Module 10: Mercury - Planet of Extremes

1. Module 10: Mercury -Planet of Extremes Activity 1:Observing Mercury

2. Summary: • In this Activity, we will investigate • (a) Mercury’s vital statistics, and • (b) Mercury missions - Mariner 10 - transfer orbits & gravity assists.

3. Then in the next Activity we will look briefly at the most likely explanations of how Mercury and the Moon were formed. We will leave the Earth - Moon system for now, and investigate Mercury, a planet with a strong resemblance to the Moon.

4. (a) Mercury’s Vital Statistics Mercury follows a rather eccentric orbit (e = 0.2) nearthe Sun. It can be seen just after the Sun sets, orjust before the Sun rises, and shows phases, like Venusand our Moon. First we’ll compare Mercury’s vital statistics with those of the Moon and Earth:

5. mantle core mantle mantle crust crust Core - small, possibly partly molten Core - large, probably solid crust D 0.27 D 0.38 D M = 0.01 M M = M M = 0.056 M Moon Earth Mercury

6. Moon Earth Mercury Av. Distancefrom Sun 0.39 AU 1 AU 1 AU Being on a eccentric orbit, Mercury’s distance from the Sun varies between 46 million km and 70 million km. Mercury’s proximity to the Sun leads to temperature extremes, as we will see. Careful observations of Mercury’s orbit in the 19th century showed that something was not quite right, and people began searching for an unseen planet dubbed Vulcan. It turned out, however, that the ‘defects’ in Mercury’s orbit were due to its proximity to the Sun - which distort nearby space as predicted by Einstein’s General Relativity theory. Predicting and detecting this effect was one of the first great tests of General Relativity.

7. Moon Earth Mercury Av. Distancefrom Sun 0.39 AU 1 AU 1 AU 1 y  Length of “Year” 1 y  88 d  As we saw in the Activity Solar System Orbits, Mercury, being close to the Sun, has a high orbitalvelocity. The combination of a small orbit and a high velocity makes its orbital period (its “year”) very short indeed.

8. Moon Earth Mercury Av. Distancefrom Sun 0.39 AU 1 AU 1 AU 1 y  Length of “Year” 1 y  88 d  29.5 d 176 d Length of solar day 1 d The rotation period of Mercury (time it takes to spin on its axis) is 59 days, while the orbital period (time it takes to orbit the Sun) is 88 days. Thus there is a 3:2 relationship between the spin and orbit, which leads to a solar day of 176 days - twice as long as its year! To find out more, follow this link.

9. Moon Earth Mercury Av. Distancefrom Sun 0.39 AU 1 AU 1 AU 1 y  Length of “Year” 1 y  88 d  29.5 d 176 d Length of solar day 1 d Inclinationof axis 1.3° 23.5° 1° Like the Moon, Mercury does not seasonal effects since there is very little tilt of its rotation axis to the ecliptic .

10. Moon Earth Mercury Av. Distancefrom Sun 0.39 AU 1 AU 1 AU 1 y  Length of “Year” 1 y  88 d  29.5 d 176 d Length of solar day 1 d Inclinationof axis 1.3° 23.5° 1° Accelerationdue to gravity 0.17g 1.0 g 0.38g

11. The acceleration due to gravity on the surface of Mercuryis considerably greater than that on the Moon (0.38 g compared to 0.17 g). This is largely because Mercury is 60% denser than the Moon - a number we can calculate from its mass* and radius - which is why we conclude that it probably has a large metal core. * Follow this link to see how we estimate the mass of a planet like Mercury.

12. Moon Earth Mercury av. albedo 0.10 0.39 0.12 Mercury’s surface reflects very little light - similar to the Moon

13. Moon Earth Mercury av. albedo 0.10 0.39 0.12 Almost none atmosphere No permanent atmosphere, “borrows” solarwind particles 78% N2 20%O20.03% CO2 ~2% H2O Again, like the Moon, Mercury has almost no atmosphere.Hydrogen and helium particles from the solar wind brieflylinger around Mercury. Sodium and potassium gaseshave been detected on the side of Mercury facing the Sun,“baked out” of the rocks by the Sun’s radiation.

14. Moon Earth Mercury av. albedo 0.10 0.39 0.12 none atmosphere No permanent atmosphere, “borrows” solarwind particles 78% N2 20%O20.03% CO2 ~2% H2O surface temperature -170°C +130°C - 50°C  + 50°C - 180°C  + 430°C With its proximity to the Sun and lack of an insulating atmosphere, Mercury’s surface swings between extremes of temperature.

15. Mercury, the planet closest the Sun, receives sunlight which is about seven times more intense than does Earth. Added to this, the long solar days on Mercury means that the daytime temperatures reach about 430°C, which is hot enough to melt lead. In the long Mercury night, temperatures at the equatorplummet to below -180°C, which is cold enough for carbon dioxide to freeze!

16. (b) Observing Mercury - Mariner 10 • We do not know a great deal about Mercury - the last space mission which observed Mercury in any detail was Mariner 10, launched way back in November 1973.

17. Transfer Orbits & Gravity Assists • Mariner 10 was the first spacecraft to use the gravitational pull of one planet (Venus) to reach another (Mercury). This is an example of the technique called a gravity assist - usually used to speed a spacecraft up, but in this case, needed to slow Mariner 10 down.

18. To see why this is necessary, imagine again that we are sending the Swinburne executive spaceship from Earth to Mercury, using as little fuel as possible.

