Mar 1, 1998 - resolved large-scale albedo features [Martin et al., 1992]. The images ..... Bell, J.F., III, R.T. Clancy, P.B. James, S.W. Lee, L.J. Martin and M.J. ...
GEOPHYSICAL RESEARCH LETTERS, VOL. 25, NO. 5, PAGES 611-614, MARCH 1, 1998
Synoptic measurements of Martian the Hubble Space Telescope
winds using
Michael A. Mischna and JamesF. Bell, III Department of Astronomy, Cornell University, Ithaca, New York
Philip B. James Department of Physics and Astronomy, University of Toledo, Toledo, Ohio
David Crisp Jet Propulsion Laboratory/Caltech, Pasadena, California
Abstract. We used the Hubble Space Telescope to make wind speed and wind direction measurementsof the Martian
atmosphereduring early northern summer (Ls:97ø). Our strategy utilized a series of near-UV images with optimal temporal sampling to track observablecloud features and to determine the direction and speed of cloud motion. Several techniques were used to identify similm features between images, including Fourier transform and cross-correlation. In these measurements,which span 19-75ø N lat and 319ø E to 135ø W lon, we find averagewind speedsof 26.2 -t-10.1
m/s and a range of wind directionsfrom 51ø to 162ø with an average error of -I-23ø.
HST to take high spatial resolution images of Martian clouds over a short period of time. Tracking of the motions of these clouds was used to deduce general atmospheric motions.
Observations The images used to perform the analysis were retrieved from HST image sets taken on 30 March, 1997 as part of
HST programs7276 and 6852 [Bell et al., 1997b].We found that imagesin the UV (at 255 nm) offeredthe best contrast of cloudy and cloud-free regionsof the planet. Of the available 255 nm images, three were selectedas having the
best temporal samplingfor detectingsynopticmotion (Table 1). The imageswere photometricallycorrectedusing a
Introduction
Minnaert function with an empirically-derived coefficient of
The past several decades of detailed telescopic observations of Mars have yielded surprisingly little concrete data on the global wind field of the planet. Much of the earliest work involved observationsof wind-blown dust and poorly-
resolvedlarge-scalealbedo features [Martin et al., 1992]. The images returned from Mariner 9 and the two Viking
0.3 (seeBell et al., [1997a]). The imageswerethen mapped into a simple cylindrical projection using a method to maximize the correlation among the input images. Procedure We
identified
similar
features
that
showed
detectable
orbiters (the first high spatial and temporal resolutionim- movementin two or three maps. The images,wereblinked agesof the surface)revealedminimal atmosphericcloudiness in rapid successionand candidate features were determined but high levelsof atmosphericdustyear-round.Kahn [1983] by eye. Six cloud structures were identified as having wellcompiled these data sets and produced global and seasonal definedmotion over the 5 hour time span (Figure 1). wind direction fields based on the morphology of clouds, dust, and wind streaks. To date, there has never been a comprehensive observational study of the synoptic scale motion in the Martian atmosphere. Ground-based telescopescannot adequately resolveatmospheric features, and orbiting spacecrafthave not provided the consistent temporal resolution or wavelength coverageneeded to make a detailed investigation.
Three different techniques were used to determine the pixel offsets between these features. First, a VICAR software routine was employed which found tiepoints between two registered images by maximizing the phase and frequency correlations of Fourier transforms of the images. The second technique was to use the same VICAR routine but to manually choose matching points between the two images. A third technique involved performing a real The Hubble Space Telescope(HST) providesunprece- space cross-correlation between individual regions of adjadented spatial resolution for telescopicmeasurementsof the cent images until a maximum correlation was found within planets, as well as sensitive ultraviolet imaging capabili- the region. For each cloud feature previously identified, a ties that are difficult or impossibleto achieve through the list of coordinate pairs was created which encompassedthe Earth's atmosphere. In this paper we describe how we used entire feature. A 3x3 pixel subregion was extracted from image 1, centered around a coordinate pair from the list, •Now at The PennsylvaniaState University,UniversityPark, and correlated against every 3 x 3 pixel subregionin a region Pennsylvania. surrounding and including the same pixel in image 2, until the absolute maximum correlation was found. The process Copyright1998 by the AmericanGeophysicalUnion. was applied to each point in the coordinate pair list, generating a list of tiepoints. Papernumber98GL50358. 0094-8534/98/98GL-50358505.00 For each point identified as a moving feature, a covariance 611
612
MISCHNA
ET AL.-
SYNOPTIC
MEASUREMENTS
OF MARTIAN
WINDS
Table 1. Observational Data: March 30, 1997 HST Images Image No.
