Q&A

Do you have a question about the planets or astronomy that you have been wanting to ask but have not? E-Mail your questions to:

planets@ssec.wisc.edu.

Disclaimer: The answers are provided to satisfy your curiosity only. Notification of errors or other comments on the answers will be appreciated.
April 3, 1996

More questions from Patti Ann Rose, Newbury Park

When I look at the pictures on your web page I see a round circle with sorta stripes on it. I know that if they weren't great pictures of something you wouldn't take the trouble to put them up. 1) What do these pictures tell you that I can't see? You also noted with each picture the passes and (is it Bezel) filter number.
The pictures of Jupiter on the web page show the latitudinal bands of clouds on Jupiter. There is something lost in converting the image data to a format that is compatible with the web browsers (gif files), so the images may not show up the same way on all computers that people use as they do on the one where we make them (more graphics memory you have, the better the picture will look, to a certain degree). These particular pictures were taken with an intent to image the Galileo probe entry site, and that is one reason they are special. In terms of "absolute" picture quality, they are not the best, but certainly the best under the conditions in which Jupiter had to be observed (in daylight, very close to the sun). With more processing, we have learnt from these and other pictures that the probe entered an area which was devoid of clouds compared to other places, and would look redder than the rest of the planet. I will try to put some processed images on the web.

The "passes" associated with each image indicate the range of wavelengths that the optical filter lets through while blocking other colors (wavelengths). The wavelengths are expressed in nanometers (1 nanometer equals 10 raised to the power (-9) meters). The range of wavelengths that the filter lets in is called the "bandpass". Thus, the 750 nm filter is one that lets in wavelengths centered around 750 nm while blocking others. These are just beyond the "red" color that the human eyes can see, and are called near infrared wavelengths.

2) What does that tell the educated observer?

The images such as these are being used to aid in the interpretation of the Galileo Probe results, particularly about the abundance of water vapor. We know for example that in the near infrared wavelengths we observed more radiation compared to the rest of the planet from the region where the probe entered, indicating that the clouds were "thinner" or absent in the upper atmosphere of Jupiter, letting the more intense (warmer) radiation to reach us without being absorbed (due to the absence of clouds). There appears to be a connection between this and the relatively dry atmosphere that the probe instruments observed.

2a). You also talk extensively of the light that reaches us and the time it took to travel that far. Aren't we looking at the universe using radio waves and other such things.
3) What kinds of things do we learn from these other frequencies? (Note. These are follow up questions on the answer to an earlier question below.

All light and radio waves are electromagnetic radiation, and hence essentially similar in their behavior. The only difference is their frequency or wavelength (the two are related to the speed of these waves). All electromagnetic waves travel at the same speed in vacuum. If the medium is different, then their speed is decreased somewhat depending on the properties of the medium and the frequency or the wavelength. This is what causes the illusion of a drinking straw to appear to bend when placed in a glass- the effect is termed refraction of light. The same thing happens to radio waves when the pass through the atmosphere-- they travel along a curved path due to bending caused by refraction. This effect allowed us to measure the "density" of atmospheres of other planets and derive the profile of temperature as a function of height when the Voyager spacecraft passed behind the planets as seen from the earth. This technique is called "radio occultation".

Hello!I'm Wei San from Malaysia. Could you tell me anything about terraforming Mars?

Terraforming Mars implies altering the Martian environment for possible humans and other life forms to survive on Mars. Some research has been done regarding how it can be accomplished. For an illustrative paper (and references therein), click here

Do you think this is possible in the near future?
How near is "near"? Five years, or 50 years? In 5-20 years, the answer is probably no, given the amount of time required for planning, funding, etc. Beyond 20 years, yes, it appears to be feasible in principle, although it certainly would take a long time.

How can we do it?
By following the lessons learned in how we have impacted the earth's environment, and what we have learned about the other planets.

March 14, 1996
From Patti Ann Rose, Newbury Park (rose@rain.org): 
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One of the guys I work with says that astronomers are beginning to see so far that they think they have found the end of the universe. Is that true?

