MESSENGER Busts Mercury's Mysteries With 'Drive-By' Science

The scoop: NASA's MESSENGER spacecraft swung by Mercury twice in 2008 to get a head-start on solving some of its countless mysteries. Team leader Sean Solomon gives us the skinny on its work.

Dave on Earth (9:59 AM): Good morning Sean! Thanks for logging on.

MESSENGER2Merc (10:00 AM): Hi Dave.
I've never used this software before, so bear with me.

Dave on Earth (10:00 AM): No worries, you're doing fine already!
I've been trying to track you down for the good part of a few weeks now.
What have you been up to?

MESSENGER2Merc (10:02 AM): It's been a busy month!
We've had two very successful flybys of Mercury this year, so we're happily digesting the first data from Mercury in more than 30 years.
Right after the most recent flyby, most of the science team gathered at the Applied Physics Laboratory to discuss the new data.

Dave on Earth (10:04 AM): And you have another coming up next year, correct?

MESSENGER2Merc (10:05 AM): Yes, there's one more flyby of Mercury on Sept. 29, 2009.
Eighteen months later we'll catch up with Mercury for the fourth time, and MESSENGER will go into orbit.
As for what else I've been up to: After the second flyby I put together a talk on our findings, followed by a NASA press conference to present more discoveries in late October.
And I just returned from a meeting of the full MESSENGER science team in Boulder last week.

Dave on Earth (10:07 AM): Feel good to be home then?

MESSENGER2Merc (10:08 AM): It does, but I can't savor the moment too long...
I'm also involved in the GRAIL mission to the moon, which is holding a preliminary design review this week at NASA JPL.
So I'm off to the airport again.

Dave on Earth (10:08 AM): And I thought I was a busy person...
Of all the missions you've worked on, which would you say is your favorite?

MESSENGER2Merc (10:10 AM): MESSENGER takes up most of my time as principal investigator.
My involvement with GRAIL is as a co-investigator, and at this stage I provide scientific and technical advice. I've assisted with two earlier spacecraft missions -- Magellan to Venus and the Mars Global Surveyor -- but neither is still operational.
As for my favorite? Always the one I'm involved in at the moment.
The freshness of new observations and the challenge of figuring out what they mean is the reason I'm in this business.

Dave on Earth (10:12 AM): It's tricky business playing favorites, but that answer works for me -- suppose I'd feel the same way in your shoes.
Speaking of the current mission (MESSENGER, of course), you just pulled off an extremely close flyby.
What would you say are the biggest discoveries that have come out of it? And are more surprises in store for us?

MESSENGER2Merc (10:13 AM): Our second flyby (on Oct. 6) allowed us to view about 30 percent of Mercury's surface at close range for the first time.
Take Mariner 10 observations from 1974 through 1975 and add them to those made during our first flyby this January, and we have imaged more than 90 percent of Mercury's surface.
So one big thing is that we now have a global perspective on the planet's geological make-up.
We've confirmed that volcanism has reshaped Mercury's surface in the past, and that the entire planet has shrunk over most of its history.
That contraction has produced huge faults and scarps (or cliffs) almost everywhere we look on Mercury's surface.

Dave on Earth (10:15 AM): What other big things do we now know about Mercury?

MESSENGER2Merc (10:15 AM): In October we sampled Mercury's internal magnetic field over the western hemisphere for the first time.
We found that it's very symmetric about the planet's spin axis, indicating that Mercury's magnetic filed -- like Earth's -- comes from a magnetic dynamo in the planet's churning, molten, metal outer core.

Dave on Earth (10:16 AM): Interesting. As a young kid, I always though that Mercury's core was solid.

MESSENGER2Merc (10:16 AM): Prior to Mariner 10 that was the belief.
Compared to Earth, smaller planets such as Mercury lose interior heat rapidly. But Mariner 10 found a global magnetic field hinting of a fluid outer core.
And last year Earth-based radar supported that finding.
The contribution of MESSENGER, so far, is to show that Mercury's magnetic is best explained by an active core.
MESSENGER has given us the best-ever view of most of the planet.
We flew closer to Mercury (about 125 miles) than Mariner 10 did during its three flybys, mapped the planet's surface with lasers, and viewed Mercury's atmosphere and comet-like tail at close range (which Earth-based astronomers discovered in 2002).

Dave on Earth (10:17 AM): Thanks.
Now, I've heard Mercury is one of the hardest planets to see from Earth. Why is that?

MESSENGER2Merc (10:18 AM): Mercury is hard to view from Earth because it's so close to the sun, and for many telescopes (such as the Hubble Space Telescope) there's so much risk of sunlight damaging sensors that observations of Mercury are forbidden.
That said, conventional telescopes can view Mercury in the night sky just after sunset or just before dawn while solar telescopes can make observations in the daytime. Radio telescopes aren't time-restricted at all.
Radar observations (at radio wavelengths) taught us in 1965 that three Mercury days are equal to two Mercury years.
They also led to the discovery of Mercury's polar deposits -- radar-bright material in shadowed craters that may consist of water ice.

