According to Colwell

Exactly four years ago I was hosting a standing-room-only crowd of over 600 at the University of Colorado’s Laboratory for Atmospheric and Space Physics for the arrival of Cassini at Saturn. That means that the four-year prime mission is now over and tomorrow begins the Cassini Equinox Mission that takes us through - you guessed it - Saturn’s equinox. That phase of Cassini ends, depending on how you count it, either September 30, 2010, or March 31, 2011. That will certainly not spell the end of the mission overall as the spacecraft will still be merrily orbiting Saturn at the end of the CEM. To avoid any potential contamination of Titan and Enceladus which, due to the presence of organic compounds (Titan) or possibly liquid water (Enceladus), are considered potential abodes for life, Cassini will ultimately be destroyed, probably dumped into Saturn’s atmosphere. That will mark the end of the proposed Cassini Solstice Mission. If approved, this would happen after Saturn’s northern summer solstice in the spring of 2017. Details of that proposed mission are being worked on by the Cassini project now for presentation to NASA late this year or early next year.

In the meantime, more and more scientists are working on the tremendous volumes of data returned by Cassini’s twelve instruments. NASA has funded two rounds of proposals to its Cassini Data Analysis Program and the third round of proposals was just submitted. This summer I’m co-organizing a workshop to discuss the new discoveries related to Saturn’s rings. That will be immediately followed by a symposium to prepare for a book summarizing what we have learned about the Saturn system as a whole.

The Cassini Project Science Group (PSG) meeting #45 is taking place this week in Rome. Because of the large European participation in the international Cassini-Huygens mission, every third PSG meeting is hosted by a European participant in Cassini. The 4-year Cassini prime mission officially ends at at the end of the month, and a two-year extension to the mission begins the next day. Because a highlight of this extended mission is to take Cassini through equinox at Saturn, when the Sun is in the plane of Saturn’s rings, this is officially known as the Cassini Equinox Mission (CEM). The CEM was recently officially approved by NASA headquarters through September 30, 2010. The CEM does not end with the demise of Cassini. It will still be orbiting Saturn on October 1, 2010. Because there are planetary protection issues at Saturn (a requirement to avoid any possible biological contamination of potential abodes of life in the Saturn system), the spacecraft will ultimately be disposed of, probably by crashing it into Saturn. So there will be some sort of extension beyond the end of the CEM. Hopefully this will include further scientific study of the Saturn system, as there will be much more to learn after the end of the CEM. In large part this is due to the long seasons at Saturn, but there are also dynamical phenomena in the rings that operate on timescales of many years. An extension of Cassini beyond the CEM will enable us to study these phenomena as well as perhaps studying the atmospheres of Saturn and Titan until the northern summer solstice. This would show us the lakes region at the north pole of Titan and show if they change over the course of the seasons.

Image Credit: NASA/JPL/SSI.
This image of the F ring shows two narrow components, a structure also seen in recent stellar occultations and significantly different than the appearance of the F ring in earlier Cassini images as well as from Voyager, highlighting changes in the rings over the course of decadal timescales.

A Supernova is the explosive death of a massive star, and is also one of the coolest (not in the temperature sense!) things in the universe. Stars are basically defined by the fluctuating battle in their interiors between their own crushing weight forcing atomic nuclei to fuse and the tremendous thermal pressure generated by the energy released from that nuclear fusion. As it happens, more massive atomic nuclei are more energetically stable than less massive nuclei. This means that the mass of, say, a carbon nucleus is less than the sum of the masses of the three helium nuclei that fuse to form it inside the core of a star. How can that be? It be. Einstein famously showed the equivalence between mass and energy, so mass is a form of potential energy in the same way that the height of a boulder on a hillside represents potential energy. The boulder at the top of the hill has gravitational potential energy that is released when it rolls down the hill. Protons and neutrons, the constituents of atomic nuclei, have mass potential energy that is released when they are brought close enough together for the nuclear strong force to bind them together into a larger nucleus. Just as the rolling boulder manifests energy in a new form (rolling and falling faster) as its gravitational potential energy is released, so do the products of nuclear fusion in stars release energy that comes from its mass potential energy. In the case of nuclear fusion, that energy is released in the form of gamma ray (high energy) photons, neutrinos, and the kinetic energy of the occasional positron.

Up to a point: it turns out that it is only energetically advantageous for nuclei to fuse elements as massive as iron or less. Fusing more massive nuclei, such as silver and gold, is like rolling the boulder back up the hill: it takes more energy rather than releasing it. That’s where supernovae come in. Once the core of a massive star has fused into iron, it’s not energetically advantageous for any further fusion to take place. The iron core gets more and more massive as fusion in shells around the core dumps more heavy elements onto it. At some point the electron degeneracy pressure holding the core up cannot support its own weight, and it collapses suddenly and violently. Now we’re back to the boulder rolling down the hill: gravitational potential energy is converted to kinetic energy as the core collapses from something the size of the Earth to just a few kilometers across. This release of energy, which quickly bounces back outward into space, is a supernova. And it is in this very high energy event that atomic nuclei more massive than iron are created. Earlier this year astronomers serendipitously caught the early X-ray emission from a supernova in the galaxy NGC 2770 about 100 million light years from here. Because the bright X-ray flash is the first light to escape from the exploding star, catching it allowed the astronomers, led by Alicia Soderberg of Princeton, to alert the rest of the astronomical community to observe the supernova, providing an unprecedented record of the event from its earliest stages. Usually supernovae are not observed until days after the initial core collapse, because the brightness of the explosion in the visible part of the spectrum grows over the course of many days. They were fortunate enough to catch this one in the act because they were observing a supernova that had taken place in the same galaxy earlier. Perhaps the coolest thing about supernovae is that you probably have a fair amount of material made in an ancient supernova, whether it is a gold ring or silver necklace, every piece of that was blasted into space by a supernova explosion more than 4.6 billion years ago.

