Thursday, January 22, 2015

Lone Planet in Interstellar Space

There appears to be increasing evidence for the existence of isolated planetary-mass objects. These objects are either unbound (i.e. drifting alone in interstellar space) or orbit host stars in widely separated, distant orbits. Recently, Luhman (2014) reported the detection of a nearby brown dwarf with 3 to 10 times the mass of Jupiter and an estimated temperature of roughly 250K (-23°C). Such an object could have formed in-situ as stars do or it could be a planet that got ejected from its natal planetary system.

Using a novel technique, Freeman et al. (2014) reported the detection of an isolated planetary-mass object after re-analysis of observational data from the gravitational microlensing event MOA-2011-BLG-274. Gravitational microlensing occurs when the gravitational field of a foreground object (lens) bends and magnifies light from a background object (source). For this to happen, the observer, foreground object and background object must be in near perfect alignment.


MOA-2011-BLG-274 was previously reported by Choi et al. (2012). One indication the lens could be a planetary-mass object is the short duration of the gravitational microlensing event. At any given time, the observed amount of magnification and the time of peak magnification vary slightly from point to point on the Earth’s surface. This is known as the terrestrial parallax effect and it is particular pronounced for short duration gravitational microlensing events.

Because MOA-2011-BLG-274 was monitored from a number of separate locations on Earth, the terrestrial parallax effect could be measured by Freeman et al. (2014), making it possible to pin down the distance and hence the mass of the lens, identified in this case as MOA-2011-BLG-274L with the suffix “L”. The best fit to the data shows MOA-2011-BLG-274L is 0.8 ± 0.3 times the mass of Jupiter and located 2.6 ± 0.8 thousand light years away.

Also, the data excludes the presence of a host star out to ~40 AU (i.e. 40 times the distance of Earth from the Sun). MOA-2011-BLG-274L is either an isolated planetary-mass object drifting in the depths of interstellar space or a planet in a far-flung orbit around a host star, sufficiently far out that the host star has no effect on the gravitational microlensing light curve.

References:
- Luhman (2014), “Discovery of a ~250 K Brown Dwarf at 2 pc from the Sun”, ApJ 786 L18
- Freeman et al. (2014), “Can the masses of isolated planetary-mass gravitational lenses be measured by terrestrial parallax?”, arXiv:1412.1546 [astro-ph.EP]
- Choi et al. (2012), “Characterizing lenses and lensed stars of high-magnification single-lens gravitational microlensing events with lenses passes over source stars”, arXiv:1111.4032 [astro-ph.SR]

Monday, January 19, 2015

A Hot Giant Planet as Black as Charcoal

Kepler-423b is a hot-Jupiter which zips around its host star every 2.7 days in a close-in orbit. On each orbit, Kepler-423b crosses in front of its host star, causing an observable dip in the star’s brightness, thereby allowing the planet’s size to be measured. Kepler-423b is estimated to have 60 percent the mass of Jupiter and 1.2 times its diameter. The host star of Kepler-423b is a very old Sun-like star with an estimated age of 11 ± 2 billion years.


The orbit of Kepler-423b also periodically brings it behind its host star in what is known as a secondary eclipse. During secondary eclipse, the star occults any emission from the planet, leading to a slight decrease in the combined star-planet flux. For Kepler-423b, the drop in the combined star-planet flux is remarkably small. This indicates that Kepler-423b is a very dark object (i.e. low reflectivity). Kepler-423b reflects less than 4 percent of the light it receives from its host star, making the planet as reflective as charcoal. If Kepler-423b were more reflective, the secondary eclipse signature would be stronger since the planet would contribute more to the combined star-planet flux.

Even though Kepler-423b is as black as charcoal, the planet’s dayside would still appear utterly glaring. The extreme closeness to its host star means that the insolation the planet receives is a few hundred times more intense than what Earth receives from the Sun. Reflecting just a few percent of that intense insolation would still create nothing short of a blazing glare. Temperatures on the planet’s dayside can get as high as ~2000K.

