Primary Mission   ---------------   The primary MESSENGER mission has six guiding science   questions, which in turn correlate to specific science   objects and a set of measurement requirements related   to specific instruments. These guiding questions are:    (1) What planetary formational processes led to   the high metal/silicate ratio in Mercury?   (2) What is the geological history of Mercury?   (3) What are the nature and origin of Mercury's   magnetic field?   (4) What are the structure and state of Mercury's   core?   (5) What are the radar-reflective materials at   Mercury's poles?   (6) What are the important volatile species and   their sources and sinks on and near Mercury?    The related science objectives and instrument   measurement requirements are:    (1) Map the elemental and mineralogical composition   of Mercury's surface.   GRNS and XRS: Provide major-element maps of   Mercury to 10% relative uncertainty on the 1000-   km scale. Elements to be measured include: O, Si,   S, Fe, H, K, U (by GRS); thermal and epithermal   neutrons (by NS), and Fe, Mg, Ca, Al, Si, Ti, S   (by XRS).   MASCS (VIRS): Determine local composition and   mineralogy at the ~20-km scale.    (2) Image globally the surface at a resolution of   hundreds of meters or better.   MDIS (WAC): Provide a global multi-spectral map   at 2 km/pixel average resolution.   MDIS (NAC): Provide a global map with > 90%   coverage (monochrome) at 250-m average   resolution.   MDIS: Image > 80% of the planet stereoscopically.   Provide color images with a resolution to 1   km/pixel.   MLA: Sample half of the northern hemisphere for   topography at 1.5-m average height resolution    (3) Determine the structure of the planet's magnetic   field.   MAG: Provide a multipole magnetic-field model   resolved through quadrupole terms with an   uncertainty of less than ~20% in the dipole   magnitude and direction.    (4) Measure the libration amplitude and   gravitational field structure.   MLA & RS: Provide a global gravity field to   degree and order 16 and determine the ratio of   the solid-planet moment of inertia to the total   moment of inertia to ~20% or better.    (5) Determine the composition of the radar-   reflective materials at Mercury's poles.   GRNS: Identify the principal component of the   radar-reflective material at Mercury's north   pole.    (6) Characterize exosphere neutrals and accelerated   magnetosphere ions.   MASCS (UVVS): Provide altitude profiles at 25-km   resolution of the major neutral exospheric   species.   EPPS: Characterize the major ion-species energy   distributions as functions of local time, Mercury   Heliocentric distance, and solar activity.    First Extended Mission   ----------------   The first extended MESSENGER mission has six additional   guiding science questions, which in turn correlate to specific   science objects and a set of measurement requirements related   to specific instruments. These guiding questions are:        (1) What are the sources of surface volatiles?       (2) How late into Mercury's history did volcanism persist?       (3) How did Mercury's long-wavelength topography change with           time?       (4) What is the origin of localized regions of enhanced           exospheric density near Mercury?       (5) How does the solar cycle affect Mercury's exosphere and           volatile transport?       (6) What is the origin of Mercury's energetic electrons?    The related science objectives and instrument measurement requirements   are:    (1) Determine the morphological and compositional context of   'hollows' and their relationship to bright crater-floor deposits and   pyroclastic vents    MDIS (WAC): Image 70% of the planet in three colors at 600 m/pixel   average spatial resolution. MDIS (NAC): Acquire 100 sets of targeted   images of hollows or pyroclastic vents at 60 m/pixel average spatial   resolution. MASCS (VIRS): Acquire 20 targeted VIRS observations of   hollows and pyroclastic vents at low solar incidence angle (i).    (2) Acquire targeted, high-resolution observations of volcanic   materials of low impact crater density identified in the primary   mission    MDIS (NAC): Acquire 30 sets of targeted images of young volcanic   materials at 60 m/pixel average spatial resolution.    (3) Document changes in long-wavelength topography versus geological   time on Mercury from altimetric and complementary imaging   measurements MDIS: Image 70% of the planet at 250 m/pixel average   spatial resolution, targeting i ~ 40 deg. to 65 deg., MDIS: Image 70%   of the planet at 250 m/pixel average spatial resolution, targeting i   ~ 75 deg. to 85 deg., and MLA: Provide topographic profiles over 10   broadly elevated regions and the floors of 50 complex impact craters,   including volcanically flooded craters.    (4) Characterize regions of enhanced density versus solar distance,   proximity to geologic units, solar activity, and magnetospheric   conditions MASCS (UVVS): Survey dayside and nightside exosphere   emissions at an average rate of once every third orbit, MASCS (UVVS):   During dawn-dusk seasons, conduct repeated observations of exospheric   emission over both poles to the maximum extent permitted by   spacecraft pointing constraints, and MASCS (UVVS): Conduct   full-orbit, exosphere observation campaigns at equally spaced Mercury   true anomalies over each of four Mercury years.    (5) Measure changes in exospheric neutrals and plasma ions as solar   activity increases EPPS (FIPS): Measure the global distribution of   planetary ions and the direction of plasma flow, within operational   constraints.    (6) Infer the sources and energization mechanism from the location,   energy spectra, and temporal profiles of energetic electrons    EPPS (EPS) & MAG: Provide locations, energy spectra and pitch angles,   and temporal profiles of energetic electrons across all magnetic   longitudes in the northern hemisphere.    Second Extended Mission   ----------------   The second extended MESSENGER mission has seven additional   guiding science questions, which in turn correlate to specific   science objects and a set of measurement requirements related   to specific instruments. These guiding questions are:        (1) What active and recent processes have affected           Mercury's surface?       (2) How has the state of stress in Mercury's crust           evolved over time?       (3) How have the compositions of volcanic materials on           Mercury evolved over time?       (4) What are the characteristics of volatile emplacement           and sequestration in Mercury's north polar region?       (5) What are the consequences of precipitating ions and           electrons at Mercury?       (6) How do Mercury's exosphere and magnetosphere respond           to both extreme and stable solar wind conditions during           solar wind conditions during solar maximum and the           declining phase of the solar cycle?       (7) What novel insights into Mercury's thermal and crustal           evolution can be obtained with high-resolution measurements           from low altitudes?    The related science objectives and instrument measurement   requirements are:    (1) Characterize faulted terrain by acquiring at least one   of the following: (a) 20 NAC along-track stereo pairs or   (b) 40 MLA topographic profiles.    (2) Characterize fresh craters by acquiring at least one   of the following: (a) 20 WAC 11-color image sets or   (b) 20 NAC along-track stereo pairs.    (3) Characterize hollows by acquiring (a) UVVS observations   of exospheric species over eight clusters of hollows on three   different dates and from two different viewing geometries per   feature or (b) 20 along-track NAC stereo pairs and 20 11-color   image sets each.    (4) Characterize surface features at very high resolution by   acquiring 750 NAC images at <= 10-m pixel scale and 100 NAC images   at <= 5-m pixel scale.    (5) Search for color variations within the northern plains by   acquiring 5-color MDIS images of 75% of the surface area north   of 60 deg N at phase angles < 60 deg.    (6) Constrain the elemental composition of spectral end-member   materials by acquiring targeted XRS spectra from   (a) the large pyroclastic deposit northeast of Rachmaninoff and   (b) of at least two different portions of low-reflectance blue   plains exterior to the Caloris basin. For each target, acquire a   minimum of 1000 s of spectral integration spread over at least   five different orbits.    (7) (a) Characterize MLA-bright and dark materials by acquiring   MLA ranging and reflectance data along portions of two orbits for   which ground tracks cross each of 10 craters < 20 km in diameter, and   (b) characterize the north polar hydrogen distribution at high spatial   resolution by acquiring NS measurements for 70% of the time that the   spacecraft altitude is less than 150 km.    (8) Characterize crustal structure at high resolution by acquiring   Doppler tracking data for portions of 100 orbits at altitudes < 100 km.    (9) Characterize the structure of crustal magnetization at high   resolution by acquiring MAG and FIPS observations along portions of   100 orbits at altitudes < 50 km in the vicinity of the northern plains.    (10) Characterize magnetospheric particle flows and pitch-angle   distributions by acquiring a defined set of 970 EPPS measurements   distributed across several different pointing scenarios.    (11) Characterize the exospheric response to conditions during   solar maximum and the declining phase of the solar cycle by acquiring   a defined set of 5025 UVVS dayside and nightside observations,   including searches for species with weaker resonant emissions.    (12) Characterize the magnetospheric response to conditions during   solar maximum and the declining phase of the solar cycle by acquiring   MAG, EPPS and NS/GRS observations for 75% of the time throughout the   mission, including times at which the spacecraft altitude is < 50 km.