When Martians Attack!

Title: Meteorites from Phobos and Deimos at Earth?

Authors: P. Wiegert, M. A. Galiazzo

First Author’s Institiution: Department of Physics and Astronomy, The University of Western Ontario, London, Ontario, N6A 3K7, Canada

Status: Accepted by Planetary and Space Science

 

Well not quite. The martian invaders we are concerned with here are meteorites. The moons of Mars might provide the right conditions to exchange mass with Earth. This might sound a little crazy but its a pretty simple concept. When an impact happens, material is scattered all over the place. If the object causing the impact is traveling fast enough and the conditions are right, Phobos and Deimos would lose material via the impact and this material could very well make it to Earth.

This concept is nothing spectacular. It happens all the time in our solar system but it is particularly interesting here. The speeds at which an object must travel to scatter Mars moon material is much slower than you would think. According to Wiegert and Galiazza, the ejecta speeds are less than 1km/s for both moons. Phobos is 900m/s and Deimos is 600m/s. In comparison, the typical asteroidal speed at Mars is estimated to be at 10km/s. If that seems fast, you’ll be surprised to know that Long-period comets impact at speeds estimated at 40km/s.

This theory is partly driven by the Kaidun meteorite. The Kaidun meteorite that fell in 1980 was found to be largely made up of carbonaceous chondrite material. Spectral analysis of both Phobos and Deimos determine that their surface properties are very similar to outer main-belt D and T type asteroids. These asteroids are know to be made up of carbonaceous chondrite material. Could the weird meteorite that visited us some thirty eight years ago be from one of the moons of Mars? Quite possibly!

How did they figure this out? A trip to mars? Not quite. This is where the beauty of computers comes it to play. Simulations of course! Of course many different factors must be accounted for and simulations are not one hundred percent accurate but its the best that our modern day technology affords us. Simulations were ran for 10 martian years (18.8 Earth years, ~20,000 Phobos periods or ~5,000 Deimos periods). The point of this is to determine the likelihood of particles to escape at certain impact speeds. The faster the impact speed, the more particles there are that are assumed to have escaped. Below we can see a graph of what the simulations look like.

It may seem like a radical idea to say that some of the meteorites that visit us come from the moons of Mars. Is it that radical though? With so much space junk flying around its no surprise that a relatively slow moving object could impact Phobos or Deimos and that impact could send a little present our way in the form a meteorite. The simulations support the idea and the math is sound. Who knows, you could be walking down the street and be suddenly assaulted by a martian moon rock!

Sources: 

Meteorites from Phobos and Deimos at Earth? P. Wiegert, M. A. Galiazzo

https://www.space.com/20413-phobos-deimos-mars-moons.html

Finding a Balance

There is a lot more to habitability on a planet than being in the habitable zone. We don’t need to look beyond our own Solar System for an example. Earth’s neighbors, Venus and Mars, are both in the habitable zone but clearly unable to sustain any form of life. Looking to Earth for clues, as Valencia et. al suggest in their paper Habitability from Tidally Induced Tectonics, we know that many things need to take place beyond the location of a planet for it to be habitable.

One integral feature that makes the Earth habitable is its ability to control its own temperature (without human intervention) through plate tectonics. Plate tectonics are an essential part of the carbon-silicate cycle that has allowed for the Earth to maintain temperate climates for  the last 3-4 billion years. Other factors that we see on Earth and can look for on exoplanets is a rock and sea weathering process that allows CO2 to be stored in the rock and ocean crust. The rock exposure that allows for CO2 to be stored continuously occurs due to persistent erosion and mid ocean ridge production. However, the Earth doesn’t just absorb all the CO2, it has a way pumping CO2 back out into the atmosphere through volcanism and its continuous source of CO2 from the mantle.

