Record-breaking Storm Activity on Uranus in 2014

11.3 MB·English

by  Imke de Pater

Draft version January 8, 2015
PreprinttypesetusingLATEXstyleemulateapjv.5/2/11
RECORD-BREAKING STORM ACTIVITY ON URANUS IN 2014
Imke de Pater1,2,3, L. A. Sromovsky4, P. M. Fry4, Heidi B. Hammel5, Christoph Baranec6, and Kunio Sayanagi7
Draft version January 8, 2015
ABSTRACT
In spite of an expected decline in convective activity following the 2007 equinox of Uranus, eight
sizable storms were detected on the planet with the near-infrared camera NIRC2, coupled to the
adaptive optics system, on the 10-m W. M. Keck telescope on UT 5 and 6 August 2014. All storms
were on Uranus’s northern hemisphere, including the brightest storm ever seen in this planet at
5 2.2 µm, reflecting 30% as much light as the rest of the planet at this wavelength. The storm was
1 at a planetocentric latitude of ∼15◦N and reached altitudes of ∼330 mbar, well above the regular
0 uppermost cloud layer (methane-ice) in the atmosphere. A cloud feature at a latitude of 32◦N, that
2 wasdeeperintheatmosphere(near∼2bar), waslaterseenbyamateurastronomers. Wealsopresent
images returned from our HST ToO program, that shows both of these cloud features. We further
n
report the first detection of a long-awaited haze over the north polar region.
a
J Subject headings: Uranus – Uranus, atmosphere

6

1. INTRODUCTION and visible wavelengths, gaseous opacity is dominated
]
P by Rayleigh scattering, while methane absorption dom-
Seasonal forcing has explained hemispherical asymme-
E tries on Earth (Zachos et al. 2001), Mars (Nakamura inates in the near-infrared. At 2.2 µm absorption by
CH and H is so large that only the highest levels in
. & Tajika 2002), Jupiter (Orton et al. 1994), Saturn 4 2
h Uranus’s atmosphere are visible, as sunlight is absorbed
(Greathouse et al. 2005; Moses & Greathouse 2005; Or-
p deeper in the atmosphere. At 1.6-1.8 µm the opacity
ton&Yanamandra-Fisher2005),andNeptune(Hammel
-
is small enough that pressure levels in the atmosphere
o et al. 2007). Uranus, with its pole almost in the eclip-
r tic plane, provides a planetary obliquity extremum. Be- down to several bar8 are probed, the altitudes at which
t clouds typically form.
s cause it also lacks a measurable internal heat source, its
a weather depends more on solar energy than that of the When Uranus approached equinox in 2007 (i.e., when
[ the Sun was shining directly on the equator), cloud ac-
other giant planets. Hence, if seasonal forcing is impor-
1 tant, it should be most apparent on Uranus. We report tivity at near-infrared wavelengths (1-2.2 µm) increased
compared to earlier images at these same wavelengths.
v here extreme dynamic activity on Uranus that does not
In particular, in 2004 one cloud feature in the southern
9 fit any existing seasonal model of this planet.
hemisphere at ∼ 34◦S, known as the “Berg,” developed
0 During the Voyager encounter with Uranus in 1986,
intoapowerfulstorm9 risinguptopressuresof∼0.6bar;
3 only a handful of dim clouds were seen in its atmo-
at 1.6 µm it was seen as a large morphologically inter-
1 sphere. The typical contrast of these clouds (i.e., en-
esting cloud system, while the top of the cloud core that
0 hancement in albedo relative to the background) varied
. from <1% in the violet to ∼7% in the red (Karkoschka reached up to these high altitudes was visible as a tiny
1 (unresolved)featureat2.2µm(Hammeletal.2005). At
1998). All those features were in its southern hemi-
0
1.6µmitbecamemoreelongatedinlongitudein2007,at
5 sphere, the hemisphere facing the Sun (and Earth) at
times displaying several small cloud features at 2.2 µm
1 that time. When Uranus’s northern hemisphere came
(Sromovsky et al. 2009; de Pater et al. 2011). In 2005
: into view, HST images revealed an abundance of small
v this cloud complex started to migrate towards the equa-
cloud features at latitudes that had been in shadow for
i tor (Sromovsky et al. 2009), and it disintegrated in 2009
X ∼40 years. The contrast of these features was highest
(de Pater et al. 2011).
