MESSENGER's Discoveries at Mercury

Transcription

1 MESSENGER's Discoveries at Mercury David T. Blewett Planetary Exploration Group Johns Hopkins University Applied Physics Laboratory CAS Visiting International Scholar NAOC, Beijing July 8, 2015

2 Mercury: Planetary Oddball Smallest planet; has the highest density. Most eccentric orbit; is in a 3:2 spin:orbit resonance. Has an actively generated internal magnetic field similar to Earth's (Venus & Mars do not). Closest to the Sun, yet has ice in polar craters. Surface composition not known. Complicated surface-exospheremagnetosphere interactions. MESSENGER enhanced-color composite A lonely planet: only visited by one previous spacecraft (three Mariner 10 flybys in ). Moon, Venus, Mars, Jupiter, Saturn and asteroid Eros all had orbiters before Mercury. 2

3 A NASA "Discovery" Mission Discovery is NASA's smallest class of planetary mission. Proposal led by a scientific principal investigator MESSENGER PI is Sean Solomon of Columbia University's Lamont-Doherty Earth Observatory Selected competitively from a group of other proposals - could be for any type of planetary mission. MESSENGER was built and was operated by the Johns Hopkins University Applied Physics Lab (APL). 3

4 I was selected as a MESSENGER Participating Scientist in Competitive selection based on a research proposal submitted to NASA. Participating Scientists join the original group of Co- Investigators as full members of the science team. Funding covered the cruise and primary orbit phases of the mission, was extended for the two MESSENGER Extended Missions. 4

5 MESSENGER Initial Questions Why is Mercury so dense? What is the geologic history of Mercury? What is the structure of Mercury's core? What is the nature of Mercury's magnetic field? What are the unusual materials at Mercury's poles? What is the composition of Mercury s thin atmosphere? 5

6 Instruments MDIS MASCS MLA MAG XRS GRS NS FIPS EPPS 6

7 Mercury Dual Imaging System (MDIS): Consists of wide-angle and narrowangle framing cameras. WAC has 11 color filters covering nm. NAC is monochrome at 7 higher spatial resolution. 7

8 MASCS Mercury Atmosphere and Surface Composition Spectrometer Measure the abundances of atmospheric species, as well as collect reflectance spectra of the surface. Spot spectrometer (non-imaging). Two sensors with a common telescope: Ultraviolet and Visible Spectrometer (UVVS) nm Visible and Infrared Spectrograph (VIRS) nm ( µm) 8

9 MESSENGER: The Spacecraft Ceramic cloth sunshade provides thermal control. Powered by special high-temperature solar panels. Getting ready for vibration testing at APL 9

10 MESSENGER: The Journey On 17 March 2011, after 6.6 years of cruise, six planetary flybys and 4.9 billion miles, MESSENGER became the first spacecraft ever to orbit the planet Mercury. 10

11 MESSENGER: The Orbit Highly elliptical orbit. 12-hr period in primary mission. 8-hr in extended mission. Lowest altitudes are over the northern hemisphere. Lowest altitude ~ km, but at end of mission was as low as 15 km. 11

12 MESSENGER: The Dates Milestones ü Launch 3 Aug 2004 ü Earth Flyby 2 Aug 2005 ü Venus Flyby 1 24 Oct 2006 ü Venus Flyby 2 5 Jun 2007 ü Mercury Flyby 1 14 Jan 2008 ü Mercury Flyby 2 6 Oct 2008 ü Mercury Flyby 3 29 Sep 2009 ü Mercury Orbit Insertion 18 Mar 2011 ü End of Orbital Operations (primary) 17 Mar 2012 End of Orbital Ops (extended 1) 17 Mar 2013 Extended mission 2 ended April 30,

13 Mercury Before MESSENGER Mariner 10 image coverage About 40-45% of the surface 13

14 MESSENGER Imaging Campaigns Monochrome base map: Lighting conditions favorable for seeing surface morphology and texture, 200 m/pixel 40 km 14

