Tip of the Martian Iceberg? - The ZUBER RESEARCH GROUP · only the tip of an iceberg frozen under ground. The much lower measured hydrogen abundances at equatorial and mid-latitudes

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Surface features on Mars indicate that large amounts ofwater may have existed on the martian surface in the past.Where has the water gone? In his Perspective, Bellhighlights the reports by Boynton et al., Feldman et al.and Mitrofanov et al., who have analyzed initial data fromthe Mars Odyssey mission. The results indicate that theremay be large subsurface water ice deposits, especiallytoward the poles of our neighboring planet.

The current climate of Mars is characterized by annual cyclesof carbon dioxide (CO2) and water condensation andsublimation, dust deposition, and erosion (1–4). These cyclesare driven by large seasonal variations in solar heating causedby the planet’s high orbital eccentricity and by longer termvariations in its obliquity (or polar “tilt”) and other orbitalparameters (5).

Perhaps more intriguing, however, is the evidence forlonger term variations in climate preserved within layeredsedimentary deposits on Mars. In the 1970s, the Mariner 9and Viking orbiters discovered layered deposits of putativelyice-rich sediments alternating with dusty, ice-poor sedimentsin the martian polar regions (6). More recently, the MarsGlobal Surveyor (MGS) mission has revealed that suchlayered deposits are ubiquitous on Mars, although theirorigins are highly controversial (7).

Most exciting of all, Viking and MGS data have shownevidence for channels, valley networks, and gullies on avariety of spatial and temporal scales, indicating the action ofliquid water (8, 9). Liquid water may remain stable for longenough in the current low-pressure, low-temperature martianenvironment to form some of the observed features,especially the smaller-scale ones. However, many of the otherfeatures must have formed in a very different, more water-rich environment than exists on Mars today. Where has allthat water gone?

Isotopic evidence indicates that Mars has lost a significantamount of water through atmospheric escape (10).Nevertheless, thermodynamic models and geomorphic andcompositional evidence suggest that substantial amounts ofwater—as surface or subsurface ice and/or hydrated surfaceminerals—may still exist on Mars today (11, 12). Threereports in this issue present exciting measurements from thenewest orbital mission, Mars Odyssey, that appear to confirmthe existence of perhaps large quantities of shallowsubsurface ice in certain parts of the planet (13–15). Theauthors use measurements of the neutron flux emitted fromMars in several different energy regimes and spectra ofgamma-ray emissions induced by neutron capture reactions tomap the global distribution of near-surface hydrogen on theplanet for the first time.

The results, even after only a month of mappingobservations, are stunning. The abundance of hydrogen varieswidely. The highest concentrations occur poleward of about

60°N and 60°S and are interpreted to indicate the presence ofsubsurface water (not CO2) ice, on the basis of the specificpatterns of neutrons detected and the spatial correlation toregions where ground ice has been predicted to be stable (11,12).

Modeling of the observed neutron and gamma-ray fluxesis complex and still preliminary, and some instrumental andatmospheric effects may still be present in the data. Never-theless, initial results indicate that the best fits for theenhanced hydrogen regions are consistent with a modelsurface with a “thin and dry” [few tens of centimeters; 1 to 2weight percent (wt %) H2O] upper layer overlying a “thickand ice-rich” (several hundred centimeters; 20 to 35 wt %H2O) lower layer. Details on the thickness of the ice-richlower layer are limited by the ~1-m sensing depth of theneutron instruments, and it is not possible to determine thetotal quantity of subsurface ice present. However, if themodeling is correct, then the inferred ice concentrationimplies an extremely porous, nearly ice-filled regolith (thelayer of rocky debris and dust resulting from repeatedmeteoritic impacts) at high latitudes. Separate lines ofevidence suggest a loose and/or porous regolith that couldexceed a kilometer or more in thickness (16, 17), implyingthat the subsurface ice detected by Odyssey may representonly the tip of an iceberg frozen under ground.

The much lower measured hydrogen abundances atequatorial and mid-latitudes are consistent with telescopicand spacecraft infrared (IR) spectroscopy results (18–20),indicating the presence of 1 to 2 wt % of hydrated mineralson the surface. The specific mineralogy of this hydratedmaterial has, however, not yet been identified. The Odysseyinstruments only measure hydrogen, and thus cannotdistinguish between H2O-bearing hydrates and OH-bearinghydroxides. However, Odyssey data are sensitive to hydratedminerals at depths of tens of centimeters, whereas IR remote-sensing measurements penetrate only a few tens ofmicrometers. The combination of continuing IR observationsat higher spatial and spectral resolution, ongoing collection ofOdyssey neutron and gamma-ray data to improve signal andreduce noise, and better modeling on the basis of theseobservations may help to finally identify the specific mineralsresponsible for sequestering important volatiles such as waterand OH at low latitudes on Mars.

