Module 10: Mercury - Planet of Extremes

1. Module 10: Mercury -Planet of Extremes Activity 2:Mercury & its Evolution

2. Summary: • In this Activity, we will investigate • (a) the surface of Mercury - cratering, lava flows, rupes • (b) ice on Mercury? • (c) formation models for the Moon and Mercury.

3. (a) The Surface of Mercury • Mercury’s surface bears a strong resemblance to that of our Moon, with a couple of noticeable differences • - no dark maria, and the presence of curved cliffs called scarps.

4. Cratering on Mercury Like the Moon, Mercury’ssurface is largely coveredwith craters of all sizes, however the amount of cratering is somewhat less than that on the Moon.

5. seen here on the edgeof the terminator (thedivision between dayand night on the planet). • The largest basin on Mercury is called theCaloris Basin,

6. The Caloris Basin is approx.1300km in diameter, caused by an impact which threw ejecta600 - 800km across the planet.

7. The Caloris Basin is surroundedby rings of mountains which are up to 3km high, and partly filled with lava flows.

8. These finely structuredhills are believed to be ripples from the seismic wavesdue to the Caloris impact. They cover an area which is approximately 100km by 100km.

9. The resulting seismic shock waves would have focussed on the other side of the planet, in a region of “weird” terrain similar to the jumbled terrain on the Moon’s surface opposite its largest impact craters.

10. Lava Flows • Mercury has intercrater plains - broad plains, probably lava flows, which separate groups of craters. Collisions with planetesimals over 3.8 billion years agoprobably weakened the crust, allowing lava to well up fromthe mantle and flood low-lying areas. In particular, the Caloris impact may have single-handedlybeen the cause of many of these lava flows due to itsweakening of Mercury’s crust.

11. Scarps • Unlike our Moon, Mercury has great curved cliffs called scarps (or rupes) which are up to 3km high and 100s of km long. This is a 450 kilometer cliff called the Antoniadi Ridge. It cuts through a large 80 kilometer crater.

12. This is the Santa Maria Rupes.

13. Moon Mercury Core - small, possibly partly molten Core - large, probably solid • To see how they formed, we need to look within the core. Recall that Mercury has a relatively large core: • Scarps are common on Mercury, but not on our Moon.

14. We saw that Mercury is 60% denser than the Moon. This is probably due to its large core being madeof iron. We also saw that small Solar System bodies like the Moon and Mercury would have cooled relatively quicklyafter they differentiated. Metals like iron expand or contract noticeably as thetemperature changes - which is why railway trackssometimes buckle in extreme heat.

15. As Mercury cooled, the iron core would have contracted,and the radius of the whole planet would have shrunkcorrespondingly. As this happened relatively quickly,it is reasonable to expect that the brittle crust would have formed fault lines as it buckled - lines we see today as scarps. Why are there no shrinkage scarps on the Moon and Earth? This is because the Moon only has a small core, and the Earth’s crust is not brittle - it is kept relatively plasticby the heat flow from the interior, and any scarps whichdid form in this fashion would have been wiped out long ago by plate tectonics.

16. The following NASA movie clips show animations of the currently accepted model for the evolution of Mercury The first shows the formation of Mercury out of the Solar Nebula, its early bombardment, lava flows, differentiation and further cratering. The second shows the evolution of its surface after its crust formed, including the formation of scarps. Click on the pictures to start the movie clips. (The movie clips have sound tracks which you can listen toif your computer is equipped with a sound card and speakers.)

17. Click on these imagesin turn to see moviesabout Mercury and its evolution!

18. We will look at models for the earliest stages of the evolution of the Moon and Mercury soon. • Earth Moon Mercury • Condensation • Accretion  • Differentiation • Cratering  • Basin Flooding • (Volcanism) • Plate tectonics • Weathering • (Slow decline)

19. Earth Moon Mercury • Condensation • Accretion  • Differentiation  • Cratering  • Basin Flooding • (Volcanism) • Plate tectonics • Weathering • (Slow decline)

20. Earth Moon Mercury • Condensation • Accretion  • Differentiation  • Cratering   • Basin Flooding • (Volcanism) • Plate tectonics • Weathering • (Slow decline)

21. Earth Moon Mercury • Condensation • Accretion  • Differentiation  • Cratering   • Basin Flooding  • (Volcanism) • Plate tectonics • Weathering • (Slow decline)

22. Earth Moon Mercury • Condensation • Accretion  • Differentiation  • Cratering   • Basin Flooding  • (Volcanism) • Plate tectonics • Weathering • (Slow decline) There is no evidence for plate tectonics on either Mercury or the Moon, ...