15 MESSENGER Imaging Campaigns Color basemap coverage: 8 filters between 430 and 1000 nm, 1 km/pixel 250 km nm as RGB Enhanced color composite All data from NASA planetary missions are released to the global public through the Planetary Data System. 15

16 MESSENGER Imaging Campaigns Stereo Other campaigns targeted color 211 m/pix Seuss crater High-resolution targets of opportunity Monochrome and color Monochrome resolution as good as 10 m/pixel Three-color map Higher-res. than 8-color basemap, N hemisphere only Limb images, for shape & topography High incidence angle (Sun very low on horizon) Strong shadowing reveals subtle topographic features 60 km nm as RGB Ride-along, when other instruments are controlling pointing Satellite and Vulcanoid searches North and South pole, determine extent of permanent shadow Long-exposure imaging within polar shadows, search for ice 40 km north pole 84 m/pix Chesterton crater 16

17 Mercury Crater Naming International Astronomical Union theme for naming craters on Mercury Prominent people in the humanities Artists, poets, writers, composers, etc. e.g., Beethoven, Degas, Tolstoj, Bronte Xiao Zhao 24-km diam 18 Chinese crater names 15 named in Mariner 10 era: Li Po, Wang Meng, Lu Hsun 2 named by MESSENGER team: Xiao Zhao (painter ), Qi Baishi (painter ) Qi Baishi camels Qi Baishi 15-km diam 996, 748, 433 nm as R-G-B EW EW G 17

18 MESSENGER: Geophysics Internal Structure Solid liquid boundary is at ~410 km depth. Shallower than in earlier models. Solid FeS "anticrust" places upper bound on temperatures at core mantle boundary. Implications for core dynamics and external magnetic field. Internal structure consistent with strongly reducing conditions. McKinnon "Perspective" on Smith et al. (2012) and Zuber et al. (2012) Science. 18

19 MESSENGER Geological Discoveries Widespread Volcanism Caloris pyroclastic deposit: explosive volcanism Northern Volcanic Plains: extensive flooding by highly fluid lava flows Head et al. (2008) Head et al. (2011) Science 19

20 MESSENGER: Geological Discoveries Extensive Tectonism: Scarps Many new lobate fault scarps discovered This one is over 600 km long - only part is shown here. "Beagle Rupes": Width of image is 200 km. Sun shining from the right. 20

21 MESSENGER: Geological Discoveries Extensive Tectonism: Scarps Caused by compressive forces in the crust. One section of crust thrust over top of another. Lobate scarp schematic: thrust fault Global contraction due to cooling of the interior was even more extensive than had been thought. Planet may have contracted by as much as 7 km in radius. 21

22 MESSENGER: Geological Discoveries Major Surprise: Tectonic valleys Near the center of Caloris Hundreds of troughs (graben) radiating out from a central point Impact crater probably unrelated Nothing like this on the Moon, or on the Mariner 10 part of Mercury 40 km 22

23 MESSENGER: Geological Discoveries Major Surprise: Tectonic valleys Form by extensional forces in the crust: - Normal faulting - Graben Elsewhere, Mercury s surface is dominated by contraction Local extension due to loading by volcanic basin fill 23

24 MESSENGER: Geological Discoveries New landform: Hollows 5 km In flyby images, certain craters and basins were found to have unusual, bright material with blue color. Once in orbit, high-resolution images showed that these areas consist of irregular, shallow depressions with bright interiors and halos. Not volcanic morphology and colors are different. Extremely fresh appearance must be actively forming today. Sublimation-like process. Blewett et al. (2011) Science. 24

25 MESSENGER: Geological Discoveries New landform: Hollows I can give a separate seminar talk on hollows. 10 km Blewett et al. (2011 Science) and (2013 JGR-Planets) 25

26 Reflectance Spectroscopy Mercury s spectrum lacks diagnostic absorption bands in the visible to near-ir. Variation is in albedo and slope. No mafic mineral Fe 2+ absorption near 1 µm like the Moon, some asteroids. Fe abundance in silicates <~1%. MESSENGER VIRS spectra Izenberg et al., 2014 Icarus Average Mercury 26

