[♪ INTRO]
Venus is often described as Earth’s evil twin.
And we get it. I mean, the planets are very similar in size and mass.
Except, you know, Earth is a generally nice place
with plenty of water and green things.
And Venus is a fire world of scorching temperatures and crushing pressures,
all wrapped up in thick, sulphuric acid fog.
And that fog is kind of a pain. But not for the obvious reasons.
Because of this blanket of cloud, we didn’t know that much about Venus
until the early 1990s, when NASA’s Magellan mission arrived.
Magellan gave us unprecedented insight into Venus’s rocky surface,
and along the way, it raised a few questions, too!
And did we mention it launched from a space shuttle?
Fairly early on, modern astronomers realized that regular cameras
can’t see through Venus’s clouds, but radar can.
That’s because radar uses microwave and radio waves,
which are long enough that they don’t get scattered by Venus’s atmosphere.
That means they can pass through the clouds, bounce off the surface,
and give us an image of what lies beneath.
So, with radar, we learned a few things, like,
Venus spins backwards compared to other planets.
But even the best radio telescopes on Earth
could only image the surface to a resolution of one or two kilometers.
In other words, the sensors would miss anything smaller than New York City’s Central Park.
So, a number of spacecraft were sent to Venus to get a better look,
including ones in NASA’s Mariner program, and the Soviet Union’s Venera missions.
But even then, these spacecraft still only looked at a small part of the planet.
So if we really wanted to learn about Venus, we needed to step up our game.
Cue NASA’s Magellan mission.
Its main goal was to use radar to map a much bigger portion of Venus’s surface, and that, it did.
The spacecraft launched in 1989,
and was actually the first interplanetary spacecraft to be launched from a space shuttle.
Once the Space Shuttle Atlantis was in Earth’s orbit,
a robotic arm carefully maneuvered the Magellan out of the payload bay.
Then, when the shuttle had drifted far enough away,
Magellan fired a solid rocket booster to send it on its way to Venus.
It arrived 15 months later, and spent the next four years circling the planet.
Magellan traveled from north to south,
so between each orbit, Venus rotated a little underneath it.
That meant Magellan could measure a new strip of ground on each pass.
In fact, by doing this, Magellan covered the entire surface of Venus six times.
After its first three rounds,
it had mapped 98% of the surface to a resolution of 300 meters or better.
That meant that scientists could now pick out features the size of a few football fields,
or the size of the New York Met, to stick with the Central Park theme.
After that, Magellan spent the next three rounds mapping the strength of Venus’s gravity,
by measuring how much the spacecraft was pulled toward the surface at various spots.
And then, it was time to say goodbye.
In October 1994, Magellan was commanded to plunge toward Venus’s surface,
and it burned up in the atmosphere not long afterward.
In the end, Magellan’s data created the most complete map of Venus ever made,
and decades later, it’s still our most detailed map of the surface!
It let scientists finally see the full range and diversity of features down there,
and it also uncovered a few surprises.
Like, it showed us that Venus’s surface looks incredibly pristine
for a four-and-a-half-billion-year-old planet.
For instance, its craters look brand new.
They haven’t been eroded smooth, like they have here on Earth.
Researchers think that’s because erosion on Venus is really slow,
mostly thanks to the surface being extremely dry.
Without water to speed things along,
features can stay sharp for tens or hundreds of millions of years.
Magellan also showed us that those features are almost all volcanic.
On Earth, the crust is a mixture of rocks that come from
magma, riverbeds, high temperatures and pressures, and loads more.
But on Venus, volcanic features make up 85% of the surface.
Magellan’s maps revealed lava plains, shield volcanoes,
lava domes shaped like pancakes, and lava channels hundreds of kilometers long.
Previous surveys of Venus had shown some of this volcanism,
but Magellan showed us for the first time just how extensive it is.
And this was especially surprising,
considering that Venus doesn’t have any obvious kind of plate tectonics.
On Earth, most volcanoes form at the boundary of two tectonic plates,
since it’s easier for magma to rise there.
But from what we know so far, there’s none of this on Venus.
That suggests there are very different processes going on inside the planet,
even though its size and composition are similar to Earth’s.
Without more information from beneath the surface,
scientists can’t be sure exactly what these processes are.
But so far, the data point to plumes of magma rising from Venus’s mantle
and creating hot-spots, like the one beneath Hawai’i.
Now, a final surprise from the Magellan was that Venus has relatively few impact craters.
Compared to Mercury or Mars, it’s really smooth.
This might be because its thick atmosphere has helped shield it from meteors,
causing the rocks to burn up before they reach the surface.
But the number of craters also suggests that the surface is pretty young,
no more than about 800 million years old.
For comparison, there are parts of Earth’s crust more than four times that age.
Without faster erosion or plate tectonics, it’s not clear why the older rocks are missing.
But scientists think Venus’s surface must have undergone a huge resurfacing at some point,
where newer volcanic features replaced everything that came before.
Researchers are still unsure exactly how long this took, or why it happened.
And it’ll take even more detailed studies of rocks all over the planet to get to the bottom of this.
So Venus might still leave us with more questions than answers,
but we wouldn’t know half as much without Magellan.
Even more than 25 years after the mission ended,
it’s still one of our main tools for learning about our mysterious, next-door neighbor.
[♪ OUTRO]