Soil Moisture Active-Passive (SMAP) Mission
Missions in Brief
SMAP will make global measurements of the moisture present at Earth's land surface and will distinguish frozen from thawed land surfaces with unprecedented accuracy, resolution and coverage.
Direct observations of soil moisture and freeze/thaw state from space will allow better estimates of evaporation from the soil and transpiration from plants into the atmosphere, and of the heat energy exchanged between land surfaces and the atmosphere. Water and energy transfers between Earth's surface and atmosphere are primary driving factors for weather and climate. Thus, quantifying these exchanges over a range of scales from regional to global and at time scales of a few days or less is critical to improving our weather and climate forecast skill. Lack of knowledge of land surface water and energy fluxes is one of the primary shortcomings of climate prediction models at the present time.
In addition, how saturated land surfaces are, and whether they are thawed or frozen, are important factors in determining whether rainfall and melt water from snow can infiltrate readily into the soil or will run off at the surface, leading to excess stream flow and flooding. Soil moisture measurements are therefore of great importance in assessing flooding potential and as input to flood prediction models. Conversely, observations of widespread low soil moisture levels can provide early warning of drought conditions, reduced water supply and crop loss. SMAP observations can help mitigate these natural hazards, resulting in potentially great economic and social benefits.
The timing of the annual thawing and freezing of Earth's land surfaces controls the length of vegetation growing seasons. In the boreal forest regions of the world, changes in the length of the growing season have major impacts on whether these regions are net sources or sinks of carbon. SMAP freeze/thaw timing observations will thus reduce a major uncertainty in quantifying the global carbon balance and will help resolve the problem of the missing carbon sink.
The SMAP spacecraft will operate in a sun-synchronous, low-Earth orbit at an altitude of 670 kilometers. It will employ a microwave radiometer and a high-resolution radar that share a rotating 6-meter mesh antenna to provide high-resolution and high-accuracy global maps of soil moisture and freeze/thaw state every two to three days. The radiometer and radar operate at low microwave frequencies that can penetrate through clouds and most vegetation, allowing observations to be made in nearly all weather conditions and over all land regions where soil moisture and freeze/thaw observations are of critical importance.
Ice, Cloud and land Elevation Satellite-II (ICESat-II)
ICESat-II will use precision laser-ranging techniques to measure the topography of the giant Greenland and Antarctic ice sheets and the thickness characteristics of the Arctic and Antarctic sea ice. According to the National Research Council's recent Decadal Survey report on Earth Science and Applications from Space, ICESat-II fills an urgent need in understanding the Earth's rapidly changing ice cover and what will likely be its tremendous impact for life on Earth.
The ICESat-II mission is a successor to the current ICESat-I satellite, launched in 2003. Because of unforeseen limitations in the life of the original ICESat lasers, ICESat-I is currently collecting data during two one-month periods in spring and fall. ICESat-II will fly in the same orbit as ICESat-I at an altitude of 590 kilometers and will orbit the Earth about 14 times per day. The orbit allows the satellite to make measurements up to latitudes of 86 degrees north and south, covering most of Antarctica, all of the Antarctic sea ice, all of Greenland and most of the Arctic Ocean.
The ICESat-II payload will include a single-channel lidar with GPS navigation and pointing capabilities sufficient for acquiring high-accuracy repeat elevation data over ice and vegetation. The ability of the ICESat-II laser to make measurements with a few inches accuracy and better than an inch precision make it the ideal tool for unlocking the secrets of the Earth's changing ice cover. The design of ICESat-II will address technical problems uncovered during the ICESat-I mission, which was the first mission of its kind. As a result, the ICESat-II mission is expected to collect data continuously for five years.
The principal objective of the ICESat-II mission is to understand how and why the Greenland and Antarctic ice sheets are changing, and how these changes will influence sea level rise in this century. The ice sheets hold enough water to raise sea level by more than 200 feet if they were to disappear completely. While the total disappearance of either of these ice sheets is not a concern in this century, these vast stores of ice are changing rapidly and contributing to today's rising sea level in ways and quantities that are not fully understood. The current warming tends to increase the amount of surface melt that an ice sheet experiences and increases the rate at which the ice flows toward the sea. Both these effects raise sea level. At the same time, a warming climate can cause increased amounts of snowfall in the higher colder regions of the ice sheets, causing them to grow in some areas, and subsequently reducing their potential contribution to sea level rise. The net balance of these effects, melt, flow, and accumulation, determine how much ice sheets contribute to sea level rise.
Because changes in accumulation, melting and flow have distinct topographic expressions, this detailed elevation information also holds important clues about the mechanisms that are causing the ice to change. This information will be used by scientists to improve our ice sheet models in such a way that they will enable much better prediction of ice sheet contributions to sea level rise in the coming decades. The ICESat-II mission will provide critical information to enable scientists to predict how much and how fast the oceans will rise.
The laser can also indirectly observe the movement of liquid water under the Antarctic ice, where the liquidity is maintained by a combination of geothermal and frictional heating. ICESat-I measurements show that the movement of water under the ice sheet creates surface elevation changes that can be as large as 50 feet. Even with its limited operability, the ICESat-I laser has been used to map the distribution of under-ice lakes, which currently number about 140. The planned continuous operation of ICESat-II will permit a thorough investigation of the 'plumbing' of the Antarctic sub-ice environment.
The second major objective of ICESat-II is to provide estimates of sea ice thickness. Sea ice, the thin veneer of frozen seawater that blankets the Arctic and Antarctic oceans, has significantly shaped the climate that has been in place through much of human history. This blanket of ice reflects most of the incoming solar energy, helping to keep the planet cool. It also acts as a thin insulating barrier between the ocean and the atmosphere, significantly influencing ocean currents and atmospheric circulation and the movement of energy around the planet.
Our current visible/infrared and microwave imaging satellites measure the ice area and extent. In the Arctic, these measurements show that the ice cover has diminishing much more rapidly than the best climate models predict. To understand the significance of this decline and what it will mean for the Earth's future climate requires detailed knowledge of not just the ice area, but also the ice thickness, which until the current ICESat mission was not measurable on large scales at the needed level of detail. Like ICESat, with its precise measurement capability, ICESat-II will be able to measure the difference in height between the top of the floating sea ice and the water in which it floats, allowing scientists to determine how thick the ice is and how that thickness changes in space and time. In so doing, it will fill a critical gap in our understanding of the Earth's climate.
> NASA ICESat mission
> Greenland mass balance (conceptual animation, MPEG, 6MB)
> Antarctic subglacial lakes (conceptual animation, MPEG, 4MB)