Of Ice and Satellites
For millennia, Greenland’s ice sheet reflected sunlight back into space, but satellite measurements in recent years suggest the bright surface is darkening, causing solar heat to be absorbed and surface melting to accelerate. Some studies suggest this “dirty ice” or “dark snow” is caused by fallout from fossil fuel pollution and forest fires. But, according to a study led by Thayer adjunct professor Chris Polashenski ’07 Th’07 ’11, a research geophysicist at the U.S. Army Corps of Engineers Cold Regions Research and Engineering Laboratory in Hanover, degrading satellite sensors, not soot or dust, account for the apparent decline in reflectivity of inland ice across northern Greenland. The study, published in Geophysical Research Letters, suggests the ice sheet hasn’t lost as much reflectivity as previously thought, and that black carbon and dust concentrations haven’t increased significantly and are thus not responsible for darkening on the upper ice sheet.
Observations suggest the Greenland Ice Sheet’s albedo—its ability to reflect the sun’s energy back into the atmosphere—has declined considerably since 2001 due to black carbon and dust from increased industrialization and forest fires across the northern hemisphere. The apparent decline is greatest around the ice sheet’s edges, but it also is occurring in the high elevation interior known as the dry snow zone, where the reflectivity is effectively reset each winter by new snowfall.
In trying to explain the apparent decline in reflectivity, Polashenski and his colleagues analyzed dozens of snow-pit samples from the 2012–2014 snowfalls across northern Greenland and compared them with samples from earlier years. The results showed no significant change in the quantity of black carbon deposited for the past 60 years or the quantity and mineralogical makeup of dust compared to the last 12,000 years, meaning that deposition of these light-absorbing impurities is not a primary cause of reflectivity reduction or surface melting in the dry snow zone. Algae growth, which darkens ice, also was ruled out as a factor. The findings suggest that the apparent decline in the dry snow zone’s reflectivity is attributable to degradation of sensors in the aging NASA satellites that track land, ocean, and atmospheric features.
The study’s findings don’t apply to the ice sheet’s lower elevations, where surface melting, soot and dust result in more pronounced declines in reflectivity and where warmer temperatures may promote algae growth that further erodes reflectivity.
Image Sensor Breakthrough
In a breakthrough that may usher in the next generation of light-sensing technology with potential applications in scientific research and cell phone photography, Professor Eric Fossum and Thayer PhD candidate Jiaju Ma have developed a revolutionary Quantum Image Sensor (QIS) that can sense and count a single electron.
Fossum, inventor of the CMOS image sensor that is in billions of cell phone cameras, and Ma have worked together on the project for more than three years and shared authorship of a June 2015 paper on their invention, published in IEEE Electron Device Letters.
Their new sensor has the capability to significantly enhance low-light sensitivity. This is particularly important in applications such as “security cameras, astronomy, or life science imaging—like seeing how cells react under a microscope—where there’s only just a few photons,” says Fossum.
“When we build an image sensor, we build a chip that is sensitive to these photons. We were able to build a new kind of pixel with a sensitivity so high we could see one electron above all the background noise,” he says.
The new pixels are considerably smaller than regular pixels since they are designed to sense only one photon, but many more are placed on the sensor to capture the same amount of total photons from the image. “We’d like to have 1 billion pixels on the sensor, and we’ll still keep the sensor the same size,” says Ma.
These new pixels are able to sense and count a single electron without resorting to extreme measures, such as cooling the sensor to minus 60°C and/or avalanche multiplication. “Avalanche multiplication may be thought of as an electrically induced chain reaction, but the strong electric fields necessary lead to reliability issues, and it is difficult to make small pixels,” says Fossum.
Fossum and Ma have kept industry in mind as they solved the QIS’s technical challenges. “We deliberately wanted to invent it in a way that is almost completely compatible with today’s CMOS image sensor technology so it’s easy for industry to adopt it,” says Fossum.
The QIS project has been funded by the Silicon Valley company Rambus Inc., where Ma has served as an intern during the past two years. “A company representative offered some extraordinarily high praise for him,” says Fossum, “calling him a ‘superstar intern.’ We hope to continue our collaborations with Rambus in the future.”
Imaging Chemo’s Effectiveness
Professors Shudong Jiang, Brian Pogue, and Keith Paulsen are using an imaging technique called diffuse optical spectroscopic tomography (DOST) to enable quick assessment of breast tumor response to neoadjuvant chemotherapy given to patients to shrink tumors and reduce metastasis prior to surgery.
By measuring tumor hemoglobin and oxygen saturation levels—indicators of the presence of tiny blood vessels that cancer tumors need in order to grow—DOST can reveal whether the tumor is responding to chemotherapy. Knowing that a tumor is not responding could spare women from further chemotherapy and wasting months before moving on to surgery or other treatments.
“Today’s standard cancer imaging techniques—MRI, mammography, and ultrasound—are based on tumor volume, which can require at least three months of treatment,” says Jiang. “With DOST we are able to capture physiological variations in the tumor after only one treatment and potentially even before neoadjuvant chemotherapy treatment begins. This provides doctors with the information needed to better manage patient care at an early stage.”
Jiang, Pogue, Paulsen and their team have used DOST to evaluate 19 patients at Dartmouth-Hitchcock Medical Center before, during, and after neoadjuvant chemo treatment. The imaging technique is noninvasive, does not use ionizing radiation, and does not involve costly instrumentation. The team is working on developing a portable and compact DOST system for use in clinical settings large and small.