Astrobiology Underground

I’m a great believer in what I might call the ‘conventional’ habitable zone; i.e., a habitable zone defined by the possibility of liquid water on the surface. The definition is offered not to exclude exotic possibilities like micro-organisms floating in the clouds of Venus or aquatic life deep inside an ice-covered moon like Europa. Rather, it acknowledges that finding life is hard enough without losing our focus. In terms of exoplanets and feasible near-term study, a warm planet with liquid water — the kind we live on — would command our immediate attention.

But as we look at much broader issues of how life forms, we may indeed learn that our kind of life is but one component of a vast continuum, as recent work out of the University of Aberdeen reminds us. In a new paper published in Planetary and Space Science, researchers tackle the question of life living deep underground. Now the habitable zone starts to broaden, because things get warmer as we go deep.

We know of life here on Earth that exists more than five kilometers below the surface, and given the difficulty of probing these regions, we probably have fragmentary knowledge at best of what’s down there at deeper levels. So if we’re talking underground microorganisms, maybe a place like Gliese 581 d, evidently just past the outer edge of its star’s habitable zone in terms of liquid water, would still qualify. The Aberdeen team thinks conditions less than two kilometers below the surface there could be clement.


Image: New studies are examining habitable conditions below planetary surfaces, where liquid water might exist. Internal heat might keep even a ‘rogue’ planet moving without a parent star capable of hosting some kind of life underground. This image is an artist’s impression of the rocky super-Earth HD 85512 b. Credit: European Southern Observatory.

The Aberdeen work revolves around a computer model that estimates the temperature below the surface of planets of varying sizes at varying distances from their star. Doctoral student Sean McMahon comments on the implications in this Aberdeen news release:

“Using our computer model we discovered that the habitable zone for an Earth-like planet orbiting a sun-like star is about three times bigger if we include the top five kilometres below the planet surface. The model shows that liquid water, and as such life, could survive 5km below the Earth’s surface even if the Earth was three times further away from the sun than it is just now. The results suggest life may occur much more commonly deep within planets and moons than on their surfaces. If we go deeper, and consider the top 10 km below the Earth’s surface, then the habitable zone for an Earth-like planet is 14 times wider.”

An extension of the habitable zone indeed, taking us well past the orbit of Saturn in our own Solar System. And ponder the case of a rogue ‘super-Earth,’ a planet four or five times more massive than the Earth that has for whatever reason been ejected into the interstellar deep. We’ve talked about such places before and their prospects for generating enough internal heat to make some kind of life possible even in the absence of a parent star. And when you start extending habitable zones this far, a cosmos teeming with at least primitive life seems possible.

Since we’re pushing habitable zones in new directions, let me also mention Craig Stark (University of St. Andrews), whose work has focused on alien atmospheres extending from exoplanets to brown dwarfs. Stark argues that prebiotic molecules can form in these environments, saying “The atmospheres around exoplanets and brown dwarfs form exotic clouds that, instead of being composed of water droplets, are made of dust particles made of minerals.” Add a dose of lightning and you have charged particles and interesting possibilities:

“These charged gases are called plasmas – like those found in fluorescent lights and plasma televisions. The dust can find itself immersed in the charged gases and the charged particles stick to the dust making the dust charged. The charged dust attracts onto its surface other charges from the surrounding plasma helping grow molecules on the dust surface.”

The precursors to life in the atmosphere of a brown dwarf? Now we’re really pushing the definition of ‘habitable zone.’ The Stark paper is “Electrostatic activation of prebiotic chemistry in substellar atmospheres,” accepted at The International Journal of Astrobiology (preprint). The McMahon paper is “Circumstellar habitable zones for deep terrestrial biospheres,” Planetary and Space Science Vol. 85 (September, 2013), pp. 312-318 (abstract).


The Plumes of Ceres

The MACH-11 program (Measurements of 11 Asteroids and Comets Using Herschel) uses data from the European Space Agency’s space-based Herschel observatory to look at small bodies that are targeted by our spacecraft. With the Dawn mission on its way to Ceres, the Herschel data have now revealed the existence of water vapor on the dwarf planet. To my knowledge, this is the first time water vapor has been detected in an asteroid, or I should say, an object that used to be considered an asteroid before the International Astronomical Union decided to re-classify it because of its large size.

Herschel ran out of coolant in the spring of 2013, but not before making a series of observations of Ceres in the two previous years that show a thin water vapor atmosphere. As with so many of our missions (Kepler comes immediately to mind), we still have plentiful data to look through. In this case, we’ll be examining the increasingly fuzzy distinction between asteroids and comets as we try to figure out precisely what is happening on Ceres. Dawn’s investigations, with arrival in March of 2015 and studies continuing all that year, will obviously provide a much closer look.

