JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 102, NO. F_A,PAGES 9109-9123, APRIL 25, 1997
Mars surface mineralogy from Hubble Space Telescope
imaging during 1994-1995: Observations, calibration, and
initial
results
JamesF. Bell III, • MichaelJ. Wolff,2 PhilipB. James,
3 R. ToddClancy,
2
StevenW. Lee,4 and LeonardJ. Martins
Abstract. Visible to near-infraredobservations
of Mars were madewith the Hubble Space
Telescope(HST) during1994-1995with the goalsof monitoringseasonalvariabilityof the surface
andatmosphere
andmappingspecificspectralunitsto constrain
theplaner'ssurfacemineralogy.
Thispaperpresentsthedetailsof thecollectionandcalibrationof the data,concentrating
specifically
on thenear-IRdatathatwereobtainedexclusivelyfor thesurfacemineralogyaspectof
our HST Mars observingprogram. We alsopresentsomeinitial resultsfrom the calibrateddata
set. Our calibrationprocedures
includedthe standard"pipeline"processing
steps,supplemented
by
specialprocedures
requiredfor usewith the linearrampfilterson theWide Field/PlanetaryCamera
2 instrument,andan additionalpointspreadfunctiondeconvolution
procedureappliedin orderto
realizethe full potentialspatialresolutionof the images(23 to 64 km/pixel betweenAugust 1994
andAugust1995). The calibrationresultsin a setof imagesprojectedontoa standardmap grid
andpresented
in radiancefactor(I/F) units,havingan estimated--5% photometricaccuracybasedon
theperformance
of HST andcomparisons
with previousground-based
andspacecraft
Mars spectra.
Initial scientificanalysesof thesedatareveal (1) distinctred/bluecolor unitswithin the classical
brightregions,similar to thoseseenin Viking Orbiter imagesandpossiblyrelatedto variationsin
nanophase
and/orcrystallineferricmineralabundance;
(2) near-IRspectralslopevariations
correlatedwith albedoon a largescale(darkeris "bluer"near-IR slope)but exhibitingwider
variationsamongmany of the small-scalefeaturesvisiblein the data;(3) an absorptionat 860 nm
that occursin all regionsbut which is 3 to 5% strongerin many of the classicaldark regionsthan
in the brightregions,possiblybecauseof a greaterabundanceof a well-crystallineferric phaselike
hematiteor a very low Ca pyroxeneor opx/cpxmixture;and (4) an absorptionfrom pyroxeneat
953 nm with a banddepththatis inverselycorrelatedwith albedo(brightregions0 to 5% deep;
darkregions7 to 15% deep)andwhich showsthe highestbanddepthvaluesin individualcraters,
calderas,and othersmall geologicunitsthat are resolvedin the images.
Introduction
The surfacemineralogy of Mars providesa window into past
and present geologic, geochemical, and hydrothermal
processeson the planet. Surface minerals play an important
role in the transport, storage, and processingof volatiles. For
example, water can be stored either temporarily (adsorbed
"surface" water) or semi-permanently (bound or "structural"
water) on mineral surfacesand within mineral crystal lattices.
Atmospheric gases such as CO 2 or SO2 can be sequestered
within minerals through aqueous or other processes, thus
providing a sink for ancient atmosphericconstituents[e.g.,
Fanale et aI., 1992]. The surface-atmospherictransport of
volatiles on seasonal timescales is an important part of the
Mars volatile cycle and can substantially influence the
climatic and radiative behavior of the surface-atmosphere
system[e.g., Jakoskyand Haherie, 1992].
Previous investigators have used a combination of groundbased and spacecraft observations to detect or infer the
presenceof many different minerals on the Martian surface or
in the airborne dust (recent detailed reviews can be found in the
works by SoderbIom [1992], Roush et aI. [1993], and Bell
[1996]). Iron-bearing minerals make up the majority of these
phasesbecauseof (1) the relatively high iron abundanceof the
Martian surface [TouImin et aI., 1977; Clark et aI., 1982] and
(2) the highly spectroscopically active nature of iron in a
variety of different oxides, oxyhydroxides, and silicates [e.g.,
Burns,
1993].
In addition, observational detection of iron
111111•...,1
•...,111 L.i.i.J
o •...,Li.o 1
•...,1 ,h,,,.,
LIIL&11 that of many other materials
ß
'• .... L&i•o is l.J
,.,•,-h
......
•,:-•Department
of Astronomy,Centerfor Radiophysics
and Space becausemost of the relevant mineral absorptionfeatures occur
in the visible to short-wave near-IR (-- 400 to 1200 nm) where
Research,Cornell University, Ithaca, New York.
2Space
ScienceInstitute,Boulder,Colorado.
the solar reflected flux is highest, and recent advances in
3Department
of Physicsand Astronomy,Universityof Toledo, telescopic and spacecraft instrumentationhave allowed highToledo, Ohio.
quality CCD observations to be routinely obtained at these
4Laboratory
for Atmosphericand SpacePhysics,Universityof
wavelengths.
Colorado, Boulder, Colorado.
We are attempting to provide additional new information on
5LowellObservatory,
Flagstaff,
Arizona.
the mineralogy of the Martian surface using multispectral
observationswith the Hubble Space Telescope (HST). The
Copyright1997 by the AmericanGeophysicalUnion
HST
Papernumber 96JE03990.
0148-0227/97/96JE-03990509.00
instrument
suite available
for these observations
covered
only the UV to short-wave near-IR spectral region, so the
observations were optimized for the detection and
9109
9110
BELL ET AL.:
MARS
SURFACE
MINERALOGY
characterization of iron-bearing oxide, oxyhydroxide, and
silicate minerals. In this paper we discussthe details of the
specificobservationsthat were obtained during the 1994-1995
apparition of Mars, the data reduction and calibration efforts,
and someinitial resultsof our mineralogicinvestigation.
Observations
and Filter
FROM
HUBBLE
SPACE TELESCOPE
informationcan alsobe extractedfrom thesedata [e.g.,Bell et
al., 1995; James et al., 1996], as described in more detail
below.
With additionalobservingtime grantedto us in HST cycle
5, we were able to expandour Mars program(GO Program
5832) to include four more wavelengthsfrom 740 to 1042 nm.
Two of these wavelengths (740 and 860 nm) were obtained
Selection
usingthe WFPC2 linearrampfilters (LRFs) andthe WF4 chip
The data were obtained in 15 orbits of HST over 11 different
Mars orbital positionsbetween August 23, 1994, and August
21, 1995, using the Wide Field/Planetary Camera 2 (WFPC2)
instrument. These dates correspondedto coverage between
Mars areocentriclongitudes of Ls=336ø to Ls=145ø, or late
northern winter through mid northern summer. This time
period allowed for excellent study of the north polar regions,
as the sub-Earth latitude ranged up to 26øN; conversely,
coverageof surfaceregionsbelow about 60øS was not possible
for most of our observations. The opposition was aphelic
(Mars near aphelion at opposition), and the largest apparent
angulardiameterof Mars in our data set is 13.5 arcsec(roughly
a factor of 2 worse than during a perihelic opposition). This
correspondsto a best case spatial resolution of 2Dsin(F2/2) =
22.7 km/pixel, where D is the Earth-Mars distance in
kilometers (104 million km at closest approach in February
1995) and F2 is the angular resolution of the WFPC2 camera
(0.045 arcsec/pixelfor the Planetary Camera chip PC1). This
is approximately the same spatial resolution that was obtained
by the imaging spectrometerinstrument (ISM) on the Phobos
2 missionin 1989 [Bibring et al., 1990] and is slightly better
than the resolution obtained during the Mariner 6 and 7
mission far-encounter phase [Leighton et al., 1969].
The
observational
circumstances
of our data set are summarized
in
Table 1, and Figure 1 provides an illustration of our image
coverageand resolution.
WFPC2 has a variety of filters and observing modes
available for HST observers[Burrows, 1995]. Our program
during HST observingcycle 4 (GO program 5493) utilized the
PC1 chip and five filters from 255 to 673 nm (Table 2),
specifically concentrating on monitoring of atmospheric
activity and surface reflectivity and color changes [James et
al., 1996]. A limited amount of compositional/mineralogic
(Table 2). The LRFs are a setof continuouslyvariablenarrow-
bandpassfilters that allow imaging at --1.1 to 1.3% spectral
resolutionat almost any wavelengthbetween370 and 980 nm
[Burrows, 1995]. While it is a greatadvantageto be able to
image at precisewavelengthsthat are most diagnosticof the
specific spectral features being sought, there are some
importanttrade-offsanddisadvantages
to usingthe LRFs. For
example,images at different wavelengthsare obtainedby
placing the object at specificparts of each of the four WFPC2
detectorarrays;thusobtainingimagesat a specificwavelength
may mean the loss of a factor of 2 in spatial resolution(for
example,usingthe WF4 chip insteadof the PC1 chip resultsin
a spatial scale of 0.0996 arcsec/pixel) or may result in
vignetting problems if the object must be located close to the
edge of a chip. Additionally, the calibrationof images
obtainedwith the LRF is still being developed,and thususers
are forced to develop bootstrapcalibrationtechniques(see
Appendix A) for the images until the proper in-flight
calibrationdata are obtained and processedby the Space
TelescopeScienceInstitute (STScI).
The four additionalwavelengthsobtainedin Cycle 5
providediagnostic
information
on Mars surfacemineralogy.
