Mercury Magazine Summer 2017
In this issue of Mercury: Two spacecraft, two giant planets, and a total solar eclipse. “A Cassini Retrospective” recounts some of the amazing discoveries made by Cassini as its mission at Saturn comes to an end, while “Close Encounters of the Jovian Kind” reveals a few of the results from Juno’s recent passes by Jupiter. And “An Eclipse-Watcher’s Guide to a Total Eclipse of the Sun” describes what to expect during the August 21st eclipse.
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Table of Contents
 Close Encounters of the Jovian Kind, NASA and the Southwest Research Institute
Juno has completed its seventh swoop past Jupiter. Here are some encounter results, both visual and scientific, from its earlier passes.
 A Cassini Retrospective, NASA/Jet Propulsion Laboratory
The Cassini mission is nearly over, but the wonders it revealed will keep scientists busy for years.
 An Eclipse-Watcher’s Guide to a Total Eclipse of the Sun, Paul Deans
What to expect, and look for, during the various stages of a total solar eclipse.
 Astronomy in the News
Rethinking Earth’s ‘Little Ice Age,’ a new branch in the family tree of exoplanets, and a ringed exoplanet about to occult its star this September. These are some of the discoveries that recently made news in the astronomical community.
 Perspectives, Paul Deans
 First Word, Linda Shore
What Did the Dinosaurs Know?
 Annals of Astronomy, Clifford Cunningham
Meghnad Saha, Light Quanta, and Comet Tails
 Astronomer’s Notebook, Jennifer Birriel
The Milky Way’s Rotational Curve
 Strange New Worlds, STScI
Flares May Threaten Planet Habitability Near Red Dwarfs
 Planetary Perspectives, Emily Joseph
 Armchair Astrophysics, Christopher Wanjek
Two’s Company, Three’s a New Astronomy Discipline
 Education Matters, Brian Kruse
Astronomical Phenomena as a Provocation for Learner Engagement
 Reaching Out, Mike Simmons
You Can’t Convince Everyone
 ASP Tidings
The Bruce Gold Medal, save the date, and seeking a new Mercury editor
 Sky Sights, Paul Deans
Perseid meteors, two occultations, and a total solar eclipse
 Reflections, NASA/JPL
Twenty Years at Mars
Close Encounters of the Jovian Kind
by NASA and the Southwest Research Institute
For a very short time, there are spacecraft orbiting two giant planets in our solar system: Cassini at Saturn and Juno at Jupiter.
“Juno flies over the poles of Jupiter and it goes very close; within 2,000 miles of the cloud tops,” said Scott Bolton, Juno Principal Investigator from the Southwest Research Institute (SwRI), at a recent press conference. “Every 53 days we scream by Jupiter from north to south in about two hours [called perijove]. Most of our science is collected during these very close passes. That’s what is unique about Juno. We get so close to Jupiter and we cross over both poles. That allows us to see new and unique things about the interior and learn how the magnetosphere works.
“We knew, going in, that Jupiter would throw us some curves,” he added. “But there is so much going on here that we didn’t expect, that we have had to take a step back and begin to think of this as a whole new Jupiter.”
A Cassini Retrospective
by NASA/Jet Propulsion Laboratory
On September 15, 2017, on Cassini’s 293rd orbit of Saturn, the spacecraft will plunge into the Saturnian atmosphere and be destroyed. Yes, this was planned.
Cassini’s finale actually began November 30, 2016, when a close flyby of Titan nudged it into a series of 20 ring-grazing orbits that saw it cross Saturn’s ring plane within 4,850 miles of the center of the narrow F ring. Five months later (April 2017), another Titan encounter altered Cassini’s orbit once again, sending it through the gap between Saturn’s upper atmosphere and the inner edge of its D ring. This is Cassini’s Grand Finale mission. The spacecraft will make 22 plunges through this gap, ending with its final dive into Saturn on September 15. So as we await Cassini’s final plunge, here’s a brief look at some of its amazing finds.
An Eclipse-Watcher’s Guide to a Total Eclipse of the Sun
by Paul Deans
Totality, no matter what its duration, seems to last no longer than eight seconds. This remark, by former Sky & Telescope editor Norm Sperling, accurately describes the eclipse experience for anyone encountering totality for the first time. Even for eclipse veterans, totality always seems far too short.
Of course, the eclipse itself lasts more than two hours, with totality comprising only a small segment of the experience. And there
are times, particularly immediately prior to and during totality, when you’ll be conflicted as to what to look at, because so much is happening simultaneously.
So here are some suggestions as to what to look for (and what to expect) during the hour prior to totality, the hour after totality, and during those fleeting moments in between when the Sun is completely covered by the Moon. If you’ve never experienced totality, I hope you find this description useful.
Curiosity Peels Back Layers on Ancient Martian Lake
NASA/Jet Propulsion Laboratory
A long-lasting lake on ancient Mars provided stable environmental conditions that differed significantly from one part of the lake to another, according to a comprehensive look at findings from the first three-and-a-half years of NASA’s Curiosity rover mission. Different conditions favorable for different types of microbes existed simultaneously in the same lake.
Previous work had revealed the presence of a lake more than three billion years ago in Mars’ Gale Crater. This study defines the chemical conditions that existed in the lake and uses Curiosity’s powerful payload to determine that the lake was stratified. Stratified bodies of water exhibit sharp chemical or physical differences between deep water and shallow water. In Gale’s lake, the shallow water was richer in oxidants than deeper water was.
“These were very different, co-existing environments in the same lake,” said Joel Hurowitz of Stony Brook University, lead author of a report in the journal Science. “This type of oxidant stratification is a common feature of lakes on Earth, and now we’ve found it on Mars. The diversity of environments in this Martian lake would have provided multiple opportunities for different types of microbes to survive, including those that thrive in oxidant-rich conditions, those that thrive in oxidant-poor conditions, and those that inhabit the interface between those settings.”
Whether Mars has ever hosted any life is still unknown, but seeking signs of life on any planet — whether Earth, Mars, or more distant icy worlds — begins with reconstruction of the environment to determine if it was capable of supporting life. Curiosity’s primary goal when it landed inside Gale Crater in 2012 was to determine whether Mars has ever offered environmental conditions favorable for microbial life.