23. Earth Moon Mercury • Condensation • Accretion  • Differentiation  • Cratering   • Basin Flooding • (Volcanism) • Plate tectonics • Weathering • (Slow decline) … and the lack ofatmospheres or liquidwater on the Moon or Mercury means that weathering is essentially absent.

24. We will go on to look at models for the formationof Mercury and the Moon soon, but first let’s lookat another (surprising!) similarity between the twobodies:

25. (b) Ice on Mercury? • As we saw in the Activity The Moon and its Evolution, there is recent evidence which suggests that there is water ice present in deep, permanently shaded craters near the lunar poles. Although the Moon and Mercury share many similarities, one might not expect that a planet as close to the Sun as Mercury is could also contain locations cold enough to sustain water ice. However deep, permanently-shadowed craters at Mercury’s polar regions would have temperatures of only about -170°C.

26. Radar mapping of Mercury using the Very Large Array, a radioastronomy interferometer made of 27 dishes and located in New Mexico, USA, as a receiver ... inner dishes of the VLA

27. … has found bright reflective regions on images of the side of Mercury not imaged by Mariner 10, consistent with the presence of water ice near Mercury’s poles:

29. Mariner 10 only imaged about half of Mercury’s surface. With important issues like the presence of water icestill not resolved, another mission to Mercury is planned- the Mercury Orbiter (later renamed MESSENGER: MErcury Surface, Space ENvironment, GEochemistry, and Ranging). MESSENGER was launced on the 3rd of August, 2004. It will travel for nearly 5 years (using gravity assists from two Venus and two Mercury flybys) before being placed in an 8 hour polar orbit, coming as close as200 km above Mercury’s surface over a totalperiod of one Earth year. For Messenger Mission updates, visit:http://messenger.jhuapl.edu/

30. (c) Formation Models for the Moon and Mercury • The Moon and Mercury have much in common, as well as some important differences, and comparisons between the two should help us come to some conclusions about how they were formed.

31. In the Activity Planetary Evolution we saw a modelfor the formation of terrestrial planets in the Solar System, involving gradual accretion of solar nebula debris, differentiation, cratering and so on. This model works quite well in predicting overall similarcomposition for the interior of the Earth, Venus and Mars,but not so for Mercury and the Moon. In particular, the Moon contains too small a percentage of metals, and Mercury contains too large a percentage of metals.

32. Formation Models for the Moon • Analysis of Apollo rock samples has shown that the Moon is similar to the Earth in many ways, but is significantly different in its lack of metals and volatile compounds. These Apollo findings have been a considerable challenge to astronomers trying to form theories of how the Moon formed. We will briefly review the most popular theories:

33. (i) The Fission Theory • This theory claims that the Moon was once part of a young, rapidly-spinning Earth. Tidal forces due to the Sun made it break into two parts, with the Moon forming mainly from material thrown off from the Earth’s crust and mantle. Click here to see an animation illustrating the Fission Theory

34. However although the Moon’s composition does bear some similarities to that of the Earth’s mantle and crust, significant differences exist. • For example, the Moon • (1) has fewer volatile elements • (2) has the wrong nickel to iron and magnesium to silicon ratios, and • (3) has twice as much aluminium and calcium as does the Earth. Also, it is not clear why the Earth would have been spinning so fast to start off with, and, more importantly, where all that excess angular momentum has gone to!

35. (ii) The Binary Accretion (or double planet) Theory • This theory (also sometimes called the Sister Theory) claims that the Earth and Moon formed together as a double planet system from the same part of the Solar Nebula. • Again, this theory has a fundamental difficulty in explaining why, if the Earth and Moon formed from the same material, they now have important differences in chemical composition and chemistry. Click here to see an animation illustrating the Binary Accretion Theory

36. (iii) The Capture Theory • This theory claims that the Moon formed elsewhere in the Solar System - for example, a little inside the orbit of Mercury where the local condensation temperatures would have given it approximately the right composition - and that a gravitational interaction with Mercury would have “boosted” it out of its original orbit and brought it close enough to Earth to be gravitationally captured. Click here to see an animation illustrating the Capture Theory

37. This theory, once very popular, has fallen out of favour because: • (1) it emphasises the differences between the chemical composition of the Moon and Earth, but fails to explain many significant similarities • (2) the Moon would have been travelling too fast to becaptured by the Earth without being tidally ripped apartunless another large (unknown) nearby object was involved, • (3) if it was captured in this way, one would expect it to have taken up a highly eccentric orbit, and • (4) it relies on a sequence of unlikely events.