Let's re-phrase your question... By end of the universe you do not imply the "end" of the universe in the sense that the universe is about to cease to exist, but that you can see the "end" or the "edge" of the universe. The sense that "end" or "edge" evoke a physical limit on the size of the universe. In this sense, no, the edge of the universe has not been found, nor is it the intent to find it. Let me explain. The concept of space and time are not truly separable when you start thinking in terms of how fast we can learn about an event given that "light" is the information carrier that brings the knowledge of that event to us, and that it travels at a constant speed in vacuum.

What your colleague is referring to is that astronomers now believe that they have detected light from galaxies that emanated when the universe was young. In other words, they are so far away that the light emitted by them is just now arriving at earth. However, this interpretation is based on several assumptions about what we think we know about the origins of the universe. Briefly, there is the fundamental assumption that the universe originated in a big bang, sending matter expanding outwards. A further assumption based on some interpretation of the data is that depending on the brightness of certain class of stars, a relationship between the "intrinsic" brightness of a star and its distance from us, exists. Rather than go into those assumptions, I would like to point you to an article by George Johnson that appeared in the Sunday, March 10 1996 edition of the New York Times, Week In Review Section (Page E4, National Edition) that explains it well.

March 12, 1996

Questions from 8th Grade Science Classes, Longfellow Junior High School, Wauwatosa, Wisconsin

Have any probes been sent to the sun to make observations?

Yes. the Ulysses Spacecraft was launched some years ago to study the sun. As initially planned, the mission was to have two spacecraft, one from NASA and one from ESA, the European Space Agency. They were to approach the sun over the poles, the NASA spacecraft to fly over one pole, and the ESA spacecraft, travelling in the opposite direction, the other pole. This unusual trajectories were to have been achieved by first travelling to Jupiter, and then using Jupiter's gravity to swing the spacecraft out of the plane of the ecliptic, towards the sun. As it turned out, lack of US funding eliminated the US spacecraft. The remaining Ulysses spacecraft was launched by NASA for ESA and was tracked by JPL. It obtained data about the magnetosphere around Jupiter once it was in the vicinity before being swung on towards the sun. You may learn about the results of this mission from the JPL homepage by clicking on the highlighted hyperlink here.

If Jupiter were to become a star, could we still survive on earth?

What an interesting question. A solar system with two stars. Assuming for the moment that it is possible to have another star in our solar system (would we still call our environment the solar system?!), why not? After all, at Jupiter's orbit, the new sun would be roughly five times away than our original energy star, so its energy falling on the earth would be about twenty five times less than that from the first sun (can you tell why?) Its effect would be comparable to adding about an hours worth of extra starshine at the equinox. In the cold northern climates, that would be quite welcome!

There are other issues to keep in mind though. For Jupiter to become a start, it would have to have more mass, comparable to that of the Sun. If that happens (can you think of how?), it is likely that the orbit of the earth would be altered, since the orbital motion is about a center of mass, which in this case would be determined by the two stars. You can see what consequences this will have. Perhaps you can think about it and come up with some answers that you may want to ask about later?

What causes a planet to rotate?

This is a very good question that may not have an ultimate answer, but there is an explanation. The reason a planet rotates is due to its origins from the nebula from which it formed. This nebula had to be rotating. You probably have heard about the gyroscope and how it works- conservation of angular momentum. Angular momentum is determined by the rate of rotation and the mass of the object and its distribution as a function of distance from the axis of rotation. This is the principle behind an ice-skater spinning up when the arms are brought close to the body, or slowing down when the arms are extended. Except for dissipation and action of other torques, angular momentum is held constant. Thus the rotation of the gases and dust from which a planet formed causes the planet to keep on rotating, to conserve the initial angular momentum.

The planet's rotation is of course changed by events both inside and outside of the planet. For example, it has been recently shown that the use of dams has changed the distribution of stored water on earth at different latitudes, sufficiently to change the length of the day! The difference is small, but has been detectable.

But, why is the angular momentum conserved? And why was the nebula rotating if the universe began in a big bang?

Why does Uranus appear to tilt on its side?

Because it is tilted!

Could the asteroid belt be the remains of "dead planets"

 What are "dead planets?"

