Astronomers, from National Astronomical Observatories of Chinese Academy of Sciences (NAOC), Shanghai Astronomical Observatory of CAS and Nanjing University, revealed a wobbly and flared Milky Way disk based on LAMOST-Gaia data, which updates our understanding about the disk.
The Milky Way is a typical disk galaxy. In the classical view of the Milky Way, the disk is symmetric and flat on the whole, like a pancake. Stars in the disk rotate around the Galactic center, with mean radial and vertical velocities to be zero.
With the help of huge observational data provided by large survey projects in recent years, more and more details hidden in the Milky Way disk have become apparent, casting doubts on the traditional imagination about our host galaxy.
“The huge number of spectra obtained by LAMOST and the high-precision astrometric data released by Gaia provide a golden opportunity for re-exploring the disk structure,” said Prof. ZHAO Gang, the corresponding author of the work.
Led by Dr. DING Pingjie, the first author of the work, the researchers selected ~490,000 K-type giants from LAMOST DR8 and Gaia EDR3 as tracers. They found that in the spatial range of Galactocentric radius of 5-15 kpc and 3 kpc above and below the Galactic plane, vertical wobbles existed in the three-dimensional mean velocities.
Contrary to the classical picture that stellar motions keep symmetric about the Galactic plane, the K-type giants below the plane are found to rotate faster than those above. Meanwhile, inner disk stars migrate radially toward the outer disk, while outer disk stars exhibit alternate inward and outward radial motions, with velocities highly dependent on the vertical distance.
Moreover, a contraction-like breathing mode and an upward bending mode dominate the vertical motion of the outer disk. By comparison, there is only a weak rarefaction-like breathing mode and a trivial downward bending mode in the inner disk.
The researchers further revisited the nature of the disk flaring by estimating the scaleheight of the disk with the measured velocity ellipsoid. They found that the scaleheight increased obviously with the increasing Galactocentric radius, leading to a flaring feature. The results also indicated that north and south disks had consistent flaring strength. Similar flaring structures were also detected in some external galaxies, which means that a flared disk could be a common phenomenon in disk galaxies.
The wobbly disk probed by the LAMOST-Gaia K-type giants could be attributed to the perturbations produced by both inplane substructures (including the spiral arms and the central bar) and long-lived external perturbers (such as a satellite galaxy). The mechanism behind the flaring structure still remains mysterious.
“This study provides high-quality clues to the Galactic structure and evolution. Continuous observations will help to tell a more insightful story of the Milky Way,” said XUE Xiangxiang, the second author of the paper
A research team led by researchers at the Astrobiology Center and the University of Tokyo has observed low-temperature “ultra-short-period planets” with an orbital period of less than one day by observing with the Subaru Telescope’s near-infrared spectrometer IRD. It was discovered around a star and revealed that its internal composition consists mainly of iron and rock. The planets (TOI-1634b and TOI-1685b) found around the two low-temperature stars are both equivalent to Super-Earth (Note 1), which is about 1.5-2 times the size of the Earth, and in particular, TOI-1634b It is one of the terrestrial planets with the largest radius (1.8 Earth radius) and mass (10 Earth mass) among the ultra-short-period planets found so far. Planets of this size are on the border between rock and gas giants, especially around low-temperature stars, so how many planets have a “one year” less than the length of a day on Earth? It can be said that the most valuable celestial body was discovered in investigating whether it was formed.
Observations have revealed that about 1% of extrasolar planets (exoplanets) are planets with an orbital period of less than one day (ultra-short-period planets). It is thought that ultra-short-period planets formed in outer orbits may have moved to inner orbits due to interactions with other planets, etc., in order to understand the formation of various planets. , A rare and important celestial body.
Most of the ultra-short-period planets observed so far are small planets with a radius of 1.5 times or less that of the Earth, and it is known that their internal composition is similar to that of the Earth, which is mainly composed of iron and rocks. However, most of the ultra-short-period planets scrutinized in this way are known only around sun-like stars (solar stars), and there are only a few observations around low-temperature, small-mass stars. is. Low-temperature stars are known to have multiple small planets, so ultra-short-period planets may also be present. A closer look at the frequency and characteristics of ultrashort-period planets around low-temperature stars is expected to give a general understanding of the origins of ultra-short-period planets.
