UNIVERSITY COLLEGE LONDON

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1 UNIVERSITY COLLEGE LONDON University Of London Observatory PHAS1510 Certicate in Astronomy PHAS : Impact Craters on the Moon, Mars and Mercury Name: An experienced student should aim to complete this practical in 2 (and not more than 3) sessions. 1 Objectives The object of this experiment is to compare impact craters on three dierent planetary bodies and to interpret the causes of the dierent end products of impact. 2 Materials required 1. Lunar Orbiter photograph of the large lunar crater Aristarchus 2. Mariner 10 image of the hermian crater Brahms 3. Viking image of the martian crater Naar 4. You should also obtain and study copies of the most relevant papers in the reference list (Greeley 1985 is probably the most useful). An important part of this practical is careful reading of these papers, in order to get a good appreciation of the appearance and signicance of surface features considered in this practical. Expect to spend a half-hour or so on this familiarization exercise before attempting the worksheet. Be sure to make all measurements on the large glossy images, not the small-scale images in Figs 35! 3 Introduction Exploration of the Solar System since the mid-1960s has demonstrated that impact cratering is an important process that has sculptured the surfaces of solid planetary bodies. Craters range in size from sub-millimetre pits to gigantic basins, some of which are over a thousand kilometres across. The nature of an impact depends on a number of factors including the size and density of the impacting body, the impact velocity, the surface gravity of the body being impacted, and the target strength. As a result, craters of dierent sizes show dierent morphological characteristics, and craters on one planetary body may be dierent from those on another where impact conditions were dierent. 4 The Impact Process When a meteorite travelling at tens of kilometres per second hits the surface of a planetary body, the shock waves generated lead to an orderly and rapid sequence of events, producing a crater surrounded by the ejected material. The process is very dierent from that of, say, `Hermian' relates to the planet Mercury 1

2 Figure 1: Illustration of the spacecraft eld of view, the sun angle and shadow lengths an impacting bullet from a rie; in that case, when much weaker waves are formed, the projectile stays essentially intact and becomes embedded in the target. With a high-velocity impact, the impact process starts as the projectile touches the planetary surface. At this point, shock waves are generated which travel downwards into the planet and upwards into the meteoroid. The surface below the meteoroid becomes highly compressed. The strengths of the materials involved are insucient (by large factors) to resist the pressures generated, and the target behaves hydrodynamically, causing jetting of material at extremely high velocities away from the planet's surface. By the time the meteoroid has penetrated to its full diameter, it is consumed by the shock waves and both it and some of the target material have melted and vaporized. Next, the `shell' of shock Figure 2: Key to the widths, l, of each picture. 2

3 waves in the target excavates a rapidly expanding hemisphere that nally forms the crater. The ejected material is thrown out at progressively lower velocities, until the last material is ipped over almost intact, forming a raised rim around the crater. The main bulk of the ejecta is emplaced progressively outwards forming a continuous sheet around the crater. This sheet of continuous ejecta thins outwards as there is less material to deposit. Finally, scattered impacting blocks may form isolated secondary impact craters in the form of clusters and crater chains. In the case of large craters, it is normal for the oor of the crater to rebound upwards, producing a central peak. At the same time, the inner walls of the crater are dragged downwards and inwards, causing large-scale landslides to produce inner terraced walls. The end result of impacts is not always the same, despite the orderly character of the process. For example, the distance to which the ejecta is thrown will depend on the gravity. Thus, the higher the surface gravity, the closer the material will be emplaced to the crater. The strength of the impacted materials will also have an eect on the form of the crater, especially for smaller craters. With weaker target material the oor may be able to rebound more successfully than in stronger materials, causing the oor to oscillate up and down. If it comes to rest in a down position, a central pit results, rather than a central peak. The presence of volatiles (e.g., water or ice) in the target material may also control the stability of the continuous ejecta once impact has occurred. 5 The Images Table 1: Image Data Lunar Orbiter Mariner Viking Photograph Image Image Range (R) km km km Focal length of lens (f) 80 mm 1500 mm 475 mm Image frame size (x) 60 mm 12.3 mm 12.8 mm Sun elevation (α) The lunar crater (Picture 1) is Aristarchus, one of the freshest on the moon. This picture was taken from Lunar Orbiter 5, the last of a sequence of missions designed to look for suitable Apollo landing sites and to examine the lunar surface scientically. These missions were unusual because the images were produced using normal lm in cameras on the spacecraft. Once the pictures were taken, the lm was processed on board and then scanned by a light beam to produce the digital data that were transmitted back to Earth. These digital data were then reconstructed on strips of 35mm lm which were mosaiced to reconstruct the full image. Because this was done by hand, rather than by computer, the individual strips of lm (or `framelets') can still be seen. The hermian crater (Picture 2) is called Brahms. The crater does not appear perfectly circular because the spacecraft camera is viewing it slightly obliquely. Therefore, measurements should be made along the longest axis to avoid the eects of foreshortening. This picture was taken using a Vidicon camera (a type of TV camera). With this system, the telescope projects the image onto a small coated screen which is immediately scanned and the digital data returned to Earth. Picture 3 is the martian crater Naar, as seen from one of the Viking orbiter craft. This picture was also taken with a Vidicon, although the frame size is dierent from that of the Mariner 10 spacecraft camera. The Voyager Mission, which explored the planets of the outer Solar System, was the last 3