27 Albedo: Why is Mercury dark? Fresh materials on Mercury have reflectance 30-50% lower than fresh materials on the Moon. Iron causes darkening on the Moon (Fe +2 in minerals, and metallic Fe 0 from space weathering). Iron content of lunar highlands ~3%, up to ~14% in maria. Mercury surface has <~1-2% Fe. Darkening agent: micro-nanophase Fe 0 ; reduced minerals: sulfides (Ca- MgS), carbon; combination? Average Mercury Denevi & Robinson, 2008 Icarus 27

28 Composition of Mercury Surface elemental composition depends on starting materials and history. Critical for probing Mercury s origin and geological evolution. MESSENGER has three instruments for measuring surface elemental composition 28

29 Composition of Mercury Gamma-ray / Neutron Spectrometer (GRS/NS) measures surface composition by detecting cosmic-ray- and radioactivity-induced γ- rays and neutrons XRS measures surface composition by detecting solarinduced X-ray fluorescence 29

30 Mg/Si (wt. ratio) Major elements on Mercury Peridotites Mare basalts Cont. crust Komatiites Oceanic basalts Moon rocks Earth rocks Mercury (Nittler et al. 2011) Lunar Highlands Al/Si (wt. ratio) Ca/Si (wt. ratio) Mare basalts Komatiites Cont. crust Oceanic basalts Highlands 0.0 Peridotites Al/Si (wt. ratio) XRS flare data indicate Mercury is Mg-rich, Al- and Ca-poor, relative to Earth and the Moon's surface 30

31 Mg/Si (wt. ratio) Major elements on Mercury Peridotites Mare basalts Cont. crust Komatiites Oceanic basalts Moon rocks Earth rocks Mercury (Nittler et al. 2011) No lunar-like plagioclase flotation crust Lunar Highlands Al/Si (wt. ratio) Ca/Si (wt. ratio) Mare basalts Komatiites Cont. crust Oceanic basalts Highlands 0.0 Peridotites Al/Si (wt. ratio) XRS flare data indicate Mercury is Mg-rich, Al- and Ca-poor, relative to Earth and the Moon's surface 31

32 High Sulfur, Low Iron 0.3 XRS GRS Fe (wt%) Ca/Si XRS GRS 0.1 Number S/Si S ~1-4 wt.% S strongly correlated with Ca Presence of CaS? Fe/Si Surface Fe very low (<2 wt.%) despite large Fe core 32

33 High Sulfur, Low Iron 0.3 XRS GRS Fe (wt%) Ca/Si XRS GRS 0.1 Number S/Si S ~1-4 wt.% S strongly correlated with Ca Presence of CaS? Fe/Si Surface Fe very low (<2 wt.%) despite large Fe core Composition indicates Mercury starting materials were more chemically reduced than the other terrestrial planets 33

34 Major-Element Heterogeneity Nittler et al. (2013) Weider et al. ( ); smooth plains outlines from Denevi et al. (2013) 34

35 Major-Element Heterogeneity Evidence that smooth plains more basaltic (higher Al, Na, with lower Mg) than older intercrater plains/ heavily cratered terrain Nittler et al. (2013) Weider et al. ( ); smooth plains outlines from Denevi et al (2013) 35

36 Major-Element Heterogeneity Nittler et al. (2013) Weider et al. ( ); smooth plains outlines from Denevi et al (2013) 36

37 Major-Element Heterogeneity Coherent region with highest Mg, Ca, S, low Al Nittler et al. (2013) Weider et al. ( ); smooth plains outlines from Denevi et al (2013) 37

38 Mg/Si Crustal Thickness (Smith et al. 2012) 38

39 Mg/Si Mg-rich region corresponds to thinner crust ancient impact basin? Crustal Thickness (Smith et al. 2012) 39

40 Radioactive elements: K, Th on Mercury Mercury similar to Mars, Earth K (moderately volatile) ~1000 ppm Th (refractory) ~100 ppb K/Th ~8000±3000 No volatile depletion as seen in Moon (Peplowski et al., 2011, 2012) 40