The research team focused on two low-temperature stars TOI-1634 and TOI-1685 with transit planet candidates (Note 2) detected by NASA’s Transiting Exoplanet Exploration Satellite “TESS”. .. The mass of these stars is only about half that of the Sun. Independent analysis of TESS data and follow-up observation of transit using the MuSCAT series of multicolor simultaneous imaging cameras (Note 3), followed by spectroscopic observation using the Subaru Telescope’s infrared spectroscope IRD (InfraRed Doppler). Did. The IRD is a spectroscope that accurately measures the radial velocity of a star (radial velocity), and is a unique observation device optimized for observing low-temperature stars that appear brighter with infrared rays than visible light.
As a result of detailed analysis of the radial velocities observed by IRD, the actual ultra-short-period planets around TOI-1634 and TOI-1685 were 0.989 days (TOI-1634b) and 0.669 days (TOI-1685b), respectively. It was confirmed that it revolves in the cycle of. Furthermore, from the amplitude of the change in radial velocity, it became clear that TOI-1634b and TOI-1685b have about 10 times and about 3.4 times the mass of the earth, respectively (Note 4). When the composition of the planet was theoretically estimated based on this planet mass and the planet radius (TOI-1634b is about 1.8 earth radius, TOI-1685b is about 1.5 earth radius) obtained from transit observation, which planet It was found that, like the earth, has an internal composition mainly centered on iron and rocks (Fig. 2). Two planetary systems have been discovered in which Super-Earth, which has a composition similar to that of the Earth, revolves in the immediate vicinity of low-temperature, small-mass stars.
TOI-1634b is one of the planets with the largest radius and mass among the ultra-short-period planets confirmed to have an internal composition similar to that of the Earth, and such planets are around stars that are much lighter than the Sun. It is very interesting to find in. From the “mass-radius” relationship (Fig. 2), it was also found that there is no thick hydrogen atmosphere on both planets. On both planets, where no protoplanetary atmosphere of gas from the protoplanetary disk is left, a secondary atmosphere of gas released by the planets may be formed. It is also an interesting observation target for studying how the atmosphere of terrestrial planets that orbit the immediate vicinity of stars evolves.
Both planetary systems are located relatively close to the Earth about 100 light-years, and are particularly bright among low-temperature stars with ultra-short-period planets, making them promising observation candidates for next-generation telescopes. The lead author of the paper, Assistant Professor Teruyuki Hirano (National Institute of Natural Sciences, Astrobiology Center / National Institute of Natural Sciences, Hawaii Observatory) said, “In the future, we will observe the planetary system found in this research with the James Webb Space Telescope (JWST). By investigating the planetary atmosphere and detailed orbits, it is expected that the origin of the still mysterious ultra-short-period planets will be elucidated. Also, the planetary candidate celestial bodies identified by TESS will be intensively tracked by IRD. The observing project is still underway and many unique planets should be identified in the IRD in a year or two, “he said.
(Note 1) “Super Earth” is a planet larger than the Earth, and refers to an exoplanet whose mass is about 10 times or less that of the Earth and whose diameter is about 2 times or less that of the Earth. Since there are no planets of such weight and size in the solar system, observations of exoplanets have revealed that such planets exist for the first time.
(Note 2) “Transit” is a phenomenon in which a star appears to be dark periodically because the planet passes in front of the star. The exoplanet system in which transit is observed is called the transit planetary system. In transit exploration such as TESS, many transit-like dimmings are detected by large-scale photometric monitor observations, including false detections by “eclipsing binaries”. It is confirmed that the “transit planet candidate” detected by TESS is a real transit planet for the first time by performing follow-up observations using other telescopes.
(Note 3) Multicolor simultaneous imaging cameras mounted on the 188 cm telescope in Okayama Prefecture, the 1.52 m telescope at the Teide Observatory in Tenerife, Spain, and the 2 m telescope at the Haleakala Observatory in Maui, USA, MuSCAT, MuSCAT2 , MuSCAT3 was used for follow-up observation of transit. For all planets, this follow-up observation accurately determined parameters such as the orbital period and planetary radius that were tentatively obtained by TESS.
