This comment (below the question Is there any new water formed on, or brought to, Earth?) links to the 2002 Article in the New Scientist Was all the Earth’s water brought here by comets? which states:

Evidence that there is a constant rain of small comets, each a loose snowball held together by a carbon shell, first came in the 1980s from satellites that looked down on Earth and observed “atmospheric holes”- dark spots 50 to 100 kilometres across- that were appearing in the dayglow of the atmosphere. The dayglow is ultraviolet light emitted when sunlight interacts with the oxygen in the upper atmosphere. Water is the only common substance that could create such holes, and each could be made by a small comet containing between 20 and 40 tonnes of water that broke up into a cloud of vapour as it hit the atmosphere. The case for a cometary origin for the atmospheric holes grew in the 1990s when the trail of a cosmic snowball was picked up as it vaporised over the Atlantic by the visible imaging cameras on polar satellites.

From the number of atmospheric holes, it’s possible to calculate that the atmosphere is being hit by a 20 to 40-tonne snowball comet every 3 seconds. According to Frank, this is enough to add an inch of water to the Earth’s surface every 20,000 years. Over the 4.5-billion-year lifetime of the Earth they could have provided all the water in the oceans. (emphasis added)

The article also links to this University of Iowa page smallcomets.physics.uiowa.edu/.

Is this still believed to be happening? Have there been additional observations? Is the earth receiving water in the form of 20 to 40 ton comets ariving every three seconds?

Question: What ever happened to the "Small Comet Theory" of water influx on present-day Earth? Has it been discounted, modified, verified?

  • $\begingroup$ Just because no articles have been in the popular press on this recently doesn't stop it happening. If you do a little bit of research on NASA's Asgard system, you will see. $\endgroup$ – Rory Alsop Dec 2 '17 at 22:58
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    $\begingroup$ There are, however, some discrepancies with the theory that water was delivered to Earth by comets; namely the deuterium ratio in comets is not the same as the ratio on Earth. $\endgroup$ – Phiteros Dec 3 '17 at 1:21
  • $\begingroup$ @Phiteros thanks for your comment. Here I'm only asking about the possibility of such a high influx happening now, and observational evidence thereof. $\endgroup$ – uhoh Dec 3 '17 at 2:55
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    $\begingroup$ @RoryAlsop note: The question is about comets, not meteors. These evaporate earlier and so do not create fireballs. Asgard may not see them - the data is recorded in the UV. The interpretation of the dark spots in the UV images may have evolved, or there may be better measurements. If you are confident that a 20 ton comet enters our atmosphere every 3 seconds and that there is widespread agreement on this in the scientific community, please consider posting an answer with supporting links for other readers to see and vote on. Thanks! $\endgroup$ – uhoh Dec 3 '17 at 4:10

The "small comets" buzz was temporary and has been completely superceded by subsequent evidence that it was all noise in the detectors. A careful investigation that produced a null result looking for the comets is published in http://adsabs.harvard.edu/abs/2000JGR...10524907M. The moral is, beware of the "1 sigma effect," where a researcher sees a hint of a signal and is beguiled into seeing it as something real.


I am interested in this topic as well and have found a news item describing another survey program that may contribute to the answer to this question: http://www.neoshield.eu/protecting-earth-asteroids-comets-neos/meteor-asteroid-neo-near-earth-object-comet/

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    $\begingroup$ Welcome to Astronomy! While this may theoretically answer the question, it would be preferable to include the essential parts of the answer here, and provide the link for reference. $\endgroup$ – Glorfindel May 7 '18 at 19:07
  • $\begingroup$ Thanks! This is what is often called a "link-only answer" and is discouraged. If/when the link breaks (sites reorganize their information from time to time) the answer will no longer have any value whatsoever. Answers should at least summarize the main points that are in the link which answer the question. In other words, link or no link you should still write a minimal answer in your own words. It take s a while to get used to Stack Exchange, welcome! $\endgroup$ – uhoh May 8 '18 at 1:42

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