DPS 54: Days 1 and 2

Editor’s Note: This week we’ll be writing updates on selected events at the 54th Division for Planetary Sciences meeting in London, Ontario, and online. This post covers the first two days of the meeting, and a second post on Friday will cover the remainder of the meeting. The usual posting schedule for AAS Nova will resume on October 10th.

Table of Contents:

Astromaterials Plenary Session

Collecting Samples of Mars: The NASA Perseverance Rover’s Role in Mars Sample Return (Chris Herd)

Chris Herd (University of Alberta) kicked off the first plenary session of DPS 54 with an introduction to how the Perseverance rover will contribute to the goals of the Mars Sample Return program, which plans to deliver samples of the Martian surface to Earth in 2033.

Perseverance rover's travel path

Illustration of the path Perseverance has traveled. Click for high-resolution version. [Slide by Chris Herd]

Herd described Perseverance’s journey from its landing site in Jezero Crater to the edge of the Séítah region, which contains impassible rocks and sand dunes. En route, Perseverance made its first attempt at collecting a sample of rock, but the water-weathered rock disintegrated during the coring process, leaving only atmospheric gases inside the sample tube. After collecting a few more samples, the rover backtracked to the landing site and continued onward to an ancient river delta, a region containing sedimentary rocks formed from silt deposited 3.5–4.0 billion years ago. These sandstone samples, Herd noted, should be especially interesting to analyze when returned to Earth.

So far, 14 of Perseverance’s 38 sample tubes have been filled, containing rocks from a wide variety of geologic regions as well as one sample of Mars’s atmosphere. In addition to the sample tubes, the rover carries five “witness tubes” that will allow researchers to assess the amount of contamination within the rover.

Conceptual illustration demonstrating the stages of the Mars Sample Return program.

Conceptual illustration demonstrating the stages of the Mars Sample Return program. [NASA/JPL-Caltech]

Perseverance will soon journey to a new region to cache the samples it has collected so far before departing the delta region and Jezero Crater entirely. In the next stage of the Mars sample return program, a lander will touch down, collect the cached samples into a small rocket, and launch them into orbit. From there, another spacecraft will capture the samples and send them Earthward.

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Understand Space Weathering of Carbonaceous Asteroids Through the Analysis of Hayabusa2 Samples and Experimental Analogs (Michelle Thompson)
Illustration of the main types and effects of space weathering

Illustration of the main types and effects of space weathering. [Slide by Michelle Thompson]

Michelle Thompson (Purdue University) described how laboratory experiments can help us understand the effects of space weathering — changes to the surface of an airless solar system body by impacts from micrometeorites and solar wind particles. Space weathering typically affects the top few millimeters of a body’s surface, but the effects are far reaching, changing the spectra of these bodies in a way that laboratory experiments can help probe.

Demonstration of how space weathering changes the spectral properties of lunar materials

Demonstration of how space weathering changes the spectral properties of lunar materials. [Slide by Michelle Thompson]

To demonstrate the effects of space weathering, Thompson compared a spectrum of lunar soil, which is subjected to space weather effects, to one of ground-up lunar rock, which is protected from these effects. The weather-beaten lunar soil is darker, has weaker absorption features, and is more reflective at reddish wavelengths than bluish wavelengths.

By approximating micrometeorite impacts and buffeting by the solar wind in a laboratory context, researchers determined that the two processes can have competing effects; while both processes weaken absorption features, solar wind weathering makes surfaces redder and brighter while micrometeorite impacts make them bluer and darker. Through these laboratory experiments, researchers hope to be able to separate primary effects — those that are due to inherent properties of the sample — from secondary effects, which are due to space weathering.

side-by-side comparison of a particle collected from the surface of the asteroid Ryugu and a sample of a meteorite analyzed in a lab

Photographs of a particle from Ryugu (left) and a section of the Murchison meteorite that has been treated in a lab to approximate the effects of micrometeorite impacts. [Slide by Michelle Thompson]

In December 2020, a capsule containing more than 5 grams of material collected from the surface of the asteroid Ryugu crashed down in Australia. An analysis of just one particle from this sample revealed a surface pitted and melted by micrometeorite impacts and never-before-seen teardrop- and lentil-shaped nanoparticles, which indicated a flow in the melted region. The next major sample-return event will be the arrival of a sample of asteroid Bennu’s surface in 2023.

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From Apollo to Artemis and Beyond: How the Apollo Next Generation Sample Analysis (ANGSA) Program Helps to Prepare for Future Sample Missions to the Moon and Beyond (Juliane Gross)
cartoon showing the relationship between the Apollo lunar samples and other facets of planetary science

The Apollo samples underpin much of our knowledge of the Moon as well as other solar system bodies. Click to enlarge. [Slide by Juliane Gross]

Juliane Gross (Rutgers University) described the decades-spanning effort to study samples returned during the Apollo missions. These samples underpin much of our understanding of the Moon as well as other objects in the solar system; researchers use Apollo samples to calibrate remote-sensing data, calibrate the crater-counting curve used for nearly all planetary bodies, identify lunar meteorites that fall to Earth, and understand the space environment surrounding the Moon throughout different periods in solar system history.

