The gas giants
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Are we alone?Jupiter's moons Could there be life in the outer solar system?Previously, we looked at the possibility that life could exist on the inner, rocky planets (other than the Earth, of course). But what about the planets out beyond Mars? It is extremely unlikely that life will be found on the gas giant planets. They have no surface and their gravity is enormous. The sheer pressure of the atmosphere would squash any life forms! However, some of the moons of the gas giants may offer greater potential for life.
All the gas giants have many moons - for example, Jupiter has 28 moons; Saturn has 31. Many of these are just tiny like asteroids, but Jupiter has four big moons, which you should be able to see if you are ever able to look at Jupiter through binoculars or a telescope. Unlike the gas giants they orbit around, these moons all have rocky surfaces. Jupiter's four large moons are made out of icy rock except for its closest moon, Io. The picture on the left shows Io (on the left) and Europa on the right.
These big moons really do resemble the inner rocky planets. The image on the right shows the relative sizes of the largest moons and the smallest planets in the solar system. The two largest moons in our solar system - Jupiter's moons
Io, the moon in closest orbit around Jupiter, looks a bit like a pizza! It is one of the most fantastic (and most geologically active) planets in the solar system. It has hundreds of very active volcanoes, and the black and bright red materials show the most recent volcanic deposits, probably no more than a few years old. The reason why Io is so geologically active and hot is because it is the closest moon to Jupiter. This means it is subject to an incredible gravitational tug of war, pulled in different directions by Jupiter and its other (major) moons. Over the period of a single Io day, it is alternately extended and compressed, with its surface moving in and out by around 100m and all this continual pulling and deformation heats up the planet.
The picture on the left shows the surface of Io from a spaceprobe fly-past, and is about 250 kilometers (about 155 miles) across. North is toward the top and illumination from the Sun is from the west (left).You can see flows of molten lava which are up to 2000km long. So Io is a warm planet … but it is geologically unstable and we have not found any evidence for water on its surface, making the possibility of life less likely.
The pictures on the right show two more of Jupiter's moons - Callisto and then Ganymede to the far right. They look much like our own Moon, with classic rocky body surfaces, covered with craters of various sizes and ice. There is no evidence that either has an atmosphere or liquid water.
But the second moon out, Europa, is very interesting. It is about the same size as our Moon, but it is one of the few rocky bodies without craters. Instead it has long straight features on its surface. The Hubble space telescope has shown Europa has an extremely tenuous atmosphere of molecular oxygen, and is the only solar system object apart from Mars,Venus and the Earth, which is known to have traces of molecular oxygen in its atmosphere. When the Galileo spaceprobe flew past Europa, it analysed the light reflected from the surface, showing that the surface of this moon is composed of water. It looks very bright and smooth because it appears to have a layer of ice surrounding it, making the bands and ridges that criss-cross the surface, some curved, some straight, extending for thousands of kilometres. The cracked surface that you can see when you look at a close-up (top right image) shows very similar features to sea ice in the Arctic ocean on Earth (bottom right image). Such features occur when ice plates move on a liquid surface and molten ice (water from the ocean) escapes periodically through the fissures/cracks. This wipes the surface ‘clean’, erasing any craters - hence the lack of craters on this moon. The water then re-freezes rapidly when it comes into contact with the freezing temperatures on the outside - Europa's surface is at -162C, a temperature which could freeze an ocean over a period of several million years. So it could be that Europa had a liquid ocean underneath the frozen crust at some point in its history, but it is not clear if this ocean still exists. The fact that it is so free of impact craters indicates it has been resurfaced within the past 1 million years, and such recent activity makes it more likely that there is still liquid water underneath this crust. Scientists think the outside water layer froze solid early in Europa's history, but there are smooth, shallow craters, whose heights suggest this crust may only be 6-10 miles thick. It may be that geologically Europa is similar to Io but with less violent effects, since it is the second nearest moon to Jupiter. There could be internal warming underneath the icy surface caused by the tidal tug of war with Jupiter and its neighbouring moons. The heat generated by the expansion and contraction of the planet could be enough to melt the icy crust at depth, creating lakes or oceans below the surface. So it may be that Europa has several key ingredients favouring the evolution of life under this icy crust:
NASA is considering a possible mission to Europa that would put a lander on the surface to measure the thickness of the surface ice and to detect any underlying liquid ocean, if it exists, by bouncing radar signals through it. It could even be that a mission could be devised to drill through the layer of ice and release a probe in the liquid underneath or to map in detail the global shape of the moon, determining how much it is distorted by local tidal forces.
