Helium: Speed Brake for the Solar Wind?

Magnetic sail deployed

Someday fleets of interplanetary craft powered by the solar wind may cross the Solar System, using huge magnetic fields as their ‘sails.’ The concept is increasingly well understood, and I notice that researchers like Robert Winglee (University of Washington) have been extending it to include beamed propulsion methods as well (Winglee’s concept is called MagBeam), useful if your goal is to move deeper still into nearby space. But for all this to happen, we’ll need to learn much more about the solar wind itself and how we might ride it.

Image: Artist’s impression of a mini-magnetosphere deployed around a spacecraft. Plasma or ionized gas is trapped on the magnetic field lines generated onboard, and this plasma inflates the magnetic field much like hot air inflates a balloon. The mini-magnetosphere is then blown by the plasma wind from the Sun called the solar wind which has a speed of between about 350 to 800 km/s. Credit: Robert Winglee/University of Washington.

Enter NASA’s Solar Wind Experiment, flying aboard the Wind spacecraft launched in 1994, and designed to study such things as the stream of electrically charged particles constantly produced by the Sun. The Solar Wind Experiment can measure the speed, density and temperature of those particles. And it turns out to be particularly well placed for such work, according to John Steinberg (Los Alamos National Laboratory):

“We study the solar wind for practical reasons; the character of the solar wind blowing by Earth at any time determines conditions in the near-Earth space environment…It turned out that the Wind Solar Wind Experiment data were ideal for this particular study because of continuous data coverage that the spacecraft provided during the previous solar activity cycle minimum in 1996, through the recent solar max in 2001, and into the solar activity declining phase afterward.”

A key part of the analysis is to understand how the solar wind is accelerated to speeds between 600,000 and one million miles per hour. And it turns out that helium is implicated in the result. Most of the material in the solar corona is hydrogen, but as the hydrogen escapes the corona, it drags some heavier helium along with it, in the process slowing down.

But the huge events called coronal mass ejections show five to ten times the amount of helium normally found in the solar wind. Evidently helium building up in the solar atmosphere is suddenly expelled during these events. When the solar wind is at lower speeds, it is made up primarily of hydrogen, with little helium observed. Thus we have a lower speed limit established by the retardant effects of helium, with the coronal mass ejections showing what happens periodically to the helium that remains.

The paper is Kasper et al., “Solar Wind Helium Abundance as a Function of Speed and Heliographic Latitude: Variation through a Solar Cycle,” Astrophysical Journal 660 (May 1 2007), pp. 901-910 (abstract available).

A Galactic Collision, and the Sun’s Future

I remember a startling painting from an astronomy book I once had when I was a kid. It showed two spiral galaxies much like the Milky Way in the process of collision, and I recall the caption saying that the stars in galaxies were so widely spaced that even in an event like this, few if any stars would collide individually. The galaxies, so the writer surmised, would simply pass through each other, leaving both relatively unscathed.

What made the picture interesting was reading in the same volume that the Milky Way is eventually going to collide with the Andromeda galaxy, so that I had the vivid image of a vast galaxy getting ever closer in the night sky until the entire view was consumed by cities of stars. It was a lovely image, but the idea of galaxies merging without notable disruptive effect is long gone. And new work from the Harvard-Smithsonian Center for Astrophysics has implications for our Solar System as well.

Andromeda collides with Milky Way

In two billion years, with the Sun still firmly on the main sequence — although considerably brighter and hotter than today — Andromeda will make its first close pass, the two galaxies beginning to intermingle their stars as they swing around each other. It will take some three billion years beyond that, say the CfA’s T.J. Cox and Avi Loeb, for the two galaxies to complete their merger. Far from passing through each other and going their own ways, they would now form a single elliptical galaxy. An animation of the event can be viewed here.

Image: An artist’s impression of the collision of the Milky Way and Andromeda. Credit: James Gitlin/STScI.

By that time, the Sun will have become a red giant, with conditions in our current habitable zone far too extreme for life, and the entire Solar System will have receded from the new galactic center, from the current 25,000 light years to 100,000. And what will be the fate of our species leading up to this era? Here’s an interesting speculation from the paper. Note that the Solar System becomes problematic long before the Sun actually reaches the red giant stage or the galactic collision occurs:

Current evolutionary models (see, e.g., Sackmann et al. 1993) predict that the Sun will steadily increase its size and luminosity for the next 7 Gyr as it slowly consumes all available hydrogen and evolves towards a red giant phase. While this places a strong upper limit to the extent of life on Earth, it is likely that much smaller changes (< 50%) in the Sun’s luminosity will signi?cantly alter the Earth’s atmosphere and thus its habitability within the next 1.1-3.5 Gyr (Kasting 1988). Korycansky et al. (2001) suggested that the onset of these e?ects could be delayed by increasing the orbital radius of the Earth through a sequence of interactions with bodies in the outer Solar System, and we can not rule out the possible colonization of habitable planets in nearby stars, especially long-lived M-dwarfs (Udry et al. 2007) whose lifetime may exceed 1012 years (Adams et al. 2005). In short, it is conceivable that life may exists for as little as 1.1 Gyr into the future or, if interstellar travel is possible, much longer.

