UCLA

The giant impact hypothesis suggests that the Moon formed out of a partially vaporized disk created by a collision between an impactor and the proto-Earth. Three major models exist for this hypothesis: (a) standard model: a Mars-sized impactor hit Earth, (b) fast-spinning Earth model: a small impactor hit the rapidly-rotating Earth, (c) sub-Earths model: two half Earth-sized objects collided. Some of these models can explain several observed features, including the nearly identical isotopic ratios between Earth and Moon. However, it has not been clear if these models can explain other geochemical constraints, such as (1) the giant impact did not mix the Earth’s mantle, (2) Moon did not lose significant amount of water (hydrogen) during its formation. In this talk, I show results from giant impact simulations and investigate whether the suggested models are consistent with these geochemical constraints. I show that the standard model is more consistent with the survival of the mantle heterogeneity than the other models. I also find that water loss from the Moon-forming disk is minor in all models. Therefore, the giant impact hypothesis is consistent with the measured lunar water abundance. Finally, I will discuss implications of our model for the formation of exomoons. Our numerical simulations indicate that whether a planet can form an impact-induced moon depends on the planetary mass and composition.

The interaction between a host star and its orbiting planet can generate both radiative and gravitational effects on planetary climate. While these effects on climate are largely constrained and understood for the Earth, exoplanets fill a much more diverse range of planetary and stellar properties. To identify habitable exoplanets, it is important to understand how the climatic effects of the processes that influence the Earth’s climate might change for different host stars and planetary and orbital characteristics. I will share results from work performed using a hierarchy of models to simulate planets covered by ocean, land and water ice, with incident radiation from stars of different spectral types. Results indicate that ice extent is much greater on a planet orbiting a hotter, brighter star than on a planet orbiting a cooler, redder (M-dwarf) star at an equivalent flux distance, assuming an Earth-like atmospheric concentration of carbon dioxide. I’ll explain the reasons behind this apparent warmer planetary haven around cooler stars, address how the picture changes at the outer edge of the stellar habitable zone, and discuss the implications of our results for planetary climate and habitability. I will also explore a specific case —that of Kepler-62f, a potentially habitable planet in a five-planet system orbiting a K-dwarf star—and discuss its prospects for habitability as a function of atmospheric composition and orbital configuration, based on work performed with both N-body and global climate models. The methods presented can be used to assess the possible climates of potentially habitable planets as they are discovered.

One of the ubiquitous properties of life on Earth, and perhaps of any life elsewhere, is that it utilizes geochemical and electrochemical disequilibria and converts free energy. Planets with a water-rock interface generate disequilibria through serpentinization reactions in the form of redox, pH, and thermal gradients at hydrothermal vents. Warm, alkaline serpentinite-hosted vents continually produce fuels which can be utilized as a source of electrons for redox metabolism, and in prebiotic systems, could have formed hydrothermal precipitates (e.g., metal sulfides or oxyhydroxides) capable of catalyzing proto-metabolic reactions and concentrating biologically significant materials. Mineral precipitation chimneys associated with vents can function as chemical flow-through reactors and geochemical fuel cells, driving reactions of carbon / nitrogen / phosphorus compounds and facilitating electron transfer to/from minerals. Some key questions for astrobiology center on understanding the geochemical energy gradients produced on wet rocky planets, including icy worlds such as Europa or Enceladus, and how these abiotic redox and pH gradients may have participated in prebiotic chemistry and the emergence of bioenergetics.

The search for extra-solar planets has led to the surprising discovery of many Jupiter-like planets in very close proximity to their host star, the so-called “hot Jupiters”. Even more surprising, many of these hot Jupiters have orbits that are eccentric or highly inclined with respect to the equator of the star, and some (about 25%) even orbiting counter to the spin direction of the star. This poses a unique challenge to all planet formation models. We show that secular interactions between Jupiter-like planet and another perturber in the system can easily produce retrograde HJ orbits. We show that in the frame of work of secular hierarchical triple system (the so-called Kozai mechanism) the inner orbit’s angular momentum component parallel to the total angular momentum (i.e., the z-component of the inner orbit angular momentum) need not be constant. In fact, it can even change sign, leading to a retrograde orbit. A brief excursion to very high eccentricity during the chaotic evolution of the inner orbit allows planet- star tidal interactions to rapidly circularize that orbit, decoupling the planets and forming a retrograde hot Jupiter. We estimate the relative frequencies of retrograde orbits and counter to the stellar spin orbits using Monte Carlo simulations, and find that the they are consistent with the observations. The high observed incidence of planets orbiting counter to the stellar spin direction may suggest that three body secular interactions are an important part of their dynamical history.

Interestingly, this mechanism is applicable to many other astrophysical settings.

Throughout the history of space exploration, we’ve been improving our understanding of life and habitability on other planets and solar bodies. It is our responsibility to preserve and protect the environments under investigation as well as our own should a sample be returned. Thus, a set of policies and guidelines have been set in place to ensure that we are good custodians of the solar system and beyond. This concept of Planetary Protection Planetary Protection will be discussed in the context of past missions and its importance will be highlighted in light of the upcoming Mars Sample Return mission.

Terrestrial planet formation is generally thought to have proceeded in two main stages: The first consists of the accretion of planetesimals, which leads to the formation of several dozens of roughly Mars-sized planetary embryos, and the second stage consists of a series of giant impacts between these embryos that merge to form the Earth and other terrestrial planets. Understanding how much of the planets’ primordial atmosphere is retained during the giant impact phase is crucial for understanding the origin and evolution of planetary atmospheres. In addition, a planet’s or protoplanet’s atmosphere cannot only be lost in a giant impact, but also due to much smaller impacts by planetesimals. Therefore, in order to understand the origin and evolution of the terrestrial planets’ atmospheres I will examine the contributions to atmospheric loss from both giant impacts and from planetesimal accretion and show that planetesimal impacts are likely to dominate the atmospheric mass loss during planet formation. I will discuss the implications of these findings for the formation of the terrestrial planets and their volatile budgets.

Impact craters can be used to estimate the age of a planetary surface, given knowledge of the rate of crater accumulation, and is the primary method for age dating planetary surfaces (excluding Earth). Radiometric and cosmic ray exposure ages of Apollo and Luna samples, correlated with crater populations, anchor the lunar crater chronology and provide predicted size-frequency distributions (SFD) of crater populations for a given surface age. These predicted SFDs are scaled to Mars, accounting for the ratio of meteoroids at the top of the martian atmosphere relative to the Moon and the differences in gravity and average impact velocities of intersecting orbits. Crater derived age estimates have been called into question in the last decade as it has been recognized that secondary craters formed by debris ejected by primary impact events, may constitute a large fraction of the observed craters, thus contaminating the statistics. Images from the Mars Reconnaissance Orbiter have been used to identify fresh craters that have formed over the last 10 years providing the first direct observation of the present-day primary impact crater SFD at small crater diameters on Mars. Additionally, new high-resolution images from the Lunar Reconnaissance Orbiter provide the ability to study crater populations on the Moon in greater detail. What these new observations reveal about crater populations and how they relate to surface ages will be discussed.