Centauri Dreams

The Moon’s farside used to be a convenient setting for wondrous things. After all, no one had ever seen it, setting the imagination free to insert everything from paradisaical getaways (think Shangri-La in space) to secret technologies or alien civilizations. The Soviet Luna 3 image of 1959 took the bloom off that particular rose, but we also learned through this and subsequent missions that farside really does have its differences from the familiar face we see. More craters, for one thing, and fewer of the dark plains we call maria, or ‘seas.’

We can throw in measurements made by the GRAIL mission (the Gravity Recovery and Interior Laboratory) in 2012. GRAIL was a NASA Discovery-class mission that performed gravitational field mapping of the Moon as a way of examining its internal structure, a set of two probes that worked by analyzing measured changes in distance between the two craft as small as one micron. We wound up with a map of our satellite’s gravitational field that led to deeper understanding of its crust, its thermal evolution and its subsurface structure.

Here again we find variation between farside and what we see from Earth. The Moon’s crust is thicker and contains an additional layer on farside. In fact, the farside crust is on the order of 20 kilometers thicker than nearside, and GRAIL’s remote sensing data tell us that this extra crustal material is composed of silicate minerals, magmas, and rocks which are relatively high in the heavier elements. A collision between a dwarf planet and the Moon that occurred after the Moon had already formed a solid crust is now emerging as one explanation for these differences.

Image: Artist’s depiction of a collision between two planetary bodies. New research suggests the stark difference between the Moon’s heavily-cratered farside and the lower-lying open basins of the nearside were caused by a wayward dwarf planet colliding with the Moon in the early history of the solar system. Credit: NASA/JPL-Caltech.

The research presenting the theory is being led by Meng Hua Zhu of the Space Science Institute at Macau University of Science and Technology. Zhu and colleagues are using GRAIL data as fodder for computer simulations that work through impact scenarios for the early Moon, some 360 of them, to see whether they can reproduce the crustal variation we see today.

The work has produced a model that fits the bill, involving the impact of an object a bit smaller than Ceres (780 kilometers in diameter) striking the Moon’s nearside at 22,500 kilometers per hour. A second fit to the data is a smaller (720 kilometer) object hitting at 24,500 kilometers per hour. In both cases, material blown off the surface would fall back to bury the farside crust in kilometers of debris, which fits the additional crust layer found by the GRAIL mission.

Wouldn’t we see evidence on the nearside of such a titanic collision? Depending on conditions in the early Moon, the telltale signs may be hard to tease out, but the authors believe their model can explain the farside highlands and the nearside lowlands, noting such nearside features as “a large area of low-Ca pyroxene on the nearside observed by Kaguya mission that was explained to be formed via impact through melting a mixture of crust and mantle materials.” Let me quote more from the paper on this, talking about the formation of a ‘mega basin’ covering most of nearside:

…the mega basin structure may not be the same to the surface expression of a typical basin formed later. It is likely that the morphologies of mega basin were heavily modified and possibly even erased due to high internal temperature of the early Moon (Miljkovic et al., 2017), which easily allowed for isostatic relaxation processes (Freed et al., 2014). Therefore, it is highly possible that remnants of a giant impact on the early Moon may be less pronounced and significantly different from the impact basins formed later. To test whether a giant impact on the nearside is a plausible mechanism for the formation of Moon’s asymmetries, we performed a systematic numerical modeling study and quantified the outcome of such impact events.

Image: The basin-forming process for an impactor 780 kilometers in diameter (with a 200-kilometer diameter of iron core) with an impact velocity of 22,500 kph (14,000 miles per hour). In each panel, the left halves represent the materials used in the model: gabbroic anorthosite (pale green), dunite (blue), and iron (orange) represent the lunar crust, mantle, and core, respectively. The gabbroic anorthosite (pale yellow) also represents the impactor material. The right halves represent the temperature variation during the impact process. The arrows in (C) and (D) represent the local materials that were moved and formed the new crust together with deposits of material that was blasted from the impact. Credit: JGR: Planets/Zhu et al. 2019/AGU.

Simulating the impact scenarios produces variation in post-impact ejecta and crustal thickness that can reproduce the Moon’s current crust in the farside highlands. The giant impact also fits data on isotope variation in potassium, phosphorus and tungsten-182 between the surfaces of the Moon and the Earth, with the authors positing that these elements would have been added to the Moon’s constituents after its formation. The paper suggests that the impact of an 800-900 kilometer-sized body with the Moon is not unlikely given that the Moon received roughly half of the number of impacts as Mars, many in the form of planetesimals from the early system.

Image: The topographic (A), crustal thickness (B), and thorium distribution of the Moon show a dramatic difference between the nearside and farside. The star on the nearside represents the center of the proposed impact basin. The black dashed lines represent the boundary of Imbrium (Im), Orientale (Or), and Apollo (Ap) basin, respectively. Credit: JGR: Planets/Zhu et al. 2019/AGU.

