This video of the Moon is from the NASA/Goddard Space Flight Center Scientific Visualization Studio and uses data from the Lunar Reconnaissance Orbiter and Lunar Orbiter Laser Altimeter (LOLA) to compresses one month into 12 seconds and one year into 2.5 minutes. This is how the Moon will look to us on Earth during the entire year of 2011. We always see the same side, or face, of the moon, but because of the tilt of the Moon's axis and the shape of its orbit we see that face from slightly different angles over the course of a month and year. This "wobble" isn't apparent to us as we see the moon every night and don't think about how it was the night(s) before, but over the course of this video it is apparent.
Friday, June 24, 2011
Wednesday, June 15, 2011
I think I've stated before on this blog that I love reading studies about planets. However, when looking back through the blog archive I was surprised to find that I hadn't written any posts about Mars. All that changes today.
The current theory of planetary formation holds that planets form out of the protoplanetary disk of material left over from the formation of a star. The dust and gas in this disk are rotating around the star, and through accretion (coagulation of particles) larger and larger bodies form. When enough particles have come together they form planetesimals (100m to 10km across). The larger ones, which have more gravity, may even perturb the motions of nearby planetesimals, attracting them towards themselves in a process called gravitational focusing. Eventually one planetesimal will outpace all of the others in its orbit and become a larger body known as a planetary embryo. The terrestrial planets are thought to have formed through the collisions between large planetary embryos of diameters between 1,000 and 5,000 km. For Earth, the last collision was the one that formed the Moon approximately 50-150 million years after the birth of the Solar System.
Mars is the fourth planet from the Sun, at a distance of 1.5 astronomical units (AU). Its radius is about half that of the Earth with a mass about 10% that of the Earth. It has highly cratered highlands in the southern hemisphere with relatively smooth lowlands in the northern hemisphere. The structure of the craters in conjunction with areas that appear to have experienced erosions and degradations suggest a combination of aeolian (dust storms, wind streaks, etc.), fluvial (water), and volcanic processes in the planet's history. The crust of the planet has been estimated to be about 25-70 km thick. Below that is a silicate mantle about 1300-1800 km thick. The iron-rich metallic core has a radius of 1500-2000 km.
You add all of this information together an an interesting question comes up: Why is Mars so small?
Model simulations can explain the mass and dynamical parameters of Earth and Venus, but they fall short in explaining the size of Mars. A new(ish) paper in the journal Nature explores one explanation for the planet's diminutive size, that the planetary embryo did not collide and merge with that many other planetary embryos. A "stranded planetary embryo" origin. Now how in the world do you go about figuring that one out? I mean, you don't exactly have the planet's baby pictures.
To assess the formation of Mars the researchers had to know the planet's accretion timescale. How and how fast did it come together. So they used the 182Hf–182W decay system in shergottite-nakhlite-chassignite (SNC) meteorites.
I'm going to try to make a complex subject sound simple in a short amount of space. Chemically and mineralogically, chondrites (stony meteorites) are a good resource for testing the early composition of a planet. The timescales, mechanisms, and chemical differentiation of planets can be figured out by quantifying the radioactive decay of short-lived isotopes. Hafnium (Hf) 182 decays into tungsten (W) 182 in a half-life of nine million years. It doesn't sound like it but this is actually a relatively rapid decay process, and it means that almost all of 182Hf will disappear in 50 million years. Both elements are refractory or non-volatile, and so remain relatively constant in meteorites. They are also lithophile elements which are known to stay in the mantle when the core of Mars formed. All this makes the 182Hf–182W decay system is ideal for dating a planet's core formation.
By chemically testing 30 chondrites and another 20 Martian meteorites, the scientists are able to measure the excess abundance of 182W relative to other non-radiogenic isotopes of W (the tungsten isotopic composition) as well as the Hf/W ratio in the Martian mantle. They also measured the relationships between hafnium (Hf), thorium (Th), and tungsten (W) and generated a hafnium-thorium ratio (Th/Hf). Because Th and W have very similar chemical behaviors the researchers were able to calculate how long it took Mars to develop into a planet. They found that Mars accreted very rapidly and reached about half of its present size in about 1.8 million years or less. This is very rapid formation and consistent with their stranded planetary embryo hypothesis.
There are some other implications for these results. 26Al is known from meteorites and has a half-life of 700,000 years. Thermal modelling shows that planets accreting in under 2.5 million years incorporate enough 26Al for radioactive decay to induce silicate melting. If the time estimates of this study are correct then that would mean "Mars would have reached [about 69%] of its present size by that time and the heat generated from 26Al decay alone would have been sufficient to establish a magma ocean." Whoa! Additionally, this evidence of a quickly forming Mars could help to explain the similarities between the xenon (Xe) content of the Martian atmosphere and Earth's atmosphere, what is referred to as the "missing xenon problem." On both planets, Xe is not very abundant compared to the concentrations of other noble gases or to Xe concentrations in space. The authors suggest that part of the atmosphere of Earth was inherited from an earlier generation of planetary embryos that had their own atmospheres. "Earth may have inherited its missing Xe problem from the atmosphere of a Mars-like planetary embryo, possibly the impactor that also formed the Moon. This idea is consistent with the time when Earth became retentive for Xe, which is estimated to be, 100Myr after the birth of the Solar System and may correspond to the time of the Moon-forming giant impact."