Specifically,
860nm waschosen
to detectandmapthespatial
extentof the 6Al-->4T2(4G
) electronictransitionbandof Fe3+
(ferric iron) that is primarily characteristicof the iron oxide
mineralhematite(ct-Fe203)[Shermanand Waite, 1985;Morris
et al., 1985;Bell et al., 1990]. Imagingat 953 nm waschosen
in orderto detectand map the spatialextentof the "l-micron"
(1-gm) absorptionfeaturein pyroxenesarisingfrom Fe2+
(ferrousiron) ions primarily in M2 crystallographic
sites
[e.g., Burns,1970, 1993; Adams,1974]. The imagesat 740
nm and 1042 nm provide continuum measurementsfor these
Fe2+andFe3+ absorption
bands,andtheimagesat 1042nm
Table 1. 1994-1995 HST Mars ImagesFrom GO Programs5493 and 5832
UT Datea
YYMMDD
Time,b
Wavelengths,
UT
Diameter,
arcsec
nm
SE Latitude,
deg
Phase,
deg
L•,
deg
Resolution,c
kin/pixel
HST Cycle 4 Data
940823
2311
255,336,410,502,673
5.2
5.1
34.4
335.7
59.1
940919
1526
255,336,410,502,673
5.7
11.8
36.5
349.7
54.0
941020
1154
255,336,410,502,673
6.5
17.7
38.3
5.3
46.8
941118
0546
255,336,410,502,673
7.8
21.0
38.0
19.1
39.3
950102
1007
255,336,410,502,673
11.2
21.8
28.0
39.7
27.3
950224
1711
255,336,410,502,673
13.5
17.3
10.0
63.1
22.7
950225
0117
255,336,410,502,673
13.5
17.2
10.3
63.6
22.7
950225
0918
255,336,410,502,673
13.4
17.2
10.5
63.7
22.7
950408
1933
255,336,410,502,673
9.8
18.1
32.6
81.9
31.4
950528
0157
255,336,410,502,673
6.8
23.0
41.7
104.1
45.2
950706
0336
255,336,410,502,673,740,860,953,1042
5.6
25.7
38.9
122.1
55.1
950706
1139
255,336,410,502,673,740,860,953,1042
5.5
25.8
38.9
122.2
55.2
950711
2335
255,336,410,502,673,740,860,953,1042
5.4
25.8
38.1
124.8
54.0
950802
2137
255,336,410,502,673,740,860,953,1042
5.0
25.3
34.4
135.4
61.0
950821
0937
255,336,410,502,673,740,860,953,1042
4.8
23.5
30.8
144.6
64.4
HST Cycle 5 Data
aRead940823 as August23,1994.
bTimegivenis theapproximate
middleof the20-to 35-min.totalobserving
sequence.
CResolution
is the maximumspatialresolutionat the sub-Earthpointfor imagesobtainedon the PC chip.
BELL ET AL.'
MARS SURFACE MINERALOGY
330 o.
FROM HUBBLE SPACE TELESCOPE
0 o,
30 ø
9111
60 ø
Mars -, reocentri c Longitude (Ls)
60 ø
90 ø
150 .ø
120 ø
1
.....................
..:.
....
•":•""v:;•'
...........
'**":•::•::::':'::'
':'•$?•
•:z::'
.:. .........
'-::..::½•.:•-:•:....._.•
&::•.:
ß
-2'
.•::..........
:•:•
•
•'•½:"
•'•":'•?
.2:
•Z..•
....
.
.•2•:•
:::•
•.•:•4• •::½
..........%:
.....
.•.•.-
,,•---
Figure
1. Schematic representation of the 1994-1995 Mars •mages obtained by HST. Shown are the
seasonal date (L s) of each set of multispectral observations,the relative size of the planet when it was
observed,and the times when multiple central meridianswere observed. The observationsbetweenLs=335ø
and 104ø are the HST Cycle 4 five-color imaging sets,and the observationtaken from Ls=122ø to 145ø are the
HST Cycle 5 nine-color imaging sets (Table 1).
also provide additional characterization of the l-gm band and a
way to test for the possible presence of olivine on Mars
[Burns, 1970; Huguenin, 1987]. In combination with the
images at 410, 502, and 673 nm that characterize the shape
and curvatureof the near-UV reflectancedropoff, the additional
four filters obtainedduring HST cycle 5 provide an adequateset
of wavelengthsfor the investigationof iron-bearing minerals
on the surface of Mars.
Data
Reduction
Instrumental
Table 2. HST WFPC2 Filters and Exposure Times
Xcenter
, FWHM,
Filter
nm
nm
Exp.Time1sec.
Cycle 4
Cycle 5
F {t
W cm-2gm-1
F255W
256
41
350.0
180.0
0.01553
F336W
333
37
6.0
3.0
0.08667
F410M
409
15
4.0
2.0
0.16822
F502N
501
3
7.0
4.0
0.18775
F673N
673
5
0.7
0.6
0.15185
FR680N b
740
10
--
0.11
0.12919
FR868N b
860
11
--
0.12
0.09746
F953N
955
5
--
4.0
0.07696
F 1042M
1044
61
--
3.0
0.07125
Filter data from Burrows [1995].
aSeeAppendixB.
bLinearrampfilter.
and Calibration
Corrections
Raw images were processed using standard HST data
reductionprocedures
asoutlinedby Lauer [1989] and Holtzman
et al. [1995a,b] and using calibration files produced by the
WFPC2 InstrumentDefinition Team (IDT). The stepsincluded
correctionfor analog-to-digitalconversionerrors, subtraction
of bias,superbias,and superdarkframes,correctionfor shutter
shading effects, correction for pixel-to-pixel sensitivity
variations (flatfielding), and correction of bad pixels and
cosmic ray hits.
We remove cosmic rays through a
combination of automated (low- and high-pass filters) and
manual processes. Individual and small groups of undefined
pixels are repaired by performing an iterated fourth-order
polynomial least-squaresfit to the neighboring pixels. The
high signal-to-noise ratio (SNR; see below) and extended
nature of our target precludethe need for correctionfor charge
transfer efficiency variations [Holtzman et al., 1995a].
The instrumental SNR of our short exposure time HST
visible and near-IR Mars images is quite high and is limited,
9112
BELL ET AL.:
MARS
SURFACE
MINERALOGY
ultimately, by quantization of the 12-bit analog-to-digital
electronics and by scattering of bright Mars light in the
telescopeand instrumentoptics. An estimateof the SNR in
the raw data can be made by examining the standarddeviation
of sky pixelsfar from Mars in the imagesandof scattered-light
pixelsadjacentto the limb of Mars. The variationof the sky
far from Mars and in all wavelengthsis ñ0.6 to ñ0.8 raw data
numbers(DN; the gain was 7 e-/DN for most of our images),
and typical scatteredsky variationis ñ2.8 to ñ7.5 DN. If we
assumethat the scatteredlight componentalso occursover the
FROM
HUBBLE
SPACE TELESCOPE
The second aspect of the absolute photometric calibration
processrequiresa correctionfor the shapeof the point spread
function (PSF), which leads to "smoothing" of planetary
albedo variations because of the telescope and instrument
optics. For example, if uncorrected,this effect will result in
dark regionson Mars being spectrally contaminatedwith light
from adjacentbright regionsor polar deposits. Similar effects
occurin regionsof the imagesaroundsteepintensitygradients
associated
with
clouds
or other
albedo
features.
It is often
extremely difficult to account for this PSF effect in grounddisk of Mars itself, then it is this level of variation that
based telescopic or spacecraft imaging instrumentation.
governsthe effectiveSNR of the final data. For example,in However, one of the "advantages" of the HST primary's
the July 1995 image data the scattering-limitedSNR ranges sphericalaberrationis that the PSF is now bettercharacterized
from a low of 150-240 for dark surfaceregionsat 740 nm and for HST and WFPC2 than for perhapsany other optical system
1042 nm to a high of 740-880 for bright regionsat 673 nm ever constructed. The PSF deconvolutionsof our HST images
and 953 nm.
were performedusing 40 iterationsof the dampedRichardsonLucy algorithm with a thresholdnoise parameterof 3 [White,
1994a,b]. Details and examplesof this procedureare presented
Absolute
Photometric
Calibration
by Wolff et aI. [ 1997].
The absolutephotometriccalibrationof WFPC2 data must
take into account both the time-variable nature of the system
Registration
and
Photometric
Correction
throughput,as well as the extendedand inhomogeneous
nature
(surfacealbedo features) of Mars. For the first component,we
rely on the analysesby Holtzman et aI. [1995a,b]and Bagget
et aI. [1996] of the extensive standard star observation and
monitoringprogramscarried out by STScI. The photometric
Perhaps the most difficult aspect of dealing with Mars
multispectral data is the fact that the planet rotates
significantlyduring the time it takes to obtain a typical set of
images. Thus it is not possibleto simply overlay images at
different wavelengthsto create ratios or band depth maps; the
calibration for the WFPC2
discrete filter observations was
images must first be registeredusing map projection software.
derivedusingthe SYNPHOT referencefiles providedby STScI We performedthe transformationfrom image (x,y) coordinates
(July 1995 update[Bagget et aI., 1996]). These files allow a to projectedlatitudeand longitudeusingautomatedsoftwarewe
calibration to be determined that converts corrected DN to flux
developedin the IDL programminglanguage. First, the central
units [e.g., W cm-2 gm-1) for each filter in our observing (sub-Earth) pixel is automatically found by iteratively
program. More details on the derivationof the photometric searchingfor the limb of the planet and fitting an elliptical
calibration for the discrete filter observations can be found in
the work by Wolff et aI. [1997].
