The Phobos 2 mission provided multispectral observations of Phobos over a large wavelength range and with relatively high spectral resolution. Here, researchers integrate results from three multispectral detectors by determining the ultraviolet-visible near infrared spectral properties of color and brightness features recognized in VSK TV images. Researchers present evidence that there are two fundamental spectral units within the region of overlapping coverage by the detectors. They describe the units' spectral and reflectance properties and discuss the implications of these results for the composition of Phobos

Bibring, Jean-Pierre

Britt, Daniel T.

Erard, Stephane

Head, James W.

Langevin, Yves

Murchie, Scott L.

Mustard, John F.

Pieters, Carle M.

English

NASA Technical Reports Server

N92-10822
DISK-RESOLVED SPECTRAL REFLECTANCE PROPERTIES OF PHOBOS FROM
0.3-3.2 _tm: PRELIMINARY INTEGRATED RESULTS FROM PHOBOS 2.
Scott L. Murchie 1, Stephane Erard2, Yves Langevin 2, Daniel T. Britt I , Jean-Pierre Bibring 2, John F. Mustard I, James W.
Head 1, and Carle M. Pieters 1; I Department of Geological Sciences, Brown University, Providence, RI 02912, U.S.A.; 2
lnstitut dAstrophysique Spatiale, University of Paris, Orsay, France.
Introduction. The Phobos 2 mission provided multispe_Iral observations of Phobos over a large wavelength range
and with relatively high spectral resolution. The VSK TV cameras obtained 0A0-0.56 I.tm ("visible") and 0.78-1.1 tun
("NIR') images with resolutions of 0.3-0.5 km, covering the lungimde region 30°-250°W [1]. The KRFM instrument
obtained -0.9-kin resolution, 8-channel 0.3-0.6 Iml spectra of two groundlrucks in low-latitude regions [2]. The ISM
imaging spectrometer oblained 128-channel, 0.76-3.16 lun spectra of part of the Irailing hemisphere with a spatial
resolutioa of-0.7 km [3]. In this abelraot we integrate results from the three multispecO_ detectors, by determining the
UV-_ and NIR WecUal pmpmies ofmlor and_ features __mco__,,_ In VSKimag_ Welxmmt evidence
_ _re are two J_ademenud q_md anits w_u the _m _ _ _v_,e by _e detec_cx_we _ _
uni_ q_tnd and _Ummon pmomies,-nd we cU._ussthe_ _ theae _mlu for the ocx_qx3_3n _ Ptu_.
Synolnis of Prewio_ Amalyse#. The spectral and retleclance _ of the surface of Phobus dete_ined
previously from these experinumts are summarized below. Lalxratot7 measurements of possible meteorite mudo_ to
Phob_ provide some basis for in--on of these observatimL Tbe amlogs include hydrous CI and CM carbonacecms
chondrites,anhydrous120and CV carbonaceouschondrites, and anhyckous, optically darkem_ ordinary chondrites or "black
chondrites." (a) Color ratio images constructed from visible/NIR VSK image pairs show that the surface is spectrally
heterogeneous with a color ratio of-0.75-1.4, and may be subdivided into "red," "reddish gray," "bluish gray," and "blue"
color ratio units. The "blue" and "bluish gray" units compose the interior of the crater Stickney and a surrounding lobate
deposit extending west to -160°W, in_ as the crater's ejecta [1,4]. (b) In low phase angle images, rims of many
fresh craters west of-160°W exhibit a brightening relative to surrounding older surfaces, but bright-rimmed craters are
conspicuously absent from the region extending east from -160°W to the crater Stickney [1]. In the laboratory, analogous
brightening at low phase angies is observed in particulate _ chonckites. An analogous effect is also observed in
black chondrites, but only at small perticle sizes characZ_u_ by onnudisikally large _[lec, lan¢_ for Phobos [5]. (e) The
"red," "n_ddish gray," and "bluish gray" onlor ratio units were mnpled by the KRFM speclmme_. Tbe "red" and "mklish
gray" units have similar UV-visible spectra which are relatively flat at 0.4-0.6 fun and exhibit a falloff in reflectance below
0.4 fun, but the "bluish gray" unit has a much lesser falloff in reflectance below 0.4 tun [4]. (d) The spatial disu'ibutions
of the color ratio units, their reflectance sy_ema,lcs, and their UV-visible specmd properties indicate that color variations
originate from optical or compositional heterogeneity. The reddest and bluest materials form the interiors and flanks of
