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All posts for the month January, 2017

Join the Boise State Physics Department for a stargazing party with special guest Prof. Denise Stephens, astronomer from BYU.

Come learn about the New Horizons mission, the team, and the 20+ years it took to get this mission to Pluto. Take a closer look at Pluto, Charon, and the other 4 moons as we dive into the Kuiper Belt, and the extended mission to visit another Kuiper Belt Object.

The event takes place on Friday, Feb 3 and will start in the Multipurpose Classroom Building in room 101 at 7:30p and then move to the top of the Brady Garage at 8:30p, where telescopes will be set up for stargazing (weather permitting).

The weather forecast for this week is not promising, so we’ll cancel the stargazing portion. The lecture will still happen, though.

A friend and colleague, Prof. Hannah Jang-Condell, invited me to visit her home department, the Physics and Astronomy Dept. at University of Wyoming. Having never been to Laramie, I was happy to accept.

I gave two presentations while at Wyoming, one to the geology dept. about our work on martian dust devils and another our SuPerPiG’s work looking for ultra-short-period planets. I’ve included my presentations below.

Geology Talk

Physics/Astronomy Talk

I watched a lot of TV when I was growing up, and some shows made a strong impression on me. “Twin Peaks” stands out among them – I can still hum the theme song, and I can see the eerie photo of Laura Palmer that lingered behind the ending credits. The show had a unique blend of off-kilter comedy and claustrophobic unease that still colors my taste in TV.

I was excited to read that, true to Laura’s promise in the Black Lodge, David Lynch and Mark Frost are continuing the series later this year. While I wait for the new series, I read Frost’s recently published book The Secret History of Twin Peaks. It was just as spooky as the original series.

History fleshes out some of the underlying mythos of “Twin Peaks”, answering a few questions from the show (Who was BOB? Why did owls feature so often in the show?) but raising a lot of others. Presumably, the book is meant as an entrée into the coming series, to stir up interest in new story lines, which the book seems to hint at.

History takes the form of a dossier, written and hand-bound by a mysterious author, about the history of the Twin Peaks area, going back to shortly after the American revolution and seemingly running up to the end of the show.

It opens with a letter from a familiar FBI agent, instructing another agent to evaluate the dossier’s contents and determine its author. Throughout the dossier, the investigating agent inserts marginalia, confirming or questioning remarks and assertions by the author, all of which gives the writing a crisp, matter-of-fact tone that contrasts with the mystical and murky events in the story.

This is actually the first non-science book I’ve read in a while, and the story was so engaging, I gulped it down in a weekend. So, highly recommended reading for fans of the show or lovers of supernatural thrillers. Along with your reading, be sure to give yourself the present of a cup of hot, black coffee.

Glint from a sea on Saturn’s moon Titan. From http://www.jpl.nasa.gov/spaceimages/details.php?id=PIA18433.

Our spring semester journal club opened with a nice review paper on finding habitable planets from Tyler Robinson, NASA Sagan Fellow at UC Santa Cruz.

The traditional definition of a habitable planet is “a world that can maintain stable liquid water on its surface”, but, as astrobiologists have explored for decades, this definition involves a vast flotilla of assumptions and very narrowly focuses our search for Earth-like life.

Even with all its limitations, this definition provides a very useful and practical starting point – at the first order, whether a world can host stable liquid water on its surface depends on the amount of sunlight it receives and whether it has a sufficiently thick (but not too thick) atmosphere.

Having found countless worlds outside our solar solar, astronomers are able to assess whether those worlds satisfy these conditions using observations we can already make, and a few dozen (probably) do.

In his review paper, Robinson discusses the observational and theoretical techniques astronomers can employ in the near future to take the next steps in deciding whether a world really has liquid water. Among the different approaches he describes, one is the most striking is the search for the glint from an alien ocean.

Robinson points out that Galileo was the first person to propose how to look for an ocean on another world. In his controversial Dialogue Concerning the Two Chief Worlds Systems, he says that, if the Moon had seas, “the surface of the seas would appear darker, and that of the land brighter”, just as on Earth.

Seas can also appear very bright compared to land, given the right observing geometry: seas exhibit specular reflection – this is the same effect you see looking out a plane’s window when the Sun reflects off the ocean. So looking for the glint provides a way to find large bodies of water on a distant world. Indeed, the first extraterrestrial oceans were found on Saturn’s moon Titan using this method, and one of the sea glints now frequently observed by the Cassini mission is shown in the figure at left.

Of course, we don’t have spacecraft orbiting any extrasolar worlds (yet), so we can’t resolve individual points on their surfaces. But, as discussed by Robinson, as they orbit their host stars, some of those worlds line up the right way that we could see a spike in the total amount of light coming from the planets. Observing such a spike over and over again whenever the planet was in the right geometry would be a strong hint that it had a large body of liquid reflecting sunlight. Given a little more information about planetary conditions, we could confidently infer such a planet had liquid water on its surface.