(Note 4) If there are planets around the star, the star will fluctuate slightly due to the influence of the planet’s gravity. The radial velocity method captures this fluctuation as a periodic change in the radial velocity of a star. The larger the mass of the planet, the larger the amplitude of the change in radial velocity. The masses of the two planets found were determined by follow-up observations by the IRD.
Featured image: An image illustration comparing the sizes of the terrestrial planets discovered in this study. TOI-1685b is 1.5 times the diameter of the earth and TOI-1684b is 1.8 times the diameter. Both planets are surrounded by stars that are cooler than the Sun, so they are illuminated by reddish light. (Credit: National Institute of Natural Sciences Astrobiology Center)
A rogue planet is an interstellar object of planetary mass without a host planetary system. As they freely roam around space, could they be fertile nurseries for life?
A Florida Tech scientist believes it’s possible based on extensive research he has undertaken over the past several years.
In research highlighted this summer in Discover Magazine, university astrobiologist Manasvi Lingam (along with Harvard researcher Avi Loeb) studied how life might survive on a rogue planet via oceans prevalent underneath a thick layer of ice. The cold of interstellar space would be too much for the oceans to remain entirely liquid, but the researchers believe any putative biospheres would be protected from the cold via the ice layer, and the planet’s core would heat the planet from the inside. Underneath the ice would potentially exist Earth-like oceans that could support life.
The possibilities for rogue planets facilitating life are of deep interest to Lingam as more planets are being discovered. He noted that for every solar system discovered (each of which contains a handful of terrestrial planets), there are approximately 30-40 rogue planets traveling in the cold expanses of interstellar space. The nearest exoplanet to Earth is therefore expected to be one of these rogue planets.
“We normally think of planets bound to stars, such as Mars, that could support life, but in reality, these types of life-supporting planets could just be floating out there in the vast void of space with rich biospheres,” he said.
The next steps in the research are to do experiments on Earth to ascertain under what extreme conditions life could survive, such as low temperatures or low pressure. A way of doing this is to analyze microbes that would not need sunlight, thereby building on previous research that has conclusively shown that more microbes exist that don’t require sunlight than those that do. Another direction that merits future research is to look at rogue planets as they enter our solar system and research the planet’s conditions to see if it would facilitate life.
Lingam noted that technology would have to advance by only a modest amount to make traveling to these planets – if they are in our solar system – easier. He has published a paper on this subject, detailing how missions to these interstellar interlopers are feasible. And this subject is also covered in his recent graduate-level textbook on astrobiology, Life in the Cosmos, published by Harvard University Press in July 2021.
“You might be able to get to a rogue planet in a few decades, and, rather than looking for other planets around other stars, this might be the best chance to study these planets,” Lingam said. “Through a combination of gravity assists and suitable propulsion systems, you could reach the rogue planet in 20 years or so. Once you have a probe on the surface, you can beam the data back and it would probably take a few months to learn what it looks like on the surface.”
New data analysis has found that the sunlight filtering through Venus’ clouds could support Earth-like photosynthesis in the cloud layers and that chemical conditions are potentially amenable to the growth of microorganisms.
Biochemistry Professor Rakesh Mogul is the lead author of the study, Potential for Phototrophy in Venus’ Clouds, published online this weekin the journal Astrobiology’s October 2021 special issue focused on the possible suitability of Venus’ clouds for microbial life, and constraints that may prohibit life.
According to Mogul and his team, which includes Michael Pasillas (’21, M.S.), photosynthesis could occur round-the-clock in Venus’ clouds with the middle and lower clouds receiving solar energy similar to the Earth’s surface. Much like on Earth, hypothetical phototrophs in Venus’ clouds would have access to solar energy during the day.
In a fascinating twist, the team found that photosynthesis may continue through the night due to thermal or infrared energy originating from the surface and the atmosphere. In this habitat, light energy would be available from both above and below the clouds, which could provide photosynthetic microorganisms ample opportunities to diversify across the cloud layers. Both the solar and thermal radiation in Venus’ clouds possess wavelengths of light that can be absorbed by the photosynthetic pigments found on Earth.
The study also found that after filtering through the Venusian atmosphere, scattering and absorption scrubs the sunlight of much of the ultraviolet radiation (UV) that is harmful to life, providing a benefit like Earth’s ozone layer.
