Comet C/2022 E3 (ZTF) - why is it green?

First of all, what's with the name? The comet follows the naming conventions set out by the International Astronomical Union. C: it is a non-periodic comet -- 2022: the year of its discovery -- E: the month it was discovered in (E refers to the first half of March) -- 3: it was the third comet discovered in the first half of March 2022 -- ZTF: the observatory which made the discovery (the Zwicky Transient Facility).

The brightness of comets are quite hard to predict because the ice which latches onto the surface in cold/dark deep space starts to melt as it enters our solar system. The gas and dust trapped under the icy surface escape and this is what makes up the visible trail behind the comet. Its closest approach to Earth was on the 1st of Febuary and could be found very close to the North Star (Polaris). A super pretty green-blue hue seen from the comet is thought to be due to emission from diatomic carbon (2 carbon atoms bonded form a carbon molecule); ultraviolet rays from the Sun hit the molecule and cause it to break apart, producing energetic emission at wavelength 518nm (green).

The end of the NASA InSight mission

InSight has been studying Marsquakes (like Earthquakes but on Mars) to help us better understand the interior of Mars. The Martian surface is incredibly hostile and the solar panels powering the lander have slowly been smothered by dust over the last 4 years. We always knew that this would happen for something sitting stationary on the Martian surface; in fact, the lander has lasted much longer than the originally planned 2 years of operation. Since landing, it has detected 1319 Marsquakes, some of which were triggered by asteroid impacts!

On the 15th of December, the lander sent us its final message: "My power’s really low, so this may be the last image I can send. Don’t worry about me though: my time here has been both productive and serene. If I can keep talking to my mission team, I will – but I’ll be signing off here soon. Thanks for staying with me."

Light pollution

A study was recently published on how the light from major cities compares to the natural light from the stars in the night sky. Astronomical observatories are specifically chosen to be in areas with little light pollution ('dark sites'), but many professional observatories have been around for quite some time (up to 100 years old). So, are these observatories still in dark sites today now that cities and towns have grown as much as they have? A dark site is defined by the International Astronomical Union as being less than 10% brighter than what the sky would normally be from starlight scattering etc.

The study worryingly reveals that more than half of all professional observatories now no longer lie in a dark site -- to call this a big concern for the future of astronomy and astrophysics would be a bit of an understatement, especially since they didn't even consider the light pollution from sunlight reflected off satellites. Satellites currently orbiting Earth already result in all regions of Earth's night sky to be above the 10% threshold and this will sadly only get worse as projects like Starlink take place.

While space-based telescopes bypass this issue of pollution, they come with their own issues. Just to name a few: they're so much more expensive, they're very hard to maintain, they will contribute to space pollution, and their dimensions are restricted.

Barred galaxies seen by JWST

The shape of a galaxy reveals much of its story. James Webb Space Telescope (JWST) is able to resolve the shape of galaxies at much further distances than we've ever been able to before. If a galaxy is a big fuzzy blob, then it's likely to have had its stars scrambled up in a collision with another galaxy. If its been left alone for most of its lifetime, it's (probably) a beautiful grand spiral galaxy. We know that the early universe was a lot denser than present day, so galaxies were more likely to merge. Nowadays, most galaxies are quite far apart, so there must've been some sweet spot in the past where galaxies were equally likely to merge or be independent. At these points in the past, 'bar' structures form within galaxies. Since JWST can look into the past (as it collects light from objects many light years away), we can try to pinpoint when exactly bar formation began. We also tried to do this with the Hubble Space Telescope with its limited resolution and in 2014, Simmons et al found that barred galaxies existed at redshift z=1.5 (when the universe was approx 4 billion years old).

Recent data from JWST (Guo et al) captured the most distant barred galaxies we've ever seen at redshifts of z=2.136 and z=2.132; the light from these galaxies have been travelling for about 11 billion years! In other words, barred galaxies have existed since the universe was at least 2.8 billion years old. It's important to note that this is a lower bound -- many astronomers believe that barred galaxies could have existed 1.5 or 2 billion years into the universe's lifespan. As we continue to analyse JWST data, the story will unravel further.

Green pea galaxies observed by JWST

So called 'green pea' galaxies were first detected by citizen scientists on the Galaxy Zoo project using data from the Sloan Digital Sky Survey (SDSS). Astrophysicists determined that these green pea galaxies are incredibly compact galaxies with super intense star formation. The high energy ultraviolet from the stars cause the surrounding gas to heat up, exciting the molecules within it and making them glow. Further emission spectra analysis took place and scientists deduced that oxygen emission was also taking place in these dense galaxies, giving them their characteristic green colour and thus name.

One of the first images from JWST revealed 3 more green pea galaxies (Rhoads et al) with redshifts from 6.9 to 8.3. JWST also broke down the spectra of these galaxies, once again revealing an emission peak at the wavelength of oxygen and aligning with the SDSS galaxy data. This tells us that even after only 700 million years of the universe's existence, enough oxygen was made for us to easily detect its glow 13 billion years later. With this first step, we can start ask questions about the metallicity of these galaxies in the early universe to piece together the timescales of creation for the elements which make up our cosmic home.

Other stuff

On the 15th of February, there will be a conjunction of Venus and Neptune, bringing them very close together in the sky. Neptune is quite hard to spot, even with a high-end telescope. On the other hand, Venus is super bright, so you could use Venus as a 'latching point' to spot Neptune :) I would recommend initially using a low magnification to gauge whereabouts they lie in the night sky, then switching to a higher magnification.

Also, on the 22nd of February, a gorgeous triplet consisting of Jupiter, Venus, and the crescent Moon will light up the night sky. Hopefully it won't be too cloudy ! :)

published: 06/02/23 by kaan evcimen