Columbia landed at Kennedy Space Center at 8:58 EST on Saturday, 9 March 1996. Naturally, the Zeno instrument returned with it. And now the Zeno POCC team has all returned home, catching up on some much-needed rest. We had a most unusual event to mark the end of the mission: it was snowing (rather furiously) at the POCC just about the time we were being deactivated. Now, c'mon! This was March, in Alabama! (I wish I had a picture to show here to prove it.)

From the time when the instrument was first powered on at MET 00/03:04 (17:22 CST on Thursday, 22 February), we operated continuously for 14 days, 8 hours, and 10 minutes. During that time we issued 447 commands to the instrument, and collected 468 dual correlograms at 17 temperature set points above Tc:

Tc +

0.507 K

0.304

0.204

0.103

0.0585

0.0329

0.0207

0.0128

0.00826

0.00569

0.00443

0.00285

0.00146

0.00102

0.00079

0.00057

0.00035

A reminder: the horizontal scale is logarithmic; the vertical scale is linear. These are all simple exponential curves, and you can see that the decay times get longer as Tc is approached. You also get some idea about how the scattering intensity increases closer to Tc: the correlograms further from Tc are noisier because their run-time statistics are not so good as for the ones closer to Tc.

And now for some preliminary data. Below are forward-scattering decay rates (at a scattering angle of 12 degrees) reduced from the correlograms at the temperatures given above, plotted using our best estimate so far of the critical temperature (see below). The points are the data (generally obtained from 10-15 correlograms); the line is a theory curve.

Locating the Critical Temperature

The last day of Zeno operations was spent locating the critical temperarture (that is, its value on our instrument's thermometry scale, not in absolute temperature). Our approach was to ramp the temperature of the sample slowly at a rate of 100 microKelvin/hour, and observe the response of the scattered-light intensity and the behavior of the turbidity signal. When phase separation begins, those signals show a strong response.

The graph below is a trace, covering 5 hours, of the forward-scattered light intensity during part of the ramp. The intensity steadily increases until just past MET 13/21:00, when it starts to drop dramatically, showing that phase separation is underway.

Given the ramp rate of 100 microKelvin/hour, each 20-minute tick mark on the horizontal scale equals a change in sample temperature of 33 microKelvin. There are two points to make about this graph.

First, the fact that the intensity increases until phase saturation begins suggests to us that the sample may not have passed directly through the critical point, i.e., that there may have been a small difference between the density of the fluid that we were sampling with the laser beam and the actual critical density (by less than about 1%), causing the sample to cross the coexistence curve slightly away from the critical point. Had the sample passed precisely through the critical point, we had expected that the scattering intensity would saturate at some value just before the phase separation began.

Second, this is the sharpest phase transition we have ever seen with this instrument, with good resolution of at least 20 microKelvin, far surpassing anything we could ever hope to see on the ground in Earth's gravity. That was a nice way to finish this second Zeno experiment.