Most of the 2,196 Apollo samples were examined decades ago, but a few were set aside for the future; recognizing that our instruments and analysis techniques would improve over time, NASA developed the Apollo Next Generation Sample Analyses (ANGSA) program to connect the Apollo generation of researchers to future generations. Gross and collaborators analyzed one of these samples, 73001/2, which was collected in an interesting region at the intersection of a light-colored mantle deposit and a fault scarp (a region where part of the surface is offset vertically from surrounding areas).

photograph of the lunar surface with the paths taken by astronauts marked in white

Location of the 73001/2 sample collection site (labeled “3”) with respect to the lunar module (LM). [Slide by Juliane Gross]

Gross’s team extracted a precious sample of lunar gas from the outer sample tube before discovering that the inner sample tube hadn’t been closed properly; during the sample collection, the astronauts had overfilled the tube, preventing the locking mechanism from closing completely. Using modern X-ray capabilities and a little creative problem solving, Gross and collaborators were able to collect the sample without contaminating it, paving the way to assessing its properties and cataloging its contents to be shared with other scientists.

With the planned Artemis lunar missions approaching, Gross notes that there is much to relearn that may have been forgotten in the half century since the Apollo missions — and since Artemis will touch down in an entirely different region from Apollo, there will be plenty of new things to learn as well.

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Press Conference

plot of co to co2 ratio as a function of distance from the Sun

Relationship between CO/CO2 ratio and distance from the Sun for various comets. [From slide by Olga Harrington Pinto]

The first of two DPS press conferences (recording available here) brought us updates on comet and asteroid research. The first speaker, University of Central Florida graduate student Olga Harrington Pinto, described a project to determine the rates at which comets produce gases like carbon monoxide (CO), carbon dioxide (CO2), and water (H2O). Pinto found that beyond 3.5 au, where heating from the Sun is minimal, comets produce more CO than CO2, while eight out of nine Jupiter family comets orbiting within 3.5 au are CO2 dominated. Intriguingly, Oort cloud comets orbiting within 3.5 au show a variety of behaviors, with the CO/CO2 ratio increasing with dynamical age. This goes against expectations, and it may indicate that galactic cosmic rays process the outer layers of these comets as they make their first foray into the inner solar system. Finally, Pinto reports that the median C/O ratio is 13%, which is consistent with comets forming within the CO snow line.

plot of the shape of the asteroid Leucus derived from stellar occultation observations

Demonstration of the irregular shape of the Lucy mission target Leucus as determined through stellar occultations. [From slide by Marc Buie]

Next, Marc Buie (Southwest Research Institute) introduced a worldwide citizen science effort to observe the five targets of NASA’s Lucy mission as they pass in front of background stars. These stellar occultations provide a way for researchers to determine the size and shape of distant asteroids, as well as whether they have any moons, rings, or dust. The occultation-observation project, which started in 2018, has taken Buie across the world and involved 500 observers. A major finding from the project is that all of the Lucy mission targets have significant topography (i.e., landforms and surface features), regardless of the size of the object. The project also discovered that one of the targets, Polymele, isn’t flying solo — it’s accompanied by a small moon nicknamed Shaun. Given Polymele’s shape and size, Buie speculated that the Polymele–Shaun system may be similar to the New Horizon’s target Arrokoth, except the two bodies never merged.

animated GIF showing an asteroid's rotation

Animation of asteroid (52768) 1998 OR2’s rotation. [Arecibo Observatory/NASA/NSF]

Finally, University of Arizona graduate student Adam Battle introduced the curious asteroid (52768) 1998 OR2, which has been designated potentially hazardous due to its size (2.2 km) and orbit. In order to constrain the asteroid’s rotation period and composition, Battle used data from a variety of sources, including visible spectroscopy, archival infrared spectroscopy, photometry, and observations from Arecibo and the Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE). These data sources painted a conflicting picture of the asteroid’s nature, with evidence for the asteroid being either a carbonaceous chondrite — containing unaltered, primitive material — or an ordinary chondrite. Battle explored several causes for this discrepancy, landing on shock darkening and impact melting as the most likely causes. These processes, which don’t alter an asteroid’s composition but do affect its spectrum, can lead to a misunderstanding of the asteroid’s properties. This discovery adds to the surprisingly small sample of asteroids with known evidence of shock darkening. Press release

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Prize Talks Plenary Session

Understanding Titan’s Weather, Climate, and Paleoclimate (Juan Lora)
Cassini photograph of clouds on titan

Cassini photograph of streaky bands of clouds on Titan. [NASA/JPL-Caltech/Space Science Institute]

Juan Lora (Yale University), who was awarded this year’s Harold C. Urey Prize, described how our understanding of Titan’s surface and atmosphere has evolved. Titan, the largest moon of Saturn, has a massive, extended atmosphere mainly composed of nitrogen, which drives an active methanological cycle similar to the water cycle on Earth. And while some of Titan’s features are similar to Earth — it’s tilted by 26.7 degrees on its axis, giving it Earth-like seasons, and it’s the only other solar system body that hosts standing surface liquid — others are decidedly alien: its year lasts 29.5 Earth years, its day lasts 16 Earth days, and its abundant clouds are mostly methane (but a few are made of deadly hydrogen cyanide!). These similarities and differences make Titan an excellent candidate for comparative planetology: the study of how physical and chemical processes affect different worlds.

