Public Information Office
American Geophysical Union

September 8, 1998

"Snowball comets" are just camera noise, Berkeley researchers say after analyzing dark pixels in Iowa data

New, higher resolution images from the VIS and UVI cameras aboard the Polar spacecraft show similar clusters of dark pixels, which Frank and Dr. John B. Sigwarth, both of the University of Iowa, have recently taken as independent verification of the presence of small comets. Various critics of the comet theory have previously suggested that the simple explanation for the dark pixels is noise.

In papers scheduled for publication October 1 in the journal, Geophysical Research Letters, Prof. Forrest S. Mozer and Dr. James P. McFadden of Berkeley's Space Sciences Laboratory state that their study "differs from all others that have objected to the small-comet hypothesis in that it considers events produced by the major proponents of this hypothesis (Frank and Sigwarth) from data provided by their own Polar instrument."

Both papers analyze raw data for one day provided by Frank and Sigwarth and additional data in the form of 700,000 pixel clusters, covering 120 days, posted on the web and known as the Iowa catalog. McFadden, et al., investigate the characteristics of the dark pixels in relation to expected noise from the individual components of the two cameras. Using computer simulations, they show that the dark pixels seen in the satellite data from both cameras are entirely consistent with instrumental noise.

Mozer, et al., investigate the distribution of the dark pixels by altitude. They show that there is no appreciable height dependence. The researchers also note that the same pattern of dark pixels is seen in images of the nighttime sky as in sunlit images, which would not be the case if they were caused by external objects such as small comets. They conclude that Frank and Sigwarth's own data processing introduces those "meaningless" dark pixel clusters. Outside the radiation belt, say the authors, more than 80 percent of the dark pixel clusters "result from the process that Frank and Sigwarth employ to remove bright pixels caused by energetic particles."

GRL Space Physics and Aeronomy Editor Robert Winglee notes that Prof. Frank has been made aware of the contents of the Mozer and McFadden papers and has been invited to submit a response.

Sandia National Laboratories
Albuquerque, New Mexico

February 10, 1998

Sandia scientist, colleague suggest meteor plumes causing transient dark spots in upper atmosphere

That theory has been proffered by two space scientists Louis Frank and John Sigwarth of the University of Iowa, based on observations by the Dynamics Explorer-1 satellite of transient dark spots, or holes, in the upper atmosphere's far-ultraviolet dayglow emission.

Such a tremendous influx of small comets has never been observed. These water-bearing objects would add phenomenal quantities of water to Earth. If this could indeed be shown to be happening, much of what is known about comet creation and the origins of the oceans, terrestrial life, and perhaps even of the solar system might need revision.

The interpretations have been met with much scientific skepticism, but Frank and Sigwarth weighed in last May with more evidence -- observations by the NASA satellite Polar that they said provide new support for the existence of the dark holes. A number of former critics became converts.

Now, however, Sandia physicist Mark Boslough and Randy Gladstone of the Southwest Research Institute in San Antonio, Tex., have published a study that provides a less provocative -- but still scientifically interesting -- explanation for the so-called atmospheric holes.

They may be plumes, not holes, and meteoroids may be the source.

Their computational simulations, which make use of Sandia's shock physics code CTH and Boslough's earlier work with Sandia colleague Dave Crawford in successfully predicting the visible plumes from Comet Shoemaker-Levy 9's impact into Jupiter in 1994, suggest that the entry of ordinary meteoroids can form dark spots very similar to those reportedly observed by the satellite instruments.

Their study is published in the Dec. 15 Geophysical Research Letters, along with four other studies by other scientists that together the journal say provide "five independent tracks of evidence that are entirely inconsistent with a huge bombardment by small comets." The journal says the five studies together "refute this [the small comets] hypothesis."

Boslough and Gladstone reject the assumption, central to the small comets hypothesis, that the observed darkening is caused by absorption of water vapor above the atmosphere. Instead, they believe that atomic oxygen, which is the source of the dayglow, is momentarily displaced by the passage of meteoroids. Normal air from lower altitudes contains oxygen in its molecular form and is black in the wavelength that the satellite sees.

They propose that when a stony object as small as 50 centimeters across collides with the atmosphere and plunges into the lower layers, it ejects a very thin plume of this "black" air to as high as 1,000 kilometers. It is these dark plumes, they suggest, that are being detected by the satellites.

