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Once more, NASA leaves our sister planet out of the mix of new missions. Ouch.
Hello, my name is David, and I have a Venus problem. For decades I’ve had an unhealthy compulsion. I’ve been doing the same thing over and over again and expecting a different result. And there’s a whole community of us who share this questionable obsession.
The title of this column is the perennial answer to where NASA will send its next interplanetary spacecraft. The U.S. hasn’t launched to Venus since 1989. That mission, Magellan, revealed our sister planet to be an incredibly beautiful and geologically interesting place, and it raised many new questions: What’s in that thick atmosphere? Was there an ocean, and for how long? Could life have gotten started? In the wake of Magellan, we thought these and other burning questions would logically lead to new NASA spacecraft that would address them.
Since then we Venus researchers have proposed orbiters, entry probes, balloons, and landers. Every one has been shot down. Part of the problem is that the risk of Venus missions is perceived as too high. The observing conditions are so challenging that, judged against a mission to a more well-explored planet, we will always get less data with more risk, for the same money.
The Europeans and Japanese have helped fill the gap with small missions that have kept some vital data flowing. But without data from ambitious new NASA missions there’s less funding for new studies, fewer resources to train students, and fewer people coming into the field. Yet every time NASA has called for proposals, we’ve gone back for more. It’s the fix we can’t resist.
Recently we made the finals. In NASA’s Discovery Program competition, for missions costing up to $450 million, the agency selected two Venus contenders for the final round, out of five total. We’d been through many competitions in which no Venus missions had been selected. We’d been assured by many NASA officials that this doesn’t reflect policy or official bias, and encouraged to keep trying. Now we had 40% of the finalists. It felt like it could be our time at last.
So a lot of us took it very hard when we learned in January that NASA had chosen two missions, neither going to Venus. Both are worthwhile and exciting, flying to new kinds of asteroids never before visited. But how are we supposed to respond, emotionally and strategically, to our repeated defeat? We are like Cubs fans if the Cubs had made it to the World Series but hadn’t won. Do we buy tickets for another season? At what point are we no longer admirably committed, but merely pitiful?
Alas, we will get up, dust ourselves off, and try again. NASA is soliciting proposals for the next iteration of the New Frontiers Program, which has a higher budget than a Discovery mission. Several teams are organizing to propose new Venus sorties.
I could make up excuses for this behavior pattern. I could tell you why sooner or later the U.S. must return to Venus, because without doing so there will be limits to our ability to understand Earth, or climate, or what exoplanets are really like. I could tell you that we keep trying because sooner or later the gaps in our knowledge — compared to other places in the solar system — will become so glaring that it would be as if we’d explored the entire Earth carefully but ignored one whole continent.
But really we just can’t help ourselves. Someday, someday . . .
This article first appeared in print in the May 2017 issue of Sky & Telescope.
The Red Planet has inspired a long history of brilliant mistakes.
While preparing to give a talk recently about the impact of the New Horizons Pluto flyby, I re-read Mars and the Mind of Man, a 1972 book that I loved as a teenager. It arose from a panel discussion during which scientists Carl Sagan and Bruce Murray, together with science-fiction writers Ray Bradbury and Arthur C. Clarke, addressed the imminent arrival of Mariner 9 at Mars.
Previous Mars craft had been flybys, photographing only small areas. Mariner 9, our first orbiter, promised to revolutionize our understanding by laying bare the entire planet, which had for centuries been the subject of stories, fantasies, and overreaching attempts at scientific extrapolation. The book presents the transcript of the conversation, held as the craft rapidly closed in on Mars, as well as essays that each thinker wrote one year later (after Mariner 9 had thoroughly mapped the planet), reflecting on their earlier expectations and summing up what had been learned.
In the panel discussion, the sci-fi writers waxed poetic and mythical, invoking Edgar Rice Burroughs, H. G. Wells, Percival Lowell, and their own famous fictional Martian worlds. Bradbury declared: “[A]t this moment in history, it looks as if I must semi-retire to the wings . . . and hope to become part of some strange new mythology. This is probably true of many science fiction authors this week . . .”
Astronomer/exobiologist Sagan and geologist Murray were in sharp disagreement about the nature of the Red Planet. Sagan was bullish on the possibility of life, noting that Mars might have near-surface water. He described the planet as so poorly photographed prior to Mariner 9 that we might not yet have detected the equivalent of human civilization, let alone microbes or plants, if such existed.
