The Submillimeter Array

Solar System: Introduction

The SMA is a unique instrument with capabilities that are well matched to the often challenging requirements needed for solar system astronomy. With up to 8 GHz of total bandwidth, coupled with high spectral resolution, the SMA allows deep continuum sensitivity for imaging small and cold solar system bodies, as well as allowing observations of a multitude of spectral lines within a single tuning and enough spectral coverage to include even highly pressure broadened features. Solar system spatial scales range from about 1 arcminute (or larger for cometary comae) to well below 1 arcsecond. The SMA antennas have large primary beams suitable for imaging large fields, yet can pursue very high resolution (to below 0.2 arcseconds at 345 GHz) by moving the antennas to over 0.5 km separation. The SMA supports a small yet world-leading program of solar system research, producing pathfinding science on a variety of topics.

In one sense, science research within our solar system is as diverse as the number of objects we can observe, as each planet, moon, or small body represents a unique world to explore. SMA solar system science can be broadly broken down into two categories: (a) research dedicated toward understanding the properties of solid surfaces, and (b) research toward understanding the properties of atmospheres in its broadest application. The SMA is adept at both. On the following page is a listing of SMA projects that have been pursued in the past 5 years (with PI and year when observations were obtained).

   Solar System Research Projects with the SMA

Venus - Measurement of thermal structure and winds in the mesosphere using dual-band observations of CO (Gurwell, 2007; Sagawa, 2009)

Ceres - Observations of nearly one full rotation, showing almost no variation in brightness temperature with longitude, and thus a very uniform surface in both mm and OIR bands (Moullet & Gurwell, 2009).

Jupiter - Imaging of remnant HCN (2007) and CS (2008) in the stratosphere from the 1995 Comet Shoemaker-Levy impacts, showing evolution of distribution and effects of polar vortices where material is dragged into troposphere (Moreno, 2007, 2008)

Saturn - Detection and imaging of CO in the upper atmosphere of Saturn (Cavalie et al 2010)

Io - Imaging the spatial extent of the weak atmosphere of Io, including SO, SO2, and NaCl distributions and implications for the relative strengths of direct volcanic and sublimation processes (Gurwell, 2006; Moullet & Gurwell, 2008)

Solar System Io

Figure 01: Maps of Io's surface thermal emission (continuum) at 346.6 GHz, in 2006 (left) and 2008 (right) on the leading hemisphere. Maps from the trailing hemisphere are very similar. The contour level is 0.2 Jy/beam. The circle at the center represents the solid disk size, and the synthesized beam is sketched on the bottom-left corner. The color scale (Jy/beam) is indicated by the vertical bar on the right. Measured brightness temperature is between 93 K and 100 K (USB band). (Moullet et al. 2010)

Galilean Moons - Measurement of limb darkening and polarization properties, which can be tied to dielectric properties and density, leading to a better understanding of surface composition (Moullet & Gurwell, 2009, 2010)

Titan - Groundbreaking study on the vertical distributions of CO, HCN on Titan, along with isotopic ratios of 12C/13C (CO, HCN), 14N/15N (HCN), 16O/18O (CO) (Gurwell, 2004, 2008);- high resolution pathfinder study of the spatial distribution of CH3CN using the eSMA (Gurwell & Moullet, 2009)

Solar System Titan

Figure 02: Complex chemistry in Titan's stratophsere produces hydrocarbons and nitriles. While most nitriles can be observed and mapped by CIRS on board the Cassini spcecraft, the distibution of acetronitrile CH3CN can only be obtained from ground-based spectroscopy. The CH3CN line at 349 GHz was observed in 2009 with the extended SMA (eSMA) mode. The exquisite 0.2-0.3" resolution produced by the eSMA (SMA+JCMT+CSO) is necessary to sufficiently resolve such a small source (0.8"). The increased abundance of CH3CN at the North Pole is consistent with the distribution of other nitriles. Doppler-shifts indicate a prograde zonal circulations. Strong seasonal variations of abundance and distribution are all assessed (Gurwell et al. 2009).

Uranus - High resolution continuum imaging showing that the observed disk is warmer in polar relative to equatorial regions (Hofstadter 2006)

Neptune - High resolution spectroscopic imaging of CO and HCN in the atmosphere (in queue, not yet attempted) (Moullet & Gurwell, 2010)

Pluto - The first ever true thermal imaging that separate Pluto and Charon, which prove Pluto, at 40 K, is colder than the expected equilibrium temperature at 30 AU, a result of sublimation of N2 ices into the Plutonian atmosphere (Gurwell & Butler, 2005; Gurwell Butler & Moullet 2010)

Comet Studies - Several projects have been advanced led by C. Qi, including observations of C/Linear 2002 T7 (2004, CS, CH3OH detected), 9P/Tempel 1 ("Deep Impact". 2005, no detection of impact), 73P/Schwassmann-Wachmann 3 (2006, HCN jet activity detected), and 17P/Holmes (2007, HCN, CO, CH3OH, H2CO, H13CN, CS and jet activity detected)