Another interesting note is this: Given sufficient time, and assuming that the cosmological constant does not evolve with time (in other words, no slowing of the universe’s accelerated expansion), all galaxies not part of the Local Group will eventually recede and exit our event horizon. At that point, write Cox and Loeb, “…the merger product of the Milky Way and Andromeda (with its bound satellites) will constitute the entire visible Universe.”

The paper is Cox and Loeb, “The Collision Between The Milky Way And Andromeda,” submitted to to the Monthly Notices of the Royal Astronomical Society, and available online. As to visual effects, any observers five billion years from now will see not a strip of stars like the current Milky Way from Earth, but a huge starry bulge that defines galactic center. I like Loeb’s comment: “This is the first paper in my publication record that has a chance of being cited five billion years from now.” It’s interesting to speculate where the citation might appear!

Amateur Bags GJ 436 b Transit

One of the most exciting aspects of the exoplanet hunt is that it is not confined to huge telescopes and professional astronomers. Timothy Ferris described the remarkable advances in amateur equipment and observing techniques in Seeing In the Dark (Simon & Schuster, 2002), but he’ll need a whole new chapter to cover what’s happening not only with distributed computing (as via the systemic collaboration, for example) and the software and hardware advances that let amateurs observe exoplanet transits from sites around the world.

One of Vanmunster's telescopes

Last night Tonny Vanmunster observed the transiting ‘hot Neptune’ GJ 436 b from his CBA Belgium Observatory in Landen, reporting his results over the Net and this morning on his Web site. Ponder this: GJ 436 b orbits an M-class red dwarf that is 33 light years away. The planet itself has a mass roughly 23 times that of Earth, with a radius approximately that of Neptune. There was a time when Neptune itself would have been a tricky catch for the average amateur, but we’re entering the era of the amateur as exoplanet specialist. GJ 436 b is the sixth transiting exoplanet Vanmunster has bagged from his pastoral observatory.

Image: One of Vanmunster’s instruments. This is a 0.35-m f/6.3 telescope mounted on Astro-Physics AP1200-GTO mount. SBIG ST-7XME CCD camera. Credit: Tonny Vanmunster.

Here’s the gist of his report. Check the site for the actual light curve for the transit, doubtless the first made by an amateur:

Rather unexpectedly, the skies cleared out on May 17/18, 2007 over CBA Belgium Observatory, shortly before the start of a transit of exoplanet GJ 436 b. Photometric conditions were not ideal (bit of haze), but I started a photometry session using a 0.35-m f/6.3 telescope and SBIG ST-7XME CCD camera with V filter. ..

The times of predicted ingress and egress are indicated (taken from Transitsearch.org). The actual transit started a bit later than predicted and its duration was also a bit shorter than predicted. This is in agreement with the fact that the Transitsearch.org prediction was made for a central transit, which is not the case for GJ 436 b.

CBA Belgium Observatory

Image: CBA Belgium Observatory is located in Flanders, Belgium. It is a remotely controlled observatory, that can be operated autonomously all night long. The only human interaction required is for opening and closing of the observatory, and initialisation of the telescopes, CCD cameras and computers. The building has a roll-off roof structure, and is measuring 3m by 4m. Credit: Tonny Vanmunster.

It could be argued, of course, that Vanmunster is not your average amateur. His observatory is the Belgian node for a network of amateur and professional astronomers known as the Center for Backyard Astrophysics, and his exoplanet observations go to the XO Project as well as Transitsearch.org. He’s also the author of Peranso, a software package designed for light curve analysis, so it’s no surprise that he was the first amateur to detect the transits of TrES-1b, TrES-2b and co-discoverer of XO-1b.

But it fires the imagination to think that a small observatory using two Celestron Schmidt-Cassegrain telescopes for CCD photometry and armed with laptop computers and a wireless network can be contributing serious scientific data to one of the most advanced investigations of our time. The opportunities for non-professionals to contribute meaningfully to the exoplanet hunt should engage a new generation of researchers, with or without academic affiliation. What counts are the data, and the data are obtainable, as the work of Tonny Vanmunster continues to demonstrate.

Plumes on Enceladus: A Tidal Squeeze

An object in an elliptical, egg-shaped orbit experiences interesting gravitational stresses. Enough so that the changing forces it endures may be the cause of the plumes of water vapor that Cassini found on Saturn’s moon Enceladus in 2005. In essence, the tiny moon is being alternately squeezed and stretched as it makes its way around the planet. These tidal forces cause existing fault lines to rub against each other, producing enough heat to turn ice into water vapor and ice crystals.

The cracked surface of Enceladus

That’s the conclusion of new work by Francis Nimmo (University of California — Santa Cruz) and team, who note the warmer surface of Enceladus’ southern pole and the presence of the famous ‘tiger stripes,’ which appear to be tectonic fault lines. “We think the Tiger Stripes are the source of the plumes,” says Nimmo, “and we made predictions of where the Tiger Stripes should be hottest that can be tested by future measurements.”