Reference:
Gandolfi et al. (2015), “Kepler-423b: a half-Jupiter mass planet transiting a very old solar-like star”, arXiv:1409.8245 [astro-ph.EP]

Sunday, January 18, 2015

Strange Quark Matter (SQM) Planets

At extreme densities, normal matter may exist in the form of strange quark matter (SQM). A consequence of this hypothesis is the existence of SQM stars which are hard to distinguish from neutron stars. SQM stars and neutron stars are both ultra-compact stars that measure only several kilometres across but can contain 1 to 2 times the Sun’s mass. Unlike neutron stars which have a minimum mass limit, SQM stars can have arbitrarily small masses. Since SQM is stable in bulk, planetary-mass clumps of SQM can exist (i.e. SQM planets). The detection of SQM planets would be very useful for testing the SQM hypothesis.


Gravitational waves are ripples in the curvature of spacetime that propagate outward from their source. Sources of detectable gravitational waves include binary systems composed of compact objects such as white dwarfs, neutron stars or black holes. A more tightly bound binary system emits stronger gravitational waves. For a normal matter planet orbiting an ultra-compact star, the emitted gravitational wave power is negligible (i.e. non-detectable) since the planet cannot come close enough to the star without being tidally disrupted.

However, things become very different for a SQM planet orbiting a SQM star. Due to its extreme compactness, a SQM planet can spiral very close to its host SQM star without being tidally disrupted. Such a compact system then becomes very efficient in producing strong gravitational waves. Upcoming gravitational wave detectors such as Advanced LIGO and the Einstein Telescope can detect gravitational waves arising from the in-spiral of a SQM planet into its host SQM star.

There are a number of possible mechanisms that can result in the formation of a SQM planet. One mechanism involves newly-born SQM stars that are very hot and exceedingly turbulent. The strong turbulence can eject planetary-mass clumps of SQM. If these clumps remain gravitationally bound, SQM planets are produced. A SQM planet with 1/10th the mass of Jupiter (i.e. 32 Earth masses) would measure only ~1000m in diameter.

Reference:
J. J. Geng, Y. F. Huang, T. Lu (2015), “Coalescence of Strange-Quark Planets with Strange Stars: a New Kind of Sources for Gravitational Wave Bursts”, arXiv:1501.02122 [astro-ph.HE]

Saturday, January 17, 2015

Trio of Super-Earths Circling a Nearby Star

Following the failure of two of its four reaction wheels in May 2013, NASA’s planet-hunting Kepler space telescope was ingeniously repurposed for a new mission plan named K2. On 18 December 2014, it was announced that the K2 mission had detected its first confirmed exoplanet, a super-Earth or mini-Neptune designated HIP 116454 b (Vanderburg et al. 2014). The detection of HIP 116454 b was based on data collected during the testing run to prepare the space telescope for the nominal K2 mission.

Using data from the K2 mission covering 30 May to 21 August 2014, Crosseld et al. (2015) report the discovery of three super-Earths orbiting a nearby M dwarf star slightly larger than half the size of the Sun. This M dwarf star is designated EPIC 201367065 and it lies at a relatively nearby distance of about 150 light years. The three planets are 2.1, 1.7 and 1.5 times the size of Earth, and take 10.1, 24.6 and 44.6 days to circle the host star, respectively. From their sizes, the planets appear to span the range between rock-dominated “Earths/super-Earths” and lower-density “mini-Neptunes” with substantial volatile content.

Figure 1: Artist’s impression of a rocky planet.

Figure 2: Transit light curves of the three planets around EPIC 201367065. Top: Vertical ticks indicate the location of each planet’s transit. Bottom: Phase-folded photometry and best-fit light curves for each planet. Crosseld et al. (2015)

The outermost of the three planets, designated as planet “d”, is 1.5 times the size of Earth and receives somewhat more insolation from its host star than Earth receives from the Sun. This places the planet at the inner edge of the habitable zone where temperatures might be cool enough for the planet to support Earth-like conditions, and possibly life. If temperatures turn out to be too hot, then the planet is more likely a super-Venus with conditions too inhospitable for life.