Now, with the knowledge of how essential tectonics have been and will be for the Earth’s ability to sustain life, we can ask the pertinent questions applicable to exoplanets: are there any exoplanets that are both in the habitable zone and have a similar temperature control through tectonics? Are these tectonics tidally induced? Yes, potentially, and with exceptions. If we are to look for tidal heating in exoplanets we need to look at planets that are very close to their stars and still in the habitable zone. This is the case with M star systems. However, unlike the Sun, a G type star which brightens over time as hydrogen ignition takes place; M stars, like Trappist-1, get dimmer over time. The Trappist-1 system has three planets that are now in the habitable zone (Trappist-e,f,g), but were not when the star was much younger, brighter and hotter as seen in Figure 4. The issue then arises whether or not these planets were dried up or left with some water and atmosphere after intense photo evaporation that occurs when their star was significantly closer. Recent studies do suggest that the even with intense heat and radiation that some liquid water is in fact left.   

One way to determine if planets in the habitable zone — like in the Trappist system — could possibly have a temperate enough climate to sustain life is to determine whether or not tidally induced tectonics take place on these planets. Simple models show that tidally locked planets may have a built in system to regulate the amount of C02 in the atmosphere that could allow for sustainable surface liquid water. Not through the carbon-silicate cycle, but through continuous volcanism due to tidal heating as seen in Figure 1.

 

A promising example is the volcanic activity induced by tidal heating Jupiter’s moon Io. Although Io is far from the habitable zone it provides clues to what could occur to planets that closely orbit their stars; as is the case with M stars. Io, on a elliptical orbit, feels it’s interior being fluxed as it gets closest to Jupiter and that produces enough energy to yield a partially molten mantle. Is there enough energy to yield a molten mantle on M star planets? Planets with M stars do have a habitable zone close enough so that tidal friction occurs however, other factors, like a smaller core-mass fraction, a basalt solid and depth of water need to be taken into consideration in order to determine whether or not these exoplanets have a molten mantle that produces volcanism and pumps out CO2. Last, but not least, in order to achieve a climate equilibrium similar to Earth there must be the exposure of crust to absorb CO2 to absorb the CO2 being pumped out.

After all is said and done (and many complex equations to determine the viability of the argument not depicted above) what Valencia et. al propose is a climate-controlling feedback system that could or have occurred in 10-100 million year window which could keep liquid water stable for billions of years on M star exoplanets.

Sources: Valencia, Diana, et al. “Habitability From Tidally Induce Tectonics.” Vol. 12, no. 16, ser. 11, 20 Mar. 2018, pp. 1–14. 11 (https://arxiv.org/pdf/1803.07040.pdf) 

Just Keep Swimmming

Since the beginning of time, philosophers and scientists alike have pondered the same enormous question. To this day there is no definitive proof either way, and debates continue on as researchers attempt to answer this query. This age-old question can be simply stated as “are we alone in the universe?” Before discovering other life forms, however, astronomers are trying to discover simply habitable planets. Habitable planets are thought to have Earth-like masses, Earth-like water inventories, orbit stars that are in the main sequence of their life cycles, have CO2 and H20 as their main greenhouse gases, and have a carbonate-silicate cycle. This cycle regulates carbon dioxide between a planet’s interior, surface, and atmosphere.

An informative graphic about the Carbonate-silicate cycle

If a carbonate-silicate cycle regulates things between surface and atmosphere or surface and interior, what do you do with a planet lacking a surface?

Ocean worlds fall under this umbrella. Ocean worlds are exoplanets in which water makes up a significant percentage of the total mass and lack a substantial hydrogen/helium envelope. Often these planets have migrated so close to their star that the entire outer layer of the planet is an ocean! Traditionally defined again is a habitable zone, a circular region surrounding a star wherein rocky planets can have standing water, much like oceans, without them either evaporating or freezing.

The previously-mentioned Carbonate-silicate cycle is crucial when it comes to the stability of the planet’s temperature on its surface, and is thought to only exist in a consistent fashion on planets where at leas 5% of their surface is land.

Does this mean habitability is entirely impossible on these ocean planets, or could there be an alternate way of regulating the carbon dioxide and temperature?

