabolism.

Role of Sun light on Earth, how does light energy decrease with distance from star? We will come back to this point when we discuss habitable zones around stars.

Other sources of heat:


Explanation of tidal forces: A tidal force is the difference in the force of gravity across an object. E.g. one side of Io is closer to Jupiter than the other side. This causes Io to be squeezed by gravity. The tidal force is much stronger the closer you are to a massive object. Tidal Forces

Examples of tidal forces:

Ocean tides on Earth (question: how many high and low tides per 24 hr period, and why?)
Moon quakes
tidal locking of moon to a planet: 

- why does the Moon always face us?
- length of day on Earth



d. Do we need a planet with a solid surface to develop and sustain life? Probably so.

e. Unique property of water: ice floats on top of water. Water expands as a solid, very unusual but likely critical!

Other critical properties of water: it is abundant and a very good solvent for chemical elements which allows for transport and mixing of elements.
Also, it has charge separation properties which protect cells from dissolving in water.

METHODS AND CHALLENGES IN EXPLORATION

Human travel: we have only been in low-Earth orbit and to the Moon.
Next likely targets: back to the Moon and perhaps to Mars.

Plusses: wide appeal, it is the way our forefathers did it, on-the-spot decision capability, ability to service and fix things (within limits), human intelligence to recognize and investigate environments.

Disadvantages: Cost and Risk.


Challenge to explore Mars: hundreds times further away than Moon, so supplies and energy are critical, larger gravity than Moon: harder to come back!


Robotic space craft: less risk, considerably cheaper, ever more "clever". Other stars still too far away. Various options:

flyby's, placed into orbit, landers
sample returns (most challenging!). Succeeded from Moon and comet

To illustrate the complexity of some of the paths that robotic satellites have followed to get to the planets, here are some examples:

Voyager II path to the 4 gas giants
Galileo space craft trajectory to Jupiter
Cassini space craft trajectory to Saturn

Note: for robotic missions such long trajectories with "gravity assists" are not a problem, but what if you wanted to send people there?

Telescopes: what they can and cannot do. Some examples of telescopes:

Very Large Array
Apache Point Observatory
Keck Telescopes
Hubble Space Telescope

What they can do: observe at high resolution and sensitivity, at wavelengths the eyes can and cannot see. Telescopes need to be large to collect as much light as possible. The Earth's atmosphere is a big problem for many wavelengths because it:

distorts the image quality ("seeing")
absorbs light at many wavelengths (good for us in many cases, bad for astronomy)

The principal role of telescopes is to collect more light so we can study fainter objects than we can see with our eyes. Think of how much water you collect if you put a bucket outside or a kids pool. The pool has much larger surface area and collects more water. Therefore, a larger telescope will collect more light and we can see fainter objects (so, in general, objects that are further away). The faintness of objects we can see depends on the surface area of the primary mirror or telescope dish, coupled with its quality (how smooth is it; for optical light we need very smooth mirrors, for radio light we can do with rougher surfaces since the wavelengths of the light are longer).

Second, telescopes have detectors that provide a lasting database of the objects we observe: we take digital pictures, spectra, etc. All data are analyzed using computers, with packages referred to image analysis and processing.

Third telescopes help us take the sharpest images possible so we can see more detail of the objects we study. (Most stars are still to far away to be able to resolve them well though).  The image resolution of a telescope also depends on its size; we can mimic a very large telescope by putting smaller telescopes far away from each other and combine the light beams they receive from objects in the sky. This is called Interferometry. In radio astronomy, the largest telescope systems combined this way span the globe, with antennas in Europe, US, Canada, Hawaii etc. So, we literally create a telescope the size of Earth and can see very small details. In the optical, interferometry is only just starting to become possible, due to the very high technological challenges.