Amazingly, astronomers have used Earthshine reflected from the Moon to indirectly observe sea glints from the Earth. And so we’ve actually detected oceans on two worlds using distant spacecraft (if you let me call the Moon a “spacecraft”). As Robinson’s review implies, astronomers are probably on the cusp of finding oceans on extrasolar worlds. From there, it’s just a hop, skip, and a jump to finding life.

I just finished Don Canfield‘s book Oxygen, a sweeping exploration of the history of Earth’s most volatile atmospheric constituent.

Canfield starts by exploring how cyanobacteria generate oxygen during photosynthesis and how the process evolved. For instance, chloroplasts, the photosynthetic power plants of cyanobacteria, were once free-floating cyanobacteria themselves that took up residence in and eventually merged with eukaryotes that evolved into present-day cyanobacteria.

Subsequently, Canfield discusses Earth’s oxygen removal and renewal processes – decomposition of organics burns up oxygen, while their rapid burial preserves it.

The next several chapters present the geochemical evidence for changes in oxygen throughout Earth’s history, including variations in the ratios of different isotopes, sensitive to biological and abiological processes. Since I teach a class on astrobiology at Boise State, I focused a lot of attention on these parts, trying the piece together the interplay between biology and geology betokened by the isotopic variations.

One element of that story especially relevant to my class: variations in the carbon-13 isotope. As it turns out, one enzyme in cyanobacteria, RuBisCO, helps convert atmospheric C02 into organic carbon compounds, but it preferentially selects the lighter carbon-12 isotope 2.5% more often than the carbon-13 isotope. The organic compounds built using RuBisCO are therefore slightly depleted in carbon-13 relative to the atmosphere and when they are later incorporated into geological strata, the slight depletion gives a measure of how much life was around when the stratum was laid down.

Zooplankton salp pellets. From http://www.whoi.edu/oceanus/feature/will-ocean-iron-fertilization-work.

The book contained lots of other appealing details. For instance, it’s not exactly clear what caused an enormous variation in oxygen on Earth 580 million years ago, a sea change in Earth’s history matter-of-factly called the Great Oxygenation Event. But one explanation has to do with the evolution of a new kind of poop:

The idea is that zooplankton [newly evolved 580 million years ago] produce fast-sinking fecal pellets. These would decompose less in the upper layers of the ocean as they sink […] when compared to the smaller, slowly settling microbial biomass [that had previously predominated].  (pp. 135-136)

Since the old sinking biomass took a long time to sink to the ocean floor, it had a long time for bacteria to decompose it, using up a lot of oxygen in the process. But the new, faster-sinking poop made it to the ocean floor before it decomposed much and so left the oxygen dissolved in the ocean instead.

I did have to spend a lot of time reading and re-reading the discussions of geochemical cycles and signals because it’s been a long time since high school chemistry for me, but I was willing to struggle through these parts because I found the underlying story so interesting.

So a really fascinating and challenging read about the complex (and poop-filled) evolution of Earth’s bio-geo-atmosphere.

A hot Jupiter being ingested by its host star. From http://sen.com/thumbs/1024×576/img/47b3082d767346e8bebdb5ad99f8f33d.jpg.

Since astronomers have discovered hundreds of planets orbiting perilously close to their host stars over the last few decades, the question of what happens to planets when they wander too close to their host stars has intrigued astrophysicists.

These so-called close-in planets zip around their stars in just hours and are so close to their stars that the starlight heats their surfaces and atmospheres to thousands of degrees.

Since the planets’ atmospheres are so hot and puffy, the planets have only a tenuous gravitational hold on them, and some planets have been observed losing their atmospheres. I myself have made a hobby over the last few years of ripping apart exoplanets in the computer and of looking for their fossils in the sky.

How long it takes these hot Jupiters to lose their atmospheres and what’s leftover after the atmospheres are gone have been the subject of speculation for a while, and key to answering these questions is understanding what happens to the planets’ radii (and thus, their gravity) and what happens to the escaping mass.

A recent study by Shi Jia and Hendrik Spruit explores a new aspect of this Roche-lobe overflow process, and their results suggest the atmospheric disruption process might happen a lot faster than astronomers have thought.

Accretion disk formation for a binary star system.

Previously, it had been assumed that, as a planet’s atmosphere is ripped away, it goes streaming down toward the host star, and the gas then goes into its own orbit close to the star, forming what’s called an accretion disk. This process has been studied for decades in binary star systems, in which one star donates mass to their other.

This accretion disk can then interact gravitationally with the disrupting planet, pushing it back away from the star a bit and moderating the rate at which mass escapes from the planet.