Their work is preliminary and they acknowledge that the hypothesis doesn't account for the observed high rate of dark hole formation. But they say if they can show their idea is correct for large meteoroids, they will look into the possibility that small ones have a similar effect.

The two scientists also propose a test of their hypothesis. They say if they are right, it is just a matter of time before Department of Defense satellites detect an infrared flash from a large meteor that corresponds exactly to the time and location of one of the holes.

"We expect that satellite data on atmospheric holes in the FUV [far ultraviolet] will confirm the existence of [high-altitude] plumes that are continuously being generated by meteors."

The four other new papers that also undermine the small comets hypothesis are based on lack of any evidence for frequent comet bombardment on the moon, no sign of the high abundance of noble gases in the upper atmosphere that would be expected from a high rate of comet bombardment, absence of visible light observations, and evidence that the source of dark pixels in the Polar spacecraft's ultraviolet camera is internal to the camera system.

Three of these papers are by scientists at the Lunar and Planetary Laboratory at the University of Arizona, and one is by scientists at the Universities of Alabama and Washington and NASA Marshall Space Flight Center.

Frank and Sigwarth, however, still strongly stick to their small comets hypothesis -- they particularly dispute that the dark spots are instrumental artifacts -- and the controversy is likely to continue for some time.

Sandia is a multiprogram DOE laboratory, operated by a subsidiary of Lockheed Martin Corp. With main facilities in Albuquerque, N.M., and Livermore, Calif., Sandia has major research and development responsibilities in national defense, energy, environmental technologies and economic competitiveness.

PHOTO CAPTION:

University of Iowa

December 9, 1997

UI's Louis Frank presents additional proof for "small comet" theory

Louis A. Frank and John B. Sigwarth presented the study at the fall meeting of the American Geophysical Union (AGU) in San Francisco. The study shows dark spots (called "atmospheric holes" because of their appearance on film) captured in NASA photographs decrease in size and number as the Polar spacecraft's altitude and distance from the holes increases -- just what one would expect to find if the cameras are taking pictures of a real phenomenon. According to Frank, skeptics of the small comet theory who have maintained that the atmospheric holes are caused by electronic "noise" affecting the camera will now have to re-evaluate their position.

"This result is a marvelous confirmation of the reality of atmospheric holes," says Frank, a Fellow of the AGU and of the American Physical Society.

The latest study examines June 1, 1997 photographs of the Earth's upper atmosphere, comparing one set of pictures taken from between 3 and 5 Earth radii above the surface to another set taken at altitudes of between 5 and 8 Earth radii. A total of 5,650 atmospheric holes were observed in the images, however the high altitude photographs showed an 80 percent drop in the frequency of atmospheric holes in comparison to the low altitude data. Also a greater number of atmospheric holes were photographed during early morning hours than during evening hours.

At the spring AGU meeting in May, Frank revealed a series of photographs taken by cameras aboard NASA's Polar spacecraft as proof of the existence of the 20-to-40-ton ice comets that, over the age of the Earth, could have provided enough water to fill the oceans and plant the seeds of life. The pictures ranged from one of a small comet the size of a two-bedroom house disintegrating some 5,000 to 15,000 miles above the Atlantic Ocean to an image of light emitted by the breakup of water molecules from a small comet less than 2,000 miles above the Earth. Frank and Sigwarth, who co-discovered the small comets and designed the three Visible Imaging System (VIS) cameras aboard the Polar spacecraft, said the pictures proved the existence of the small ice comets, but some doubters remained. (Since then, a satellite trailing the Space Shuttle Discovery in August detected significant amounts of high-altitude water vapor, a finding that supports the small comet theory.)

"Despite all of the evidence that the atmospheric holes were a geophysical phenomenon and not an artifact of the camera, many members of the scientific community refused to accept the reality of the atmospheric holes because of the immense implications of the large fluxes of small comets in the vicinity of our planet," says Frank.

Frank first announced the small comet theory in 1986, after examining images recorded in photographs taken by NASA's Dynamics Explorer 1 spacecraft. A specially-made camera had been designed to take pictures of the northern lights, a mission it completed successfully when it captured the first images of the complete ring of the northern lights from above the north pole. But some of the images contained unexplained dark spots, or atmospheric holes. After eliminating the possibility of equipment malfunction and numerous other explanations, Frank and Sigwarth concluded that the atmospheric holes represented clouds of water vapor being released high above Earth's atmosphere by the disintegration of small ice comets.

