BSU Journal Club

The geysers near Enceladus' south pole. From

The geysers near Enceladus’ south pole. From

Saturn’s moon Enceladus has inspired fascination since Herschel found it in the late 1700s. The discovery of active cryovolcanoes geysers** on its surface by the Cassini mission in 2006 raised that fascination to a feverous intensity.

Although the source of energy powering the volcanoes geysers** was not (and is still not) understood, their implication was clear: Enceladus could have a sub-surface ocean, like his big sister Europa.

The follow-up discovery of salty particles in the geysers all but clinched the existence of a sub-surface water source. Salt in the eruptions would probably require liquid water to dissolve rocky materials to produce the salt. But it wasn’t clear whether the sub-surface source is a small pocket of water directly beneath the geysers or a more global-scale ocean.

In our journal club today, we discussed a paper that points to the existence of a global ocean. The recently published study from Peter Thomas and colleagues analyzed Enceladus’ rotation to study its internal structure. By tracking the motion of hundreds of control points on Enceladus’ surface, they found that its outer icy crust oscillates back and forth during its rotation much more than it should if the moon were solid all the way through.

Thomas and colleagues show that the large oscillations they found (called libration) require a large layer of fluid within Enceladus to lubricate the space between its outer, icy shell and its rocky interior. Otherwise, the moon would oscillate a lot less. Difficult to say exactly, but Thomas and colleagues estimate the ocean could be as thick as 30 km beneath an 20-km thick icy crust.

Just as Europa’s ocean, a sub-surface ocean in Enceladus could represent an enormous harbor for life. Even on this tiny moon, such a deep ocean is only about a tenth the size of the Earth’s oceans*.

Although Enceladus’ surface would be a tough neighborhood for life, sub-surface biota (if they ever evolved) would be protected by a thick layer of ice from the vacuum of space and interplanetary radiation. In fact, this same radiation could impinge on the surface and produce a steady supply of biologically useful oxidants, which could then trickle down into the subsurface ocean and help power the alien biosphere.

Thomas and colleagues suggest a more detailed analysis of Enceladus’ surface geology, newly inspired by their discovery, might help unravel the history of the ocean. Similar analyses of Europa’s complex tangle of surface ridges and cracks helped piece together that moon’s geological history.

Journal club attendees today included Jennifer Briggs, Hari Gopalakrishnan, Tyler Gordon, and Jacob Sabin.

*Earth’s oceans are, on average, 4 km deep, much smaller than Earth’s radius of 6,400 km. Therefore, I approximated their volume as pi*(6,400 km)^2 (4 km) ~ 500 Mkm^3. A similar calculation for Enceladus’ ocean — pi*(50 km)^2 (30 km) ~ 24 Mkm^3.

**Dr. Thomas kindly pointed out that there is some debate over the nature of the eruptions on Enceladus, and volcanoes is probably not the term to use.

Warm_Season_Flows_on_Slope_in_Horowitz_Crater_(animated)First, I would like to thank Michael Blanchard and Rena Drennon for their generous support of our PonyUp Campaign at Boise State.

At journal club this week, we had to talk about the recent confirmation of flowing water on the surface of Mars. How did scientists confirm that water is flowing on Mars? They studied a well-known martian phenomenon, recurring slope lineae (RSL).

These lanes of darkened martian surface occur seasonally, usually on the slopes of craters facing the Sun, and have been observed on Mars for a few years. Their seasonal occurrence on Sun-facing slopes pointed to the possibility that they result from flowing water, but since flowing water on Mars could be such a huge discovery, scientists have been very cautious about their origins; hence the obscure name.

In the recent paper, Ojha and colleagues analyzed high-spatial resolution observations of Mars from the HiRISE camera to carefully localize the flows on Mars’ surface. They then analyzed infrared observations from HiRISE’s sister instrument CRISM to look for spectral evidence of salts dissolved in water.

The salts they looked for aren’t like table salt but instead consist primarily of perchlorates. These chemicals can be toxic for us, but they are well-documented on Mars. Ohja and colleagues showed that the lineae exhibit spectral features consistent with dissolution of perchlorate salts.

These salts are known to significantly lower the freezing point of water, so they could easily keep water liquid, despite Mars’ cold temperatures. Eventually, even this briny water sublimates at Mars’ low surface pressures, and the spectral features found by Ohja and colleagues fade as the linaea fade, presumably as the water evaporates on the surface.

Why is this discovery so exciting? It points to the very real possibility that there is microbial life on Mars in liquid water deposits deep beneath the surface.