The upshot of this is that formation of the accretion disk can slow the disruption process, and so it could take billions of years for a Jupiter-mass planet to lose its entire atmosphere – a long time on human timescales but relatively short on astronomical timescales.

However, in their study, Jia and Spruit point out that these hot Jupiters undergo disruption very close to their host stars – so close, in fact, that, between the planet and the stellar surface, there might not be room enough for an accretion disk to form at all. Instead, gas that escapes the planet might just crash directly into the host star.

That’s a problem for the planet because if there’s no disk around, it can’t slow the mass loss rate, and so the planet might shed its entire atmosphere not in billions of years but just a few weeks – less time than it would take me to explain the process during a semester of astrophysics class.

Although the disruption process might be very fast, Jia and Spruit’s results suggest the remnants might be left around to be observed. What exactly they look like is still unclear, but the K2 and upcoming TESS missions will probably find lots of them (and may already have).

It’s been miserably cold in Boise, and we’ve had record snowfall during the last few weeks.

Sunday, however, it was relatively warm — warm enough, in fact, that the snow that was slated to fall fell as sleet instead.

So I took my daughter over to a friend’s house for a playdate, and as we walked over, we heard the beautiful sound of the tiny icy pebbles clattering to the ground. The sound was so distinctive and regular that I whipped out my phone to record it.

When I got home later, it seemed like there must be something interesting I could do with the recording, and the first thing that struck me was to analyze the frequency of the clattering sound. In other words, how often did I hear a sleet pellet strike the ground?

To begin, I imported the m4a file from my iphone and then convert it via command-line into a wav file (since I planned to use python and it was easier to find ways to manipulate wav than m4a files):
ffmpeg -i Sleet_Falling.m4a Sleet_Falling.wav

Then I fired up ipython notebook and imported and plotted the wav file:

%matplotlib inline
 
import numpy as np
import wave
import matplotlib.pyplot as plt
 
from scipy.io import wavfile
# Load the data and calculate the time of each sample
samplerate, data = wavfile.read('Sleet_Falling.wav')
times = np.arange(len(data))/float(samplerate)
 
# Make the plot
# You can tweak the figsize (width, height) in inches
fig = plt.figure(figsize=(6, 4))
ax = fig.add_subplot(111)
ax.plot(times, data, lw=0.5) 
ax.set_xlim([0,10])
ax.set_ylim([-2000,2000])
ax.set_xlabel('time (s)')
 
fig.savefig('Sleet_Falling.png', dpi=500

The distinctive rapid crackling is apparent in the waveform.

Next, I calculated a Fourier transform of the signal:

from numpy.fft import rfft
 
ft = np.fft.rfft(data)
n = data.size
timestep = 1./samplerate
freq = np.fft.rfftfreq(n, d=timestep)
period = 1./freq
 
fig = plt.figure(figsize=(6, 6))
ax = fig.add_subplot(111)
ax.semilogx(period, abs(ft), lw=0.5)
 
mx_arg = abs(ft).argmax()
print(period[mx_arg])
 
fig.savefig('Sleet_Falling_fft.png', dpi=500)

which shows a distinct peak at about 1 millisecond.

Assuming the sleet I saw is about 5 mm in radius, I estimate a terminal velocity of about 5 m/s. If I imagine (unrealistically) that the sleet particles are falling in a single column, with one directly above another and traveling at 5 m/s, then one striking the ground every millisecond means the sleet balls are basically packed end to end, as tightly as they can be [1 ms = (5 mm)/(5 m/s)]. Of course, if the pellets were spread out and striking the ground at random places, they could be way more spread out and not as many would have to fall at a time in order to make the sound I heard.

Turns out I’m not the first person to try estimating the precipitation rate using sound. NASA deploys microphones in the ocean to record the distinctive sound of raindrops.

From http://earthobservatory.nasa.gov/Features/Rain/rain_2.php.

In fact, different size raindrops make different sounds because some sizes of drops generate bubbles and others do not, and so scientists can actually estimate the sizes of raindrops by just looking at the distribution of frequencies.

Unfortunately, I can’t make the same kinds of measurements using my iPhone because I haven’t calibrated the microphone using a sound of known amplitude. Maybe something for the future.

I was probably first introduced to Marcus Aurelius by Hannibal Lector:

So as the last book that I’ll get to during the break, I read the classic Meditations and finished it during the AAS conference.

I’m conflicted about this book. There were parts I enjoyed and found inspiring, but other parts I found confusing and repetitive.

The book presses a message of self-assurance and poise: “Be like the promontory against which the waves continually break; but it stands firm and tames the fury of the water around it.” As someone who struggles with self-doubt, _Meditations_ provided potent language and powerful imagery to help tame my anxieties, and reading it was like standing in a cold, bracing wind.