They calculated that about 20 comets enter the atmosphere each minute. At that rate, the steady stream of comets would have added about one inch of water to the Earth's oceans every 20,000 years -- enough to fill the oceans over billions of years. The theory was immediately controversial, with people asking why such objects hadn't been observed previously. Frank countered that not only their small size -- 20-to-30-feet in diameter -- makes observation difficult, but also that water striking the upper atmosphere glows very faintly as compared to the bright glow of metal and rock in solid meteors.

Not until the 1996 launch of Polar, with its two sensitive visible light cameras and one far-ultraviolet light camera, was there a chance to photograph the small comets with greater resolution.

University of Arizona News Services

December 9, 1997

Arizona scientists highly skeptical of 'small comet' theory

University of Iowa physicist Louis A. Frank and his former graduate student announced last May in a NASA news release and at an American Geophysical Union news briefing that images from their Visible Imaging System on the Polar spacecraft show Earth is showered by a steady stream of water-packed objects, small comets that bombard our planet at a rate of between five and 30 per minute. They published the results in the Oct. 1 Geophysical Research Letters. If true, the discovery would force a rethinking of the origins of the oceans, terrestrial life and the formation of the solar system.

In five independent studies to be published Dec. 15, scientists -- including three teams from The University of Arizona -- conclude that theoretical calculations and observational evidence rule out the small-comet hypothesis.

If the small-comet theory were correct, the Earth's sky would be a continual display of bright clouds and flashes, according to calculations by Bashar Rizk and Alex J. Dessler of the UA Lunar and Planetary Laboratory. If 30,000 small comets bombard Earth daily, as the theory says, constant meteor-like displays would be visible even during the day.

The expanding cloud of tiny ice particles that small-comet theory suggests is created when a 30-ton, 40-foot-diameter comet breaks up high in the atmosphere would have a brightness somewhere between that of Venus and the full moon, Rizk and Dessler calculate. (Dessler in the late 1980s published a review of several earlier scientific studies that rigorously tested small-comet theory.)

If, as small-comet theory says, a small comet strikes Earth about every three seconds, it would be visible for at least a minute to the naked eye, readily seen by anyone looking up, Rizk and Dessler add. "Where are they? We should see them," the LPL researchers puzzle.

"A whole-Earth flux of 20 comets per minute implies the sudden appearance of at least two bright patches of light every five minutes," they report in GRL. "The two-hour periods after sunset and before sunrise ought to produce the most spectacular sightings -- intermittent punctuations of bright, rapidly moving points of light." Twilight would be much more exciting in Tucson, Cairo, Sydney, Capetown and other communities, say Rizk and Dessler. Citizens of Fairbanks, Montreal, Moscow and Stockholm would be treated to near all-night meteor shows, they add.

Small-comet theory requires that the bombarding comets were formed in very cold regions far the from sun, Timothy D. Swindle and David A. Kring of the LPL note in their paper. Comets that formed far out in space condensed from the same dust and gas that accreted into planets, trapping "noble" gases in the same ratio as the sun and the rest of the solar system. Noble gases are inert, or non-reactive gases, not easily removed from the atmosphere. They include argon, krypton, xenon, as well as the more common nitrogen, helium and neon.

Swindle and Kring analyzed how much noble gas the small comets would have delivered to Earth's atmosphere over the lifetime of the solar system. "We know that if the Earth's atmosphere were bombarded according to small planet theory, it would have a dramatically different composition," Kring said in an interview.

At the current rate of supposed small comet bombardment, Earth should have 500 times as much krypton and xenon and 30,000 times as much argon in its atmosphere, Swindle and Kring calculate. Put another way, all the kypton and xenon in Earth's atmosphere would have been delivered by small comets in 10 million years. All the argon present would have been added in 100,000 years. The scenario for Mars' atmosphere is an even more enigmatic: Small comets would have delivered the known martian inventory of krypton and xenon in 500 years and the known inventory of argon in about 60,000 years.

Either the rate of supposed small-comet bombardment is today 30,000 times greater than it has been over the 4.5-billion-year lifetime of the solar system, or the comets formed much nearer the sun, about the distance of Jupiter, for the theory to fit the observed noble gas inventory, Swindle and Kring conclude. Comets can be greatly depleted in noble gases if they form closer to the sun, near Jupiter. "The problem with that idea is that it is completely inconsistent with several other physical conditions that Frank's team require to explain other features of their hypothesis," Kring said in the interview.