Murray was mistrustful of Sagan’s optimism. He recounted the lengthy record of wishful thinking among scientists who wanted Mars to be Earth-like and life-friendly. He clearly placed Sagan’s ebullient speculations in this category. Murray pointed out that, given the pictures we had up to that point, Mars seemed much more like the cold and dead Moon than the vibrant Earth.
In the essays written a year later, their disagreements are not resolved. Moreover, it is remarkable how wrong both of them still are about Mars — judged by today’s knowledge. Sagan remains much more optimistic. He concedes that Mariner 9 did not find life and had definitively ruled out a human-level civilization. But he believes that the polar caps hold enough H2O and CO2 to intermittently give Mars an atmosphere as thick as Earth’s (wrong). He expresses optimism about the upcoming 1976 Viking landers’ search for life.
Murray, for his part, concludes that Mars never had a more Earth-like past but is in the process of coming to life geologically and may have an Earth-like future (wrong and wrong).
Mariner 9 played an important role in helping us achieve our current comprehension, but obviously these insights didn’t come instantaneously. The fact that these two brilliant scientists, when presented with so much good new data, both so completely misinterpreted them, makes me strongly suspect that whatever we think we know about Mars today will surely not appear so correct given another 50 years of exploration.
This article first appeared in print in the January 2017 issue of Sky & Telescope.
With all the interest in August’s sky spectacular, it’s no surprise that you can find lots of great information about solar eclipses. Here are some favorite resources chosen by the editors of Sky & Telescope magazine:Books About Eclipses
In the Shadow of the Moon: The Science, Magic, and Mystery of Solar Eclipses by Anthony Aveni (Yale Univ. Press, 2017, 328 pages)
Sun Moon Earth: The History of Solar Eclipses from Omens of Doom to Einstein and Exoplanets by Tyler Nordgren (Basic Books, 2016, 264 pages)
Total Addiction: The Life of an Eclipse Chaser by Kate Russo (Copernicus, 2012, 208 pages)
Totality: The Great American Eclipses of 2017 and 2024 by Mark Littmann and Fred Espenak (Oxford Univ. Press, 2017, 288 pages)
Your Guide to the 2017 Total Solar Eclipse by Michael Bakich (Springer, 2016, 395 pages)
Detailed Guides to the 2017 Eclipse
Atlas of the Great American Eclipse by Michael Zeiler (2017, 52 pages, available from greatamericaneclipse.com)
Eclipse Bulletin: Total Solar Eclipse of 2017 August 21 by Fred Espenak and Jay Anderson (2015, 156 pages, available from shopatsky.com)
Road Atlas for the Total Solar Eclipse of 2017 by Fred Espenak (2015, 50 pages, available from shopatsky.com)
Simple Guides to the 2017 Eclipse
Eclipses Illustrated by Jay Ryan (graphic-oriented eBook series, available via americaneclipseusa.com)
Get Eclipsed: The Complete Guide to the American Eclipse by Pat and Fred Espenak (32 pages, includes viewing glasses, available from astropixels.com)
All-American Total Solar Eclipse by Andrew Fraknoi and Dennis Schatz (8-page PDF, available here)
See the Great American Eclipse of August 21, 2017 by Michael Zeiler (44 pages, includes viewing glasses, available from greatamericaneclipse.com)
Books for Kids & Families
The Big Eclipse and The Big Eclipse Activity Book by Nancy Coffelt (2016, 16 and 24 pages, available from orbitoregon.org)
Total Eclipse or Bust! A Family Road Trip by Patricia Totten Espenak (2015, 50 pages, available from astropixels.com)
When The Sun Goes Dark by Andrew Fraknoi and Dennis Schatz (2017, 36 pages, available from store.nsta.org)
Websites About Eclipses (including 2017)
aa.usno.navy.mil/data/docs/Eclipse2017.php U.S. Naval Observatory website generates predictions for your location
eclipse.aas.org comprehensive resource for all aspects of the August 2017 solar eclipse; includes interactive guide to eclipse-related events across the U.S.