4 planetary mission to use Vidicon cameras. All subsequent missions have used charge coupled devices (CCDs). References 1. Greeley, R. Planetary Landscapes. London: Allen and Unwin, 1985, pp 39-43, 95-98(Moon), (Mercury), (Mars). 2. Guest, J.E. et al. Planetary Geology. London: David and Charles, 1979, pp 20-32(Moon), 73-74(Mercury), (Mars). 3. Murray, B., Malin, M., Greeley, R. Earthlike Planets. San Francisco: Freeman and Co., 1981, pp See also: Guest, J.E. et al. 1975, J. Geophys. Res., 80, 2444; Guest, J.E. et al. 1977, J. Geophys. Res., 82,

5 Impact Craters on the Moon, Mars and Mercury: Worksheet Please be careful when handling the photographs provided. Make no marks on them and do not write or draw on any pieces of paper laid on top of them. 1. (a) On Figs. 3, 4, and 5, annotate features such as the central peak (or pit), inner terraced walls, raised rim, continuous ejecta, secondary impact craters and any other features you consider important. Figure 3: Image of the Lunar Orbital photograph of the large lunar crater Aristarchus. (Use this image only for annotating features, not for making measurements!) 5

6 1. (b) Figure 4: Image of the Mariner 10 photograph of the hermian crater Brahms. (Use this image only for annotating features, not for making measurements!) 6

7 1. (c) Figure 5: Image of the Viking photograph of the martian crater Naar. (Use this image only for annotating features, not for making measurements) 7

8 1. (d) By considering four types of feature, compare and contrast the craters you have studied in the following table. (You should include brief descriptive comments, not just `yes/no' answers.) Moon Mercury Mars Feature (Aristarchus) (Brahms) (Naar) Central feature? Terracing? Ejecta Blanket? Secondary cratering? 8

9 2. The size (in km) of each image can be calculated if we know f, the focal length of the spacecraft camera lens; x, the size of the original frame; and R, the range of the spacecraft from the planetary surface. From Figure 1, you can see that the true distance across each frame in km, y, is given by y = R x/f. (a) Using the information given in Table 1, determine y (in km) for each picture. (b) Measure the width in mm, l, of each picture; take care to measure l along the correct side, as indicated in Figure 2 for each picture. (c) From y and l, determine the scale for each picture, in km/mm. (d) Using the scale, determine the diameter of each crater in km. Show your working in the space below the table; be careful to quote your nal results for crater diameters to an appropriate level of precision. Quantity: y l Scale Diameter Diameter (km) (mm) (km/mm) (in mm) (in km) Moon (Aristarchus) Mercury (Brahms) Mars (Naar) 9

10 3. (a) Measure the lengths of the shadows, s, cast by the crater rim both inside and outside the crater, and by the central peak. s cast by central peak/pit s cast by crater rim s cast by crater rim (mm) inside crater (mm) outside crater (mm) Lunar Orbiter (Aristarchus) Mariner Image (Brahms) Viking Image (Naar) (b) From this, together with the scale and sun's elevation (the angle above the horizon; α in Table 1), calculate the maximum depth of each crater, the depth below the surrounding surface, and the height of the central peak. Give your results in the following table, showing your working in the space at the bottom of the page. Again, quote your results to a sensible level of precision (e.g., by considering how accurately you can make measurements on the prints). Lunar Orbiter (Aristarchus) Mariner Image (Brahms) Viking Image (Naar) Height of peak/ Height of rim Depth of oor Depth of oor Depth of pit above surface (km) below rim (km) below surface (km) above/below oor (km) ( N.B.: For Aristarchus, the outer slope of the crater wall is in sunlight, so it is impossible to measure the shadow cast by crater rim outside crater. You should, instead, make an estimate of the width of the rim (in mm on the print); you should do this from its appearance all around the crater. Since the outer crater wall is in sunlight, the slope of that wall must be less than, or equal to, the sun's elevation. In order to estimate the height of the rim above the surroundings you should assume that the external slope of the rim is the same as the sun's elevation, α.) 10

Introduction. Background

Introduction. Background Introduction In introducing our research on mars we have asked the question: Is there a correlation between the width of an impact crater and the depth of that crater? This will lead to answering the question: