Thus began a hunt for clues that has turned out to be Cassini's most riveting detective story.
"Enceladus was so exciting that, instead of just three close flybys planned for our four-year primary mission, we added 20 more, including seven that went right through the geysers at the south pole," said Linda Spilker, Cassini project scientist at NASA's Jet Propulsion Laboratory, Pasadena, California.
By following the trail of scientific breadcrumbs, Cassini eventually found that Enceladus harbors a global ocean of salty water under its icy crust, possibly with hydrothermal vents on its seafloor. The trail of clues that began with a puzzling magnetometer reading led to an understanding that the moon -- and perhaps many small, icy moons like it throughout the cosmos -- could potentially have the ingredients needed for life.
"Half the excitement of doing science is that you sometimes find yourself going in a totally different direction than you expected, which can lead to amazing discoveries," said Spilker. "That little anomaly in Cassini's magnetometer signal was unusual enough that it eventually led us to an ocean world." NASA: Snow Science Supporting the US Water Supply
Researchers have completed the first flights of a NASA-led field campaign that is targeting one of the biggest gaps in scientists' understanding of Earth's water resources: snow.
NASA uses the vantage point of space to study all aspects of Earth as an interconnected system. But there remain significant obstacles to measuring accurately how much water is stored across the planet's snow-covered regions. The amount of water in snow plays a major role in water availability for drinking water, agriculture and hydropower.
Enter SnowEx, a NASA-led multi-year research campaign to improve remote sensing measurements of how much snow is on the ground at any given time and how much water is contained in that snow. SnowEx is sponsored by the Terrestrial Hydrology Program at NASA Headquarters in Washington and managed by NASA's Goddard Space Flight Center in Greenbelt, Maryland. The first year of the campaign is taking place this month in western Colorado.
"This is the most comprehensive campaign we have ever done on snow," said Edward Kim, a remote sensing scientist at NASA Goddard and the SnowEx project scientist. "An army of nearly 100 scientists from universities and agencies across the U.S., Europe and Canada are participating. Our goal is to find and refine the best snow-measuring techniques."
More than one-sixth of the world's population relies on seasonal snow for water. In the western United States, nearly three-quarters of the annual streamflow that provides the water supply arrives as spring and summer melt from the mountain snowpacks. Right now, predictions of streamflow can vary widely due to limited ground measurement sites. That's one reason scientists and resource managers are interested in a comprehensive view from space of what they call snow water equivalent -- the amount of liquid water contained in snow cover. Scientists use snow water equivalent to estimate the amount of water that will melt into rivers and reservoirs.
Snow also affects and is affected by the climate. Scientists have detected changes in snow quantity and snowmelt timing that track with other changes prompted by Earth's warming climate. While satellites are not able to measure snow-water equivalent accurately over all snowy landscapes, satellites have monitored the extent of seasonal snow-covered areas for decades. Since 1967, Northern Hemisphere spring snow cover has declined by about 1 million square miles. Loss of snow cover results in Earth absorbing more sunlight, accelerating the planet's warming.
In the air, on the ground
The instruments and techniques developed in campaigns such as SnowEx could one day result in a snow-observing space mission. "We will also figure out a better way to optimize the use of existing satellites to make measurements," said Jared Entin, program manager of the Terrestrial Hydrology Program at NASA Headquarters in Washington.
Five aircraft with a total of 10 different sensors are part of the SnowEx campaign. From an operations base at Peterson Air Force Base, Colorado Springs, SnowEx will deploy a P-3 Orion aircraft operated by the Scientific Development Squadron ONE (VXS-1), stationed at the Naval Air Station Patuxent River, Maryland. High-altitude NASA jets will fly from NASA's Johnson Space Center in Houston and Armstrong Flight Research Center in Palmdale, California. A King Air and a Twin Otter will fly out of Grand Junction, Colorado.
The King Air carries the Airborne Snow Observatory from NASA's Jet Propulsion Laboratory in Pasadena, California. ASO is the first remote sensing system ever to measure snow depth, snow water equivalent and snow albedo across entire mountain basins, and has uniquely quantified snow water equivalent over mountainous regions since 2013.
The other planes will carry five different microwave sensors that are good at measuring snow-water equivalent in dry snow but less optimal at measuring snow in forests or light snow cover; a thermal infrared camera and a remote thermometer for measuring surface temperature; a laser instrument that is good at measuring snow depth and snow water equivalent through trees; an imaging spectrometer that measures snow albedo -- the amount of sunlight reflected and absorbed by snow, which controls the speed of snowmelt and the timing of its runoff.
The field portion of the campaign is based on Grand Mesa and in the Senator Beck Basin. Teams of 50 researchers are making ground measurements, rotating in and out of the field each week for three weeks. The scientists will use measurement and sampling procedures to validate the remotely sensed measurements acquired with aircraft. They will be working above 10,000 feet in potentially windy and freezing conditions up to 10 hours a day. The teams use snowshoes, skis and snowmobiles to access the ground locations. The Senator Beck Basin study area is near the headwaters of the Rio Grande River. "Its research areas are the first major mountain systems downwind of the desert Southwest and Colorado Plateau, making it an ideal place to study the effects of dust on snowmelt," said Hans-Peter Marshall of Boise State University, Idaho, who is leading operations in the basin. Grand Mesa was chosen for its flatness and range of forest conditions, according to Chris Hiemstra, a research physicist with the U.S. Army Corps of Engineers and the lead for the Grand Mesa operations.
Ground equipment was installed in September 2016, before snow started to fall. A ground site near a campground will host specialized equipment too large to move around. This Local Scale Observation Site effort is led by Ludovic Brucker from NASA Goddard.