19. The most efficient trajectoryfor the spaceship is tocoast along part of an ellipse, called a transfer ellipse. aphelion * Spaceship Earth The aphelion(point furthestfrom the Sun)of the ellipseis at thepoint wherethe spaceshipleaves Earthorbit. Sun Mercury * coast means engines off

20. aphelion Spaceship Earth The perihelion(point nearestto the Sun)of the ellipseis at thepoint where the spaceshipmeets Mercury. Sun Mercury perihelion

21. aphelion To get the spaceship to follow the transfer ellipseinstead of Earth’salmost circular orbit, the spaceshiphas to be slowed downat aphelion. Spaceship Earth Sun Mercury perihelion

22. aphelion This can be achievedby launching the spaceship in the oppositedirection to the Earth’srotation,and then making fineadjustmentsby firing spaceship rockets to slow itdown even more. Spaceship Earth Sun Mercury perihelion

23. aphelion As the spaceship reachesperihelion, it speedsup, and, unlessslowed down,would betravelling toofast to be able to observe Mercury for any length oftime. Spaceship Earth Sun Mercury perihelion

24. aphelion Again, spaceship rocketsmust be fired in theopposite directionto the spaceship’smotion to slowit down even more. Spaceship Earth Sun Mercury perihelion

25. Firing rockets for any length of time at Mercury’sorbit means that a considerable amountof fuel needs tobe carried,increasing the‘payload’(total weight) aphelion Spaceship Earth Sun - whichincreases theexpense of placing the spaceship in Earthorbit in the first place. Mercury perihelion

26. Our spaceship can use another method to slow down, while still on the way to Mercury: a “gravity assist”. aphelion Spaceship Earth Sun Mercury perihelion

27. If the transfer orbit is timed so thatthe spaceship comes upclose behind Venusin its orbit,then the effectof passing inand out of Venus’s gravitationalfield is to slowthe spaceshipdown. aphelion Spaceship Venus Earth Sun Mercury perihelion

28. This means that less fuelis needed to slow thespaceship downfurther at perihelion. aphelion Spaceship Venus Earth Sun Mercury perihelion

29. Our spaceship’s encounter with Venus is a little like a car colliding into the back of a van - the car is definitely slowed down. (The aim of course is not to collide the spaceship withthe surface of Venus! Its encounter with Venus’s gravitational field has the required effect of slowingit down without destroying it.)

30. The orbit of Mariner 10 was more complicated than that of our hypothetical spaceship, for example, Mariner 10 made three flybys of Mercury- to closest approach distances of 704km, 47,000km and 327km, however the basic principles of the orbit were similar to those in our example above.

31. In the next Activity we will study what information we have about Mercury’s surface and evolution, gleaned largely from the Mariner 10 flybys. Note though, that Mariner 10 only imaged approximatelyhalf the surface of Mercury - we do not know what the other half looks like.

32. Image Credits • NASA: Mariner 10 mosaic of one hemisphere of Mercury • http://nssdc.gsfc.nasa.gov/image/planetary/mercury/mercuryglobe1.jpg • Mariner 10 spacecraft • http://nssdc.gsfc.nasa.gov/thumbnail/spacecraft/73-085A.gif • Goldstone/VLA Maps of Mercury • http://wireless.jpl.nasa.gov/RADAR/mercvla.html

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

35. A day on Mercury To understand the relationship between Mercury’sday and year, we need to remember a few definitions: • A planet’s “year” (or orbital period) is the time it takes to make one complete orbit around the Sun. • A planet’s “sidereal day” is the time it takes to rotate once, relative to the fixed stars - e.g. the time from when a particular star is at the zenith, to when it is next back in that position. • A planet’s “solar day” is the time it takes to rotate once, relative to the Sun - e.g. the time from when the Sun is at the zenith, to when it is next back in that position. (If you want to revise these concepts, look again at the Activity Night and Day)

36. Welcome to sunny Mercury! In order to understand how Mercury’s solar and siderealdays compare to its year, we will imagine that the Swinburneexecutive branch is travelling on a fact-finding tour to Mercury in the Swinburne executive spaceship, with a view to establishing a campus there.

37. When the Swinburne spaceship touches down, the Sun happens to be directly overhead ... Sun Spacecraft Mercury (not to scale!)

38. … and so it is midday on Mercury at the landing site. midday

39. We’ll follow what happens at the landing site over acomplete solar day, also keeping track of how manysidereal days pass. We will see that, on Mercury: 1 Mercury solar day = 2 complete Mercury years = 176 Earth days, and 1 Mercury sidereal day = 1/3 of a Mercury solar day = about 58 Earth days (actually 58.65)

40. Remember that solar days have to do with wherethe Sun is in relation to the zenith, Whereas sidereal days have to do with where the starsare in relation to the zenith. It may help if you first go through the following animation,focussing particularly on the length of the solar day, then repeat the animation and focus on the length of the sidereal day.

42. midday Sun Spacecraft Mercury (not to scale!) Earth day: 0

43. early afternoon (not to scale!) Earth day: 14

44. late afternoon (not to scale!) Half a Mercurysidereal day Earth day: 29

45. sunset (not to scale!) Earth day: 44

46. early night (not to scale!) One complete Mercurysidereal day Earth day: 58

47. Before midnight (not to scale!) Earth day: 73

48. One half a Mercury solar day 1 Mercury year later midnight (not to scale!) One and a half Mercurysidereal days Earth day: 88

49. late night (not to scale!) Earth day: 102

50. before sunrise (not to scale!) Two complete Mercurysidereal days Earth day: 117