Carol Raymond is deputy principal investigator for Dawn at the Jet Propulsion Laboratory:

“We’ve got a spacecraft on the way to Ceres, so we don’t have to wait long before getting more context on this intriguing result, right from the source itself. Dawn will map the geology and chemistry of the surface in high resolution, revealing the processes that drive the outgassing activity.”


Image: Dwarf planet Ceres is located in the main asteroid belt, between the orbits of Mars and Jupiter, as illustrated in this artist’s conception. Observations by the Herschel space observatory between 2011 and 2013 have revealed the first unambiguous detection of water vapor around an object in the asteroid belt. Credit: JPL/Caltech.

Finding water vapor is not totally unexpected, given the long-standing assumption that Ceres contains a mantle of ice in its interior, one that may include as much as 200 million cubic kilometers of water, more than the amount of fresh water on Earth. Herschel’s work in the far infrared is what it took to make the call, complicated by the fact that the signature proved to be elusive. This JPL news release notes the emerging view that Ceres releases water vapor in plumes at about 6 kilograms per second when orbiting closer to the Sun, while remaining frozen tight when further out.

Thomas Müller (Max Planck Institute for Extraterrestrial Physics) discusses the finding in an MPE release:

“The intensity of the water line is linked to certain dark regions on the surface; these are either warmer areas or craters where an impact has exposed some layers of ice deeper down. Moreover, we were able to track how the amount of water changes along the asteroid’s way around the sun: if it passes nearer to the sun the water signature increases and then decreases again in the more remote sections of its orbit.”

Dawn will have specific targets to examine when it arrives thanks to the Herschel work, because the space observatory was able to see how the strength of the water vapor signature varied over time as Ceres spun on its axis. Those dark areas that Müller talks about had also been identified by earlier Hubble observations, and those of ground-based telescopes as well. Clearly, we’ll count on Dawn to teach us a lot more about them and how they operate, just as we used Cassini to investigate the geyser activity on Enceladus. The plumes of Ceres are going to get plenty of attention in the year ahead.


HD 142527: An Unusual Circumstellar Disk

Conventional models of planet formation involve core accretion, where dust grains accumulate into protoplanets whose subsequent collisions and interactions produce planets, or gravitational instability, involving a rapid collapse from dense disk debris into a planetary core. But how far from the parent star does planet formation occur? The more we learn about protoplanetary disks, the more questions individual systems pose, as illustrated by the discovery highlighted today.

I’m looking at the image of a young star called HD 142527 in the constellation Lupus, some 450 light years from Earth. The T Tauri star, some five million years old, is thought to be of about two solar masses. A team of Japanese astronomers using observations from the Atacama Large Millimeter/submillimeter Array (ALMA) has found an asymmetric ring of dust that appears, based on the density of dust in the densest part of the ring, to be producing planets. A previously discovered inner disk is confirmed by this work, but the kicker is that there appears to be a possible planet formation region about 160 AU out, five times the distance between Neptune and the Sun.

The paper on this finding notes that “…the pile-up of disk material beyond 100 AU is quite surprising in the classical scenario of planet formation.” Munetake Momose, a team member and a professor at Ibaraki University, adds this:

“Seeing the site of planet formation directly is one of the most important goals for ALMA. Our observations successfully located a unique candidate in an unexpectedly distant place from the central star. I believe that ALMA will bring us more surprising results.”


Image: Dust and gas disk around HD 142527. The dust and gas distributions observed by ALMA are shown in red and green, respectively. A near-infrared image taken by the NAOJ Subaru telescope is shown in blue. The image clearly shows that the dust is concentrated in the northern (upper) part of the disk. The circle in the image shows the position of the dust concentration, in which planets are thought to be formed. Credit: ALMA (ESO/NAOJ/NRAO), NAOJ, Fukagawa et al.

This news release from the National Astronomical Observatory of Japan points out that near-infrared observations cannot penetrate the innermost part of the dense region of a protoplanetary disk — near-infrared light is easily absorbed by large amounts of dust. ALMA’s millimeter and submillimeter wavelength observations — these wavelengths are poorly absorbed by dust — allow better resolution of the inner part of the disk. The ring around HD 142527 shows one side that is thirty times brighter than the other. Misato Fukagawa leads the team:

“The brightest part in submillimeter wave is located far from the central star, and the distance is comparable to five times the distance between the Sun and the Neptune. I have never seen such a bright knot in such a distant position. This strong submillimeter emission can be interpreted as an indication that large amount of material is accumulated in this position. When a sufficient amount of material is accumulated, planets or comets can be formed here. To investigate this possibility, we measured the amount of material.”

