Water in the Universe: Abundant? Yes - But Not Where We Thought it Would Be

SWAS TargetsSWAS Mission: Results

Depending on where you look in the universe, there is more - or less water than astronomers had been predicting. These findings come as a result of observations made by the Submillimeter Wave Astronomy Satellite (SWAS). SWAS is operated by the Harvard-Smithsonian Center for Astrophysics as one of NASA's Small Explorer Projects (SMEX) and was designed to study the chemical composition of interstellar gas clouds. Its primary objective is to survey water, molecular oxygen, carbon, and isotopic carbon monoxide emission in a variety of galactic star forming regions.

SWAS was launched on 5 December 1998 from Vandenberg Air Force Base aboard a Pegasus-XL launch vehicle. The spacecraft has been operating flawlessly ever since. According to the mission's home page, SWAS was designed to focus on the following spectral lines:

(1) Water (H2O) at 556.936 GHz
(2) Molecular oxygen (O2) at 487.249 GHz
(3) Neutral carbon (CI) at 492.161 GHz
(4) Isotopic carbon monoxide (13CO) at 550.927 GHz
(5) Isotopic water (H218O) at 548.676 GHz

The SWAS spacecraft will make detailed 1 degree x 1 degree maps of at least twenty giant molecular and dark cloud cores during the first 2 years of the mission.

In a series of papers published this week in the Astrophysical Journal Letters, some contradictory information has emerged. In regions of space where the temperature is only 30 degrees above absolute zero, water was found to be far less plentiful than expected. Conversely, according to a press release, "within gas clouds where new stars are being born, the gas can be heated to temperatures of several thousand degrees Fahrenheit; here the water concentration seems to be as much as 10 thousand times larger."

In addition to detecting water, SWAS is also designed to search for other elements - with the molecular couds that spawn stars being one of the mian observational targets. So far the amount of Oxygen is much lower than expected leading one researcher to state that "most of the oxygen atoms in interstellar space remain hidden in some form that we have yet to detect."

SWAS has also pointed its sensors at targets much closer to home - planets in our own solar system. Water observed in the atmospheres of Jupiter and Saturn is thought to come from cometary bombardment of these gas giants. Data derived from observations made of Comet C/1999 H1 differed somewhat from what had been expected from previously suggested models.

SWAS also confirmed a long standing model of Mars' atmosphere which predicted a relative humidity of 100%. In a paper published in the Astrophysical Journal Letters SWAS researchers found that "the distribution of water in the Martian atmosphere matches a profile of constant, 100% saturation from 10 to 45 km altitude."

SWAS Mission: Termination

Despite these important findings from SWAS and the questions that remain unanswered as a result, NASA now belives that SWAS has accomplished its main goals and that it is to be terminated at the end of its original mission. The following is the assessment of SWAS conducted as part of the Report of the Senior Review of Origins and Structure and Evolution of the Universe Mission Operations and Data Analysis (MO&DA) Programs, NASA Office of Space Science:


The Submillimeter Wave Astronomy Satellite (SWAS) is designed to study several important spectral lines in the submillimeter region, to investigate the role of cold water and molecular oxygen in star formation and to observe neutral carbon and carbon monoxide. The mission has performed well since launch in December 1998. On-orbit performance meets or exceeds design specifications and there is no evidence of degradation of the critical components.


SWAS has made several significant discoveries. It found that the water abundance in the interstellar medium is very variable. Observations of several giant molecular clouds indicate that the water abundance in these objects is more than an order of magnitude lower than the predictions of models of the interstellar medium. The water abundance in 'translucent clouds', observed in absorption toward Sgr B2, is approximately as predicted, as is the water abundance in regions that have been compressed by shock waves associated with protostellar outflows. SWAS has also detected water in the circumstellar envelopes of evolved stars, in the atmospheres of Jupiter, Saturn, Mars, and in Comet Lee. It has been predicted that molecular oxygen plays a major role in the chemistry of molecular clouds. SWAS has not yet detected molecular oxygen in the interstellar medium, indicating that the abundance of molecular oxygen is at least a factor of 10 to 100 lower than predicted by steady-state models.


The major science goals for SWAS have been accomplished. While additional observations may add new sightlines and improved signal to noise, the potential for additional scientific breakthroughs is judged to be small.


We recommend that SWAS complete its primary mission and be granted the nine month no-cost extension. It appears that the mission can be terminated at that point with little scientific impact and need not go into an extended phase."


Every time we look closer at the chemical composition of the universe that surrounds us, be it on the planet next door, or within a dust cloud on the other side of the galaxy, we find more of the basic components that comprise our own bodies - and that of all life on Earth. The more stars we examine, the more planets we find circling them. Even the skeptics have to pause and wonder why the universe seems to be seen as an ever-more hospitable place for planets and the basic building blocks of life.

Related Links:

° Cosmic Gas Clouds Yield Puzzling Concentrations of Water, Harvard-Smithsonian Center for Astrophysics
° The Submillimeter Wave Astronomy Satellite (SWAS), Harvard-Smithsonian Center for Astrophysics
° Report of the Senior Review of Origins and Structure and Evolution of the Universe Mission Operations and Data Analysis (MO&DA) Programs, NASA Office of Space Science (adobe Acrobat)

Background Information

° Astronomers Find Sugar Molecules In a Dust Cloud at Our Galaxy's Center, SpaceRef
° Meteorite Found to Contain Water From Our Solar System's Infancy, SpaceRef
° Ex astra: Life From the Stars, Ad Astra magazine
° Astrochemistry, SpaceRef Directory
° Astrochemistry Branch, NASA Goddard Space Flight Center
° NASA's Origins Program, NASA Jet Propulsion Laboratory
° Astrochemistry, NASA Ames Research Center
° NASA's Astrobiology Program, NASA Ames Research Center

SWAS Mission Results

The following papers are from a special issue of The Astrophysical Journal Letters devoted to SWAS (Volume 539, Number 2, Part 2; 20 August 2000). Abstracts can be read online but a subscription is required ot gain access to the entire article online:

° The Submillimeter Wave Astronomy Satellite: Science Objectives and Instrument Description
° Observations of Water Vapor toward Orion BN/KL
° Submillimeter Wave Astronomy Satellite Observations of Extended Water Emission in Orion
° The Distribution of Water Emission in M17SW
° Water Abundance in Molecular Cloud Cores
° Observations of Interstellar Water Vapor in Outflow Regions
° Observations of Absorption by Water Vapor toward Sagittarius B2
° An Analysis of Water Line Profiles in Star Formation Regions Observed by the Submillimeter Wave Astronomy Satellite
° Water Abundance and Velocity Structure in S140, Oph A, and B335
° O2 in Interstellar Molecular Clouds
° Implications of Submillimeter Wave Astronomy Satellite Observations for Interstellar Chemistry and Star Formation
° Large-scale 13CO J = 5 4 and [C I] Mapping of Orion A
° Extended [C I] and 13CO (5 4) Emission in M17SW
° Submillimeter Wave Astronomy Satellite Observations of the Martian Atmosphere: Temperature and Vertical Distribution of Water Vapor
° Submillimeter Wave Astronomy Satellite Observations of Jupiter and Saturn:Detection of 557 GHz Water Emission from the Upper Atmosphere
° Submillimeter Wave Astronomy Satellite Observations of Water Vapor toward Comet C/1999 H1

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