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Project Progress

Testing Solar System Formation Models (with Jacqueline Keane, Jan Kleyna, Bin Yang, Svetlana Berdyugina, Bin Yang, Olivier Hainaut, Alessandro Morbidelli, Marco Micheli, Richard Wainscoat)

In Aug 2013 the PanSTARRS 1 (PS1) survey discovered an asteroidal object, C/2013 P2, on a long-period (LP) comet orbit. This was very unusual as LP comets are typically very active at 3.3 AU (e.g. C/1995 O1 Hale-Bopp). Objects on these orbits have not likely experienced volatile depletion having spent little, if any time in the inner solar system and should be active for up to ~1000 perihelion passages. Immediate follow up with Gemini N showed a very weak dust coma, atypical of a LP comet so close to the sun. This triggered the nickname “Manx comet”, after the tailless cat. We obtained a spectrum with Gemini that showed a surprisingly red object—more like the ultra-red mate-rial seen in trans-Neptunian objects. A second Manx, C/2014 S3, was soon discovered and it too was active, but unlike C/2013 P2, the spectral reflectivity was consistent with an inner solar system S-type object!

Why is this interesting? Comets and trans-Neptunian objects are believed to have formed in the volatile-rich outer solar system, and their range of colors from neutral to red reflects a mix of organics, ice and surface weathering processes. S-type material, on the other hand is believed to have formed relatively dry in the inner solar system. Modern planet formation models predict planet migration can scatter material both outward and inward. The Grand Tack model follows giant planets’ growth and migration in a gas disk. During their inward migration, giant planets initiate significant movement of icy planetesimals to the inner solar system; likewise inner solar system material moves outward. The inner solar system material scattered into the outer solar system was later redistributed to the Oort cloud during the late dynamical instability, presented in the Nice model. This scattering can explain the origin of S-type material in the Oort cloud. There are models, however, that do not require large-scale planetesimal movement to explain the masses and chemistry of the terrestrial planets, predicting no S-type Oort cloud objects. The implication of activity on S-type objects moving inward from the Oort cloud is that we may be seeing “fresh” early inner solar system planetary building blocks that were dynamically scattered outward. Dynamical models make testable predictions regarding the fraction of inner solar system material (S-type) on LP orbits. Determining this fraction will constrain dynamical models. Our team has begun a large project to investigate these objects as a class with the goal of distinguishing between solar system formation models.

Carbon Dioxide, Carbon Monoxide, Water and Comet Activity (with Jacqueline Keane, Jan Kleyna, James Bauer, Mike Mumma, Mike Disanti, Geronimo Villanueva, Lucas Paganini, Boncho Bonev)

Recent comet missions have shown the importance of CO₂ to understanding how volatiles were distributed in the early solar system. We have been using an ice sublimation thermal model to characterize the brightness behavior of an active comet as the activity develops and declines on its orbit around the sun. Our sublimation modeling is providing a unique way to determine which comets have activity that is controlled by ices more volatile than water, such as CO and CO₂. This is particularly important because CO₂ cannot be detected from the ground due to Earth’s atmosphere. This requires constraints on the physical characteristics of the comet and of measurements of gas production. Water production rates are published by many teams and we are collaborating with the Goddard team to use high resolution near-IR spectroscopy to detect CO in bright comets and are additionally getting CO through sub-mm observations. We are collaborating with the NEOWISE team to obtain visible-wavelength photometry of comets observed in two near infrared band passes with the WISE observatory in space. The WISE W2 band pass at 4.7 μm contains transitions from CO₂ and a weaker CO line. We are using NEOWISE measurements to test our model predictions for CO₂ outgassing in comets.

We are collaborating with the Goddard team to investigate parent volatiles in comets using high resolution near-IR spectroscopy, and are using this to both characterize the behavior of specific comets, and to work toward a taxonomy of comet chemistry for both minor organic volatile species and for the major drivers of activity. The UHNAI work is focusing in particular on looking at the cor-relation of CO in comets with dynamical class and orbital properties. We collaborated on observations of C/2012 S1 ISON, C/2013 R1 Lovejoy and C/2014 E2 Jacques. The change in water production was monitored for comet ISON from heliocentric distances of 1.21 to 0.34 AU. Four of the trace molecules didn’t change in abundance relative to water over these distances, whereas other species did. The CO abundance of 1.4% was similar to what we had inferred from our sublimation model. Data on C/2013 R1 Lovejoy show that it is relatively CO-rich, whereas C/2014 E2 Jacques is relatively CO-poor.