At the same time, the book repeated the same ideas over and over again, often using very similar wording. If you come to this book expecting a cogent philosophical treatise, as I did initially, then this repetition is puzzling. As I read the book, though, I also did some background reading and discovered the book was written as a diary by Marcus Aurelius. In this case, it makes sense to me that he repeated the same admonitions – he was trying to instill them in his own mind.

There are lots of references to philosophers and ancient cities, which I also found confusing. Instead of buying this bare bones edition of _Meditations_, it might help to have an annotated version that can provide historical context.

Although some aspects of the Stoic philosophy described in the book were stirring — “Do not act as if you were going to live ten thousand years. Death hangs over you. While you live, while it is in your power, be good.” and “Such as are your habitual thoughts, such also will be the character of your mind; for the soul is dyed by the thoughts.” — I found unconvincing the numerous assertions that “everything that happens, happens justly”, that nature is implicitly ordered and fair to be. The admonitions to remain true to one’s nature didn’t give me a lot of insight because Aurelius never says how to determine one’s true nature.

So I came away from _Meditations_ with mixed feelings. Its philosophy of equanimitous withdrawal from the world reminded me of an empty marble temple: resilient against the eons but austere and sterile.

Figure from Gao’s talk, showing the spectrum of a planet before (blue and black lines) hazes and after (red line). The molecular features are almost completely wiped out by the hazes.

On day two of the AAS 229th meeting, I attended the morning session on exoplanet characterization and theory, which focused on atmospheric characterization. Several great talks, but one that made an impression on me was Peter Gao‘s presentation on sulfur hazes in hot Jupiter atmospheres.

Gao discussed work from Kevin Zahnle at NASA Ames showing that UV photolysis can transform even small amounts of gaseous sulfur in a hot Jupiter’s atmosphere into significant amounts of polymer haze. Something that has become a running motif in exoplanet atmosphere studies, these hazes discombobulate the spectrum of light emerging from a planet’s atmosphere, completely masking the signatures of other atmospheric components. This is bad news if, say, you wanted to determine the composition of a planet using light reflected from its atmosphere.

In the afternoon, I was fortunate to attend Sean Carroll‘s plenary talk “What We (Don’t) Know About the Beginning of the Universe”. It was a fascinating tour of all the different ideas about the origin of the universe, including The Big Bounce, baby universes hidden inside black holes, and the idea that the universe may have no beginning and no end. The best part of the talk, for me, was the end, when Carroll showed us a stern letter from a 10 year old skeptic sent to him in a response to an NYT article in which Carroll was quoted:

I Don’t know if you Exist But I Do! I bo not Agree with your Articl and I Do not Beleave that “MOMBO-JOMBO” if you do … Well! it’s Disturbing thought But I know How to Deal with it! I will Not let the Wolb Disiper under My Nose But if you Do I cant say I’m sorry!

Sincerely

a ten year old who knows a little more than some Pepeol!

George Wing

ps. some peopl Have a little to Much time.

Just brilliant!

Unfortunately, I had to depart for home shortly thereafter, so I’m missing the rest of the meeting. So here endeth my blog series on the meeting. Our fall semester at Boise State starts next week, though, and I plan to have weekly (maybe even semi-weekly) updates on the blog, so stay tuned.

A very active and engaging morning session on detecting exoplanets via the transit method on AAS 229 Day 1. Lots of good talks (although all of the talks were by male astronomers) and probing but polite questions (again, mostly by male astronomers – interesting study on these trends here). A few talks stuck out in my mind.

Aaron Rizzuto from UT Austin looked for transiting planets in stellar clusters spanning a range of ages using data from the K2 Mission and found there seem to be fewer hot Jupiters in clusters 10 million years old than there are in older (hundreds of millions of years old) clusters. He suggested that this may mean it takes 100s of millions of years for the migration that makes hot Jupiters to take place. That would probably rule out one standard model for making hot Jupiters, namely gas disk migration, since that probably takes place within a few 10s of millions of years.

Dave Kipping of Columbia University discussed his search for transits of Proxima b, the recently discovered, Earth-sized planet just four light years from Earth. Unfortunately, the host star, Proxima, is a highly variable star due to almost constant flaring. As a result, it would be very difficult to see the planet’s transit – as Kipping said, it requires wading through “a sea of variability”. However, Kipping and his group struggled valiantly to recover the transit using data from the Canadian MOST satellite, and it looks like the planet just does not transit. So we probably won’t know the planet’s radius (at least not for a long time). Bummer.

The last talk of the session was from George Ricker, the PI of the TESS mission, the follow-up mission to Kepler, about TESS’s status and prospects. Apparently, the mission will observe more than 2 million stars! Orbiting many of those stars will be nearby habitable planets, and Ricker showed an amazing simulation of where those stars might be found (courtesy of Zach Berta-Thompson of UC Boulder), a still from which is shown below.