The photometriccalibration of the LRF has not yet been
fully determinedby the STScI calibrationprogram.Thus we
deviseda bootstrapcalibrationtechniquefor theseimagesthat
relies on calibrated ground-based and spacecraft spectra of
curveuntil a X2 fit parameteris minimized. Next, the date and
time from the image headers are automatically used to
determine planetary ephemeris information necessary for the
map projection (i.e., sub-Earth and subsolar latitude and
longitude, north polar angle, distance). Finally, the images
can be projected to one of 14 types of cylindrical, equal-area,
Mars and the information from STScI that is available on the
conic, or polar azimuthal projections. Our software uses fully
LRF system throughput. This LRF photometric calibration ellipsoidal map projection formulae
[Snyder, 1985;
schemeis outlined in Appendix A.
Bugayevskiy and Snyder, 1995], as HST images are sharp
The flux values for both the discrete filter images and the
enoughto detect the small but nonzero flattening of Mars, and
LRF imageswerethenconvertedto radiancefactoror I/F using spherical projection formulae would result in detectable
the methods of Roush et al. [1992] and Bell et aI. [1994] (I is
mapping errors.
the actual irradiancereceived from Mars within each HST pixel
Our map projection software also outputs images of the
and •F
is the theoretical irradiance received within each HST
incidence
and emission angles for each pixel, thus allowing
pixel from a perfectlydiffusingLambertiansurfaceilluminated
by the Sun and viewed at normal geometryat the heliocentric photometric corrections to be applied in order to properly
interpret absolutereflectancelevels in all regions within about
distance of Mars [Hapke, 1981]). Details are presented in
60 ø of the sub-Earth and subsolar points.
For the
Appendix B.
photometrically
corrected
images
discussed
here,
we
used a
The absolutephotometricerrors in this calibration process
simple Minnaert correction with a constant "typical" k
are conservatively estimated to be approximately 2 to 5% for
parameter of 0.7 [e.g., Harris, 1961; de Grenier and Pinet,
the discretefilter observations[Holtzman et al., 1995b; Wolff
1995].
et aI., 1997], but are likely 5 to 10% for the LRF observations
The end result of the data reductionand calibrationprocess
because of the additional assumptions and uncertainties
is a setof co-registered
and map-projected
imagecubes(spatial
discussedin Appendix A. We have not applied throughput
x spatial x spectral), calibrated to absolute I/F units, that can
correctionsfor UV contamination effects, thus worsening the
photometricaccuracy for the F255W filter data by possibly be analyzed using a variety of spectroscopicand imaging
spectroscopyanalysis tools.
more than 5%.
However, this filter is not critical for
mineralogicstudies,and flux values at 255 nm and 336 nm will
vary by 5-10% or more anyway, dependingon the dust and
cloud opacity and the Martian airmass [e.g., Clancy et aI.,
1996a, Wolff et al., 1997]. The magnitude of the UV
contamination
effect is less than 1% for all of the other filters
that we used [Holtzman et aI., 1995b]. Despite these
uncertainties, at this current level of accuracy, these data
represent some of the best calibrated Mars observations ever
obtained
from Earth.
Resultsand Interpretations
Spectra
Some representativespectrafrom our July 1995 nine-color
observations are shown in Figure 2. Figure 3 presents a
comparisonbetween the HST spectra and previous calibrated
Mars reflectancespectra obtained by McCord and WestphaI
BELL ET AL.'
MARS
SURFACE
MINERALOGY
FROM
HST Bright Region Spectra
0.70.•'"
•'"
•"'
•'"
HUBBLE
9113
HST Dark Region Spectra
0.70
•"'
SPACE TELESCOPE
I'"l'"l'"l'"l'"
(A)
0.60
0.60
6
9
5
0.50
0.50
7
5
9
0 4O
0.40
6
5
7
,.5
4
.
0.50
O3O
•
6
5
0.20
0.20
4
5
0.10
o.1o
2
1
0.00
400
600
,.,
0.00
1,,,I,,,•,,,•,,,•,,,
200
1
800
1000
1200
Wavelength(nm)
200
400
600
800
1000
1200
Wavelength(nm)
Figure 2. Representative
radiancefactor(I/F) spectraextractedfrom the observations
on July 6, 1995, at
1139UT. (a) Spectrafrombrightregions:(1) Moab, (2) Xanthe,(3) Chryse,(4) Tempe,(5) NorthPolarCap,
(6) Ares/TiuValles (NASA Mars Pathfinderlandingsite). (b) Spectrafrom dark regions:(1) NorthernAcidalia,
(2) SouthernAcidalia, (3) Sinus Meridiani, (4) Oxia Palus, (5) Margaritifer Sinus, (6) Mare Australe, (7)
westernsideof northpolarsandsea,(8) easternsideof northpolarsandsea,(9) SinusSabaeus.Each spectrum
is from a 3x3 pixel box, and the errorbar shownrepresents
the varianceof the spectrawithin that box. Each
spectrumis offsetby 0.05 unitsfrom the one below.
[1971] and Mustard and Bell [1994]. The HST spectraare
generallyconsistentwith previousmeasurements.There is a
systematic
increasein the reflectivityof the HST spectraat the
shortestwavelengthsas comparedto the 1969 and 1988-1989
data. This is likely a manifestationof the increasedcloudiness
of Mars during the aphelic apparition of 1994-1995 [e.g.,
Martin et al., 1995; Clancy et al., 1996b; Jameset al., 1996]
relative to the earlier observations that were obtained closer to
perihelion. The effect of cloudsis to increasethe reflectivity
preferentially at the shortestwavelengthsbecause of the
increasedRayleigh scatteringefficiency and also becausethe
surfaceitself is extremelydark in the blue and near-UV.
The HST nine-color spectra display interesting and
diagnosticcharacteristicsthat are consistentwith previous
spectroscopic investigations [e.g., McCord and Westphal,
1971; McCord et al., 1977a; Singer et al., 1979; Bell et al.,
1990]: (1) The slope of the near-UV absorption edge that
gives Mars its distinctive ruddy color varies with reflectivity
such that bright regions are typically "redder" than dark
regions; (2) There is a reflectivity maximum near 750 nm with
a position that is not a function of absolutereflectivity; (3)
There is a broad absorption band in the short-wave near-IR
between the reflectivity maximum near 750 nm and the longwavelength extent of our data at 1042 nm. The near-IR
spectral slope between 750 nm and 1042 nm is "red"
(reflectivity increasing at longer wavelength) for bright
regionsbut is neutralto "blue"for dark regions; (4) There is a
weak absorption/inflection at 673 nm superimposed on the
9114
BELL ET AL.' MARS SURFACE MINERALOGY FROM HUBBLE SPACETELESCOPE
Typical Dark Regions
TypicalBrightRegions
.4
.4
McCordand Westphal (1971): Spot 69-1
McCordand Westphal (1971): Spot 69-6
Mustardand Bell (1994): Spot 88-41
Mustard and Bell (1994): Spot 88-22
HST July 1995: ChrysePlanitia
HST July 1995: MargaritiferS•nus
LL
c'..,.
-o
cO
0
2
ß
O
(A) _
(B) _
0
200
400
600
800
1000
1200
Wavelength (nm)
2OO
400
600
800
1000
1200
Wavelength (nm)
Figure 3. Comparisonof spectraextractedfrom our HST data set and previousground-based
and spacecraft
calibratedspectraof Mars. The McCord and Westphal[1971] data are geometricalbedoat 5ø phaseangle,the
Mustard and Bell [1994] compositespectraare in reflectance,and the HST data are in I/F.
near-UV absorptionedge in the HST spectraof bright regions
that is usually absent in the spectra from dark regions
(however, see spectrum4 (Oxia Palus) in Figure 2b); (5) The
reflectivity of the residual north polar cap is considerably
higher than that of the surface at wavelengthsshorterthan 673
nm and is comparable to or slightly higher than that of the
surfaceat 673 nm and longer. Despite the usual whitish/bluish
appearancein typical images of Mars, the cap is in fact quite
red.
Similar overall spectral character is also observed in the
other Cycle 5 nine-color HST image sequencesobtained in
August 1995. The five-color image sequences obtained
betweenAugust 1994 and May 1995 (Cycle 4) are not able to
detect spectralvariationslongward of 673 nm, but the spectra
from
255 to 673 nm exhibit
color
variations
consistent
with
the Cycle 5 data.
Global
Color
Variations
Rather than individually examine tens of thousands of
spectra, a more fruitful way to explore spectral variations is
through the use of color ratio images and similar imageoriented analysis techniques(see Appendix C). James et al.
[1996] provided an initial analysis of red to blue (673 nm to
410 nm) color variations and spectral units from the HST
Cycle 4 images obtainedin February 1995. Analysis of twodimensional (2-D) histogram scatter plots of 673 nm versus
410 nm I/F valuesby Jameset al. [1996] revealeda numberof
distinctcolor units and showedthat the ubiquitouscloud cover
observed near aphelion can substantially influence the
interpretation of multispectral images obtained in blue and
near-UV wavelengths. Specifically, clouds produce elevated
reflectance
values shortward of 502 nm and thus frustrate
efforts to derive the true surface color ratio values.
