What does the discovery of the atmosphere and water on Enceladus mean?
24.6.2005
By: Haim Mazar
Direct link to this page: https://www.hayadan.org.il/mazarenceladus.html
introduction
The fourth of Saturn's moons that Xsini passed by is Enceladus. It is a small moon, 500 km in diameter and close to it in size are Mims, another moon of Saturn, which is 392 km in diameter, and Miranda, a moon of Uranus, which is 484 km in diameter. The first photographs of this moon arrived in Israel when Voyager 2 passed at a distance of 90,000 km and transmitted 22 images. In fact, the spacecraft took more photographs, but some of them with the high resolution of 2 km per pixel (let's not forget that this is a technology of the 70s of the 20th century) were lost . 43% of the surface was photographed, mainly from that part of the moon's surface that is not visible from the direction of Saturn and part of the North Pole.
Cassini passed by Enceladus on 17.2 at a distance of 1180 km and its images had resolutions from 1.5 km per pixel to 70 meters per pixel and on 9.3 it passed at a distance of 500 km and the resolutions were higher, from 300 meters per pixel to 40 meter per pixel. This is the closest pass planned during Cassini's lifetime. A third crossing is expected on 14.7 this year and a fourth crossing on 12.3.2008 at a distance of 1000 km. The intention is to reach full coverage and mapping of the moon.
In terms of its basic astronomical conditions, Enceladus orbits Saturn once every 33.4 hours at a distance of 238,000 km and its density is 1.2 g/cmXNUMX. Its low density can indicate a large presence of water in it.
atmosphere?
In both passes near the Moon, the spacecraft discovered unusual activity in Saturn's magnetic field. The field lines curved around the moon. The plasma in Saturn's magnetosphere is slowed and deflected by the moon and envelops it. It was possible to notice that the magnetic field was oscillating. Such a phenomenon can occur when ionized molecules interact with the magnetic field while moving around the field lines. The significance of this discovery was surprising. Despite its size, Enceladus has an atmosphere (Cassini finds 17.3.2005/XNUMX/XNUMX: web). Where does this atmosphere come from? The assessment was that it was ionization of water vapor and that there was activity of gases around the moon. Where do they come from? Several options have been put forward. The gases are a product of the activity of volcanism and geysers, the gases are emitted from the surface or they come from within the star itself. It should also be taken into account that the three sources work at the same time.
This phenomenon was not observed by Voyager 2 when it passed by the Moon. This can be explained in two ways. One possibility is that the discovery of this phenomenon exceeded the limits of the measurement capability of the spacecraft's instruments, and the second possibility is that during that period between 1981, the date of the spacecraft's passage by the moon, until today, something happened that led to the formation of the atmosphere. What is clear is that due to the small dimensions of the moon, the life span of the atmosphere is short, unless there is constant activity within the star that allows for a continuous supply of gases. Confirmation of the existence of the atmosphere can be made from Earth. For this purpose, the movement of the moon must be monitored using powerful telescopes equipped with Adaptive Optics technology or using the Hubble telescope. When the moon hides any star, see if there is any delay in the arrival of the light from this star. If this is what will happen, it will be clear that the moon has an atmosphere. It would be desirable to perform several measurements of this type. These measurements will also allow us to measure the density of the atmosphere. What is clear is that this is a very low density like that of Pluto. Pluto's atmospheric pressure is 6 ˉ 10 bar. The axial speed of the moon is 47 km/h similar to that of Pluto 44.9 km/h. Voyager 2 while passing by the Moon measured a temperature of -200°C on the side illuminated by the sun (Morison 1982). When Cassini passed by Paba the range of temperatures measured was between -161°C immediately after noon to 195°C just before sunrise. to give an indication of temperature differences on Enceladus as well. What are the gases of the atmosphere? Since nitrogen and CO2 are found on Pluto, it is possible that these gases also make up the atmosphere of Enceladus. The existence of this atmosphere, if indeed it is the case, could indicate that atmospheres can also exist on Other small bodies in the solar system and perhaps on Ceres, the largest of the asteroids.
Dust particles
In the first two passes of the spacecraft, a very large number of dust particles that hit it were counted. The thickness of these particles does not exceed that of a human hair, so it does not suffer any damage. In the first pass, thousands of hits were measured for 39 minutes. The origin of the particles is in dust clouds around the moon or the E ring. In the coming months, measurements will be made to find their source.
Ammonia and water
The low density of the moon and the high brightness of the surface have given rise to the thought that the presence of silicates on the surface is less than 15% (Squyers et al 1983). It was estimated that the density of the crust is too low to allow liquid water to reach the surface. A possibility was raised that there must be other volatile substances below the surface that would significantly lower the density of the liquids that would erupt towards the surface. The option they thought of was a combination of ammonia and water. The model built is that over the years as the moon cooled, only the clean water crystallized out of the water-ammonia compound. The concentration of ammonia will gradually increase in the interior residual material of the moon. The measurements the Voyager made did not reveal any presence of ammonia on the surface of Enceladus. The visible and infrared mapping done by Hexini also did not reveal any traces of either ammonia or CO2. On the other hand, many plains were discovered covered with water ice (Saturn's moons 18.2.2005 :web ). Given the fact that Xsini made these measurements at a smaller distance than those made by Voyager 2, the chances of finding ammonia on the surface decrease, although this does not rule out the possibility that the ammonia is inside the crust.