If the small-comet hypothesis is right, a small comet hits the moon at a rate of almost one per minute, Jennifer A. Grier and Alfred S. McEwen of the UA Lunar and Planetary Laboratory report in their paper. That is, scientists should see evidence of 400,000 comet hits on the moon annually.

Even a small, low-density comet would excavate a crater at least 50 meters in diameter and spread bright ejecta over an area of at least 150 meters in diameter, Grier and McEwen calculate. (The lunar surface darkens over time; the underlying, unexposed soil is lighter in color.)

Grier and McEwen compared Apollo 17 images taken in late 1972 to Clementine images taken 22 years later for a 52,000 square-kilometer area of the moon, which is about half the size of Kentucky and more than one-tenth of one percent of the lunar surface. Any crater and bright spot seen in the 1994 Clementine images but not visible in the 1972 Apollo photos might record a small impactor hit.

Each of the 3,920 bright spots seen over the study area in 1994 by the Clementine spacecraft was also recorded by Apollo. If the small-comet theory were correct, Clementine imaging should have discovered more than 10,000 new bright impact spots over this area.

Grier and McEwen calculate from the spacecraft observations an upper limit of 33 impacts a year for the entire moon, not 400,000 hits per year as expected according to the small-comet hypothesis. Small comets with properties hypothesized by Frank's team are probably more than a billion times less abundant than predicted, Grier and McEwen further conclude.

In another research article published in the Dec. 15 Geophysical Review Letters, a team of researchers who use a Polar spacecraft camera similar to the Frank team's Visible Imaging System also report seeing the dark pixels, or black points, that Frank interprets as evidence for small comets. This team, however, concludes the dark pixels are an inherent camera feature, or "noise" rather than real features.

A fifth paper in the journal suggests meteorites as small as 50 centimeters in diameter create plumes of atmospheric gas that Frank and his team interpret as small comets. The large-scale analogy to this phenomenon is the Comet Shoemaker-Levy impacts on Jupiter, they report.

University of Washington

December 9, 197

Giant Snowballs In Space?
No, Says Researcher,
They're Simply Black Snow On The TV Screen

But the snowballs may not exist. University of Washington geophysicist George Parks has analyzed Frank's ultraviolet (UV) camera images and has concluded that the white snow in space is no more than black "snow" on the television screen.

After a close analysis of one hour of data supplied by Frank, Parks says he and his collaborators are certain that Frank has been looking at "instrument noise." It is very similar, says Parks, "to the static you hear on your hi-fi."

Frank and Parks will debate the real vs. phantom snowballs here today at the fall meeting of the American Geophysical Union in San Francisco (Dec. 9 at 4 p.m.).

Frank first proposed his theory of the cosmic snowballs -- actually small comets -- in 1986, but the idea was widely discredited. Then, earlier this year, he presented evidence from the Polar satellite, which carries an instrument that can produce both UV and visible light images. Frank compared the same spots on both types of image and concluded that these were clear evidence of the existence of the comets.

Parks says that at first he was "agnostic" towards Frank's data. But when he saw the far more detailed images from the Polar camera he became suspicious. It was simply unlikely, he says, that the clusters of spots on the images could have been caused by snowballs in space. Parks began an analysis of his own images taken with the Ultraviolet Imager (UVI) on the NASA Polar satellite. There he found the same dark spots that Frank had found on his images.

He grew even more uneasy about Frank's analysis when he found that the UVI had recorded the same dark spots while pointed at a UV light in the laboratory.

When Parks began a minute examination of the images, made by breaking the clusters of spots down into tiny picture point, or pixels, he found statistical evidence that he was seeing not real events, but what he calls an "instrument artifact."

After Parks had detailed his analysis in an article for Geophysical Research Letters, Frank released one hour of data that overlapped with Parks' UVI images. Parks has made a comparison of the two and now believes, even more emphatically, that Frank has been attempting to interpret background noise.

What is causing the spots on the images? Park blames the very complexity of the cameras themselves, which consist of a number of parts, including optics, an image intensifier that includes a device for multiplying electrons, a TV screen and a light-gathering charge-coupled device. Parks suspects that the dark spots change character as the camera's high voltage is varied.

Parks claims that Frank has taken complex images and selected only one tiny area as evidence of the comets' existence. "He nevers shows the full image because it always looks corrupted by noise," he says.

