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ESS 250: MARS. Dave Paige / Francis Nimmo. 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

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ess 250 mars

ESS 250: MARS

Dave Paige / Francis Nimmo

ESS 250 Winter 2003

lecture outline
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

ESS 250 Winter 2003

getting to mars
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 < Type II < 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

ESS 250 Winter 2003

deep space network dsn
Deep Space Network (DSN)
  • NASA’s “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

ESS 250 Winter 2003

getting into orbit
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

ESS 250 Winter 2003

orbits
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

ESS 250 Winter 2003

landing
Landing
  • At the “top” of the atmosphere Vtop = Vinf + Vesc, where Vinf is the vehicle’s 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

ESS 250 Winter 2003

cartography latitude
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…)

ESS 250 Winter 2003

cartography longitude
Cartography - Longitude
  • Mapping longitude requires a reference longitude, and a sense of direction
  • The British established the Earth’s 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 don’t use Martian Minutes and Martian Seconds when specifying fractional longitudes….

ESS 250 Winter 2003

map projections
Map Projections
  • Mars is basically spherical, but it’s 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)

ESS 250 Winter 2003

visible imaging
Visible Imaging
  • Imaging is a key component of most Mars missions
  • The images can be used in fairly unprocessed forms for “seeing” what’s 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 2“Beautified”, and geometrically corrected individual images with associated timing and solar geometry data
    • Level 3Photometrically 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

ESS 250 Winter 2003

near ir spectroscopy minerals
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

ESS 250 Winter 2003

near ir spectroscopy volatiles
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)

ESS 250 Winter 2003

thermal ir radiometry and spectroscopy
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)

ESS 250 Winter 2003

atmospheric properties from thermal ir observations
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

ESS 250 Winter 2003

surface properties from thermal ir observations
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

ESS 250 Winter 2003

surface properties from thermal ir observations1
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!

ESS 250 Winter 2003

martian daily surface temperature variations
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

ESS 250 Winter 2003

global albedo and thermal inertia
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

ESS 250 Winter 2003

temperature variations with depth
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)

ESS 250 Winter 2003

annual average temperature and rock abundance maps
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

ESS 250 Winter 2003

effects of slopes on annual average temperatures
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)

ESS 250 Winter 2003

thermal emission spectroscopy
Thermal Emission Spectroscopy
  • <10% 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

ESS 250 Winter 2003

laser altimetry
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

ESS 250 Winter 2003

mola map products
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

ESS 250 Winter 2003