Akos Kereszturi (teacher)
"Catch a Star!" 2004
Table of contents:
Closer look at the surface
Comparison with Mimas
Imagine you are on Enceladus…
Parameters of Enceladus
We wrote our essay about Enceladus, the moon of Saturn, because this is not a very well-known, but a very interesting satellite. It is probably the second most active satellite of that planet. It was photoghaphed by Voyager 1 and 2, and now it will be analized by Cassini in the near future. There are some hypoteses about Enceladus, which will be proved as we hope. Enceladus is not only a single Saturn moon, but part of a complex system. It is connected to Dione with an orbital resonance which causes tidal heat and the particles from the satellite probably take part in the maintenance of the E-ring. It's craterfree and young terrains are produced probably by cryovolcanism. We have observed this moon inspite of the difficulties from bad weather and the bright Saturn.
In the following pictures we show the position in the Saturn system, and the connection with the other satellites which are close to Enceladus.
On the 28th of August, 1789 Friedrich Wilhelm (William) Herschel looked at the Saturn with a reflector, which had 122 cm diameter and 11.9 meter focal length. At that time only five moons were known. These were: Titan, Rhea, Dione, Thethys, Iapetus. Still on this day he discovered Enceladus, and twenty days later (on the 17th of September) Mimas.
The names of these new satellites come from his son, John, who named them only in 1847, after two figures of the Greek and Roman mythology. They were siblings, their father was Uranus, and their mother Gaea. Enceladus was a giant, with one hundred hands. He rebelled against the gods, but finally Pallas Athene defeated him, and pushed him under the Etna. There are superstitions about him: each time when he takes a breath the volcano will erupt and if he turns from one side to the other, an earthquake happens.
After its discovery people did not know too much about it, and noone thought that in such a small moon there could be anything interesting. Not even in 1971, when John S. Lewis published his treatise about icy moons' thermal history. He presumed that a celestial body with a size of Enceladus could keep so infinitesimal internal energy from its formation, which is definitely not enough to cause geological processes.
The first mission which dealt with Enceladus was the Voyager 1. It took the photo on the right from a distance of 202040 km in November, 1980. It is not a good quality picture and moreover it was taken from the night side of the moon. Although it was not very particular, but the surface seemed smooth, and this raised scientists' curiosity.
Nine months later - in August 1981 - Voyager 2 took much better pictures (the resolution was 0.97 km/ pixel) (on the left) only 87140 km from Enceladus.
In these pictures scientists discovered that the surface is much more free of craters than the satellites which were thought to be similar to Enceladus, like Mimas, Dione, Tethys, Rhea. It has some smaller craters, but they are scattered unevenly on the surface.
Among other things, this is what the Cassini have to map, it will approach Enceladus four times on its way, much more closer, than the earlier spacecraft.
These flybys will be on: 17th February, 2005 - 1179 km; 9th March, 2005 - 500 km; 14th July, 2005 - 100 km; 12th March, 2008 - 995 km. In the program this Saturn satellite will get the biggest attention after the Titan. It has already taken a photo from a great distance (on the right).
Closer look at the surface
Its flavour is that Enceladus reflects almost 100 percent of the sunlight that strikes it. In the Solar system this body has the largest albedo (~1). Not even the fresh snow has got that kind of brightness. It is interesting, because the ice would have darkened because of the radiation which has been hitting the surface for a long time. There is only a little difference (~15 %) between the leading and the trailing hemispheres. Scientists think because of the high albedo the surface can not be very old. It is confirmed by the images of the surface because some areas are craterfree, while others are nearly as full of craters as Saturn's similar satellites. The smoothest areas have four times fewer craters than on Triton, which is surely an active satellite. The areas with different crater density are often separated from each other with a sharp boundary, as we can see it in the picture:
We can divide the territories into three large categories:
- - the oldest areas with a lot of craters are (CT1, CT2),
- - middle-aged plains with some craters are (CP1, CP2, CP3)
- - the youngest ridged plains (RP).
Among the features we can see craters which are partly embayed which shows, that new surface evolved in the course of time. The lack of great craters (the two largest craters are about 35 kilometers in diameter) makes us suppose this too. Scientists presume from this, that the original surface has totally been changed already.
At some craters (for example on Peri-Banu and on Ahmad) distortion can be noticed. If the surface was constituted by very hard and for a long time frozen ice, the features could not be so much deformated. So the temperature of the ice must be high enough that the viscous relaxation could take place.