Is Parks then denying the existence of cosmic snowballs? "The burden is on Frank, he's got to prove they exist," Parks says. "He is seeing things that are scientifically not permitted. It would, for example, be easy for me to say these dark spots are UFOs, but it would be up to me to prove it."

NASA Headquarters, Washington, DC
Goddard Space Flight Center, Greenbelt, MD

May 28, 1997

POLAR SPACECRAFT IMAGES SUPPORT THEORY OF INTERPLANETARY SNOWBALLS SPRAYING EARTH'S UPPER ATMOSPHERE

Using Polar's Visible Imaging System (VIS), a research team led by Dr. Louis A. Frank of the University of Iowa in Iowa City has detected objects that streak toward Earth, disintegrate at high altitudes and deposit large clouds of water vapor in the upper atmosphere. Frank's research is being reported in a news briefing at 10 a.m. today at the spring meeting of the American Geophysical Union at the Convention Center in Baltimore, MD.

The incoming objects, which Frank estimates to be the size of a small house, pose no threat to people on Earth, nor to astronauts in orbit. "They break up and are destroyed at 600 to 15,000 miles above the Earth," Frank noted. "In fact, this relatively gentle 'cosmic rain' -- which possibly contains simple organic compounds -- may well have nurtured the development of life on our planet."

"This is an intriguing result that requires further scientific investigation," said Dr. George Withbroe, science director for the Sun-Earth Connection program in NASA's Office of Space Science. "We need to look closely at measurements from other sensors to find out if they see related signatures in the atmosphere, now that we have learned more about what to look for."

The Polar cameras have imaged trails of light in both ultraviolet and visible wavelengths as the objects disintegrate above the atmosphere. Using a filter that detects visible light emitted only by fragments of water molecules, Frank has shown that the objects consist primarily of water.

"The Polar results definitely demonstrate that there are objects entering the Earth's upper atmosphere that contain a lot of water," commented Dr. Thomas M. Donahue, a noted atmospheric physicist and professor at the University of Michigan in Ann Arbor.

"The images show that we have a large population of objects in the Earth's vicinity that have not been detected before," said Frank, who designed the VIS instrument. "We detect these objects at a rate that suggest Earth is being bombarded by five to 30 small comets per minute, or thousands per day." Comets are known to contain frozen water and are sometimes called "dirty snowballs".

Frank's new observations are consistent with a controversial theory he proposed in 1986 to explain the existence of dark spots, which he termed "atmospheric holes", in images of the sunlit atmosphere of the Earth. He first detected these holes while analyzing data from an ultraviolet imager flown on NASA's Dynamics Explorer 1 spacecraft. He theorized that the holes were caused by the disintegration of small icy comets in the upper atmosphere. The water vapor they produce momentarily absorbs the ultraviolet solar radiation scattered from oxygen atoms in the upper atmosphere, preventing it from reaching his camera and resulting in a dark spot on the image. These holes have diameters of 15 to 25 miles.

His theory of a new class of objects in the Solar System ignited a wide-ranging controversy. Many colleagues discounted the appearance of the holes as an instrumental problem. But the new images from Polar also include observations of atmospheric holes in much greater detail than before, suggesting that they are real. "These results certainly vindicate Lou Frank's earlier observations", said Donahue.

"These remarkable images cap a great first year for Polar," added Dr. Robert Hoffman, Project Scientist for Polar, which is operated and managed by NASA's Goddard Space Flight Center, Greenbelt, MD. "I am pleased that Polar's instruments were able to actually detect these objects streaking towards the Earth and disintegrating into clouds of water vapor. They give scientists a fascinating new and important phenomenon to take into account in theories of Solar System evolution."

Uit KIJK, december 1988:

DE OCEANEN KWAMEN UIT HET HEELAL

Aan de uiterste rand van de atmosfeer, waar de sterren ongehinderd in de ruimte schijnen worden complete zwembaden over de aarde uitgestort. Per keer minstens 100.000 liter water. Alleen gaat het natuurlijk niet om echte zwembaden, maar om kosmisch water dat wordt ingevangen door de aarde. Het zijn minikometen die als reusachtige sneeuwballen op de aarde botsen. Blootgesteld aan de zwaartekrachtswerking van de aarde, overleven zij die botsing niet. Op 1.500 tot 3.000 km hoogte vallen de minikometen in brokstukken uiteen. Wat overblijft zijn uitgestrekte wolken sneeuw en ijs, die overgaan in waterdamp. De waterdampwolken dringen met grote snelheid in de atmosfeer, en uiteindelijk regenen ze neer op het aardoppervlak.

