Dave Paige / Francis Nimmo

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ESS 250 MARS

• Dave Paige / Francis Nimmo

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Lecture Outline

• Mars Mission Basics
• Getting to Mars
• The Deep Space Network
• Getting Into Orbit
• Orbits
• Landing
• Cartography
• Selected Instrument Techniques
• Visible Imaging
• Near IR Spectroscopy
• Thermal IR Radiometry and Spectroscopy
• Laser Altimetry
• Particle and High Energy Photon Spectroscopy
• RADAR
• Organics and Life Detection
• Sample Return

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Getting to Mars
Lowest Energy Hohman Transfer Orbit
Pork Chop Plot

• Mars launch opportunities occur every 26
  months
• Type I trajectories require less than 180 deg
  transfer, 180 lt Type II lt 360, etc.
• 8 months for Type 1 Transfer, 4 months using
  nuclear propulsion.
• Launch vehicle requirements are determined by
  C3, the excess energy per unit mass (km/s)2
  required after reaching Earth escape velocity
  required to make the transfer

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Deep Space Network (DSN)

• NASAs Pioneer Deep Space Network provides
  radio communications via three principal stations
  at Goldstone, CA, near Madrid, Spain, and near
  Canberra, Australia
• Each station has 70m and 34m antennas that can
  monitor multiple Mars spacecraft simultaneously
• Data rates are a function of power, sensitivity,
  noise, encoding efficiency, and the directivity
  of antennas etc.
• Landers can communicate to orbiters using UHF
  (Ultra High Frequency) 65 cm wavelength signals
  which are then relayed by the orbiters to the DSN

Band Wavelength Frequency Name Origin Uses
L 23 cm 1.3 GHz Long Long wavelength RADAR and low data rate
S 10 cm 3 GHz Short Low data Rate
X 3 cm 10 GHz X marks the spot for weapons targeting High data rate direct to earth communications
K, Ka and Ku 1.5 cm 20 GHz Kurt (German for short) K band has highest data rate, but significant water absorption, Ka (higher) and Ku (lower) frequency reduces effects of water, but still cant be used in bad weather
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Getting Into Orbit

• Mars Orbit Insertion (MOI) can be accomplished
• Purely Propulsively - safest, larger rockets,
  more fuel)
• Aerobrake Assisted (using the atmosphere to help
  slow down) - more risky, smaller rockets, less
  fuel
• Aerocapture (using the atmosphere to do all the
  slowing down) - requires a first pass at 25 km
  altitude look out for Olympus Mons!, requires
  no fuel, but good heat shield and nerves of
  steel..
• Aerobraking can be used to gradually circularize
  orbits, at the cost of time and some risk

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Orbits

• Orbits can be tailored to meet specific mission
  needs (mapping, communications, planetary
  protection etc.)
• Orbits with periapses less than 200 km above
  the surface interact with the atmosphere and is
  not stable

Orbit Type Characteristics Pros/Cons Examples
Sun Synchronous Mapping 300 km circular, near polar, 1.52 hour period, with precession rate that matches Mars orbital period to give observations close to two times per day Pros Excellent global coverage at consistent and close range, consistent illumination Cons No nadir coverage within 2 degrees of poles, very high energy, incomplete hourly coverage, long eclipse durations MGS, Odyssey
Elliptical 250 km at periapsis and up to 40,000 km at apoapsis, periods from 8 to 24 hours Pros Low energy, stable, potential for improved time of day coverage, potential for unique science opportunities at periapsis Cons Instruments spend most of their time far from planet, with inconsistent surface resolution Viking (24 hour equatorial and high inclination) Mars Express (8 hour high inclination)
Synchronous Circular, Equatorial, period matched to planetary rotation rate (aerostationary) Pros Continuous visibility of fixed surface location, excellent for communications with the surface Cons Cant see other sides of planet, very far from planet (30,000 km) Future communications orbiters
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Landing

• At the top of the atmosphere Vtop Vinf
  Vesc, where Vinf is the vehicles approach
  velocity at infinite distance (not including the
  gravitational effects from Mars itself), and Vesc
  is the Martian escape velocity (5 km/sec).
• Direct from Earth trajectories have Vinf that
  are greater than Vinf from orbit
• The energy required to slow the vehicle from
  Vtop to 0 goes as the square of Vtop
• The atmosphere of Mars is an aid to landing as
  it provides aerodynamic resistance
• The atmosphere of Mars hinders landing due to
  unpredictable density variations and winds

Mars Pathfinder Accelerometer Data

• Most of the energy is taken out by the aeroshell
  heat shield high in the atmosphere
• Three types of terminal descent and landing
  systems. Rockets and landing legs, Airbags and
  Penetrators (no airbags or legs)
• Score so far, Earth Vs. Mars Rockets and Legs
  (2 to 1), Airbags (4 to 2), Penetrators (0 to 2)
• Landing failures can be caused by malfunction of
  landing system and by landing hazards/design
  weaknesses

