The Solar System
Planets | moons | asteroids | comets | origins of water
David Tholen is tracking near-Earth asteroids to help determine which of them may represent an impact hazard to Earth, observing them until they are removed as impact threats. Special attention is being given to (99942) Apophis (right), with the goal being to detect the Yarkovsky acceleration on this object, which is the largest source of uncertainty of its future impact threat.
Tholen’s group has also measured the density of a small near-Earth asteroid, 2009 BD, via the detection of radiation pressure effects acting on the object. The results indicate a nominal density that is less than that of water, suggesting a very porous internal structure, which has also been seen in a couple of other smallasteroids. If these results can be shown to apply to larger near-Earth asteroids, the potential impact energy estimates could be revised downward, making them less of a threat.
Jan Kleyna and Richard Wainscoat
Jan Kleyna and Richard Wainscoat are using the giant Hyper Suprime-Cam imager on the 8.2 meter Subaru telescope to search for near-Earth objects that are undetectable using normal observing strategies, because they have orbits that rarely take them outside of the Earth’s orbit. These objects, with orbits that may bring them close to the Earth, represent an important but poorly surveyed class of PHAs (potentially hazardous asteroids). By observing close to the horizon in the early morning or late evening using the wide-field deep imaging ability of Hyper Suprime-Cam, it is possible to detect such objects well inside the earth’s orbit, and to characterize their sizes and shapes using their light curves.
Robert Jedicke also studies Near-Earth Objects and has provided evidence that some small, dark objects catastrophically self-destruct when they approach too close to the Sun
Three main-belt, calcium-aIuminum-rich asteroids showing the 2-micron absorption band diagnostic of aluminous spinel.
Some of the oldest materials in the solar system are the refractory minerals contained in calcium, aluminum-rich inclusions that are found in chondritic meteorites.
Bobby Bus has been using infrared spectroscopy to estimate the concentration of these minerals in a number of main-belt asteroids. He has found several asteroids with concentrations of these minerals 2-3 times higher than in any meteorite. These asteroids must therefore be older than any known sample in our meteorite collections, making them prime candidates for future sample return missions.
Some near-Earth asteroids can be temporarily captured by the Earth’s gravity if they have a slow relative speed while passing through the Earth-Sun Lagrangian points. Robert Jedicke has been calculating the population characteristics of these irregular natural Earth satellites.
He finds that any given time there should be at least one such object of at least 1 meter diameter orbiting Earth. His results are consistent with the single known natural minimoon, 2006 RH120, a few-meter diameter object that was captured for about a year starting in June 2006. Jedicke estimates that about 0.1% of all meteors impacting the Earth were once minimoons.
The Origin of the Earth’s water
Ices in the Outer Asteroid Belt
The distribution of organic material and ice (in particular water) during the growth of planets is crucial to the development of a habitable planet. Of particular interest for us is the origin of Earth’s water. There are three leading scenarios for its origin:
- Direct capture from nebular gas
- Delivery from icy planetesimals that formed beyond the protoplanetary disk snowline
- Chemical reactions between oxides in a magma ocean and an early hydrogen atmosphere
Comets provide one of the mechanisms for large-scale transport and delivery of water within our solar system, and asteroids provide another source of volatiles. However, neither comets nor asteroids can explain both Earth’s water and its noble gas inventory. A recently discovered new class of icy bodies in the outer asteroid belt, the main-belt comets (MBCs), may be the key. Dynamics suggest they formed in situ, beyond the primordial snow line, and as such represent a class of icy bodies that formed at a distance from the Sun that has not yet been studied in detail and that could potentially hold the key to understanding the origin of water on terrestrial habitable worlds. Karen Meech and her team are developing a Discovery satellite mission concept to explore ices in the outer asteroid belt.
Simulated collision between a water-carrying planetismal and a dry object
The Transfer of Water and Other Compounds from the Asteroid Belt to the Inner Solar System.
Nader Haghighipour explores the mechanism by which water was acquired by the embryonic Earth by modeling collisions between wet and dry planetismals. He has shown that these collisions can facilitate the transfer of water and other compounds from the asteroid belt to the inner solar system.
A Manx comet, spends most of its early life in the distant Oort coud, but later gets moved to a smaller orbit
The Pan-STARRS survey
Because comets are the left-over remnants of the planet-building process that have been preserved cold since the time of formation, they can provide clues about the chemistry and dynamics in our solar system’s protoplanetary disk. The Pan-STARRS survey has been exceptional at discovering comets with low activity levels at a variety of distances. One new class, the Manx comets (named after the tail-less Manx cat), may represent inner solar system material that was tossed to the Oort cloud during giant planet growth. These may represent inner solar system material that built our habitable planet. Spectroscopically, they are similar to rocky inner solar system asteroids, but have a small level of comet activity.
Understanding the origin of Earth’s water requires an interdisciplinary approach, with insight from the fields of protoplanetary disk chemistry, dynamics, cosmochemistry and geochemistry. Karen Meech’s team studies deep mantle rocks to determine the isotopic composition of Earth’s primordial water and is combining this with information from protoplanetary disk chemical and dynamical models, comet observations, and hopefully, in the future a space mission, to explore how habitable worlds form.
Astrobiology reserach at UH is coordinated under the University of Hawaii NASA Astrobiology Institute
HST image of Pluto and four of its moons: Pluto and Charon are saturated while three of the outer satellites are circled.
Pluto and its Moons
Pluto is now known to have at least five moons, which David Tholen and his colleagues have been studying using the Hubble Space Telescope.
Each of the four outer small satellites of Pluto are in near resonance with the much larger innermost moon Charon. The resonalnces are aproximately: Styx 3:1, Nix 4:1, Kerberos 5:1, and Hydra 6:1, The reason they are not in a perfect resonance is because Charon is massive relative to Pluto; it is effectively a binary system.
Nader Haghighipour models the formation process of planets.
In the case of gas-giant planets his main goal is to understand why the concentration of high-z elements is much higher than that of the Sun.
In the case of terrestrial planets his focus is on understanding how repeated destructive collisions among Moon to Mars-sized objects resulted in fully-formed planets, and why Mars and Mercury are so small.
Snapshots of the collision of two Ceres-mass bodies