The researchers, from Osaka University and Ibaraki University, estimate based on the submillimeter emission strength that the dense region under investigation is massive enough to produce giant planets more massive than Jupiter through gravitational instability. But a high enough density of dust in the same region could readily produce smaller rocky planets or the cores of gas giants through core accretion processes. It’s worth noting that a 2013 paper has examined planet formation close to this star; the new work extends to the outer disk.

So HD 142527 seems a promising place for follow-up work, an opportunity to observe critical aspects of the planet formation scenario at considerable distances from the host star. Most ring-like disks under observation have proven to be smaller in mass, lacking the intensity of the brightness fluctuations found around this star, not to mention the disk’s striking asymmetry. New measurements of the gas in this disk using ALMA are being undertaken as the team tries to resolve which of the planet formation processes is taking place around the star.

The paper is Fukugawa et al., “Local Enhancement of Surface Density in the Protoplanetary Ring Surrounding HD 142527,” published in Publications of the Astronomical Society of Japan, on December 25th, 2013 (preprint). The 2013 paper on this system is Casassus et al., “Flows of gas through a protoplanetary gap,” Nature 493 (2 January 2013), pp. 191-194 (abstract).


A Brown Dwarf Benchmark

Couple the Keck I 10-meter telescope on Mauna Kea with HIRES (the High-Resolution Echelle Spectrometer) and you get extremely high spectral resolution, making the combination a proven champion at finding planets around other stars. But it was when Justin Crepp (University of Notre Dame) and team followed up seventeen years of HIRES measurements with new observations using NIRC2 (the Near-Infrared Camera, second generation), mounted on the Keck II telescope with adaptive optics, that a nearby brown dwarf could be directly imaged.

HD 19467 B is a T-dwarf more than 100,000 times fainter than its host, a nearby star whose distance (roughly 101 light years) is well established. The team believes the discovery will allow scientists to establish benchmarks that will help define objects with masses between stars and planets. Says Crepp:

“This object is old and cold and will ultimately garner much attention as one of the most well-studied and scrutinized brown dwarfs detected to date. With continued follow-up observations, we can use it as a laboratory to test theoretical atmospheric models. Eventually we want to directly image and acquire the spectrum of Earth-like planets. Then, from the spectrum, we should be able to tell what the planet is made out of, what its mass is, radius, age, etc., basically all relevant physical properties.”


Image: Direct image detection of a rare brown dwarf companion taken at Keck Observatory. Stellar speckles have been removed using PSF subtraction [used to study faint features around bright objects]. The companion is 100,000 times fainter than its host star in the K-band. Credit: Crepp et al./ 2014 APJ.

The work grows out of TRENDS (TaRgetting bENchmark-objects with Doppler Spectroscopy), a high-contrast imaging survey using adaptive optics to target older objects orbiting nearby stars. Imaging surveys like these have in general targeted young stars, but TRENDS focuses on older targets for which the existence of an unseen companion has been suggested by earlier radial velocity data. The paper on this work notes that our ability to see these older, fainter objects is improving with recent advances in high-contrast imaging techniques and hardware.

TRENDS looks for faint companions and thus far has uncovered a number of high mass ratio binary stars, a triple star system (HD 8375) and a white dwarf companion orbiting HD 114174. But here’s why Crepp speaks of using this brown dwarf discovery as a model. From the paper:

By connecting the properties of directly imaged companions to that of their primary star (such as metallicity and age), these objects serve as useful test subjects for theoretical models of cool dwarf atmospheres… Further, the combination of Doppler observations and high-contrast imaging constrains the companion mass and orbit, essential information that brown dwarfs discovered in the field or at wide separations by seeing-limited instruments do not provide.

I learned from the paper that while many nearby brown dwarfs have been discovered by surveys scanning large areas of the sky at optical, near-infrared and mid-infrared wavelengths, only a few are members of multiple systems. Even these are at large separations from the host star given that the glare of the primary makes it so difficult to see ultra-cold dwarfs in closer orbits. The significance of HD 19467 B, then, is that this is the first directly imaged T-dwarf orbiting a Sun-like star with a measured Doppler acceleration, meaning it will be among the first to have a dynamically measured mass. As studies continue, what Crepp and team have found should turn out to be an important benchmark in the investigation of how brown dwarfs evolve.