Why do planets revolve in elliptical orbits instead of circular orbits?

A circle is a special case of an ellipse, whose eccentricity is zero. So a circular orbit is an elliptical orbit with zero eccentricity. So your question really is "why is the eccentricity not zero for all the orbits?" The answer to this lies in the laws that govern the "motion under the action of a central force." In other words, when a body is moving under the influence of a force that has a definite "source" or a point of origin. For planets, or any object for that matter, this force is the gravitational force. The solution of the possible trajectories includes the general conic sections - hyperbola, parabola, and the ellipse. Only the ellipse is a closed trajectory. All planets around the sun move in closed trajectories, with the central force at one of the focii of the ellipse. This is very close to the location of the sun. In reality, the planets move about the center of mass of the planets and the sun, called the barycenter. The barycenter is close to the sun center, but not quite located at it, since the sun is so massive compared to all the planets combined.

J. Kneisel, Longfellow Middle School, Wauwatosa, Wisconsin asked:
Is there a planet X ?

The belief that a planet may exist beyond the orbit of Pluto was based on the difficulty in modelling the orbital motions of the outer planets, mainly Uranus and Neptune. The reasoning was that a massive object or a planet beyond Pluto would exert enough gravitational attraction to disturb the motions of these planets. Uranus and Neptune themselves were discovered in the last two hundred years in this manner, along with Pluto in this century. However, since 1989 there has been a significant change. The Voyager 2 fly-by of the outer planets, from Jupiter to Neptune, improved our knowledge of the masses of these planets significantly. This knowledge, coupled with the observational errors in the positions of the Uranus, Neptune and Pluto since their discovery, show that their orbital motions can now be modelled well within the observational uncertainties. There is thus no reason to expect a large, massive planet, often called planet X. Since the orbital periods of Neptune and Pluto are approximately 164 and 248 years respectively, we still have not observed them to go around the sun since their discovery.
David Clarkson, South Africa, asked:

Can you please answer two questions that I have regarding the planet Jupiter. I have heard from different unscientific sources the "theory" that Jupiter may be a dormant Sun, or at least it consists of gases that are needed in the composition of a Sun.

Question 1. Is the above science-fiction or is there any truth in it? If there is any truth in it can you please give a brief explanation.

The wording of your question is such that it could be ambiguously interpreted. Let me explain how. If by dormant, you imply that at some future point Jupiter could turn into a "sun", i.e. a star that radiates a lot of energy and has nuclear processes going inside of it (namely conversion of hydrogen into helium that produces much of the energy of the sun), then the answer is no.

If you interpret the question to mean that Jupiter is radiating more energy than it receives from the sun, then yes, Jupiter has this star-like quality. This excess energy is believed to come from the primordial heat of formation of the planet itself from the hot nebula from which the planet was formed. A small fraction may be due to ongoing gravitational compression.

One of the theories of its formation is that the planet formed from the solar nebula, and hence its composition is believed to be sun-like, which is valid to a large degree. Jupiter is mostly hydrogen and helium with trace amounts of water, ammonia, etc. The other theory is that Jupiter accreted by bombardment of comets onto a planetesimal (in which case the composition need not be solar like), and then accreted the gas surrounding it (solar composition). The precise abundance of trace gases is thus one means of distinguishing between these two hypotheses.

Question 2. Does Jupiter have a solid core or is it made up entirely of gases?

Jupiter is believed to have a very small rocky core at its center, extending to about t 10,000 km from the center, or about 15% of its equatorial cloud top radius. Beyond this is a thick layer of metallic hydrogen and helium that contains most of the mass of Jupiter. The boundary between the metallic state and the liquid/gas phase is believed to be approximately 54,000 km from the center. The last layer that extends as much as 17,000 km, is the liquid/gas layer, It is at the opt of this layer that the visible clouds are seen, at a distance of about 71,400 km at the equator from the center. Jupiter is quite oblate, i.e.,. its polar radius is smaller than the equatorial radius by approximately 5,000 km, primarily because of centripetal acceleration due to its rotation rate (once every 9h 55min).

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