eclipse2017.nasa.gov in-depth resource for the August 2017 solar eclipse, with emphasis on NASA activities
eclipses.info website of the International Astronomical Union’s Working Group on Eclipses
eclipsewise.com vast listings of tabulations and maps for solar (and lunar) eclipses from 1900 to 2100
eclipse2017.org website of eclipse enthusiast Dan McGlaun; specific pages for towns within the totality's path
greatamericaneclipse.com extensive collection of detailed maps for 2017, along with historical eclipse records
marketplace.skyandtelescope.com classified astro-related listings; includes dozens of eclipse-viewing lodging offers and venues
skyandtelescope.com/2017-eclipse articles about how to observe and record eclipses with links to many other sites
totalsolareclipse.org solar astronomer Jay Pasachoff's website about past and current eclipses
xjubier.free.fr/en/site_pages/Solar_Eclipses.html interactive Google-based maps of past and future solar eclipses that provide specifics for any location
Videos About Eclipses (Including 2017)
Solar Eclipse 2010 July 11 Easter Island (National Geographic documentary, five parts, youtu.be/Vc0UpnGaMvg)
Still Hooked 20-minute film by eclipse chaser David Makepeace to convince you to see totality, vimeo.com/214773716
eclipse2017.nasa.gov/video-gallery extensive collection of eclipse animations, interviews, and live eclipse-day coverage
Weather Statistics & Forecasts
eclipsophile.com Canadian meteorologist Jay Anderson’s comprehensive analysis of eclipse-day weather prospects and other logistics
spotwx.com easy-to-use interactive website that provides weather predictions for your location
www.goes.noaa.gov real-time satellite imagery from NOAA satellites
How to Photograph the Solar Eclipse: A Guide to Capturing the 2017 Total Eclipse of the Sun by Alan Dyer (2017, 290-page eBook, available for download from amazingsky.com)
mreclipse.com/SEphoto/SEphoto.html helpful, time-tested advice and detailed tables for taking solar-eclipse photos
eclipse.aas.org/resources/solar-filters reputable sources of solar filters for handheld use and for telescopes, binoculars, and cameras
Solar Science: Exploring Sunspots, Seasons, Eclipses and More by Dennis Schatz and Andrew Fraknoi (NSTA Press, 2016, 260 pages)
Getting Ready for the All American Eclipse: An NGSS Storyline Approach to Classroom Instruction by Brian Kruse (2016, available from astrosociety.org)
Información en Español
https://is.gd/seguridad_en_espanol Como Ver el Eclipse Solar del 2017 con Seguridad (viewing the solar eclipse safely)
youtube.com/watch?v=1jdDnJ8fZoc La Experiencia de Ver un Eclipse Total de Sol (the eclipse experience)
www.youtube.com/watch?v=i80Otoxq28o En Camino al Eclipse Total de Sol del 2017 (where 2017’s eclipse will be seen)
Astronomers have confirmed the existence of the seventh planet around the ultracool dwarf star TRAPPIST-1.
The modest M8 red dwarf star TRAPPIST-1 became famous after astronomers discovered seven small exoplanets in orbit around it. At the time the discoverers made the announcement in February, they couldn’t say much about the outermost world, labeled h: The astronomers had seen the planet — or, at least something they thought was a planet — pass in front of the star only once.
Rodrigo Luger (University of Washington, Seattle) and colleagues, including members of the original discovery team, have now confirmed planet h’s existence and some of its specs.
The team used more than 70 days of data from NASA’s repurposed Kepler spacecraft, taken as part of its K2 mission. The craft detected h crossing in front of its star four times, with an orbital period of 18.77 — just what the researchers were expecting, based on their previous observations. (They analyzed the data three different ways, too, just to be sure.) This orbit places the exoplanet well outside TRAPPIST-1’s habitable zone: The amount of energy planet h receives from the little star is on par with what dwarf planet Ceres receives from the Sun at its home in the main asteroid belt.
The transits reveal that planet h is 75% as wide as Earth, or about 40% larger than Mars. But we still don’t know the world’s mass. Researchers used tiny shifts in the other six exoplanets’ transit times to estimate their gravitational influence on one another, and hence their masses. Unfortunately, planet h’s measured transits aren’t clean enough to reveal timing shifts due to its siblings’ gravitational tugs, Luger says.
