Glaciers under observation – a laser journey back in time

• Researchers from Wrocław decided to reconstruct how Earth's gravitational field has changed.
• A novel method is based on observing anomalies in the motion of artificial satellites and on "orbital memory."
• The ice mass in the polar regions was stable until the 1990s.
• The shape of the Earth is still changing due to geological and hydrological processes.
• Filip Gałdyn from the Doctoral School at UPWr combines SLR data with data from the GRACE mission.
• The Wrocław team is involved in further satellite projects.

Geodesists from UPWr developed a method to measure the extent of ice loss deep in Greenland and Antarctica. – This is a serious problem related to climate warming, and in these difficult-to-access polar regions, measurement equipment cannot be installed, so there were no observations there. Hence, other methods of monitoring glacier melting are needed – says Professor Krzysztof Sośnica from the Institute of Geodesy and Geoinformatics, co-author of the article "Long-Term Ice Mass Changes in Greenland and Antarctica Determined Using Laser Ranging to Satellites," published in the prestigious journal Remote Sensing of Environment. The publication is the result of joint research with scientists from the University of Bern.

The GRACE mission – twins in orbit

Above our heads, hundreds of thousands of artificial satellites orbit the Earth. They serve meteorological, geodetic, and agricultural purposes, allow for the observation of water circulation and salinity in oceans, transmit TV and internet signals, and enable the use of GPS navigation. We know what has been happening to Earth in recent years partly thanks to GRACE (Gravity Recovery and Climate Experiment). This American-German satellite mission measures Earth's gravitational anomalies. NASA and the German Aerospace Center scientists gather and analyze data that is far more accurate than before.

– We didn’t previously have the technical capabilities for such precise measurements – says Professor Sośnica.

The twin GRACE satellites were launched into a polar orbit in the spring of 2002, separated by 220 kilometers. Orbiting Earth multiple times a day, they measured the distance between each other with an accuracy of 10 micrometers.

– That's about one-tenth the thickness of a human hair – the professor added.

The satellites maintained a constant distance until they encountered a gravitational anomaly caused by, for example, greater mass. If satellite A flew over such an irregularity in Earth's shape, it was more strongly attracted and slowed down. Satellite B would also slow down, but later. The change in the mutual position of the twins, measured using microwaves, allowed the creation of a map of Earth's gravitational anomalies. These anomalies result from varying mass distribution on Earth's surface, including the movement of water and ice masses. This enables scientists to determine the impact of ocean circulation on the gravitational field and the change in ice mass, which, when melting, alters sea levels.

GRACE accomplished its mission: the first satellite burned up in Earth's atmosphere in 2017, followed by the second one a year later.

– Some satellites burn up, while others are moved to a 'graveyard orbit,' where they avoid collisions with other satellites. They orbit passively, and when they run out of fuel, they become uncontrolled space debris. Unfortunately, space is becoming increasingly cluttered with debris, and there are no effective methods to eliminate these inactive satellites, rocket parts, or fragments – the geodesist explains.

The successor to GRACE is the twin pair GRACE Follow-On, launched in 2018, a year after the "space death" of its predecessor. It still orbits above us, measuring Earth and providing data used by geophysicists, oceanographers, hydrologists, and glaciologists. But what happened to Earth before 2002, before the first GRACE was launched? How can we obtain missing data from the gap year between GRACE and GRACE Follow-On and during the satellite eclipse seasons of GRACE? Researchers from Wrocław and Switzerland decided to go back in time and reconstruct what had previously happened to Earth's shape, its gravitational field, and mass changes.

Laser pulses back and forth

In this journey through time, they utilized satellite laser ranging (SLR) measurements, particularly to satellites such as LAGEOS, Starlette, Stella, LARES, and Ajisai. These geodetic satellites have been orbiting the planet, some since the mid-1970s. They are spherical, with radii ranging from 12 to 110 centimeters, covered in prisms made of special quartz glass.

– This allows them to reflect laser light in the same direction from which the light beam was sent. This helps determine their position using laser rangefinders – explains Filip Gałdyn from the Doctoral School at UPWr, the first author of the article. The light comes from an observatory equipped with laser rangefinders. The station sends a laser pulse to the satellite, which reflects it back to the station. This allows the distance between the satellite and the laser station to be measured, which in turn helps measure Earth's shape.

There are only 20 operational laser stations worldwide, including one in Poland, located in Borowiec near Poznań. – It’s a very expensive technology – the PhD student explains.

Unfortunately, there are no laser stations in the polar regions we studied that could measure distances to satellites, Gałdyn added. – That’s why we used the so-called 'orbital memory.' When a satellite passes over an area with an unusual mass distribution, it changes its orbit, and its motion is disturbed.