at 1.6-1.8 µm, where it reached values over 2 orders of
r Thenorthernhemisphere,whichhasbeenrotatinginto
a magnitudehigherthanthe∼1%seenbyVoyageratvis-
view since the 1990s, contained a greater number of
ible wavelengths. The observed contrast is a function
bright cloud features than seen during previous decades
of atmospheric opacity and cloud reflectivity. At UV
in the southern hemisphere, with the brightest long-
lived feature at ∼30◦N, usually a double spot known as
1Astronomy Department, 501 Campbell Hall, University of
the“BrightNorthernComplex.”Thisfeaturebrightened
California,Berkeley,CA94720,USA;[email protected]
2FacultyofAerospaceEngineering,DelftUniversityofTech- considerablyattimes, andwasusuallyalsovisibleat2.2
nology,NL-2629HSDelft,TheNetherlands µm. At the peak of its brightness, the feature’s cloud-
3SRON Netherlands Institute for Space Research, 3584 CA
top reached altitudes up to near 300 mbar (Sromovsky
Utrecht,TheNetherlands
4Space Science and Engineering Center, University of et al. 2007). The brightest discrete features on Uranus
Wisconsin-Madison,Madison,WI53706,USA have morphologies – long extended sweeps of clouds and
5AssociationofUniversitiesforResearchinAstronomy,1212 multiplefeaturestravelingtogetherasagroup–thatare
NewYorkAvenueNW,Suite450,Washington,DC20005,USA
6Institute for Astronomy, University of Hawai‘i at Ma¯noa,
Hilo,HI96720-2700,USA 8 InMKSunits,1bar=105 Pascals
7DepartmentofAtmosphericandPlanetarySciences,Hamp- 9 Weusetheword“storm”torefertoextremelybright(relative
tonUniversity,Hampton,VA,23668,USA tothebackgroundatmosphere)andoftenlargecloudfeatures.
2 de Pater et al.
suggestiveofunderlyingvortexsystemsdeeperintheat- UT 17 and 20 August, as indicated. However, without
mosphere. continuous time coverage we cannot be absolutely sure
We have periodically imaged Uranus at high spa- these are the same features.
tial resolution since its 2007 equinox to track the sea- Table 1 summarizes the latitudes of these eight fea-
sonal evolution of its atmosphere (this is the first post- tures. Features labeled 2 and 3 are in a latitude range
equinoctial season observed with modern astronomical that makes them possible candidates to be remnants of
technology). The cloud morphology on Uranus has not the Bright Northern Complex (Sromovsky et al. 2007).
seen many changes, with occasional small cloud features Zonal banding is seen throughout the atmosphere, with
in addition to the Bright Northern Complex. While the the northern hemisphere being “hazier” than the south.
south pole has been covered in a featureless haze since For the first time, we saw a thin haze layer forming over
ourfirstgoodviewsfromVoyager2,whenthenorthpole the north pole.
cameintoviewitwaspepperedwithsmalldiscretecloud In the H band, numerous smaller cloud features were
features (Sromovsky et al. 2012a). This difference may visibleaswell. OnUT5August,someclouds(Feature2,
be a seasonal effect: the south pole has been seen dur- as well as some smaller clouds) show what look like nar-
ing its summer and fall period, while the north pole has row plumes of cloud particles trailing to the east (down
onlybeenseenduringspringtime. Basedondecades-long in the picture) at 1.6 µm. On this date, a small cloud
records of Uranus brightness (Lockwood & Jerzykiewicz can be seen inthe southern hemisphereas well. For alti-
2006), we expect its north pole to become as hazy as its tude determination, the images captured on UT 5 and 6
south pole (Hammel et al. 2007), and perhaps cover the August were complemented with images taken through