An alternate theory is presented, as Levi et Al. Argues that carbon dioxide can be mobilized between the atmosphere and interior of a world, not needing the surface layer previously crucial to the carbonate-silicate cycle. This is further supported by the observation that sea ice, when full of CO2 clathrate, can sink, due to a CO2 pressure that passes a certain threshold. This subpolar ice takes care of the other function of the carbonate-silicate cycle in that it helps to moderate the climate.

That being said, finding the sweet-spot to create and sustain this subpolar ice is rather challenging and specific. Planets must rotate, at a minimum, three times faster than the Earth does, or the difference between the temperatures of the equator and poles of the planets will not be varied enough to support a subpolar region and a tropical, warmer region. Without these regions, the planet would not have freeze-thaw cycles, and evolution would be nearly impossible, which is further complicated by the already challenging environment and continent-less, deep-oceaned world provides.

A final stipulation is that these planets cannot be tidally locked, suggesting that they will be relatively young planets. Though it seems like the perfect accidental storm must transpire to create a habitable planet, water-rich planets are widely found around stars with low masses, of which there are many in the universe.

As with the philosophers and astronomers inquiring as to our solitary state in the universe, the impact of these ocean planets and their ability to meet these parameters of habitability will take time. While waiting for the answer, though, it doesn’t hurt to hypothesize.

 

Sources:

Click to access 1803.07717.pdf

https://www.google.com/search?q=carbonate+silicate+cycle&safe=active&rlz=1C9BKJA_enUS713US713&hl=en-US&prmd=isnv&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjoh8fOyZfaAhUB22MKHcj-AvYQ_AUIESgB&biw=678&bih=909#imgrc=XblrrohPHZiiwM:

 

Ross 128-b Highlights New Technologies and Techniques in Searching for M Dwarf Exoplanets

Title: A temperate exo-Earth around a quiet M dwarf at 3.4 parsecs*

Authors: X. Bonfils1, N. Astudillo-Defru2, R. Dıaz3,4, J.-M. Almenara2, T. Forveille, F. Bouchy2, X. Delfosse1, C. Lovis2, M. Mayor2, F. Murgas5,6, F. Pepe2, N. C. Santos7,8, D. Segransan2, S. Udry2, and A. Wunsche

First Author’s Institution: Institute of Planetology and Astrophysics, University of Grenoble, Grenoble, France

Status: Accepted by Astronomy & Astrophysics, 26 October 2017

 A team of scientists have announced the discovery of a nearby planet which may possess many highly desired ingredients for life. Ross 128-b, which orbits the M dwarf star “Ross 128” only 11 light-years from our Sun, is a potentially habitable planet that could be close to Earth in size (at a minimum of 1.35 Earth masses), is temperate, and could have oxygen in its atmosphere. For all its potential similarities with Earth, though, Ross 128-b’s discovery highlights not simply the potential to find life on an extrasolar planet, but an opportunity to develop and utilize myriad new techniques and technologies in exoplanet characterization.

Artist interpretation of Ross 128-b and its star. M. Kornmesser / European Southern Observatory

The Advantages of M Dwarfs

In a paper titled A Temperate exo-Earth around a quiet M dwarf at 3.4 parsecs*, the team behind the discovery of Ross 128-b – led by Xavier Bonfils of The Institute of Planetology and Astrophysics in Grenoble, France, — looked to an M dwarf star about twice as old and only 1/5^th the size of our Sun. In doing so, they noted a few such advantages in these systems, which were for years ruled out by many scientists in their search for life on other planets, although they are the most common stars in our galaxy: critical information like the depth of the planet’s transit across it’s star, the star’s reflex motion (its slight, detectable wobble due to an orbiting planet as it moves around its center of gravity) and the contrast-ratio between the star and its orbiting planet are all conditions made more favorable by the relatively low mass and luminosity of M dwarf stars.

