To find features like groundwater under Earth's surface — or under the
surface of another world — scientists can sense the subtle marks those
features leave in the planet's gravitational field.
But those measurements aren't easy to get; you need very sensitive
instruments, and even the slightest vibrations can throw off the
measurements. Now, a group of physicists has demonstrated an hourglass-like
gravity-measuring device that they say helps to overcome this
challenge.
Gravity-measuring devices, called gravimeters, themselves aren't new.
They're used for everything from probing physical constants to mapping
rugged landscapes. Modern, cutting-edge gravimeters use atoms. If you pulse
two atoms with lasers and send them out to different points, a gravitational
field will affect the two in slightly different ways. You can measure that
gravitational field by overlapping those two atoms and puzzling out the
differences in their quantum properties.
But when physicists try to boost the resolution in attempts to see objects
the size of a few meters, such as pipes and passages underground,
conventional gravity sensors hit a wall. Ground variations, temperature
shifts and even slight magnetic fields can throw them off.
So the new sensor takes a different approach. The researchers call it an
hourglass; each "bulb" contains a cloud of rubidium atoms trapped in a
magnetic cage, pulsed through with a laser. The dual clouds mean that that
device effectively has two separate gravimeters. As a result, the
researchers can not only measure a gravitational field but also measure it
at two different heights.
It's not the most sensitive quantum gravity sensor in the world, but it is
one of the first to leave the lab. In a real-world test, this hourglass-like
gravimeter detected a utility tunnel buried under a road in Birmingham,
England.
"As far as we know, our instrument has been the first to detect a real
underground target of relevance to civil engineering outside of the
laboratory environment," study co-author Kai Bongs, a physicist at the
University of Birmingham in the United Kingdom, told Space.com. "This is
really a breakthrough in making quantum technology practical."
The new gravimeter could become a wonderful tool for mapping built-up
features underground.
And these gravimeters aren't limited to use on Earth. In fact, the European
Space Agency (ESA) is already interested in taking them to the launchpad.
ESA's next generation of Earth observation satellites might carry sensors
like these, measuring things like underground water, the circulation of the
world's oceans and how these things are being affected by climate change.
"This might be extended to the exploration of other planets in the solar
system, understanding more about their inner structure," Bongs told
Space.com.
Sending gravimeters to study other worlds isn't new. In 2012, NASA's GRAIL
mission sent a pair of spacecraft to map the moon's gravitational field and
peel away its surface. That mission probed the layers of the moon's interior
with unprecedented accuracy, studied the material under impact basins and
found what might be the signatures of underground caverns.
Now, if ESA's interest is any indication, these next-generation gravimeters
could be used to find underground water on the moon — or on other worlds,
like Mars.
The researchers published their work Feb. 23 in the journal Nature.
Reference:
Stray, B., Lamb, A., Kaushik, A. et al. Quantum sensing for gravity
cartography. Nature 602, 590–594 (2022).
DOI: 10.1038/s41586-021-04315-3
Tags:
Physics