Yeon Joo Lee, a co-author of the study, used a radiative transfer model to show that the present-day middle and lower cloud layers above Venus receive significantly less UV, 80-90% less flux in the UV-A when compared to Earth’s surface, and are essentially depleted of radiation in the UV-B and UV-C, which represent the most harmful components of the UV.
To gauge the nighttime photosynthetic potential via Venus’ thermal energy, Mogul and his team compared the photon fluxes rising from Venus’ hot atmosphere and surface to the photon fluxes measured within low-light phototrophic habitats on Earth – hydrothermal vents in the East Pacific Rise, where geothermal emissions are reported to support phototrophy at depths of 2400 meters, and the Black Sea, where solar powered phototrophs are found at depths of 120 meters. These comparisons showed that photon fluxes from Venus’ atmosphere and surface exceed the fluxes measured in these low-light phototrophic environments on Earth.
While a recent report by Hallsworth et al. 2021, concluded that Venus’ clouds were too dry to support terrestrial life, Mogul and his team found that the chemical conditions of Venus’ clouds could be partly composed of neutralized forms of sulfuric acid, such as ammonium bisulfate. These chemical conditions would exhibit dramatically higher water activities when compared to Hallsworth’s calculations and much lower acidities when compared to current models for Venus.
“Our study provides tangible support for the potential for phototrophy and/or chemotrophy by microorganisms in Venus’ clouds,” said Mogul. “The acidity and water activity levels potentially fall within an acceptable range for microbial growth on Earth, while the constant illumination with limited UV suggests that Venus’ clouds could be hospitable for life. We believe that Venus’ clouds would make a great target for habitability or life detection missions, like those currently planned for Mars and Europa.”
Pasillas, who recently graduated with a M.S. focused on chemical education from the Chemistry & Biochemistry Department, began his work on the study in a graduate seminar course (CHM 5500) taught by Mogul. Students in the class worked on small literature projects and were offered the opportunity to continue with the research. Pasillas ultimately worked on the assessments of acidity and included the work in his thesis. He is currently teaching chemistry at Mt. San Antonio College.
The co-authors of the study are Sanjay S. Limaye (University of Wisconsin, Madison), Yeon Joo Lee (Technische Universitat Berlin, Berlin, Germany) and Pasillas (M.S., chemistry ‘21).
Featured image: Night on Venus in Infrared from Orbiting Akatsuki. Source: ISAS, JAXA
New telescope will see planetary neighbors’ atmospheres
hen the world’s most powerful telescope launches into space this year, scientists will learn whether Earth-sized planets in our ‘solar neighborhood’ have a key prerequisite for life — an atmosphere.
These planets orbit an M-dwarf, the smallest and most common type of star in the galaxy. Scientists do not currently know how common it is for Earth-like planets around this type of star to have characteristics that would make them habitable.
“As a starting place, it is important to know whether small, rocky planets orbiting M-dwarfs have atmospheres,” said Daria Pidhorodetska, a doctoral student in UC Riverside’s Department of Earth and Planetary Sciences. “If so, it opens up our search for life outside our solar system.”
To help fill this gap in understanding, Pidhorodetska and her team studied whether the soon-to-launch James Webb Space Telescope, or the currently-in-orbit Hubble Space Telescope, are capable of detecting atmospheres on these planets. They also modeled the types of atmospheres likely to be found, if they exist, and how they could be distinguished from each other. The study has now been published in the Astronomical Journal.
Study co-authors include astrobiologists Edward Schwieterman and Stephen Kane from UCR, as well as scientists from Johns Hopkins University, NASA’s Goddard Space Flight Center, Cornell University and the University of Chicago.
The star at the center of the study is an M-dwarf called L 98-59, which measures only 8% of our sun’s mass. Though small, it is only 35 light years from Earth. It’s brightness and relative closeness make it an ideal target for observation.
Shortly after they form, M-dwarfs go through a phase in which they can shine two orders of magnitude brighter than normal. Strong ultraviolet radiation during this phase has the potential to dry out their orbiting planets, evaporating any water from the surface and destroying many gases in the atmosphere.
“We wanted to know if the ablation was complete in the case of the two rocky planets, or if those terrestrial worlds were able to replenish their atmospheres,” Pidhorodetska said.
The researchers modeled four different atmospheric scenarios: one in which the L 98-59 worlds are dominated by water, one in which the atmosphere is mainly composed of hydrogen, a Venus-like carbon dioxide atmosphere, and one in which the hydrogen in the atmosphere escaped into space, leaving behind only oxygen and ozone.