All three planets around EPIC 201367065 are probably tidally-locked. This means the same side of each planet always faces the host star (i.e. permanent day and night sides). Since M dwarf stars are much cooler and fainter than the Sun, a planet around an M dwarf star needs to be much closer-in to get a similar amount of insolation as Earth gets from the Sun. As a result, the planet experiences stronger tidal forces from the host star and is expected to be tidally-locked.

A study by Yang et al. (2013) suggests that a tidally-locked planet can support thick water clouds on its dayside as the high amount of insolation drives strong dayside convection. The water clouds reflect away incoming radiation from the host star, leading to lower surface temperatures. If such a stabilizing cloud feedback mechanism works on planet “d”, then its surface temperatures can be lower than would otherwise be, allowing the planet to support cool and clement conditions even though its receives somewhat more insolation than Earth.

The trio of planets around EPIC 201367065 can be conveniently studied in further detail as the host star is relatively bright and nearby. Both the Hubble Space Telescope (HST) and the upcoming James Webb Space Telescope (JWST) have the capabilities to reveal more about this planetary system. Such a discovery so early in the mission shows the ubiquity of planetary systems and that the K2 mission will extend the legacy of Kepler for years to come.

References:
- Vanderburg et al. (2014), “Characterizing K2 Planet Discoveries: A super-Earth transiting the bright K-dwarf HIP 116454”, arXiv:1412.5674 [astro-ph.EP]
- Crosseld et al. (2015), “A nearby M star with three transiting super-Earths discovered by K2”, arXiv:1501.03798 [astro-ph.EP]
- Yang et al. (2013), “Stabilizing Cloud Feedback Dramatically Expands the Habitable Zone of Tidally Locked Planets”, arXiv:1307.0515 [astro-ph.EP]

Saturday, November 15, 2014

Trapping Water on the Night Side of a Planet

The presence of liquid water on a planet’s surface is a prerequisite for habitability. Planets circling within the habitable zone of red dwarf stars are believed to be tidally-locked. This is because red dwarf stars are much cooler than stars like the Sun and a planet must be situated much closer in to receive a similar amount of warmth Earth gets from the Sun. As a result, strong tidal interaction between the planet and its host star quickly drives the planet into a tidally-locked configuration. A tidally-locked planet always presents the same hemisphere towards its host star, resulting in a permanent day side and a permanent night side.

On the planet’s permanent night side, large amounts of water can become trapped in kilometres-thick ice sheets. This mechanism is known as water trapping and it can potentially cause the planet’s day side to be depleted of water. The consequence is that the planet becomes less habitable or not habitable at all since it is only on the planet’s day side where photosynthesis is possible. A study by Yang et al. (2014) suggests that water trapping is unlikely to remove all the water from the day side of a tidally-locked planet.


For a planet that is mostly covered by ocean, surface winds transport sea ice toward the day side and ocean currents transport heat toward the night side. As a result, sea ice on the planet’s night side remains thin and water trapping is insignificant. Water trapping starts to become significant on a planet whose water content and continental coverage is similar to Earth’s. Ice sheets with thickness ~1,000m can form on continents located on the planet’s cold night side. The trapping of so much water on the night side creates a large decrease in the planet’s sea level.

Furthermore, if plate tectonics happen to move all continents to the night side, water trapping would become more severe. Nonetheless, if the planet’s geothermal heat flux is similar to Earth’s, the thickness of the ice sheet on the planet’s night side would be limited and complete removal of water from the planet’s dayside is unlikely. For water trapping to remove all water from a planet’s dayside, a combination of special conditions must be met. These conditions include the planet having a geothermal heat flux lower than Earth’s, most of its surface is covered by continents and its surface water content is only ~10 percent of Earth’s.