There are some models to estimate the age of the youngest parts of the surface. These are based on the impact flux. If this has been decreasing in direct proportion since the heavy bombardment era, then this age is 5-20 x 10 6 years, but if the decline has been exponential, then the surface is 2 x 10 8 years old. Although we do not know exactly, which model is the correct one, the surface is too young anyway, for us to explain the features without one-time active geological processes.
The features of Enceladus
- Craters: these are not very big, and are found only in certain areas. As far as we know, there are 14 relatively big craters (bigger than 10 km in diameter). These are named after the figures of Arabian Nights, for example Aladdin, Ahmad, Shahrazad, Sindbad.
- Planitia: these are regions which are very smooth, and are lowlands. There are only two: the Diyar and the Sarandib Planitia.
- Sulci: System of prolonged, almost parallel ridges, heights and lows. Enceladus has two of them: Harran and Samarkand Sulci.
- Fossae: long, narrow fosses. These are: Bassorah, Isbanir, Daryabar Fossae.
The positions and shapes of the features could be seen in the following map:
All the signs make us suppose that Enceladus was a geologically active satellite in the near past and presumably it still is. To be active, Enceladus needs some kind of internal enegy. Scientists did not understand it for a long time, how it is possible, because it is too small, and too far from the Sun, and it had to get frozen a long time ago. Sometimes radioactive materials give the internal heat (for example on the Earth), but here we can preclude this possibility, because Enceladus's density (1.24 g/cm 3 ) is too small that there could be enough radioactive materials.
So other explanation had to be found. S. J. Peale - who investigated the volcanism of Io at that time - was the first man who had a new idea. He thought that there is tidal enegy on Enceladus too, as in Io. But here it comes from Saturn gravitation and from the resonance (2:1) of Enceladus and Dione. Dione orbits around Saturn two times longer, than Enceladus, so they will be closest to each other always in the same point, at regular intervals. In such cases Dione tries to deformate the orbit of Enceladus. The continuous production of heat can be guaranteed by the movement of the tidal bulge produced by Saturn. According to the estimations some parts of the mantle are liquid in the depth of 10-30 km. It is not sure that this heat source is enough to melt the ice permanently. But scientists calculated, that if Enceladus contains metan and ammonia ice too, then the melting point can fall even 100 degrees. Nevertheless the chemical constitution of the satellite can only be surmised, we do not know it exactly.
Before 1979 the existence of the E (and also the D) ring was disputed, because it could not always be seen.. But when the Pioneer 11 made a flyby of Saturn (in the autumn of 1979) it confirmed the existence of the rings. This ring is 180000 km from Saturn, and 300000 km wide. Following this the ring was observed from the Earth in March 1980 and found to be the densest and brightest at the orbit of Enceladus. Scientists think the material of the ring is from Enceladus partly because the lifetime of the particles is very short. Unless the production of new material, the ring would disappear in 10 3 - 10 4 years. This suggest the ring is replenished time after time. If we supposed Enceladus is geologically active it can serve as source of the ring. There are two theories for this: It is generally accepted that impact events can produce substantial material for the ring. But nobody knows if it is enough for the maintenance of the ring. And arises the question 'why do none of the other icy satellites of Saturn contribute to an extended ring system'? The size of the particles of the ring is between 2 and 2.25 micrometers and shaped like frozen teardrops. So A. F. Cook and R. J. Terrile American scientists supposed that the materials of the ring might come from the satellite, when icy particles escape by water blow-up. They get frozen in the area of the ring. Based on certain theories there can be geysers and volcanic eruptions, but it has not been proved yet.
The probable volcanoes of Enceladus are different from those on the Earth, because the magma contains water ice and volatiles. Most of the mass of Enceladus is water ice thus there cannot be silicate volcanism. This process can reform the surface and maintain the E-ring. Some researchers think some of the craters have volcanic origin. This is supported by some very high and large central peaks, sometimes higher than the crater wall, and they often entirely fill the crater floor and have flattened summit regions. For example in Aladdin the central peak is 2, in Ali Baba 1 km high respectively. We can see their vertical profiles on the right.
Comparison with Mimas
Mimas is one of the innermost moons of Saturn. It was discovered by William Herschel in 1789. Mimas is an icy moon, and has similar size as Enceladus, so scientists thought that they have similar physical aspects. But the statement does not square with the reality. The later pictures showed, that Mimas's surface is full of craters which are usually greater than 40 kilometers.
