by Paul Gilster | Oct 18, 2008 | Missions, Outer Solar System |
Interstellar Boundary Explorer (IBEX) may be the perfect name for the mission to be launched on Sunday the 19th, but the word ‘interstellar’ has some people thinking this is a precursor mission, headed out for deep space in the fashion of the Voyagers or New Horizons. Nothing could be further from the truth. IBEX is destined for a sedate though distant orbit reaching 240,000 kilometers above the Earth. Its instruments are the interstellar component, enabling the spacecraft to study the ever-changing boundary between the heliosphere and the true interstellar medium.
Two Energetic Neutral Atom cameras are the operative tools, capable of detecting atoms emitted from this distant region. This is a fascinating mission for interstellar advocates, for we’re looking at the effect of the solar wind as it collides with the cloud of interstellar materials through which the Earth moves. The shock wave that occurs where the solar wind meets the edge of the ‘bubble’ of materials streaming out from the Sun is what the spacecraft will study.
“These atoms represent the sound of the shock, and they are barely audible. We will only measure a few thousand atoms each day, so we had to make a jumbo camera. Only our camera
detects atoms instead of light,” said Herb Funsten, Los Alamos’s principal investigator on the instrument. “Every six months we will make global sky maps of where these atoms come from and how fast they are traveling. From this information, we will be able to discover what the edge of our bubble looks like and learn about the properties of the interstellar cloud that lies beyond the bubble.”
The solar wind, of course, plays into propulsion concepts that might one day harness its 1.6 million kilometer per hour flow to push deep into the Solar System and beyond, using magsail or other technology. Recent studies using data from the Ulysses spacecraft have shown that the solar wind has diminished markedly and is in fact lower than in any previous measurements. That has implications for galactic cosmic rays, the energetic particles from outside the Solar System that create potential health risks for astronauts moving beyond Earth orbit.
A diminished solar wind may weaken the heliosphere, allowing more galactic cosmic rays into the inner system. And variations in wind strength are obviously critical for futuristic designs that would attempt to use solar wind energies for controllable propulsion.
Image: This image shows the known and unknown distances between our heliosphere, the termination shock, heliopause and bow shock. The termination shock creates a sphere around our solar system. It is located about 100 AU away from the Sun. Solar wind particles begin to interact with the interstellar medium (ISM) here. The outward movement of the solar wind creates pressure, much as air filling a balloon creates pressure on the outside of the balloon, causing it to expand. The interstellar medium also has its own pressure of moving plasma. The point where the pressure of the solar wind is equal to the pressure of the interstellar medium hitting the solar wind is called the heliopause. A particle directly at the heliopause will feel equal pressure from both the solar wind and the interstellar medium. The outermost layer, called the bow shock, is where the ISM first interacts with the solar wind. Credit: NASA/IBEX.
The Voyager spacecraft have already sketched in early information about the heliosphere, with Voyager 1 evidently crossing the termination shock in 2004 at a distance of 94 AU. That event was flagged by simultaneous increases in the intensity of energetic particles and magnetic field strength, but the failure of Voyager 1’s plasma detector made it impossible to gauge the velocity of the solar wind after crossing the shock. It may be up to Voyager 2, itself now approaching the termination shock, to provide the needed measurements of the plasma’s velocity.
What IBEX brings to the table is the potential for creating large-scale maps of the solar wind’s boundary with interstellar space, complementing what the Voyagers have done with their useful but necessarily limited spot measurements. Launch will be via a Pegasus XL rocket dropped from an Orbital Sciences L-1011 aircraft about 125 miles north of Kwajalein Atoll, in the Marshall Islands. The launch window opens at 1748 UTC (1348 EDT), with live coverage (no TV) to be provided via webcast available here. Streaming video and audio will begin at 1615 UTC (1215 EDT) and will conclude some twelve minutes after launch.
by Paul Gilster | Oct 17, 2008 | Exoplanetary Science |
Directly imaging a terrestrial planet is going to be a tough challenge. Suppose you were thirty light years from the Sun, looking back at our star in the hope of seeing the Earth. You would face the problem that the Earth and its star show an angular separation of 100 milliarcseconds, a spacing so tiny that the far brighter Sun would render its third planet (and all the others) invisible. Indeed, in optical wavelengths the Earth is ten billion times less bright than the Sun. How to go about seeing it?
Observing at other wavelengths offers some help. The Sun is only a million times brighter than the Earth in the mid-infrared, which is why our first glimpse of planets like ours will probably be in this range.
And it may be that our first catch is not a mature, established planet potentially offering a habitat to living organisms. Instead, it may be a clump of molten rock still glowing brightly from the heat of formation. Even after surface magma solidifies — and new work suggests this could take five million years, rather than the hundreds of thousands previously thought — the planet might stay hot enough to be an unusually bright target in the infrared for tens of millions more.
This is the conclusion of Linda Elkins-Tanton (MIT), whose work implies that a glowing, molten planetary surface may be the most feasible find for early terrestrial planet hunter missions. As to the processes producing all that magma, they’re initially the result of radioactivity in the planet’s interior and the heat of planetary formation created by millions of rocky collisions in the early system. But a second process, causing iron-rich materials to sink toward the core, may force hotter materials from within back up to the surface, keeping the landscape molten much longer.
