62 votes

Why do Jupiter’s moons have so much water?

tl;dr: They have more water because they captured it as ice, and it’s easier to hold onto ice than water vapor. Planets (and by extension, moons) beyond the frost line were formed with ice as a part ...
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  • 2,647
20 votes

What are the orbital velocities of the other planets? For objects in a 'Low-Earth-Orbit' around planets other than Earth, e.g.?

The formula for orbital velocity is $\sqrt{GM/r}$ and for a "low" orbit you would mean orbit at, or close to the surface, ie with a radius equal to the radius of the planet. This makes ...
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  • 91.4k
15 votes

At what point in history was the idea of planets being spit out by the sun abandoned?

That specific idea doesn't seem familiar to me, and I work in that field. Particularly this idea would only be called 'theory' when making testable predictions. The only thing that comes to mind here ...
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15 votes

How scientists find the direction of rotation of planets?

I'll focus first on the question of the title: "how do we find / measure rotation?" The easiest method is the same as everyone else: look and see. Take images some time apart and you will ...
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  • 12.3k
12 votes
Accepted

At what point in history was the idea of planets being spit out by the sun abandoned?

Between 1965 and 1980 I recall a book from the "Ladybird" series of short non-fiction books for children which included two possibilities for the formation of the planets. One was "...
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  • 91.4k
11 votes

Which JWST instrument modes are compatible with observations of the bright trans-Earth planets Mars, Jupiter, and Saturn? Which aren't?

There are approved proposals for Cycle 1 to point the JWST at The Jovian system Jupiter's great red spot Mars Saturn and its moons and rings In those PDFs, they describe exactly what instruments ...
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9 votes

What is k2, how does it relate to Io's volcanism and how can Juno constrain its value?

It is called Tidal love number. The definition is as follows: In Newtonian gravitational theory, a tidal Love number relates the mass multipole moment created by tidal forces on a spherical body to ...
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  • 3,243
9 votes

At what point in history was the idea of planets being spit out by the sun abandoned?

Spit out? In 1955 my parents gave me an illustrated book on Astronomy, which gave two hypotheses: the Kant-Laplace (nebular) hypothesis, and Sir James Jeans' idea that a passing star pulled some ...
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7 votes

Which JWST instrument modes are compatible with observations of the bright trans-Earth planets Mars, Jupiter, and Saturn? Which aren't?

All modes can be used. But for bright targets, observations are limited to specific filters, subarrays, regions of the target planet, or spectral intervals. James Norwood and colleagues wrote a paper ...
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  • 1,774
7 votes

What are the orbital velocities of the other planets? For objects in a 'Low-Earth-Orbit' around planets other than Earth, e.g.?

"Low-Earth-Orbit" is kind of arbitrarily defined, and I don't believe there's a widely accepted general definition of a low orbit that can be applied to other planets. If you know what the ...
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6 votes
Accepted

What is k2, how does it relate to Io's volcanism and how can Juno constrain its value?

$k_2$ is one of three tidal Love-Shida numbers related to how gravitation of another body (Jupiter in this case) changes a planet-like body's second degree spherical harmonics (Io in this case). Three ...
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  • 28.3k
6 votes

How do rocky planet break up? Would they fragment into "a gazillion" rocky pieces pieces, or crack open like an egg?

Energy Intro I think any good discussion of planetary destruction should start by talking about energy! Our SI base unit of energy is a Joule, which is defined as $\mathrm{J} = \mathrm{kg} \mathrm{m}^...
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  • 14.1k
6 votes

Is the Michael H. Hart range for the Habitable Zone (Goldlocks Zone) still used?

A number of contemporary researchers in this field consider Hart's model plain erroneous at the lower temperatures (i.e. upper range on the AU), among them David Catling who reasons that the main ...
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  • 231
6 votes

Calculating net gravitational force on Uranus

In the particular case of Uranus, (and considering Uranus and its moons a single system) You only need to consider the Sun + perturbations by the other outer planets. As a more general procedure, ...
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  • 91.4k
5 votes

Could a magnetosphere be created for Venus by recreated by spinning-up the planet to a 24 hour day?