Early in the solar system’s history, the life-giving soup of chemicals that rained down in meteorites on Earth probably fell on Mars. Conditions on early Mars could have been similar to Earths’, so life could very likely have arisen. Whether life still exists on Mars may just be a question of whether it could escape to subsurface oases bearing liquid water, whose existence is strongly suggested by this new discovery. It may be only a matter of a few years before we find life on Mars.

In attendance at journal club this week were Jennifer Briggs, Karan Davis, Hari Goppalakrishnan, Tyler Gordon, and Emily Jensen.

Image of 51 Eri b (indicated by arrow) in the near-infrared, 1.65 microns.

Image of 51 Eri b (indicated by arrow) in the near-infrared, 1.65 microns.

A little behind the times on this one, but we finally managed to discuss the exciting discovery of a young, Jupiter-like planet in a Jupiter-like orbit by the Gemini Planet Imager (GPI) team at journal club today.

Using high-precision optical instrumentation and some sophisticated data processing to block out the host star’s glare, the team was able to directly image the planet 51 Eri b in infrared wavelengths.

No mean feat, given that the star is more than a million times brighter than the planet and is only one ten-thousandth of a degree away in the sky. This is a little like trying to see the glow of a firefly in the end zone against the glare of a football stadium light from the 50 yard line when the two are separated by the width of a human hair*.

51 Eri b is only 20 million years old, so it’s much hotter and glows much more brightly than our own Jupiter-like planets, making it easier to see. Jupiter-like planets tend to cool in a more-or-less well-behaved way that depends partially on their masses — bigger planets start out hotter. However, the planet is much cooler than predicted by some planet formation models, which provides strong constraints on the ways in which gas giants form.

So using 51 Eri b’s estimated temperature, 700 K (400 C) and age, Macintosh et al. put its mass somewhere between 2 and 12 Jupiter masses — solidly in the planet category.

The GPI observations also show us the planet has methane and water in its atmosphere. In fact, the methane detection for this planet is the most prominent so far seen for an exoplanet, according to Macintosh.

The GPI instrument is positioned to find many more planets like this one in the coming years, so expect lots of exciting results in the next few years.

Today’s journal club attendees included Jennifer Briggs, Hari Gopalakrishnan, Tyler Gordon, and Emily Jensen.

*Macintosh et al. estimate 51 Eri b’s luminosity is about 1 millionth that of our Sun. Wikipedia indicates the star 51 Eri is about 5 times brighter than our Sun. I had a lot of trouble finding the luminosity of a firefly — this page is the best I could do, and it estimates that one firefly emits about 2 mW. Stadium lights look to emit about 1,000 W, so that gives my factor of one million in luminosity.

51 Eri b has a projected separation from its host star of 13 AU, and the star is about 96 light years away, giving an angular separation of about 2 microradians. A human hair is about 100 microns across, so it would subtend 2 microradians from a distance of about 50 yards.

A hot Jupiter being ingested by its host star. From

A hot Jupiter being ingested by its host star. From×576/img/47b3082d767346e8bebdb5ad99f8f33d.jpg.

In journal club today, we discussed the recent study by Matsakos and Königl that investigated the possibility that hot Jupiters can be ingested by their host stars.

The idea that stars might ingest hot Jupiters has been around since the planets were first discovered. The putative accomplice in this type of astrophysical murder is tidal interaction between the planet and host star (the same kind of interactions that cause the Moon to recede from the Earth).

Tides cause the hot Jupiters to slowly spiral into their host stars, while spinning up the host star, but the strength of the interactions drops off rapidly with distance between the planet and star. The first hot Jupiters were far enough from their stars that they are probably safe from tidal destruction.

However, astronomers have continued to find planets closer and closer to their host stars, raising again the specter of planetary tidal destruction.

These same tidal interactions also align a host star’s equator to its planet’s orbital plane. So stellar equators that start out highly inclined to their hot Jupiter’s orbit (and there are a surprisingly large number) can end up completely aligned, but, as Matsakos and Königl argue, only at the cost of the planet’s orbital angular momentum.

The upshot of this is that many of the exoplanet host stars with equators aligned to their planets’ orbital planets may have eaten a hot Jupiter early in their lives. Under some reasonable assumptions, Matsakos and Königl show that the observed distribution of inclination angles for host star equators is consistent with about half of the stars having eaten a hot Jupiter.

Fortunately, the planets in our solar system will not suffer the same fate — at least not for a few billion years. But once the Sun leaves the Main Sequence and enters stellar senescence in a few billion years, its radius will blow up, destroying Mercury and Venus. Whether the Earth is also consumed by the approaching cloud of plasma is not clear, but if exoplanet studies have taught us anything, it’s that the universe is a tough place to be a planet.