We presentadditionalvisible-wavelengthcolor ratio data in
Figure 4 and Plate 1. Figures 4a and 4b show the reflectances
at 673 nm and 410 nm for the Syrtis Major-centered
hemisphereas imaged in February 1995, and Figure 4c shows
the ratio of these two images. Plate l a presents a 2-D
histogramplot of the 673/410 nm color ratio (ordinate) versus
the reflectanceat 673 nm (abscissa). The ratio image and
histogram both reveal a number of distinct color units,
delineatedin Plates l a and lb. The 2-D histogramshows a
large and diffuse cluster correspondingto the dark, moderate
673/410 nm ratio regions Syrtis Major, Hesperia Planum, and
VastitasBorealis (blue in Plates la and lb); a compactcluster
corresponding to the bright, very high 673/410 nm ratio
region encompassingparts of Elysium and Utopia Planitia
(magenta);and anotherrather diffuse clustercorrespondingto
the bright, high 673/410 nm ratio regions Arabia and Isidis
BELL ET AL.' MARS SURFACE MINERALOGY FROM HUBBLE SPACE TELESCOPE
9115
Planitia (yellow). Plate l a also shows there to be a substantial partsof theplanetnot imagedin Cycle4 (Figure5a). The fine
amountof spectralmixing between thesecolor units, and Plate detailsare slightlydifferentbetweenthe February1995 Cycle
ø) andJuly1995Cycle5 data(Ls=122
ø) because
of the
lb demonstratesthat the mixing occurs primarily along the 4 (Ls=63
boundaries
between
the individual
units.
Other
outlier
color
units in the 2-D histogram correspond primarily to the polar
cap and other bright condensateregions (green), and regions
along the limb and at high emission angles, where
wavelength-dependentlimb darkening accountsfor most of the
color variation (red and cyan).
These same general color ratio units can also be identified
in our Cycle 5 HST images in the Syrtis region and in other
different distribution of clouds at this later seasonal date. In
general,our Cycle 5 HST imageshavethe ability to extend
previous
ground-based
andspacecraft
colorunitresultsintothe
short-wave near-IR, where additional mineralogic information
can be obtained from the broad region of 750-1050 nm
absorption
thatresultsfrom a combination
of ferricandferrous
minerals.Figures5b, 5c, and5d showexamplesof three-color
ratio and 2-D histogrampairsfrom our near-IR data.
The ratio between 740 nm and 1042 nm (Figure 5b) is a
measure of the overall near-IR spectral slope. The near-IR
slope is sensitive to mineralogy (especially Fe2+-bearing
minerals with a strong 1-pm absorption feature), the opacity
and composition of Mars atmospheric aerosols, and the
presence of particle coatings or rinds [e.g., Fischer and
Pieters, 1993; Erard et al., 1994]. Figure 5b shows that the
740/1042 nm color units occur in two primary clusters (linked
by a well-defined mixing trend) and that the low-albedo
regions have approximately 20% higher 740/1042 nm ratio
values than the high-albedo regions. This result can also be
seen in the individual spectraof Figure 2: darker regions have
flat or negative near-IR spectral slopes, while brighter regions
have positive near-IR spectral slopes.
The ratio between 860 nm and 953 nm (Figure 5c) is a
measure of the relative strengths of the 860-nm ferric
absorption band and the 953-nm ferrous absorption band.
Figure 5c (and Figure 2) reveals that the low-albedo regions
generally exhibit flat spectra between 860 and 953 nm,
althoughthere is a roughly +5% 860/953 nm color ratio value
variation among dark regions. The brightest regions exhibit
"red" 860/953 nm spectral slopes (860/953 ratio values
< 1.0), and also show +5% color ratio variations. There is a
clear mixing trend between these endmembercolor ratio units.
The ratio between953 nm and 1042 nm (Figure 5d) provides
a way to characterize the shape of the 1-pm pyroxene
absorptionfeature, which is centered near 920 to 950 nm for
low-Ca orthopyroxenes and near 950-1000 nm for high-Ca
clinopyroxenes[e.g., Adams, 1974; Pinet and Chevrel, 1990;
Mustard et al., 1993]. The global variation in the 953/1042
nm ratio is small (mean+1o = 0.92+0.03), and there is only a
O*
weak
correlation
between
albedo
and
953/1042
nm
ratio.
However, Figure 5d demonstratesa stunningexampleof one of
the greatestassetsof our data set: spatial resolution. A number
of small surface regions (from 60 to 100 km in size) exhibit
significantly lower 953/1042 nm ratio values than the rest of
the planet, meaning that these regions have increased953-nm
+60'
+30':'
c
Figure 4. Red/blue color ratio resultsfrom the February
1995 HST observations near opposition. All images are
shown in a Molleweide projection of the Martian eastern
hemispherenorth of-60 ø, with grid lines at every 30ø of
longitude
and15ø of latitude. (a) Map of calibratedI?F at 673
nm, showing prominent Syrtis Major dark albedo feature
(center) as well as the bright regionsIsidis and Arabia, the
retreatingnorthpolar cap, and the Hellas Basin(-45ø, 300ø),
(b) Map of calibratedI?F at 410 nm, showingthe loss of
surfacealbedofeature contrastin the blue, exceptfor the north
polar cap, cloudsforming along the morningand evening
limbs and in the southpolar region, and a discretecloud over
the volcano Elysium (+20ø, 220ø), (c) Ratio of 673 nm
(Figure 4a) to 410 nm (Figure 4b). The image has been
enhancedso that black corresponds
to a ratio value of 0.5 and
white corresponds
to a ratio value of 6.2.
9116
BELL ET AL.'
MARS SURFACE
MINERALOGY
FROM HUBBLE
SPACE TELESCOPE
I
!
I
I
I :::"l'" '1' I
! --
m
--
--
m
--
---
--
m
--
,--
m
--
C)
--
m
---
--
--.
..
---
m
--..
,-,--
..
.-..
0.00
0.10
0.20
0.30
0.40
67,%nm I/F Values
,i
ß
ß
ß,,......,:'....,::..:,:+60
o.
ß....70
ß
ß
ß
:0
-•,•
ß ....... ß: ...............
ß
ß
e"
ß
......:.½
.......:
ß
ß
ß
ß
:;.......•...... +3;0o...•
ß
ß
ß
ß
ß
ß
ß
ß
ß
ß
ß
ß
ß
ß
ß
ß........ ß; ......... ß• ........ •ß........ •ß ........ -ß ......... ß• ....
ß
ß
.
:
*
: ........ ß...................
ß
ß
ß=........
ß.
ß
......
ß
=.........
.
ß
; ........
%
ßß
ß
'..
:
.**.........
=.........
• ........
.:.......
ßß
ß
ß
:
%
:
-
' ..... 30' .... -:......... ß
ß
ß
:
0 ß.... eß
.....
ß
o
ßß
' ..........
e'lle
:
:ß..................
ß
el
ß
ß
ß
.
ß........... ,...............•.......•.......•..... ,........
'"'"'% .....:......:'.....•'-60' ......
0ø
: .... '"
300'
.•.'..•::
:'.... •; "'"
240 ø
180'
Plate 1. Red/bluecolorratioresultsfrom the February1995HST observations
nearopposition.All images
are shownin a Molleweideprojectionof the Martianeasternhemisphere
northof-60 ø, with grid linesat every
30ø of longitude
and 15ø of latitude. (a) Two-dimensional
(2-D) histogram
of the673/410nm ratio(Figure4c)
versusradiancefactorat 673 nm (Figure4a). This andsubsequent
2-D histograms
are shownso thata higher
frequencyof occurrenceis seen as a darker and densercluster. Here, black correspondsto five or more
occurrences
of eachdata value in eachparticular(x,y) bin, and white corresponds
to 0 occurrences.
Different
clustersof datahavebeengroupedby color,(b) Spectralunit mapderivedfrom the histogramunitsin Plate l a.
This unit map providesa representative
exampleof how spectralvariationsseenin ratio imagescan be
correlatedwith specificsurfaceregionsor geologic/albedounits. See text for details.
absorption.Theseregionsare associated
with specificimpact
craters in the Acidalia hemisphere,with the Nili Patera and
Meroe Pateracalderaeof Syrtis Major, and with the small dark
teardrop-shapedfeature to the northeast of Syrtis Major
(referredto as Nubis Lacusand/orAlcyoniusNodusin classical
albedomaps;it doesnot appearto have any uniquegeologic
characteristic).
Absorption
Band
Depth
Variations
While color ratios provide diagnostic information on
spectral slope variations, we chose our Cycle 5 imaging
wavelengthsto allow for the mappingof the actual absorption
features associatedwith iron-bearing minerals on the Martian
surface. Specifically, we can use the images at 740 nm and
BELL ET AL.'
2t.0•
150'
90*
30•W
MARS SURFACE MINERALOGY
•0'*
270'
210'
FROM HUBBLE SPACE TELESCOPE
21.0
•
t50'
90
•*W
5'•0'
27'0•
9117
ZlO"
1.10
1,05
.,..,
E
.• 0.95
o
}...
Ec0.90
.............
:*'- •
".,.';':.::':i-i
,'"'-:,-:•.:..,..,...,....;,: -:....
