Sun-Scorched Mercury

1. Sun-Scorched Mercury

2. 5.430 g/cm3 MESSENGER's suite of instruments have provided insight into the mineral makeup of the surface terrain and detected ultraviolet emissions from sodium, calcium and hydrogen in Mercury's exosphere.

3. Physical Properties

7. Guiding Questions • What makes Mercury such a difficult planet to see? • What is unique about Mercury’s rotation? • How do the surface features on Mercury differ from those on the Moon? • Is Mercury’s internal structure more like that of the Earth or the Moon?

8. 88 days

9. Orbital Properties Phases of Mercury can be seen best when Mercury is at its maximum elongation:

10. Earth-based optical observations of Mercuryare difficult • At its greatest eastern and western elongation, Mercury is never more than 28° from the sun • It can be seen for only brief periods just after sunset or before sunrise

11. Greatest Elongations Apparition Date Elongation Magnitude Morning 2012 Dec 4 20.6°W -0.3 Evening 2013 Feb 16 18.1°E -0.2 Morning 2013 Mar 31 27.8°W +0.5 Evening 2013 Jun 12 24.3°E +0.7 Morning 2013 Jul 30 19.6°W +0.4 Evening 2013 Oct 9 25.3°E +0.2 Morning 2013 Nov 18 19.5°W -0.3 Evening 2014 Jan 31 18.4°E -0.3 Morning 2014 Mar 14 27.6°W +0.4 Evening 2014 May 25 22.7°E +0.7 Morning 2014 Jul 12 20.9°W +0.6 Evening 2014 Sep 21 26.4°E +0.3 Morning 2014 Nov 1 18.7°W -0.3

12. Solar Transit Transits occur about twelve times a century when the sun, Earth and Mercury are aligned There will be a transit on May 9, 2016

13. Best Earth-based Views of Mercury Difficulties observing Mercury from Earth led early astronomers to incorrectly decide that Mercury always kept the same face towards the sun in synchronous orbit Note phases like the moon

14. Rotation and Revolution Mercury was long thought to be tidally locked to the Sun; measurements in 1965 showed this to be false. Like Earth’s moon (tidally locked to revolution around Earth), Mercury’s rotation has been altered by the sun’s tidal forces, but not completely tidally locked: Revolution period = 3/2 times rotation period Revolution: ≈ 88 days Rotation: ≈ 59 days

15. Rotation of Mercury Because of the highly elliptical orbit, during perihelion the sun appears to stop in the sky, move backwards (for several Earth days), and then return to normal motion. 3:2 Spin-Orbit Coupling

16. Strong tidal effects, Mercury’s slightly elongated shape and its very eccentric orbit cause this strange 3-to-2 orbit • A “day” of solar light on Mercury would be 88 earth days

17. Images from Mariner 10 reveal Mercury’s heavilycratered surface • Most of our detailed information about Mercury’s surface is from this fly-by mission in 1974/1975. • Mariner only saw one side of the planet. • Presently, Messenger is in orbit. • Volcanics at surface are mostly basaltic • Silicate rocks are low in Fe • BepiColombo mission, scheduled to launch to Mercury in 2014.

18. Mercury Very similar to Earth’s moon in several ways: • Small; no atmosphere • lowlands flooded by ancient lava flows • heavily cratered surfaces (Less dense cratering than moon) • Gently rolling plains • Scarps • No evidence of tectonics View from Earth

19. Mercury Note the lava flows. Note the density of craters. Note the similarity in color.

20. Note how much more densely the craters occur on the moon’s surface.

21. At the terminator, shadows give us crater wall heights.

22. The previously unseen side of Mercury. (From Messenger)

23. Note how much more densely the craters occur on the moon’s surface.

24. Moon vs. Mercury 1) There are few maria on Mercury and they are small. No large impact era like the Moon. Therefore, Mercury must have cooled faster. 2) Cratering is less heavy, more plain region between craters. Due to the higher surface gravity on Mercury. This means impacts did not throw debris as far, fewer secondary craters and more concentrated around primary crater. 3) Long scarps or wrinkles are found on the crust and the tops of craters (i.e. after cratering epoch). After Mercury cooled, its crust solidified first. Scarps are believed to have been formed as Mercury cooled.