small cloud features. a narrowband CH S filter; visually, these images look
4
very similar to the H band images, but probe somewhat
2. OBSERVATIONSANDDATAREDUCTION different atmospheric depths.
Observations of Uranus were obtained on UT 5 and
3. DISCUSSION
6 August 2014 with the 10-m W. M. Keck II telescope
on Maunakea (Hawaii), using the near-infrared cam- The brightness and morphology of Uranus’s cloud ac-
era NIRC2, coupled to the adaptive optics (AO) sys- tivityonUT6Augustisunprecedentedinoveradecade
tem (Wizinowich et al. 2000). NIRC2 is a 1024×1024 of2.2-µmimagingwithKeck. Figure2showsFeatures1,
Aladdin-3 InSb array, which we used in its highest an- 2(UT5August)andBr(UT6August)ascolorcompos-
gular resolution mode, i.e., the NARROW camera at ites(K’=red;CH4S=green;H=blue)afterprojection
9.94±0.03 mas per pixel (de Pater et al. 2006), which onto a rectangular latitude-longitude grid. Feature 1 is
translates roughly to 140 km/pixel on these dates. Be- composed of several compact clouds, visible at both 1.6
cause the AO system was operating in a sub-optimal and 2.2 µm, extending almost ∼ 15◦ in longitude and
fashion, we did not achieve the expected (diffraction- ∼3◦ in latitude, while Feature Br extends over ∼25◦ in
limited) resolution. longitude, i.e., almost 10,000 km in extent, and ∼ 7◦ in
Data were taken in the broadband H (1.48-1.78 µm) latitude. Interestingly, the west side of Br is much red-
andK’(1.95-2.30µm)filters,andthenarrowbandCH S der (i.e., relatively brighter at 2.2 microns, and thus at
4
(1.53-1.66µm)filter,overa4.5-hrperiod(11 15:30UT) higher altitudes in the atmosphere) than the east side,
on each night. One additional image in each H and J indicativeofadifferenceincloudaltitude. Feature1also
(1.17-1.33 µm) band was obtained on UT 17 August at shows such a gradient, but here the east side is reddest.
∼15:10 and in H and K’ band on UT 20 August 2014 Since the winds blow in opposite directions at these two
at ∼13:40. All images were processed using standard latitudes (e.g., Sromovsky et al. 2012a), both features
near-infrareddatareductiontechniques(flat-fielded,sky- haveincommonthatthecloudsinthedown-winddirec-
subtracted, with bad pixels replaced by the median of tion are higher in the atmosphere than in the up-wind
surrounding pixels). We corrected geometric distortion direction, which perhaps may indicate the effect of ver-
using “dewarp” routines provided by Brian Cameron of tical wind shear on a convective updraft.
the California Institute of Technology10. Photometric Figure 2c shows a color composite of Feature 2. This
calibrationswereperformedusingthestarHD1160(Elias image has been processed both to enhance the contrast
et al. 1982). We converted the observed flux densities andtakeoutartifactsinducedbytheAOsystemworking
to the dimensionless parameter I/F as in Hammel et al. inasub-optimalfashion. Thedouble-lobedstructurein-
(1989). ducedbythelatterisseenparticularlywellinthetrailof
BecauseUranushasnotexhibitednotableatmospheric Feature2(Fig.1). Wedeconvolvedtheimagesbyusinga
events over the past several years, we were surprised double Gaussian PSF with offset of 6 pixels and relative
that the northern hemisphere of Uranus was unusually amplitude of 0.4 for the secondary lobe. Eight consec-
active on UT 5 and 6 August, and the activity contin- utive H band images were processed this way, as well
ued throughout at least UT 20 August (Fig. 1). A total as an image close in time in the K’ and CH4S bands.