The exoplanet’s orbital period makes a complicated pattern with the periods of those around it, the authors explain May 22nd in Nature Astronomy. Normally, when we talk about such resonant orbits, we think of situations like that of Jupiter’s Galilean moons: For every circuit Ganymede makes around Jupiter, Europa makes two. TRAPPIST-1’s planets have a more complicated arrangement, called a higher-order Laplace resonance, in which the pattern is a combination of three periods that doesn’t exactly produce the straightforward, integer multiples we usually think of. For those interested in the math, the relationship is
x/P1 – (x+y)/P2 + y/P3 = 0
where x and y are integers and P1, P2, and P3 are the orbital periods of planet 1, planet 2, and planet 3 in the trio of neighboring bodies you’re comparing.
For those not interested in the math, just know that for every two laps planet h makes around TRAPPIST-1, planet g makes about 3, and planet f makes (more roughly) four. The exoplanets would have migrated into this complex chain arrangement sometime after the system formed, then gotten gravitationally stuck.
How Old Is TRAPPIST-1?
The above animation shows a simulation of the TRAPPIST-1 exoplanets over 90 Earth-days, then focuses on the outer three after 15 days. The three-body resonance of the outer three planets causes the planets to repeat the same relative positions. Astronomers used this expected resonance to predict the orbital period of TRAPPIST-1h. Credit: Daniel Fabrycky / University of Chicago
Luger’s team also tried to constrain TRAPPIST-1’s age. Dating stars as puny as this one is tough. The way a star ages depends on its mass; at a measly 8% the Sun’s mass, TRAPPIST-1 will age very slowly.
Thanks to the K2 data, the astronomers could use starspots to clock the dwarf’s rotation period at 3.3 days (about twice as long as the period we previously reported). That’s middle-of-the-road for nearby, ultracool dwarf stars. Kepler also didn’t reveal much activity, but it did catch at least one notable flare. Based on the spin and activity level, the authors estimate the star’s age is between 3 and 8 billion years.
Other M dwarf astronomers agree that that’s a reasonable range. Elisabeth Newton (MIT) says that most nearby stars are younger than 8 billion years. She and her colleagues recently surveyed nearly 400 nearby M dwarfs, finding that those with periods less than 10 days generally had ages of less than 2 billion years. But she cautions that the red dwarfs her team looked at were more massive than TRAPPIST-1, and the relationship between age and rotation period depends on the star’s mass. “I don’t think that the current data we have on the rotation periods of red dwarf stars is too useful for pinning down the ages of stars as small as TRAPPIST-1,” she warns.
John Bochanski (Rider University) agrees. TRAPPIST-1’s activity level implies that it’s not “really” old, he says, but beyond that it’s hard to say. It wouldn’t surprise him if the star was a little outside the range. Meanwhile, Jeffrey Linsky (University of Colorado, Boulder) puts his bet on 2 to 5 billion years, based on the star’s heavy-element content, X-ray output, and motion through the Milky Way.
Whatever the exact number, it’s likely that TRAPPIST-1 is about as old as the Sun. That permits all sorts of speculation about habitability and alien life, but given how much remains unknown about this system, I prefer not to dabble in such musings.
Reference: Rodrigo Luger et al. “A Seven-Planet Resonant Chain in TRAPPIST-1.” Nature Astronomy. May 22, 2017.
Visit the TRAPPIST-1 system with NASA's Visions of the Future poster.
Mars was once far wetter than it is now — but just how wet?
Once upon a time (about 3.7 billion years ago, to be exact), there was rain on Mars. In fact there was enough liquid water on the Red Planet to create vast valley networks and overflowing crater lakes. But these conditions didn’t last long. The poor planet lost its atmosphere and became a frozen wasteland.
Based on geologic evidence, scientists are confident rain fell on ancient Mars. But how much rain? In the September 1st issue of Icarus, Robert Craddock (Smithsonian Institution) and Ralph Lorenz (Johns Hopkins University Applied Physics Laboratory) take a fun foray into the past, exploring how Martian rainfall might have changed with time and how much of an impact the rain could have had on the surface.
Two forces act on a raindrop: gravity (down) and air drag (up). A thicker atmosphere produces more drag, which means raindrops will fall more slowly. Denser atmospheres also produce smaller raindrops. That’s because as falling drops become larger, aerodynamic drag flattens their bottoms, until the drop “assumes a shape similar to the top of a hamburger bun,” Craddock and Lorenz write. Boost the drop size more, and it’ll deform into an umbrella shape, until the drag force fragments it.