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Cartography - Latitude

• Mapping surface features to coordinate systems,
  and keeping track of conventions can be tricky.
• The northern hemispheric peoples won the battle
  over which hemisphere should be positive latitude
  years many years ago

• Because of Mars oblate shape, there are two
  ways to measure latitude

Aerographic Latitude (f)
Aerocentric Latitude (f)

• Aerographic latitude is favored by imaging teams
  because the local zenith angle is perpendicular
  to the surface (this makes measuring your
  latitude easier if you ever need to take your
  bearings using a sextant while on a boat at
  sea..)
• Aerocentic latitude is favored by gravity and
  topography teams, and most modern, right-minded
  people, because spacecraft orbit the center of
  mass (this makes measuring your latitude easier
  when using modern spacecraft technology using the
  fewest assumptions)

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Cartography - Longitude

• Mapping longitude requires a reference
  longitude, and a sense of direction
• The British established the Earths reference
  longitude at Greenwich
• The International Astronomical Union (IAU) has
  established Mars zero longitude based on the
  position of the small crater Airy, or the Airy-0
  frame
• The Mars reference longitude has been updated
  through time as the position of Airy has been
  better determined in Mars inertial frame
• Early telescopic observers used a West-positive
  longitude system for Mars so that longitude would
  increase as they observed through the night
• The convention has held on in some of the more
  backward circles (that include telescopic
  astronomers and Mars geologists that are
  unfamiliar with the most basic principles of
  algebra and geometry)
• Most right minded and right-handed individuals
  prefer the East-positive longitude system because
  of the obvious and natural benefits of using a
  mathematically-sound, right-handed coordinate
  system.
• Unbelievably, the different experiment groups on
  the MGS mission have archived their data using
  different conventions for both latitude and
  longitude, which makes comparison of datasets
  difficult for the uninitiated!
• At least we dont use Martian Minutes and
  Martian Seconds when specifying fractional
  longitudes.

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Map Projections

• Mars is basically spherical, but its difficult
  to print out a sphere.
• Desired qualities of a map projection
• Equal Area preserves size relationships
  between large and small features
• Conformal preserves the shapes of features,
  maps great circles as straight lines

Common Mars Map Projections
Mercator (conformal, non equal area)
Robinson (nonconformal, non equal area)
Stereographic (conformal, non equal area)
Sinusoidal (non-conformal, equal area)
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Visible Imaging

• Imaging is a key component of most Mars missions
• The images can be used in fairly unprocessed
  forms for seeing whats there
• Quantitative work with images requires multiple
  levels of processing
• Level 1
• Raw, unprocessed unmerged spacecraft data
  acquired from different ground stations
• Level 0
• Compressed, raw, unprocessed whole images
• Level 1
• Decompressed images merged with associated
  spacecraft and instrument geometry and timing
  data
• Level 2Beautified, and geometrically corrected
  individual images with associated timing and
  solar geometry data
• Level 3 Photometrically calibrated individual
  images with Level 2 geometry
• Level 4
• Higher-order image products, often employing
  multiple images to create color images, mosaics,
  stereo images, movies, maps, spectra etc.
• Note The definitions of these various levels
  vary from experiment to experiment

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Near IR Spectroscopy (Minerals)

• Near IR spectroscopy can provide considerable
  information about the presence of various
  minerals
• Caveats
• Mixtures of minerals and/or minerals in low
  abundance can cause problems for whole rock or
  whole region spectra
• Dust on top of rocks obscures rock signals
• Atmospheric H20 and CO2 gas absorption can
  hinder orbital measurements

Laboratory reflectance spectra of (a) pure
igneous minerals, (b) iron oxides/hydroxides, (c)
anhydrous carbonates, (d) sulfates, (e) clays and
(f) nitrates
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Near IR Spectroscopy (Volatiles)

• CO2 gas and water vapor in the Martian
  Atmosphere absorbs strongly at 1.37, 2.0, and 2.7
  microns.
• Water ice and CO2 ice have distinct absorption
  features, whose shapes are sensitive to grain
  size

Mars Express Omega Spectrometer Results Water Ice
(left), CO2 Ice (middle), Visible (right)
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Thermal IR Radiometry and Spectroscopy

• Objects at Martian temperatures emit radiation
  at infrared wavelengths (peak at 15-20 microns)
• Emission can be measured from orbit, or from
  surface
• Resulting spectra are determined by
• Blackbody function for surface temperature
• Absorption, emission by atmospheric gas and
  aerosols
• Emissivity of the surface as a function of
  wavelength
• Measurements can be made by
• Spectrometers (resolved spectral features)
• Radiometers (unresolved spectral features in
  spectral bands)