HARPS, K2, and ASAS: Confidence in the Data

In the case of Ross 128-b, Bonfils and his team used data from the land-based spectrograph High Accuracy Radial Velocity Planet Searcher (viewed to the side, Fig. 2) dating back to 2005 combined with archive photometry from Kepler’s K2 mission, which observed Ross 128 for 82 days, and from the All Sky Automated Survey, which observed Ross 128 for more than 9 years (viewed below, Fig. 1). The Radial Velocity (RV) method — captured here by HARPS — measured Ross 128’s reflex motion, while the photometry methods focused on establishing its stellar rotation period. The RV data showed a power excess at the period of Ross 128-b (9.9 days) much higher than the expected photon noise level, while establishing the stellar rotation period (about 100 days) allowed them to ensure that the RV detection of 128-b was not a false positive due to Doppler shifts induced by various phenomena originating from the star itself.

Through this data, they became confident in the existence of Ross 128-b; a planet orbiting an extraordinarily quiet M dwarf a mere 3.4 parsecs (about 11 light years) from our Sun. Ross 128 seems not to be plagued by the explosive and atmospherically-destructive solar flares characteristic of other M dwarfs like TRAPPIST-1 and Proxima Cen. This is great news for 128-b, as the planet orbits its star at a mere .049 AU and with a period of only 9.9 days. Because Ross 128 is about 280 times less luminous than our sun, its habitable zone is much closer in. Thus, despite being so close to its star, Ross 128-b receives about 1.38 times the flux of the Earth, causing more excitement among scientists about the potential for this newly discovered planet to harbor life.

Non-Transit Methods: Limitations and Potential Solutions

Bonfils and his team urge caution, as many questions about Ross 128-b still remain. As the planet is not detectable by transit, they do not yet know anything about the composition of its possible atmosphere, and there remains contradictory data regarding whether it lays within its star’s habitable zone or not. Citing these issues, the team has turned to the potential to capture Ross 128-b’s phase curve as the most promising means of resolving its atmospheric composition and discovering its potential for water. The phase curve, which is derived from characterizing an object’s phase angle (e.g. the angle observed between the light bouncing onto and off of an object from an observational perspective), could provide the necessary information about the atmospheric and even surface characteristics of the planet. They estimate that this could be done in a reasonable time frame and at a reasonable cost by taking advantage of the anticipated contrast improvements in the next generation of Extremely Large Telescopes (particularly the ground-based European-ELT currently under construction) to search for biomarkers on the planet.

Despite the challenges inherent in characterizing the atmosphere and surface composition of planets that do not transit, non-transiting planets can be found much closer to our sun, as is the case with Ross 128-b (Ross 128 being one of the 20 closest stars to us). Although our understanding of the habitability of Ross 128-b remains low, it represents an exciting opportunity for astronomers to both explore a close earth-like world with a mature and stable star, and to further develop new theories and techniques regarding exoplanets as we move into the next generation of astronomical detection and imaging technologies.

Citations:

Click to access aa31973-17.pdf

https://en.wikipedia.org/wiki/Ross_128_b

 

Advanced Photoshop Finds Water on Phoebe

Article: Phoebe: a surface dominated by water
(https://arxiv.org/abs/1803.04979)

Authors: Wesley C. Fraser¹ and Michael E. Brown²
1. Queen’s University, Belfast
2. California Institute of Technology

Background

Phoebe is one of Saturn’s (and possibly our Solar System’s) most interesting moons. It was first discovered in August of 1898, by American astronomer William Pickering. When the Cassini spacecraft first arrived at the Saturn system in 2004, its first target was Phoebe. For this reason, the moon has been unusually well-studied for an satellite its size.

Phoebe has an irregular and wide orbit at a distance of 12,952,000 kilometers. The moon is roughly spherical, with a mean radius of only 106.5 kilometers, or about one-sixteenth the radius of our Moon. It completes a full rotation about its axis every 9 hours, and completes a full orbit around Saturn every 18 Earth months. Phoebe’s irregular, elliptical and retrograde orbit is inclined about 175 degrees to Saturn’s equator, and it is one of two moons that does not orbit closely to the plane of Saturn’s equator.¹

Recently, Phoebe’s low albedo (or darkness) and irregular orbit have led scientists to believe it is a captured body, originally from an outer region of the Solar System such as the Kuiper Belt.² Some scientists believe that Phoebe could be a captured Centaur, an intermediate small body found somewhere between the asteroid belt and the Kuiper Belt. Centaurs are commonly believed to be objects dating back to the formation of the Solar System. They are the “building blocks” of the Solar System that never accreted onto a planet. Furthermore, because of Phoebe’s small size, the object may never have heated up enough to change its chemical composition, which must be akin to early Solar System objects. Thus research on Phoebe could teach us an immense amount about our own origins.¹

Why are we back at Phoebe? 

