Lunar world in a different light
This image is a radio observation of the Moon, taken by the Green Bank Observatory (GBO) in West Virginia- this telescope is 20 meters in diameter and can detect wavelengths of light between 1300 MHz and 1800 MHz.
In my previous post, we talked about our process of taking a picture of the lunar surface and the information that we can retain from it. However, what happens when some parts of the sky are too bright to observe things that we want? Well, this is just one application of another type of picture we can take with telescopes! I am talking about radio imaging, the same way we take pictures with the light we use to see; we can capture an “image“ of the Moon in radio light. Once again, thanks to the Skynet Robic Telescope Network, we were able to construct a stunning radio picture of the Moon that reveals numerous amounts of information about our near celestial neighbor.
Unlike traditional visible light photography, radio imaging uses radio waves to create an image. The picture of the Moon taken shows a series of colorful blotches and swirls that are caused by variations in temperature and composition of the Moon's surface. Different types of rock and soil on the Moon absorb and release heat differently, which creates these distinctive patterns when viewed in radio waves.
So the whole basis of this observation is to determine whether the radio light we see from the Moon is reflected sunlight like the optical light we are used to seeing. Although we are only showing a sliver of why radio astronomy is a useful tool, below, I explain a bit more about why examining radio light is a revolutionary tool for our scientists today.
About Radio light and why is it helpful?
One of the advantages of radio imaging is that it allows us to see things that are invisible to the naked eye and traditional optical telescopes. Radio waves can penetrate through dust clouds and atmospheric interference, which means we can observe objects that might be obscured in visible light.
Radio astronomy is also useful in studying objects that emit radio waves, such as pulsars, quasars, and galaxies. These objects often emit radio waves at frequencies that are not visible to the human eye but can be detected by radio telescopes.
In addition, radio waves can provide information about an object's composition, temperature, and magnetic fields, which can be difficult to discern through optical observations alone. By combining radio and optical data, astronomers can get a more complete understanding of the objects they are studying.
Furthermore, radio astronomy has applications beyond just studying celestial objects. It is also used in communication and navigation, as radio waves can travel long distances through space without significant degradation.
Overall, radio astronomy provides a valuable complement to optical astronomy, offering a unique perspective on the universe and revealing insights that might be hidden from optical telescopes alone.
How did we construct the image?
This section may be a bit technical for anyone that has never worked with telescopes; however, I will try my best to simplify as much as possible. Feel free to skip this part and go straight to what we can conclude from looking at the image in the following sections!
Creating a radio map of the Moon is a fascinating process that involves using a telescope and specialized equipment to detect radio waves that are emitted by the Moon. This process provides us with a unique perspective on our closest celestial neighbor and allows us to learn more about the Moon's physical properties and characteristics.
The radio waves that the Moon emits can be detected using a specialized telescope equipped with a radio receiver. In the case of the radio map of the Moon taken through the Skynet network, the telescope used was the GBO 20m’s L-band receiver. This receiver detects radio waves with frequencies between 1300 MHz and 1800 MHz and has a high spectral resolution mode with an "HI" bandpass filter that narrows the detector's response to only 1355 – 1435 MHz. This removes frequencies that are known to have a lot of human-made interference.
Once the radio waves are detected, they are converted into images using a software tool called Afterglow. The images that are produced allow researchers to measure the brightness of the Moon. However, the brightness measurements are initially in arbitrary units and change slowly with time. To convert these measurements to standard units, researchers need to map a "calibration" source around the same time. This is a source of known flux density, which is the amount of light received by the telescope. For the radio map of the Moon, the calibration source used was Vir A (3C 274).
To create the radio map of the Moon, the telescope is positioned in a way that allows it to capture the radio waves efficiently. This involves designing the observation's field of view (FOV) and then sweeping around to sample this FOV efficiently. In the case of this radio map, the FOV was only 6 x 6 beam widths since only a single source was being mapped.
After obtaining all four images of the Moon, we need to measure the brightness of the Moon in each image and calculate the flux density for each image. This is done using a method known as Aperture Photometry, which uses a set of measurements to calculate the brightness of an object.
Overall, creating a radio map of the Moon is a complex and fascinating process that provides us with valuable insights into the physical properties and characteristics of the Moon. It allows us to learn more about our closest celestial neighbor and opens up new avenues of research for scientists around the world. Compared to optical astronomy, radio astronomy is useful for studying objects that are invisible to optical telescopes and for detecting emissions that are not visible to the human eye.
The Scientific Process
As I said before, we are doing the observation to determine whether the radio light we observe is reflected sunlight, just as the optical light we are used to seeing from the Moon. To solve this mystery, we took two pictures of the Moon, one when it was a bit less than half-lit (waning crescent) and another when it was more than half-lit (waning gibbous). The first picture only captured 44% of the Moon's surface being lit up by the Sun, while the second picture captured 66%. Interestingly, even though the second picture showed more of the Moon being lit up, it wasn't that much brighter than the first one. In fact, the first picture was slightly brighter in radio waves, measuring at 927 Jansky, compared to 845 Jansky for the second picture. However, this is where our calibration observations come into play. We can compare the observations of Vir A to its corresponding Moon radio image, ensuring nothing weird is happening. And sure enough, we are able to see that Vir A radio light also slightly changes with its corresponding Moon observations. Thus, we can say that the variation could be caused to the fluctuations of radio light throughout space.
What information can we retain?
I will not bore you with all the number crunching we did; however, I will provide some of the conclusions we came to. The first is that throughout our observations, the amount of sunlight the Moon is exposed to does not affect the radio light that we receive. Secondly, the Moon is fairly chilly, coming in at around -17 F to 37 F!
So what can this information tell us about the Moon? Well, the first tells us that the radio light we are observing is not reflected sunlight but rather absorbed sunlight that was being re-emitted in radio light. If it were reflected sunlight, the more sunlight is exposed to the Moon, the more radio light we would observe, but we observed the opposite! Next, the temperature of the Moon tells us a plethora of things; however, one thing we can draw from this is that there is no atmosphere to hold any heat, thus making the surface of the Moon much colder than here on Earth. Being the good scientists we are, obviously we checked previously done work with some of the researchers in the field, and our values align fairly well with theirs, so we know we are getting numbers that make sense.
In conclusion, the radio picture of the Moon offers a unique and fascinating view of our closest celestial neighbor. It highlights the beauty and complexity of the Moon's surface while revealing important insights into its composition and geological history.
Acknowledgments
I would like to thank my Professor, Dr. Reichart, and my TA, Mae Dubay, for the opportunity to capture these amazing images and the opportunity to access the amazing telescopes within the Skynet Network! Also, special thanks to my group mates for all of their help and support throughout this process!