Planetary magnetic fields are not produced by their rotation, they are produced by convection in the core. The rotation has an effect on the patterns of convection through Coriolis forces, but is not ...
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5 votes
Accepted

Do moons of rocky oblate planets migrate to low inclination over time? If so, does the time it takes depend on the Moon's size?

To answer this question, we have to recall the definition of the Laplace surface. Conventionally called Laplace plane, this surface is not actually a plane. In the vicinity of a planet, it coincides ...
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5 votes
Accepted

Calculate Earth Zenith Point of Another Planet with Azimuth and Elevation of another Observer Point

Let's call the point on the Earth's surface where Jupiter is at the zenith as the "sub-Jupiter point". The latitude of the sub-Jupiter point is equal to the declination of Jupiter. Therefore,...
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  • 7,160
4 votes
Accepted

Are there any SuperCam spectra available?

Yes, the spectra from SuperCam along with that from many other instruments is available from the NASA Planetary Data System (PDS) Geosciences Node. The overall landing page for all of the Mars2020 ...
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  • 7,250
4 votes
Accepted

What's the expected distribution of planet masses in a given system?

At zeros order such assumption might be made, but a powerlaw relation is more common and accepted. Also a protoplanetary disk is more complex as is the planet formation process which may include ...
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  • 12.3k
4 votes

If Theia did collide with Earth, would it have produced rings? If so, would we expect to still see at least some remnants today?

First things first, but in this case, second things first. When an asteroid or moon, passes the Roche Limit, it breaks apart and forms rings around the primary body. This is a widely believed ...
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  • 28.3k
4 votes
Accepted

For how many solar system bodies do recent JPL Development Ephemerides include libration?

The short answer is, at the moment, only for the Moon. I include below a brief description of the JPL DE format to arrive to this conclusion. This link explains quite well the format of JPL DE ...
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  • 344
4 votes

Could a magnetosphere be created for Venus by recreated by spinning-up the planet to a 24 hour day?

Earth's magnetic field is more about the rotation of the Earth's core than the Earth itself. Earth's core rotates at a slightly higher rate than Earth itself. It's two-thirds of a second faster. ...
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  • 1,499
4 votes
Accepted

Calculating velocity of Earth from JPL DE

It should be quite obvious that you compute the velocity of the Earth the same way you compute the position. It's multiplying a function by a constant, and that constant will be unaffected by the ...
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  • 2,552
4 votes

Why does the factor of $\frac{1}{4}$ appear in this equation?

It's simple geometry: A planet absorbs energy from its star with its geometric cross-section $\pi r^2$. The same planet re-radiates the energy via its complete surface $4\pi r^2$. In equilibrium, in ...
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  • 12.3k
4 votes

What else can we learn from a Foucault pendulum? Have they ever been used to determine anything more than that the Earth rotates on its axis?

Well, you can calculate your Latitude by observing how long the pendulum takes to complete a full circle. With many thanks to "d_e" for correcting my original statement. the time to ...
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4 votes

Does magnetosphere have an influence on where meteorites would impact on Earth?

There is some evidence that meteoric events are actually distributed unevenly, with the poles experiencing fewer meteoric events per given area than the more central latitudes. NewsInScience(...
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  • 624
3 votes
Accepted

Why does the factor of $\frac{1}{4}$ appear in this equation?

The $\mathrm{\frac{S}{4}}$ represents the area- and time-averaged incident solar flux and the whole term on the LHS represents solar flux emitted by the planet. The factor of 1/4 comes from the fact ...
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  • 3,243
3 votes

How does the Hubble Space Telescope measure the speed of the wind inside Jupiter's Great Red Spot?

There's nothing special happening to determine the velocity: Several pictures are taken, e.g. each Jovian day and the distance some features in the clouds have moved is calculated. For that, image ...
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  • 382
3 votes

Is the boulder on the peak of Tycho Crater the core of the impactor, or is it a random rock?

The photo shows a mix of dark basalt, light anortosite from under the basalt, and a thin layer of impact melt (shiny and rippled) that the Boulder is sitting in. On the slope of the peak towards the ...
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3 votes

If Theia did collide with Earth, would it have produced rings? If so, would we expect to still see at least some remnants today?

tl;dr Maybe it made rings, but they’re certainly not around today. The whole reason the Roche limit exists is because of tidal forces, and the whole reason tidal forces exist is because we choose to ...
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  • 2,647

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