craters, and are interpreted to have been excavated from a heterogeneous interior overlain by a"reddish gray" surficial layer
[4]. (e) The very blue color ratio of "blue" material supports a marie, black chondrite-like composition rather than a
earhonaceous chondritic onmposition. UV-visible spectra of"bluish gray" material are mote similar to speclra of black
chondrites than _ alum&ires. Specua of"reddish gray" and'red" materiais also more resemble speclra of black
chondrites, but are comparable to spectra of some carbonaceous chondrites [4,6]. (0 ISM observed only the "red" and
"reddish gray" color ratio units. The 3-tun absorption due to hydration is ver_ weak or negligible [3], suggesting that
Phobos's regolith is anhydrous. (g) The density of Phobos, 1.901-0.I g/cm _, is much lower than that of meteorite
analogs, indicating significant internal porosity [I]. The amount of porosity would be more realistic if high-density
anhydrous carbonaceous chondrite or black chondrlte were diluted by lower-deusity CM-like carbonaceous chondrite [4,7], if
this material is has been dehydrated by regolith processes to accountfor HIR spectral properties.
Calibration. VSK: VSK images were calibrated initially using ins_umental parameters measured on-ground and
in-flight [1]. Visible/NIR color ratio images were recafibrated on lhe basis of a portion of Mars observed in the background
of Phobos in some images, employing telescopic spectra resampled into the visible and NIR bandpasses as described by
Murchie et ol. [4]. KRFM: Initially published locations of the two groundiracks [2,8] have been termed by correlating
along-track brighmess pmf'des with lxmions of visible-band VSK images [4]. Previously published spectra [4] for the
longer, more southern gronndtrack (track 1) were recalibrated by dividing by a "standard" area with an "average" color ratio,
and multiplying by the disk-integrated Caverage") speclrum [9]. When recalibrated in this way, specwa of Mars obtained at
the end of the track (beyond the satellites drab) are consistem with telescopic spectra. Speclra from track 2 could not be
recalibrated because this track lacks a suilable "standard" area, and because no spectra of Mars were acquired at either end of
it [4], and so are therefore not used in the present analysis. ISM: These spectra have been calibrated using insm,mental
parametexs measured on-ground and in-flight [10]. We axe attempting several methods of recalibration, including (a)
dividing by a "standard" area and multiplying by spectra of anhydrous meteorites whose spectral properties in VSK and
KRFM bandpasses closely match those of the "standard" area, and (b) deriving revised gains and offsets for each channel
using spectra of various regions on Mars.
Propertios of Spectral Units on Phobos. The area west of Stickney lacking bright-rimmed craters is closely
correlated spatially with materials having color ratios of >1.0; the western oudiers of this bluish material coincide with
dark patches between the easternmost bright-rimmed craters, and the bright-rim craters in the distal portion of the bluish
material form oudiers of the reddea matexiul prevalent to the west. The magnitude of the falloff in reflectance below 0.4
lun is also related to color ratio (Figure 1). Materials with a color ratio of >1.0 exhibit a relatively flat spectrum with
249
https://ntrs.nasa.gov/search.jsp?R=19920001604 2020-03-19T16:25:36+00:00Z
INTEGRATEDSPECTRALPROPERTIESOF PHOBOS: Murchie, S. et al.
only a I0q6% UV falloff; materials with a color ratio <1.0 exhibit a 20-26% falloff in reflectance, independent of color
ratio within this range. These results are interpreted as revealing two basic spectral units in this region of Phobos:
(a) "bluer" Stickney ejecta with a color ratio of >1.0, a relatively flat UV-visible spectrum, and lacking bright material on
rims of young craters; and (b) "redder" material with a color ratio of <1.0, a greater falloff in UV reflectance, and
exhibiting pervasive bright material on rims of young craters.
Of these two basic units, ISM observed only the "redder" one. We have focused our analysis of this data set on the 1-