On the other hand on Enceladus the biggest crater is only 35 kilometer wide. Mimas has an extremely big crater which is named after Herschel. It is 130 kilometers wide, one-third diameter of Mimas. Enceladus has very smooth regions without craters, while on Mimas the south pole is a bit freer of craters, but there are still craters about 20 kilometers in diameter.
These big craters show that Mimas's surface is much older than Enceladus's. This is interesting, because they were formed at the same time. In this picture we can see the difference between the two surfaces. The top of the image shows Mimas and at the bottom we can see Enceladus.
In the following diagram we formulated some data of the two satellites to compare them. We took the rates of Enceladus 100% and the other rates compared to this.
|The relative sizes of Earth and Enceladus. You could put Enceladus 16588 times to fill the volume of the Earth. If our Moon was as small as Enceladus, its visible diameter would be seven times less. The area reflecting sunlight would be 49 times smaller, the brightness of certain area would be 9 times brighter because of higher albedo. As a result our Moon Enceladus would be about 5 times fainter than the real Moon.|
|The visible size of the Sun from the Earth is on the bottom of the image. If you are on Enceladus you would see the Sun as small as the circle on the top. The Sun from Enceladus is 90 times fainter than from the Earth.|
|To compare the size of the other moons of Saturn, you can see them in front of Titan. Enceladus is on the top, then clockwise direction Tethys, Dione, Iapetus, Rhea and Mimas.|
Here we have a video which imitates the surface of Enceladus. We do not know the whole surface and we have not pictures with high resolution of it but it is still useful to imagine in 3D. This video is copyrighted by Calvin J. Hamilton.
Theories of possible life
Just like in the case of other planetary bodies there are theories of possible life in Enceladus too. These are only speculations, but very interesting ones. Craig Levin, professor of physics at Bradley University wrote an article about it. He thinks Enceladus had and still has the necessary chemicals, so life had a good chance to start. For the continue presence of life we need an energy source and liquid water. Because both of them are in Enceladus he supposes that life could be there. He says: "This leads me to believe that our first aliens are to be found as Europans and Enceladians, fellow members of the Solar system of which we ourselves are a part."
Here are some space artist pictures:
We observed Enceladus from the Polaris Observatory in Hungary with a Dobsonian telescope. We waited for a long time to have clear weather, but after the Christmas holidays on Thursday there was no cloud in the sky. But it still was very difficult to find Enceladus. First we checked the position of Enceladus with Guide software, then we tried to spot it in the telescope. Unfortunately Saturn was too bright and Enceladus was too close to the ring. Because of that, we put Saturn out of the field of view, and watched as it was coming into the field. In the last seconds before Saturn entered the faint moon appeared next to the ring. Both of us could see it individually.
The parameters of the observation:
- Observers: Edina Budai, Andrea Szabo
- Location: Budapest, Hungary
- Date: 6th January, 2005, 20:47 UT
- Telescope: 250/1200 Newton
- Magnification: 200x
- Visual limiting magnitude: +5 m
On the far left of Saturn Rhea, at the top Dione is visible.
Enceladus is the small faint spot right beside the ring.
Now that we got to know this satellite, let us delineate the most important things of Enceladus: Although this is very far from the Sun and it is very small, it is still charasterictic in its own way. It is full of ice, but probably still has liquid water inside which may come from the tidal energy, the resonance with Dione. It is connected to the E-ring, which is very interesting, because we still do not know surely, how the particles get there. We can see some strange features on its surface, some of them are explained by a special kind of volcanism, the cryovolcanism. We are full of theories, but these are not confirmed by proofs yet. The topic is very relevant, and in the next months a lot of questions will be answered by Cassini.
Parameters of Enceladus
|Discovered by||William Herschel|
|Date of Discovery||1789|
|Mass (kg)||8.40 x 10 19|
|Equatorial radius (km)||250|
|Mean density (gm/cm 3 )||1.24|
|Mean distance from Saturn (km)||238020|
|Rotational period (days)||1.370218|
|Orbital period (days)||1.370218|
|Mean orbital velocity (km/sec)||12.64|
|Orbital inclination (degrees)||0.02|
|Escape velocity (km/sec)||0.212|
|Visual geometric albedo||0.99|
|Mean surface temperature||-201°C|
|Atmosphere||Not yet detected|
References (Sources of images)