With some assumptions thrown into the mix, the team explored how intense ancient Martian rain could have been. The duo calculated max drop size, terminal velocity, and rainfall intensity across a range of atmospheric pressures from 0.5 to 10 bars, where 1 bar is the air pressure at Earth’s sea level. This range encompasses a variety of proposed conditions for early Mars.
Depending on how well Martian soil would soak up rain (or not), Craddock and Lorenz suggest that in atmospheric pressures greater than 4 bars, precipitation would have had the intensity of fog — too gentle to erode the landscape. But as Mars lost its atmosphere over the millennia, raindrops could have grown larger and fallen harder, reaching drizzle levels. Conditions would have hit the sweet spot at 1.5 bars, with intensity levels on par with terrestrial storms — although, the combination of larger drops and slower terminal velocities on Mars compared with Earth would mean that a storm on the Red Planet would be only 70% as intense as its terrestrial twin, the authors note. (If you’re curious, the maximum drop size would be around 7½ mm, 1 mm bigger than on Earth.)
Rain’s intensity would have continued to rise until 0.5 bar, when the atmosphere should have collapsed.
The calculations suggest that there would have been a brief window in Martian history when rain could have spurred serious erosion. This inference makes sense: The planet’s deep valley networks formed around 3.7 billion years ago, which is around the time the atmosphere was peacing out. A 2012 study by Timothy Goudge (then of Brown University) and others also found only limited signs of water-related mineral transformations in more than 200 Martian lake basins, suggesting that the lakes didn’t last long.
Craddock and Lorenz go on to argue that their prediction of how rain’s intensity changed with time matches the change in crater erosion patterns between the Noachian (4.1 to 3.7 billion years ago) and Hesperian (3.7 to 3.3 billion) periods on Mars, with less erosion during the Noachian and more during the Hesperian. But a lot of other processes have eroded craters on Mars, notably volcanism: The 2012 Goudge study found that volcanism had resurfaced more than 40% of the lake basins investigated, and volcanism was especially active in the Hesperian period.
Still, while we should take the connection to erosion with a grain of salt (or perhaps a drop of salty water), the rainfall study is a delightful romp through Mars’s potential past.
Robert A. Craddock and Ralph D. Lorenz. “The Changing Nature of Rainfall During the Early History of Mars.” Icarus. September 1, 2017.
Timothy A. Goudge et al. “An Analysis of Open-Basin Lake Deposits on Mars: Evidence for the Nature of Associated Lacustrine Deposits and Post-Lacustrine Modification Processes.” Icarus. May 2012.
Explore the Red Planet with your own eyes with Sky & Telescope's Mars globes.
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Tabby's star, otherwise known as the most mysterious star in the galaxy, is dipping drastically in brightness, giving astronomers an opportunity to figure out what has been causing this star's weird behavior.
Ever the Kepler space telescope captured a series of random-seeming dips from a certain mysterious star, astronomers and the public alike have been baffled by its behavior. Then, following the end of the main Kepler mission, the star went quiet. Now, at long last, the star has begun a steep decline in brightness — it’s already 2% dimmer after a single night of observation — and telescopes all over the world are at the ready!What We Already Know About Tabby’s Star
What is now famously known as Tabby’s Star is a normal-looking F-class star in the field of the Kepler space telescope. Kepler’s mission was to monitor more than 150,000 stars, watching for the minute dips in brightness that would signal an exoplanet moving across the face of its parent star from Earth’s perspective. But in Tabby’s Star, Kepler — and the watchful eyes of citizen scientists involved in the Planet Hunters project — found something completely different.
Tabby’s Star was observed to dim 10 times, sometimes by 1% (typical of a giant exoplanet transit) and sometimes by 10% to 20% (not at all typical of exoplanet transits, or anything else for that matter), each dimming lasting days to weeks at a time. The dips were irregular both in terms of how long they were and when they occurred.
Explanations for the star’s behavior ranged from the mundane (starspots) to the interesting (a comet breaking up around the star) to the sci-fi-inspired (a Dyson sphere syphoning the star’s energy for an alien civilization).
Eventually, various astronomers involved in the project, including Tabetha Boyajian (Louisiana State University) and Jason Wright (Penn State), seemed to settle on two main explanations: a circumstellar object of some sort, such as a giant comet in an elliptical orbit, or some dusty clump in the stuff between stars.