of about eight sizable cloud features (labeled on the im- Following Fry et al. (2012), all images were projected
ages)wereseeninthenorthernhemisphereineachofthe on a rectangular latitude-longitude grid, and then com-
broadband H and K’ filters on UT 5 and 6 August. As- bined. Feature 2 is moving to the west and faster than
sumingthefeatureswouldfollowSromovskyetal.(2009) the atmosphere at latitudes just to the south. If it in-
13-term Legendre polynomial fit to Uranus’s wind pro- jected cloud material to its south, it would indeed make
file, we were able to identify a few of these features on the long streak we see trailing to the east. The merid-
ional wind shear at this latitude is about -0.08 degree
10 http://www2.keck.hawaii.edu/inst/nirc2/forReDoc/post_ of longitude per hour per degree of latitude. Assuming
observing/dewarp/nirc2dewarp.pro the streamer is 3◦ S of the storm center, it would ex-
Record-breaking Storm Activity on Uranus in 2014 3
Figure 1. ImagesofUranusatH(1.6µm)andK’band(2.2µm)obtainedwiththe10-mKecktelescopeonUT5and6August2014(top
andmiddlerow);thebottomrowshowsHbandimagestakenonUT17and20August2014. On5Augusttheimageswerebothobtained
near UT 12:30. On 6 August they were obtained at UT 15:29 (H) and UT 15:32 (K’). Note the extremely large storm system (labeled
Br) on 6 August. Seven other features that are discussed in the paper are indicated as well. A feature seen in H band on the southern
hemisphereon5AugustismarkedwithanarrowandthesymbolS1. NumerousotherfeaturesarealsovisibleintheHbandimageswhen
suitablyenhanced. Alongitude-latitude(planetocentric)gridhasbeensuperposedontheK’bandimageswithagridintervalof30◦.
4 de Pater et al.
Figure 2. ColorcompositeimagesofFeatures1,2andBr,projectedonarectangulargrid. TheK’bandimage,sensitivetothehighest
altitudes,isshowninred;Hband,sensitivetothedeepestlevels,isshowninblue,andCH4Singreen. TheimagesofFeature1weretaken
onUT5August2014near14:30,Feature2onUT5August2014near11:30,andofBronUT6August2014near14:20. TheboxesA-D
for Feature 1, and A-F for Br indicate the areas where cloud altitudes were determined (Table 1). Feature E is the same as Feature 4 in
Fig. 1.
Record-breaking Storm Activity on Uranus in 2014 5
Figure 4. Models of the vertical 2-way transmission of several
filters(top)andthetransmissionratios(bottom)forazenithangle
Figure 3. The ratio (expressed as a percent) of the differential of0.8. Inaclearatmosphere,theobservedI/Fshouldfollowthe2-
integrated brightness of the cloud as a function of the emission waytransmission. Suchmodelsareusedtodeterminethealtitudes
angle, µ. The intensity scale for K’ band is given on the left; for of the various features, using brightness ratios in different filters,
H and CH4S bands on the right (the I/F for the CH4S band was followingdePateretal.(2011)andSromovskyetal.(2012b).
multipliedby1.5). NotethatthecloudinK’bandreaches30%of
tivity of a Uranus without the presence of discrete cloud
thetotalreflectedlightfromtherestoftheplanet.
features. InordertodeterminethereflectivityofUranus
tend eastward (relative to the storm) at a rate of 0.24◦ without clouds, we constructed an image from all data
hour−1 or5.8◦ day−1,andthustakeabout10daystoex- combined. By using a combination of median averaging
tend backward by ∼60◦. The morphology of this cloud, and removal of excess cloud brightness above the back-
with its long slightly slanted tail (∼1◦ in latitude over ground we were able to construct an image of Uranus
∼60◦ in longitude towards the east) reminds us of the without clouds. The advantage of using this method,
early morphology of the 2010 storm on Saturn, that de- ratherthanusingimagestakenondifferentnightsinpre-
velopedintoastormsystemencompassingtheentirecir- viousyearsthatshowednocloudfeatures,isthatitkeeps
cumference of the planet (Sa´nchez-Lavega et al. 2011, thesameviewinggeometryandphotometriccalibration.