They found that the two telescopes could offer complementary information using transit observations, which measure a dip in light that occurs as a planet passes in front of its star. The L 98-59 planets are much closer to their star than Earth is to the sun. They complete their orbits in less than a week, making transit observations by telescope faster and more cost effective than observing other systems in which the planets are farther from their stars.
“It would only take a few transits with Hubble to detect or rule out a hydrogen- or steam-dominated atmosphere without clouds,” Schwieterman said. “With as few as 20 transits, Webb would allow us to characterize gases in heavy carbon dioxide or oxygen-dominated atmospheres.”
Of the four atmospheric scenarios the researchers considered, Pidhorodetska said the dried-out oxygen-dominated atmosphere is the most likely.
“The amount of radiation these planets are getting at that distance from the star is intense,” she said.
Though they may not have atmospheres that lend themselves to life today, these planets can offer an important glimpse into what might happen to Earth under different conditions, and what might be possible on Earth-like worlds elsewhere in the galaxy.
The L 98-59 system was only discovered in 2019, and Pidhorodetska said she is excited to get more information about it when Webb is launched later this year.
“We’re on the precipice of revealing the secrets of a star system that was hidden until very recently,” Pidhorodetska said.
In the recent paper, Postnov and colleagues proposed an idea to detect electromagnetic signal from annihilation in outer layers of antistars. Their idea is to search for antistars in the Galaxy through X-rays in the ∼ (1–10) keV energy band. The reason is, prior to annihilation, protons and antiprotons could form atomic-type excited bound states (‘protonium’, Pn), similar to 𝑒+𝑒¯-positronium (Ps) atoms, and in the process of de-excitation of protonium, an antistar could emit not only ∼ 100-MeV gamma-rays but a noticeable flux of X-rays with energies in the keV range. Their study recently appeared in Arxiv.
Antistars are objects that could have form from smaller high baryonic number (HBB). They were created in the very early universe after the QCD phase transition at 𝑇 ∼ 100 MeV & should also populate the galactic halo. Such stars are not only too old, but also they are moving very fast, and have a highly unusual chemical content. Present observations also favored the possibility of their existence.
Now, Postnov and colleagues explored the possibility that, when antistars interact with interstellar medium (ISM) gas it can give rise to excited protonium atoms. Formation of these atoms takes place most effectively during interaction of protons with neutral (or molecular) antimatter. This can happen if an antistar has a noticeable wind mass-loss.
“These (protonium) atoms rapidly cascade down to low levels prior to annihilation giving rise to a series of narrow lines which can be associated with the hadronic annihilation gamma-ray emission.”
— wrote authors of the study.
They have also shown that these protonium atoms cascade to the 2p-state producing mostly L (Balmer) 3d-2p X-rays around ∼ 1.7 keV line before the 𝑝𝑝¯ hadronic annihilation.
While, antistars formed in higher HBBs should have an enhanced helium abundance. Therefore, the 4.86 keV M (4-3) and 11.13 L (3-2) narrow X-ray lines from cascade transitions in ⁴He𝑝¯ atoms can also be associated with gamma-rays from hadronic annihilations.
“These lines are interesting from the observational point of view because the protonium 3d-2p transition line energy 1.73 keV is close to the Si K-shell complex lines, which could hamper its disentangling from the background.”
— wrote authors of the study.
Finally, it has been suggested, these lines can be probed in dedicated observations by forthcoming sensitive X-ray spectroscopic missions XRISM and Athena and in wide-field X-ray surveys like SRG/eROSITA all-sky survey.
Reference: Bondar et al., “X-ray signature of antistars in the Galaxy”, pp. 1-10, 2021. arXiv:2109.12699
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New study reveals that early galaxies have no fuel, and something is stopping them from refilling the tank
Early massive galaxies—those that formed in the three billion years following the Big Bang —should have contained large amounts of cold hydrogen gas, the fuel required to make stars. But scientists observing the early Universe with the Atacama Large Millimeter/submillimeter Array (ALMA) and the Hubble Space Telescope have spotted something strange: half a dozen early massive galaxies that ran out of fuel. The results of the research are published today in Nature.