Reference:
Yang et al. (2014), “Water Trapping on Tidally Locked Terrestrial Planets Requires Special Conditions”, arXiv:1411.0540 [astro-ph.EP]

Thursday, October 30, 2014

A Black Hole Lurks in the Trapezium

The Orion Nebula Cluster (ONC) is a young star cluster whose age is estimated to be less than ~3 million years. Due to its proximity, the ONC is one of the best studied star clusters. At the heart of the ONC is the Trapezium, a tight cluster of several massive OB stars. A study by Subr et al. (2012) suggests that a massive black hole with more than 100 times the mass of the Sun might be lurking in the Trapezium. This is due to the large velocity dispersion observed for the 4 brightest Trapezium stars - Θ1A, Θ1B, Θ1C and Θ1D.

Considering the small number of stars in the sample, such a velocity dispersion measurement is not particularly robust. Nevertheless, the large velocity dispersion indicates there is more mass holding the cluster together than can be accounted for by just the stars. As a result, the presence of a black hole with more than 100 times the Sun’s mass is hypothesized. A black hole of this mass would represent the collapsed remnant of what was once a massive runaway-mass star. At 1300 light years away, this black hole would be the closest known to Earth.

This image shows the heart of the ONC. Clearly visible are the 4 Trapezium stars. These 4 hot and massive stars dominate the core of the ONC. Image credit: Hubble Legacy Archive, Robert Gendler

The ONC was once more compact that it currently is. For instance, the ONC does not contain wide stellar binaries with separations larger than ~1000 AU which would have been disrupted when the cluster was more compact. A consequence of being more compact is that gravitational interactions among massive OB stars would have been frequent enough to cause a fraction of them to be ejected from the cluster and a fraction of them to undergo ‘runaway’ stellar collisions to form a massive runaway-mass star. Subsequently, the massive runaway-mass star collapses directly to form a black hole, which can still continue to experience runaway growth by accreting more mass.

Interestingly, the observed number of massive OB stars in the ONC appears to be fewer than predicted. This is consistant with the loss of massive OB stars through ejection from the cluster and ‘runaway’ stellar collisions, leading to a deficit in massive OB stars. If a massive black hole is indeed lurking in the Trapezium, it probably isn’t growing much at present. The intense radiation from the hottest and most massive stars in the ONC would have driven much of the star-forming gas out of the cluster, causing the cluster to expand in size as there is less mass keeping the cluster together. As the cluster swells, stars collide less frequently and the runaway growth of the black hole slows to a halt.

Reference:
Subr et al. (2012), “Catch me if you can: is there a runaway-mass black hole in the Orion Nebula Cluster?”, arXiv:1209.2114 [astro-ph.GA]

Wednesday, October 29, 2014

Tantalizing Possibility of a Hidden Ocean on Mimas

Mimas is the smallest and innermost member of Saturn’s mid-sized icy moons, a group which includes Enceladus, Tethys, Dione, Rhea and Iapetus. With a diameter of 396 km, Mimas is also one of the smallest objects known in the Solar System whose self-gravitation is sufficiently strong to keep it rounded in shape. Mimas whizzes around Saturn about once every 23 hours. As it goes around Saturn, Mimas wobbles back and forth. This type of motion is called libration.

By carefully analysing images of Mimas taken by NASA’s Cassini spacecraft using a technique known as stereophotogrammetry, one particular component of Mimas’ libration was found to have an amplitude roughly twice as large as predicted. Since this component of libration depends on the interior structure of Mimas, its large value suggests that Mimas has a ‘weird’ interior. A few interior models of Mimas have been proposed to explain the large libration.

An image of Saturn’s moon Mimas taken by the Cassini spacecraft on 13 February 2010. Credit: NASA/JPL/Space Science Institute.

The 140 km wide Herschel impact crater on Mimas makes it resemble the Death Star from the Star Wars franchise. One interior model suggests the presence of a large mass buried beneath the Herschel impact crater, making Mimas more massive on one side. However, this model is inconsistent because the presence of such a large buried mass would have permanently reoriented Mimas such that the Herschel impact crater would face more towards Saturn, which is not the case.