:,::.;,::,:½:;•?%.:.,..;::d'
.. :: "-
0.85
:% .:.::..:
0,80
O. '0
0.20
0.30
0_40
0 50
Figure5. Initialcolorratioresults
fromtheJuly1995HSTobservations.
All of theratioimages
presented
herearestretched
to encompass
thesamerangeasthey axisof theircorresponding
histogram,
andareshown
ona Molleweide
mapprojection
centered
on30øWlongitude,
0ølatitude.Pixelshaving
anincidence
angleor
emission
anglegreaterthan60ø wereremoved
fromboththemapsandthehistograms.
An estimate
of the 1-•
x axisandy axiserrorbaris shownfor each2-D histogram
(seeAppendixC). (a) The 673/410nm colorratio
imageand 2-D histogramof 673/410 nm ratio versus673-nmradiancefactor, (b) The 740/1042 nm color
ratioandhistogram
of ratioversus
1042-nm
radiance
factor,(c) The860/953nmcolorratioandhistogram
of
ratioversus953-nmradiance
factor, (d) The 953/1042nm colorratioandhistogram
of ratioversus953-nm
radiance
factor.
1042 nm as continuumpointsand spatiallymap the strength
of the 860-nm (ferric) and 953-nm (ferrous)absorptionbands
usingthe banddepthmappingtechniquesdefinedby Clark and
Roush [1984] and Bell and Crisp [1993]. Details of the
method and the techniqueused to estimateerrors in the band
depthmapsare providedin AppendixC.
Figure 6a displays a map of the 953-nm pyroxene
observations
absorption feature relative to a linear continuum defined
absorptionin Mars dark regions[e.g., Bell, 1992; Murchie et
between740 nm and 1042 nm. This map revealsthe powerof
HST as a mineralogic mapping tool. It is apparentthat the
953-nm band is much strongerin the dark regionsthan in the
brightregions,consistentwith the spectraof Figure 2 and the
colorratio dataof Figure5. However,the strengthof the 953-
al., 1993; Mer•nyi et al., 1996].
strengthof the 860-nm band seen in our HST Mars spectra
exhibitsa weak inversecorrelationwith albedo:dark regions
generallyhave a stronger860-nm featurethan bright regions
by an averageof 3 to 4%. This relationshipis difficult to
derivefrom examiningjust a few spectra(as in Figure2). This
result is consistentwith previousground-basedand Phobos2
that showed
evidence
for increased
"ferric"
Anotherinterestingresultis shownin Figure 6c, which is a
map of the depth of the 673-nm band seenin the spectraof
Figure 2 relative to a linear continuumdefined between 502 nm
and 740 nm. The map and the histogramreveal the curvatureof
nm bandvarieswithinthe darkregions:the bandis strongest the Martian spectrumbetween 502 and 740 nm, which may be
in SyrtisMajor andothernearbyequatorialdark regions,but it relatedto the degreeof crystallinity of surfaceiron mineralogy
is weaker in the northern dark regions Acidalia and [e.g., Guinnesset al., 1987; Morris and Lauer, 1990; Morris et
Utopia/Borealis.
al., this issue]. While there is no systematictrend above the
Figure 6b shows a map of the 860-nm ferric oxide errors in 673-nm band depth versus albedo, the 673-nm band
absorption feature relative to the same linear continuum varies suchthat the spectraof most surfaceregionsare convex
between740 nm and 1042 nm. This map reveals that the (negativeband depth in Figure 6c). Some bright regionsand a
9118
BELL ET AL.' MARSSURFACEMINERALOGYFROMHUBBLESPACETELESCOPE
70.'"'
1 .'t0
1,0:5
0,95
'•
1,00
.,,•
0,90
,.
'% 0,95
::::'*:L:;
....
E
0,9-0
o 85
0.85
0.10
8. i5
0.20
0.25
0 30
0,35
0.40
0.10
O, •.5
0.20
0.25
0.,30
0,35
0.40
953
953 nm I/F volue.•
Figure
(continued)
similar to the ratio map of Pinet and Chevrel [1990]: bright
regions are spectrally flat in the near-IR, while dark regions
are "blue." However, Figures 5d and 6a reveal individual
craters and caldera with much stronger 953-nm pyroxene
absorption features than typical dark regions. Comparison
Discussion
with the 953/1042 nm ratio image (Figure 5d) indicates that
The resultspresentedabove provide new informationon the these regions of increased 953-nm absorptionare likely not
spectroscopic
variability of the Martian surface. For example, causedby the presenceof olivine or very high-Ca pyroxene
color ratio images and 2-D histogram analysisreveals that the becausethere is no associatedstrong increasein the 953/1042
classical bright regions can be subdivided into at least two
nm ratio value for these areas. The lack of atmospheric
distinct red/blue color units that occur in spatially distinct interference/turbulence and the exceptional ability to model
regionsin Isidis and Utopia Planitia (Plate la and Figure 5a). and correct the HST PSF [see Wolff et al., 1997] allow this
This result is consistentwith the color ratio and 2-D histogram level of spatialdetail to be obtainedin the near-IR for the first
analysesof Viking Orbiter imaging by Soderblom et al. time. The implication is that there are small regions of the
[1978] and McCord et al. [1982]. While the Viking data have Martian surface that exhibit a very strong pyroxene
higher spatial resolution and thus can reveal finer details absorptionband either because (1) there is simply a higher
associatedwith specific craters or other features,it was abundanceof pyroxene in the rocks and soils of these regions;
restrictedin wavelength to only three bandsbetween 450 nm
(2) the pyroxenein theseregionsis "fresher"or less alteredto
and 590 nm. The HST images can thus extend the color ratio ferric phasesthan in other regions, and/or (3) the particle size
results into the near-IR and provide additional diagnostic of the pyroxene-bearing surface minerals is larger, thus
informationkeyed to specific mineralogicvariations.
leadingto increased953-nm absorption.
Our HST near-IR color ratio results are consistent with
The ubiquity of the 860-nm absorption feature and its
with albedo
came as somewhat
of a
previous ground-based near-IR imaging observations by inverse correlation
McCord et al. [1977b] and Pinet and Chevrel [1990], and they surprise, especially considering the well-known correlation
extend these results by providing higher spatial resolution between red/blue color ratio and albedo. However, the
measurements that are free from the typical terrestrial apparent increase in ferric mineral content in some dark
atmospheric contamination problems encountered between regions has been noted by previous ground-basedobservers,
750 and 1050 nm (e.g., 02, H20). For example, the overall and these HST data provide confirmation. How could dark
appearanceof the HST near-IR slope image (Figure 5b) is regions have a stronger 860-nm absorption band yet a
smaller number of isolated intermediate and dark regions
exhibit concave spectra(band depth > 0) between 502 and 740
nm (cf. Figure 2), however.
BELL ET AL.' MARS SURFACEMINERALOGY FROM HUBBLE SPACETELESCOPE
.
9119
.
:::::::::::::::::::::::::
:::::S•
: x:.3•
:•.'
..............
,'•:•
.... %..
210ø
1_0
90•
30•W
3%½
270ø
210ø
0.20
0.16
0,10
0.I0
0.05
0,00
0,05
O. lO
0.20
0.30
0.40
9'53 nm I/F vo:u•:
0.10
0.15
0.20
0.25
030
0.35
0.40
860 nm I/F volues
Figure 6. Initial banddepthmappingresultsfrom the July 1995 HST observations.All of the banddepth
imagespresentedhere are stretchedto encompassthe same range as the y axis of their corresponding
histogramand are shownon a Molleweidemap projectioncenteredon 30øW longitude,0ø latitude. Pixels
having an incidenceangle or emissionangle greaterthan 60ø were removedfrom both the maps and the
histograms.An estimateof the 1-o x axisandy axiserrorbar is shownfor each2-D histogram.SeeAppendix
C for a discussion
of the banddepthmappingtechniqueand error analysismethod. (a) The 953-nm banddepth
defined relative to a linear continuum between 740 nm and 1042 nm.
There is a clear trend above the error
indicatingthat the low albedo regionsgenerally exhibit a 5-10% deeper 953-nm absorptionthan bright
regions, (b) The 860-nmbanddepthdefinedrelativeto a linearcontinuumbetween740 nm and 1042 nm.
There is a weak correlation above the 1-o level between 860-nm band depth and albedo such that low-albedo
regionsexhibita =3-5% deeper860-nmbandthan high-albedoregions, (c) The 673-nm banddepthdefined
relativeto a linear continuumbetween502 nm and 740 nm. There is no systematiccorrelationbetween673nm banddepthand albedo,althoughthe largestvariationis seenamongintermediate-and low-albedoregions.
shallowervisible spectral slope? One possible solution is
that the visible spectral slope is dominated by poorly
crystallineferric-richmaterialsimilarto the nanophase
ferric
oxidepigmentsstudiedby Morris andLauer[1990], Morris et
al. [1993, 1997], and others. This pigmentingmaterial does
not have an 860-nm absorptionfeature but it does have a
strongred spectralslope. The 860-nm bandcouldthen arise
from a well-crystalline ferric oxide phase like hematite
occurringpreferentiallyin the lower albedo regions. The
additionalabsorption
at 860 nm in the dark regionsmay result
from moreof this ferric phaseoccurringin theseregions. This
possibilityis supported
by the observation
of greater673 nm
band depth variability in low albedo regions (Figure 6c)
becausemany ferric oxides also exhibit an absorptionin the
600 to 700-nm region as well as the strongerfeaturecentered
near 850 to 900 nm.