The paper is Crepp at al., “The TRENDS High-Contrast Imaging Survey. V. Discovery of an Old and Cold Benchmark T-dwarf Orbiting the Nearby G-star HD 19467,” The Astrophysical Journal Vol. 781, No. 1 (2014), p. 29 (abstract / preprint).


Waking Up Rosetta

In the first post of 2014, I wrote about what the following year — 2015 — would bring, the New Horizons flyby of Pluto/Charon as well as the arrival of the Dawn spacecraft at Ceres, a fascinating object with a possible internal ocean. But let’s not forget about the European Space Agency’s Rosetta spacecraft, which is now nearing the end of a decade-long journey to comet 67P/Churyumov-Gerasimenko. The spacecraft is scheduled to awake from a two-year stretch in sleep mode today, with arrival at the comet’s core in November. The orbiter will operate there until the end of 2015.

We’ve had missions to comets before, many of them discussed in these pages, but none as ambitious as this one. Rosetta’s Philae lander will attempt a landing on the comet in November while the orbiter will continue tracking it as the comet is transformed by its approach to the Sun into an erupting, churning mass of ice and dust. With gravity about a thousand times less than that of Earth, this is a tricky object to land on, but the visual rewards should be great, according to Michael Combi (University of Michigan), a co-investigator on several instruments aboard the craft:

“On the lander, there’s a camera that can look straight down like you’re standing up and looking at the ground. Then there’s a panoramic camera that can look out and see a picture of the horizon. It’ll be fun to see what this landscape looks like. It’ll be like standing on a comet.”


Image: An artist’s interpretation of the Rosetta mission lander, named Philae, on the core of comet 67P/Churyumov-Gerasimenko. It’s expected to land in November. Credit: ESA / AOES Medialab.

Rosetta’s investigations will be numerous, and many bear directly on issues we routinely discuss here. We’ve been looking, for example, at Pekka Janhunen’s concept of an electric sail that would ride the solar wind — a stream of charged particles flowing outward from the Sun — to distant destinations in the Solar System, reaching perhaps 100 kilometers per second. Rosetta will be studying the interactions of the solar wind with cometary gases to learn more about the composition of the charged particles and help us better understand solar storms.

That’s the kind of space ‘weather’ an electric sail would confront as it makes its long journey to system’s edge. This University of Michigan news release notes that the solar wind travels more slowly from the area of the Sun’s equator, but moves much faster at higher latitudes. Comets pass through a wide range of solar wind conditions and thus offer an ideal way to study the phenomenon. Accurate control and navigation of future craft riding the solar wind will depend upon our understanding of its turbulent interactions.

Comets are also useful in teaching us about the origin and evolution of the Solar System, as they were present in the nebula from which the system grew and have been orbiting far from the Sun ever since. We still have much to learn about their role in delivering water to Earth’s oceans and possibly organic materials. Combi adds: “People use the analogy that it’s been in the freezer for the past 4.5 million years and brought in for convenient study. So we’re looking as much as we can at the way the way the solar system was 4.5 billion years ago.”

This recent NASA news release offers information about the three instruments the agency has contributed to the ESA mission:

  • An ultraviolet spectrometer called Alice, which analyzes the ultraviolet part of the spectrum, examining gases in the coma and tail to measure the water, carbon monoxide and carbon dioxide it finds there, along with other readings on the surface composition of the nucleus;
  • The Microwave Instrument for Rosetta Orbiter, combining a spectrometer and radiometer to read temperatures and identify chemicals on the comet’s surface and the dust and ices around it. This will be a key instrument in tracking changes as the comet approaches the Sun;
  • The Ion and Electron Sensor, used in characterizing the plasma environment of the comet and the interactions of the solar wind with the comet’s gases.


Image: An artist’s view of Rosetta, the European Space Agency’s cometary probe. The spacecraft is covered with dark thermal insulation in order to retain its warmth while venturing into the coldness of the outer solar system, beyond Mars orbit. Credit: ESA.

The European Space Agency’s Rosetta page is here, a place you’ll want to bookmark as the year progresses and Rosetta moves ever closer to its encounter with 67P/Churyumov-Gerasimenko. Thus far the spacecraft has made three Earth flybys and one flyby of Mars as it established its trajectory to the comet, with encounters with asteroids Steins and Lutetia along the way. Assuming all is going well, the spacecraft’s star trackers are now warming up, a six hour process, and adjustments will soon be made to its orientation to keep Rosetta’s solar arrays facing directly toward the Sun. Follow @ESA_Rosetta for confirmation that the wake-up procedure is complete, probably between 1730 and 1830 UTC. Keep an eye on this page for live video updates.