Mimas is basically comprised of a shell of icy material overlying a denser rocky core. A more plausible interior model to explain the large libration involves Mimas having an elongated rocky core. However, such an elongated rocky core is expected to have an effect on the global shape of Mimas. If the icy shell is fully relaxed over the elongated rocky core, then the overall shape of Mimas should appear more elongated, which is not the case. Nevertheless, a low gravity object like Mimas can maintain large internal porosities which can create space for an oddly shaped core without affecting its overall shape.

A more exciting interior model of Mimas suggests that this small icy moon of Saturn might have an internal global ocean of liquid water located 24 to 31 km beneath its battered icy surface. For a small object like Mimas, it is difficult to keep an internal ocean from freezing. Heat generated from the decay of radioactive isotopes in the rocky core of Mimas would easily escape through the icy shell and cause the internal ocean to quickly freeze.

However, such an internal ocean on Mimas can still be kept liquid through heat generated from tidal heating. This is because Mimas’ orbit around Saturn is somewhat eccentric and the eccentricity may even have once been higher. As a result of its eccentric orbit, Mimas is sometimes closer to Saturn and sometimes further away. This causes Mimas to feel a difference in the gravitational pull from Saturn, which has the effect of alternately squeezing and stretching Mimas. Such a flexing motion creates friction in the interior of Mimas and friction generates heat. A tidally heated internal ocean of liquid water on Mimas is not inconceivable. Its neighbour, Enceladus, is known to have an internal ocean of liquid water sustained by tidal heating.

Reference:
Tajeddine et al., “Constraints on Mimas’ interior from Cassini ISS libration measurements”, Science 17 October 2014: Vol. 346 no. 6207 pp. 322-324

Tuesday, October 28, 2014

Polar Ice Deposits on Mercury

NASA’s MESSENGER spacecraft has obtained the first ever optical images showing the presence of water-ice and other frozen volatiles within the permanently shadowed interiors of craters near Mercury’s north pole (Chabot et al., 2014). It may come as a surprise that water-ice is present on Mercury since Mercury is the closest planet to the Sun and surface temperatures at its equatorial regions can soar above 400°C. However, near Mercury’s poles, there are a number of craters whose interiors are permanently shadowed from the Sun. Since Mercury does not have an atmosphere to transport heat around the planet, the permanently shadowed interiors of these craters serve as cold traps where water-ice and other volatiles can remain frozen there.

Figure 1: Mercury.

 Figure 2: Locations of water-ice deposits in the shadowed interiors of craters on Mercury.

Over two decades ago, Earth-based radar observations provided the first indications that water-ice might be present on Mercury’s poles. MESSENGER entered orbit around Mercury on 18 March 2011 and has been observing the planet from orbit ever since. In late 2012, the presence of polar water-ice deposits on Mercury was confirmed by MESSENGER through a combination of observations involving neutron spectrometry (Lawrence et al., 2013), measurements of surface reflectance at the near-infrared wavelength of 1064 nm (Neumann et al., 2013) and thermal modelling (Paige et al., 2013).

The polar deposits of water-ice and other frozen volatiles on Mercury were imaged using the Wide-Angle Camera (WAC) on MESSENGER’s Mercury Dual Imaging System (MDIS). Although the polar deposits never receive direct sunlight, they could still be imaged by taking advantage of the very low levels of sunlight scattered off illuminated crater walls. Images of the permanently shadowed interior of the 112 km wide Prokofiev crater, the largest crater near Mercury’s north pole, show an area with widespread surface water-ice deposits. The area shows up in the WAC images as a region with higher reflectance compared to its surroundings.

Numerous smaller craters cover the floor of Prokofiev crater. The area with surface water-ice deposits within Prokofiev crater has a similar cratered terrain as the neighbouring sunlit surface. This indicates that the water-ice deposits were placed there after the formation of the underlying craters, suggesting that the water-ice deposits were placed there relatively recently instead of billion of years ago. Furthermore, the water-ice deposits appear to be uniform, again implying a recent emplacement. Because if the water-ice deposits were there before impacts excavated the craters, a patchy appearance would result since the craters and their ejecta would have buried parts of the water-ice deposits.