No one can live on the surface of Jupiter, but it is possible that we could build colonies on some of these large, rocky moons. What do you think it would feel like to live on one of them? In the daytime, the Sun would be so far away, it would appear as no more than a bright star. At night, there wouldn’t just be one small moon, but lots of moons with Jupiter itself taking up a lot of the sky. The picture on the right shows an artist's impression of what it might be like. Saturn's moons
Saturn has even more moons than Jupiter, which are particularly topical right now (January 2005). The last time a spaceprobe visited Saturn was over 20 years ago when Voyager 1 and 2 flew past. The latest mission to visit Saturn started to orbit Saturn during the summer of 2004, having travelled across the solar system for 7 years to get there. The spacecraft is called Cassini, and many of the pictures on these webpages of the Saturn system have been taken by it over just the last few months. The picture on the left shows Saturn with one of its moons, Enceladus.
Saturn has many moons - 31 at the last count - but most are no bigger than asteroids. The picture on the right shows a collage of Saturn and its moons. Cassini has been looking closely at some of the larger moons, such as Enceladus, Rhea, Dione and Mimas, to see what they are like. They are nearly all just like our own moon, heavily pitted with craters, and without an atmosphere. The far right image shows one of the small asteroid moons, Phoebe, which looks a bit like a potato - this short was also taken from Cassini.
The moon of Saturn we are particularly interested is the largest one, Titan. It is the second largest satellite in the Solar System, larger than both Mercury and Pluto. It doesn't look very interesting from this photo, since you can’t see lots of features on the surface, but this is what makes it exciting! We can’t see the surface because it is covered in cloud - it is the only moon in the solar system with a thick atmosphere surrounding it.
The Huygens space probe landed on Titan on Friday 14 January 2005, collecting data as its parachutes slowed it down. The mission only lasted about 3 hours - 2.5 hours to descend to the surface, and then another half-hour on Titan’s surface before the the batteries ran out! The heat generated by friction as Huygens fell through Titan’s thick atmosphere was immense, so it needed a heat shield in front of it that could withstand temperatures of up to 18,000 degrees C. However, during its descent, Huygens' camera captured many images, and its other five instruments sampled Titan's atmosphere to determine its composition. This is the first probe that has ever been landed on a world in the outer solar system, and it is the largest interplanetary spacecraft ever built. The picture on the left is a painting of the probe descending into the atmosphere of Saturn's moon Titan after being released by NASA's Cassini spacecraft. Once Huygens landed, it measured the wind, weather, energy flux and surface features, relaying the information back to the Cassini spacecraft.
Titan’s atmosphere is similar to the Earth's, as it is mostly made up of nitrogen, although atmospheric pressure at the surface of Titan is 60% greater than at sea-level on Earth. However, the atmospheric composition could be very similar to that of the early Earth when life began - oxygen was only acquired as a by-product of photosynthesis in plants. Underneath these clouds the rocky planet is probably made of equal amounts of rock and ice, but it might well have oceans and rivers, although these would be formed from more complex liquids than water, such as liquid hydrocarbons like methane and petrol. The picture on the right shows the first colour photo of Titan's surface, sent back by Huygens when it landed on Titan on 14 January 2005.