Alternately, the additional 860-nm
absorptionin the dark regions may be causedby a band
centeredlongwardof 860 nm that has a broad wing extending
down through 860 nm. In this case, the origin of the
additional860-nm absorptioncould be ascribedto a very low
Ca pyroxene,an orthopyroxene-clinopyroxene
mixture [e.g.,
Mustard and Sunshine, 1995] or an iron oxyhydroxide phase
like goethite. Given the coarsespectralsamplingprovidedin
our HST data set and the broad overlap in absorptionbands
possiblefor variousferric and ferrousphases[e.g., Morris et
al., 1995], it is not possibleto uniquely determinethe origin
of this additional 860-nm absorption. The optimal way to
providethis discriminationwould be with a combinationof
high spectral resolution and high spatial resolution
observations.
9120
BELL ET AL.'
MARS SURFACE
MINERALOGY
FROM HUBBLE
SPACE TELESCOPE
from 5% to 15% deep. The spatialdistributionof this band is
not a strongfunction of albedo, unlike the red/blue spectral
slope. These HST data confirm previous ground-based
observations, indicating that many of the classical dark
regions(includingAcidaliaand Syrtis)exhibit--3 to 5% deeper
860-nm absorptionfeature than the classicalbright regions.
The origin of this additional860-nm absorptionin the dark
regionsmay be related to a greater abundanceof a wellcrystalline ferric phase or a very low Ca pyroxene or
orthopyroxene/clinopyroxenemixture.
4. Mappingof the depthof the 953-nm ferrousabsorption
band showslarge variationsin the depth of the 1-pm feature.
Bright regionstypically have either no band or only a weak
absorption,
while dark regionsexhibit a 953-nm bandranging
from --7% to 15% deep. The increasesin 953-nm banddepth
within individual craters, calderas, and other small geologic
units in the dark regionsare interpretedas indicatingeither an
increasein the abundance, "immaturity," and/or particle size
of pyroxenein theseareas.
..c •0.00
Appendix A' Photometric Calibration of LRF
Images
The scheme that we developed to determine the LRF
photometric calibration proceeded as follows. We used the
STScI/WFPC2 Exposure Time Calculator (ETC) software for
extended objects to estimate the expected Mars DN value for
each LRF wavelength (at the appropriate gain and exposure
time settings). The current version of the ETC uses preflight
calibration data in order to provide an estimate of the
photometricperformance of the LRFs and thus does not take
into accountthe likely differences between preflight and inflight performance and calibration.
To estimate the
photometric calibration of the LRF images, we input the
E .-0.05
-0.1
o
-0,15
o. 15
0•10
0 20
o. 25
0.50
67,,,5nm I/F values
Figure 6.
appropriateMars surfacebrightness(mag/arcsec
2) and use a
(continued)
spectraltype G5 stellar spectrumas the sourceto approximate
reflected sunlight. An estimated photometric scale factor (in
W cm-2gm-• DN-•) is thenobtainedto convertDN to flux by
Conclusions
This paper presents the details of the collection and
calibrationof imagesof Mars obtainedby HST during 19941995, as well as some initial analyses. The images were
obtainedbetween 255 nm and 1042 nm as part of a long-term
HST investigation of seasonalphenomenaon Mars. The
primarygoalof obtainingthe near-IRimagesdiscussed
hereis
the study of Martian surface mineralogy. The calibration
exerciseperformedon the data resultedin a spectacular
set of
imagesthatexhibit--5% photometricaccuracyfrom 410 nm to
1042
nm.
Our
initial
scientific
examination
of
these
calibrated data shows the images to compare favorably with
previous ground-based and spacecraft imaging and
spectroscopic
observations.Some of the most salientresults
found to date include the following.
1.
Discrimination
of at least two distinct red/blue
color
units within the classicalbright regions, with one unit having
a 20-25% higher 673/410 nm ratio value than the other,
possibly becauseof an increase in nanophaseferric iron
abundance.
2. Mapping of near-IR spectral variations on a scale
unprecedented
in previousEarth-basedtelescopicobservations
and coveringmany regionsnot yet imagedby spacecraftin the
near-IR.
The maps reveal general correlation between
1042/740 nm spectralratio and albedo, and the increased
spatial resolution allows the near-IR color properties of
individual cratersand small geologicunits to be investigated.
3. Mapping of the depth of the 860-nm ferric absorption
band reveals that all surface regions exhibit a band ranging
dividing the input Mars surface brightnessby the DN value
estimatedby the ETC.
In comparison with calibrated ground-based spectra of
Mars, the ETC-based LRF calibration schemeyields 740 and
860-nm flux values that are systematicallyapproximately20%
too high. This is likely the result of compoundingerrorsfrom
the various system throughput estimates made by STScl in
developing the ETC in the absence of a completely
characterizedLRF in-flight calibration and from the derivation
of the solar flux convolved through the LRF bandpasses(the
transmission functions of the LRF filters were only
approximated, and the in-flight system efficiency of the
WFPC2-LRF combination has not yet been determined; see
Appendix B). Given the various uncertaintiesinvolved, a 20%
absoluteerror is not unexpectedlylarge.
In order to correct for this systematicoffset as best we can,
we usedthe ground-based/ISMcompositereflectancespectraof
Mustardand Bell [1994] to help determinea correctionfactor
for the 740 and 860-nm LRF data. This was a two-step
process: first, we used the LRF photometriccalibration values
derivedaboveand the solarflux valuesdeterminedin Appendix
B to calibratethe data to I/F, and then we extracteda typical
bright region spectrumfrom the Isidis region in the July 6,
1995, image cube from 0336 UT (Table 1). We then examined
the Mars bright region composite spectrum of Spot 41
(Olympus-Amazonis) from Mustard and Bell [1994] and
determinedthe ratios of the compositespectrum'sreflectances
to the HST I/F valuesat 740 and 860 nm. The averageof these
ratios was found to be 0.80. Thus the LRF photometric
calibrationvalues derived above are multiplied by 0.8 to yield
BELL ET AL.:
MARS SURFACE MINERALOGY
FROM HUBBLE SPACE TELESCOPE
9121
Error propagation using standard derivative-based error
a more accurate estimate of the actual I/F values of the HST
data. Scalingbothof the LRF imagesin this way by the same formulae [e.g., Bevington, 1969] yields the following
amountalso preservesthe value of the ratio betweenthese equationsfor calculatingerrorsin ratios (I R) and band depth
wavelengths.We notethat it is not criticalthat the exactsame maps(IsD):
regionof the planetis usedfor this scalingtechnique:what
reallymattersis thattwo spectraarechosenthatarebothfrom IR = Is / I L
"typical" bright regions (thus likely having similar
mineralogy)
andthatbothhavesimilarreflectance
levels. The
-- IL!
Olympus-Amazonis
spectrum
of MustardandBell [1994]and øR
_ •/Ors
2+IRCrL
22
(Cla)
(Clb)
the HST Isidis spectrumsatisfythesecriteria.
AppendixB: Determinationof I/F Values
The flux values for both the discrete filter images and the
IBD: 1- (IB / Ic)
(C2a)
_••foB
2+(IB/Ic)[Os(1-f)
2 2+o•f
22]
OBD-Ic
(C2b)
LRF imageswereconverted
to radiancefactoror I/F usingthe
methodsof Roushet al. [1992] and Bell et al. [1994] modified
Knowledgeof the errorson eachof the images(Os, os, OL)
for the squarepixelsof HST (as opposed
to circularapertures). should come from a rigorous formal propagationof errors
This modification results in the expression for F M, the
along the data reduction pipeline. However, this is not
theoretical irradiance received within each HST pixel from a
possiblefor our HST databecausethe errorson manyof the
perfectlydiffusingLambertiansurfaceilluminatedby the Sun standardSTScI calibration productsand reductionalgorithms
and viewedat normalgeometryat the heliocentricdistanceof
usedin the pipelineare unknownor indeterminate.
Instead,for
Mars [Roushet al., 1992, equation 8], becoming
our analysis(Figures5 and 6) we rely on a more empirical
approachby adoptingthe error bars in the representative
FM ----
4Fs
sin
2(l-l/2)
•D
2
(B1)
wherenFs is the solarirradianceat 1 AU, gl is the angularsize
of an HST pixel in arcsec,andD is the heliocentricdistanceof
Mars in AU. Values of F$ at each of our HST wavelengths
(Table 2) were obtainedby using the World Meteorological
Organization(WMO) solar flux spectrumof Wehrli [1985,
brightand dark regionspectraof Figure3 as the "typical"
errors for our HST data. These are not true instrumental errors,
per se, but are measuresof the variationof homogeneous
surfaceunits over a 3x3 or 5x5 pixel surface region. These
errorsareusedas the [os, os, oœ]valuesin equationsClb and
C2bandoRandOBDare calculated
separately
for bright(Figure
3a) anddark (Figure3b) regionsfor eachratio imageor band
depthmap. The largerof the calculatederrorsfor brightand
darkregionsis shownasthe verticalerrorbar in Figures5 and
1986]. For the discreteHST filters, the WMO solar spectral
6; the horizontal error bar on the I/F valuescomesdirectly
data were convolved with the system efficiency function for
from the data in Figure 2.
eachWFPC2 filter as describedby Wolflet al. [1997]. For the
LRFs, the systemefficiencyof the filters in flight has not yet
beenprovidedby STScI or theWFPC2 IDT, sowe estimatedFs
by simply convolvinga gaussianfilter transmissionprofile
havinga centerat eachof the LRF wavelengths
anda full width
at half maximum appropriatefor each wavelength [Burrows,
1995] with the full-resolution WMO solar flux spectrum.