My own interest in this paper is not so much the Moon itself as the fact that our abundant data on our satellite may help us understand the kind of asymmetries between hemispheres that can occur on large objects as the period of planet formation is drawing to its close. All that is part of learning about the origins of our own Solar System as well as conditions in the young systems we are now beginning to measure in circumstellar disks like the one at HD 163296 that I looked at on Friday (see HD 163296: Emerging Insights into Circumstellar Disks).

The paper is Zhu et al., “Are the Moon’s nearside?farside asymmetries the result of a giant impact?” Journal of Geophysicl Research: Planets 20 May 2019 (abstract).

We should be glad to run into the unexpected when doing research, because things we hadn’t foreseen often point to new understanding. That’s certainly the case with infant planetary systems as observed through the circumstellar disks of gas and dust surrounding young stars. ALMA (the Atacama Large Millimeter/submillimeter Array) has been central to the study of such targets. An array of 66 radio telescopes in Chile’s Atacama Desert, the facility works at millimeter and submillimeter wavelengths to provide detailed imaging of emerging systems.

Because it has been revealing a variety of small-scale structures within circumstellar disks, ALMA is giving us insights into planet formation as we observe gaps, rings and spiral arms and their interactions with young planets. This is where the unexpected comes in. For researchers looking at a 5 million year old star called HD 163296 are seeing an unusual amount of dust, more than 300 times the mass of the Earth, despite the detection of at least three proposed planets whose masses range between that of Jupiter and twice that of Uranus.

HD 163296 is a Herbig Ae/Be star, a pre-main-sequence object still embedded in a gas-dust envelope. The star has about twice the mass of the Sun, with a massive disk accounting for approximately one tenth of the Sun’s mass. The disk is about 500 AU wide, making it perhaps twice the outer boundary of our own Kuiper Belt. It’s noteworthy that the disk remains as abundant as it is given the age of the system and its production of at least three planets.

Image: Artist’s impression of protoplanets forming around a young star. Credit: NRAO / AUI / NSF / S. Dagnello.

The conventional model of dust distribution in this environment is migration from the outer regions of the disk inward because of coupling and friction with abundant gas, and the assumption has been that this migration would be stopped by the presence of massive planets. The hypothesized result: Dust piling up outside the orbits of the massive planets, while clearing out inside the orbit of the innermost planet as the dust migrates onto the star.

But the new ALMA observations, in tandem with the researchers’ dynamical simulations of this system and their application of a collisional model to test various outcomes, show a different result. The orbital regions inside the first planet, as well as between the first and second planet, have some of the highest concentrations of dust anywhere in the disk. The new work looks at this dust distribution in terms not only of interactions with gas and the new planets but also with what must be a large population of planetesimals, small objects that provide the building blocks of planet formation.

Diego Turrini is lead author of the study and a researcher at the Institute for Space Astrophysics and Planetary Science (IAPS) of the Italian National Institute of Astrophysics (INAF):

“From the study of the Solar System we know that mature circumstellar disks like HD 163296 are not composed only by gas and dust, but also contain an invisible population of small planetary objects similar to our asteroids and comets. We also know that the appearance of giant planets affects these planetesimals by causing in their orbital evolution a brief but intense spike of dynamical excitation that, while short from the point of view of the long life of a planetary system, can have a duration comparable to the life of circumstellar disks.”

Image: The circumstellar disk surrounding HD 163296 and the system of gaps and rings created by its young giant planets as recently imaged by ALMA (DSHARP Project). Credit: ALMA (ESO/NAOJ/NRAO), S. Dagnello.

Thus the dynamical simulations calculated to show these interactions and test their limits. What emerges is a regime of planetesimal collisions that remains relatively calm until the giant planets reach their ultimate masses. As the planets grow, more and more planetesimals are driven into eccentric, inclined orbits. At this point, planetesimal collisions become intense. We now have an explanation for the renewed dust in the system, and an orbital distribution of that dust markedly different from before, one where dust accumulates in the inner orbital region of the first planet and also forms a ring between the first and second planets.

These findings jibe with the ALMA observations, linking simulations to a living system. Thus Danai Polychroni, co-author of the study and at the time professor at the Universidad de Atacama and adjunct researcher at INAF-IAPS:

“The fast rate at which ALMA is providing new and more detailed data on HD 163296 allowed us to expand our study beyond its original scope. We noticed that many planetesimals are excited to supersonic velocities with respect to the surrounding gas of the disk and can create shocks waves that can heat both planetesimals and gas. While we could not yet model this process in detail, recent observations reported the unexpected presence of CO gas in regions characterized by temperatures where it should be frozen solid and of possible anomalies in the thermal structure of the disk. Both findings can in principle be explained thanks to the presence of these supersonic planetesimals and the shock waves they create.”

Image: The disk of icy planetesimals hidden in HD 163296’s circumstellar disk seen from above and the side. The young giant planets rapidly create a large population of exocomets acting as high-speed projectiles for the other bodies. Credit: D. Turrini (INAF-IAPS).

We’ve learned, then, that the formation of planets in a circumstellar disk can produce dynamical excitation leading to a resurgence in the ratio of dust to gas that halts the decay of the dust by creating second-generation dust grains through high-velocity collisions. This is useful stuff, for we’re observing more and more possible planets emerging in disks in which planetesimals should be widespread. The disk around HD 163296 may be showing us a stage of planet and system evolution that is common, and one we can now look for in a range of young systems.