Notably, the comet scenario predicts a dimming event this very month. From Boyajian and colleagues’ paper, published in January 2016: “A more robust prediction is that future dimming events should occur roughly every 750 days, with one in 2015 April and another in 2017 May.” Read Benjamin Montet (University of Chicago) and Boyajian’s article in the June 2017 issue of Sky & Telescope for a full rundown of all the possible explanations.
There are clear ways to tell these scenarios apart, but those ways require spectra during the dips — and we don’t have that kind of data from Kepler. For example, if it’s dust, then the star will dim more at bluer wavelengths. A Dyson sphere, on the other hand, is presumably a solid object and so the star would dim the same at all wavelengths. Certain spectral fingerprints, such as those left from sodium or calcium, could also enable astronomers to learn more about the obstruction.
Infrared data will also be key, as any material close to the star ought to be hot — and therefore ought to show up as excess infrared radiation.
So Boyajian, Wright, and several others made their predictions and settled in for a wait: All they needed was for the star to stop pretending to be ordinary. And sure enough, Tabby’s Star dropped its act.Tabby’s Star Dips
At 4 a.m. on May 19th, Boyajian called Wright: Fairborn Observatory in Arizona had issued an alert that Tabby’s star had dimmed by 3% — a big dip in the star’s brightness. The team immediately sent out the call for more observations.
ALERT:@tsboyajian's star is dipping
This is not a drill.
Astro tweeps on telescopes in the next 48 hours: spectra please!
— Jason Wright (@Astro_Wright) May 19, 2017
As soon as the sun sets around the world, astronomers will train their telescopes on Tabby’s Star — from the amateur astronomers involved in the American Association of Variable Star Observers (AAVSO) to the spectroscopists at the Keck I and II telescopes in Hawai‘i. Additional spectroscopy will come from the MMT Observatory in Arizona. The Las Cumbres Observatory Global Telescope Network (LCOGT), which was already regularly monitoring the star, has stepped up its observing cadence.
The Green Bank Observatory may get in on the action, too, to collect radio observations. Even space-based telescopes are slewing toward Tabby’s Star. While Spitzer can’t point in that direction of the sky, the Swift space telescope will be monitoring the star’s brightness at optical and ultraviolet wavelengths.
And this is far from an exhaustive list — many more telescopes will be participating in follow-up observations. That in itself is no small feat considering that telescopes are typically scheduled weeks in advance.
“This is the first time we’ve seen a clear dip since the Kepler mission, and also the first we’ve caught in real time,” says Wright. “The changes are as steep as we ever saw it change brightness with Kepler.”
“It’s going to be a busy couple of weeks.”
Watch a live stream from earlier today with Wright and SETI Director Andrew Siemion as they discuss the recent changes and incoming observations:
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It might seem intuitive that water always flows downhill. But that axiom provides important clues to the tectonic histories of Mars, Titan, and Earth. A team of researchers led by Benjamin Black (City College of New York) recently compared the topographies of these three bodies, all of which show evidence of fluvial, or river-based, influences on their surface features. The team’s study, published this week in Science, used global drainage patterns of each object’s surface to determine the likelihood of recent tectonic activity.
On Earth, a tectonically active body (hello earthquakes and subduction zones), the results seem to defy physics: water appears to flow along level surfaces or uphill about 40% of the time. Of course, this doesn’t actually occur. The misleading results arose when the researchers blurred topographic data from Earth and Mars to match the resolution of Cassini’s Titan data. At this lower resolution, the researchers suggest, only larger, continent-scale features that formed over a longer timescale will be detectable. “Short-wavelength” mountain ranges and other features formed by tectonic activity will be blurred out, sometimes creating the illusion that fluid is flowing against gravity.
Mars and Titan, on the other hand, have much better “topographic conformity”— that is, the fluvial features seem to flow downhill (65% of the time or better). On Mars, high conformity indicates that little topographic reshaping has occurred since Martian river networks formed. So apparently there’s been little tectonic activity or intense impact barrages to disrupt drainage patterns that had aligned with ancient topographic gradients.