41 MESSENGER: Surface Geochemistry Volatile Element Enrichment Potassium (K) is Gamma-Ray Spectrometer (GRS) moderately volatile Thorium (Th) is refractory Mercury not depleted in potassium relative to thorium. Peplowski et al. (2011) Science. Moon is depleted: formed in hot giant impact 41

42 Na, Cl on Mercury Mercury similar to Mars, Sun Na ~3 wt% on average (Evans et al. 2012; Peplowski et al. 2013) Cl ~0.14 wt% on average (Evans et al Icarus) Both elements increase in north polar latitudes Earth 42

43 Formation of Mercury Terrestrial planets shared a common formation process: accretion Dust -> Rocks -> Planetesimals -> Planets Why is Mercury so iron-rich, relative to other planets? 43

44 Formation models High-T accretion Lewis (1973) Evaporation of larger mantle/crust by early active Sun -Cameron (1988), Fegley & Cameron (1988) Stripping of larger mantle/crust by giant impact Wetherill (1988), Benz (1988) Metal-silicate fractionation in nebula - Weidenschilling 1978, Wurm et al. (2013) Precursors enriched in C-rich, waterpoor dust (e.g. cometary dust component) Ebel & Alexander (2011) 44

45 Formation models High-T accretion? Lewis (1973) Predict very high Evaporation of larger Al, mantle/crust low K, S, Na, Cl by early active Sun? -Cameron (1988), Fegley & Cameron (1988) Stripping of larger mantle/crust by giant impact? Wetherill (1988), Benz (1988) Metal-silicate fractionation in nebula? - Weidenschilling 1978, Wurm et al. (2013) Precursors enriched in C-rich, waterpoor dust (e.g. cometary dust component)? Ebel & Alexander (2011) 45

46 Formation models High-T accretion Lewis (1973) Evaporation of larger mantle/crust by early active Sun? -Cameron (1988), Fegley & Cameron (1988) Stripping of larger mantle/crust by giant impact Wetherill (1988), Benz (1988) Predict very high Al, low K, S, Na, Cl? Solar-like Cl/K may argue against Metal-silicate fractionation in nebula - Weidenschilling 1978, Wurm et al. (2013) Precursors enriched in C-rich, waterpoor dust (e.g. cometary dust component) Ebel & Alexander (2011) 46

47 Formation models High-T accretion Lewis (1973) Evaporation of larger Al, mantle/crust low K, S, Na, Cl by early active Sun -Cameron (1988), Fegley & Cameron (1988) Stripping of larger mantle/crust Solar-like Cl/K by giant impact Wetherill (1988), Benz (1988) Predict very high may argue against Metal-silicate fractionation in nebula - Weidenschilling 1978, Wurm et al. (2013) Precursors enriched in C-rich, waterpoor dust (e.g. cometary dust component) Ebel & Alexander (2011)? 47

48 MESSENGER: Polar Regions Polar Shadowed Craters North pole to 82 N It has been known since the early 1990s that areas near Mercury's poles have radar characteristics consistent with the presence of water ice. A special campaign took images of Mercury's north polar region during the course of a Mercury solar day. Images combined into a map of areas of permanent shadow. Areas of ice-like radar characteristics fall within these permanently shadowed locations. Radar-bright areas (yellow) and areas of persistent shadow (red) Chabot et al. (2013) J. Geophys. Res.-Planets Neutron spectrometer detected hydrogen in abundances consistent with water ice in permanent shadow cold traps. 48

49 MESSENGER: Polar Regions Polar Shadowed Craters North pole to 82 N I can give a separate presentation on study of the polar ice deposits and comparisons with permanently shadowed areas on the Moon. Radar-bright areas (yellow) and areas of persistent shadow (red) Chabot et al. (2013) J. Geophys. Res. 49

50 MESSENGER: Exosphere-Magnetosphere Complex Interconnections The surface-exosphere-magnetosphere system is coupled and dynamic. Mercury's magnetosphere fluctuates on time scales much more rapid than Earth's. 50