These images shows the surface of Titan from Cassini as it went past in December 2004 - the area shown on the right-hand image is approximately 2,000 kilometers (1,200 miles) across. It shows a complex interplay between dark and bright material on Titan's surface. The surface appears to have been shaped by multiple geologic processes. Although a few circular features can be seen, there are no features that can be definitively identified as impact craters. The far-left image was taken through the cloud using radar to show the surface of the planet. Titan has been imaged in a special light (infrared) which can see through the thick haze of the atmosphere. This image spans an area of about 75km across. Again, these may be geological flows of some kind - it is interesting that there doesn’t seem to be any sign of any craters. But while Titan may contain one of the richest stores of organic molecules in the solar system, it is unlikely that these compounds will have nurtured the origin of life on the surface of the moon because of the lack of liquid water. Any water would be frozen solid, and chemical reactions would occur at a sluggish pace because of the low temperature. Titan is much colder than the Earth, at a chilly -180C, because it is 800 million miles away from the Sun. It has been suggested that some heat is possible from heating due to the impact of a meteorite, which could have provided enough heat to liquefy water for perhaps a few hundred or thousand years, but this is only supposition. Titan is still one of the best possibilities for life in the solar system outside the Earth, however. Life in extreme environmentsLet's revisit what we know about what we need for life like that on Earth. Some discoveries right here on Earth in the past decade or so that show that we must not be too rigid in our definition of conditions for life - life exists on Earth under the most unlife-like and difficult conditions. First, we need to be clear that we’re talking about microscopic life-forms, such as bacteria. Microscopic life has penetrated environments on this planet that appear very inhospitable - at great extremes of temperature, pressure, darkness, and toxicity.
To start with, not all living things depend on oxygen - deep on the floor of the Earth’s oceans are ‘smokers’, or hydrothermal vents. These are associated with ocean volcanoes, and are an underwater version of natural hot springs/geysers on land. Hot chemical-rich water rushes up from volcanic rocks, and the minerals in the water crystallize directly from the hot water onto the rocks to form hollow chimney-like structures through which the hot water continues to flow from under the earth’s crust. There are bacteria that live around these smokers, and larger living things that feed on the bacteria. An example are these tubeworms (bottom image), which can be up to 2m long. Bacteria help the tubeworms by making food from the chemicals that come out of the water and the vent. So there is a whole ecosystem/marine community on Earth thriving deep beneath the surfaces of the oceans in a very harsh environment - these life-forms live in extremely hot regions, with temperatures of 120 degrees C, where it is completely dark. Sunlight only reaches down to about 100m under the surface of the ocean, and these 'smokers' are found about 2-3 miles down on the ocean floor. They get their energy not from sunlight, but from the sulphur compounds in the hot water. It is also an extremely high pressure environment, as there is the weight of 2-3 miles of water sitting on top it. It is possible that life on other planets may have formed in environments similar to these. In Antarctica, there is a lake the size of Lake Ontario two miles under the ice, so these strange marine life-forms could provide us with a possible model for how similar organisms might survive on Europa! Microscopic life has also penetrated other environments on this planet that appear very inhospitable - for example, extremes of temperature. Even the North and South Poles are not barren wildernesses. Microbe colonies have been discovered living under rocks in the Arctic and Antarctic. They have very little protection from the atmosphere so the levels of UV light they are exposed to are very damaging. The temperatures are consistently sub-zero, there are harsh winds and a very dry atmosphere. This type of life could be a good model for micro-organisms on Mars. The possibility that microbes could grow in clouds raises the potential for life in the clouds of Venus’s atmosphere - this would lessen the hideous temperatures and pressures down on the surface.
We have now finished our tour of our solar system, looking to see where life might be possible. But we shouldn't forget that astronomers have now detected planets in orbit around other stars near to our own. And in each galaxy - like this one here - there are millions of stars! There are many more solar systems out there!
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(Copyright NASA - source: 
(Copyright Calvin J Hamilton)
(Copyright NASA public domain)
(Copyright NASA public domain)
(Copyright Calvin J Hamilton)
(Main picture copyright Calvin J Hamilton
(Illustration credit and copyright David A Hardy, PPARC)
(Copyright NASA public domain)
(Left-hand picture copyright Calvin J Hamilton
(Copyright Calvin J Hamilton)
(Copyright NASA public domain)
(Picture credit and copyright ESA/NASA/Univ of Arizona, source 
(Copyright NASA public domain)
(Top left image obtained from University of Delaware, Graduate College of Marine Studies
(Picture credit Gemini Observatory, GMOS Team
