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ameter. Radar studies have shown that theprimaries usually spin in ~2 hours, near thespin period at which a cohesionless bodycan no longer be held together by gravity;are roughly spherical; and have lowalbeit

poorly constraineddensities. Orbits are al-most circular and close (~5 primary radii).Because NEAs are more often binaries thanother objects and traverse planetary orbits,

the suspicion is that they formed when frag-ile rubble piles were tidally torn asunder(12), like comet Shoemaker-Levy 9 (11).

In the outer solar system, a satellite cir-cles 617 Patroclus, one of six Trojans (theasteroids that orbit at Jupiters distance)surveyed so far with adaptive optics. Seventrans-Neptunian objects (TNOs, residentsof the Kuiper Belt or Centaurs), reflecting

perhaps 1% of the TNO population, are bi-nary (2). These outer solar-system binariesare distinctly different from those closer tothe Sun, implying alternative formationmechanisms. The companions have com-

parable masses on eccentric, widely sepa-rated orbits. It remains unclear how suchloosely bound objects can be formed. Theymay have been produced in the early solar

system, when TNOs were more numerous.Well-determined masses and shapes al-

low reliable estimates of density. Comparedto the measured densities of carbonaceous(2.3 to 2.8 g/cm3), stony (3 to 4 g/cm3), oriron (5 to 6 g/cm3) meteorites (12, 18), as-teroids (the putative source of meteorites)have remarkably low densities. For exam-

ple, the main-belt asteroid 45 Eugenias

density is 1.2 (+0.6/-0.2) g/cm3 (2), that ofthe NEA 2000UG11 is ~1.0 0.5 g/cm3

(19), and even the density of the metallic22 Kalliope is just 2.3 0.4 g/cm3 (2).These low densities confirm estimates fromspacecraft flybys and from direct gravita-tional tugs of other bodies. Asteroids musttherefore contain significant pore space (40to 60%) (18), suggesting an unconsolidatedrubble-pile structure, as inferred from rota-tional studies (20).

As the mutual orbits of more solar-sys-tem binaries are tracked, we will be able touse the densities of the solar systems

smallest members to understand their ori-gins. Along with asteroid families, bina-ries are providing a code book to decipherthe pivotal role of impacts in forming and

destroying solar-system bodies. Like stel-lar binaries, solar-system pairs are reveal-ing much about the processes that havegiven us todays rich world.

References1. C.R. Chapman et al., Nature374, 783 (1995).2. W.J. Merline et al., in Asteroids III,W. F. Bottke et al.,

Eds. (Univ. of Arizona Press,Tucson, AZ, in press).3. A. Batten, Binary and Multiple Star Systems (Perga-

mon, Oxford,1973).4. C.Veillet et al., Nature416, 711 (2002).5. W.J. Merline et al., Nature401, 565 (1999).6. M. Brown,J.-L.Margot, IAU Circ. 7588, 1 (2001).7. A.Storrs et al., IAU Circ. 7599, 1 (2001).8. P. Pravec et al., Icarus146, 190 (2000).9. J.-L. Margot et al., Science296, 1445 (2002).

10. S. J. Weidenschilling et al., in Asteroids II, R. Binzel etal., Eds. (Univ. of Arizona Press,Tucson, AZ, 1989), pp.643658.

11. E.Asphaug,W. Benz, Nature370, 120 (1994).12. D.C. Richardson et al., Icarus134, 47 (1998).13. P. Michel et al., Science294, 1696 (2001).14. B. Chauvineau et al., Icarus94, 299 (1991).15. D.J. Scheeres, Icarus110, 225 (1994).16. D.P. Hamilton, J.A. Burns, Icarus92, 118 (1991).17. W.F.Bottke,H. J.Melosh, Icarus124, 372 (1996).18. D.T.Britt, G. J. Consolmagno, Icarus146, 213 (2000).19. M. C. Nolan, personal communication.20. P.Pravec,A.W. Harris, Icarus148, 12 (2000).21. I thank W. J. Merline and J.-L. Margot for sharing

manuscripts before publication and for comments onan earlier draft.

W

ater vapor is by far the most im-portant greenhouse gas. By ab-sorbing outgoing thermal radia-

tion from Earth, it provides some 30 K ofheating (1). Water vapor also absorbs about10 to 20% of incoming solar radiation (2).The weak overtone bands of water (see thefigure) play an important role in this absorp-tion because the solar photon flux peaks at~630 nm, in the yellow-red part of the visi-

ble spectrum. Both the amount of water va-por in our atmosphere and the strength andposition of its absorption bands thus help tomaintain Earths energy balance (1).

