According to Colwell

Alan Stern resigned as NASA’s Associate Administrator of the Science Mission Directorate. In his short tenure as AA Alan had embarked on an ambitious program to overhaul how SMD operates. Speaking from the perspective of a university researcher, his changes to the Research and Analysis programs were a great improvement: faster and better communication between NASA HQ and proposers, longer terms for typical awards coupled with new “on-ramps” for young researchers, new science programs to capitalize on the new exploration initiative, and new programs for small space experiments, such as sounding rocket experiments. Of course, anytime there is something “new” without an increase in the budget means there’s going to be a cut to something “old”. Alan addressed the American Astronomical Society’s Division of Planetary Sciences meeting last October and said that in a zero-sum budget environment, his plan to get new missions and programs started was to hold the line on budget overruns on existing programs. Many high profile missions are running over their budgets. His departure suggests that he may not have had the flexibility he needed to deal with those cost overruns. Hopefully some of the changes he did manage to institute during his short tenure will persist into the new administration.

Earlier this month a paper was published showing evidence of rings around Saturn’s moon Rhea. This would be the first case of rings or other natural material orbiting a planet’s moon, though asteroids and Kuiper belt comets have been observed to have natural satellites. The Cassini project issued a press release announcing the results. The press release is titled “Saturn’s Moon Rhea Also May Have Rings” and includes phrases like “this is the first time rings may have been found around a moon”. The careful wording stems from the nature of the observation and the lack of a visual confirmation of the rings (so far, at least). Among Cassini’s dozen instruments are charged particle detectors that measure the energy and abundance of electrons in Saturn’s magnetosphere. Moons plowing through the magnetosphere usually leave a wake in the magnetosphere - a region downstream of the moon that is relatively depleted in charged particles. On one of Cassini’s close flybys of Rhea, however, the Magnetospheric Imaging Instrument (known as MIMI) detected localized regions near Rhea with fewer electrons, indicating that some material near that region absorbed those electrons. The most intriguing aspect of the MIMI measurements is that the instrument detected dips on each side of the moon at locations consistent with the electron absorption being produced by a circular ring around Rhea. Rhea itself, while large, is a relatively unremarkable moon (putting aside the issue of its possible rings).

Cassini image PIA09841 of the moon Rhea.

The next most intriguing aspect of the discovery is that no images show any material orbiting Rhea. That doesn’t mean there aren’t rings. Cassini’s cameras (like all cameras) see the surfaces of things. The more surface area something has, the easier it is to see it with a camera. MIMI, however, indirectly measures the mass of an object. The more massive it is, the better it is at absorbing electrons. So the objects that absorbed MIMI’s electrons must be relatively large. To have enough dust particles to produce the observed absorptions, those particles would have been detected by Cassini’s cameras. The puzzle is compounded because larger particles would naturally produce dust as a byproduct of meteoroid bombardment on the larger particles. We are left with a mystery wrapped in a conundrum. Future observations of Rhea are planned. Images with greater sensitivity may reveal the rings. Cassini’s dust detector will sample the dust population near Rhea during Cassini’s extended mission. Stay tuned.

This lighthearted romp of a movie follows the dowdy and down-on-her-luck governess Miss Guinivere Pettigrew (Frances McDormand) through 24 hours in London on the eve of World War II. Desperate for work, she takes the place of another worker from an employment agency to become the social secretary for Delysia Lafosse, a flighty aspiring American actress. Amy Adams plays Delysia with an almost irresistible effervescence. She lives in the penthouse sweet of a wealthy bar-owner, Nick, while seducing a young play producer in the hope of getting a lead role. Lurking in the wings is her true love, Michael (Lee Pace of Pushing Daisies). There is little suspense over the eventual outcome, but a lot of fun on the way there.

Today Cassini flies only 50 km over the surface of Saturn’s moon Enceladus, and grazes the edge of the water vapor geysers spewing ice from cracks near the moon’s south pole. NASA has set up a blog to chronicle the flyby here, where there is detailed information on the geometry and promises to be some exciting images and results in the next day or two.

