#surface gravity

[thecurioustale]

Worldbuilding: Something I Learned About Planetary Surface Gravity

Last year (hehe; it's always fun to say that at the start of a new year; actual time about 4 months ago), I made a strange discovery!

I won't slow-roll you. Here's the bottom line: In reality, i.e. based on observational data, there appears to be a plateau of surface gravity for planets up to about 500 times more massive than Earth. Most planets between 1 Earth mass and ~500 Earth masses have a surface gravity that is close to our own, i.e. 1 g. So if you're worldbuilding, and need a planet with a surface gravity that is more than a few percentage points higher than Earth's, be careful.

All right, on to the story!

I had need of such a planet for Galaxy Federal. About 125% percent or so, i.e. 1.25 g. And it was in the back of my mind that Neptunian planets often have a "surface" gravity similar to that of Earth's. "Okay," I figured, "that's probably because their giant atmospheres reduce the average planetary density. On a rocky world that wouldn't be an issue. But I'd better check to be safe."

Oh, it is always a good idea to check!

I found a paper which was reporting the plateau that I mentioned in the TL;DR. Here's what it looks like:

Gravity vs mass for Solar System bodies and exoplanets. Ballesteros, Fernando & Luque, Bartolo. (2016). Walking on Exoplanets: Is Star Wars Right?. Astrobiology. 16. 10.1089/ast.2016.1475.

What they found is that Super Earths, i.e. rocky planets with a mass greater than Earth's, did not exhibit the increase in surface gravity seen in smaller celestial bodies. There is this "plateau" where increases in planetary mass and radius are such that the surface gravity essentially levels out for a while.

This isn't something I think you could predict from amateur astronomical principles. What you would predict (certainly, what I predicted) would be what we see on the left side of the above figure, where more mass equals more surface gravity. All of the small rocky bodies in our own Solar System fall onto that left side of the graph. The plateau only really begins in the Earth range of planetary mass. You can see how Venus is included in the plateau almost perfectly, but the much less massive Mars is not.

This is a truly remarkable finding about planetary formation, and has serious implications both for fictional worldbuilding and someday for planetary colonization. That is, if these observations are truly representative of the Universe. This is a pretty big "if" that I don't have the knowledge to be able to check, except to say that there may be some bias unaccounted for because of how we detect exoplanets, such that perhaps the ones we detect are not representative of the ones we do not—so take it all with a grain of salt. This paper is 8 years old but I couldn't find anything newer following up on it.

Fully understanding how it would work is still a question mark for me. I mean, it's a question mark for the professional planetary scientists, too, so there's that, but, in addition to the unknowns that curtail their comprehension, there's a lot I don't know about astronomy that further curtails mine. My understanding is that this isn't really about the intrinsic density of the materials (i.e. substances that are more massive or less massive per mole); it would mainly have to be a consequence of how planets form, how the geological processes change in different planetary mass categories; that sort of thing. But I don't really know! I'm out of my depth.

Anyway, you can download a copy of the full paper on its entry page on ResearchGate; look for the button that says "Download full-text." And the website Centauri Dreams has a nice write-up which is where I originally discovered this.

[brianbrownnet]

A new article has been published on www.brianbrown.net

New Post has been published on http://www.brianbrown.net/2014/10/01/moons-hidden-valley-system-revealed/

Moon's hidden valley system revealed

1 October 2014 Last updated at 19:44

By Jonathan Amos Science correspondent, BBC News

The Moon as we see it (L), in terms of height variation (C), and from surface gravity variations

Scientists have identified a huge rectangular feature on the Moon that is buried just below the surface.

The 2,500km-wide structure is believed to be the remains of old rift valleys that later became filled with lava.

Centred on the Moon’s Procellarum region, the feature is really only evident in gravity maps acquired by Nasa’s Grail mission in 2012.

But knowing now of its existence, it is possible to trace the giant rectangle’s subtle outline even in ordinary photos.

Mare Frigoris, for example, a long-recognised dark stripe on the lunar surface, is evidently an edge to the ancient rift system.

“It’s really amazing how big this feature is,” says Prof Jeffery Andrews-Hanna.

“It covers about 17% of the surface of the Moon. And if you think about that in terms relative to the size of the Earth, it covers an area equivalent to North America, Europe and Asia combined,” the Colorado School of Mines scientist told BBC News.

“When we first saw it in the Grail data, we were struck by how big it was, how clear it was, but also by how unexpected it was.

“No-one ever thought you’d see a square or a rectangle on this scale on any planet.”

The full Moon as seen from the Earth, with the Procellarum border structure superimposed in red

So how was this extraordinary feature produced?

Andrews-Hanna and colleagues note that the Procellarum region contains a lot of naturally occurring radioactive elements, such as uranium, thorium and potassium.

On the early Moon, these would have heated the crust, which, when it cooled would have contracted.

Mare Frigoris is evidently an edge to the ancient rift valley system

This shrinking, they propose, would have ripped the surface, opening deep valleys. The geometry is the giveaway.

On Earth, cooling and contraction will preferentially produce hexagons containing 120-degree angles.

The famous Giant’s Causeway in Northern Ireland is a classic example on the small scale, but even in bigger settings, such as in East Africa’s rift valleys, geological lines tend to intersect in this way.

Procellarum’s giant rectangle does the same, too – because the entire feature is draped over a sphere. This means the angles at the corners are wider than 90 degrees.

“What we’re seeing is a clever trick of spherical geometry. For structures on this scale, a polygon with 120-degree angles at the corners actually has four sides instead of six,” explained Prof Andrews-Hanna.

The team cannot tell when the rift occurred, but the dating of Moon rocks brought back by Apollo would suggest the valleys were filled by volcanic lavas about 3.5 billion years ago.

Giant’s Causeway: Cooling basaltic rock naturally fractures into hexagons

The Grail satellites sensed very subtle variations in the pull of gravity across the Moon’s surface

The new study goes some way to resolving arguments over the origins of Procellarum, which looks different to other, more circular mare (dark regions) on the Moon’s surface.

For these regions, big asteroid impacts were more important in sculpting their forms.

The study is also further proof of the value of the Grail mission, led from the Massachusetts Institute of Technology.

This comprised two, near-identical satellites that chased each other around the Moon over the course of a year.

They mapped changes in the pull of gravity as they flew over areas of differing mass.

Big mountains will have a different signal to deep depressions, obviously. But the data also reveals those locations that have different rock types and densities.

In the case of Procellarum, the pair sensed an excess of mass stemming from the presence of all the basaltic lava filling the rift valleys.

The feature stands out in this Mercator map projection of the changes in gravity

[email protected] and follow me on Twitter: @BBCAmos

Link to this story: 

Please share with your friends:

[christinetheastrophysicist]

A brighter method for measuring the surface gravity of distant stars

Astronomers have found a clever new way to slice and dice the flickering light from a distant star in a way that reveals the strength of gravity at its surface.

That is important because a star’s surface gravity is one of the key properties that astronomers use to calculate a star’s physical properties and assess its evolutionary state.

Read More.