Earth Scientists

Probe Geological

Processes of the

Moon and Beyond

Earth Scientists

Probe Geological

Processes of the

Moon and Beyond

Geophysicist Catherine Johnson examines NASA Mars Rover

images at Scripps’s Visualization Center on iCluster, a “wall of

information” composed of 12 high-resolution computer displays.

B Y C H U C K C O L G A N

ScrippsIn Space

ScrippsIn Space

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O R S O M E E A R T H S C I E N T I S T S A T ScrippsInstitution of Oceanography, investigating

one world isn’t enough. They turn their atten-tion to the sky and wonder about the geologicalprocesses taking place on planets and moonsthroughout the solar system.

Using scientific techniques developed forunderstanding Earth’s internal configuration,these planetary geoscientists are discoveringthat some other nearby heavenly bodies havehistories and structures similar to those at home.Their efforts also are helping to advance designof sophisticated instruments for remotely sens-ing geological data with applications for moni-toring conditions on Earth.

They’re finding that Earth’s geologicalprocesses are not entirely unique within thesolar system. So far, evidence of volcanism, seis-mic activity, gravitational tides, magnetic fields,tectonic deformation, and other geophysicalforces are variously found on the three otherinner planets—Mercury, Venus, and Mars; ourMoon, and the four largest moons of Jupiter.

But why would an oceanographic institutionbe interested in extraterrestrial geology?

F

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Accord ing to geophys ic i s tCatherine Johnson, studying otherparts of the solar system helps us deci-pher our own planet’s evolution. Putmore simply, she said, “Exploration iswhat we do at Scripps, and it’s not justhere on Earth.”

F R O M E A R T H T O S PA C E

Planetary geoscience is by its nature ahighly cross-disciplinary field. Inorder to interpret the various types ofremote sensing and other data fromspace instruments and surface landermissions, the planetary geoscientistmust have a solid background in suchdiverse subjects as astronomy, miner-alogy, geochemistry, atmospheric sci-

Below, Graduate student Renee Bulow found new moonquake information in 30-year-old NASA data.

E A R T H Q U A K E S O N T H E M O O N

NASA astronauts

deployed seismic instru-

ments on the nearside of

the Moon during four

Apollo Missions between

1969 and 1972. The

network supplied moon-

quake data until 1978,

providing insight into the

Moon’s internal structure

and composition. One

discovery was seismic

activity clustered in

regions deep within the

Moon believed to be the

result of the buildup and

release of strain generat-

ed by Earth-Moon gravi-

tational forces. This

moonquake zone is 700-

1200 kilometers (435-

745 miles) below the

moon surface.

se i smic s ta t ion

moonquakecluster region

moonquake zone

core

mant le

crus t

ences, and computer programming.Supplying that foundation is therole of academic programs such asthose of the Cecil H. and Ida M.Green Institute of Geophysics andPlanetary Physics (IGPP), wherestudents train in terrestrial earthsciences and can apply similar con-cepts to planetary studies. There isconsiderable expertise at IGPP incollecting, processing, and model-ing large sets of geophysical datafor understanding Earth’s interiorthat can be translated and utilizedfor studying other planets andmoons.

Johnson’s Earth-based researchinterest is geomagnetism, specifi-cally the behavior of Earth’s mag-netic field over the past 5 million

years. Her space interests have varied from the topography ofVenus to magnetic fields on Marsand lunar seismology. A 1994Scripps graduate, she cites a simi-larity between today’s planetarystudies and the advancementsmade in earth sciences in the 1960swhen the first measurements ofdeep-earth seismology and gravityconfirmed continental drift andplate tectonics. Space geophysics,she said, allows researchers to “goback to asking fundamental ques-tions about how planetary bodiesdevelop and evolve.”

THE EARTH-MOON SYSTEM

From the beginnings of IGPP atScripps in 1959, the focus of plane-tary research has been the Earth-

Moon system. IGPPfounder and oceanographerWalter Munk conducted pioneeringstudies of the gravitational torquebetween the two bodies and theMoon’s effects on tides and Earth’srotation. Many others followed,leading investigations into the ori-gin of the Moon, its composition,and internal structure.

Today, two of Johnson’s stu-dents are continuing to ask basicquestions about Earth’s Moon. Onehas found that even 30-year-old dig-ital data can yield new information,and another is the first to apply amodern laboratory technique tolunar rock samples to help

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Astronaut Edwin Aldrin heads back to the

lunar lander after installing a seismometer

during an Apollo mission..

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resolve the Moon’s geological history.Renee Bulow, a fifth-year student, decided to analyze lunar

seismic data from the Apollo missions to look for informationthat possibly escaped the attention of scientists who did theanalysis in the 1970s. Her advantage was that the originalmoonquake data couldn’t be adequately processed withoutpresent-day computers.