IJJn and 2-1am absorptions diagnostic of marie minerals. The depth of the l-lain absorption, defined as the drop in
reflectance at 0.91-1.09 tim relative to a continuum between 0.8 and 1.2 ltm, was found to vary laterally by up to -5%.
Depth of the absorption is independent of illumination (Figure 2), but is correlated with visible/NIR color ratio
(r, 0.71i_0.01) (Figure 3). Areas with relatively redder color ratios have the weakest absorption, and areas with relatively
bluer color ratios have the deepest absorption. If the current calibration is nearly correct, then the l-lxm absorption is weak
or absent in areas with the reddest color ratios (<0.8). A weak 2-1xm absoq_ion feature is also present in some of the areas
with a relatively smmg l-tun absorption and relatively bluer color ratio. Spectra of a mixed population of meteorite
analogs [11,12] reveal a similar relationship betweea color ratio and the depth of the l-lun absorption. Cazbonaceons
chondfites generally have weak or negligible l-tim absmptions; black chmu_tes generally have Idun absorptions with
depths of-2-7%, and as a da_ of matexisl$ have a bluer color ratio llum do caflxmace(m$ _ [4].
Implications for the ComlmSltlon of Phobos. Tentatively, we interpret these results as follows: "Bluer"
material considered to be Stickney ejecta is interpreted here as having a black chondrite-lflm composition, based on its UV-
visible spectral pmpe_es, its rely blue color ratio, and its lack of bright-rimmed craters. "Redder" material is a mix of a
red component with a weak or negligible 1-tun absorption, and a gray component possibly conlaining olivine and (locally)
pyroxene. Based on the correlation of the depth of the l-fun absorption with color ratio, the character of the UV-visible
spectrum, and the p_ of bright-rimmed craters throughout the "reddex" unit, we propose that its gray component is
black chondrite-like in composition and that its red component is analogous to lower-density carbonaceous chondritic
material such as CM chondrites. This mixed composition reconciles spectral evidence of a black chondrite-like
composition in some areas with the satellite's low density, which requires a more reasonable porosity (-35%) if a porous
interior has a large component of low-density carbonaceous chondritic materiaL The implied slruclnre of Phobos's interior
is a rubbly mixture of caflxmaceous chondrite and either ordinary chondrite or some other marie material containing a
similar mineral assemblage. This association is not thought to be compatible with formation within a single parent body
[13], and implies that the material composing Phobos originaw_! from more than one nstemidal and/or planetlry source.
References: [1] Avaaemv et al., Planetary and Space _ 0.30
Science, in press, 1990. [2] lGumfomality et al., Planetary and
Space Science,inpress,1990.[3]LangevinetaI.,Lunar E FigureI
Planet. Sci. XXl, 682-683, 1990. [4] Murchie et al., J. _q
Geophys. Res., in press, 1990. [5] Shkuratov et al., Abstracts
of Papers Submitted to the 12th Soviet-American I
Microsymposium, pp. 78-79, Moscow, 1990. [6] Brits et al.,
Lunar Planet. Sci. XXI, 129-130, 1990. [7] Hartmann, Lunar E
Planet. Sci. XXI, 457-458, 1990. [8] Ksanfomality et ai.,
Nature, 341,588-591, 1989. [9] Pang et al., Science, 199, 64- _
66, 1978. [10] Erard et al., Proc. Lunar Planet. Sci. Conf. 21,
in press, 1990. [11] Gaffey, J. Geophys. Res., 81, 905-920, v
1976. [12] Britt, D. and C. Pieters, unpublished spectra.
[13] Dodd, R., Meteorites: A Chemical-Petrologic Synthesis, o
Camb. Univ. Press, 1981.
>
D
0.25
0.20
O.15 -
o. 10
0.05 -
0.00
0.80
• t
• %: * .
1 I I I I I
0,84 0.88 0.92 0.96 1.00 1,04
Vis./NIR Rotio
1.08
t-
.l.+
a 0.00
IJ
c_
"O
t-
O
m 0.02
C
U
I 0.04
0.
I I I
30. 60. 90.
I-- Micron
Figure2
I I I I l
120. 150. 180. 210. 240.
Brightness (DN)
0.00
o.
w 0.01Q
"10
¢ 0.02
o
m
c 0.03
U
i o.o4
I
0.05
0.70
Figure 3
I I I I
0.75 0.80 0.85 0.90
Vis./NIR Rotio
0.95
250 .


Disk-resolved spectral reflectance properties of Phobos from 0.3-3.2 micron: Preliminary integrated results from Phobos 2

Disk-resolved spectral reflectance properties of Phobos from 0.3-3.2 micron: Preliminary integrated results from Phobos 2

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server