Image Name
Time, UT
L8
SE Lon
SE Lat
Resolution,km/pixel
I
u3gi7101
10:40:14
97.56
18.65
23.38
21.9
2
n3m00401
12' 17:14
97.59
42.07
23.39
21.9
3
u3gi7201
15:35:14
97.65
90.37
23.39
21.9
was also found and compared to the covariances of the other pixels in the region. Points with a covariancefalling outside of one standard
deviation
of the mean covariance
value of the
regionwere eliminated to removeboth anomalouspoints and insignificantcorrelationswith low covariance.The offsetsof the remaining pixels were converted into wind speeds and directions. Errors could be confined to one-half pixel or less, which corresponded in many casesto a windspeed error of roughly 35 percent and a directional error lessthan 20ø
averageof all data points. In feature A, direction varied between 55ø southwardto 200•, with different directionsbeing prevalent in different pockets of the cloud. Small features such as features B and D display more uniform results than the larger features.
Discussion
Wind
Speed
The results found using HST imagery agree well with the
Results
observationsof Kahn [1983], and other atmosphericmod-
We should expect to see high levels of similarity between els. If we apply the morphology scheme set forth initially adjacent pixels within a singlecloud feature. This is consis- in Frenchet al., [1981]and againin Kahn [1984],and draw comparisons with terrestrial clouds, it can improve our intent with a cloud that primarily translates its position with tuitive understanding of the meteorological conditions on time. Figure 2 displaysour results for a single cloud feature (feature A) for all three observationtimes. There is indeed Mars and give some indication of what we should expect of a high level of similarity between most of the high-variance the various features. Feature A appears to be a plume with probable forced convection as it passes around the Tharsis points, lending confidenceto our results. region. Kahn observeda concentration of these types of feaTable 2 lists the results of the analysis of the six distinct cloud features observed in these images. There is a high tures in the Tharsis region, and they are evident here and in degree of similarity between the observed northern hemi- other HST imagingdata [Jameset al., 1994],[Jameset al., sphere features. Winds appear to blow from the southeast- 1996]. Our resultsfoundan averagespeedof 24.4+ 9.0 m/s. ern quadrant in all cases. For the large cloud features, there Features B, C and E occur in a band of low-latitude clouds is a chance for a higher degree of variability in direction analogous to the bands of cloudiness in the Earth's tropics. and speed throughout the cloud, and findings are merely an Most likely this occurs as a result of localized convection. The wind speedsof these features are between 18.1 and 26.4
m/s, and appear self-consistent through the entire feature, unlike feature A which has a higher internal variability. The remaining features are located in the mid-latitudes and are very tenuous and faint. They have no distinct shape
and resemblethe cloud streaksfound by Viking [Frenchet al., 1981]. They form as a result of localizedcondensation and also, in the case of feature F, are influenced by the local
topography.On average,feature D movedat 36.0 m/s and feature F at 37.7 m/s. The variance between the three techniques used for measuring wind speed is modest. Both VICAR methods gave
a range of wind speedof 10.0-42.0 m/s, while the crosscorrelationtechniqueproducedresultsfrom 15.5-43.9m/s. In determining wind speed, we need to be cautious in regionswith topographic influence. Coupling of cloud features
with topography(i.e. lee waves)has long been observedin the Tharsis region [Leovyet al., 1973]. Thus, the visible cloud motion may not be indicative of the wind field in the entire region, and may be a source of error. In this study, features
A and F are both
clear
of the
leeward
side of the
terrain, so such effects should be negligible.
Figure 1. HST Mars imagen3m00401(12:17 UT, March 30, 1997) at a wavelength of 255 nm, orthographicprojection, illustrating the location of each of the six features studied
here.
North
is indicated
for reference.