This is the type of paper that reminds me of how cool I think planetary science is while also reminding me why I became an ecologist rather than a chemist. Overall, very interesting results!
You can read the paper for yourself here:
N. Dauphas, and A. Pourmand. (2011) Hf–W–Th evidence for rapid growth of Mars and its status as a planetary embryo. Nature, 2011; 473 (7348): 489. (DOI: 10.1038/nature10077)
Also, check out these stories:
(image via nasa.gov)
Tuesday, June 14, 2011
|Lunar Eclipse Timing Chart (via Universe Today)|
The eclipse visibility path will be rising over South America, western Africa and Europe, and setting over eastern Asia. If you are in western Asia, Australia, or the Philippines the eclipse should be visible just before sunrise.
Still not sure if you are in the lunar eclipse zone? Check on the map HERE to find out.
Here are your eclipse start and end times:
Penumbral Eclipse Begins: 17:24:34 UT Partial Eclipse Begins: 18:22:56 UT Total Eclipse Begins: 19:22:30 UT Greatest Eclipse: Begins: 20:12:37 UT Total Eclipse: Ends: 21:02:42 UT Partial Eclipse: Ends: 22:02:15 UT Penumbral Eclipse: Ends: 23:00:45 UT
Did none of that make sense? Click HERE for an easy-to-read explanation of of shadow zone terms.
If you are like me and live in a place where you won't be able to see the eclipse that's OK. You can still see it via the Intenet. Check out the AstronomyLive.com website and SLOOH for live broadcasts!
If you want the technical specs of the eclipse then visit NASA's 2011 eclipses page: http://eclipse.gsfc.nasa.gov/OH/OH2011.html
Monday, June 13, 2011
If you like the 2005 movie March of the Penguins then you will like today's post. The movie, and today's post, is about Emperor penguins.
Emperor penguins (Aptenodytes forsteri) are the only vertebrates that breed during the austral winter, in Antarctica. At the approach of winter these penguins journey in large numbers, single file, from the sea inland to their breeding grounds. There they find mates through courtship song and dance. Once they have paired off into monogamous couples they mate and the females produce a single egg. The females transfer the egg to the tops of the males feet where it can be kept warm and off of the ice. Exhausted and having gone weeks without nourishment the females return to the sea leaving the males to guard and hatch the egg. The males must survive the brutal Antarctic winter by huddling in large groups to keep warm and protect the eggs.
A new paper in the journal PLoS ONE looks at the coordinated movements of an Emperor penguin huddle. To study this they first picked a medium sized penguin colony consisting of approximately 2000 animals. Then, from an elevated, high, distant position they set up a camera that took high resolution time lapse images every 1.3 seconds for 4 hours. During that time they recorded environmental conditions such as air temperature and wind speed. Once they had the images they analyzed them in the lab, detecting and tracking penguin positions.
They found that Emperor penguins have a surprising strategy to prevent jamming while still remaining in a densely packed (and so warm) configuration. As the sun sets and the temperatures drop from −33 to −43°C (-27 to -45°F) the penguins aggregate into multiple huddles. These multiple huddles are tightly packed and remain relatively motionless, with the penguins within a single huddle all facing the same direction. The jammed state of the huddle is interrupted every 30 to 60 seconds by small 5-10 cm coordinated steps of the penguins. Sort of like doing the wave at a sports event if each time you sat down you sat in the seat next to you. It slowly moves the crowd in a single fluid direction and rotates every bird through the warmest parts of the huddle.
The authors propose that these small, regular steps serve a three-fold purpose. First, they help the penguins pack together in the highest density. Second, the small steps lead to a forward motion of the entire huddle. This allows smaller huddles to merge into larger huddles. Third, the steps lead over time to a slow large scale huddle reorganization. The huddle is reorganized but individual penguins do not change positions relative to their neighbors and do not force their way out of a huddle. It is unclear whether the the penguin wave is started by a single individual or a few leading penguins and follows a well-defined hierarchy among group members. But similar wave patterns have been seen in other grouping/flocking animals such as pigeons, fish, and locusts.
Check out a video of the time lapsed penguin huddle. And because the article is published in PLoS ONE (linked at the bottom of the post) it is free to everyone, and you can access additional movies at the bottom of the article in the Supporting Information section.
Daniel P. Zitterbart, Barbara Wienecke, James P. Butler, Ben Fabry. (2011) Coordinated Movements Prevent Jamming in an Emperor Penguin Huddle. PLoS ONE: 6 (6): e20260 (DOI: 10.1371/journal.pone.0020260)
Thursday, June 9, 2011
This is a story about sleep. It is also a story about blindness. Do the blind sleep more or less than the sighted? Is that a result of having no sight, a result of the environment, a genetically controlled characteristic, or a little bit of everything? Today's post involves a study about sleep in fish.