Dave on Earth (10:22 AM): You mentioned water ice existing at the poles of Mercury...
As the closest planet to the sun, how is that possible?

MESSENGER2Merc (10:23 AM): It does seem weird -- at Mercury's equator, the temperature can exceed 800 degrees F.
But Mercury's spin is almost perpendicular to its orbit, so sunlight just grazes the two poles.
Inside the deep floor of an impact crater -- surrounded by a rim of mountains -- the surface is in permanent shadow.
Because the atmosphere is so thin, there's no transport of heat from the equator to the poles as you get on Earth... which means the craters are -300 degrees F.
That's plenty cold to keep water and other material in permanent deep freeze.

Dave on Earth (10:25 AM): If there is water ice, when might it have arrived there?

MESSENGER2Merc (10:26 AM): A current idea is a comet impact some tens of millions of years ago.
The water ice could be stable for billions of years, but radar shows pretty "clean" ice. If the deposits were, older they'd have mixed with soil by now from by smaller meteorite impacts.
We'll have to wait for MESSENGER's insertion into a nearly polar orbit before we can learn much more, though.

Dave on Earth (10:28 AM): I see. Now another curious thing you mentioned: Mercury has an atmosphere.
Is this from volcanoes, or something else?

MESSENGER2Merc (10:28 AM): Mercury isn't thought to be volcanically active today, so there must be some other supply.
We now know hydrogen, helium, oxygen, sodium, magnesium, potassium and calcium are all in the atmosphere. Hydrogen and most helium are probably just solar wind trapped by the planet, but the heavier elements must come from the planetary surface.
Heat from the sun, impact by solar wind, and micrometeorite impacts are all contenders for tossing this material up.

Dave on Earth (10:29 AM): So the jury is out on Mercury's atmosphere, but do we know how thin it is in relation to Earth's or Mars'?

MESSENGER2Merc (10:30 AM): Very thin -- more than a billion times thinner than the Earth's.

Dave on Earth (10:31 AM): So you're mapping the surface, looking for icy deposits, studying the planet's atmopshere, etc.
What else is on the books?

MESSENGER2Merc (10:34 AM): One of the biggest goals is to find out what Mercury's surface is made of, exactly.
It's high density means it has a much higher fraction of iron than any other planet, including Earth.
Why it's so iron-rich is a mystery: Most of Mercury's outer rocky shell could have been vaporized, or a giant impact may have removed it. Or it could be the manner in which the planet accreted (formed).

Dave on Earth (10:34 AM): So Mercury may just be a shadow of its former planetary self...

MESSENGER2Merc (10:35 AM): Possibly. We have many instruments to help us, but they require long observation times to extract chemical signatures of the surface. The flybys were just too short to do this.
Not only that, but the planet's surface, atmosphere and magnetosphere change over days, hours, and minutes.
Flybys do give us fascinating snapshots, but once we're in orbit we'll be able to track all of those changes in detail.

Dave on Earth (10:38 AM): So there's still plenty of work to be done around Mercury, then.
Which I'm sure is exciting :)

MESSENGER2Merc (10:42 AM): Yes! From the outset MESSENGER was designed to be the first spacecraft to orbit Mercury, so most of goals will be met then.
The flybys have been extremely exciting -- they've validated our instruments and they've provided us with armloads of data that help address some big questions.
But now we have more questions, so we're hard at work figuring out how to address as many of them as we can once in orbit.

Dave on Earth (10:44 AM): Thanks Sean.
We've visited every planet in the solar system now, and New Horizons is off to Pluto while Dawn is on its way to Ceres in the Asteroid Belt.
Where in the solar system would you like to see a spacecraft sent, and what would it do there?

MESSENGER2Merc (10:46 AM): The next big robotic mission to be selected by NASA will be to the outer solar system, either the Jupiter or Saturn system.
Both planets have satellites that are each like small worlds, and several have lakes or oceans where prebiotic chemistry is possible.
Are Titan or Europa habitable to life of some form? It's a fascinating question that we won't be able to answer without new missions.

Dave on Earth (10:47 AM): Which mission would you vote for?

MESSENGER2Merc (10:48 AM): I've followed solar system exploration closely since the mid 1960s, and one of the recurring lessons is this: every destination offers its own set of surprises.
I'd like us to learn more about Titan, I'd be engaged by a lander on Europa, and there are new things to learn about Mars, the moon, and every other solar system object.
I'd like to see samples from asteroids and planets returned to laboratories on Earth because we can only do so much with a spacecraft.
Choosing a target is important, but so is deciding what to do once we arrive.
Thankfully there are plenty of attractive possibilities, so I'll be delighted to see us pick a few and do the best job we can.

Dave on Earth (10:50 AM): Thanks Sean!

MESSENGER2Merc (10:51 AM): My pleasure, Dave.
It's been an interesting way to have a conversation!

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