2012; Garc´ıa-Melendo et al. 2013; Sayanagi et al. 2013). Hencethesenumbersdonotsufferfromphotometriccal-
Features 1 and Br are both relatively compact and ac- ibration errors (which typically are of order 10-15%).
company cloudless areas around them; such morpholo- In the absence of gas opacity, the brightness of a per-
gies suggest that these features might be coupled to vor- fectly reflecting cloud of finite dimensions would vary
tex systems deeper in the atmosphere. The clouds likely with µ µ (µ and µ are cosines of the emission and
0 0
form when gas rising in the atmosphere becomes cold incidence angles), due to foreshortening of the projected
enough for particular constituents, such as methane gas, area both with respect to the Sun and Earth. Increased
to condense and form clouds, similar to the orographic gasopacitynearthelimb(i.e,methaneandhydrogenab-
clouds associated with dark spots on Neptune, which sorption, which is strongest in K’ band) would enhance
have been modeled numerically (Stratman et al. 2001). theeffect(thepathlength,andhenceone-wayopacityin-
As shown, by UT 17 August, the storm’s brightness had creases with 1/µ). An optically thin cloud, on the other
decreased substantially. If indeed these features are oro- hand, would brighten near the limb (with 1/µ) due to
graphic clouds, drastic fluctuations in brightness would an increase in optical depth, τ, of cloud particles along
not be surprising. the line of sight (until τ ∼ 0.15; see Dunn et al. 2010).
Rectilinear projections of the images from UT 6, 17 We found a near-linear increase in Br’s brightness as a
and 20 August show that the morphology of the feature function of µ (Fig. 3), suggesting that the clouds must
had changed so much that it was not possible to make be optically thin.
unambiguous measurements of the feature’s drift rate. The most remarkable result is the cloud’s total bright-
Usinglocalbrightcomponentsofthefeatureoneachdate ness in K’ band: it represents ∼30% of the total light
wefoundtwopossibledriftrates: 0.183◦±0.001◦ hour−1 reflected by the rest of the planet at this wavelength.
or 0.148◦±0.001◦ hour−1 eastward, The former is more This is ∼2 times larger than what was seen in 2005 for
probable as it is very similar to the 0.18◦ hour−1 drift the “brightest cloud feature ever observed” on Uranus
ratepredictedforalatitudeof15◦NbasedonSromovsky (Sromovskyetal.2007). Theintegratedbrightnessratio
etal.(2009)13-termLegendrepolynomialfittoUranus’s in H band is a factor of 10 lower than that in K’ band,
wind profile. ∼3%. This is somewhat less than was observed at H in
The differential integrated brightness was obtained by 2005, which suggests that a much larger fraction of the
integrating the brightness (in units of reflectivity, I/F, 2014 cloud resides at very high levels in Uranus’s atmo-
above the background level) in each pixel of the cloud. sphere.
Wethentooktheratioofthisnumbertothetotalreflec- Byusingtheratioofthecloudexcessreflectivityinthe
6 de Pater et al.
Figure 5. a)HSTimagetakenonUT14October2014inthe845Mfilter. b)3-colorcompositeafterprojectiononarectangularlatitude-
longitudegrid. Thecolorsandfiltersareindicatedatthetopoftheimage(FQ889Ninred;Q658Ningreen;FQ937Ninblue). AsinFig.
2,inthiscomposite,highaltitudefeaturesareredanddeepfeaturesareblue.
K’ vs. H bands, and in the H vs. the CH S bands, we seenonUT6August; itwascloudFeature2seenonUT
4
can determine an effective pressure (i.e., altitude) of the 5,6and20August(Fig.1),theonlycloudthatwasmuch
various cloud features. To obtain a rough estimate, we deeper in the atmosphere, and that displayed a tail rem-
calculatetheratiooftheintegratedI/Fvaluesrelativeto iniscent of the early morphology of the 2010 storm on
the background (Table 1) and compare this with calcu- Saturn.