Astyanax mexicanus is a characin fish commonly known as the Mexican cave fish or Mexican tetra, and it occupies a wide variety of freshwater habitats within both it's native range and naturalized range. They are typically carnivorous, but has also been reported to be omnivorous in some parts of it's range. The species has a characteristic adipose fin, a strongly compressed body, a relatively small number of scales on the lateral line, and is approximately 12 cm (4.7in) long. Most notably, it has two forms: the normal form and the blind cave form. The blind cave form is albino with no eyes. The embryos are sightless and as the animal ages the organs decay and the eyes are scaled over. They live in the freshwater pools or streams deep within caves where no light penetrates. As such, they have lost their eyes, but are able to navigate using their lateral lines, which are highly sensitive to fluctuating water pressure. This is a pretty common pattern that you see in cave dwellers worldwide. They tend to converge on a suite of traits including eyelessness, loss of pigmentation, metabolic changes, and changes in feeding behavior. This type of convergence on traits in unrelated lineages is called convergent evolution.
A new study published in Current Biology takes a look at both blind and normal A. mexicanus populations. There are numerous cave populations known that are largely independent in their origins. This makes this fish ideal for studying the genetic basis of convergent evolution. Additionally, ecological conditions change as you go from surface to cave and this change is likely to have an impact on the fishes' sleep. This makes the fish good for studying the variability in patterns of sleep.
The researchers reared fish collected from isolated, blind cave populations in Pachon, Tinaja, and Molino as well as surface sighted fish. The Pachon and Tinaja populations are derived from the same ancestral stock but are geographically distant and hydrologically isolated by surface and subsurface drainage divides. The Molino population is derived from a different stock than the Pachon and Tinaja and is also isolated. Hybridization experiments were conducted where each population of fish was mated to each of the other populations of fish to create F1 hybrids (first cross/generation). They also reared an F2 (the following generation, mated F1's), and backcross (a hybrid to the original/wildtype) hybrids. Then the researchers tested whether or not a fish could see (just having eyes doesn't mean you can see out of them). To do this they immobilized a fry (recently hatched fish), placed them in a cylinder that flashed alternating patterns of black and white stripes, and watched to see if their eyes moved according to the color divisions. Eye movement equals sight. Gene assays were run to assess the complemation of the genes between populations. Then they developed an assay to characterize sleep in the F1, F2, and backcross hybrids.
In the inital crosses they found that, in some cases, the F1's could see. They found that the genetic mutations causing blindness are different in different lineages of fishes. So, why the restoration of sight as quickly as one generation? That is because populations from different caves are blind for different reasons. That means that in each population a different set of genes is nonfunctional, causing the blindness. Even though the fish were blind they still had basic functional visual systems, they had just been deactivated. The activated genes in the normal fish were able to overcome the inactivated genes of the blind fish. Interestingly, this relationship was stronger in populations that were more geographically distant from one another. The more distant the populations the less overlap in blindness-causing genes. The genetics results showed that the three cave populations also converged on the phenotype of reduced sleep.
So they then conducted a few sleep experiments:
1. They tested the threshold of arousal after inactivity they measure the responsiveness of a fish to repeated mechanical stimulus. Basically, they wait until the fish falls asleep (they stop moving, sink to the bottom of the tank, and drop their tail), wait a certain period of time, and they try to wake them up by tapping on the tank.
2. They tested if the fish were really sleeping by depriving them of sleep for one night and measuring their level of activity the next day.
3. They tested the day and night cycles by simulating them in alternating 12-hour light and dark periods.
4. From their breeding experiments, they looked at the sleep duration and cycles of the hybrids. This would give a genetic link to the sleep habits.
In the first, wake-the-fish-up, experiment they found that both surface and cave fry transitioned toward a higher wake up threshold as the period of inactivity increased. Once the fish had been inactive for 60 seconds it was in a different state entirely (it was sleeping) and it took more taps on the tank to wake them up. The second, keep-the-fish-awake, experiment found that all of the sleep deprived fish were less active the next day, and that depriving a fish of sleep is a good way to induce subsequent sleep behavior. A sleep rebound, if you will. The third, lights-on-lights-off-cycle, experiment found that the three populations of cave fish slept significantly less (110-250 min/day) than the surface fish (800 min/day). This is probably because the cave fish are in dark environments all the time while on the surface the light and dark cycles are tied to food availability and predator activity. So surface fish have a greater need to be attuned to light/dark cycles than do cave fish. The fourth, hybrid-sleep, experiment showed that the F1's slept like the cave fish. This indicates that the gene for less sleep is dominant. The F2 generation showed an intermediate sleep behaviors. Put these two results together and it suggests that sleep is controlled by a few specific genes.
Overall, a pretty cool experiment. Kinda reminds me of how I sleep. Which, ultimately was the purpose of the study - to equate fish sleep patterns and genetic control to human patterns and genes.
Yawn. Kinda makes me want to take a nap.
Here is the paper:
Duboué, Eric R., Alex C. Keene, Richard L. Borowsky. (2011) Evolutionary Convergence on Sleep Loss in Cavefish Populations. Current Biology: 21(8), 671-676. (DOI: 10.1016/j.cub.2011.03.020)
And here are a few story links:
(image via the Wired story linked above)