The solar system is the closest and the best-studied planetary system. Observations of the planets, satellites, and small bodies in the Solar system provide indispensable information about planet formation and evolution processes that remain unattainable for other planetary systems. The advent of TMT will enable us to tackle long-standing questions concerning the formation of the solar system, the origin of planetary volatiles, the physics of the ice and gas giants, and the unraveling of the complex dynamical history recorded in the Kuiper belt.
Beyond Neptune, a much larger population of small bodies exists in the Kuiper belt (Figure 11.1), KBOs outnumber asteroids by 1000:1. These objects result from low temperature accretion in the outer regions and they are thought to be ice-rich. About 1500 KBOs are known but physical properties have been established for only a 100 or less. TMT will provide dramatic improvement for optical detection of these bodies. Study of collisional processes there will have impact on the interpretation of the much more distant (but numerous) main-sequence debris disks, about which much less is known.
Density is a fundamental property for the understanding of their composition and internal structure. The mean density of distant planetesimals is expected to be markedly different from that of native main-belt asteroids. The most promising method to estimate the shape, and subsequently infer the density, is via analyzing light curves of binary systems. For ground-based measurements, the precision on the diameter is the limiting factor of the most accurate density estimates. Giving the advantage of its high-angular resolving power (~7mas), TMT will be able to resolve a dozen large Trans-Neptunian objects and ~50-100 Trojan asteroids, providing a good estimate of their size. If a moon is detected, TMT will also derive high-precision densities of these objects. This study will greatly improve the number and quality of density measurements of the outer-belt asteroids, to a degree not possible before TMT.
Because of the vast distance (30-50 AU) of this region from the Sun and the Earth, even with the largest telescopes currently available, only the largest KBOs (> several hundred km) are observable. The Kuiper Belt is therefore the region about which we know the least in the solar system. On the other hand, due to the low temperature and the relatively slow dynamical evolution, the Kuiper belt can be considered a “debris disk” of our planetary system, comparable to the counterparts of other stars (e.g., Kalas et al., 2006; Trilling et al., 2008). The belt contains essential information about the planetary formation processes, including both the “cold disk” that harbors the objects that are thought to formed in situ with the whole planetary system, and the “hot/scattered disk” that is the refuge of objects that are dynamically scattered into it during the dynamical evolution of the inner solar system. Comparisons of the Kuiper belt with the debris disks around other stars provide important indications about both the Kuiper belt itself and the planetary environment around other stars. TMT allows the characterization of more small KBOs by providing greater sensitivity. Compositional data will also greatly aid the study of dynamically families, which are generally formed during collisions of the parent bodies. Finally, spectroscopic analysis may also reveal evidence of evolutionary processes on KBOs, such cryovolcanism, volatile loss and surface gardening. The large aperture of TMT allows for the collection of the spectra of relatively smaller KBOs, vastly increase the number of samples for which we can study surface compositions.
Centaurs are recently (within 10 Myr) escaped Kuiper belt objects on their way to becoming Jupiter family comets. They are relatively nearby (5-30AU), compared to the belt, making them relatively easier to observe using the high resolution capability of TMT. They also remain pristine, enabling us to determine the composition of the solar nebula. The IRIS instrument with its diffraction limited imaging and spectroscopic capabilities make it ideal to study the actual shapes and composition of these objects, resolving complex structure such as the rings around Chariklo (Braga-Ribas et al. 2014) that have a diameter of ~0.1”. The increase in sensitivity will enable us to characterize several new centaurs as small as 20km in size at a distance of 10 AU and larger ones further away.
Comets represent the icy planetesimals that are left over building blocks from the collapse of the solar nebula (Figure 11.2). A majority have been stored at temperatures ranging from 10 K to 40 K in one of two reservoirs: the Kuiper belt that extends from Neptune’s orbit at 30 AU to at least several thousand AU and the Oort cloud, a spherical assemblage that reaches 50,000 to 100,000 AU from the Sun. These reservoirs contain, respectively, 1 billion and 100 billion comets larger than about 1 km in scale. Comets are important as carriers of the most primitive material in the solar system. Their study allows us to probe the chemical make-up of the system at its origins.