De gemiddelde minikomeet is onbeduidend als het gaat om zijn afmetingen. In tegenstelling tot echte kometen, met doorsneden tot vele kilometers, hebben minikometen een diameter van ongeveer 12 meter. Maar wat ze missen in afmeting, maken ze goed in getalssterkte. Als een vloot botsen ze op de aarde: 20 per minuut, 1.200 per uur, ofwel 28.000 per dag. Zo gaat dat dag in dag uit, vermoedelijk al 4,5 miljard jaar lang sinds het ontstaan van de aarde. Een snelle rekensom leert hoe al dat water tenslotte tot één groot golvenbad is uitgegroeid. De hoeveelheid water die de minikometen op aarde brachten is zo groot, dat het moet gaan om bijna àl het water. Uit de rivieren, meren, zeeën en oceanen.

Tot voor kort werd algemeen aangenomen dat het water op aarde door condensatie is ontstaan: uit de hete en dichte oeratmosfeer die de aarde vroeger omringde. Deze atmosfeer bestond voornamelijk uit waterdamp en kooldioxyde, afkomstig van de gassen die tijdens het afkoelen en stollen van de aarde vrijkwamen. En dat water, voegt de theorie er aan toe, is langzaam maar zeker aan het verdwijnen. Waterdamp rijst immers op in de atmosfeer, waar een deel onder invloed van het zonlicht in afzonderlijke atomen wordt afgebroken. De lichtste daarvan, waterstofatomen, ontsnappen in de ruimte.

Louis A. Frank en zijn collega's John Sigwarth en John Craven van de Universiteit van Iowa in de Verenigde Staten, zeggen van niet. Het water kwam niet van beneden, maar van boven, neerdruppelend uit de ruimte. De hoeveelheid water op aarde neemt daardoor langzaam toe. De hele wereld baadt zich nu, en heeft dat altijd al gedaan, in water van buitenaardse oorsprong.

Het was in 1981 dat Frank en Craven een uitzonderlijk werk voltooiden: de ontwikkeling van een optisch systeem voor de Dynamics Explorer 1 (DE-1): de eerste kunstmaan voor het waarnemen van de buitenste lagen van de dampkring. Met deze satelliet hoopten zij foto's te kunnen maken van eventuele veranderingen in die buitenlagen. Ook van de elektrische verschijnselen die zich daarin afspelen. Frank was met name geïnteresseerd in het sterk veranderlijke poolicht. De DE-1 werd daartoe uitgerust met drie videocamera's. Twee voor waarnemingen in de golflengten van zichtbaar licht en een die ultraviolette straling moest vastleggen die onzichtbaar is voor het menselijk oog.

De van de satelliet ontvangen beelden waren in alle opzichten spectaculair. Foto's van de aardatmosfeer, compleet met een ring van noorderlicht, verschenen spoedig in alle tijdschriften Overal ter wereld werd Frank uitgenodigd lezingen te houden en zijn opnamen te vertonen.

Al die tijd was er één hinderlijk probleem. Gefilmd in het ultraviolet, en naar een zichtbare kleur vertaald door een computer, zou de atmosfeer boven de dagzijde van de aarde een egaal, oranje kleurverloop moeten hebben. Maar in plaats van een egaal kleurverloop, zagen de wetenschappers een atmosfeer, die werd onderbroken door talloze zwarte vlekjes. Elk vlekje verscheen plotseling, bleef een paar minuten zichtbaar, en verdween dan weer geleidelijk uit het beeld. "Telkens als ik een diavertoning hield," herinnert Frank zich, was er wel iemand onder de toeschouwers die zich afvroeg wat die vlekjes betekenden. Mijn antwoord was dan altijd dat het vermoedelijk ging om een soort van elektronische ruis. Het was een redelijk antwoord, en zowel de vragenstellers als ik waren er mee tevreden."

Zo ging het voor een tijdje. In 1982 echter bestudeerde John Sigwarth, toen nog een student van Frank, de opnamen. Sigwarth zocht naar zwaartekrachtsgolven, rimpels in de atmosfeer die soms volgen op het optreden van poollichtverschijnselen. Maar telkens als het computerprogramma de opnamen analyseerde, staakte het zodra het bij een van de zwarte vlekjes kwam. Sigwarth vroeg Frank of er een manier was om de ruis te verwijderen. Dat was het begin van een jarenlang onderzoek.