So we may have a ‘magma ocean’ that lasts at least a few million years longer than had previously been thought. It’s an interesting model, and one that clearly has implications for detectability since it lengthens the window for observation. Surprisingly, the theory may gain support when the MESSENGER mission settles in around Mercury. Earth’s crust is too dynamic for material from such early epochs to survive, but Mercury’s surface may offer up minerals that Elkins-Tanton’s model says should be there. Even better, of course, would be the direct detection of a molten, young Earth analog, but for now Mercury may have to do.
by Paul Gilster | Oct 16, 2008 | Exoplanetary Science |
If the weather on Uranus, examined here yesterday, isn’t exotic enough for your taste, consider the situation on Jupiter-class worlds around other stars. A ‘hot Jupiter’ orbiting extremely close to its star spawns weather like nothing we’ve ever experienced, as modeled by computer simulations coming out of the University of Arizona. And while we can’t actually image these objects yet, we can certainly deduce a great deal about them from observations made during the times they transit their star.
On that score, well-studied HD 189733b is an early example of pushing the envelope. Located 63 light years from Earth, this transiting planet orbits once every 2.2 days, scooting along a mere three million miles from its primary. Spitzer Space Telescope data culling variations in starlight during the frequent planetary transits have allowed us to peg daytime temperatures on worlds like these, usually in a range somewhere between 2000 and 3000 degrees Fahrenheit (1300 and 1900 degrees Kelvin). What stands out in studies of HD 189733b, though, is the nightside, where temperatures reach almost 1300 degrees Fahrenheit (1000 degrees Kelvin) despite the obvious lack of light.
Considering how close this planet is to its star, that nightside reading is deeply interesting. It implies a robust heat transfer mechanism in the form of strong winds, a finding that may be generalized across the entire family of hot Jupiters. Much work has already gone into this. Have a look, for example, at this story on David Charbonneau’s work on HD 189733b’s atmosphere, followed by Geneva studies by Frédéric Pont that seem to identify haze there. Adam Showman (University of Arizona), who led the work we’re looking at today, is now producing the computer models that firm up the hot Jupiter picture. Says Showman:
“These planets are 20 times closer to their star than Earth is to the Sun, and so they are truly blasted by starlight… Because these planets are so close to their stars, we think they’re tidally locked, with one side permanently in starlight and the other side permanently in darkness. So, if there were no winds, the dayside would be extremely hot and the nightside would be extremely cold.”
Showman’s group performed 3-D simulations that factored in the absorption of all that blazing starlight and the ways in which a planet loses heat to space. The models explain the observed data for HD 189733b and suggest the kind of winds we’re talking about — jet streams with speeds reaching over 11,000 kilometers per hour. Winds like that, moving from west to east, push the hottest regions on the planet away from the ‘high noon’ region, Showman adds, and move them further east by about thirty degrees of longitude.
And I used to think that Venus was a good description of hell… Maybe it still is, but ‘hot Jupiters’ with supersonic winds and dayside temperatures that would melt lead now seem an even better way to view it. In any case, note the considerable distance we’re moving from early exoplanet work, which produced the first mass and orbital information about these distant worlds. Now we’re actually looking at planetary weather patterns for objects we cannot yet see directly. The science of exoplanet mapping is indeed in its infancy, but great things are coming.
by Paul Gilster | Oct 15, 2008 | Outer Solar System |
No one ever said that Uranus was anything but a strange world. Nineteen times farther from the Sun than the Earth, the planet’s equator is tilted 98 degrees from its orbital plane. The tilt is so profound that if you work out the averages, the Uranian poles get more sunlight than the equator. That could lead to interesting weather patterns on a world with an 84-year orbit where seasons last twenty-one years. Such seasonal subjects have been the subject of recent study using imagery from the Keck II instrument in Hawaii, the results presented at the Division for Planetary Sciences meeting this week in Ithaca, NY.
Uranus reached equinox in 2007 when the Sun attained a position directly over the planet’s equator. Having equal amounts of sunlight over northern and southern hemispheres is obviously not a routine occurrence for this planet, but it’s a good chance to look at what’s happening on the meteorological front. Lawrence Sromovsky (University of Wisconsin) notes that seasonal weather changes are driven by the variance in solar energy caused by the tilt of a planet on its axis (the term for this is ‘seasonal forcing’). But the data gathered by Keck shows that Uranus features a unique lag in responding to that input. Says Sromovsky:
“Although both hemispheres were symmetrically heated by sunlight at equinox, the atmosphere itself was not symmetric, implying that it was responding to past sunlight instead of current sunlight, a result of Uranus’s cold atmosphere and long response time.”