It is particularly impressive that the three reports in thisissue were generated after the Odyssey spacecraft hadcompleted only ~30 days of its planned multiyear mission,and during a phase of the mission before the Gamma-RaySpectrometer (GRS) had been deployed to its nominalmapping configuration. The GRS was originally flown toMars on the Mars Observer spacecraft, which stoppedworking just 3 days before entering Mars orbit in 1993. ManyOdyssey investigators have been waiting more than 15 years

Tip of the Martian Iceberg?Jim Bell

The author is in the Department of Astronomy, Cornell University, Ithaca, NY 14853, USA. E-mail: jimbo@astro.cornell.edu

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to finally collect these data. One can hardly blame them fortheir enthusiasm and excitement over their early findings.

Within the next few weeks, the GRS is to be extended outon a ~6-m boom to isolate it from neutron and gamma-raysignals originating in the Odyssey spacecraft itself and thusboost the instrument’s sensitivity to the surface of Mars. Theresults from that configuration are sure to provide additionalinsights into subsurface ice and surface hydrated minerals,and yield unique new information on the planet’sgeochemistry from global maps of rock-forming elementssuch as Fe, Si, and Mg. The results will also be used to guidethe selection of landing sites for future rovers and landers,sample returns, and eventual human exploration. In thatsense, the most important implications of the detection ofsubsurface water ice deposits on Mars may not be realized fordecades. It is likely to be worth the wait, however.

References 1. P. B. James, B. A. Cantor, Icarus 154, 131 (2001). 2. B. M. Jakosky, A. P. Zent, R. W. Zurek, Icarus 130, 87(1997). 3. R. T. Clancy et al., Icarus 122, 36 (1996). 4. R. W. Zurek , L. J. Martin, J. Geophys. Res. 98, 3247(1993). 5. J. Touma, J. Wisdom, Science 259, 1294 (1993). 6. K. R. Blasius, J. A. Cutts, A. D. Howard, Icarus 50,140 (1982). 7. M. C. Malin, K. S. Edgett, Science 290, 1927 (2000). 8. S. W. Squyres, Icarus 79, 229 (1989). 9. M. C. Malin, K. S. Edgett, Science 288, 2330 (2000).10. B. M. Jakosky et al., Icarus 111, 271 (1994).11. F. P. Fanale et al., Icarus 67, 1 (1986).12. S. M. Clifford, J. Geophys. Res. 98, 10973 (1993).13. W. V. Boynton et al., Science, 30 May 2002(10.1126/science.1073722).14. W. C. Feldman et al., Science, 30 May 2002(10.1126/science.1073541).15. I. Mitrofanov et al., Science, 30 May 2002(10.1126/science.1073616).16. L. A. Soderblom, D. B. Wenner, Icarus 34, 622(1978).17. J. F. Mustard, C. D. Cooper, M. K. Rifkin, Nature 412,411 (2001).18. J. R. Houck et al., Icarus 18, 470 (1973).19. J. F. Bell III, D. Crisp, Icarus 104, 2 (1993).20. S. M. Murchie et al., Icarus 105, 454 (1993).

Published online 30 May 2002; 10.1126/science.1074025

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All stacked up. High-resolution images of the surface ofMars have revealed evidence of layering all over the planet,hinting at complex, perhaps episodic or periodic variations ingeologic activity and/or climatic conditions. In this MarsOdyssey image, acquired on 17 March 2002, spectacularlayering can be seen in the floor of Ganges Chasma, a part ofthe Valles Marineris canyon system. Different layers appearto have different levels of susceptibility to erosion, suggesting

physical and/or compositional differences. The origin of thislayering may be related to deposition of either windblown orwater-borne sediments in the canyon floor. The absence ofimpact craters indicates a relatively young surface. [MarsOdyssey THEMIS image V01126002 (Release 20020329 athttp://themis.la.asu.edu)]

Flying high. The Mars Odyssey spacecraft joined the MGS inorbit around the Red Planet in February . This artist’sconception shows the spacecraft in its mapping configuration,after the GRS boom has been deployed this month. Odysseywill obtain maps of the planet’s elemental chemistry,thermophysical properties, and visible and IR color propertiesuntil 2004. It will also serve as a communications relaysatellite for other U.S. and international Mars missions in2003 and 2004.

Credit: first figure, NASA/JPL/Arizona State University;second figure, NASA/JPL

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