Today’s attendees included Jennifer Briggs, Emily Jensen, and Tyler Wade.

Artist's conception of Kepler-452 b. From

Artist’s conception of Kepler-452 b. From

Exciting discovery reported last week of a planet a little bigger than Earth orbiting a star very like our Sun.

The planet, Kepler-452 b, was discovered by the Kepler mission and has a radius 60% larger than the Earth’s. It receives about 10% more light from its star than we do here on the Earth, and it’s probably about 2 billion years older. Together, these qualities mean it may be the most Earth-like exoplanet found to date (although there are lots of other similar planets).

Unfortunately, the host star is so distant, 1,400 lightyears from Earth*, that the usual method for directly estimating the planet’s mass, radial velocity observations, is not feasible. Instead, the planet’s discoverers constrain the planet’s mass by considering a range of compositions, calculating the radius expected for each of those compositions, and comparing it to the observed radius. Based on this analysis, they estimate at least a 49% probability that the planet is rocky, like the Earth.

Based on the amount of light it receives from its host star, there’s a good chance Kepler-452 b is habitable. This means, given a long-list of assumptions about the planet and its atmosphere, liquid water would be stable on its surface. Thus, Kepler-452 b joins a short but rapidly growing list of planets that might host life.

With our success finding potentially habitable planets, it’s probably only a matter of time (maybe just a few more years) before we find a planet that’s not just habitable but inhabited. Children in school right now might be the first generation to grow up in a universe where they know we’re not alone.

Today’s journal club attendees included Jennifer Briggs, Hari Gopalakrishnan, and Jacob Sabin.

*This website is the only reference I can find that gives the distance to Kepler-452 b from Earth. The paper itself doesn’t say 1,400 light years. The catalog gives a stellar magnitude V = 13.7 (also not given in the discovery paper). Converting that V magnitude to a flux and then using the stellar parameters given in the paper, I estimate a distance of 2,400 light years.

The recent study has removed Lord Helmet's original skepticism about buckyballs as the originator of DIBs.

The recent study has removed Lord Helmet’s original skepticism about buckyballs as the originator of DIBs.

At journal club today, we discussed the recent paper from Maier and colleagues which has solved the long-standing mystery of diffuse interstellar bands or DIBs.

These light absorption features were originally discovered by Heger way back in 1919, only a few months after the end of World War I. Their discrete absorption peaks are pervasive throughout visible wavelengths, indicating they are not simply due light-scattering by interstellar dust. The fact that they appear unchanged no matter the nature of the star whose light they absorb also suggests they don’t arise from the star itself. Instead, they must lay somewhere in the vast space between the Earth and the star.

Astronomers proposed DIBs might arise from dust grains, carbon chains, and even floating bacteria. Bucky balls, large soccer-ball-shaped carbon molecules, had also been proposed as candidates since they were discovered in white dwarf stars.

But deciding which candidate was actually the culprit required meticulous and highly sensitive lab work to recreate the extreme conditions of outer space, where temperatures are near absolute zero and gas pressures can be 10 million times smaller than at Earth’s surface. After twenty years of work, Maier and his team in Switzerland and Germany finally managed to create a little pocket of interstellar space in their lab.

By carefully ionizing buckyballs and introducing them into a cold He gas, they showed the spectral features created by the buckyballs in association with He matched almost exactly the spectral features of some DIBs.

The upshot of this is that the spectral forest of DIBs found at other wavelengths likely points to the prevalence of other large and complex molecules self-assembling in space, so this discovery is just the tip of the chemical iceberg. It has even been suggested that the complex molecular precursors for life originated in interstellar space in the same way as the buckyballs.

Whether that’s true or not, this discovery shows that the vast and lonely spaces between the stars aren’t quite as empty as they seem.

Journal club attendees included Jennifer Briggs, Emily Jensen, and Tyler Wade.

At journal club, we discussed the discovery of two new hot Jupiters using data from ESA‘s CoRoT mission, with the names CoRoT-28 b and -29 b. Both systems seem a little off.

The host star CoRoT-28 has an inflated radius, suggesting it is ancient and on its way off the main sequence. But it has a lot more lithium than we’d expect for an old star, and its rotation rate is similar to the Sun’s, much faster than we would expect.

Equally puzzling is the transit light curve for CoRoT-29 b (shown below at left). Most transit curves are u-shaped, but CoRoT-29 b’s is strangely asymmetric. The asymmetry resembles what has been seen for a planet transiting a rapidly rotating star — rapid rotation reduces the gravity at the stellar equator, resulting in a cooler, darker region. Barnes et al. (2013) looked at the transit light curves for such a Kepler system and actually used the light curve to study the planet’s orbital inclination.