51 The Exosphere Unlike the Earth, Mercury has no appreciable atmosphere Neutral atoms do come off the surface and spill into space. Sodium (Na) exosphere Densities are so low that the atoms do not collide with each other only with Mercury s surface called a surface bounded exosphere Sunlight on the Na atoms causes excitations which lead to characteristic light emission 51

52 Mercury s Sodium and Calcium Exospheres Sodium Calcium observa.ons point to a persistent dawn source and suggest an origin associated with meteoroid impacts Sodium observa.ons in the tail region are remarkably consistent from (Mercury) year to year (~88 Earth days) illustra.ng the dominance of seasonal variability 52

53 Magnetosphere Like the Earth, Mercury has an active planetary dynamo in its large core. The resulting magnetic field is sufficiently small that the planet dominates this plasma/magnetic field structure in interplanetary space. 53

54 Comparing Earth and Mercury Auroral Zones? Mercury Earth Earth s Van Allen Belts Van Allen Belts? [Adapted from Baker et al., 1986] 54

55 MESSENGER: Magnetic Field Dipole field, ~1% strength of Earth s. 500 km Magnetic equator is shifted northward relative to geographic equator. Related to the internal dynamo. The southern hemisphere experiences greater bombardment by energetic particles. 55

56 MESSENGER: Summary 1 Mercury is distinctly different from the Moon, an object to which it was often compared. - The Moon and Mercury differ their major characteristics: internal structure, surface composition, geological history, magnetic field, interactions with space environment. - However, Moon-Mercury comparative planetology is very informative. MESSENGER enhanced-color composite color composite from Galileo 56

57 MESSENGER: Summary 2 Mercury is a strange planet. Many aspects are poorly understood. MESSENGER has raised many fundamental questions. Models for formation of Mercury (and all the inner planets) need to be reconsidered. If we want to claim that we understand in general how planets form and evolve, we need to be able to explain Mercury. Ultimately, these studies will inform observations of planet formation around other stars. ESA-JAXA BepiColombo mission to launch in

58 End. 58

59 Mg/Si (wt. ratio) Major elements on Mercury Peridotites Mare basalts Cont. crust Komatiites Oceanic basalts Moon rocks Earth rocks Mercury (Nittler et al. 2011) +Weider et al unpub No lunar-like flotation crust Lunar Highlands Al/Si (wt. ratio) Ca/Si (wt. ratio) Mare basalts Komatiites Cont. crust Oceanic basalts Highlands 0.0 Peridotites Al/Si (wt. ratio) XRS flare data indicate Mercury is Mg-rich, Al-Ca-poor, relative to Earth and the Moon's surface 59

60 Mg/Si (wt. ratio) Major elements on Mercury Peridotites Mare basalts Cont. crust Komatiites Oceanic basalts Moon rocks Earth rocks Mercury (Nittler et al. 2011) +Weider et al unpub No lunar-like flotation crust Lunar Highlands Al/Si (wt. ratio) Ca/Si (wt. ratio) Mare basalts Komatiites Cont. crust Oceanic basalts Highlands GRS results (Evans et al 2012) 0.0 Peridotites Al/Si (wt. ratio) XRS flare data indicate Mercury is Mg-rich, Al-, Capoor. Gamma ray data is consistent with XRS. 60

61 Magnetic Equator and Dipole Offset Assume distribution of plasma and magnetospheric currents is symmetric about the magnetic equator. Asymmetries due to the IMF average out over time. Magnetic equator, Z ρ0, can be determined from B ρ =0 crossings without correction for plasma and external field effects. Determination from 141 passes demonstrates northward displacement of the dipole from the planet center by 484 ± 11 km. 61

62 MESSENGER: Polar Regions Reflectance in permanent shadow measured by laser altimeter Neumann et al. (2013) Science Areas of higher reflectance (arrows) are in unusually cold spots where water ice is predicted to be stable at the surface: bright frosts/ice present Also see areas that are darker than average: organic compounds delivered by comets or volatile-rich asteroids. 62