by Debra L. Davis
Impact cratering is a cataclysmic process that occurs on every body throughout the Solar System. The effect it has on each body, however, is not the same. There are many factors to be considered when studying craters on a planet. There is the type of impactor, as well as the characteristics of the planet that is impacted.
In this paper, similarities and differences of craters on Mars and Europa are discussed.
Europa is a prime candidate for the possibility of life in its sub-surface ocean. Mars is also a candidate for microbial life. They are both planets that warrant further research as humans venture out into the Solar System. Before those first steps on another planet are taken by humans, however, a better understanding needs to be obtained, as was the case with the Apollo program in the early 1960s.
Though all planets experience geologic processes, not all of those processes occur on all planets. One process, however, does. That process is impact by comets or asteroids which leave scars on the terrain in the form of craters.
The method used in this report was meta-analysis of previous studies of craters on Europa and Mars. A visual comparison was made of types of craters, such as simple and complex craters, as well as impactor populations. Some images were processed to enhance visual comparison using Image J software.
No original research such as data collection, other than reading journals and articles, was conducted for this project.
Europa and Mars Compared
Europa is less than half the size of Mars, being closer in size to Earth’s moon. The reason these two planets were chosen for comparison in this paper is due to similarities in crater structures, most notably lobate ejecta features. On Mars, these features are thought to be an indication of a sub-surface layer of permafrost. On Europa, it is believed that there is a sub-surface ocean under a layer of ice. Though these two planets are of different size and composition, it is of interest to survey craters on these two bodies because of the possible presence of water at depth, a liquid ocean for Europa and a permafrost layer for Mars.
Some basic characteristics for both planets are as follows. Of interest is the similar density of each planet. Ganymede and Callisto, in comparison, have an average density of 1.9 g/cm-3 and 1.8 g cm-3, respectively.
Both planets, undoubtedly, were heavily impacted during the early development of the Solar System, as well as the period of late heavy bombardment approximately 4 billion years ago. On Europa, however, due to the relatively young surface age, there is no remaining record of these impacts.
The impact populations on Europa and Mars are currently thought to be of different origins. Impactors on Europa are cometary and “generally thought to originate in the Kuiper Belt” and those on Mars are believed to originate from the Main Asteroid Belt. Though there may be craters formed by cometary origin on Mars, most impacts are believed to be dominated by asteroids.
A kilometer sized object will make a crater of approximately 20 kilometers in diameter. As of the Galileo Europa Mission (GEM), there are only seven craters on Europa that are in excess of 20 kilometers. This contrasts greatly with Mars where large impact craters number in the hundreds and basins are found well in excess of 1,000 km, such as the Argyre (image on the right) and Hellas Basins. Impacts which create craters in the 20 kilometer size are thought to occur on a timescale of approximately 1.4 million years. Based on this information, the surface of Europa would be less than 10 million years old, though no older than 100 million years old.
Most small craters on Europa are thought to be secondary craters, ejecta from larger events, however, “trojan asteroids” may contribute to the population of small craters.
Due to the different origins of the population of impactors on Europa and Mars, and the young surface age of Europa compared to the much older age of Mars, it is difficult to relate cratering rates for these two planets.
The final crater created by an impact event is dependent on many factors, including, but not limited to, the type of impactor (e.g., comet, asteroid), impact velocity, impact angle, and the target material.
There are three main types of crater morphologies that are found on all planets. They are:
Due to the different characteristics of Europa and Mars, there are differences to be found in crater morphology. The most noticeable difference is the relaxed nature or lower topography of features on Europa. Craters on Europa in excess of 30 kilometers, notably Callanish and Tyre, have no crater rims or central uplifts and are instead surrounded by “concentric troughs and ridges” which may indicate a “fundamental change in the properties of Europa’s icy crust at increasing depths.”
Europa’s crater morphology even differs from that of the other two icy Galilean satellites, Ganymede and Callisto, perhaps due to its thinner ice shell. Europa’s ice shell is thought to be no less than 4 kilometers thick, and possibly as thick as 25 kilometers. In comparison, the ice shells of Ganymede and Callisto are thought to be between 100-200 kilometers thick.
One similarity between craters found on Europa and Mars are the lobate appearance of ejecta found surrounding some craters. On Mars, this ejecta structure has been studied in depth and is believed to be melting of a layer of sub-surface permafrost. There are single-lobed and double-lobed ejecta, as well as multiple-lobed and pancake-shaped ejecta. As cited by Strom (1992) from a source by Mouginis-Mark in 1979, some general conclusions for lobate ejecta surrounding Martian craters have been determined. It was found that:
There are examples of lobate ejecta surrounding craters found on Europa, such as that seen surrounding Manannàn, a ~22 kilometer crater found at 3°N 240°W. Upon examining additional crater images of Europa, but not available for reprint in this paper, a very distinctly lobate ejecta surrounds a 13.5 kilometer crater tentatively named Grainne, found at -60°N 95°W.