– Based on laser distances to satellites, we determined the orbital changes. Then, we calculated what forces must have acted to alter the satellite’s orbit before and after passing over the polar regions. These forces are related to Earth's mass, that is, changes in its shape. We converted the changes in Earth's shape into the ice mass – says Professor Sośnica. – In short, we developed a method to recover the time-variable gravitational field, even in the polar regions, by observing what happened to the satellites before and after they flew over the poles. Normally, satellites would need to observe a given area, like capturing a photo of Earth’s surface. Our method is somewhat unconventional because we didn't have direct satellite observations over Greenland and Antarctica.

To ensure precision, measurements used the "memory" of not just one but up to nine satellites simultaneously. This allowed researchers to estimate changes in the ice cover of Greenland and Antarctica between 1995 and 2021, even without direct satellite observations over the poles. Before 1995, some satellites like Stella or LAGEOS-2 were not yet in orbit, and early laser observations were sparse. As a result, stable and accurate solutions could only be obtained from 1995 onward. The findings, however, were far from optimistic.

Greenland is disappearing at an alarming rate

The numbers are frightening: six thousand tons of ice disappear from Greenland every second! This means that, on average, 190 gigatons—billion tons—of ice melt there every year.

The melting island is not just a problem for its inhabitants—the water flowing from Greenland into the North Atlantic disrupts ocean and atmospheric circulation. This is one of the reasons behind heatwaves and other climate disturbances in Europe in recent years.

According to SLR data, Greenland's ice mass was relatively stable until the 1990s. After this period of stability, more rapid changes began, and the ice loss accelerated. The most significant ice loss occurred between 2005–2010 (–213.9 billion tons annually) and 2010–2015 (–287.2 billion tons annually). Over the next four years, the melting slowed down slightly, but from 2019 until today, it has picked up again. The island’s coastlines are melting particularly fast, while the center still accumulates some ice. In West Antarctica, melting began later, but between 2010 and 2015, the largest ice loss was observed.

– The period from 2015 to 2020 saw a slowdown in Antarctic ice loss, a reduction in the negative trend, and a return to the situation of the 1990s, when there were no significant changes in ice mass – says Professor Sośnica. Unfortunately, the latest data shows that the ice loss is increasing once again.

– Thus, we see that the ice mass balance in polar regions is a dynamic process – the professor adds. – There’s significant loss at the coastlines, but also moderate growth in the central parts.

For example, in East Antarctica, ice is likely accumulating, albeit slowly. – However, what we perceive as increased ice cover might be misleading, as Earth's crust is rising. Estimates of how quickly the crust is uplifting under the ice are very uncertain. Even if ice is accumulating, it’s certainly not enough to compensate for the losses in the western part – cautioned the geodesist.

– In general, climate warming is intensifying. The concentration of carbon dioxide in the atmosphere is higher than ever before since humans have been on Earth – he added. – We must do everything we can to keep the planet in balance. The melting of glaciers in polar regions reveals dark rocks that heat up very quickly. Ice reflects sunlight, creating the so-called albedo effect. The warming rocks and the absence of albedo lead to particularly high heat accumulation in polar areas.

Our planet – Earth, a potato, or a pear?

The results of the research were published in Remote Sensing of Environment, ranked number one in environmental research journals. Models of Earth's shape changes, derived from satellite data, were placed on the platform of the International Centre for Global Earth Models (ICGEM), managed by the German Research Centre for Geosciences (GFZ) in Potsdam.

What is the shape of the Earth? Throughout history, there have been many speculations, starting with ancient beliefs that Earth was a flat disk floating on the ocean or resting on the back of a giant turtle. Eratosthenes, Pythagoras, and Aristotle all demonstrated Earth's roundness. However, our planet is not as round as the globes we learned from in school. It is more of an ellipsoid, as its rotation causes centrifugal force that flattens the Earth at the poles and bulges it at the equator.

– We got a more accurate understanding of Earth's shape in 1957 when the first satellite, the Soviet Sputnik I, was launched into orbit. Today, thanks to precise measurements, we know that our planet is even more irregular than an ellipsoid. Scientifically, it’s called a 'geoid,' – Professor Sośnica says, holding a green 3D-printed model resembling a potato. – The irregular shape is due to the distribution of rock densities. Tectonic plates overlap and spread apart.

The most significant surface deviations are visible in the Himalayas.

– This shape is not fixed – said Professor Sośnica. – It is constantly changing due to geological processes, such as the uplift of mountain ranges and continents. Hydrological cycles, especially in the Amazon and Central Africa, also affect the shape recorded by satellites. The water cycle in nature causes, for example, the Congo Basin or the Amazon to gain so much water during the rainy season that it changes Earth's shape by a few centimeters.

The ice loss observed by scientists, however, is not cyclical but long-term. Since the start of satellite measurements in Greenland and Antarctica, the ice has been consistently decreasing. The GRACE mission has also been able to observe changes in Earth's shape caused by events like earthquakes, such as the one near the coast of Sumatra in 2004.