This tense heist thriller is based on the true story of a London bank robbery in 1971. Four days after the robbery, the U.K. government issued a “D-notice” that requests the media to stop reporting on the story. The media complied, meaning that relatively little is known about the actual robbery. This gives the movie a fair amount of leeway while still maintaining a claim of veracity. Not that it really matters. The story as told in the movie, however much it might deviate from or adhere to the actual events, is gripping and taut. Jason Statham plays Terry, a car mechanic with a petty crime past who, with his mates, jumps at the chance to make one big score and get out of small-time crime and low-income labor. That chance is served up by Martine, played by Saffron Burrows. Martine is a model and one-time friend of Terry’s who gets busted for a drug offense. A government official tells her that if she can steal some compromising photos of a member of the royal family that is held in a safe deposit box at a London bank, she’ll be free and clear. The problem is that it must be done with no official government involvement, so the bank job must still be pulled off in spite of the best efforts of the bank and the police to prevent that sort of thing.

What no one bargained for is that lots of people have a tendency to put embarrassing and compromising material in their safe deposit boxes, and many of them can get extremely upset when that material goes missing. Pulling off the robbery is not the most challenging aspect of the operation. The movie is tense and suspenseful, and I have to confess that the link to a real bank robbery added to the intensity for me. Some aspects of the movie are certainly fiction, but the basic events and historical figures are real.

Expectations play a larger role than they should in my appreciation of a movie. How else to explain how much I enjoyed the forgettable “Wild Hogs” than that I was expecting a terrible movie and found myself laughing at the goofy gags instead? Persepolis, on the other hand, is an example of a good movie that disappointed due to elevated expectations. Publicity for the movie left me cold, but rave reviews and an Academy Award nomination prompted me to see it. The movie brings to the screen the autobiographical graphical novel (okay, it’s really a “graphic novel”, but how often do you get to have “graphical” twice in a row?) of Marjane Satrapi. And “autobiographical” is the key and sufficient adjective to describe this movie, which follows Marjane from the age of eight in Teheran Iran during the final days of the Shah’s rule. It follows, episodically, her life through the revolution that toppled the Shah and instituted a repressive Islamic regime. Had I been ignorant of the events that marked that tumultuous period in Iran, the movie would have been fascinating on a historical level. As a story, the movie exhibits the characteristics of real life: it is complicated, messy, and without a clear resolution. This is not necessarily a flaw in a movie (which usually simplifies life to arrive at that clear resolution), but in this case it left me unaffected.

These images taken by the Mars Reconnaissance Orbiter’s HiRISE camera, which has exquisitely high resolution, show avalanches off the layered icy terrain near Mars’s north pole. Noticed by HiRISE team member Ingrid Spitale, wife of my Cassini ring scientist colleague Joe Spitale, these slides are part of Mars’s seasonal changes which are more extreme than the Earth’s. Like Earth, Mars’s rotation axis is tilted relative to its orbital axis resulting in more direct sunlight on one hemisphere than the other for half of a Martian year. Unlike the Earth, however, Mars’s orbit is significantly elliptical, meaning that it is closer to the Sun during summer in the Southern hemisphere making it particularly warm, and further from the Sun during northern summer. It is currently northern summer on Mars, and these avalanches are a product of the seasonal breakup of the ice on the north polar cap. The martian polar caps consist of water ice underneath carbon dioxide ice (”dry ice”).
Context image showing two plumes of dust caused by material falling down the face of Mars’s polar ice caps.

In a new paper in Earth and Planetary Science Letters John Huw Davies (Cardiff University) postulates that a giant impact in the late stages of planet formation is responsible for Venus’s hot and dry climate. The impact would have been between two planet-sized objects, not dissimilar to the impact believed to be responsible for the formation of the Earth’s Moon. That latter giant impact, now accepted as the standard model for the origin of the Moon, was off center and resulted in a disk around the proto-Earth and gave the Earth a rapid spin. Davies’ model would have a near-central impact that would leave Venus with virtually no rotation and vaporize any water it might have had at that point. The paper was reported on by USA Today via Space.com with the headline “Venus mysteries blamed on colossal collision”.

