The Case of the Disappearing Neutrinos

Following up on yesterday’s intriguing antimatter results at Fermilab, a neutrino study called the Main Injector Neutrino Oscillation Search (MINOS) is providing independent confirmation of a critical idea: neutrinos have mass. This is significant news because it helps to illuminate earlier experiments that suggested neutrinos oscillate between three different types, something that could occur only if they do have mass, and an effect that, given the sheer abundance of neutrinos in the universe, may provide clues to why antimatter has disappeared and how galaxies originally formed.

Neutrinos are odd things indeed; they can pass through the entire Earth without interacting with matter. MINOS studies them by producing neutrinos at Fermilab using protons accelerated in a 4000 foot tunnel pointing toward a second detector some 450 miles away in Soudan, Minnesota. The neutrinos are measured first with a detector below the Fermilab site, with a second measurement being taken at a 6000-ton particle detector at Soudan. Researchers are examining how many muon neutrinos have disappeared along the way, presumably by turning into another type of neutrino and thus confirming that neutrinos have mass.

The result, based on only the first months of data: a signficant number of muon neutrinos are disappearing in ways consistent with the idea that neutrinos regularly transform themselves into different types. The mass difference between two of the types of neutrino is now found to be 0.056 eV, just 0.00001% of the mass of the electron. Further MINOS work should confirm whether actual neutrino oscillation is occurring, or whether we are observing some form of neutrino decay or, a more exotic possibility, the influence of extra dimensions.

Clues About the ‘Antimatter Wars’

With 700 physicists from 90 different institutions in 20 countries working on an experiment, you expect interesting results. And the DZero experiment at Fermi National Accelerator Laboratory is living up to the expectation. Scientists at Fermilab have been studying a subatomic particle known as the B_s meson (pronounced ‘B sub s’). Their work suggests that this particle actually oscillates between matter and antimatter more than 17 trillion times per second.

The data come from over 1 billion events at Fermilab’s Tevatron particle accelerator, and more precise results are expected soon from a different Fermilab collaboration. And the more we learn, the better: exactly how particles turn into their own antiparticles, and with what frequency, is a major issue that could point to answers in an even bigger one, the balance between matter and antimatter in the universe. For if matter and antimatter appeared in equal numbers at the time of the Big Bang, their mutual annihilation should have left nothing behind but energy.

So how did matter survive? One solution is that there was, for reasons unknown, an imbalance between matter and antimatter. Back in 1985, physicist John Cramer, in one of his engaging columns for Analog, said that the early universe could have had 100,000,001 protons for every 100,000,000 antiprotons, making the universe we see around us “…the few ragged survivors of the ‘antimatter wars’ of 16 billion years ago.” We’ve revised the date on that, knowing from the WMAP results that the universe’s birth occurred roughly 13.7 billion years ago. But the idea of an early imbalance remains, even if unsatisfying.

But the fluctuation of particles into their own antiparticles may be giving us clues about a more robust solution. The frequency of matter/antimatter oscillation has never been measured to this degree of confidence. Studies of such oscillations go back to the 1980s, when a different kind of meson (the B_d meson) was found to oscillate at a higher rate than predicted by theory. The current studies aim to firm up our knowledge of how B_s mesons fit into the picture, with an eye toward uncovering new interactions that any deviation from prediction might reveal. Out of such results could come ways to evaluate exotic supersymmetry theories, whose predictions may be put to the test.

Henry Hsieh on Main-Belt Comets

Centauri Dreams recently discussed the discovery of so-called ‘main-belt comets’ — icy objects found in asteroid-like orbits that apparently formed in the inner Solar System rather than on its outer edges. The work, performed by Henry Hsieh and David Jewitt (University of Hawaii) raises questions about the origins of Earth’s water supply, which had been thought to have been delivered by cometary impacts on the primordial Earth. Could this water have, in fact, been delivered by main-belt comets, and could a mission to one of them yield the answer?

A sharp-eyed reader wanted to know more: assuming we flew such a mission, how could we pin down the main-belt comets as the source, as opposed to the huge population of long-period comets with their highly elliptical orbits? Henry Hsieh was kind enough to respond:

In recent years, the debate over the origin of the Earth’s water has focused on the so-called D/H (deuterium to hydrogen) ratio of ocean water, comet water, and meteorite water (in the form of hydrated minerals; used as a proxy for “asteroid” water). The two following links provide technical and layman’s discussions, respectively, on this issue:

Jewitt, Chizmadia, Grimm et al., Water in the Small Bodies of the Solar System (see Section 4.1)

Harder, Ben. Water For the Rock: Did Earth’s Oceans Come from the Heavens?

Essentially, measurements of long-period comets seem to show that they are overabundant in deuterium as compared to terrestrial water. Therefore, there must be another source of water, perhaps the dominant one, that has a D/H ratio closer to that of “standard ocean water”. The asteroid belt is a possibility (recent orbital models also show that asteroids from the main belt were more likely than comets from the outer solar system to strike the Earth in sufficient quantities to supply the volume of water we see here today), but until now, the only D/H measurements we have of asteroids were derived indirectly from hydrated minerals (which are not quite the same as actual water ice) in meteorites.

Now that the MBC [main-belt comet] class has been identified, we would obviously very much like to know what the D/H ratio in MBC ice is. We surmise that it may be different from that measured for other comets since these objects formed at a different location (and therefore at a different temperature, which may affect deuterium abundance/retention in forming planetesimals) in the protoplanetary disk that was the early solar system, and might be closer to the D/H ratio measured for Earth water. This of course needs to be confirmed or refuted by observational evidence yet to be obtained.