WAC images of other craters with permanently shadowed interiors show areas of lower reflectance believed to be water-ice deposits covered by a thin, overlying layer of dark, organic-rich volatile material. These lower reflectance deposits extend to the edges of the permanently shadowed regions and terminate sharply. The sharp boundaries indicate that the deposits are relatively young since the long process of lateral mixing by impacts has yet to smudge the boundaries. WAC images of surface volatile deposits in Mercury’s polar craters show that these deposits are relatively young. The deposits were either delivered to the planet recently or continuously restored at the surface through an ongoing process.

References:
- Chabot et al., “Images of surface volatiles in Mercury’s polar craters acquired by the MESSENGER spacecraft”, Geology 15 October 2014, v. 42, no. 10
- Lawrence et al., “Evidence for Water Ice Near Mercury’s North Pole from MESSENGER Neutron Spectrometer Measurements”, Science 18 January 2013: Vol. 339 no. 6117 pp. 292-296 DOI: 10.1126/science.1229953
- Neumann et al., “Bright and Dark Polar Deposits on Mercury: Evidence for Surface Volatiles”, Science 18 January 2013: Vol. 339 no. 6117 pp. 296-300 DOI: 10.1126/science.1229764
- Paige et al., “Thermal Stability of Volatiles in the North Polar Region of Mercury”, Science 18 January 2013: Vol. 339 no. 6117 pp. 300-303 DOI: 10.1126/science.1231106

Monday, October 27, 2014

Brown Dwarf in a Distant Orbit around an A-Type Star

Brown dwarfs are substellar objects that populate the gap between the most massive planets and the least massive stars. ζ Del B is a newly discovered brown dwarf around the A-type star ζ Del A. Before this discovery, ζ Del A was simply known as ζ Del without the suffix “A”. ζ Del A is a main-sequence star with 2.5 ± 0.2 times the mass of the Sun and about 50 times the Sun’s luminosity. A main-sequence star is basically a star that is currently generating energy by fusing hydrogen into helium and it is neither at the start nor near the end of its life. The Sun is one example of a main-sequence star.

Figure 1: Artist’s impression of a brown dwarf.

Only a small number of brown dwarfs are known to orbit stars that are significantly more mass and luminous than the Sun. ζ Del B is the latest addition to this short list of brown dwarfs. ζ Del A and its brown dwarf companion, ζ Del B, are both estimated to lie at a distance of roughly 220 light years away. Estimates show that the ζ Del system is 525 ± 125 million years old. Spectroscopic observations of the spectrum of ζ Del B indicate it is an L-type brown dwarf (L5 ± 2) with an effective temperature of 1650 ± 200 K. Brown dwarfs are classified into 4 spectral types - M, L, T and Y. M-type brown dwarfs are the hottest, while Y-type brown dwarfs are the coolest.

Based on its near-infrared brightness, temperature and age, ζ Del B is estimated to be 50 ± 15 times the mass of Jupiter. This gives the brown dwarf a mass ratio of 0.019 ± 0.006 with respect to its stellar companion. Additionally, ζ Del B has a projected separation of 910 ± 14 AU from ζ Del A. Such a projected separation means the orbital period of ζ Del B around ζ Del A is probably on the order of ~10,000 years. ζ Del B is one of the most widely-separated and lowest mass ratio substellar companions known around a main-sequence star.

Figure 2: Estimated mass of ζ Del B based on its near-infrared brightness, temperature and age. De Rosa et al. (2014).

 Figure 3: Mass ratio as a function of separation for brown dwarf companions (blue open circles), directly imaged brown dwarf and planetary companions (red open squares), and brown dwarf and planetary companions detected by radial velocity and transit techniques (black points). ζ Del B (black filled star) is among the most widely separated, lowest mass ratio companions imaged to date. De Rosa et al. (2014).

A number of different scenarios might explain the formation of ζ Del B and other widely-separated substellar companions around main-sequence stars. The formation of ζ Del B at its current location would require an unusually massive circumstellar disk at a large distance around ζ Del A and such a formation scenario is quite unlikely. Nevertheless, it cannot be ruled out that ζ Del B might have formed much closer to ζ Del A before migrating outward due to interactions with the circumstellar disk or with an unseen companion.