There is obviously much uncertaintyin this determinationof
F$ for theLRFs(seeAppendixA), butthe valuescanbe refined
once the resultsof the ongoing STScI LRF calibrationprogram
are completed.
Final calibratedI/F values were derived by dividing the Mars
flux values determined using the photometric calibrations
Acknowledgments. We are extremely grateful to WFPC2 IDT
members David Crisp and Karl Stapelfeldt for their assistancewith
determiningthe best possibleflatfields for the LRF images. We thank
Andy Switala and Tom Daley for crucial help in developingthe
automated map projection and data analysis software, and Paul
Helfenstein for assistance with the voodoo art of photometric
calibration. We thank B. Ray Hawke and a mystery reviewer for
providinga careful review of the initial manuscript,and JohnMustard
for reviewingan on-line versionof the revisedpaper. Fundingfor this
researchwas providedby grantsfrom the NASA PlanetaryGeologyand
GeophysicsProgram(NAGW-5062) and the SpaceTelescopeScience
Institute.
This research was based on observations with the NASA/ESA
Hubble Space Telescope obtained at the Space Telescope Science
Institute,which is operatedby Associationof Universitiesfor Research
definedby Wolffet al. [1997] andin AppendixA above(IM) by
the valuesof FM determinedusingequation(B 1).
in Astronomy under NASA contractNAS5-26555.
Appendix C: Color Ratios, Band Depth Maps,
References
and Error Analysis
Color ratios are formed by simple coregistration and
divisionof two images.Band depth maps are calculatedusing
Adams, J.B., Visible and near-infrareddiffuse reflectancespectraof
pyroxenesas appliedto remotesensingof solid objectsin the solar
system,J. Geophys.Res., 79, 4829-4836, 1974.
threecoregistered
imagesand a techniquebasedon Bell and Bagget, S., W. Sparks, C. Ritchie, and J. MacKenty, Contamination
correction in SYNPHOT for WFPC-2 and WF/PC-1, WFPC2
Crisp [1993]: images on the short-wavelengthside (I s,
wavelength•,s) and long-wavelength
side (IL, wavelength•,L)
of an absorptionband at wavelength•,B are usedto constructa
continuumimage(I½) that represents
the value of eachpixel at
•,s alonga line definedby the pixel valuesat •,s and•,L. The
lnstrum.Sci. Rep. 96-02, SpaceTelescopeSci. Inst., Baltimore, Md.,
1996.
Bell, J.F., III, Charge-coupleddevice imaging spectroscopy
of Mars, 2,
Resultsand implicationsfor Martian ferric mineralogy,Icarus, 100,
575-597, 1992.
fractionaldistance,f, betweenthe absorptionband wavelength
andthe short-wavelength
continuumpoint is (•,s- •'S)/(•'L-•'S), Bell, J.F., III, Iron, sulfate,carbonate,and hydratedmineralson Mars, in
Mineral Spectroscopy:A Tribute to Roger G. Burns,Geochem.Soc.
andthusthe continuumimage I½ is simplyequalto (1 -f)Is +
fIœ. Thebanddepthis thendefinedas 1 - (I B/ Is), whereIB is
the imageat •,s. This definitionof band depthallows for an
intuitive display of results: areas of an image having more
absorptionappearbrighter.
Spec. Pub. 5, edited by M.D. Dyar, C. McCammon, and M.W.
Schaefer, 359-380, 1996.
Bell, J.F., III, and D. Crisp, Ground-basedimagingspectroscopy
of Mars
in the near-infrared:Preliminary results, Icarus, 104, 2-19, 1993.
9122
BELL ET AL.:
MARS SURFACE
MINERALOGY
Bell, J.F., III, T.B. McCord, and P.D. Owensby, Observationalevidence
of crystalline iron oxides on Mars, J. Geophys. Res., 95,
14,447-14,461, 1990.
Bell, J.F., III, J.B. Pollack, T.R. Geballe, D.P. Cruikshank, and R.
Freedman, Spectroscopyof Mars from 2.04 to 2.44 [tm during the
1993 opposition: Absolute calibration and atmospheric vs.
mineralogic origin of narrow absorption features, Icarus, 111,
106-123, 1994.
SPACE TELESCOPE
The performanceand calibration of WFPC2 on the Hubble Space
Telescope,Publ. Astron. Soc. Pac., 107, 156-178, 1995a.
Holtzman, J.A., C.J. Burrows, S. Casertano,J.J. Hester, J.T. Trauger,
A.M. Watson, and G. Worthey, The photometricperformanceand
calibration of WFPC2, Pub. Astron. Soc. Pac., 107, 1065-1093,
1995b.
Huguenin, R.L., The silicate componentof martian dust, Icarus, 70,
162-168, 1987.
Bell, J.F., III, P.B. James,L.J. Martin, R.T. Clancy, S.W. Lee, and D.
Crisp, Mars surface mineralogy from Hubble Space Telescope
multispectralimaging: 1994 pre-oppositiondata (abstract),Lunar
Planet. Sci. Conf., XXVI, 95-96, 1995.
Bevington,P.R., Data Reduction and Error Analysisfor the Physical
Sciences,336 pp., McGraw-Hill, New York, 1969.
Bibring,J.-P., et al., ISM observationsof Mars and Phobos:First results,
Proc. Lunar Planet. Sci. Conf., 20th, 461-471, 1990.
Bugayevskiy, L.M., and J.P. Snyder, Map Projections: A Reference
Manual, Taylor and Francis,Bristol, Pa., 1995.
Bums, R.G., MineralogicalApplicationso.fCrystalField Theory, 224 pp.,
CambridgeUniv. Press,New York, 1970.
Bums, R.G., Origin of electronic spectraof minerals in the visible-near
infrared region, in Remote GeochemicalAnalysis: Elemental and
Mineralogical Composition, edited by C. Pietersand P. Englert, pp.
3-29, CambridgeUniv. Press,New York, 1993.
Burrows, C.J. (Ed.), Wide Field and Planetary Camera 2 Instrument
Handbook, version 3.0, Space Telescope Sci. Inst. Pub., Baltimore
Md., June 1995.
Clancy, R. T., M. J. Wolff, P. B. James,E. Smith, Y. N. Billawala, S. W.
Lee, and M. Callan, Mars ozone measurements near the 1995
aphelion:Hubble SpaceTelescopeultraviolet spectroscopywith the
Faint Object Spectrograph,J. Geophys.Res., 101, 12,777-12,784,
1996a.
Jakosky,B.M., and R.M. Haberle, The seasonalbehavior of water on
Mars, in Mars, edited by H.H. Kieffer, B.M. Jakosky, and M.S.
Matthews,pp. 969-1016, Univ. of Ariz. Press,Tucson, 1992.
James,P.B., J.F. Bell III, R.T. Clancy, S.W. Lee, L.J. Martin, and M.J.
Wolff, Global imaging of Mars by Hubble Space Telescopeduring
the 1995 opposition,J. Geophys.Res., 101, 18,883-18,890, 1996.
Lauer, T.R., The reduction of Wide Field/Planetary Camera images,
Publ. Astron. Soc. Pac., 101, 445-469, 1989.
Leighton, R.B., N.H. Horowitz, B.C. Murray, R.P. Sharp, A.G.
Herriman, A.T. Young, B.A. Smith, M.E. Davies, and C.B. Leovy,
Television observations from Mariners 6 and 7, in Mariner-Mars
1969: A Preliminary Report,NASA Spec.Pub. 225, 37-82, 1969.
Martin, L.J., J.F. Bell III, P.B. James, S.W. Lee, and D. Thompson,
"Mars Watch" observations from the Lowell Observatory, and
observed cloudiness in 1994-95: Possible implications for dust
activity, Mars TelescopicObservationsWorkshop,LPI Tech. Rep.,
95-04, 19-20, 1995.
McCord, T.B., and J.A. Westphal, Mars: Narrowband photometry,
from 0.3 to 2.5 microns, of surface regions during the 1969
apparition,Astrophys.J., 168, 141-153, 1971.
McCord, T.B., R.L. Huguenin, D. Mink, and C. Pieters, Spectral
reflectance of Martian areas during the 1973 opposition:
Photoelectricfilter photometry, 0.33-1.10 [tm, Icarus, 31, 25-39,
1977a.
Clancy,R. T., A. W. Grossman,M. J. Wolff, P. B. James,D. J. Rudy, Y.
N. Billawala, B. J. Sandor, S. W. Lee, and D. O. Muhleman, Water
vapor saturationat low altitudes around aphelion: A key to Mars
climate?, Icarus, 122, 36-62, 1996b.
Clark, B.C., A.K. Baird, R.J. Weldon, D.M. Tsusaki, L. Schnabel, and
M.P. Candelaria, Chemical composition of Martian fines, J.
Geophys.Res., 87, 10,059-10,067, 1982.