The 2.9-5.0 µm region (3450- 2000 cm-1) is the single most important region for spectroscopy of simple molecules (up to 8 atoms). Virtually all simple gases have at least one vibrational fundamental band in this spectral region, which spans ~1450 cm-1. For emission spectra, resolving powers to 100,000 (even 300,000) are preferred in order to discriminate densely packed and blended lines. Some important work can be done in the 1 - 2.5 µm region - mainly on absorption spectra of combination bands, which are weaker than fundamental bands by large factors (typically 100 or more). With NIRES it will be possible to measure emission lines from the dominant primary volatile (H2O) at 2.0 µm in (active) distant comets. It may be possible to measure hot bands of CO2, the CO overtone (2.4 µm), and various other species as well. And ices can be investigated, through their solid-state absorption bands, in a variety of objects - from active comets to satellites and TNOs.
At optical wavelengths, high resolution spectroscopy can be used to determine nuclear spin temperatures in NH3 and H2O through emission lines of their dissociation products NH2 and H2O+. The nuclear spin temperature is believed to be preserved indefinitely after the formation of a molecule, and hence gives an estimate of the temperature prevailing at the time of the last condensation of the ice. Extending these measurements to fainter objects and to a larger sample will allow us to examine temperature differences that might correlate with formation location in the protoplanetary disk.
The TMT with its high spatial resolution brings to the ground based observer a unique observing technique long held as only possible from spacecraft, planetary limb sounding. The limb sounding method offers an observer the ability to probe a planetary atmosphere at distinct tangent points giving remarkable vertical resolution. These types of observations can be used to look for vertical variations of the chemical constituents and temperature structure. Many of the planetary scale atmospheric waves are expressed as thermal variations on the background atmosphere. These waves would be easily observable with the TMT in this limb sounding geometry. As an example, Mars orbits the Sun at a mean orbital distance of 1.52 AU and thus can be observed as close as ~0.5 AU from the Earth. Looking at shows that the spatial resolution afforded by the TMT in the near infrared (~1-5 µm) is more than adequate to resolve the 11 km scale height of the Martian atmosphere. At 8 microns, where mapping of the HDO and H2O2 abundance, the TMT would offer 20 km spatial resolution to sample the vertical extent of the Martian Atmosphere, >100 km.
Atmospheric features of the planets change on a range of timescales that can be probed using TMT. For example, Mars experiences global dust storms in addition to its annual cycles of atmospheric freeze-out at the poles. The cloud decks of the giant planets evolve in dramatic and unpredictable ways, as recently shown by the emergence of a super-storm on Saturn (Sayanagi et al 2014). Multi-wavelength imaging and spectroscopy with TMT will bring capabilities for high-resolution observations of sudden atmospheric changes that cannot be matched by telescopes in space.
Titan maintains a high pressure (> 1 bar) nitrogen atmosphere and an active hydrological cycle driven not by water, as on Earth, but by hydrocarbons. Its atmosphere is often compared to that of the young Earth, albeit cooled too much lower temperatures (<90 K) than ever found on our planet. It offers a valuable opportunity to study a high mass atmosphere on a solid-surface planetary body, and so to advance models of atmospheric circulation, precipitation and seasonal response. Current observations suggest that Titan has lakes mainly in the northern polar region and has tropospheric clouds mainly in the southern middle latitudes and polar region. Recent numerical simulations suggested the lake formation is due to the cold-trapped methane accumulated in the polar region, and predicted prominent clouds will form within about two (Earth) years while lake levels will rise over the next fifteen years due to the seasonally varying solar radiation on Titan. TMT will be a powerful telescope with which to observe and monitor exciting climate change on Titan. In particular, the high spatial and spectral resolution offered by TMT will reveal the spatial distribution and temporal variation of methane clouds, and separate the high clouds generated by deep convection from the low clouds formed over the surface methane reservoirs. When combined with General Circulation Models, such observations would be essential in understanding the hydrological cycle and seasonal variation on Titan.