Disintegration of Comet C/2012 S1 ISON (with Jacqueline Keane, Karen Meech, and GSFC NAI team)

Jacqueline Keane led a joint UHNAI and NASA Goddard comet ISON daytime observing campaign using the SCUBA 2 sub-millimeter camera at the James Clerk Maxwell Telescope (JCMT) located on Maunakea during the week of perihelion (Nov 23-29 2013). The variability and time resolution obtained in these images has revealed significant dust outbursts and have likely captured the onset of the final disruption event of comet ISON. SCUBA 2 images simultaneously at 850 μm and 450 μm, wavelengths at which large cometary particles (mm to cm sized) efficiently radiate. Deconvolving the images with gaussians enhances discrete structure that is generally hidden within the overall emission at 850 μm. The method reveals that when comet ISON was first detected, the 1mm-sized dust particles were tightly bound to the comet nucleus. Three days later the dust was less tightly bound and as comet ISON approached perihelion it became elongated and diffuse, spreading a rubble pile over as much as 120 arc seconds (approximately 80,000 km) in the anti-solar direction. The total brightness of the comet faded throughout the observations, and comet ISON was not detected after Nov 28.04, 17 hours before perihelion implying that mm-sized dust was rapidly destroyed as comet ISON disintegrated.

Main Belt Comet Discovery, Observation and Characterization (Jan Kleyna, Karen Meech, Olivier Hainau, Henry Hsieh, Nader Haghighipour)

Active asteroids are a relatively new (~1996) class of active objects on Main Belt asteroid orbits. Some are caused by asteroid-asteroid collisions collision, but others have recurring activity driven by volatile sublimation. This group is called the Main belt comets (MBCs). MBCs are indicative of the existence of a new reservoir of water in the inner solar system, in bodies that were previously thought to have no volatiles. They could have played a role in delivering volatiles to the habitable zone. MBCs are difficult to detect, because their activity is observed as a faint tail or coma superimposed on asteroid images. The Pan-STARRS 1 telescope is making more frequent discoveries of interesting objects, and two objects discovered during mid 2013 are new main belt comets: P/2013 P5, and P/2013 R3. Intensive follow up work to characterize both of these objects has been under-taken. The first of these objects has been suggested by some to be caused by activity from rotational spin up shedding of material, but our team’s work (lead by UHNAI collaborator, O. Hainaut) suggests that the dust structures are more suggestive of continued activity such as that produced by outgassing. The second object, P/2013 R3, is the first occurrence of a split main belt comet, and we have obtained follow up spectra and photometry to search for volatiles. Additionally our team participated in a study of the activity and dynamics of main belt comet 313P/Gibbs. We find that the data are consistent with volatile driven activity, and that this is the second main belt comet to be-long to the Lixiaohua asteroid family.

Additionally, UHNAI members analyzed observations of main-belt objects obtained by the Pan-STARRS 1 survey telescope between 2012 May 20 and 2013 November 9 to estimate the upper limit of the comet discovery efficiency rate for PS1. From this statistical analysis of PS1 survey data, the inferred number of MBCs among the outer main-belt asteroid population is ~140 between absolute magnitudes of 12

Work has begun to explore the origin of Main belt comets based on their interactions with the giant planets, using the Tisserand parameter to determine the place where they originated from. The model suggests that MBCs are native to the asteroid belt, and they could not have been implanted from outer regions during the early evolution of the solar system.

Characterizing Comet Asteroid Transition Objects (with Megan Ansdell, Karen Meech)

3200 Phaethon exhibits both comet- and asteroid-like properties, suggesting it could be a rare transitional object such as a dormant comet or previously volatile-rich asteroid. Our detailed study of (3200) Phaethon’s physical properties provides a better understanding of asteroid-comet transition objects and insight into minor body evolution. Using data taken over 15 nights from 1994 to 2013, we utilized light curve inversion to: (1) refine (3200) Phaethon’s rotational period (2) to estimate a rotational pole orientation; and (3) derive a shape model. We also used our extensive light curve dataset to estimate the slope parameter of Phaethon’s phase curve showing that it was consistent with C-type asteroids. We discuss how this highly oblique pole orientation supports previous evidence for (3200) Phaethon’s origin in the inner main asteroid belt as well as the potential for deeply buried volatiles fueling impulsive yet rare cometary outbursts.