Life in Solar System

Chapter 7


METHODS OF AND CHALLENGES IN EXPLORATION


In our solar system, we can in principal travel to the objects and study them from up close. This can be done two different ways:


1. Human travel: we have only been in low-Earth orbit and to the Moon. Next likely targets: back to the Moon (?) or an asteroid, and eventually to Mars.

Plusses: wide appeal, it is the way our forefathers explored Earth, ability for on-the-spot decision making, 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! Cosmic rays present in space make travel challenging too.


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

- flyby's, placed into orbit, or landers
- sample returns (most challenging!). So far we have done this successfully from the Moon and a comet(s)


To illustrate the complexity of some of the paths that robotic satellites have followed to get to the planets I show an example below in the link. The strange trajectories, using "gravity assists" from other planets, are to save fuel (hence weight).

Voyager and Pioneer space craft trajectories. - these were "straight" shots towards the destinations. Note the trajectories are NOT straight due to the always present gravity of the Sun and planets.
Cassini space craft trajectory to Saturn - this was a complicated path, first going inwards before moving out. This saves fuel by using a "gravity assist", also called "sling shot".

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


TELESCOPES


For studies outside the solar system, we have to rely so far on telescopic observations. Of course, telescopes are also used to study objects in the solar system.


Telescopes: why do we need them?


Clicker questions. What do you consider to be the most important role of telescopes and their detectors?

a. Magnify images so we can see more detail.

b. Gather more light each second so we can see fainter objects

c. Enable long integrations so we can detect faint objects.

d. Orbit the Earth to allow observations at all wavelengths.



Very Large Array
Gemini Observatory: an example of a modern 8-m optical telescope
Hubble Space Telescope

What telescopes can do: observe at high spatial and spectral resolution and sensitivity, observe 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)


A 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 special software packages that can do image analysis and image 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.


Radio telescopes can also be used to send signals to other stars and planets, but so far we don't know if anyone might be listening...

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Now that we have the tools, let's start exploring. What are we looking for when it comes to finding life?


"Environment requirements" for life: what are suitable conditions? We can list at least 5 requirements, number a through e below:

a. Presence of chemical elements for life (especially oxygen, carbon, hydrogen, nitrogen, phosphorus). Not an issue, since these chemical elements are found all around the Milky Way in stars and interstellar medium, so likely too in other planets (although perhaps not in "usable" form, e.g. local physical conditions may not enable certain molecular forms to be present).

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 from cosmic rays coming from outer space. Water can also shield elements inside it from destruction by UV light or cosmic rays.


c. Energy source to fuel metabolism.

Sun light on Earth is the principal energy source for life.

How does light energy decrease with distance from star? We will come back to this point when we discuss habitable zones around stars. We can't be too close to a star (too hot) or too far away (too cold) for life to thrive.


Clicker question. Jupiter is 5 times further away from the Sun than Earth. The amount of Sun light that Jupiter receives per unit area is hence:

a. 5 times less than Earth

b. 25 times less than Earth

c. 125 times less than Earth

d. Less than on Earth but the temperature is still good since Jupiter is much bigger than Earth and therefore intercepts more Sun light in total.


Other sources of heat that could help sustain life in certain environments:


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



Clicker questions.

How often does the high tide on ocean beaches occur on Earth?

a. Once a day.

b. Twice a day.

c. A little less than twice a day.

d. Once a week.


What is the principal cause of tides on Earth?

a. Gravity from the Moon.

b. Gravity from the Sun.

c. Gravity from the other planets in the Solar System.

d. All of the above.


Examples of effects of tidal forces in action:


d. Do we need a planet with a solid surface to develop and sustain life? Well, perhaps a planet that only has water and ice at the surface might be fine too. But certainly human technological development has greatly benefited from living on land. It would be much more difficult to imagine an advanced civilization living completely under water.

e. Water, preferably in liquid form. Note that ice floats on top of water. Water expands as a solid, very unusual but likely critical as we discussed previously.

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.

SOLAR SYSTEM OVERVIEW

Time for a more detailed look at the planets in our search for life in the solar system!


A few clicker questions to get us started.


What is the planet that looks most like our Moon? a. Mars b. Venus c. Mercury d. Earth


Which of the these planets has the least amount of atmosphere?


Which of these planets has the highest average temperatures?



The relative sizes of the planets and Sun
Mercury, from the Messenger Mission
Venus properties, visual light and other images

Surface of Venus unveiled
Venus evidence of ancient lava flows
Earth from space
Mars overview
What the Pluto fuss about? Earth and the small worlds
Jupiter gallery

Saturn gallery
Uranus gallery
Neptune gallery


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 (History of the "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.


Clicker question. What makes Mars the most likely planet other than Earth to host life?

a. It has the same size as Earth.

b. It has a similar atmosphere to Earth.

c. It has similar temperature to Earth

d. It has polar caps and water ice.

e. It has seasons like Earth.



Pay attention to the following issues:

a. Mars has seasons like Earth. Make sure you understand the origin of the seasons. It is not principally 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. Mars does have a more eccentric orbit than Earth, so the summer on the Martian southern half is short and hot, while the summer on the Northern hemisphere is longer and not as hot (why?)

b. Mars has strong evidence for liquid 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 in substantial amounts. Why not? What happens with liquid water at the surface? It looks like Mars has been very different in the past. Recent evidence does point to some evidence even at the present time for occasional "damp flows" on Mars, not exactly a river but a temporary salty flow. Read about the evidence here.

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 is one big and one still active small rover on Mars today and future missions will include more rovers, possibly a sample return mission and talk of manned missions (which would seem to be at least 20 years away).

Image gallery of Mars from earlier missions:

Mars photo gallery

Mars pathfinder images

Phoenix lander mission to Mars


New rover: Mars Curiosity Mission

Evidence for past surface water on Mars 2.

Caves on Mars



All missions to Mars, planned, current and future: Click here.


And some videos about the Mars Rovers and other missions:

NASA Solar System exploration site.



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.



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:



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 measure where the ice is bulging in and out (suggesting it is thin) by Jupiter's tidal forces. Or, as has been discovered, Europa may have water plumes shooting out from the planet so that could be a lot easier to probe.


Enceladus


Recently it has been discovered that the 6th moon of Saturn, Enceladus, click here, also has liquid water, and the water plumes seem more prominent and might be more easily accessible than on Europa: Water plumes on Enceladus


DISCUSSION questions.


1. If we know there is liquid water on (or rather in...) Europa and Enceladus why would NASA spend more money exploring Mars?


2. What would you consider the odds for life in the solar system outside Earth, after learning about the planets and moons?

a. <5% b. 25% c. 50/50 d. pretty sure it should exist