SOLAR SYSTEM OVERVIEW

It is about time we look at the planets in our search for life in the solar system!
The relative sizes of the planets and Sun
Mercury
Venus in visible, Surface of Venus unveiled,
Venus lava flows
Earth from space
Mars overview
What the Pluto fuss about? Earth and the small worlds
Jupiter gallery
Uranus gallery
Neptune

Chapter 8. Searching for life on Mars

This is an excellent chapter in the book, starting with a great historical discussion on early "evidence" for civilizations on Mars, from Herschel's speculations to the "canali" by Schiapelli and Lowell's fiasco of the canal network(Lowell's canals on Mars). All this was enough to put fear in the hearts of citizens, as brilliantly exploited by a 1938 radio show by Orson Welles, based on the 1898 novel "War of the Worlds" by H.G. Wells.

Another major fluke in the perception of Mars was the face on Mars, which subsequently was proven to be this, as scientists had of course always stated.


Pay attention to the following issues:

a. Mars has seasons like Earth. Make sure you understand the origin of the seasons. It is not related to the distance between Mars and the Sun, but  like for Earth is caused by the tilt of Mars' rotation axis compared to the plane in which it orbits the Sun.

b. Mars has strong evidence for water on the surface in the past. See many features that look very much like water gulleys and streams. Present conditions on Mars do not allow liquid water to be present at the surface
(why not? What happens with liquid water at the surface?). It looks like Mars has been very different in the past.

c. The Martian atmosphere is very much less dense than Earth's atmosphere and very different in chemical composition (being mostly CO2).

d. Gravity on Mars is only 38% of what it is on Earth (a quick way to lose weight!).

Why is that so?

What consequences might this have for astronauts visiting Mars?

e. Mars has polar caps. What are they made of?

f. Mars has extensive dust storms that can cover most of the planet.
What consequences would this have for astronauts visiting Mars?

g. Mars has had two major experiments to search for life: in the 70's the Viking missions did experiments on the surface to look for life. By today's standards, and knowing more about extremophiles, those experiments are very primitive and could be improved significantly. Second attention came with the suspicion of evidence for life in a Mars meteorite found on Earth; this remains controversial and is not generally accepted as evidence for past life.

h. Mars is the primary object of study now in NASA's missions to the solar system planets with continuing missions for the past and next decade. There are two small rovers active on Mars today and future missions include more rovers, possibly a sample return mission and talk of manned missions (which would seem to be at least 25 years away).

Image gallery of Mars from earlier missions:

Mars photo gallery

Mars pathfinder images

Phoenix lander mission to Mars

Evidence for past surface water on Mars 2.

Valles Marineris, Mars Express mission



Chapter 9. Prospects for life on the largest Jovian Moons

Note that we do not only include the 4 largest of Jupiter's moons here (Io, Europa, Ganymede, Callisto), but also Titan (Saturn's largest moon) and Triton (Neptune largest moon). All of these moons are larger than Pluto, some are larger than Mercury.

These Jovian moons are orbiting their planets in one plane, in the same direction, much like a miniature solar system. The one exception is Triton, which orbits Neptune in opposite direction as expected; Triton is probably a captured moon! And it may be a lot like Pluto in its properties. The large moons were formed from pieces of ices and rocks circling the planet as it was forming.  The moons may well contain significant amounts of water (mostly in form of ice).

The moons rotate synchronously with the planet, due to tidal forces. We have mentioned this before and discussed how this happens in class.

Questions:

Why do we consider the moons of the large planets as possible places where life might occur?

Several aspects are relevant:


We have already discussed the consequences of the tidal forces on Io, the most volcanically active place in the solar system. Clearly, conditions for life are not good there; the next moon out from Jupiter offers more promise. Coincidentally, it is the one called Europa.

Europa

Characteristics:


Explanations and pictures of Jupiter's large moons

Summary of evidence for liquid ocean under water ice surface on Europa:


Summarizing case for life on Europa:

Future exploration of Europa:

Ice crust is expected to be 5-25 km thick.
Can we drill through this ice to sample the water underneath? Clearly not easy, we need to find spots where the ice is thin. This can be done using an altimeter to meausure where the ice is bulging in and out (suggesting it is thin) by Jupiter's tidal forces.