Professor Sośnica emphasizes that the GRACE missions were not without problems. After eight years in orbit, the batteries in the GRACE satellites failed. Whenever the satellites entered Earth’s shadow, measurements could not be taken. In the GRACE Follow-On mission, the accelerometer, which measures perturbations such as air drag, solar pressure, and satellite heating effects, also failed. Data from the accelerometer on the second satellite, with the necessary corrections, were used as a workaround.

– The failures of the accelerometers and the uneven heating of GRACE and GRACE Follow-On satellites rendered the missions unsuitable for determining Earth's flattening or its 'pear-shapedness,' that is, the degree to which the southern hemisphere bulges more than the northern hemisphere. –This problem, too, was solved using laser measurements to satellites, the professor explains.

Taking the best of every method

In a project funded by the National Science Centre, the researchers from Wrocław collaborated with scientists from the Astronomical Institute of the University of Bern, including physicist and astronomer Professor Adrian Jäggi and satellite geodesist Ulrich Meyer.

– That’s where one of the world’s 20 SLR observatories is located, and it’s where I completed my PhD in physics and astronomy on laser measurements to geodetic satellites – explains Krzysztof Sośnica, head of the Department of Geodesy.

He established the Laser Ranging Analysis Centre at Wrocław, which specializes in monitoring the accuracy of satellite orbits, including for the European Galileo navigation system.

Professor Krzysztof Sośnica, one of the youngest professors in Poland (awarded the title at age 35), is a member of the Scientific Advisory Committee for GNSS at the European Space Agency and chairman of the "Coordination of Space Techniques" Subcommission in the International Association of Geodesy. He has won numerous awards, including the Minister’s ("Science Genius"), the Polish Academy of Sciences, the European Space Agency, and the European Union of Earth Sciences awards for outstanding contributions to global geodesy. He was also named one of the "30 Creative Minds of Wrocław" in a city project a few years ago.

– It’s important to have motivated and creative people to work with – says the professor. – I’m lucky to have been able to create such a cohesive team at the university. He was the doctoral supervisor of dr inż. Radosław Zajdel, a co-author of the article, and is currently guiding the doctoral research of Filip Gałdyn as an assistant supervisor. In his next paper, the PhD student is combining SLR data with data from the GRACE and GRACE Follow-On missions. In the future, he hopes to integrate data from Global Navigation Satellite Systems (GNSS) as well.

– By combining these approaches, we can take the best from each method. For example, GRACE struggled to determine Earth's flattening, but this comprehensive approach yields more precise results. It not only allows us to observe the state of glaciers but also changes in water levels – says the doctoral student. He is working to identify areas where adverse hydrological processes are accelerating, mainly due to human activity.

– This could include water loss in lakes or groundwater depletion. For example, the Caspian Sea has been losing water for a long time, and this process has accelerated over the past 10 years. The same is true for the Aral Sea, which is shrinking due to extensive irrigation for agriculture – Gałdyn explains. – Unfortunately, large-scale agriculture exploits resources to the limit in these regions.

Moonlight – a journey to the future

One of the advantages of using spherical satellites is that they don’t require any onboard electronics. As a result, they can survive for hundreds of thousands of years, orbiting Earth. Observing their motion helps researchers understand the processes occurring on our planet.

– This is incredibly important because, without knowledge of our negative impact on the environment, humans would continue exploiting natural resources without restraint. Planet Earth is our home, and for now, we don’t have an alternative, although many countries are preparing to expand to other celestial bodies. The first target will be the Moon – Professor Sośnica explained. His team completed a satellite mission project last year, commissioned by the European Space Agency, for positioning, navigation, and communication on the Moon. The project has entered the implementation phase under the Moonlight program. Astronauts will likely be able to use the first version of a "GPS system" for the Moon by 2027.

In addition to the Moonlight mission, the team is involved in the GENESIS project. This will be the first satellite mission observed not only with traditional techniques like GPS, Galileo, or SLR but also by radio telescopes. These telescopes usually observe distant galaxies, particularly those with active black holes at their centers. A black hole, consuming surrounding matter, emits energy that we observe as microwaves on Earth. These active galaxies, known as quasars, are used in geodesy as fixed points in the sky to determine how Earth rotates relative to the celestial system. The GENESIS mission will be the first satellite mission to integrate the four main geodetic observation techniques, including radio telescope observations, on board a single satellite.

– The future holds many inspiring satellite missions, and our Wrocław team is deeply involved in their preparation – says the professor.

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  Prof. Sośnica: – Copernicus changed our perspective on the universe

  The year 2023 has been declared the Year of Copernicus to mark the 550th anniversary of the astronomer's birth. So in celebration of him, Prof. Krzysztof Sośnica talks about the significance of the Copernican breakthrough, the search for beauty in science, space missions and the participation of UPWr scientists in the construction of a base on the Moon.