The intensities of the absorption linesare determined by the electric dipole mo-ment surface, which describes the chargedistribution in the water molecule as afunction of the two OH bond lengths andthe bond angle. A reliable dipole momentsurface is thus essential for the calculationof a molecular spectrum from first princi-

ples. Several dipole moment surfaces areavailable for water (3, 4). On page 993 ofthis issue, Callegari et al. (5) report a

clever method for testing their quality.The prediction of the water spectrum on

the basis of ab initio potentials and dipole

moment surfaces is very important. Current-ly available experimental data cannot accountfor the atmospheric absorption of sunlight(6). In astronomy, the spectral energy distri-

bution emitted from cool stars and browndwarfs is strongly modulated by water ab-sorption (7), which cannot be simulated withexisting experimental data. Only theory can

generate the many millions of weak absorp-tion lines that are required to bring observa-tions and simulations into agreement (8). Abinitio predictions of the water spectra have al-so been used to assign hot water spectra insources such as sunspots (9).

The prediction of an absorption spec-trum requires as minimum input a set ofline positions, their intensities, and a line-

shape function. Often these data are derivedfrom experiment and tabulated in publicdatabases (10). Water line positions can rou-tinely be measured to eight significant dig-its, whereas line intensities typically have anaccuracy of only 5%. Hence, empiricaldipole moment surfaces deduced from ex-

perimental data have modest reliability (4).Dipole moment surfacesderived from state-of-the-art ab initio calcula-tions (5) are probablymore accurate.

What molecular prop-erties of water deter-mine the positions andstrengths of the absorp-tion lines? From a theo-retical perspective, theline positions are deter-mined by the changes inthe total electronic ener-gy as a function of thetwo OH bond lengthsand the bond angle, cap-tured in the potential en-ergy surface (11). Simi-

P E R S P E C T I V E S : L A S E R C H E M I S T R Y

Water Vapor Gets ExcitedPeter F. Bernath

0.9

0.8

0.7

0.6

0.5

0.4

0.3

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24,000 18,000

Wavenumber (cm-1)

7 +

6 + 4 +

4

5 + 5

6

7

Intensity

(arbi

trary

units)

16,000 14,00022,000

420 nm 500 nm 600 nm 700 nm

20,000

O

HH

Now you see it. The visible absorption spectrum of water, recordedwith a high-resolution Fourier transform spectrometer and a long-pathabsorption cell.The overtones are the small modulations of the spec-trum.(Inset) The water overtone OH stretch. [Adapted from (15)]

The author is in the Department of Chemistry,University of Waterloo, 200 University AvenueWest, Waterloo, Ontario N2L 3G1, Canada. E-mail:bernath@uwaterloo.ca

S C I EN C E S C O M P A S S

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larly, the dipole moment surface deter-mines the line intensities (3, 11).

In contrast to the ab initio calculationof line intensities, line positions from abinitio potential energy surfaces are still farfrom spectroscopic accuracy. Problems in-clude the breakdown of the Born-Oppen-heimer approximation (which enables theseparation of nuclear and electronic mo-

tions) because of the light, rapidly movinghydrogen atoms. Furthermore, relativisticand quantum electrodynamic effects makenonnegligible contributions to the energylevels (7, 12).

Despite this somewhat discouragingnews on the theoretical front, Partridgeand Schwenke have shown that a smallempirical adjustment of the high-qualityab initio potential surface yields calculatedline positions to better than 0.05 cm1 onaverage (11). This result is somewhat de-ceptive, however, because such spectro-scopic accuracy is achieved for calcula-

tions within the range of data used to de-rive the correction only; the quality of thepredictions deteriorates rapidly for line po-sitions beyond the range of the availableexperimental data.

Callegari et al. (5) have devised a high-ly precise method for testing the reliabilityof dipole moment surfaces that does notrequire the measurement of line intensi-

ties. The trick is to apply an electric fieldto a sample of water vapor and monitor thesplitting of the lines. The field causes eachrotation-vibration level to split into com-

ponents because of the partial removal ofthe degeneracy associated with spatialquantization of rotation.

This Stark effect is well known. Aver-age dipole moments have previously been

measured in the low-lying vibrationalstates of water (13). Callegari et al. (5)have now found a way to measure smallStark splittings in very weak water over-tone transitions.

The authors use a sequence of threepowerful laser pulsesthe first to excitethe overtone transition, the second to dis-sociate H2O into H + OH, and the third tomonitor the OH free radicalfor the ultra-sensitive detection of visible water bands.Because the Stark splittings are relativelysmall compared to the laser line widths,the interference of two coherently excited

molecular wave functions was monitoredwith the quantum beat method (14).The work is a tour de force in modern

chemical physics. As is often the case, thetake-home message depends somewhat onyour personal perspective. The good newsis that the best current dipole momentfunctions for water are generally in goodagreement with experimental observations.

However, discrepancies of up to about 10%indicate that there is room for improve-ment. Such work is already under way (5).