It may be that these ice plains have something to do with the ionization phenomena of the water vapor that we mentioned earlier. The obvious conclusion is that there must be a supply of water vapor. Where do they come from? This moon may still be geologically active. Inside it there are convection currents which, together with tectonic processes, refine the surface and the evidence that many different types of grooves, the grooves, have been observed on the surface of the moon. When cracking occurs, a lot of water bursts out to the surface. Most of the water freezes, but a small percentage evaporates and creates the ionization phenomenon observed by Hexini.
Makhteshim
In the new photographs of Enceladus, Cassini revealed to the researchers additional parts of the surface that were not observed by Voyager 2. Although additional craters were observed, these did not change the general picture regarding the size of the craters and their distribution on the surface. In some of the photographs, the photographic coverage reached in some places as far as latitude S º 65 compared to Voyager 2 which covered part of the North Pole. This spacecraft photographed 50% of the North Pole up to the 55th parallel. Here is a large part of the craters that scarred the surface. It should be taken into account that in future photographs of the moon by the Cassini, craters will also be found at the South Pole, although it is difficult to predict their size and density. Most of the craters are located between latitude N º ºN-9057 and longitude 30º to longitude 215º. Two other concentrations are found one between 30ºN-57ºN and the longitudes 160º-270º and a second concentration is between the latitudes 25ºN-90ºN and the longitudes 330º-30º. The largest crater on this moon has a diameter of 35 km in contrast to other moons of Saturn that have craters with a diameter of 100 km from and more Only a few craters have a relief in their center. No crater has rays, material thrown out from the impact site in a radial manner like on the Earth's moon. In the Dunyazad crater located at 35ºN 115º and which is 21 km in diameter, there is a dome-shaped bump similar to the one found in the Har crater on Callisto, a moon of Jupiter (photos 1,2, 2). A crater count made by Voyager 43 within the limits of resolution showed that within the photographed area - 453% of the moon's surface - 5.66 craters with a diameter exceeding 2 km were counted. Many of the craters imaged by Voyager 40 are shallow compared to Earth's moon. The high resolution of the spacecraft up to XNUMX meters per pixel allows to see smaller craters up to hundreds of meters in size. At first glance it can be seen that these small craters are similar to their counterparts on the Earth's moon.
tectonic activity
Voyager 2's photographs show a section of the moon rich in tectonic activity, grooves tens of kilometers long. This area is mostly bounded between the equator to latitude 30ºN and the longitude 335º to longitude 180º. In the north of the complex are two sinusoidal grooves, Samrakand Sulci in the west and Harran Sulci in the east. In the center is a complex of short grooves and on both sides are plains. This is how they were seen by the spacecraft's cameras given their resolution limits. To the west of it Sarandib Planitia and to the east of it Diyar Planitia. Northwest of Harran Sulci are 3 ridges parallel to each other with a length of about 100 km each. This region seems to have gone through a process of wrinkling similar to the alpine wrinkling on Earth. North of the Samrakand Sulci a flow configuration is seen reaching as far as the North Pole. This configuration is narrow at its southern end and at its northern end (near the North Pole). Its maximum width is near latitude 57ºN 65.5 km. It looks like a lava flow. What can support this possibility is the cutting of craters through which the current passed and covered some of them.
Various studies have been done to measure the height of the moon's relief. Some of the measurements showed that the relief is between 500-1500 meters above the surface and other measurements showed that the maximum height of the relief is 2000 meters (Kargel 1996). The impression is that this non-uniformity originates from the resolutions of the photographs which did not allow reaching unanimity.
When Cassini made its flybys near Enceladus it became clear that the tectonics of this moon is much more massive and complex than thought. An area located south of Diyar Planitia and which is bounded between latitudes 0º-60ºS and between longitudes 254º-296º within the limits of the resolving power of Voyager 2 appeared to be a clear plain plowed with grooves (photo no. 3 PIA06191). A system of grooves tens of kilometers long cross each other. One slot is extremely wide, 5 km. The number of craters here is small. The large crater that stands out here is 5 km in diameter, it is very shallow and looks as if it has been filled with dust. The grooves are convoluted and a careful examination of the way they cross each other can indicate their relative age. Who preceded whom and there are some in them to remind the grooves of Jupiter's moon Ganymede. In photo no. 4 (enlargement of the right part of image no. 3). Prominent formations are visible rising above the surface. They have a wide arc and are tangent to each other. It may be the remains of two craters. If they are indeed craters, they are very shallow. At the western end of the photograph you find a complex of reliefs that indicate a crease that this area has undergone and it ends in a trapezoid configuration (Photo no. 5 n00028194.jpg ) and inside it are parallel grooves and one groove crosses them in a northeast-southwest direction. Another closed tectonic formation is at the eastern end of Diyar Planitia (photo no. 6). These formations have no parallel in the solar system. The height of the relief here is 50-100 meters. There are several craters here and you can see that they have undergone any kind of weathering.