(left) CoRoT-29 b transit light curve. (right) Planet transiting star spot.

(left) CoRoT-29 b transit light curve. (right) Planet transiting star spot.

But CoRoT-29 doesn’t appear to be a rapid rotator. So instead Cabrera et al. suggest that perhaps the star has a large, nearly stationary star spot and that the planet transits the spot over and over again. However, this scenario would require a nearly stationary spot with a very long lifetime (~90 days), neither of which is expected.

So a few more astrophysical conundra to add to the growing list of puzzling exoplanet discoveries.

Journal club attendees included Jennifer Briggs, Emily Jensen, and Hari Gopalakrishnan.

Artist's conception of a hot Jupiter shedding mass.

Artist’s conception of a hot Jupiter shedding mass.

At journal club today, we discussed a recent paper by Valsecchi et al. (2015) that looks at mass loss from hot Jupiters. These planets are so close to their host stars that the stars can blast away and rip the planets’ atmospheres apart.

By employing the sophisticated star/planet evolution model MESA, Valsecchi and colleagues found that the planets can shed most of their atmospheres, leaving behind a small sub-Neptune planet in a short period orbit. However, gravitational interactions between the planet and escaping gas actually push the planet away from the star as the planet is shedding mass, potentially out to orbital periods of a few days.

The upshot of this is that, based on these calculations, the recently discovered population of small ultra-short period planets probably did NOT originate from atmospheric stripping of more massive planets. So it’s not totally clear how these little planets originated, although Kevin Schlaufman suggested one still viable possibility.

Today’s attendees included Jennifer Briggs, Emily Jensen, Charlie Matthews, Jacob Sabin, and Tyler Wade.

Phase-folded and phase-binned light curve for KELT- 3, from Zhang+ (2015).

Phase-folded and phase-binned light curve for KELT- 3, from Zhang+ (2015).

At research group meeting on Thursday, we discussed a recent paper by Zhang and colleagues that investigated the performance of Canon’s EOS 60D and whether it was suitable to use for precision photometry to look for exoplanet transits.

Although the authors found the camera exhibited a few peculiarities (that are apparently not described in any of Canon’s documentation), they showed that it could be used to observe exoplanet transits — a really great result.

It means that astronomers, amateur or professional, who want to do transit observations don’t need to spend $10,000 to buy a high-end CCD camera. Instead, they can spend just a few hundred to produce reasonable quality transit light curves.

One especially tantalizing result from the paper: Zhang and colleagues mention having seen exoplanet transit-like signals for four of the target stars they studied, only one of which is known to host a planet — KELT-3 b. That means they may not only have recovered known transiting with the Canon EOS 60D; they may also have found three new ones. Presumably, they’re in the process of trying to confirm whether the other three are new planets.

UPDATE: The authors kindly updated me to say that follow-up observations indicated these three candidates are all false positives. But they would have discovered KELT-3 b with their survey, if it hadn’t already been discovered. So a pretty amazing achievement.

Attendees included Jennifer Briggs, Andrew Farrar, Nathan Grigsby, Emily Jensen, and Tyler Wade.

In anticipation of the upcoming New Horizons fly-by of the Pluto system, a really exciting result from Mark Showalter of SETI and Doug Hamilton of UMD — complex gravitational interactions among the moons of Pluto, Charon, Styx, Nix, Kerberos, and Hydra, have driven Nix mad and make it rotate chaotically. A simulation of its rotational evolution is shown in the youtube video above.

Showalter and Hamilton analyzed Hubble images of the Pluto system to understand how the moons’ orbits evolve as the result of the gravitational tugs between the moons. The three moons Styx, Nix, and Hydra are very near to and probably in a three-body resonance reminiscent of the Galilean satellites. Computer models of that interaction allowed them to constrain the masses of the moons, somewhere in the range of Mars’ moon Deimos‘ mass.

We would expect that the complex gravitational environment as applied to such elongated moons would likely lead to complex rotation, and indeed, Showalter and Hamilton find that the phase curve for Nix, observed from 2010 to 2012, varies erratically, consistent with a chaotic rotation. Their analysis also shows that Hydra rotates chaotically; probably some of the other moons (except for Charon) as well.

One bit of good news for the New Horizons flyby emerges from all this: the systems’ chaotic dynamics probably keep it clear of rings or additional small moons that would pose hazards for New Horizons. It seems nature has decided the system is complicated enough already.

Attendees at today’s journal club included Jennifer Briggs, Emily Jensen, and Tyler Wade.