Known as “quenched” galaxies—or galaxies that have shut down star formation—the six galaxies selected for observation from the REsolving QUIEscent Magnified galaxies at high redshift, or the REQUIEM survey, are inconsistent with what astronomers expect of the early Universe.
“The most massive galaxies in the Universe lived fast and furious, creating their stars in a remarkably short amount of time. Gas, the fuel of star formation, should be plentiful at these early times in the Universe,” said Kate Whitaker, lead author on the study, and assistant professor of astronomy at the University of Massachusetts, Amherst. “We originally believed that these quenched galaxies hit the brakes just a few billion years after the Big Bang. In our new research, we’ve concluded that early galaxies didn’t actually put the brakes on, but rather, they were running on empty.”
To better understand how the galaxies formed and died, the team observed them using Hubble, which revealed details about the stars residing in the galaxies. Concurrent observations with ALMA revealed the galaxies’ continuum emission—a tracer of dust—at millimeter wavelengths, allowing the team to infer the amount of gas in the galaxies. The use of the two telescopes is by careful design, as the purpose of REQUIEM is to use strong gravitational lensing as a natural telescope to observe dormant galaxies with higher spatial resolution. This, in turn, gives scientists a clear view of galaxies’ internal goings-on, a task often impossible with those running on empty.
“If a galaxy isn’t making many new stars it gets very faint very fast so it is difficult or impossible to observe them in detail with any individual telescope. REQUIEM solves this by studying galaxies that are gravitationally lensed, meaning their light gets stretched and magnified as it bends and warps around other galaxies much closer to the Milky Way,” said Justin Spilker, a co-author on the new study, and a NASA Hubble postdoctoral fellow at the University of Texas at Austin. “In this way, gravitational lensing, combined with the resolving power and sensitivity of Hubble and ALMA, acts as a natural telescope and makes these dying galaxies appear bigger and brighter than they are in reality, allowing us to see what’s going on and what isn’t.”
The new observations showed that the cessation of star formation in the six target galaxies was not caused by a sudden inefficiency in the conversion of cold gas to stars. Instead, it was the result of the depletion or removal of the gas reservoirs in the galaxies. “We don’t yet understand why this happens, but possible explanations could be that either the primary gas supply fueling the galaxy is cut off, or perhaps a supermassive black hole is injecting energy that keeps the gas in the galaxy hot,” said Christina Williams, an astronomer at the University of Arizona and co-author on the research. “Essentially, this means that the galaxies are unable to refill the fuel tank, and thus, unable to restart the engine on star production.”
The study also represents a number of important firsts in the measurement of early massive galaxies, synthesizing information that will guide future studies of the early Universe for years to come. “These are the first measurements of the cold dust continuum of distant dormant galaxies, and in fact, the first measurements of this kind outside the local Universe,” said Whitaker, adding that the new study has allowed scientists to see how much gas individual dead galaxies have. “We were able to probe the fuel of star formation in these early massive galaxies deep enough to take the first measurements of the gas tank reading, giving us a critically missing viewpoint of the cold gas properties of these galaxies.”
Although the team now knows that these galaxies are running on empty and that something is keeping them from refilling the tank and from forming new stars, the study represents just the first in a series of inquiries into what made early massive galaxies go, or not. “We still have so much to learn about why the most massive galaxies formed so early in the Universe and why they shut down their star formation when so much cold gas was readily available to them,” said Whitaker. “The mere fact that these massive beasts of the cosmos formed 100 billion stars within about a billion years and then suddenly shut down their star formation is a mystery we would all love to solve, and REQUIEM has provided the first clue.”
Featured image: Credit: ALMA (ESO/NAOJ/NRAO)/S. Dagnello (NRAO), STScI, K. Whitaker et al.
An international team of researchers has used seasonal variations to identify likely sub-surface deposits of water ice in the temperate regions of Mars where it would be easiest for future human explorers to survive. The results are being presented this week by Dr Germán Martínez at the European Planetary Science Conference (EPSC) 2021.
Using data from NASA’s Mars Odyssey, which has spent almost 20 years orbiting the Red Planet, Martínez and his colleagues have identified two areas of particular interest: Hellas Planitia and Utopia Rupes, respectively in the southern and northern hemisphere. Seasonal variations in levels of hydrogen detected suggests that significant quantities water ice can be found in the metre or so below the surface in these regions.