Cassini’s flyby acquired optical imaging with an Imaging Sub System (ISS) and Visible Infrared Mapping Spectrometer (VIMS). The ISS sent back some beautiful pictures (see above), revealing Cassini’s spherical body peppered with large impact basins. With the VIMS observations, on the other hand, only rough compositional mappings have been done. The most notable discovery thus far has been of very variable levels of water-absorption on Phoebe’s surface, leading the authors of this paper to describe the moon simply as an “icy rock”.

But flyby VIMS images can be difficult to decipher. Years after Cassini, Fraser and Brown use what they call an “automatic technique” to correct the geometry of each VIMS flyby image, thereby producing clear and comprehensive compositional maps of Phoebe’s surface. They assert that their imaging technique critically includes “a geometry correction routine that enables pixel-by-pixel mapping of visible and infrared spectral cubes directly onto the Phoebe shape model, even when an image exhibits significant trailing errors.” Fraser and Brown’s analysis allows them to infer new properties of water distributed across Phoebe’s surface.

The actual computerized techniques used are described in Section 2 of the paper, but a few points are of note. Phoebe’s shape is nearly ellipsoidal, so Fraser and Brown used a best-fit ellipsoid as the base of their shape model of the moon. They also assigned albedo levels to each face of the shape model of Phoebe, using an albedo map produced by previous images taken by the Voyager spacecraft. The authors additionally used a technique to count and measure craters on Phoebe’s surface. Albedo and the presence of craters are clearly connected with the level of water absorption and absorption depth on the surface of a body, so these techniques help create the most thorough maps of water on Phoebe’s surface.

Results

The images produced are fascinating. Below is the water-ice absorption on Phoebe’s surface projected onto the shape model. More red sections indicate sections with deeper water.

Below are two other images of a full water absorption depth map at a particular place on Phoebe, with each image showing different levels of water absorption.

What does this mean?

Clearly, these images show that water can be found anywhere on Phoebe’s surface. Crater mapping shows that the regions richest in water are impact basins, where asteroids and other objects are thought to have collided with Phoebe. The iciest areas of the moon are found just beyond the outer edges of its large Jason and South Pole Basins. In general, areas with larger craters have higher levels of water-absorption. The albedo map mentioned earlier shows a positive correlation between water-ice absorption depth and visual albedo, confirming the results of Fraser and Brown’s revision to VIMS.

So what?

Because water is so concentrated around Phoebe’s large impact basins, the impacts that formed those basins must have also affected water distribution by enhancing water absorption on Phoebe’s surface. It is plausible that at one point, early Phoebe had a water-poor surface, with richer layers underneath. Because depth and water absorption is positively correlated on Phoebe, impacts must have exposed these deeper, water-rich layers and increased water absorption around impact basins.

Remember the theories cited earlier? They posited that Phoebe is most likely an escaped Kuiper Belt Object (known as a KBO), or possibly a Centaur. If this is true, the authors argue that KBOs must exhibit a range of water concentrations similar to those observed on Phoebe. Large impacts must also be responsible for enhancing the surface-level water concentrations on KBOs. Because impacts are so stochastic (or random), there must be a lot of variability in the levels of water absorption on KBOs, with more impacted KBOs exhibiting higher water concentrations on their surface. Thus high water absorption levels on KBOs must come from impacts, what the authors refer to as a “dredge-up” of the surface.²

What I think:

Reading this paper really got me thinking: how much water is out there in our Solar System that we just haven’t discovered yet? If it takes giant impacts to uncover water-rich layers on Phoebe, how much water must be beneath its surface? This may seem far-fetched, but have any KBOs or Centaurs been able to heat up enough to have liquid water somewhere inside? If so, could that presence of liquid water support simple forms of life outside of Earth, life from the earliest days of our Solar System?