lations of ratios for reflecting layers at different pressure These amateur observations were used to trigger
levels, as shown in Figure 4 (de Pater et al. 2011; Sro- our Target of Opportunity (ToO) program on HST
movsky et al. 2012b). The pressures tabulated for the (GO13712, PI: K. Sayanagi). These observations were
sub-elements of larger features in Table 1 were derived carried out on UT 14 October 2014; images are shown
with the same model using a linear-regression fit (Sro- in Figure 5. An image taken at 845 nm (845M band),
movsky et al. 2012b), where for each point in the boxes where the contrast of features on Uranus’s disk is large,
in Figure 2 the I/F values in the K’ band were plotted isshowninPanela. Panelbshowsa3-colorcompositeof
against those in the H band, and similarly in the H vs. a portion of the image after projection on a rectangular
CH S band. latitude-longitude grid: The highest levels in the atmo-
4
As expected from the numerous clouds detectable in sphereareprobedintheFQ889Nfilter(red);thedeepest
the K’ band, most features are at relatively high al- levels in the atmosphere are probed in the FQ937N fil-
titudes. Feature 2, which was not detected at K’, is ter(blue);theQ658Nfilterprobesintermediatealtitudes
the deepest feature, located at altitudes below the 1-bar (green).
pressure level. All other features are at altitudes above Feature 2, with its extended streamer, is visible just
the methane condensation level at 1.2 bar (Lindal et al. north of the center of the disk at a latitude of 32◦N. At
1987), and hence we suggest that these high clouds are this time it looks like a complex feature, visible in all fil-
composed of methane ice. ters, i.e., it appears to extend from the deep atmosphere
Interestingly,somepartsofthebrightfeatureBrreach tothehighestlayersintheatmosphere. Ratherthanjust
altitudesupto∼330mbar,whereasotherpartsaremuch Feature2,however,weseeinsteadthechanceappearance
deeper in the atmosphere, near the 700-mbar level. The of both Features 1 and 2 at nearly the same longitude;
fact that such a large cloud is visible at such high alti- Feature 1, being higher in the atmosphere (Fig. 1), gives
tudessuggeststhattheremusthavebeenstrongupdrafts the complex feature the red color near 35◦N. Feature 2’s
or waves at or (shortly) before the time of observation. extended streamer is visible only in the blue (FQ937N
This result, combined with the observation of so many filter).
clouds at high altitudes during these observations, sug- The extended tail on the south side shows a slightly
geststhatUranuswasgoingthroughaperiodofextreme steeperincline,by∼1◦inlatitudeover∼40◦oflongitude.
dynamic activity. Thetripletoffeaturesnear17◦Nismostlikelyconnected
In order to learn more about this unusual activity, we to feature Br seen on UT 6 August (and perhaps on the
shared our data immediately with the amateur commu- limbonUT17and20August). Itisvisibleathighlevels
nity,andissuedanalertthroughtheInternationalOuter in the atmosphere. These HST and amateur images will
PlanetsWatch. Theamateurcommunityrespondedwith be analyzed in detail in a future paper.
an extensive observation campaign in close coordination Neither our Keck or HST observations revealed a dark
with our team. Throughout the 2014 fall season they spot. Such spots have been seen in the past (Hammel
observed Uranus. By early October several amateur as- etal.2009;Sromovskyetal.2012b),andmaybeasigna-
tronomers had reported the detection of a bright cloud ture of a vortex system deeper in the atmosphere. The
feature on the planet’s disk , using telescopes varying in non-detection in our Keck images is not surprising, as
size from 14-inch up to the 1-m telescope at Pic-du-Midi such detections require an excellent AO correction. We
andbroadbandfiltersspanning∼650-850nmrange. The also did not detect a dark spot in the HST ToO images.
clouddetectedbythem,however,wasnottheFeatureBr Note, though, that not detecting a dark spot does not
Record-breaking Storm Activity on Uranus in 2014 7
imply that there is no vortex system. edgessupportfromNASAPlanetaryAtmospheresGrant
Circulation models predict the largest hemispheric NNX14AK07G and NSF AAG Grant 1212216. The au-
asymmetry at equinox, and the most significant convec- thors wish to recognize and acknowledge the very sig-
tive events in the late winter (Sussman et al. 2012), i.e., nificant cultural role and reverence that the summit of
they would be visible when the region first comes into Maunakea has always had within the indigenous Hawai-
view after a long cold winter, as the cloud features that ian community. We are most fortunate to have the op-
were seen in the 1990s with HST, as mentioned in the portunity to conduct observations from this mountain.
introduction(Karkoschka1998). Thesemodelsthuspre- Facility: Keck:II (NIRC2-NGS), HST
dict that cloud activity most likely would subside after
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