Martínez, of the Lunar and Planetary Institute, said: ‘Data from Mars Odyssey’s Neutron Spectrometer showed signs of hydrogen beneath the surface Mars from mid to equatorial latitudes, but we still had the challenge of working out whether this is in the form of water ice, which can readily be used as a resource, or locked away in mineral salts or in soil grains and minerals. This is where the seasonal variation provides an important clue. As the coldest ground temperatures occur at the same time as the largest observed increase in hydrogen content, it suggests that water ice is forming in the shallow subsurface of these regions during the fall and winter seasons, and then sublimating into gas during the warm season of each hemisphere.’
Water ice in the shallow subsurface has been found in plentiful supply at the poles. However, the frigid temperatures and the limited solar light make polar regions a hostile environment for human exploration. The areas from equatorial to mid latitudes are much more hospitable for both humans and robotic rovers, but only deeper reservoirs of water ice have been detected to date, and these are hard to reach.
To survive on Mars, astronauts would need to rely on resources already available in-situ, as sending regular supplies across the 55 million kilometres between Earth and Mars at their closest point is not an option. As liquid water is not available in the cold and arid Martian environment, ice is a vital resource. Water will not only be essential for life-support of the explorers, or the growth of plants and food, but could also be broken down into oxygen and hydrogen for use as rocket fuel.
Two other regions are rich in hydrogen: Tharsis Montes and the Medusae Fossae Formation. However, these do not display seasonal variations and appear to be the less accessible forms of water.
‘Definitely, those regions too are interesting for future missions,’ added Martínez. ‘What we plan to do now for them or Hellas Planitia and Utopia Rupes, is to study their mineralogy with other instruments in the hope of spotting types of rock altered by water. Such areas would be ideal candidates for robotic missions, including sample return ones, as the ingredients for rocket fuel would be available there too.’
Featured image: Global map of Mars with overlaid topography indicating areas with significant seasonal variations in hydrogen content during northern spring (top) and fall (bottom). Green (red) represents increase (decrease) in hydrogen content. The areas highlighted in orange are Hellas Planitia in the southern hemisphere, and Utopia Rupes in the northern hemisphere. These are the only extended regions undergoing a significant variation throughout the Martian year. Credit: G. Martínez.
What doesn’t stick comes around: Using machine learning and simulations of giant impacts, researchers at the Lunar and Planetary Laboratory found that the planets residing in the inner solar system were likely born from repeated hit-and-run collisions, challenging conventional models of planet formation.
Planet formation – the process by which neat, round, distinct planets form from a roiling, swirling cloud of rugged asteroids and mini planets – was likely even messier and more complicated than most scientists would care to admit, according to new research led by researchers at the University of Arizona Lunar and Planetary Laboratory.
The findings challenge the conventional view, in which collisions between smaller building blocks cause them to stick together and, over time, repeated collisions accrete new material to the growing baby planet.
Instead, the authors propose and demonstrate evidence for a novel “hit-and-run-return” scenario, in which pre-planetary bodies spent a good part of their journey through the inner solar system crashing into and ricocheting off of each other, before running into each other again at a later time. Having been slowed down by their first collision, they would be more likely to stick together the next time. Picture a game of billiards, with the balls coming to rest, as opposed to pelting a snowman with snowballs, and you get the idea.
The research is published in two reports appearing in the Sept. 23 issue of The Planetary Science Journal, with one focusing on Venus and Earth, and the other on Earth’s moon. Central to both publications, according to the author team, which was led by planetary sciences and LPL professor Erik Asphaug, is the largely unrecognized point that giant impacts are not the efficient mergers scientists believed them to be.
“We find that most giant impacts, even relatively ‘slow’ ones, are hit-and-runs. This means that for two planets to merge, you usually first have to slow them down in a hit-and-run collision,” Asphaug said. “To think of giant impacts, for instance the formation of the moon, as a singular event is probably wrong. More likely it took two collisions in a row.”
One implication is that Venus and Earth would have had very different experiences in their growth as planets, despite being immediate neighbors in the inner solar system. In this paper, led by Alexandre Emsenhuber, who did this work during a postdoctoral fellowship in Asphaug’s lab and is now at Ludwig Maximilian University in Munich, the young Earth would have served to slow down interloping planetary bodies, making them ultimately more likely to collide with and stick to Venus.
