Arecibo, A Giant in the Field

This week, Conor Hayes pays tribute the Arecibo Radio Telescope. You may know it in connection with SETI (or perhaps from movies!) but its ability to make observations along the ecliptic made it one of Planetary Science's most effective instruments. Unfortuately, the telescope recently experienced a structural failure which will require its demolition. 

Photo above:  The Arecibo Observatory as seen in June 2019. (CC BY-SA 4.0, https://commons.wikimedia.org/wiki/File:Arecibo_Radiotelescopio_SJU_06_2019_7472.jpg)

by Conor Hayes

On November 19, 2020, the astronomical community lost a (literal) giant. Following the failure of two support cables that made repair work dangerously unsafe, the National Science Foundation announced that they would be decommissioning the Arecibo Observatory. This 305-metre radio telescope has faithfully watched the skies for nearly 60 years, during which time it became perhaps the most well-known telescope on the planet, appearing in popular media like GoldenEye, Contact, and The X-Files. Its loss will be felt by all of us, most acutely by those who have dedicated their lives to radio astronomy. Given this, I felt it was appropriate to write a little bit about radio telescopes in general, as well as some of the notable discoveries made using Arecibo.

Why was Arecibo so large? When designing a new telescope, you need to balance the quality of the data it can output with how expensive it will be to build. The resolution of images taken by a telescope is covered by the following equation: R = λ / D, where R is the resolution, λ is the wavelength of the light you are observing with, and D is the diameter of the telescope’s primary mirror or objective lens.

From this equation, you can see the two-front war that radio telescopes are fighting. As with all telescopes, the larger your light collection area is, the higher resolution your data will be. However, radio telescopes face an additional problem, in that radio waves have the longest wavelengths of all electromagnetic radiation. Because the wavelength appears in the numerator of the resolution equation, observations done on, say, a 10-metre radio telescope will be significantly lower resolution than observations done on a 10-metre optical telescope. This means that radio telescopes must be significantly larger than those used to observe at shorter wavelengths if they want to achieve a comparable resolution. Fortunately, because radio waves are less impacted by small-scale imperfections in the telescope’s surface, we can get away with constructing them out of sheets of metal (or even a fine metal mesh!) rather than carefully-polished mirrors, greatly reducing the cost.

Of course, actually building and maintaining such a large structure is still massively expensive, to the point that Arecibo spent much of the last 20 years of its life under constant threat of being shut down due to funding shortfalls. To avoid this, many radio observatories, like the Very Large Array (VLA) in New Mexico, the Atacama Large Millimeter Array (ALMA) in Chile, and the upcoming Square Kilometre Array (SKA) in Australia and South Africa, use a technique called very-long-baseline interferometry (VLBI) to combine a large number of smaller telescopes into one telescope with a much larger effective collection area. VLBI was most recently leveraged to combine many radio telescopes across the planet into the Event Horizon Telescope, which took the first direct image of a black hole.

As the largest radio telescope in the world from its completion in 1963 to the construction of the Five-hundred-metre Aperture Spherical Telescope (FAST) in 2011, the Arecibo Observatory was at the forefront of radio astronomy and contributed to a number of important scientific discoveries.

In our own Solar System, Arecibo was used to determine that Mercury has a rotational period of 59 days, rather than being tidally locked as was previously assumed. Relevant to my own research into ice in the permanently shadowed regions (PSRs) of the Moon, Arecibo made some of the first radar measurements of ice in the PSRs of Mercury, observations that were later confirmed by the MESSENGER spacecraft.

Elsewhere in the universe, Arecibo was responsible for a number of other “firsts.” In 1968, scientists using Arecibo measured the rotational period of the Crab Nebula pulsar, which provided the first direct evidence of the existence of neutron stars. Just six years later, Arecibo produced its first Nobel Prize-winning research in the form of the discovery of a binary pulsar system. This system was found to have a gradually decreasing orbital period, which is consistent with energy loss in the form of gravitational waves (though gravitational waves would not be directly measured until 2016). Though the search for exoplanets has been popularized by space telescopes like Kepler and TESS, it was actually Arecibo that first found extrasolar planets due to irregularities in the measured rotational period of a pulsar. To date, one of these still holds the record for the smallest known exoplanet, with a mass around twice that of the Moon.