Unfortunately, the activity of the MBCs discovered thus far is too faint to hope to make a meaningful D/H measurement from the ground. The close proximity of the MBCs to the Earth, their stable, predictable orbits, and the fact that they possess surface or near-surface ice that even occasionally sublimates, releasing water vapor into space, makes them attractive spacecraft targets, however. A visit to one or more MBCs by a spacecraft equipped to make D/H measurements would certainly shed more light on this issue.

Centauri Dreams’ note: A mission to measure deuterium/hydrogen ratios in the main-belt comets so far identified seems well within the reach of current technology, and as we noted yesterday, NASA has just reversed its cancellation of the DAWN mission to the two largest main-belt asteroids. The measurements that could result from a main-belt comet mission would provide information that is more than purely historical. We’ve seen that at least one nearby star of high astrobiological interest, Tau Ceti, seems to be surrounded by a vast cometary cloud (see The Comets of Tau Ceti). Our understanding of what it takes to create terrestrial water worlds could be materially advanced by further information on the delivery mechanism of water to the habitable zone surrounding such stars.

Icy Moons and Their Oxygen

Gas giant moons like Europa offer the tantalizing hint of life-sustaining conditions, with oxygen supplied by their abundant ices. But without sufficient heat, how is the oxygen to be coaxed from their frozen surfaces? So far, the explanation has been that high-energy particles bombarding such a moon’s surface could help to release the gas, which would have already been molecularly bound with hydrogen.

Views of Europa

But a study at the Pacific Northwest National Laboratory suggests a different explanation. Simulating high-energy bombardment of a moon’s surface, researchers there found that the process is much more complex. “We found that a simpler two-step could not account for our results,” said PNNL staff scientist Greg Kimmel. “Our model is a four-step process.”

Here’s how a PNNL news release explains what’s going on:

First, the energetic particle produces what is known as a common “reactive oxygen species” called a hydroxyl radical, or OH. Next, two OH molecules react to produce hydrogen peroxide. Third, another OH reacts with the hydrogen peroxide to form HO2 (hydrogen coupled to two oxygen atoms), plus a water molecule. And, finally, an energetic particle splits an oxygen molecule from the HO2.

Image: Distant Europa and its frozen surface. New clues are emerging about the composition of such distant moons. Credit: NASA.

This work was presented at the annual meeting of the American Chemical Society on Monday. It tells us more about the processes at work on such distant satellites, but until we get a spacecraft with a full science package onto the surface of one of these moons, we’ll still face daunting questions about their chemistry. And with recent NASA budget cuts — and growing constraints on ESA’s finances — such a mission once again seems a long way off (although the DAWN mission to Ceres and Vesta seems to have won a reprieve, thanks to progress on resolving the spacecraft’s xenon fuel tank problems).

‘Main-Belt Comets’ A Clue to Earth’s Water

How did a newly formed Earth, supposedly hot and dry, wind up with oceans? Comets have been the leading candidate for the needed delivery mechanism, given their large ice content. But ice from the asteroid belt may make a better fit to Earth’s water supply, and the discovery of a new class of comets there may mean that at least some objects in that part of the Solar System have ice at their surfaces.

Asteroids and comets, in other words, may in some cases be more closely related than anyone realized. These conclusions come from work performed by University of Hawaii graduate student Henry Hsieh and professor David Jewitt, who have christened the newly discovered objects ‘main-belt comets.’ These are comets with asteroid-like orbits; they seem to have formed in the inner Solar System rather than in the frigid reaches of the Kuiper Belt or the Oort Cloud.

The evidence? An object called asteroid 118401 is ejecting dust, just like a comet. And so is another mysterious comet known as 133P/Elst-Pizarro and a third, designated P/2005 U1. The original ‘main-belt comet’ (i.e., the first discovered) was 133P/Elst-Pizarro, which was found almost a decade ago. Hsieh and Jewitt’s work on the other two seems to identify a new class of cometary objects.

Icy main-belt comets

Image (click to enlarge): The three known main-belt comets (at center of each panel). Other objects shown are stars and galaxies smeared by the motion of the telescope while tracking each comet. Credit: H. Hsieh and D. Jewitt (Univ. of Hawaii).

“The discovery of the other main-belt comets shows that 33P/Elst-Pizarro is not alone in the asteroid belt,” Jewitt said. “Therefore, it is probably an ordinary (although icy) asteroid, and not a comet from the outer solar system that has somehow had its comet-like orbit transformed into an asteroid-like one. This means that other asteroids could have ice as well.”

Centauri Dreams‘ take: This is provocative work indeed. Perhaps we’ve made too rigid a distinction between comets, with their elongated orbits and vast temperature swings as they approach the Sun, and asteroids, with their essentially circular orbits and rocky, water-free composition. In adjusting our definitions, we may be on the way to explaining how water gets delivered to rocky, terrestrial worlds as Solar Systems evolve.

But three comets make for a tiny sample; we’ll need much more evidence before drawing too firm a conclusion about how these main-belt comets came to be where they are. The Oort Cloud, after all, is believed to be home to comets in the trillions. As Hsieh and Jewitt note, a mission to one of the main-belt comets would tell us much about their ice content.

The paper is Hsieh and Jewitt, “A Population of Comets in the Main Asteroid Belt,” with abstract and PDF available here. The work will appear in the April edition of Science. Also see Henry Hsieh’s Web page on main-belt comets, which contains links to the original article and news coverage on the story.