“We think that during solar system formation, the early Earth acted like a vanguard for Venus,” Emsenhuber said.
The solar system is what scientists call a gravity well, the concept behind a popular attraction at science exhibits. Visitors toss a coin into a funnel-shaped gravity well, and then watch their cash complete several orbits before it drops into the center hole. The closer a planet is to the sun, the stronger the gravitation experienced by planets. That’s why the inner planets of the solar system on which these studies were focused – Mercury, Venus, Earth and Mars – orbit the sun faster than, say, Jupiter, Saturn and Neptune. As a result, the closer an object ventures to the sun, the more likely it is to stay there.
So when an interloping planet hit the Earth, it was less likely to stick to Earth, and instead more likely to end up at Venus, Asphaug explained.
“The Earth acts as a shield, providing a first stop against these impacting planets,” he said. “More likely than not, a planet that bounces off of Earth is going to hit Venus and merge with it.”
Emsenhuber uses the analogy of a ball bouncing down a staircase to illustrate the idea of what drives the vanguard effect: A body coming in from the outer solar system is like a ball bouncing down a set of stairs, with each bounce representing a collision with another body.
“Along the way, the ball loses energy, and you’ll find it will always bounce downstairs, never upstairs,” he said. “Because of that, the body cannot leave the inner solar system anymore. You generally only go downstairs, toward Venus, and an impactor that collides with Venus is pretty happy staying in the inner solar system, so at some point it is going to hit Venus again.”
Earth has no such vanguard to slow down its interloping planets. This leads to a difference between the two similar-sized planets that conventional theories cannot explain, the authors argue.
“The prevailing idea has been that it doesn’t really matter if planets collide and don’t merge right away, because they are going to run into each other again at some point and merge then,” Emsenhuber said. “But that is not what we find. We find they end up more frequently becoming part of Venus, instead of returning back to Earth. It’s easier to go from Earth to Venus than the other way around.”
To track all these planetary orbits and collisions, and ultimately their mergers, the team used machine learning to obtain predictive models from 3D simulations of giant impacts. The team then used these data to rapidly compute the orbital evolution, including hit-and-run and merging collisions, to simulate terrestrial planet formation over the course of 100 million years. In the second paper, the authors propose and demonstrate their hit-and-run-return scenario for the moon’s formation, recognizing the primary problems with the standard giant impact model.
“The standard model for the moon requires a very slow collision, relatively speaking,” Asphaug said, “and it creates a moon that is composed mostly of the impacting planet, not the proto-Earth, which is a major problem since the moon has an isotopic chemistry almost identical to Earth.”
In the team’s new scenario, a roughly Mars-sized protoplanet hits the Earth, as in the standard model, but is a bit faster so it keeps going. It returns in about 1 million years for a giant impact that looks a lot like the standard model.
“The double impact mixes things up much more than a single event,” Asphaug said, “which could explain the isotopic similarity of Earth and moon, and also how the second, slow, merging collision would have happened in the first place.”
The researchers think the resulting asymmetry in how the planets were put together points the way to future studies addressing the diversity of terrestrial planets. For example, we don’t understand how Earth ended up with a magnetic field that is much stronger than that of Venus, or why Venus has no moon.
Their research indicates systematic differences in dynamics and composition, according to Asphaug.
“In our view, Earth would have accreted most of its material from collisions that were head-on hits, or else slower than those experienced by Venus,” he said. “Collisions into the Earth that were more oblique and higher velocity would have preferentially ended up on Venus.”
This would create a bias in which, for example, protoplanets from the outer solar system, at higher velocity, would have preferentially accreted to Venus instead of Earth. In short, Venus could be composed of material that was harder for the Earth to get ahold of.
“You would think that Earth is made up more of material from the outer system because it is closer to the outer solar system than Venus. But actually, with Earth in this vanguard role, it makes it actually more likely for Venus to accrete outer solar system material,” Asphaug said.
The co-authors on the two papers are Saverio Cambioni and Stephen R. Schwartz at the Lunar and Planetary Laboratory and Travis S. J. Gabriel at Arizona State University in Tempe, Arizona.
Featured image: Artist’s illustration of two massive objects colliding.NASA/JPL-Caltech