Finally, Arecibo played a key role in the Search for Extra-Terrestial Intelligence (SETI) and Messaging to Extra-Terrestrial Intelligence (METI) programs. Arecibo was one of the primary telescopes used by the SETI Institute to listen for possible communications from extraterrestrial life, and in 1974 broadcast a short message containing information about humanity and our location in the galaxy to M13, a globular star cluster located 25,000 light years away. Although seen as reckless and possibly dangerous by many, it is widely accepted that nothing will come of this message due to the vast distances involved.

 

The Arecibo Message, which was sent from the observatory towards the globular cluster M13 in November 1974. It contains information on the genetic structure of humans, our physical characteristics, and how to find us. Many felt this was a dangerous amount of information to be sending to a hypothetical alien civilization whose motivations were unknown. (CC BY-SA 3.0, https://commons.wikimedia.org/wiki/File:Arecibo_message.svg)

The Arecibo Observatory will certainly be missed, but it leaves behind a vast and rich legacy of scientific discoveries. With FAST now operational and the 100-metre Green Bank Telescope still in good working order, along with a number of existing and new interferometric arrays around the planet, the world of radio astronomy continues to be well-served. Though the end of this particular telescope’s career was perhaps more abrupt than some might have imagined, it is leaving us having done more than its fair share of the hard work of advancing our knowledge of the universe.

Mars’ Family of Rovers

In this week's post, Grace discusses the different rovers that have been to Mars in response to a query from her sister. One of the most important things we can do as scientists is to translate the excitement and meaning of what we do for the public. (above) Image credit NASA/JPL-Caltech.

By Grace Bischof

A few days ago, I received a text message from my sister asking for a fun space fact to tell the 5-year-olds she teaches who are “super into space”. I thought about all of the interesting things I’ve learned since joining PVL, from both my own and other lab members’ work, but was pretty certain that the seasonal variation of methane on Mars might not be very interesting to young children. Having just finished shadowing my first MSL shift, I had the Curiosity rover on my mind. I realized that Curiosity has been roaming the Martian surface for longer than the kids in her class have been alive. I told my sister to explain to the students that Curiosity is older than them; they might not have found it a very fun fact, but I sure did.

 

So what other rovers are there on Mars, and how old are they? Let’s work from youngest to oldest.

 

Assuming the landing goes to plan, the Perseverance rover should touch down in Jezero Crater on February 18th of 2021 as part of the Mars 2020 mission. If the age of the rover starts at landing, Perseverance might still be considered a zygote rover. Regardless of its infant age, Perseverance is set to be the largest rover to touch down on the Martian surface. Weighing in at 2260 lbs, Perseverance is approximately the size of an SUV. Perseverance is equipped with 7 instruments and has the ultimate goal of searching for signs of past or present life on MArs.

 

Curiosity is the second youngest rover that is currently on Mars. Landing on the red planet in 2012, Curiosity is set to reach its 3000^th sol in early 2021. Curiosity resides in Gale Crater and has traveled more than 21 km in its 8 years of operation. The MSL mission had 8 main objectives, which largely centered around determining if Mars was ever hospitable to life. The rover is equipped with 10 instruments, including the Rover Environmental Monitoring System and the Sample Analysis at Mars, which help to understand the meteorology and atmospheric gases on Mars, respectively. Curiosity’s many cameras have captured interesting Martian features, such as the major dust storm in Martian Year 34. In its 8 years on the planet, Curiosity has been an indispensable asset to understanding the habitability of Mars.

 

Leaping back 17 years, the twin rovers, Spirit and Opportunity, landed at separate locations on Mars in January 2003. Smaller than Perseverance and Curiosity, the twins are about the size of a golf cart. Spirit and Opportunity are the fastest rovers to roam mars, moving at a neck-breaking speed of 0.16 km/hour. Primarily, Spirit and Opportunity were to identify rocks and soil from the Martian surface. Though originally planned as a 90-sol mission, Spirit traveled the dusty plains of Mars until May 2009, when it became stuck in soft soil. Attempts to free Spirit from the soil were carried out for 9 months, but were eventually abandoned. Spirit acted as a stationary instrument until it lost connection to Earth in March 2010. Opportunity stayed active until June 2018, losing its signal to Earth after its solar panels were covered in dust from the MY 34 dust storm. Opportunity holds the record for most distance traveled on a non-Earth world, having driven a total of 42.2 km during its 11 active years. 