One observation upon examining craters on Europa and Mars with lobate ejecta is the size of the feature. On Mars, these features extend from the crater rim from a range of approximately one radius, as seen in a 7 kilometer crater located at about 22°N 32°W, to over four diameters seen in an 18 kilometer crater with pancake-shaped ejecta located in the Oxia Palus Ut area. These features, as observed for Europa, appear to be less than one diameter from the crater.
Small craters are those with diameters less than 4 kilometers and the “classic” bowl shape (image on right). This class of crater is “probably a mixture of primary, secondary, and endogenic craters. This class of craters is found in abundance on Mars. On Europa, however, they are far less numerous, probably due to the much younger age of Europa’s surface
On Mars, simple craters originate from the above described mixture. On Europa, however, they are believed to be mostly secondaries, based on “the steepness of the average differential power-law slope (-4.2) of the small-crater size distribution.” Bierhaus et. al. (2001) indicates that “the amount of mass ejected by Pwyll and the other large craters on Europa is potentially enough to create the majority of the small crater population via the secondary cratering process”.
On Mars, particularly in the southern highlands, the number of small craters may reach saturation levels. “One of the central issues concerning saturation is whether the heavily cratered terrains of the terrestrial planets essentially display a production crater population or whether the production function has been changed by attaining saturation.”
On both planets, secondary craters are seen in ray systems from larger impacts. An example on Europa of such secondaries can be seen in the Conamar Chaos region where a ray from Pwyll crater located 1000 kilometers to the south has deposited debris. Such a “high” concentration of small craters, as seen in this area, is unusual for Europa. This high concentration is probably due to the relative young age of Pwyll.
On Europa, though they may not be associated with any visible ray system, “many small craters appear in clusters or clumps,” instead of a random distribution and are, therefore, thought to be secondary in nature.
To date, there are only 28 large craters seen on Europa. Of this number, 14 are less than 10 kilometers in diameter; seven are between 10 to 20 kilometers in diameter; and seven more are greater than 20 kilometers in diameter, with Tyre being the largest at approximately 43 kilometers. Mars, in comparison, has hundreds of large craters, some of them to the scale of being a basin, such as Argyre Basin at over 1,000 kilometers and Borealis at well over 7,000 kilometers in diameter.
As defined earlier, complex craters typically have central peaks on level floors and terraced walls. Complex craters on Mars fit this description. Though complex craters on Europa also fit this description, they are much more “relaxed” than similar sized features found on Mars. On Europa, topographic features are mere meters compared to kilometers on Mars. This relaxation is due to the elastic nature of its surface ice layer.
The transition from simple to complex craters on Europa is approximately 5 kilometers. On Mars, the transition occurs at 8-10 kilometers.
The large crater population on Europa is sparse compared to its sister planets, Ganymede and Callisto. It would stand to reason that cratering rates would be fairly uniform for the Galilean planets. This difference in cratering populations is probably due to Europa’s relatively young surface age and shallow ice layer, as compared to Ganymede and Callisto.
Pwyll, located at -23°N 137°W, (image below) at a diameter of approximately 24 kilometers, is the youngest large impact on Europa. This is evidenced by its expansive ray system which is visible in excess of 1,000 kilometers from the crater. It is believed to be 18 million years old or younger.
Tyre, located at 34°N 146°W, is almost twice as large as Pwyll at approximately 43 kilometers in diameter and has a much different appearance. It has a central feature approximately 15-20 kilometers in diameter and is relatively flat. It has no distinct crater rim. The crater is then encircled by concentric troughs and ridges which radiate to a distance in excess of 100 kilometers. Another crater of similar size, Callanish, located at -16°N 334°W, has a similar appearance. One hypothesis for the appearance of these craters is that they “penetrated through an icy crust into a less brittle layer.”
The low number of craters seen on Europa will make accurate dating of the planet difficult. “There are too few large craters (>10 km) on Europa to provide statistically meaningful crater density and age information on geological units, making a better understanding of the small crater population vital to surface age calculations. If secondary craters were mistakenly interpreted as primary craters, then the derived surface age would be greatly overestimated.” This small population of craters, both large and small, also has implications for understanding the impactor population on Europa, as well as the possibility of a liquid ocean.
While researching this paper, it became apparent that there was a vast amount written about cratering on both Mars and Europa, as well as cratering in general. It was difficult to distinguish what would be of benefit and what was repetitious. Hence, I was overloaded with data and unsure in what direction to take this paper. In retrospect, I believe a paper that focused entirely on lobate ejecta would have been more productive, as well as original research and data collection on these interesting features.
What was apparent is that there is still a lot more work to be done regarding craters. As more spacecraft venture further out into the Solar System, there will be new sights to see and new insights into old problems. I, personally, will be on the lookout for more research on lobate ejecta around craters on Europa and Mars.
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When originally written, they included citations. These have been removed to protect the innocent and to deter