Io is the most active volcanic world in our solar system. Spacecraft imaging shows a world littered with volcanic eruptions. Current research has shown that the atmosphere of Io is supported both by SO2 frost sublimation as well as SO2 and other gasses erupting from the volcanoes found there. However, the relative dominance of the frost or volcanic input is still a subject of controversy. SO2 has molecular bands at 7, 8 and 19 µm. At these wavelengths Io can just barely be resolved by an 8 meter telescope without AO (the case for TEXES on Gemini North). However, with the TMT Io will be resolved by 26 and 10 spatial resolution elements across its disk at 7 and 20 µm, respectively. This will transform the way we study Io.
Eclipse observations, where Jupiter eclipses Io for a few hours approximately every two days, also offer another way of testing atmospheric support. Eclipse events inhibit solar heating required for frost sublimation. Observing the atmosphere during eclipses will help to see if and how much the atmosphere condenses onto the cold surface. This process has never been observed for the primary atmosphere due to the lack of telescope light collecting power. Ultimately, MICHI on the TMT will be able to measure the spatial variation of SO2 on Io both meridionally (with latitude) and zonally (with longitude), but critically as a function of local time and during Jupiter eclipses that are both strong functions of frost sublimation, and will allow us to determination of how much of Io’s atmosphere is supported purely by sublimation and how much by volcanic activity.
Additionally, we can expect that individual plumes will be recognizable using MICHI as well. During the New horizons flyby of Jupiter the volcano Tvashtar was imaged with a plume height varying between roughly 320 and 360 km and a full width of about 1,100 km, consistent with the diameter of the pyroclastic deposits. With spatial resolutions ranging from 100 km at 5 µm, 240 km at 12 µm and 400 km at 20 µm, we should expect to be able to directly measure the composition of volcanoes with TMT and MICHI.
Ice giants Uranus and Neptune have only been visited once (by Voyager 2). They remain the least understood and most mysterious planets in the solar system and yet Kepler has already shown that planets of similar mass (in between that of Earth and Jupiter) are widespread outside the solar system. Thus, TMT observations will play a very important role in understanding these ice giants. High angular and high spectral resolution observations by TMT will be used to constrain the bulk composition and the mean vertical distribution of temperatures and gas abundances, to shed light on how planetary atmospheres form and evolve as a function of distance from their host stars.
TMT observations can also determine the different spatial variability of temperature and gaseous abundance on Uranus and Neptune. The spatial variability is strongly connected to the atmospheric dynamics, such as the vertical propagation of waves from the troposphere to the stratosphere. TMT can observe these waves by taking direct optical images of the planets. In combinations with numerical simulations, these observations can help constrain the properties of the waves, as well as the background states for wave propagation, for instance, the atmospheric stratification. Thus, they are crucial for investigating the different atmospheric conditions on Uranus and Neptune.
The internal structures of giant planets are much less well known than those of main-sequence stars because of uncertainties in the equation of state of degenerate gas, the composition (typically non-solar), the interaction with the magnetic field and, in the upper layers, the relative magnitudes of internal heat and energy deposited from the sun. Giant planet interiors are inaccessible to direct study from above, but oscillations excited by asteroid and comet impact can generate waves that are potentially observable. Such waves will propagate through the planetary interiors, allowing giant planet seismology to constrain internal structure in much the same way as done for our planet using earthquakes. The model is the impact of comet Shoemaker-Levy 9 on Jupiter in 1994, although the state of technology then did not permit the detection of planet-wide waves. Asteroid impacts, especially the large ones, can excite atmospheric waves capable of revealing information about the internal structures of the planets that probably cannot be obtained in any other way. TMT would be able to measure the propagation direction, propagation speed, as well as the energy containing wave number of the atmospheric waves. Such measurements will probe the atmospheric structure and composition, providing unique information useful not just in the solar system but also in the study of Jupiter-like exoplanets, where no comparable data will be available for the foreseeable future.