 

Reference:

https://arxiv.org/abs/1803.04979

Other References

1. Solar System Exploration: NASA Science: In Depth. (2017, December 05). Retrieved March 20, 2018, from https://solarsystem.nasa.gov/moons/saturn-moons/phoebe/in-depth/

The Possibility of Habitable Snowballs

Title: Habitable Snowballs: Generalizing the Habitable Zone

Authors: Adiv Paradise, Kristen Menou, Diana Valencia, Christopher Lee

First Author’s Institution: Department of Astronomy and Astrophysics, University of Toronto, St. George, Toronto, ON M5S 3H4, Canada

Status: Published in arxiv.org, astro – ph.EP – Earth and Planetary Astrophysics

When you think terrestrial habitable zone planets, you usually don’t think snowball planets. A terrestrial habitable zone planet is defined as a terrestrial, telluric, or rocky planet that is composed primarily of silicate rocks or metals. The habitability of earth sized exoplanets is usually determined through climate models. Climate models use a quantitative method to simulate the main of climate like, atmosphere, oceans, land, surface, and ice. Snowball episodes of terrestrial planets are considered to be unable to sustain life. But, the authors of the article point out that there is no evidence of decreased biodiversity during Earth’s two snowball episodes. This means that it is not necessarily the case that very low global temperatures imply snowball conditions everywhere on the planet. This is the key to the current study. Previous research has shown that planets with high obliquity and eccentricity can be partially / temporarily habitable. This makes sense if you think about it. If a planet is highly eccentric, at some point it’s going to be getting more sun that at another, there are more opportunities for temperate weather. In terms of obliquity, this is also the case, at a really oblique angle there is going to be one part of the planet more affected by the sun than the other, this provides an opportunity for temperate weather as well. In the current study, they used an intermediate – complexity GCM (general circulation model) to explore the possibility that small scale melting could occur in snowball planets, making them habitable.

The negative feedback mechanism happening on planets without vascular land plants (Earth has vascular land plants) between temperature and CO₂ causes low outgassing rates and low temperatures. But over time the planet may outgas enough CO₂ to deglaciate. And if weathering were possible during snowball states, there would be small pockets of warm regions in the planet, habitable regions.

The General Circulation Model

The experimenters used PlaSim, a 3-D general circulation model that simulated these snowball climates. They used the same land configuration, obliquity, and eccentricity as Earth in their experiment. They also used a range of insolations (an insolation is the portion of the sun’s output of electromagnetic energy that is received by the Earth at the outermost part of our atmosphere) and CO₂ partial pressures ranging from cold to warm start conditions. They slowly increased the sample resolution at the snowball’s transition points. The models that reached equilibrium in snowball conditions just before they were able to start deglaciation were deemed to be at deglaciation threshold. They mimicked a weathering model similar to a previous studies model. They set the parameters for the precipitation rates in terms of surface temperature. This was the equation they used:

In words, this meant that the weathering increased when the amount of carbon dioxide increased, and the rate at which the weathering happened on the surface also increased. The constant k was set to account for any deviation from Earth’s weathering rate that they were using, and to bring some accountability for inaccurate representation of small scale processes inside the planet.

Findings

One of the Earth – like models (1300 W m^-2 and 24 mbars of CO₂, 1.025 AU, axial tilt), in “July” had 35% of its land surface in the northern hemisphere in temperate conditions. See Figure a (figure below on the left). Figure a shows the levels of weathering, surface temperature, precipitation, and evaporation in this model. All pointing to higher levels of all the variables in the northern hemisphere. This led to the other proven hypothesis that if a planet with an axial tilt had temperate conditions during its summer, a planet with zero obliquity could support temperate conditions all year round. See Figure b(figure below on the right). Figure b shows the average temperature of a planet with zero obliquity hitting towards the middle close to 16 degrees Celsius (60.8 degrees Farenheit).