Allereerst werd gekeken of er sprake was van storingen door een andere satelliet, bliksemontladingen of een radiobron op aarde. Toen die niet konden worden gevonden, werd nagegaan of de pixels op de juiste manier werden doorgeseind door het ruimtevaartuig. Pixels zijn de individuele punten waaruit een elektronisch beeld is opgebouwd. De pixels van de ultravioletcamera aan boord van de DE-1 werden bewerkt en uitgezonden door twee afzonderlijke apparaten. Uit voorzorg voor een eventuele hapering, seinden deze apparaten steeds een wisselende helft uit. Maar welk apparaat de betreffende pixels ook uitzond, telkens verschenen dezelfde zwarte vlekjes. De onderzoekers konden dus niet anders dan concluderen dat ofwel de twee apparaten telkens dezelfde fouten gaven, wat elektronisch gezien nagenoeg onmogelijk is, ofwel dat de zwarte vlekjes echt waren.

Als de zwarte vlekjes echt waren: waardoor werden ze dan veroorzaakt? Frank, Sigwarth en later ook John Craven, verzamelden radarwaarnemingen van meteoren van de afgelopen tien jaar. De posities waar deze meteoren in de atmosfeer kwamen, vergeleken ze met die van de raadselachtige zwarte vlekjes. De meeste meteoren kwamen voor aan de ochtendzijde van de aarde. Aangezien dat de zijde is die ligt in de bewegingsrichting van de aarde om de zon, worden ze daar aanmerkelijk vaker door de aarde "opgeveegd". Hetzelfde verschijningspatroon deed zich voor bij de zwarte vlekjes. "Het was met die ontdekking, die we eind 1985 deden, dat we ons onderzoek naar elektronische ruis of stoorsignalen afsloten," vervolgt Frank. "Wat we waarnamen was duidelijk iets dat te maken had met inslagen in de aardatmosfeer."

De volgende stap was natuurlijk het identificeren van de objecten die verantwoordelijk waren voor de zwarte vlekjes. Inmiddels was komen vast te staan dat die een oppervlakte vertoonden van ongeveer 3.000 vierkante kilometer. Een aantal mogelijkheden werd onderzocht en afgewezen. Waren het bijvoorbeeld meteoren, die de lucht op lage hoogten verhitten, en waardoor reusachtige gasbellen in de atmosfeer omhoog stegen? "Zo'n rotsblokachtige meteoor zou een kilogram of honderd moeten wegen, en dan hebben we het over twintig meteoren per minuut," zegt Frank. "Als het om dergelijke meteoren zou gaan, zouden we hier aan het oppervlak allemaal in inslagkraters leven. Daarom dachten wij aan uit waterijs bestaande minikometen, die door de getijdekrachten van de aarde al op grote hoogte werden uiteengetrokken. Het plotselinge verschijnen en na ongeveer 3,5 minuten weer oplichten van de zwarte vlekken in de atmosfeer kon dan door de absorptie van zonlicht door de vrijgekomen waterdamp worden verklaard. Er vormt zich eerst een rond zwart gat als de waterdampwolk de atmosfeer treft, en vervolgens wordt dit gat weer opgevuld als die damp zich lager in de atmosfeer verspreidt."

Zou het werkelijk zo gaan, dat neerregenen van grote hoeveelheden kometenwater op aarde? Toen de drie onderzoekers hun resultaten publiceerden (in Geophysical Review Letters, van mei 1986), brak een stortvloed van kritiek los uit de rest van de wetenschappelijke wereld. Thomas Donahue van de Universiteit van Michigan bijvoorbeeld, een specialist op het gebied van planeetatmosferen, verwoordde die kritiek als volgt. "Als de oceanen niet van het begin af aan hier waren, waarom zijn er dan fossielen afgezet op de oceaanbodem, die teruggaan tot 3,8 miljard jaar geleden? Waar is al het water op Venus? En als die kometen in dergelijke getalen rondzwermen: waar is dan de oceaan op Mars? Ik heb altijd veel respect gehad voor Frank, maar dit idee gaat lijnrecht in tegen alle andere waarnemingen, en de aan de hand daarvan opgestelde modellen voor planeetatmosferen."