25. The Plains of Mercury No large maria, but intercrater plains: Marked by smaller craters (< 15 km) and secondary impacts Smooth plains: Even younger than intercrater plains

26. The Caloris Basin is evidence of a large impact • ~960 miles in diameter (from new estimates of Messenger images)

27. Caloris Basin = 1300 km

29. The sprawling Caloris basin on Mercury is one of the solar system's largest impact basins. Created during the early history of the solar system by the impact of a large asteroid-sized body, the basin spans about 1,500 kilometers and is seen in yellowish hues in this enhanced color mosaic. The image data is from the January 14th flyby of the MESSENGER spacecraft, captured with the MDIS instrument. Orange splotches around the basin's perimeter are now thought to be volcanic vents, new evidence that Mercury's smooth plains are indeed lava flows. Other discoveries at Mercury by NASA's MESSENGER mission include evidence that Mercury, like planet Earth, has a global magnetic field generated by a dynamo process in its large core, and that Mercury's surface has contracted significantly as its core cooled.(APOD,Credit: NASA, Johns Hopkins Univ. APL, Arizona State U., CIW)

30. Caloris Basin Floor

31. “The spider" appears to be an impact crater surrounded by more than 50 cracks in the surface radiating from its center. It is on the Caloris Basin.

32. The seismic waves from the impact that caused the Caloris Basin caused this deformation on the opposite side of Mercury. This terrain covers 500,000 km2 (twice the size of Wyoming)!

33. The Caloris Impact Caloris Basin, very large impact feature; weird terrain on opposite side of planet:

34. Orientale Since studying Caloris, we have come to understand that the same process has been at work elsewhere. This is the large crater basin Orientale on the far side of the Moon. It too has jumbled terrain opposite it on the Moon.

35. Radar of Mercury Dominated by ancient lava flows and heavy meteorite bombardment. Radar image suggests icy polar cap.

36. Mercury's North Pole. These unusual images of what looks likes ice have sparked the imagination of scientists. It has been suggested that a tiny flow of ice from comets and meteorites could be cold-trapped in these polar deposits over billions of years, or that the polar deposits consist of sulfur that has seeped from minerals in the surface rocks over the eons.

37. Scarps are cliffs This one is more than a km high They probably formed as the planet cooled and shrank

38. Scarps Some distinctive features: Scarp (cliff), several hundred km long and up to 3 km high

40. January 14, 2008. Part of an old, large crater occupies most of the lower left portion of the frame. An arrangement of ridges and cliffs in the shape of a "Y" crosses the crater’s floor. The shadows defining the ridges are cast on the floor of the crater by the Sun shining from the right, indicating a descending stair-step of plains. The main, right-hand branch of the "Y" crosses the crater floor, the crater rim, and continues off the top edge of the picture; it appears to be a classic “lobate scarp” (irregularly shaped cliff) common in all areas of Mercury imaged so far. In contrast, the branch of the Y to the left ends at the crater rim and is restricted to the floor of the crater. Both it and the lighter-colored ridge that extends downward from it resemble “wrinkle ridges” that are common on the large volcanic plains, or "maria," on the Moon.

41. Mercury has an iron core and a surprisingmagnetic field • Most iron-rich planet in the solar system with a core that is 85% of the diameter (New # from Messenger data, March 2012.) • The earth’s core is 55% of its diameter and the moon’s core is 20% of its diameter • Highest uncompressed density for the planets • Weak magnetic field indicating part of the core is liquid • Magnetic field causes a magnetosphere similar to Earth’s but weaker

42. Interiors Mercury is much denser (over 60%) than the Moon, and has a weak magnetic field – not well understood!

43. 85% 61%

44. Mercury may have a very • Different core: • Out solid iron sulfide layer • Deeper liquid iron layer • Possible solid Fe core

47. If the core is so big, why does Earth have a higher density? • We failed to take account of the mass of the object. • What happens is that more massive objects have stronger gravities. As a result, more massive objects get more compressed than less massive objects. • This compression (decrease in the size of the object), means that the density of the object will be greater. Thus, even for objects which have the same composition, if one is more massive, then the objects will have different densities. • This effect needs to be accounted for when using densities to determine chemical compositions. • A quantity known as the Uncompressed Density is defined which is free of this mass dependence. • The uncompressed densities of Mercury 5.3 g/cm3 Venus 3.95 g/cm3 Earth 4.03 g/cm3 Moon 3.3 g/cm3 Mars 3.71 g/cm3 • Mercury is significantly denser than the Earth and thus composed of heavier elements; Mercury has significantly more iron and nickel than the Earth.

48. How do you get such a big core? Hypotheses: • Close to Sun, so more metals • New Sun blew upper portions off • Impact exploded upper portions off