Implications and Limitations

The experimenters found that their results were sensitive to the geological (glacial) history and the specific dynamics of each of these snowball planets. While they were looking at planets that had specifically the same histories as Earth, because those were the numbers they were plugging in, other planets may not have this same history. They also found that results would be different if the planets were further out of the habitable zone and had higher insolations. Basically, each of these snowball planets’ climates are a big part of the limitations to this study. But, this study does open doors. The experimenters did find that snowball climates do actually feature some temperate weather areas and therefore potential habitable climates. The study also tells us that weathering equilibrium more heavily depends on glaciology, erosive processes, and topography. Through this study, the “habitable planet” definition has been widened, and through that we can look at snowball planets as opportunities, not dead ends.

Citations:

https://arxiv.org/abs/1803.00511

Paradise, A., Menou, K., Valencia, D., & Lee, C. (2018, March 07). Habitable          Snowballs:Generalizing the Habitable Zone. Retrieved March 30, 2018, from https://arxiv.org/abs/1803.00511

 

Did the Moon’s Magnetosphere Overstay its Welcome?

There are many ways in which earth is unique, for one it is home to the only known life in the universe. The hostility of space creates environments that are inhospitable for known life; however, earth is teeming with millions of different species of plants and animals. So why is that? One of the many factors can be attributed to earth’s magnetosphere, which shields life on earth from harmful, positively charged particles in the solar wind.

Unlike earth, the moon does not have a magnetosphere, but this has not always been the case. As of 2017, perplexed scientists discovered that the moon’s magnetosphere survived for at least a billion years past what was originally thought. As a matter of fact, the moon’s magnetic field may have actually endured 2 billion years, a exceptionally long time given the moon’s relative size.

In terrestrial bodies, or those primarily composed of rocky compounds, magnetic spheres are produced when molten metals within the core churn about. A paper titled A two-billion-year history for the lunar dynamo, published by Sonia Tikoo and her team at Rutger’s University, questions the known mechanisms in which the moon could have maintained a magnetic field for longer than expected. Smaller terrestrial bodies, such as the moon, should cool faster as their reduced size means that more energy from the convection cells in the core rises upward and reaches the mantle more quickly. The energy is thus transferred to the surface via conduction, and later radiated off into space. The hastened cooling of smaller bodies via these processes indicates that the moon should have lost its magnetic field much sooner than it did.

To get a better understanding of the moon’s magnetic past, Tikoo and her team sampled the the moon’s ancient magnetic field frozen in rock. As molten rock solidifies, it creates minerals, which align with the the body’s magnetic field at the time of formation. 

In one sample, the research team examined a glassy rock named 15498 (Fig 2), which was retrieved from the moon’s surface during the 1971 Apollo 15 mission. With the help of a magnetometer, an instrument that measures the magnetism of rock samples, Tikoo and her associates determined that he rock formed when the moon had a magnetic field of about 5 microteslas, about 1 billion to 2.5 billion years ago according to its radiogenic ⁴⁰Ar (Fig. 1); however, 3.56 billion years ago the moon’s magnetic field ranged anywhere from 20 to 110 microteslas.

Fig. 1 multi-domain predictions for diffusion of radiogenic ⁴⁰Ar models experienced by a 1-cm-diameter of 15498 resulting from breccia formation between 650 and 3300 Ma, the exposed ⁴⁰Ar levels prove that 15498 formed between 1 – 2.5 billion years ago. Reading ⁴⁰Ar/³⁹Ar levels is a form of radiometric dating, a method that measures the abundance of the radioactive isotope to the decayed product in order to determine the age of the artifact.

The data collected from the rock sample demonstrate that the moon’s magnetic field lasted about a billion years longer what was previously thought, raising the question about other possible dynamos that could be responsible for generating the field. On earth, most of the magnetic field is generated from the convection cells of molten metals, but the intensity of the field is also somewhat compounded by the energy released when the inner core solidifies. Tikoo posits that the moon’s core crystallization, or other processes besides from convection, must also be responsible for the moon’s enduring magnetic field. 