Image: This is a mosaic of Enceladus compiled from 21 images taken by the Cassini spacecraft as it swooped past the moon’s south pole on July 14, 2005. The Tiger Stripe region appears as a series of long cracks toward the bottom. The mosaic is in false color, and includes images taken in various types of light, including ultraviolet and infrared. Credit: NASA/JPL/Space Science Institute.

The process at work is sublimation, which does not require water to go through a liquid state before becoming a vapor (as a guy who used to live in upstate New York, I’m reminded of the way snow can dissipate over time even when the temperature never gets above the freezing point — that’s sublimation). Interestingly enough, the study does suggest that Enceladus contains a liquid ocean deep beneath its ice, under a shell perhaps tens of kilometers thick. The theory almost demands an ocean to be there because otherwise tidal forces couldn’t generate enough movement in the faults to produce the needed heat.

A second paper in the same issue of Nature examines tidal forces and their effect on Enceladus, with this comment by lead author Terry Hurford (NASA GSFC), discussing his team’s computer model for calculating stress effects on the Tiger Stripes:

“We found that because of the way the Tiger Stripes are oriented on the surface, when Enceladus is farthest from Saturn, the stresses in the region pull most of them open, and when Enceladus is closest to Saturn, the stresses force most of them to close. Different stripes open at different times in the orbit. Assuming they erupt as soon as they open, exposing liquid water to the vacuum of space, we can predict which stripes will be erupting at certain times in the orbit. Also, because most of the stripes are open when Enceladus is farthest from Saturn, we expect the eruptive activity to be greatest at this time.”

The Nimmo paper is “Shear heating as the origin of the plumes and heat flux on Enceladus,” Nature 447 (17 May 2007), pp. 289-291; abstract here. The second paper is Hurford et al., “Eruptions arising from tidally controlled periodic openings of rifts on Enceladus,” in the same issue, pp. 292-294 (abstract) Future Cassini observations will be needed to get better imagery of the Tiger Stripes to differentiate the sources of the eruptions.

Transiting ‘Hot Neptune’ Found

Whether or not Gliese 581 c, that intriguing world that may or may not offer temperatures conducive to life, will make a transit of its star is not yet known. But the principle that radial-velocity searches can identify a planet that is subsequently studied via transit received further validation today with the detected transit of a Neptune-class world around GJ 436. This is the smallest and least massive planet ever examined through transit methods, and it bodes well for future such studies of M-class stars.

The new transit comes courtesy of the Swiss team that includes Michel Mayor and Didier Queloz, recently in the news due to their work on Gliese 581 c — do these guys ever get any sleep? As Andy noted in a comment this morning, this ‘hot Neptune’ orbits closer to its star than the innermost planet of Gliese 581. GJ 436 is an M-class red dwarf, a type of star whose small radius makes the detection of such worlds by transit methods easier than would be the case for solar-type stars. And it gives credence to the idea that the Canadian MOST instrument might be able to detect a transit of Gliese 581 c if the planet is properly aligned.

Transits are chancy, and even here, the planetary passage is said to be ‘almost grazing,’ showing a duration half as long as would be expected for a central transit in front of the star. But it’s enough to yield helpful information indeed. GJ 436 b turns out to have a radius comparable to that of Uranus or Neptune. Here is what we know of its composition, from the discovery paper:

The mass and radius that we measure for GJ 436 b indicate that it is mainly composed of water ice. It is an “ice giant” planet like Uranus and Neptune rather than a small-mass gas giant or a very heavy “super-Earth”. It must have formed at a larger orbital distance, beyond the “snow line” where the proto-planetary disc is cool enough for water to condensate, and subsequently migrated inwards to its present orbit.

This planet, which may hold a small hydrogen/helium envelope, will not light up any media switchboards with questions about habitability. Its atmosphere is assumed to be hot, with temperatures ranging from 520 K to 620 K depending upon the albedo value, and the authors note that greenhouse effects may heat it to still higher temperatures.

So while the discovery team believes there are scenarios in which it could be considered an ‘ocean planet,’ GJ 436 b could hardly sport the kind of oceans we’re familiar with here on Earth:

Because of the high surface temperature, this would imply a steam atmosphere above supercritical water rather than an Earth-like situation. As methane and ammonia have very low condensation temperatures, this scenario would imply migration from a wide orbit.

Supercritical water is formed at high temperatures and pressures, showing no real distinction between its liquid and gaseous state. No gently lapping ocean waves in this scenario — in fact, supercritical water is used on Earth as a medium for destroying toxic substances without releasing emissions into the atmosphere. GJ 436 b would be one hostile place indeed, but what promise it holds for future transit studies. For that matter, are there other planets in this very system? Stay tuned.

The paper is Gillon et al., “Detection of transits of the nearby hot Neptune GJ 436 b,” accepted for publication in Astronomy & Astrophysics Letters, with abstract available.