Vanzelfsprekend was Frank het niet met die kritiek eens. Hij wist niet hoeveel water nodig was voor het laten ontstaan van sedimenten, maar het was niet onredelijk te veronderstellen dat 3,8 miljard jaar geleden al iets van een oceaan op aarde aanwezig was. Venus is door het daar heersende, extreme broeikaseffect gewoon te heet om een oceaan vast te houden. En wat Mars betreft: daar was eens en zal ooit weer een zee zijn aan het oppervlak. Het wachten is alleen op een nieuwe warme periode, waardoor de poolkappen zullen afsmelten en in de atmosfeer genoeg waterdamp aanwezig is om een broeikaseffect op gang te brengen. "Dan is er al snel weer stromend water op Mars. Genoeg om nieuwe kanalen uit te slijpen naast degene die we daar al zo lang waarnemen."

Dit alles speelde zich tweeënhalf jaar geleden af, toen Frank en zijn collega's hun bevindingen nog maar net openbaarden. Maar het interessante van deze wetenschappelijke controverse is, dat zij dit jaar weer opnieuw in alle hevigheid is losgebarsten. Allereerst ging Thomas Donahue, de felste kritiekhebber, overstag. Hij onderzocht de gegevens van de ultraviolet-camera aan boord van de Voyager ruimtesondes, en vond dat in de ruimte tussen de planeten veel meer "schone" waterdamp voorkwam dan op grond van de aanwezigheid van stoffige kometen kon worden verondersteld. Donahue stelde een nieuwe groep kometen voor die voor 90% uit losse sneeuw zouden bestaan en die doorsneden moesten hebben van niet meer dan een tiental meters. Precies dus die kometen, die waren voorgesteld door Frank. Later werd Donahue op zijn vingers getikt door collega's. Hij had een rekenfout gemaakt bij het uitwerken van de Voyagergegevens. In plaats van het getal 3 x 10^-2 uit een tabel had hij 3 x 10^-3 overgeschreven. Zijn minikometen smolten daarmee als sneeuw voor de zon.

Hierna voerde Christopher F. Chyba, van het laboratorium voor planeetonderzoek van de Cornell-universiteit, tellingen uit aan inslagkraters op de maan. Uit de grootte van de inslagstructuren, die tussen de 3,8 en 4,5 miljard jaar geleden ontstonden, leidde hij de massa's van de ingeslagen objecten af. De totale massa rekende hij om naar de veel grotere, totale massa die er toen op aarde zou moeten zijn neergekomen. De aarde heeft immers een sterkere aantrekkingskracht en een groter botsingsoppervlak. Zo vond hij dat in de periode tussen 3,8 en 4,5 miljard jaar geleden ongeveer 10^13 miljoen ton aan objecten op de aarde moet zijn gebotst. Het grootste deel van de inslaande objecten zouden planetoïden zijn geweest: rotsblokken die tijdens het proces van planeetvorming overbleven en grotendeels uit gesteenten, ijzer en koolstof bestaan. Waterijs bevattende kometen ontstonden vermoedelijk eerder uit de echte oermaterie van het zonnestelsel. Als nu wordt aangenomen dat 10% van alle inslaande objecten grote kometen waren en dat kometen voor 50% uit ijs bestaan, dan zou dit betekenen dat 3,8 miljard jaar geleden al 40% van het huidige oceaanwater op aarde aanwezig was.

In mei kwam tenslotte een aanwijzing van waarnemende astronomen dat kleine kometen zich in de ruimte moeten ophouden. Clayne Yeates van het Jet Propulsion Laboratory van de NASA en Tom Gehrels van het Lunar and Planetary Laboratory van de Universiteit van Arizona fotografeerden de hemel met een zeer gevoelige detector. Zij gebruikten de "Spacewatch" telescoop op Kitt Peak. De telescoop werd halverwege de afstand aardemaan gericht en volgde de veronderstelde beweging van de minikometen. Op de opnamen verschenen sporen van kleine, donkere objecten, in dezelfde aantallen als volgens de theorie van Frank en zijn collega's.

Is het raadsel van het kometenwater nu definitief opgelost? De meerderheid van de astronomen en geofysici staat nog steeds zeer afwijzend tegenover de nieuwe theorie. Het water op aarde, zeggen zij, kan immers net zo goed worden verklaard uit het vrijkomen van waterdamp tijdens vulkanische uitbarstingen in de oertijd van de aarde. Maar een andere verklaring voor het ontstaan van de gaten in de dampkring hebben zij echter niet.

Tekst: Carl Koppeschaar

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