NASA has formally approved a two-year extension to Cassini’s mission at Saturn. While not a surprise (we’ve been planning the extended mission for nearly two years, and are a significant way through the process of the detailed observation plans for that time period), it is welcome news. The nominal mission ends June 30 of this year, four years after Cassini arrived at Saturn. The extended mission takes Cassini through Saturn’s equinox and nearly doubles the number of orbits or “revs” of Cassini around Saturn. Each rev brings close observing opportunities for Saturn’s atmosphere as well as its retinue of moons. Ring observations will be particularly hectic at the beginning of the extended mission (which hopefully will soon be known as the Equinox Mission), and then again around equinox as Cassini observes the thermal response of the rings as the Sun moves from the southern hemisphere to the northern hemisphere of Saturn. The equinox period is also when shadows cast by small vertical warps in the rings will be longest and easiest to observe, providing new measurements of the dynamics of the ring system.

Coming up: further discussions of what we can learn with an extension of the mission after the two-year Equinox Mission.

The Mars Reconnaissance Orbiter with its powerful HiRISE camera onboard came within 6000 km of one of Mars’s tiny moons, Phobos, providing some of the best images yet of this moon.

The Stickney crater on Phobos imaged from MRO. Credit: NASA/JPL/Univ. Arizona.

The large crater on the right, called Stickney, is comparable to the size of the moon, suggesting that had the impactor that formed the crater been a little more massive (or traveling a little faster) it would have shattered the moon, leading to a debris ring around Mars. Phobos survived that impact, but its fate is sealed. Because Phobos orbits Mars in less than a Martian day (only 7 hours and 40 minutes compared to the Martian day of about 24 hours and 40 minutes), it races ahead of the tidal bulge that its gravity produces on Mars. This tidal bulge then produces a torque that, unlike that of the Earth on our Moon which is causing it to move slowly away from the Earth (because the Moon orbits the Earth much slower than the Earth rotates, causing it to lag behind the tidal bulge it raises on the Earth, expressed by the ocean tides) retards the motion of Phobos causing it to spiral inward toward the planet. This will cause Phobos to hit Mars in less than 100 million years. Before that happens it is likely to be torn apart by tidal forces making a debris ring around Mars which itself will quickly decay into Mars’s atmosphere.

Phobos, like Mars’s other moon Deimos, is likely a former asteroid, captured into orbit around Mars through a combination of tidal dissipation and perhaps atmospheric drag from an earlier, denser Martian atmosphere. Although Phobos orbits quite close to Mars, its small physical size means that it is not able to fully block the Sun as seen from the surface of Mars. Nevertheless, the Mars rover Opportunity took this series of images showing Phobos transiting in front of the disk of the Sun. Phobos’s aspherical shape can be clearly seen in silhouette.

Phobos transits the Sun

Credit: NASA/JPL/Cornell.

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.

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.

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.

Last night we had a beautiful view of the total lunar eclipse from Orlando. In some ways it was more impressive to see it during the partial phase when only part of the Moon was in the shadow of the Earth. The curved shadow of the Earth across the face of the full Moon provided ample evidence that the Earth was a sphere long before it was finally circumnavigated. Maybe Sherri Shepherd of “The View” got a look.

Lunar eclipses occur when the Moon is on the opposite side of the Earth than the Sun so that the Earth’s shadow falls on the Moon. If the orbit of the Moon were in the same plane as the orbit of the Earth around the Sun, we would have a total lunar eclipse every full Moon. The Moon’s orbit is inclined to the plane of the Earth’s orbit (the ecliptic) by a little over 5 degrees. So the Moon crosses the ecliptic twice each month, but for a lunar eclipse to occur, that crossing point must occur when the Moon is full (meaning that it is on the opposite side of the Earth than the Sun). In rough numbers that occurs twice a year. However, the size of the Earth’s shadow at the distance of the Moon is only a couple of times the size of the Moon, so depending on the exact timing of the Moon crossing the ecliptic, the Moon may not pass through the shadow entirely (resulting in a partial eclipse), or it may only pass through the penumbral shadow (where only part of the disk of the Sun is obstructed by the Earth). Nevertheless, I could not help but wonder how the local Orlando television station got the idea that it would be 40 years before the next total lunar eclipse. In fact the next total lunar eclipse will be in December 2010. The gap between this one and that 2010 total eclipse is actually unusually long. There will be 85 total lunar eclipses in the 21st century, or nearly one every year.

Because the Moon’s orbit is not only inclined to the ecliptic, but is also not circular, there are additional differences in the frequency of eclipses as the orientation of the Moon’s elliptical orbit changes with time and the distance of the Moon from the Earth (and hence the size of the shadow) also changes. Combining the time for the Moon to go around the Earth, the time between two full Moons, and the time for the Moon’s elliptical orbit to return to its same orientation with respect to the Earth and the Sun, we get a cycle for when eclipses will repeat with nearly the same duration and visibility (this holds for solar eclipses as well, which are much more finnicky than lunar eclipses because the shadow cast by the Moon is much smaller than the shadow cast by the Earth). This cycle is called the Saros cycle and is a little over 18 years. So I still don’t know where the local weatherman got his 40 year figure, but the station’s web site has the correct information now.

The more spectacular solar eclipses are far more rare due to the small size of the Moon’s shadow. There is a nice total solar eclipse visible across the United States on August 21, 2017, and another one on April 8, 2024. In the meantime, you’ll have to go quite far to catch one.