Fig. 2 (A) Chips 15498,274, 15498,282, and 15498,287. (B) Chip 15498,313. (C) Chip 15498,314. The sample contains abundant mare basalt fragments (blue arrows and labels) within a glassy matrix (purple arrows and labels) containing magnetized minerals that help scientists determine moon’s past magnetic life

Understanding how magnetic fields come and go is crucial to discovering potential life beyond earth. Mars once had a magnetic field and water to boot, but it possibly lost both once its magnetosphere dissipated and could no longer shield the planet from incoming radiation, stripping the planet of water and leaving it dry as a bone. Scientists who concern themselves with finding earth-like exoplanets may need to question the presence of magnetic fields as an indication of hospitable environments for life. Comprehending the mechanisms by which these magnetic fields function may crucial for understanding how something as delicate as life can survive in space. 

Citations:

http://advances.sciencemag.org/content/3/8/e1700207

What happened to the moon’s magnetic field? (n.d.). Retrieved March 20, 2018, from https://www.pri.org/stories/2017-09-02/what-happened-moon-s-magnetic-field

Instructions

astrobites is a superb blog run by astronomy graduate students who post short summaries of recent papers on their website. Imitation is the best form of flattery and so we are going to imitate the blog but focus just on planets, hence the name planetbites!

Your task will be to post on this blog page your own summary of a recent (generously defined as the last 5 years) scientific peer-reviewed paper about “planets” either within the Solar System or beyond. By “planets”, I am being quite broad and including anything directly related to a planetary system, such as (but not exclusive to)…

• Exoplanet detection
• Exoplanet characterization
• Moons, rings, trojans and other minor bodies in planetary systems
• Astrobiology
• Habitability
• Effects of stars or environment on planets
• Planetary interiors/dynamics

The only topic off-limits you might think of is a mission concept, since this has too much overlap with our last activity together. If you are unsure if your topic is appropriate, please ask me!

Your blog post should be 500-1000 words, include one or two figures with explanations, and written in the style of a typical astrobites article. (Some) example posts on planet-related stuff over at astrobites:

• https://astrobites.org/2018/01/25/diagnosing-the-amnesia-of-exoplanetary-systems/
• https://astrobites.org/2018/01/05/titanic-exoplanets/
• https://astrobites.org/2017/09/08/a-history-of-water-loss-in-the-trappist-1-exoplanets/
• https://astrobites.org/2017/08/28/lightning-on-exoplanets/
• https://astrobites.org/2017/08/01/detecting-exoplanet-life-in-our-proximity/
• https://astrobites.org/2017/01/13/when-planets-wont-stay-put/

Logistics: you *should* have permission to post to this wordpress. You are free to post and edit as you see fit up to the date of submission 3rd April. At that date, I’ll print the blog off as a PDF so no further edits will affect the grading. The blogs are public so you can delete your post if you wish after this time or edit it to correct small mistakes.

Grading: This exercise is worth 8.5% of your final grade and should be completely independently. It’s OK if multiple people pick the same paper. If you plagiarize a blog summary from elsewhere, then you will receive no credit for this exercise. The grading rubric has been posted on CourseWorks, so please look there. Essentially I am looking for 1) conciseness 2) clear writing 3) teaching/summary quality of paper’s core concepts 4) technical accuracy. It is not expected that you fully understand all of the methods are details used. It is expected that you got what the main point of the paper was and can communicate broadly how the authors came to this conclusion.

Resources:

• Papers are archived at https://arxiv.org/archive/astro-ph/Astrophysics
• Recent papers posted related to exoplanets can be viewed here: https://arxiv.org/list/astro-ph.EP/recent
• Recent exoplanet bibliography maintained independently: http://exoplanet.eu/bibliography/

The Journey Begins

Thanks for joining me!

Good company in a journey makes the way seem shorter. — Izaak Walton