Clark, R.N., and T.L. Roush, Reflectance spectroscopy:Quantitative
analysis techniquesfor remote sensing applications, J. Geophys.
Res., 89, 6329-6340, 1984.
de Grenier,M., and P.C. Pinet, Near-oppositionMartian limb darkening:
Quantification and implication for visible-near-infrared
bidirectional reflectance studies, Icarus, 115, 354-368, 1995.
Erard, S., J. Mustard, S. Murchie, J.-P. Bibring, P. Cerroni, and A.
Coradini, Martian aerosols: Near-infrared spectral properties and
effects on the observation of the surface, Icarus,
111, 317-334,
1994.
Fanale, F.P., S.E. Postawko, J.B. Pollack, M.H. Carr, and R.O. Pepin,
Mars: Epochal climate changeand volatile history, in Mars, edited
by H.H. Kieffer, B.M. Jakosky,and M.S. Matthews,pp. 1135-1179,
Univ. of Ariz. Press, Tucson, 1992.
Fischer, E.M., and C.M. Pieters, The continuum slope of Mars:
Bidirectionalreflectanceinvestigationsand applicationsto Olympus
Mons, Icarus, 102, 185-202, 1993.
Guinness,E.A., R.E. Arvidson, M.A. Dale-Bannister,R.B. Singer, and
E.A. Bruckenthal, On the spectral reflectance properties of
materialsexposedat the Viking landing sites,J. Geophys.Res., 92,
E575-E587,
FROM HUBBLE
1987.
Hapke, B., Bidirectional reflectance spectroscopy, 1, Theory, J.
Geophys.Res.,86, 3039-3054, 1981.
Harris, D.L., Photometryand colorimetryof planetsand satellites,in The
Solar SystemIII: Planetsand Satellites,pp. 272-342, editedby G.P.
Kuiper and B. Middlehurst,Univ. of ChicagoPress,Chicago,Ill.,
1961.
Holtzman, J. A., J. J. Hester, S. Casertano,J. T. Trauger, A.M. Watson,
G. E. Ballester, C. J. Burrows, J. T. Clarke, D. Crisp, and E. Evans,
McCord, T.B., R.L. Huguenin,and G.L. Johnson,Photometricimagingof
Mars duringthe 1973 opposition,Icarus,31, 293-314, 1977b.
McCord, T.B., R.B. Singer, B.R. Hawke, J.B. Adams, D.L. Evans, J.W.
Head, P.J. Mouginis-Mark, C.M. Pieters, R.L. Huguenin, and S.H.
Zisk, Mars: Definition and characterizationof global surfaceunits
with emphasison composition,J. Geophys.Res., 87, 10,129-10,148,
1982.
Mer6nyi, E., R.B. Singer,and J.S. Miller, Mapping of spectralvariations
on the surface of Mars from high spectral resolution telescopic
images,Icarus, 124, 280-295, 1996.
Morris, R.V., and H.V. Lauer Jr., Matrix effects for reflectivity spectra
of dispersed nanophase (superparamagnetic) hematite with
application to Martian spectral data, J. Geophys. Res., 95,
5101-5109, 1990.
Morris, R.V., H.V. Lauer Jr., C.A. Lawson, E.K. Gibson Jr., G.A. Nace,
and C. Stewart, Spectral and other physicochemicalpropertiesof
submicronpowdersof hematite(c•-FeaO3),maghemite(¾-Fe203),
magnetite (Fe304), goethite (c•-FeOOH), and lepidocrocite
(¾-FeOOH),J. Geophys.Res., 90, 3126-3144, 1985.
Morris, R.V., D.C. Golden, J.F. Bell III, H.V. Lauer Jr., and J.B. Adams,
Pigmenting agents in Martian soils: Inferences from spectral,
M6ssbauer,and magnetic propertiesof nanophaseand other iron
oxides in Hawaiian palagonitic soil PN-9, Geochim. Cosmochim.
Acta, 57, 4597-4609, 1993.
Morris, R.V., D.C. Golden, J.F. Bell III, and H.V. Lauer Jr., Hematite,
pyroxene,and phyllosilicateson Mars: Implications from oxidized
impact melt rocks from ManicouaganCrater, Quebec, Canada, J.
Geophys.Res., 100, 5319-5329, 1995.
Morris, R.V., D.C. Golden, and J.F. Bell III, Low-temperature
reflectivity spectra of red hematite and the color of Mars, J.
Geophys.Res.,this issue.
Murchie, S., J. Mustard, J. Bishop, J. Head, C. Pieters, and S. Erard,
Spatial variations in the spectralpropertiesof bright regions on
Mars, Icarus, 105, 454-468, 1993.
Mustard,J.F., and J.F. Bell III, New compositereflectancespectraof
Mars from 0.4 to 3.14 gm, Geophys.Res. Lett., 21, 353-356, 1994.
BELL ET AL.:
MARS SURFACE MINERALOGY
Mustard, J.F., and J.M. Sunshine, Seeing through the dust: Martian
crustalheterogeneityand links to the SNC meteorites,Science,267,
1623-1626, 1995.
Mustard, J.F., S. Erard, J.-P. Bibring, J.W. Head, S. Hurtrez, Y.
Langevin,C.M. Pieters,and C.J. Sotin, The surfaceof SyrtisMajor:
Compositionof the volcanicsubstrateand mixing with altereddust
and soil,J. Geophys.Res.,98, 3387-3400, 1993.
Pinet, P., and S. Chevrel, Spectralidentificationof geologicalunitson the
surface of Mars related to the presence of silicates from
Earth-based near-infrared telescopic charge-coupled device
imaging,J. Geophys.Res., 95, 14,435-14,446, 1990.
Roush, T.L., E.A. Roush, R.B. Singer, and P.G. Lucey, Estimates of
absoluteflux and radiancefactor of localizedregionson Mars in the
2-4[tm wavelengthregion, Icarus, 99, 42-50, 1992.
Roush,T.L., D.L. Blaney, and R.B. Singer, The surfacecompositionof
Mars as inferred from spectroscopic observations, in Remote
GeochemicalAnalysis: Elemental and Mineralogical Composition,
editedby C. Pietersand P. Englert, pp. 367-393, CambridgeUniv.
FROM HUBBLE SPACE TELESCOPE
9123
Toulmin, P., III, A.K. Baird, B.C. Clark, K. Keil, H.J. Rose Jr., R.P.
Christian,
P.H. Evans, and W.C.
Kelliher,
Geochemical and
mineralogicalinterpretation of the Viking inorganic chemical
results,J. Geophys.Res.,82, 4625-4634, 1977.
Wehrli,C. (Ed.), WorldRadiationCenterPub.615, July 1985.
Wehrli, C. (Ed.), World Climate Research Programme Publication
Series, No. 7, WMO ITD-No. 149, 119-126, Oct. 1986.
White, R. L., Image restorationusing the Damped Richardson-Lucy
method,in AstronomicalDam AnalysisSo.
ftware and SystemsIII,
editedby D. R. Crabtree,R. J. Hanisch,and J. Barnes, ASP Conf
Ser., 61, 292-295, 1994a.
White, R. L., Image restoration using the damped Richardson-Lucy
method,in The Restorationo.fHST Imagesand SpectraII, editedby
R. J. Hanischand R. L. White, pp. 104-110, SpaceTelescopeSci.
Inst., Baltimore, Md., 1994b.
Wolff, M.J., S.W. Lee, R.T. Clancy, L.J. Martin, P.B. James,and J.F.
Bell III, 1995 observationsof Martian dust stormsusingthe Hubble
SpaceTelescope,J. Geophys.Res., 102, 1679-1692, 1997.
Press, New York, 1993.
Sherman,D.M., and T.D. Waite, Electronicspectraof Fe3+oxidesand
Am. Mineral., 70,
J.F. Bell III, Departmentof Astronomy,Centerfor Radiophysics
and
Space Research,424 Space SciencesBuilding, Cornell University,
Singer,R.B., T.B. McCord, R.N. Clark, J.B. Adams, and R.L. Huguenin,
Mars surface composition from reflectance spectroscopy: A
summary,J. Geophys.Res., 84, 8415-8426, 1979.
Snyder,J.P.,Map ProjectionsUsedby the USGS,U.S. Geol. Surv.Bull.,
Ithaca, NY 14853-6801. (email: [email protected])
R.T. Clancy and M.J. Wolff, Space Science Institute, 1234
oxide hydroxides in the near-IR to near-UV,
1262-1269, 1985.
1532, 1985.
Soderblom, L.A., The compositionand mineralogy of the martian
surfacefrom spectroscopic
observations:
0.3-50 [tm,in Mars, edited
by H.H. Kieffer, B.M. Jakosky,and M.S. Matthews,pp. 557-593,
Univ. of Ariz. Press, Tucson, 1992.
Soderblom, L.A., K. Edwards, E.M. Eliason, E.M. Sanchez, and M.P.
Charette, Global color variations on the martian surface, Icarus, 34,
446-464, 1978.
Innovation Drive, Suite 294, Boulder, CO 80303-7814.
P.B. James,Departmentof Physicsand Astronomy,University of
Toledo, Toledo, OH 43606.
S.W. Lee, LASP, University of Colorado,Boulder, CO 80309.
L.J. Martin, Lowell Observatory,Flagstaff,AZ 86001.
(Received June 7, 1996; revised December
accepted December 19, 1996.)
12, 1996;