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Last year, @giadaplanet, @tonhingm, J. Stocke, K. France, @akiroberge, and I got #Hubble DD time to observe the Jan 2019 total lunar eclipse. Our paper about it is out TODAY on the Astronomical Journal, arXiv, and is described in a few press releases! 1/15
The press releases describing the project for a public audience are at https://hubblesite.org/contents/news-releases/2020/news-2020-30, https://www.spacetelescope.org/news/heic2013/ ,
and https://www.colorado.edu/today/2020/08/06/researchers-take-ultimate-earth-selfie. But this thread is for my astronomer colleagues - see the paper at https://iopscience.iop.org/article/10.3847/1538-3881/aba0b4 or ARXIV LINK. 2/15
and https://www.colorado.edu/today/2020/08/06/researchers-take-ultimate-earth-selfie. But this thread is for my astronomer colleagues - see the paper at https://iopscience.iop.org/article/10.3847/1538-3881/aba0b4 or ARXIV LINK. 2/15
Lunar eclipses have been recognized for a while for their similarity to exoplanet transit observations. Using the moon as a mirror, Earth-bound telescopes not meant for staring at the Sun can observe the Earth transiting the Sun as if it were a transiting planet. 3/15
Earth is our only example of a habitable/inhabited planet, so we better know what it looks like with the same techniques astronomers use for exoplanets! Our study particularly targeted the near-ultraviolet and blue-optical to detect the important ozone (O3) molecule. 4/15
O3 can be a signpost of life (although not necessarily!), and the Hartley-Huggins band in the near-UV is O3's strongest spectral signature, meaning a little O3 goes a long way in terms of absorption depth. This could be particularly useful for studying young exo-Earths. 5/15
1-2 billion years ago, Earth had ~1000x less oxygen than today, but with the strong H-H band in the near-UV, this small amount of O3 (a photochemical byproduct of O2) could be detectable. 6/15
On Jan 21, 2019, Hubble spent 3 orbits observing the penumbral and umbral phases of the eclipse, as well as an out-of-eclipse baseline of the full moon. This was the first lunar eclipse observation by a space telescope (as far as I can tell) and the first in the ultraviolet! 7/15
We got multiple spectra from 170-550 nm with the STIS CCD, allowing us to make Earth transmission spectra. We detected ozone from the Chappuis band and Rayleigh scattering in the umbral and penumbral phases, and ozone from the Hartley-Huggins band during the penumbral phase. 8/15
During the umbral phase (or total lunar eclipse), the moon was COMPLETELY dark below ~420 nm. We also didn't detect any spectral signatures below ~300 nm in any phase! This was a combination of bad timing (missing the window when sunlight is passing through most ... 9/15
of the Earth's O3 layer - we were observing with the optical grating during that time) and contamination from residual solar features (due to solar center-to-limb variations) with amplitudes larger than the expected O3 signatures. 10/15
Are you thinking, "wait, Hubble can point at the MOON?" Yes, it's not too bright for the CCDs! HST has observed the moon before. Observing a lunar eclipse from space is nice bc there's no contaminating atmosphere above the telescope, but there were other problems... 11/15
HST can't stably point at the moon! Minor problem: HST's FOV is too small to see any guide stars, so gyro guiding is necessary (if more of the gyros go, no more moon observing for HST). Major problem: HST's software only allows it to track in STRAIGHT LINES! 12/15
The moon is so close that its apparent motion is quite wiggly (black curve), and we had to approximate that motion with multiple linear tracks (colored lines). However, the more linear tracks you do, the less science observing time you get, so it was a balancing act. 13/15
The moon is not a great (uniform) mirror... so while HST's pointing was zipping around all over the moon, we were picking up some contaminating signals. The colors in this figure match the last figure. Black star was our goal, yellow line is STIS slit size/orientation. 14/15
Are you still here? Go flip through paper! Thanks for reading. 15/15
Or watch this really neat video created by the talented Paul Morris at NASA Goddard!