The “mysteries” that the headline refers to are (1) Venus’s slow, backwards rotation, and (2) its lack of water. Venus’s famously hot surface temperature (nearly 900 degrees Fahrenheit) is due to a crushing atmosphere of nearly pure Carbon Dioxide, famous back home for being a greenhouse gas produced by (among other things) burning organic matter such as oil, coal, and wood. Venus’s atmospheric pressure at the surface is 90 times that of Earth, and it is basically entirely a greenhouse gas. This gas prevents heat from radiating away from the surface of Venus freely into space. CO2 absorbs some of that radiant energy, trapping it in the atmosphere and making it hotter. A little greenhouse effect is nice to have and is responsible for Earth’s current comfortable temperatures. The standard model explains Venus’s high temperature and mystery number (2) above (lack of water) through a runaway greenhouse effect.

The runaway greenhouse effect occurs because like CO2, water is also a greenhouse gas. Take a planet with a lot of surface water (like the Earth) and heat it up a little, and you can drive water out of the oceans and into the atmosphere. In the atmosphere, it acts as a greenhouse gas and makes things a bit warmer which leads to more evaporation of water which makes it even hotter. Before you know it all the water is in the atmosphere and it’s hot as hell. CO2 dissolves in water (think Coke or Perrier), so without oceans there is a missing reservoir for CO2 and more of it ends up in the atmosphere. Also, as temperatures increase it drives CO2 out of rocks (think Tums or chalk (Calcium Carbonate)) making it hotter still. So why isn’t there a lot of water vapor in Venus’s atmosphere now? Solar radiation breaks water into Hydrogen and Oxygen atoms, and the light Hydrogen atoms can easily escape to space. A key piece of evidence in this story is that Deuterium (D), the isotope of Hydrogen that is twice as massive as vanilla Hydrogen (H), is far more abundant relative to H on Venus than it is on Earth. That is, D/H on Venus is much larger than D/H on Earth. Since D and H behave the same way chemically, the easiest way to explain a difference in that ratio is through thermal escape of H: the less massive H atoms have an easier time escaping Venus’s gravity than the more massive D atoms because at a given temperature, the H atoms will be moving faster than the D atoms. Thus, while both escape, the H escapes faster, and with less H, the ratio D/H gets big.

The runaway greenhouse model thus explains the D/H ratio, the lack of water, and the high temperatures on Venus quite nicely. The other mystery, Venus’s slow backwards (compared to most of the other planets) rotation, may not be a mystery after all. Certainly, giant impacts must have occurred and Venus may have suffered some whoppers. In fact, in addition to the formation of the Earth’s Moon, scientists have invoked giant late-stage impacts to explain Mercury’s high density (giant impact removes and vaporizes the outer layer of lower density material) and Uranus’s odd rotation (it is tipped over on its side relative to the other planets). These giant impacts undoubtedly have a significant, er, impact on the planet’s final rotation. However, some models of planet formation show that without giant impacts you end up with very slow rotation and the relatively rapid spin of Earth and Mars may be the mysterious ones. (And for the Earth, we know we can thank the Moon-forming impact and the Moon’s subsequent tidal evolution for our current 24 hour day.) Venus’s rotation is, almost by definition, a product of the angular momentum it received from all the impacts onto it during its formation (though a tidal interaction between the Sun and Venus’s atmosphere can change that significantly over the age of the solar system and can produce the slow retrograde rotation observed today).

The smoking gun for a giant impact onto the Earth is the Moon with its geochemical signatures of a terrestrial origin. The smoking gun for a giant impact onto Mercury is its large density, while for Uranus it is a tilted spin axis for which we have no other plausible explanation. For Venus, the case is much less clear. While there must have been major impacts late in its formation, neither its slow rotation nor its lack of water require it. In fact they are explained as a natural consequence of other mechanisms in the planet’s evolution. However, there is still much unknown about our sister planet. Its recent geological history remains somewhat controversial, with some evidence that it might have been volcanically active in the recent past. The problem is that it’s so hot there, and the atmosphere is opaque, so it is impossible to see the surface except with radar, and it is nearly impossible to put a spacecraft on the surface. Still, it will require dedicated spacecraft visits to Venus to untangle its recent history and clearly resolve the issue of how and why its climate diverged so dramatically from that of the Earth.