Direction
It is possible to distinguish between two primary directional forcings that control the Martian wind field in these data. During the early northern summer, maximum solar
MISCHNA
ET AL.: SYNOPTIC
MEASUREMENTS
OF MARTIAN
WINDS
613
Table 2. ObservedWind Speeds:March 30, 1997HST Images
Feature a A1,2 A2,3 B•,2 B2,3 Cx,• C•,3 D1,2 D•,:• E•,2 F•,•
Lat Range 34-55 33-60 19-32 23-33 20-30 20-30 42-59 40-48 24-34 59-75
LonRange
Speed,m/s
50-81 45-99 357-8 0-13 10-28 12-26 13-27 14-26 319-327 90-135
22.3 + 8.3 26.4 + 9.7 25.0 + 10.5 22.7 + 9.7 23.5 + 9.9 18.1 + 7.4 43.9 q- 17.0 28.0 + 8.3 15.5 q- 8.4 37.7 q- 12.1
Direction, degrees 110.0 + 21.9 162.8 + 21.6 113.9 + 24.8
108.3 116.7 121.2 104.3 149.9 51.3 154.0
+ + + + + + +
25.3 24.9 24.1 22.8 17.2 32.8 18.7
aFeaturecharacteristics aremeasured betweenthe two subscripted images
radiation occursin the northern mid-latitudes. On earth, port forthe MGCM calculations of Haberleet al. [1993],who the large thermal inertia doesnot alter the pole-to-equator predict easterly motion at nearly all locationsand at all altithermal gradient, and the mid-latitude winds continue to tudes in the northern hemisphereduring summer,as well as blow from the west. On Mars, however, the low ther- a weakeasterlyjet at approximately35km,with wind speeds mal inertia produces a temperature maximum in the midlatitudes. This results in a negative pole-to-equatortemperature gradient in the mid- and low-latitudes. Winds be-
taperingoff graduallyapproaching the surface.Kahn [1984] wasableto identify 10 featuresappearingunambiguously in two imagesand to derive wind speedsand directions.Only
ing forcedby this thermal gradientwill blow retrogradeto
two of these pairs occurred near the same seasonas the im-
the planetary motion. This atmosphericbehavior was con-
ageswe used in this study. One (Ls:101.5 ø) occurredin the southernmid-latitudesandmovedat 26.3+ 9.0 m/s in a
firmed by Mariner 9 and Viking experiments[Hanel et al., 1972],[Leovy, 1982].The influenceof semi-diurnaland diur-
direction 114+ 38ø. Another occurredduring late northern spring (L8=50.3ø) in the northern tropics and moved in a umented[Leovy,1982],[Hess et al., 1977],[Leovy and Zurek, directionof 99 + 16ø at 22.5+ 14.3 m/s. This is consistent 1979],[Zureket al., 1992],and its effectson regionalwinds with our findingof generallySE windsat 30 m/s. (suchasnear the Tharsisridge)havebeenshownto be influConclusions encedby localtopography[Ryanet al., 1978]. Subsequently, the Coriolisforcewill deflectthe windsslightlypoleward,esThe findingsfrom UV imagery of Mars showthat it is inpecially at lower elevations. This type of motion is seen in deedpossibleto measurewind fieldsusingsynoptic-scale reour results. Clouds located in the northern tropics move mote sensingmeasurementssuchas thoseobtained by HST, from the ESE, as expected. contingenton the appropriatechoiceof temporal sampling. In addition, orographiceffectsof the Tharsis regionplay We find northern summerwind speedsin the range of 15a major role in definingthe wind field in these images. In 44 m/s blowingfrom the southeastacrossa wide rangeof particular, the northern edge of Tharsis Montes interferes latitudes. There is noticeable orographic influence in the with the flow of visible cloudsin the region of the planet Tharsis region. Excellent agreement exists between these nal thermal tides on the wind regime has also been well doc-
we analyzed. If we look at the motion of cloud features sur-
results and both observational and theoretical models of the
rounding this high terrain, including features A and F, we noticethat there is a significantlygreaternorthward component to the flow, possiblyimplying that the wind has been deflected poleward, around the mountains. Our wind speedand directionmeasurements providesup-
Martian atmosphere.Descriptionsof observedatmospheric motions during this seasonusing Viking and Mariner 9 imagery are confirmed, and values from this study fall well within the limits describedby recent Mars generalcircula-
(10:40 UT)
tion
models.
t 2(12:17 UT)
t3(15:35 UT)
Figure 2. FeatureA illustratedin eachof the threeimagesusedin this study. The crosshairs are a fixedset of latitude/longitude pointsthat canbe usedasfiducialsto detectcloudmotionsfromthesedata. Duringthe 5 hoursbetween
tx andt3, featureA moved withan average speedof 24.4+ 12.8m/s towards thenorthwest (direction = 136.4+ 30.8ø).
614
MISCHNA ET AL.: SYNOPTIC MEASUREMENTS OF MARTIAN WINDS
Upcoming Mars orbital spacecraft measurementsfrom the Mars Global Surveyor and Mars-98 missionswill provide additional information on Martian cloud morphology and opacity. However,the sun-synchronous,low polar orbits of these spacecraftpreclude a truly synoptic analysisof the Martian wind field. Thus, continued observationsusing the capabilities of HST and high spatial resolutiongroundbased observing facilities will provide unique measurements,sampling a wide and nearly continuousrange of seasons,with which to assessthe atmospheric dynamics of Mars.