The ζ Del system could also have formed from the fragmentation of a single pre-stellar core of gas and dust into two cores, where one core is much more massive than the other. The more massive core collapsed to form ζ Del A, while the less massive core collapsed to form ζ Del B. Such a scenario can produce companions with separations on the order of ~1,000 AU. Another formation scenario, albeit with a low probability of occurring, involves ζ Del B forming independently before being captured by ζ Del A to form a low mass ratio binary.

Reference:
De Rosa et al. (2014), “The VAST Survey - IV. A wide brown dwarf companion to the A3V star ζ Delphini”, arXiv:1410.0005 [astro-ph.SR]

Sunday, October 26, 2014

Adaptation of Antarctic Lichens to Conditions on Mars

Billions of years ago, Mars was a warm and wet planet. Life could have evolved on Mars and then receded to micro-habitats as the planet subsequently became colder and dryer. Present-day micro-habitats for life on Mars can include subterranean aquifers and cracks or fissures in rocks. J.-P. de Vera et al. (2014) conducted a study using the lichen Pleopsidium chlorophanum and found that this Antarctic lichen can adapt, within a span of 34 days, to the conditions expected to be present in the micro-habitats on Mars today.


The sample used in the study was collected from the granites and volcanic rocks of North Victoria Land in Antarctica during the 10th German North Victoria Land Expedition in 2009/2010. Pleopsidium chlorophanum is an extremophile that lives in very cold and dry places. Its native habitat in Antarctica somewhat approximates the conditions on Mars. Pleopsidium chlorophanum is usually found within cracks and fissures in rocks. It can remain metabolically active down to -20°C and can absorb water directly from snow.

To simulate Mars-like environmental conditions, the lichens were placed in the Mars Simulation Chamber (MSC) at the Mars Simulation Facility (MSF) of the DLR Institute of Planetary Research in Berlin. The atmosphere in the MSC was 95 percent carbon dioxide, 4 percent nitrogen and 1 percent oxygen. The pressure was held at 800 Pa, with a diurnal relative humidity cycling of 0.1 to 75 percent and a diurnal temperature cycling of 21°C to -50°C (i.e. similar to the temperatures observed in the equatorial to mid-latitude regions on Mars).

The lichens were embedded within a Mars analogue soil material. In the experiment, 3 samples of lichens were subjected to Mars-like niche conditions (i.e. conditions expected in the micro-habitats on Mars) and another 3 samples of lichens were subjected to the unprotected Mars-like surface conditions for 34 days. For lichens in the unprotected Mars-like surface conditions, they were subjected to much more intense UV irradiation.

Results from the experiment indicated that for Mars-like surface conditions, photosynthetic activity dropped to 18 percent of pre-experiment levels and it was unclear if the lichens remained photosynthetically active at the end of the 34 days. However, lichens that were subjected to Mars-like niche conditions and experienced a much lower radiation dose fared very differently. For these lichens, photosynthetic activity only dropped to 55 percent of pre-experiment levels after the 34 days. In fact, photosynthetic activity at the end of the experiment was 17 percent higher than what was measured for the lichens in their native habitat in Antarctica.

Under simulated Mars-like niche conditions, the lichens appeared to have experienced an initial period of shock lasting ~7 days. Following that, the lichens rapidly adapted. Photosynthetic activity increased over the subsequent days and the increase continued to the end of the experiment. It appears that the lichens were much more sensitive to the intensity of UV irradiation than to other Mars-like parameters such as high carbon dioxide concentration, very low temperatures, extreme humidity fluctuations and very low atmospheric pressure. This study supports the notion that Earthly life can adapt to the present-day conditions on Mars. Furthermore, life which may have originated during the early warm and wet Mars might still survive and thrive in present-day micro-habitats on Mars.

Reference:
J.-P. de Vera et al., “Adaptation of an Antarctic lichen to Martian niche conditions can occur within 34 days”, Planetary and Space Science 98 (2014) 182-190