Lora notes that our knowledge of Titan has come a long way, both in terms of what we’re able to observe as well as what we’re able to model. Early models were able to approximate the current distribution of surface liquid, but these models required certain characteristics to be imposed to begin with to achieve that outcome. Today, self-similar models reproduce the north polar methane seas without these restrictions. Models also give us a way to explore Titan’s past climate, or paleoclimate; while the north pole is dotted with seas, the south pole contains dry seabeds, indicating that the southern hemisphere likely hosted seas in the past. While researchers have suggested that Saturn’s eccentric orbit drives Milankovitch cycles on Titan, resulting in a net northward movement of methane, there are still plenty of questions to be answered.

Luckily, the next decade will bring us a means of answering them: the Dragonfly mission. Among the mission’s many objectives is to understand how the surface and atmosphere interact, which will help scientists understand Titan’s past and present climate. Looking ahead, the Titan Orbiter recently endorsed by the Planetary Decadal Survey would provide long-term monitoring of Titan’s atmosphere.

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Current Constraints on Ancient Venus (Martha Gilmore)

Martha Gilmore (Wesleyan University) was awarded this year’s Claudia J. Alexander Prize. Gilmore delved into the uncertain history of our neighboring planet, Venus. Today, Venus’s surface is a scorching 450°C, its atmosphere presses down with a crushing force more than 90 times stronger than Earth’s atmosphere, and all but 4% of its air is carbon dioxide. That’s Venus as we know it — but could it have been a more hospitable place billions of years ago?

view of tessera region on Venus

Perspective view of Venus’s Fortuna Tessera constructed from NASA Magellan data. The color scale shows the emissivity of the region. [NASA/JPL/USGS]

The answer might lie in Venus’s unusual tesserae: rough, deformed, high-altitude regions of Venus’s surface. The average age of Venus’s 900 craters is somewhere between 300 million and 1 billion years (Gilmore uses 500 million years as a rough estimate), indicating that the plains regions were resurfaced around that time ago, and the tesserae are likely older. But just how old the tesserae are is up for debate — they could be 500 million +1 years old or 4 billion years old. Gilmore notes that the tesserae witnessed the creation of the plains, which means they were subjected to the chemical and thermal conditions that existed on Venus during that period of extensive volcanism.

More information about the tesserae came from the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) aboard the Venus Express orbiter. VIRTIS observations at a wavelength of 1 micron — a wavelength sensitive to changes in iron content in minerals — supported the idea that the tesserae contain felsic igneous rocks, which require water and plate recycling to form. This contrasts with present-day Venus, which is devoid of both water and plate tectonics. Given that Venus’s tesserae potentially overlapped with the presence of water, Gilmore encouraged the audience to consider how rocks on Venus would react not just to present-day runaway greenhouse conditions, but how they would react to water and then the greenhouse.

Future missions and laboratory work will further our understanding of Venus’s history (a Venus sample return mission isn’t impossible!), and, as Gilmore notes, finding a Venusian meteorite would go a long way!

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From Pinpoints of Light to Geologic Worlds: The Magic of Photometry (Bonnie Buratti)
zoomed-in image of europa's surface

Photometry of Europa’s surface hints at a mystery yet to be solved. [NASA/JPL-Caltech/ SETI Institute]

This year’s Kuiper Prize goes to Bonnie Buratti (NASA’s Jet Propulsion Laboratory), who introduced the many instances of photometry being ahead of its time, hinting at discoveries that didn’t follow until much later. Buratti began by describing how researchers can use photometry to assess the roughness of surfaces in the solar system, noting that this technique gives us access to particles far below the resolution limit of our cameras.

There are many examples of photometry giving us the first hint at a significant discovery (too many to detail in this short summary!). Using photometric tools, researchers correctly predicted the presence of carbon dioxide on Saturn’s moon Iapetus, the methane-soaked surface of Saturn’s moon Titan, the plumes of icy Enceladus, and a population of retrograde moons around Uranus.

While photometry has been a major player in many discoveries, it may yet play a role in several unsolved mysteries as well. One example is the surface of Jupiter’s moon Europa. Europa exhibits limb darkening (it’s brighter at the center of its disk than at the edges), but it should be the same brightness across the disk, and it’s very forward scattering (i.e., photons striking particles on its surface are scattered roughly in the direction of the incident light rather than being reflected back). Buratti speculated on what the upcoming Europa Clipper mission might discover to explain these observations. Could icy slurries seeping up from below be compacting the moon’s surface? Or could particles from plumes be filling in cracks in the surface? Time will tell exactly how the mystery posed by these photometric observations is solved!

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