From the paper:

The collisional production of second-generation dust in circumstellar disks hosting giant planets …likely represents a common evolutionary phase marking the transition from a circumstellar disk dominated by primordial dust to a debris disk dominated by second generation dust. Whether the amount of collisionally produced dust is high enough to produce observable signatures, like our results suggest being the case for HD 163296’s disk…depends on the characteristics of each specific system, first of all the masses of the giant planets and of the planetesimal disk.

The paper is Turrini et al., “Dust-to-gas ratio resurgence in circumstellar disks due to the formation of giant planets: the case of HD 163296,” Astrophysical Journal Vol. 877, No. 1 (23 May 2019). Abstract / Preprint.

A planet labeled NGTS-4b has turned up in a data space where astronomers had not expected it, the so-called ‘Neptunian desert.’ Three times Earth radius and about 20 percent smaller than Neptune, the world was discovered with data from the Next-Generation Transit Survey (NGTS), which specializes in transiting worlds around bright stars, by researchers from the University of Warwick. It turns out to be a scorcher, with temperatures in the range of 1,000 degrees Celsius.

NGTS-4b is 20 times as massive as the Earth, and its orbit takes it around its star, a K-dwarf 920 light years out, every 1.3 days. The planet is getting attention not so much because of what it is but where it is. Lead author Richard West (University of Warwick) comments:

“This planet must be tough – it is right in the zone where we expected Neptune-sized planets could not survive. It is truly remarkable that we found a transiting planet via a star dimming by less than 0.2% – this has never been done before by telescopes on the ground, and it was great to find after working on this project for a year. We are now scouring out data to see if we can see any more planets in the Neptune Desert – perhaps the desert is greener than was once thought.”

Image: An artist’s conception of exoplanet NGTS-4b. Credit: University of Warwick/Mark Garlick.

The paper on NGTS-4b has me thinking about all of our exoplanet detection methods in terms of their strengths and weaknesses, for there are some things that each of them can’t do well. Consider Jupiter-class worlds in orbits comparable to the Solar System’s largest planet. Depending on where the planet was in its orbit, an astronomer might look for a transit of such a world for years and never see it, even if it were the dominant planet in its system.

Even an Earth-sized planet in a habitable zone comparable to the Sun’s is going to take a year or so to complete its orbit, so we’d need several years to confirm it. When the exoplanet hunt began to take off in the energizing months and years after the discovery of 51 Pegasi b, we were finding ‘hot Jupiters’ in unexpectedly close orbits and more than a few of them. Here again observational bias was in play — we found these early on because radial velocity methods could detect these more easily than the tougher, smaller targets further out in the system.

But the Neptunian desert does not seem to be the result of observational bias. What we discover in looking at Kepler statistics is that there are few short-period Neptune-sized worlds, a ‘desert’ that the paper on this work defines as a lack of exoplanets with masses around 0.1 Jupiter mass and orbital periods less than 2-4 days. The reason this isn’t considered observational bias is that such planets should be easy to spot, and we’ve found many Neptunes with longer orbits from missions like Kepler and CoRoT. We may still find, however, that short-period Neptunes produce transits too shallow for most ground-based surveys to detect.

The issue of ‘deserts’ may well remind you of the better known ‘brown dwarf desert,’ which refers to the lack of such objects in orbits closer than 5 AU around solar mass stars. This desert turned up statistically at a time when numerous free-floating brown dwarfs were being found. Explanations include the possibility of brown dwarf migration into the primary star, but we’re a long way from fully understanding migration within a protoplanetary disk. Various formation models produce different outcomes as the investigation of the phenomenon continues.

Meanwhile, as the University of Warwick researchers note, the transit of NGTS-4b is the shallowest ever detected from the ground, giving us the promise of further such discoveries. From the paper:

The discovery of NGTS-4b is a breakthrough for ground-based photometry, the 0.13 ± 0.02 per?cent transit being the shallowest ever detected from a wide-field ground-based photometric survey. It allows us to begin to probe the Neptunian desert and find rare exoplanets that reside in this region of parameter space. In the near future, such key systems will allow us to place constraints on planet formation and evolution models and allow us to better understand the observed distribution of planets. Together with future planet detections by NGTS and TESS we will get a much clearer view on where the borders of the Neptunian desert are and how they depend on stellar parameters.

The scientists offer two possibilities for the survival of this planet in the Neptunian desert, and in particular for the persistence of its atmosphere in these hellish conditions. NGTS-4b may have an unusually high core mass, or it may have migrated in to its current close orbit, probably within the last one million years, so that its atmosphere would still be evaporating. The question would then be whether the ‘desert’ is elsewhere marked by a mechanism stopping such migration.

The paper is West et al., “NGTS-4b: A sub-Neptune transiting in the desert,” Monthly Notices of the Royal Astronomical Society, Volume 486, Issue 4 (July 2019), pp. 5094–5103 (abstract / full text).