woensdag 6 mei 2015

Measuring mass changes with satellites

In my latest paper, GRACE gravity observations constrain Weichselian ice thickness in the Barents Sea, I use an incredible satellite data set. Starting from 2003, NASA's GRACE satellite mission is producing monthly "images" of the Earth's gravity field. These observations are so accurate that certain mass transport is visible, if you look at the differences between these "images". This new data set gives tons of information about many different natural processes. Oh and the figures are so cool!

For this blog, I downloaded GRACE data (click here for information, and here for the data) from the period between 2003 and 2013, CSR release. For every spot on the Earth, I calculated the linear gravity change in this period, so not the seasonal variations. The results are shown in the figure below:

GRACE gravity change during 2003 and 2013 using a 'white' noise filter of ±0.5 microGal/year.
The red areas show localised gravity increase (or mass increase) and the blue areas show localised gravity decrease (or mass decrease). I have capped the negative areas (blue) at 2 mircoGal/yr, but some areas, like West Antarctic, can go up to -8 mircoGal/year. However, smaller signals are made visible by performing this truncation. Of course, mass can neither be created nor destroyed, so integrating this signal should give zero. It now looks a bit of to the negative side. The large localised negative mass losses in Greenland, Alaska, and West Antarctic are of course linked to melting of glaciers and land ice. The meltwater is redistributed over the entire oceanic surface of the Earth. This generates a long wavelength signal with a very small amplitude, which is not visible in this map, because of my 'white' noise filter. However, when this would be taken into account the integral of this signal would be zero.

But enough physical and mathematical mambo jambo! Look at the figure (click on it to zoom in), what do you see? The melting of land ice is prominent in Greenland, Alaska, and West Antarctic, but can also be seen in the Iceland, Pantagonia (South of Chili), and localised areas on the east coast of Antarctic. Other, negative areas are related to non-ice hydrology, such as the Mississippi basin and the drying of the Caspian Sea. Also, the signals in the Himalaya can be treated as groundwater motion. Some positive hydrological signal can be seen in the Amazon basin, Zambezi river basin, and in Ghana. 

Larger scale areas with positive gravity change are related to Glacial Isostatic Adjustment (GIA), which I am working on in my PhD research. The biggest signal is in Laurentia (Canada). Here, GRACE has given evidence for a multi-dome ice sheet during the last ice age. Can you find the two maxima? Another well known GIA area is Fennoscandia (Scandinavia and Finland), also here the positive signal is mantle material flowing back to the area due to the uplift of the crust, responding to the isostatic equilibrium. Antarctic shows GIA related positive gravity change, but due to the ongoing ice transport it is difficult to tell what is present-day ice transport and GIA-related magma transport. 

Moreover, the figure shows the effects of three very large earthquakes in the period of 2003-2013. Can you spot them? I will help you. The easiest visible is the 2004 Sumatra earthquake (9.1), which is clearly visible as an elongated gravity dipole. The gravity dipole of the 2011 Honshu, Japan earthquake (9.0) is also visible. The last one is a bit tricky, because of its smaller magnitude, the 2010 Chili earthquake (8.8), where the positive part of the gravity dipole is barely visible above the noise.

It is incredible to see that by measuring the tiniest movements of orbiting satellites, scientists and engineers have created this new and incredible data set, which allows us to observe and learn more about our home planet.