 

 

Opportunity: Planetary Marathoner. The Opportunity rover has covered more ground than any other rover on an extraterrestrial surface.
Credit: NASA/JPL-Caltech - https://mars.nasa.gov/resources/6471/driving-distances-on-mars-and-the-moon/

 

The oldest rover to land on Mars is called Sojourner, and landed on the planet in July 1997 (which makes Sojourner older than me by one year). Sojourner is much smaller than its younger siblings, resembling a microwave-oven in size. Carrying only 2 instruments, Sojourner took hundreds of pictures of the Martian surface, and sampled Martian rocks and dirt. Sojourner operated for 83 sols, surpassing its original mission by 53 sols. By the end of its lifetime, Sojourner had traveled 100 m.

 

The family of Mars rovers has been growing since 1997 and is set to become a family of 5 once Perseverance arrives early next year. Through incredible science and engineering feats, we have been able to explore the surface of an entirely different planet from Earth for the past 23 years. I, for one, am excited for the next 23 years of Mars exploration – and maybe by then I can think of a cooler space fact to tell a bunch of 5-year-olds.

 

For more interesting information on the rovers, see: https://spaceplace.nasa.gov/mars-rovers

K2-141b: I can see your halo

 

This week, PhD Student Giang Nguyen talks about his recent paper discussing lava planet K2-141b. This planet's extreme atmospheric conditions set up a stable ring of clouds just beyond the edge of a large lava ocean centered on the subsolar point. Examples of other large stable annular features on planets include the auroral ovals of the Earth and Jupiter. The photo above was captured by the Imager for Magnetopause to Aurora Global Exploration or IMAGE satellite.

By Tue Giang Nguyen

A few weeks ago, my paper on modelling the atmosphere of K2-141b was published. There was a press release and quite a few news organizations picked up on the story such as CNN and CBS. The public seemed excited by the idea of a scorching planet half engulfed in lava with supersonic winds and molten rock raining from above. Many have started to call K2-141b Mustafar, a fiery world Darth Vader calls his home as envisioned by George Lucas. Others, correctly so, shared how grateful they live on this blue Earth rather than K2-141b. Although K2-141b is described as a hellish landscape, from afar, I think there’s something to be admired from our little lava planet.

 

As K2-141b is tidally locked, there is a permanent dayside and nightside on the planet. This means that certain meteorological processes exist in only specific parts of the planet, mainly clouds. Near the sub-stellar point where it is hottest, the air is not saturated with mineral vapour so nothing condenses. Far away from the sub-stellar point, it is too cold and virtually all of the atmosphere has collapsed back onto the surface and nothing condenses. This means condensation, or cloud formations, can only exist in an annulus centered around the sub-stellar point.

 

In essence, when looking at K2-141b from afar, we should see a ring of clouds around the sub-stellar point. And if you adjust your perspective such that the star is directly overhead on K2-141b, the ring faces upward which makes it a halo! K2-141b has a halo of clouds which is made up of quartz-based gems, how cool is that? This should be the case for other lava planets as well where the planet’s crust, and atmosphere, is dominated by oxidized silicon. Although with this analogy, Earth also has a cool “halo” which is the aurorae (Borealis and Australis).

 

An "eye-ball" planet in which the heat of the star creates a large ocean centered on the subsolar point. In the case above, the working fluid is water and the solid ice, but if you get close enough to the parent star, rock will also melt and form an ocean, ringed by clouds. Image Credit: NASA/JPL-Caltech https://commons.wikimedia.org/w/index.php?curid=56552366

 

For atmospheric modelling, the cloud halo acts similar to the “eye-ball” icy Earth scenario where the sub-stellar point of the icy Earth is an ocean that slowly freezes as you move further away. The middle circular ocean resembles an eyeball that is surrounded by an icy surface that has a much higher albedo. A reflective cloud halo would behave similarly to ice as it reflects more light than then bare magma ocean of K2-141b. Although this is a classically hard problem in climate modelling, we can study some limiting cases that arise from K2-141b’s halo. The albedo of the clouds can be measured which helps us determine cloud properties such as particle size and back-scattering efficiency (dependent on cloud composition and crystal geometry).

 

Now that my work on K2-141b has expanded to include radiative transfer and cloud formations, I am also collaborating with more scientists. Among them are astronomers from the Max Planck Institute, climatologists from Oxford and Chicago. Their expertise on modelling and observing exoplanets will help to make much more accurate descriptions of the K2-141b’s meteorology, surface composition and interior dynamics. While what we predicted about the weather on K2-141b is frankly mind-boggling, I believe the planet has even more interesting things to tell us.