On Titan, where rock-hard water ice shapes the landscape and liquid methane and ethane fill its rivers, the drainage networks follow the prevailing slopes in mid-latitude and equatorial regions. So the topography there has been stably in place since before the river networks formed. However, its north polar region doesn’t conform as well — hinting that some kind of deformation (cryovolcanoes?) happened in the geologically recent past.
Based on Cassini’s infrared and radar imagery, Titan does show evidence of recent or ongoing geologic activity — perhaps a consequence of tidal heating or melting deep down where its ice crust and rocky core meet — that results in global changes in the ice’s thickness. In general, surface material on Titan seems to migrate poleward — hydrocarbons in the atmosphere travel from mid-latitudes to the pole and five out of six rivers drain to the poles. Evidence also suggests that a substantial amount of sediment drifted from high locations to low ones, conceivably erasing some short-wavelength evidence of Titan’s geologic past.
In short, as far as Titan, Earth, and Mars are concerned, if you’ve got topographic conformity and your liquid flows downhill, then your topography is positively ancient.
A magnetic field appears to span the space between the Large and Small Magellanic Clouds, the two dwarf galaxies being consumed by our Milky Way Galaxy.
For stargazers in the Northern Hemisphere, it’s easy to forget that the Milky Way is actively consuming two dwarf galaxies. Those in the Southern Hemisphere have a front row seat to watch our galaxy wreak havoc on the Large and Small Magellanic Clouds (LMC and SMC). But there’s more to the story — the dwarfs are not only gravitationally interacting with the Milky Way but with each other as well.
The gravitational effects evident from these interactions can tell us a lot about the history and evolution of these galaxies as well as the environments surrounding them, but gravity isn’t the only force at work here. Magnetic fields play a role as well, one astronomers are still trying to puzzle out. Now, for the first time, researchers using the Australia Telescope Compact Array radio telescope in New South Wales, Australia, have detected a magnetic field in the space between the Magellanic Clouds. Called the Magellanic Bridge, this structure is a 75,000 light-year long filament of gas and dust that stretches from the LMC to the SMC. These results are published in the Monthly Notices of the Royal Astronomical Society (full text here).Detecting the Invisible
Magnetic fields can be found within and around planets and stars but also on the scales of galaxies. We’ve detected galactic magnetic fields in both our own galaxy and in several other disk galaxies, but an extragalactic magnetic field is something else. This is the first magnetic field detected “outside” of a galaxy.
To detect the presence of a magnetic field associated with the Magellanic Bridge, Jane Kaczmarek (University of Sydney) and colleagues observed 167 known radio sources in the same area of the sky, located far beyond the Magellanic Clouds themselves (the LMC and SMC are 160,000 and 200,000 light-years away, respectively). Some of these radio sources lay directly behind the bridge along our line of sight and some of them were off to either side.
Light from radio sources is often partially polarized to begin with, so that the light waves tend to undulate along a certain direction. But if the light passes through a medium (such as a large gas filament) on its way to our telescopes, that passage can change the polarization. How much it changes tells us about the intervening medium. From the observations, the astronomers calculated the magnetic field to be 0.3 microgauss — a million times weaker than Earth’s magnetic field on our planet’s surface.The Magellanic Cloud Connection
Interpreting the data isn’t straightforward though. The Milky Way has its own magnetic field, as does Earth, the Sun, and several other planets in the solar system. So the team had to subtract out possible contributions from all other sources to isolate the effect due to the gas in the Magellanic Bridge alone. To do this, the team made assumptions and simplifications that may or may not be accurate, as magnetic fields still aren’t very well understood.
We know that the LMC and SMC had a close fly-by in the past. Astronomers can’t quite agree on exactly when or how close they got, but the event left both of them literally bent out of shape, their once spiral shapes now unrecognizable. The Magellanic Bridge is probably a remnant of this interaction, comprised of gas torn out of both galaxies as they passed by each other.
The authors of this paper argue that this newly discovered magnetic field is similarly made up of both galaxies’ magnetic fields, which were dragged into the bridge structure along with the gas. If true, this result would confirm the existence of a pan-Magellanic field — a magnetic field that spans both galaxies. The implications of such a field could speak to the history and future of the entire Magellanic system.
The Square Kilometer Array (SKA), currently in the final design phase, will probe magnetic fields surrounding interacting galaxies like the LMC and SMC in even more detail, as well as look for potential signs of magnetism in the intergalactic medium, when it comes online in 2021.
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