A new phenomenon was found south of the Dunyazad crater, narrow and parallel grid lines in a north-south direction crossing craters along their entire length (photo no. 7 PIA06217), a phenomenon that has also not been observed anywhere in the solar system. At the southern latitudes of Enceladus in photo no. 8(PIA060207) you see a concentration of craters some of which have undergone weathering and narrow grooves a few kilometers long.
discussion
The origin of the grooves is from the stretching of the soil in opposite directions. The final product is a subsidence of the soil for tens of meters to tens of kilometers. On Earth, you can find fissures that can reach thousands of kilometers in length, and a well-known example from our region is the Syrian-African rift, which is 2000 kilometers long. In this case it is a movement of plates. Is this what happened on Enceladus? Such a possibility was indeed discussed in the past (Kargel 1983). The photographs that the Kasini sent to Israel show that the grooves including the Samrakand Sulci and the Harran Sulci are local in nature. They do not have global dimensions such as the Ithaca Chasma canyon that will fly by another moon of Saturn. The grooves are shallow and cross each other at different angles, which means that the same intramembranous tensions acted in different directions. From the nature in which they cut "and lie" on top of each other, it is possible to build a stratigraphic map and determine the order of their occurrence. Such mapping today will be only partial. Full mapping will only be possible when the moon is fully photographed.
The ridges to the north of the Harran Sulci testify to the likes of folding processes found on Earth. But how can the closed configurations be explained? The impression obtained is that each "wall" of a closed configuration was created in an "independent" process and separately from the other "walls". If we look at photo no. 5 In the trapezoidal formation we will notice that to the right of the trapezoid is a ridge line which, upon entering the formation, splits into two arms from which additional arms emerge that close the formation. It could be that the crease started on the right side ran to the left and then for whatever reason it split and moved to both sides until the configuration was closed. It sounds very strange, but we must not forget that we are watching different worlds where unfamiliar processes are taking place on Earth. What are the forces that brought about the occurrence of this process? It seems necessary to develop new tools to explain this geological phenomenon. An attempt to explain the configuration in photo no. 6 becomes more complicated since the configuration has 6 sides and within it there are also wrinkling processes.
In the flow configuration north of the Samrakand Sulci, as we mentioned, the flow cuts several craters. Another phenomenon that is noticed is the furrows inside the fluid material. what happened here? Lava in its flow covers everything in its path, so an explanation must be found for why the craters here were not fully covered. 3 explanations are possible. One explanation is that the craters sit on a slope and the lava in its flow covered those parts sitting on the lower part of the slope. This explanation can give a hint about the topographical structure of the place before the eruption of the lava and its flow. A second explanation is that the craters are so shallow that the lava during its flow covered those parts of the craters that were pierced in its path. A third explanation is that both possibilities occurred simultaneously. What is clear is that the furrows appeared after the lava solidified. It seems there was a series of earthquakes here that cracked the ground. And here it is possible to distinguish based on these difficulties the order of events that took place there.
The settlement of the craters in photo no. 7 is in a north-south direction and is found in other places in this crushing area. The grooves are in an east-west direction and are also crossed by this chipping. The upper rim of the large crater in the picture is cut by a groove, which means that the crater was formed after the formation of the groove. Since both the groove and the crater as the whole area are covered by these crack lines, it is clear that the crack lines were formed after the appearance of those on the surface. The interesting thing is that all the craters in the area, including the groove, did not undergo any deformations after cracking. There are no strike-slip fragments here. It seems that there was a series of earthquakes here that cracked the area without reaching the same threshold that resulted in moving "blocks" relative to each other.
Another question to ask is where the ice plains come from. The answer is to be found in cryovolcanism. Although no volcanoes were found on Enceladus, there may have been and still are Hot-Spots that emit water ice and these freeze immediately upon reaching the surface (to remove the water that evaporates). If this is what really happened, then layers of ice were piled on top of each other here, similar to lava flows piling on top of each other as a result of volcanic eruptions.
Summary
In light of the spacecraft's findings, it was decided to change its flight path in preparation for its passage by the moon on July 14.7. Instead of passing at a distance of 1000 km from it, it will pass at a distance of 175 km from the surface. Due to the geological richness of this moon, it will be possible at the end of the Hexini operation to use the abundance of data to create a virtual geological laboratory for comparison with moons of a similar size in the solar system to understand Different geological phenomena not only at the edge of the solar system, but also on Earth.