During Apollo missions from 1969 to 1972, astronautsinstalled four seismometers at locations on the Moon. Theywere the first space instruments to collect data in a digital for-mat and telemeter it back to Earth. The network operateduntil 1978 when NASA permanently shut down the instru-ments because of budget issues and the overwhelming amountof data. No computer at that time could handle 13 gigabytes ofdata. That’s a quantity small by today’s standards when asmuch information can be processed from a single earthquake ina matter of hours. In the 1970s, scientists converted a subset ofthe digital data from more than 10,000, nine-track magnetictapes to paper printouts that were reviewed visually on lighttables. Some 12,500 seismic events were cataloged. The dataset, however, contained many sections of “noisy” data with noway to distinguish internally generated moonquakes from mete-or impacts and surface thermal expansion and contraction.

Unlike the Earth, the Moon does not have active plate tec-tonics, so in contrast to the hundreds of earthquakes recordedeach day, fewer than 10 moonquakes occur daily. The Apollo-era scientists identified shallow quakes ranging 20 to 30 kilo-

moon rock

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meters (12.4-18.6 miles) below thesurface and deep quakes at depths ofabout 700 to 1,200 kilometers (435 to745 miles), about halfway to theMoon’s center. The shallow quakeswere relatively rare; there were fewerthan 30 over eight years, but some ofthem were intense, measuring up tomagnitude 5.5 and lasting more than10 minutes. More significantly, re-searchers distinguished 6,500 smallerand deeper quakes that appeared tobe clustered at some 75 locations andthat happened at somewhat regularintervals.

Using modern seismological dataprocessing techniques and contem-porary computers, Bulow filtered andcleaned up the noisy data from themost active of the deep moonquakeclusters. So far, data from five regionshave been analyzed, with the resultof about 30 percent more moon-quakes being found. She identified apeak period of activity at 27.2 days,the time it takes the Moon to orbitEarth, which indicates that gravita-tional forces from Earth generatemoonquakes. She also was able tobetter pinpoint moonquake centersand correlate recordings from theseismometer stations.

“It’s still not understood howmoonquakes occur at such greatdepths, where temperatures arebelieved to be 1,000° C (1,832° F)”,Johnson said. “That’s typically atmuch higher temperatures and deep-er than earthquakes, which gives useven more to wonder about the inte-rior of the Moon.”

T H E M O O N R O C K S

Third-year student KristinLawrence studies moon rocks fromthe Apollo missions to better under-stand their origin and development.Working with Johnson and geo-physicist Lisa Tauxe, an expert on

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geomagnetic fields, Lawrence is thefirst to apply modern lab methods todetermine the history of lunar sam-ples based on their magnetic proper-ties, or paleomagnetism. By measur-ing the orientation and intensity ofmagnetic minerals in rocks, it is pos-sible to establish the magnetic fieldsthat existed when the rock formedand gain insight about ancient geo-logical conditions.

Planetary scientists generallyagree that the Moon was formedsome 4.6 billion years ago when amassive asteroid or other object col-lided with a very young Earth, knock-ing free the material that later con-densed into the Moon. This explainsthe similar mineral structure andcomposition of the Earth and Moon.

Data from Apollo missions havesupplied a basic understanding of thegeological history of the Moon.Initially the Moon was molten rock,or magma. As it cooled, the first

Graduate student Kristin Lawrence (opposite

page bottom and above) puts minute sam-

ples of moon rock (opposite page top)

through laboratory processes that allow their

original magnetism to be measured. The

samples are heated to 800° C (1,472° F),

cooled, and measured in a magnetometer at

-263° C (-442° F). Small Earth rock sample

cores (above) are similarly examined.

pieces of crust formed, which we stillsee today as the lighter areas on theMoon’s surface. Meteoroids and otherobjects bombarded the early Moon,resulting in a surface riddled withcraters. The Moon also had what isestimated to be a billion-year periodof volcanism that formed basalt rocksthat we can see as dark areas.

One aspect of the lunar geologythat still puzzles planetary scientistsis the Moon’s core. Most likely the

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lunar core is made primarily of ironand nickel like the Earth, but is itsolid or molten? It’s the Earth’sdynamic, liquid outer core that gen-erates the strong magnetic fields onour planet that extend tens of thou-sands of kilometers into space as themagnetosphere, shielding us fromdestructive effects of charged parti-cles from the solar wind. Measure-ments show the Moon has no inter-nally generated magnetic fieldtoday; however, regions of the lunarsurface are magnetic. Were theseareas magnetized by externalsources or did the Moon once havean internal magnetic field, and if so,how intense was it?