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Atmospheric Properties from Thermal IR
Observations
The radiative effects of atmospheric temperature,
dust, water vapor etc. discussed in the last
lecture can be exploited to retrieve these
atmospheric properties from infrared spectra
MGS TES Multi-Year Atmospheric Retrieval Results
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Surface Properties from Thermal IR Observations

• IR observations can be used to infer the thermal
  state of the surface and near-subsurface, as well
  as aspects of the bulk thermal properties of
  materials
• Key parameters

• Surface Albedo As
• Controls surface solar heating
• Thermal Inertia I ( k ? c )1/2
• Controls heat flux
• Controls amplitude of temperature variations
• Significant regional variations due to soil
  particle size, rock and ice abundance
• Thermal inertia and Albedo determine daily
  average and annual average temperature
• Thermal Skin Depth D ( ( k P ) / ( ? ? c ) )
  1/2
• Controls penetration of diurnal and seasonal
  temperature waves
• Annual skin depth is 26 times diurnal skin depth
• For low thermal inertia soil, D (diurnal) 6.6
  mm, D (seasonal) 17 cm
• For solid ice, D (diurnal) 25 cm, D (seasonal)
  6.5 meters

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Surface Properties from Thermal IR Observations
For most geologic materials in the Martian
environment, the ?c product varies by less than
a factor of 2, whereas the thermal conductivity
varies by factors of 100, primarily due to the
effects gain size variations and atmospheric gas.
Implication Significant spatial variability in
thermal behavior good for remote sensing!
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Martian Daily Surface Temperature Variations
A 0.2, I 50 (Low Thermal Inertia) A 0.2, I
250 (Mars Average) A 0.2, I 1000 (High
Thermal Inertia) A 0.5, I 250 (High Albedo)

• The effects of albedo and thermal inertia can
  generally be separated
• Albedo affects daily average temperature
• Thermal inertia controls amplitude of daily
  temperature variation, with second-order effect
  on daily average temperature (low I soils have
  colder average temperatures)
• Albedo and thermal inertia can be uniquely
  determined from two surface temperature
  measurements (day and night)
• Thermal inertia can be guessed from a single
  pre-dawn surface temperature measurement, and an
  estimate of surface albedo

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Global Albedo and Thermal Inertia

• Albedo variations caused by distribution of
  bright surface dust relative to darker sand and
  rocks
• High albedo regions generally correlated with
  low thermal inertia more bright fine-grained
  particles
• Large low thermal inertia regions centered on
  Tharsis, Arabia, Elysium and South Polar regions

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Temperature Variations With Depth
Surface D 5 mm D 20 mm D 37 mm D 67 mm

• Surface temperature measurements can be fit with
  the results of models to infer
• Annual Average Temperature
• The presence of high thermal inertia material
  close to the surface
• Mixtures of high and low thermal inertia
  material in the instrument field of view (rock
  abundance)

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Annual Average Temperature and Rock Abundance Maps
The annual average temperature is equal to the
temperature at great depth (excluding the effects
of planetary heat flow)
Mid-Latitude Rock Abundance Map from Viking IRTM
Radiometer Data
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Effects of Slopes on Annual Average Temperatures
Topographic slopes magnitudes and orientations
can affect insolation, and annual average
temperatures
0 K
Latitude
Latitude
Northward Slope (deg)
Eastward Slope (deg)
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Thermal Emission Spectroscopy

• lt10 variations in the infrared emissivity
  result in thermal emission spectra
• Energies of IR photons similar to lattice
  vibration energies for many minerals
• Thermal IR spectroscopy generally superior to
  Near IR spectroscopy for mineralogy

• Identifying minerals on Mars is complicated by
• Atmospheric gas and aerosols
• Particle size effects (example spectra are for
  homogeneous slabs)
• Mixing within spectrometer field of view
• Rocks and minerals identified thus far from TES
  orbital spectra
• Basalt
• Andesite
• Hematite
• Carbonate

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Laser Altimetry

• MGS MOLA laser altimeter provides range to
  surface and clouds
• MOLA spot size is 130 m, along track shot
  spacing is 330 m
• Topographic profiles require MOLA data plus
  detailed spacecraft ephemeris based on radio
  tracking
• Topographic maps require gridding of profiles
  from multiple orbits. MOLA gridded topographic
  maps available with resolutions as high as 1/128
  degree (500 m)
• By measuring returned pulse width and inter-shot
  variability, MOLA data can be used to estimate
  surface roughness

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MOLA Map Products

• MOLA map products provide excellent quantitative
  information regarding elevations, slopes etc
• MOLA maps can also be displayed as shaded relief
  maps, providing detailed morphology etc.
• Using MOLA data as basemaps for dataset analyses
  instead of images has several advantages
• Consistent resolution, lighting angles
• No atmospheric, camera or mosaic artifacts (but
  no color or albedo info either..)
• Extremely accurate feature locations in
  planetocentric coordinates