References

1. R.P.Wayne, Chemistry of Atmospheres(Oxford Univ.Press,Oxford,ed. 3, 2000),pp. 5058.

2. V. Ramanathan, A. M. Vogelmann, Ambio26 , 38(1997).

3. D.W. Schwenke, H. Partridge, J. Chem. Phys. 113,

6592 (2000).4. R. B .Wattson, L . S. Rothman, J. Quant. Spectrosc.

Radiat. Transfer48, 763 (1992).5. A.Callegari et al., Science297, 993 (2002).6. W. Zhong, J. D. Haigh, D. Belmiloud, R. Schermaul, J.

Tennyson, Q. J. R. Meteorol. Soc. 127, 1615 (2001).7. P. F. Bernath , Phys. Chem. Chem. Phys. 4, 1 50 1

(2002).8. F. Allard, P. H. Hauschildt, D. Schwenke, Astrophys. J.

540, 1005 (2000).9. O.L. Polyansky et al., Science277, 346 (1997).

10. For example, the HIgh resolution TRANsmission(HITRAN) Database of L. Rothman et al. (www.hitran.com).

11. H. Partridge, D.W. Schwenke, J. Chem. Phys. 106,4618 (1997).

12. P. Pyykko, K. G. Dyall,A. G. Csaszar,O. L. Polyansky, J.Tennyson, Phys. Rev. A 63, 024502 (2001).

13. S. Shostak, W. L. Ebenstein, J. S. Meunter,J. Chem.

Phys. 94, 5875 (1991).14. This method detects splittings in lines by monitoring

periodic time-dependent changes in a molecularproperty such as fluorescence caused by interferenceof the wave functions of the coherently excited ener-gy levels.

15. M.Carleer et al.,J. Chem. Phys. 111, 2444 (1999).16. Thanks to R. Colin for assistance with the figure.

Financial support was provided by the Natural Sci-ences and Engineering Research Council of Canada(NSERC), Canadian Space Agency (CSA), and NASA.

S C I EN C E S COMPAS S

In the transmittance of genetic materialfrom parent to offspring, accidents occa-sionally happen. Such accidents may re-

sult in the duplication of a chromosomal seg-ment that then becomes separated from theoriginal segment, ending up in a differentchromosomal location. A number of humangenetic disorders are known to be associatedwith the increased expression of genes con-tained within such duplications. However,evolutionary biologists have long been en-thralled with the idea that duplicate genescould provide the ultimate substrate on whichevolution could work. There are two ways inwhich gene duplication could generate a sub-strate suitable for adaptive evolution. Eitherone member of the duplicate gene pair couldtake on a new function, or two duplicategenes could divide the multiple functions ofthe ancestral gene between them, with natu-ral selection then refining each copy to a

more restricted set of tasks (1). On page 1003of this issue, Bailey and colleagues (2) con-tribute to our understanding of gene duplica-tion by calculating the number of segmentalduplications in the human genome.

The most common fate of duplicate genesappears to be the simple silencing of onemember of the pair. The average time beforesilencing of one duplicate gene pair memberis ~4 million years in animals (3, 4). Byrestoring the content of a genome to its origi-nal state, the silencing of duplicate genes haslittle direct effect on adaptive evolution. But,

because either the ancestral or the descendantcopy can be silenced, recurrent duplication ofgenes at unlinked chromosomal locations can

passively give rise to small-scale chromoso-mal rearrangements (5, 6). When combinedwith geographical isolation, these small-scalegene rearrangements may contribute to theemergence of new reproductively isolatedspecies.

Consider a pair of unlinked copies of anessential gene in an ancestral species. If,

because of functional redundancy, one ran-

dom member of the duplicate gene pair isdestined to become silenced in each popu-lation, there is a 50% chance that differentcopies will be silenced in two geographi-

cally isolated populations, thereby result-ing in different chromosomal locations forthe gene (see the figure). The contributionof this process to the evolution of geneticincompatibility between the two popula-tions depends on the rate at which gene du-

plication takes place. Thus, it is noteworthythat all recently characterized eukaryoticgenomes harbor substantial numbers ofvery young gene duplicates, many showingless divergence than gene copies (alleles)at the same chromosomal position (locus).

By applying demographic principlesand genome sequence analysis to species-wide surveys of duplicate genes, scientistshave calculated an average duplication rateof ~1% per gene per million years (3). Thisestimate holds up under a reanalysis of ad-ditional and better curated genome se-quences (4). Using a new statistical ap-

proach to infer the presence of duplicatedregions in pools of random human genomesequences, Bailey et al. estimate that atleast 5% of the human genome consists ofsegmental duplications. They calculate thatthe span of the segmental duplications inthe human genome ranges from tens to

P ERSP ECT IVE S : G ENOM ICS

Gene Duplication and EvolutionMichael Lynch

The author is in the Department of Biology, IndianaUniversity, Bloomington, IN 47405, USA. E-mail:mlynch@bio.indiana.edu