Some 382 kilograms (842pounds) of lunar material were

brought back to Earth during six Apollo sur-face missions. More than three-quarters ofthe samples remain in pristine condition in avault at the Johnson Space Center inHouston, Texas, and on occasion smallamounts are given out for research. In the1970s, several scientists measured remnantmagnetic fields in Moon rocks and said thattheir magnetization was of lunar origin andbillions of years old. Unfortunately, laterimprovements in test methodologies showedthe results for paleomagnetic intensities to be inconclusive.

Lawrence received 6.5 grams of moon fragments, about the size of asmall sugar cube, because she and Johnson proposed to examine them withnewer techniques. She is using instruments in Tauxe’s paleomagnetics lab-oratory, sort of a time machine for rocks housed in a metal-shielded roomwhere the walls cancel out Earth’s magnetic fields. Over the past five years,Tauxe and Scripps colleagues Peter Selkin and Jeff Gee have establishedan elaborate system of checks that are performed during various stages ofpaleomagnetic analysis to assure reliability. This method has become the

standard for terrestrial rock analysis. During processing, Lawrence seals tiny amounts of

lunar samples in evacuated quartz tubes, heats them to800° C (1,472° F), cools them to room temperature in themagnetic field-free room, and then measures them in amagnetometer at temperatures of 10° Kelvin or -263° C (-442° F). The procedure removes unwanted secondarymagnetism and allows the original magnetization of therocks to be measured.

Lawrence is still in the early stages of processingthe lunar samples and is months away from publishingresults; however, she is accomplishing her major objec-tive in coming to Scripps by studying planetary geo-physics while participating in hands-on fieldwork. Shehas also collected samples from ancient lava flows inMexico and analyzed rocks from Antarctica.

“Earth materials are easier to analyze for theirpaleomagnetic proper-ties because you knowexactly where theycame from and havelots of samples to com-

Left, NASA scientist Bruce Bills is on

loan to Scripps as a liaison between

the space agency and the UC San

Diego science community.

pare their histories,” Lawrencesaid. “What’s exciting about themoon rocks is that they are suffi-ciently old that they provide cluesto formation of the solar system andprocesses that created the innerplanets.”

F R O M S PA C E B A C K T O E A R T H

Some two dozen Scrippsresearchers have NASA-fundedprojects ranging from using satelliteinstruments to observe coastalwater quality to searching for tracesof life in Earth’s oldest rocks. NASAspends about $5.5 billion on scienceprograms annually, but hasannounced it is limiting growth to1.5 percent next year and 1 percenteach year through the end of thedecade. The agency also recentlydeclared a preference for fundingspace missions rather than Earthmissions. The scientific challengethen becomes, according to visitingresearch geophysicist Bruce Bills, to“look for ideas for scientific tech-niques and instruments that can bedeveloped both for sending to otherplanets and for monitoring Earth.”

Bills is on quasi-permanent loanto Scripps from the Goddard SpaceFlight Center in Greenbelt,Md., as a liaison between NASA

Left, Geophysicist David Sandwell uses space

instruments in earth studies and teaches an

undergraduate course called Physics of Surfing.

Below, NASA's ICESat spacecraft

and the UC San Diego sciencecommunity. He cites many exam-ples of space instruments that haveproven beneficial for terrestrial pur-poses. The Magellan spacecraftlaunched in 1989 carried the firstsynthetic-aperture radar to viewdetails of the surface of Venus.Today, such instruments are usedfor a wide variety of environmentalapplications, such as monitoringcrops, deforestation, ice flows, andoil spills. The Mars Global Surveyorlaunched in 1996 carried the firstlaser altimeter to precisely mapsurface topography, yielding atechnology for mappingon Earth today.

Geophysicist DavidSandwell’s experienceoffers insight into how evensecret military satellite programsoccasionally result in space instru-ments coming into general scientif-ic use.

In 1985, the Navy launchedGeosat, a satellite carrying a radaraltimeter to measure the bumpsand dips in the ocean surfacecaused by subtle variations in thepull of gravity. By combining thisinformation with the 40-year recordof soundings from research vessels,it is possible to map the topography

of the seafloor kilometers below thesurface. The Geosat data were clas-sified and unavailable for public useuntil 1995, when the EuropeanSpace Agency launched a radaraltimeter satellite and began releas-ing ocean-surface data.

Soon after the Geosat data wereavailable Sandwell and Walter Smithof the National Oceanic andAtmospheric Agency produced themost complete, high-resolution mapof the world seafloor available thathas become a standard referencetool for marine science and educa-tion. An interactive version can beviewed by selecting “global topogra-phy” at http://topex.ucsd.edu/. Thisglobal model is also used for theocean component of Google Earth.

Johnson lamented that the costsof planetary satellite missions arelarge and the logistics complex, butis optimistic that many opportuni-ties will continue in the future.“When dealing with scientific pro-jects in space,” she said, “you haveto not only secure funding, but you

have to be flexible when thingsat NASA change.”

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