Hess, S.L., R.M. Henry, C.B. Leovy, J.A. Ryan, and J.E. Tillman, Meteorological results from the surface of Mars: Viking 1 and 2, J. Geophys. Res., 82, 4559-4574, 1977. James, P.B., R.T. Clancy, S.W. Lee, L. Martin, R. Kahn, R. Zurek, R. Singer, and E. Smith, Monitoring Mars with the Hubble Space Telescope: 1990-1991 observations,Icarus, 109, 79-101, 1994.
James, P.B., J.F. Bell III, R.T. Clancy, S.W. Lee, L.J. Martin, M.J. Wolff, Global imaging of Mars by Hubble spacetelescope during the 1995 opposition, J. Geophys. Res., 101, 1888318890, 1996.
Kahn, R., Someobservationalconstraints on the global-scalewind system of Mars, J. Geophys.Res., 88, 10189-10209, 1983. Acknowledgments. We gratefullythank PeterGierasch Kahn, R. The spatial and seasonal distribution of Martian clouds and Don Banfield for their instructive comments. We also thank and some meteorological implications, J. Geophys. Res., 89,
Mike
Wolff
for assistance
with
the
initial
HST
data
reduction
and Tom Daley for assistancewith the HST mapping software. This research was supported by grants from the Space Telescope
ScienceInstitute (GO-02370.05-87A) and the NASA Planetary Astronomy and AtmospheresProgram (NAGW-5117). This research was based on observationswith the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by Association of Universities for Research in Astronomy under NASA contract NAS5-26555.
References Bell, J.F., III, M.J. Wolff, P.B. James, R.T. Clancy, S.W. Lee, L.J. Martin, Mars surface mineralogy from Hubble Space Telescope imaging during 1994-1995: observations, calibration, and initial results, J. Geophys. Res., 102, 9109-9123, 1997a. Bell, J.F., III, R.T. Clancy, P.B. James, S.W. Lee, L.J. Martin and M.J. Wolff, Hubble Space Telescope observations of Mars during 1996-1997, LPSC, 28, 85, 1997b. French, R., P. Gierasch, B. Popp and R. Yerdon, Global patterns in cloud forms on Mars, Icarus, J5, 468-493, 1981. Haberle, R.M., J.B. Pollack, J.R. Barnes, R.W. Zurek, C.B. Leovy, J.R. Murphy, H. Lee and J. Schaeffer,Mars atmospheric dynamics as simulated by the NASA Ames General Circulation Model, 1, The zonal-mean circulation, J. Geophys. Res., 98, 3093-3123, 1993.
Hanel, R., B. Conrath, W. Hovis, V. Kunde, P. Lowman, W. Maguire, J. Pearl, J. Pirraglia, C. Prabhakara, B. Schlachman, G. Levin, P. Straat and T. Burke, Investigation of the Martian environment by infrared spectroscopy on Mariner 9, Icarus, 17, 423-442, 1972.
6671-6688, 1984.
Leovy, C.B., G.A. Briggs, and B.A. Smith, Mars atmosphereduring the Mariner 9 extended mission: television results, J. Geophys. Res., 78, 4252-4266, 1973. Leovy, C.B. and R.W. Zurek, Thermal tides and Martian dust storms: direct evidence for coupling, J. Geophys. Res., 8•, 2956-2968, 1979.
Leovy, C.B., Martian meteorological variability, Advances in Space Research, œ19-44, 1982. Martin, L.J., P.B. James, A. Dollfus, K. Iwasaki, and J.D. Beish, Telescopicobservations: Visual, photographic, polarimetric, in Mars, edited by H.H. Kiefer et al., pp. 34-70, The University of Arizona Press, Tuscon, Arizona, 1992. Ryan, J.A., R.M. Henry, S.L. Hess, C.B. Leovy, J.E. Tillman and C. Walcek, Mars meteorology: three seasonsat the surface, Geophys. Res. Left., 5, 715-718, 1978.
Zurek, R.W., J.R. Barnes, R.M. Haberle, J.B. Pollack, J.E. Tillman, and C.B. Leovy, Dynamics of the atmosphereof Mars, in Mars, edited by H.H. Kiefer et al., pp. 835-933, The University of Arizona Press, Tuscon, Arizona, 1992. J. F. Bell, III and M. A. Mischna, Department of Astron-
omy, Cornell University, Ithaca, NY 14853. (e-maih jimbo• marswatch.tn.cornell.edu; mischna(•astrosun.tn.cornell.edu) P. B. James,Department of Physicsand Astronomy,University of Toledo, Toledo, OH.
D. Crisp, Jet PropulsionLaboratory/Caltech, Pasadena,CA.
(ReceivedAugust26, 1997; revisedNovember25, 1997; accepted December30, 1997.)