38. (iv) The Large-Impact Theory • Developed in the 1980s in response to the Apollo data, this theory claims that a large planetesimal (perhaps as large as Mars) impacted the Earth, largely merging with the Earth but throwing off a disk of debris in the process. The debris, mainly composed of iron-deficient mantle and crust material, would have gradually aggregated together to form the Moon. Click here to see an animation illustrating the Impact Theory

39. This theory has several advantages: • (1) The ejected material would have been low in metal content as it originated in the crusts and mantles of the two colliding bodies. It would initially have been very hot, and volatiles would have evaporated off, explaining the relative lack of volatile compounds on the Moon. • (2) To eject sufficient material to form the Moon, the collision had to occur at a steep angle - not head-on. Such a collision would have “spun-up” the resulting Earth - Moon system, explaining its high angular momentum. • (3) As the Moon is composed of crust and mantle material, this explains the many similarities between the Moon and Earths’ chemical compositions.

40. There do not appear to be any fundamental problems with the Large-Impact Theory, and supercomputer simulations of such a collision support it. • However it has to stand up to testing with data from future lunar missions: it is always easier to form a theory after the experimental data is gathered and analysed!

41. One of the reasons astronomers were initially reluctant to support such a theory was because it relied on an extremely violent collision in the early Solar System - the size calculated for the impacting body, approx. one-tenth the mass of Earth, is almost the largest impact that the Earth could have suffered without being totally broken apart. However now astronomers can use the power of supercomputers to simulate events like these. Some simulations suggest that as many as 100 planetesimalslarger than the Moon were loose in the inner solar system, as well as many more smaller surviving planetesimals.

42. If so, the early Solar System’s history would have been marked by many collisions, and near encounters, and the cratering history of the terrestrial planets contains a number of the scars of giant impacts to support this.

43. Formation Models for Mercury • Superficially, the accretion model which we have used to describe the formation of the terrestrial planets should fit Mercury. However as we have seen, Mercury has a much higher percentage of metal as compared to rocky constituents than would be expected from this model. It is tempting to suggest that Mercury and the Moon might have one more thing in common - the effects of giant impacts in the early Solar System.

44. Early in Mercury’s history, a giant impact might have stripped off much of its lower-density rocky crust and mantle material. The remaining denser core material could have attracted some (but not most) of the debris to reform a thin mantle and crust, leaving Mercury rich in metals but short on rocky mantle material. Until further missions visit Mercury, we can only speculate. Figure 10.16 in the textbook ‘Universe’ illustratesa supercomputer simulation of the formation of present-day Mercury by collisional stripping.

45. In the next Module, we will investigate our nearest planetary neighbour, Venus.

46. Image Credits • NASA: Mariner 10 mosaic of one hemisphere of Mercury • http://nssdc.gsfc.nasa.gov/image/planetary/mercury/mercuryglobe1.jpg • Mosaic of the Bach area of Mercury • http://nssdc.gsfc.nasa.gov/image/planetary/mercury/bach.jpg • Mosaic of the Caloris Basin and surrounding area • http://nssdc.gsfc.nasa.gov/image/planetary/mercury/caloris.jpg • Hills of Mercury • http://learn.jpl.nasa.gov/projectspacef/ME_03.jpg • Antoniadi Ridge • http://learn.jpl.nasa.gov/projectspacef/ME_08.jpg • Large Faults on Mercury (Santa Maria Rupes) • http://learn.jpl.nasa.gov/projectspacef/ME_07.jpg

47. Image Credits • NASA: Mercury • http://pds.jpl.nasa.gov/planets/welcome/thumb/merglobe.gif • Earth http://pds.jpl.nasa.gov/planets/welcome/earth.htm • Three-filter color image of the Moon (Galileo)http://nssdc.gsfc.nasa.gov/image/planetary/moon/gal_moon_color.jpg • Mercury evolution animations (Space Movie Archive) • http://graffiti.u-bordeaux.fr/MAPBX/roussel/anim-e.html • Goldstone/VLA radar maps of Mercury • http://wireless.jpl.nasa.gov/RADAR/mercvla.html • Mercury Orbiterhttp://umbra.nascom.nasa.gov/SEC/secr/missions/meo.html

48. Now return to the Module home page, and read more about Mercury and its Evolution in the Textbook Readings. Hit the Esc key (escape) to return to the Module 10 Home Page