ESS 250: MARS
Dave Paige / Francis Nimmo
ESS 250 Winter 2003
Lecture Outline
• Mars Mission Basics
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Getting to Mars
The Deep Space Network
Getting Into Orbit
Orbits
Landing
Cartography
• Selected Instrument Techniques
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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 < 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
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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
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 can’t
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,
1.5–2 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: Can’t 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 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
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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 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….
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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)
Sinusoidal (non-conformal, equal area)
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Robinson (nonconformal, non equal area)
Stereographic (conformal, non equal area)
Visible Imaging
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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 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
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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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
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
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Mars Express Omega Spectrometer Results
Water Ice (left), CO2 Ice (middle), Visible (right)
Thermal IR Radiometry and
Spectroscopy
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• 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)
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 nearsubsurface, as well as aspects of the bulk thermal properties of materials
• Key parameters:
Surface Albedo As
1.
Controls surface solar heating
Thermal Inertia I = ( k  c )1/2
1.
Controls heat flux
2.
Controls amplitude of temperature variations
3.
Significant regional variations due to soil particle size, rock and ice abundance
4.
Thermal inertia and Albedo determine daily average and annual average temperature
Thermal Skin Depth D = ( ( k P ) / (   c ) ) 1/2
1.
Controls penetration of diurnal and seasonal temperature waves
2.
Annual skin depth is 26 times diurnal skin depth
3.
For low thermal inertia soil, D (diurnal) = 6.6 mm, D (seasonal) = 17 cm
4.
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)
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• 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
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:
0K
La
tit
ud
e
La
tit
ud
e
Northward Slope (deg)
Eastward Slope (deg)
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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
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• 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
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
ESS 250 Winter 2003