Cold indeed. We’re talking about atmospheric temperatures at the cloud tops of minus 215 degrees Celsius. We’re also talking about winds that can reach speeds of 900 kilometers per hour. The image below displays recent atmospheric features and their changes:
Image: Near-infrared images from the Keck II telescope show the planet Uranus in 2005 (left), with the rings at an angle of 8 degrees, and at equinox in 2007 (right pair), with the planet’s ring system edge-on. In all images, the south pole is at the left and the equator is directly below the rings. Credit: Imke de Pater, University of California, Berkeley; Heidi Hammel, Space Science Institute; Lawrence Sromovsky and Patrick Fry, University of Wisconsin-Madison. Obtained at the Keck Observatory, Kamuela, Hawaii.
Notice the cloud structure in the planet’s southern hemisphere (at left of the image and near the bright band at bottom), which may be dissipating as it drifts north, a motion that is probably the result of seasonal change. The vortex has evidently been in existence for years, if not decades, at between 32 degrees and 36 degrees south latitude. At the same time, the bright feature in the northern hemisphere in the 2005 image seems to correspond to the smaller bright area in the rightmost 2007 image. Note that the southern band is not as bright as it was in 2005, while a new northern band is now brightening. The expectation is that the bands will completely reverse by the time of the next equinox.
by Paul Gilster | Oct 14, 2008 | Exoplanetary Science |
How tides affect habitability has become a sub-genre within exoplanetary studies, a theme pushed hard by the gifted trio of Brian Jackson, Rory Barnes and Richard Greenberg (University of Arizona). You may want to browse through earlier Centauri Dreams entries on their work, especially this fascinating take on habitability around M dwarfs, in which the authors consider the possibility that Gliese 581 c was once a relatively benign place, but is now in an orbit that renders life impossible. Orbital evolution is the broad issue, sustained complex life demanding planets with low eccentricities. And orbital evolution can take a lot of time to operate.
Now I see that Brian Jackson has presented new work on tides and habitability at the 40th annual meeting of the Division of Planetary Sciences in Ithaca, NY. Here we push into interesting questions about planets already inside a habitable zone that are nonetheless too hellish to support life, and planets outside that zone that seem too cold to sustain life, but may be able to do so because of tidal effects. Planets in elongated orbits (unlike those in our Solar System, where orbits are relatively circular) undergo tidal stretching when near their star, an effect that diminishes as they move away from it. The result: Friction, generating internal heat that keeps the planet geophysically active.
What we find is that we shouldn’t be too quick to make judgments based upon the presence of liquid water at the surface. Take a planet up to ten times as massive as the Earth — a ‘super Earth’ — and consider the effects of tides upon conditions there. A likely prospect is extreme volcanic activity, which can render a planet already within a star’s habitable zone more like Jupiter’s moon Io than our mild and pacific Earth. This can occur even at relatively low eccentricities. Here’s Jackson on what we may discover when we study the first extrasolar terrestrial planets:
Given the wide range of masses and eccentricities that potentially give rise to extreme volcanism, we might expect that many terrestrial planets will be too volcanically active for life… [T]he most massive terrestrial planets may also be the most heated and thus the most volcanically active. Since the first extra-solar terrestrial planet that is likely to be confirmed will probably be much more massive than the Earth, we might expect it will be volcanically active. Such volcanic activity may be recognizable in the planet’s atmospheric transmission spectrum, similar to Io, whose tenuous atmosphere is largely made of sulfur…
This is drawn from the paper on the team’s work, which also examines the flip side of these effects, that a planet that might otherwise be too cold to sustain life may benefit from tidal effects, which would cause the outgassing of volatiles that could keep its atmosphere viable. For that matter, planets with an ocean under a crust of ice — we can think about Europa as the nearest analog of this process — could maintain warmer water temperatures than would otherwise be possible. Throw in plate tectonics, which can stabilize planetary atmospheres and surface temperatures, and you may have generated what you need to produce a functional biosphere.
And here’s an interesting scenario: A planet whose tidal evolution makes it pass through alternating periods of heating and cooling, such that it may go through an early habitable period, possibly including the development of life. And then, after a long period in which life has been extinguished because of volcanism, the same world may once again become habitable when its orbit circularizes. Thus two separate epochs for the emergence of life may occur on the same planet, although occurring billions of years apart.
The range of outcomes is extensive, as the paper’s summation suggests:
Even with the many simplifying assumptions employed here, these results suggest a wide range of geophysical scenarios. As advancement is made in the understanding of the processes of tidal evolution, in modeling of the geophysics of hypothetical planets, and eventually in the discovery and characterization of actual terrestrial-type planets, these calculations will need to be revisited. In any case, the calculations here show that tidal heating has the potential to be a major factor in governing the internal structures, surfaces and atmospheres of extra-solar terrestrial planets. Accordingly, the effects of tidal heating must be given consideration when evaluating the habitability of such planets.
We shouldn’t be too doctrinaire about the exact specifications for a living planet. As our own system shows, even bizarre outliers like Enceladus may eventually show evidence of a second outbreak of life within reachable distance of our own. Who knows how wide a range of living planets we may find as we zero in on exoplanetary systems? The paper is Jackson et al., “Tidal Heating of Terrestrial Extra-Solar Planets and Implications for their Habitability,” accepted for publication in Monthly Notices of the Royal Astronomical Society and available online.
